Recycling and Reuse of Wastes as ConstructionMaterial through Geopolymerization

Item Type text; Electronic Dissertation

Authors Ahmari, Saeed

Publisher The University of Arizona.

Rights Copyright © is held by the author. Digital access to this materialis made possible by the University Libraries, University of Arizona.Further transmission, reproduction or presentation (such aspublic display or performance) of protected items is prohibitedexcept with permission of the author.

Download date 26/04/2018 04:55:59

Link to Item http://hdl.handle.net/10150/223338

1

RECYCLING AND REUSE OF WASTES AS CONSTRUCTION

MATERIAL THROUGH GEOPOLYMERIZATION

by SAEED AHMARI

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF CIVIL ENGINEERING AND ENGINEERING MECHANICS

In Partial Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

WITH A MAJOR IN CIVIL ENGINEERING

In the Graduate College

The University of Arizona

2012

2

UNIVERSITY OF ARIZONA GRADUATE COLLEGE

As members of the Dissertation Committee, we certify that we have read the dissertation

prepared by Saeed Ahmari

entitled RECYCLING AND REUSE OF WASTES AS CONSTRUCTION MATERIAL

THROUGH GEOPOLYMERIZATION

and recommend that it be accepted as fulfilling the dissertation requirement for the Degree of

DOCTOR OF PHILOSOPHY

_____________________________________________________________Date: April 13, 2012

Dr. Lianyang Zhang

_____________________________________________________________Date: April 13, 2012

Dr. Jinhong Zhang

_____________________________________________________________Date: April 13, 2011

Prof. Muniram Budhu

_____________________________________________________________Date: April 13, 2011

Prof. George N. Frantziskonis

Final approval and acceptance of this dissertation is contingent upon the candidate’s submission

of the final copies of the dissertation to the Graduate College.

I hereby certify that I have read this dissertation prepared under my direction and recommend

that it be accepted as fulfilling the dissertation requirement.

_____________________________________________________________Date: April 13, 2012

Dissertation Director: Dr. Lianyang Zhang

3

STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of requirements for an

advanced degree at the University of Arizona and is deposited in the University Library

to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided

that accurate acknowledgment of source is made. Requests for permission for extended

quotation from or reproduction of this manuscript in whole or in part may be granted by

the author.

SIGNED: Saeed Ahmari

4

ACKNOWLEDGEMENTS

I would like to acknowledge the people who have helped me complete this dissertation. I

truly appreciate my major advisor Dr. Lianyang Zhang of the Department of Civil

Engineering and Engineering Mechanics for his sustained attention to my research and

the priceless supervision through which I have learned how to conduct research to reach

lofty objectives. I thank him for sharing his knowledge with me and showing me how to

think when I want to solve a problem. I also thank my minor advisor Dr. Jinhong Zhang

of the Department of Mining Engineering for his warm support, precious advice, and

helping me to learn inter-disciplinary subjects.

I express my gratitude to my dissertation committee members, Prof. M. Budhu, Prof. G.

Frantziskonis, and Dr. Jinhong Zhang for their very constructive comments to align our

research methodology with the goals.

I also thank my best friends Rui Chen and Xin Ren for being helpful in the experiments. I

am so grateful to David Streeter of the Department of Mining Engineering for being

supportive and helping me perform part of the experiments.

At the end, I send my sincere regards to my parents Ashraf and Maryam to whom I owe

everything I have. My parents are heroes of my life since they have dedicated their life to

raise me and help me pursue my academic career. Definitely, their constant persuasion

has taken me to this level of education. I express my earnest appreciation to my older

sister Sorayya for being an advisor of my life and to my younger sisters Sanaz and

Samira for their endless love.

5

DEDICATION

I would like to dedicate this dissertation to my parents Ashraf and Maryam, who have

devoted their life to raise me, have supported me in all stages of my life, and have

encouraged me to pursue my passions through academia. Undoubtedly, I would never

reach at this level of education without their support and attention. Although I have not

been able to visit them during my Ph.D. study, I have been feeling their presence beside

me with every single cell of my body encouraging and giving me hope through the

hardships of my academic life. I am glad that I can please them by accomplishing this

phase of my scholarly life.

6

TABLE OF CONTENTS

ABSTRACT ………………………………………………………… 8

CHAPTER 1 INTRODUCTION………………………………….... 10

1.1 Background……………………………………………. 10

1.2 Research Objectives…………………………………… 13

1.3 Research Methodology………………………………... 13

1.3.1 Macro-scale Study…………………………………….. 14

1.3.2 Micro/nano-scale Study……………………….………. 16

1.4 Dissertation Layout……………………………………. 17

CHAPTER 2 PRESENT STUDY...………………………………….. 19

2.1 Research Performed……………………………………. 19

2.2 Conclusions…………………………………………….. 20

2.3 Recommendations for Future Research………………… 23

REFERENCES ………………………………………………………….. 26

APPENDIX A SYNTHESIS AND CHARACTERIZATION OF FLY

ASH MODIFIED MINE TAILINGS-BASED

GEOPOLYMERS ……………..……………..….…….

29

APPENDIX B EFFECTS OF ACTIVATOR

TYPE/CONCENTRATION AND CURING

TEMPERATURE ON ALKALI-ACTIVATED

BINDER BASED ON COPPER MINE TAILINGS .… 66

APPENDIX C PRODUCTION OF ECO-FRIENDLY BRICKS FROM

COPPER MINE TAILINGS THROUGH

GEOPOLYMERIZATION..………………………..….

104

APPENDIX D LEACHING BEHAVIOR OF MINE TAILINGS-

BASED GEOPOLYMER

BRICKS....……………………………………….…....

143

7

TABLE OF CONTENTS – Continued

APPENDIX E UTILIZATION OF CKD TO ENHANCE MINE

TAILINGS-BASED GEOPOLYMER

BRICKS…………..…………………………………...

156

APPENDIX F PRODUCTION OF GEOPOLYMERIC BINDER

FROM BLENDED WASTE CONCRETE POWDER

AND FLY ASH…………..………………………..…..

182

8

ABSTRACT

Global warming as the most challenging problem in the 21st century and the need for

sustainable development due to the diminishing natural resources have urged recycling

and reuse of wastes. Each year, a huge amount of waste is generated from different

sectors including mining, power and energy, and construction. The significant amount of

mine tailings from mining operations has led to growing concerns about their ecological

and environmental impacts such as occupation of large areas of land, generation of dust,

contamination of surface and underground water. Much of the concrete waste from the

construction industry is still landfilled, leading to different environmental and ecological

problems. Researchers have attempted to reuse wastes as construction material by

utilizing ordinary Portland cement (OPC) to stabilize them. This method, however, has a

number of limitations related to OPC. In this research, a recent technology called

geopolymerization is used to stabilize mine tailings and concrete waste so that they can

be completely recycled and reused. The research includes three main parts. The first part

studies the effect of different factors on the mechanical properties, micro/nano structure,

and elemental and phase composition of mine tailings-based geopolymer binder and

investigates the underlying mechanism of geopolymerization of mine tailings at different

conditions. The second part investigates the feasibility of producing geopolymer bricks

using mine tailings. The physical and mechanical properties, micro/nano structure,

durability, and environmental performance of the produced bricks are studied in a

systematic way. Moreover, the enhancement of the mine tailings-based geopolymer

bricks by adding cement kiln dust (CKD) is studied. The third part of the research

9

investigates the recycling of the fines fraction of crushed waste concrete to produce

binder through geopolymerization in order to completely recycle concrete waste. The

results indicate the viability of geopolymerization of mine tailings by optimizing the

synthesis conditions such as the Si/Al ratio, curing temperature, NaOH concentration,

activator type and composition, initial water content, and pre-compression pressure. By

properly selecting these factors, mine tailings-based geopolymer bricks can be produced

to meet the ASTM standard requirements and to be environmentally safe by effectively

immobilizing the heavy metals in the mine tailings. The physical and mechanical

properties and durability of the mine tailings-based geopolymer bricks can be further

enhanced by adding a small amount of CKD. The results also show that the fines fraction

of crushed waste concrete can be used together with fly ash to produce high performance

geopolymer binder. Incorporation of calcium in the geopolymer structure and coexistence

of the calcium products such as CSH gel and the geopolymer gel explains the

enhancement of the mine tailings-based geopolymer bricks with CKD and the high

performance of geopolymer binder from the waste concrete fines and fly ash. The

research contributes to sustainable development by promoting complete recycling and

utilization of mine tailings and concrete waste as construction material.

10

CHAPTER 1

INTRODUCTION

1.1. Background

Each year, the mining industry generates a tremendous amount of mine tailings,

accounting for nearly half of the total solid wastes generated in the United States [1,2].

Mining tailings are generated from mineral processing of ore via which valuable minerals

such a copper and gold are separated. Mine tailings are commonly disposed of in slurry

form in impoundments constructed on large areas of land. Storage of mine tailings in

impoundments has major disadvantages including failure of the impoundment dam,

costly construction and maintenance, surface erosion and dust generation, and release of

heavy metals due to acid mine drainage (AMD). To overcome these disadvantages and

reduce the risk of contamination, research has been conducted on stabilization of mine

tailings using pozzolanic materials such as cement, lime and fly ash and utilization of the

stabilized tailings as construction material [3,4]. Although the results show improved

mechanical properties, calcium-based stabilization is associated with a number of

disadvantages including poor immobilization of heavy metals, especially at high content,

low acid resistance, and energy intensiveness and generation of greenhouse gases related

to the production of Ordinary Portland Cement (OPC) [5-10].

11

Waste concrete resulted from construction and demolition of concrete structures is

another type of industrial waste, which needs special attention considering its

environmental impacts and economical aspects related to landfilling. Sustainable

development in construction industry requires complete recycling and reuse of waste

concrete; but in current practice only the coarse aggregate part is recycled with the fines

still landfilled. It is also noted that the recycled coarse aggregates can only be used in

low-specification applications because the cement paste/mortar from the original concrete

remains attached to stone particles in the concrete aggregate and leads to new OPC

concrete with inferior strength, durability, and shrinkage properties. On the other hand,

recycling and reuse of concrete aggregates with OPC will have the limitations related to

OPC itself as stated earlier.

To completely recycle and reuse mine tailings and waste concrete in a sustainable and

environmentally friendly way, a technology called “geopolymerization” is adopted in this

research. Geopolymerization is an alkaline activation of silica and alumina containing

materials to produce an amorphous to semi-crystalline polymeric structure. In this

method, sodium hydroxide (NaOH) and sodium silicate (SS) are commonly used as the

activating agent and metakaolin, fly ash and furnace slag as the aluminosilicate sources.

The silica and alumina components of the source material are attacked by and dissolved

in the alkaline solution followed by the formation of a polymeric gel called geopolymer.

Geopolymers have significant advantages over OPC, including [5-15]:

12

Geopolymers can be produced from wastes and thus help preserve natural

resources

Geopolymers can be produced at ambient or slightly elevated temperatures but

OPC is produced at extremely high temperature and thus consume large amount

of energy and generate greenhouse gasses. Production of each ton of OPC

generates about one ton of greenhouse gases

Geopolymers have improved physical and mechanical properties like high

strength, high acid resistance, high resistance to freeze-thaw, and high thermal

resistance

To be workable, OPC typically needs water to solid ratios above 0.4, but

geopolymers are workable even at much lower water to solid ratios

Geopolymers have low permeability around 10-9

cm/s

Geopolymers exhibit effective immobilization of heavy metals

So far, most researchers have focused on using metakaolin, fly ash, and slag as the

aluminosilicate source material (see, e.g., [16-19]). Since mine tailings and waste

concrete have high content of silica and alumina, they can be a potential source material

for geopolymerization. Through geopolymerization of the wastes, not only high

performance binder is produced but the heavy metals in them are effectively

immobilized.

Research has shown that inclusion of calcium can enhance the behavior of geopolymer

[20-23]. Cement kiln dust (CKD) is a byproduct of cement production and contains high

13

calcium content. Therefore, CKD can be used as an economical calcium additive material

for geopolymerization.

1.2. Research Objectives

The major goal of the research is to elucidate the underlying mechanism of

geopolymerization of mine tailings and waste concrete and promote their utilization as

construction material in large scale and in an environmentally friendly way. Specifically,

the research has the following objectives:

Study at macro-scale the physical and mechanical properties, durability and

environmental performance of mine tailings- and/or waste concrete-based

geopolymers by conducting mechanical experiments and leaching analysis to

investigate the effect of different factors.

Investigate the micro/nano-scale structure and elemental and phase compositions

of mine tailings- and waste concrete-based geopolymers at different conditions to

better understand the mechanism through which the geopolymer is formed from

mine tailings and waste concrete.

1.3. Research Methodology

The research takes a multi-scale and multi-disciplinary approach involving systematic

experimental studies as outlined in Fig. 1.1.

14

Fig. 1.1: Research program for investigating geopolymerization of mine tailings and

waste concrete.

1.3.1. Macro-scale study

The macro-scale study investigates the physical and mechanical properties, durability,

and environmental performance by conducting unconfined compression tests, water

absorption/durability tests, and leaching analysis. The unconfined compression tests are

performed to measure the unconfined compressive strength (UCS) of mine tailings- and

waste concrete-based geopolymers prepared at different conditions. The tests investigate

how the different factors such as NaOH concentration, activator type and composition,

initial water content, curing temperature affect the UCS of the geopolymer product.

Macro –Scale Study

Uniaxial

Compression TestsSEM/EDX

Micro /Nano–Scale

Investigation

XRD

FTIR

Water Absorption/

Durability Tests

Leaching Analysis

Leaching KineticsImmobilization

Effectiveness

Kinetics of

Dissolution/Geopolymerization

Linking macro-scale behavior and micro/nano-scale

properties to better understand the underlying mechanism of

geopolymerization

15

The water absorption tests are conducted by immersing specimens in water for different

times to study the capability of specimens in absorbing water, which depends on the

microstructure and porosity of the specimens. Besides that, water absorption can be an

indicator of the degree of geopolymeric reaction. The immersed specimens are also used

to evaluate the durability by measuring the weight loss and UCS of specimens after

immersion.

To study the environmental performance of mine tailings-based geopolymer, leaching

analysis is performed by immersing the specimens in solutions with pH respectively of 4

and 7 and then measuring the concentration of heavy metals in the leachate after different

immersion times. The results can be used to evaluate the effectiveness of geopolymer in

immobilizing heavy metals at different conditions. The leaching behavior is also studied

by fitting the first order reaction/diffusion model to the measured data and comparing the

back-calculated parameters for different heavy metals and specimens. In order to study the

kinetics of geopolymerization, leaching analysis is also performed on the mine tailings

immersed in NaOH solutions of different concentrations and at different temperatures. The

results clarify the effects of alkalinity and curing temperature on the degree of

geopolymerization and thus the mechanical properties of the final geopolymer product.

16

1.3.2. Micro/Nano-scale Study

The macro-scale behavior is closely related to the micro/nano-scale characteristics –

micro/nano-scale structure and elemental and phase compositions. Therefore, systematic

micro/nano-scale study is performed in order to better understand how the micro/nano-

scale characteristics affect the macro-scale behavior.

The micro/nano-scale study consists of SEM/EDX, XRD, and FTIR. The SEM imaging

is used to study the morphology of geopolymer gels and the microstructure of the matrix

after geopolymerization. It provides useful information about the reaction between the

source material particles and the activating solution to form the geopolymeric matrix at

different conditions. Moreover, the failure mechanism can be better understood by

imaging the cracked surfaces. The EDX analysis is performed to identify the surface

constituting elements and evaluate the Si/Al and Na/Al ratios of different phases in the

geopolymer matrix. Incorporation of other types of cations in the geopolymer gel can also

be verified by EDX analysis. An FEI INSPEC-S50/Thermo-Fisher Noran 6 microscope is

used throughout the study to perform the SEM/EDX analysis. Uncoated and unpolished

samples are used in order to minimize the disturbance.

The XRD analysis is performed to study the phase composition of the source material and

the geopolymer. The effect of different conditions on the phase composition of the

geopolymeric matrix is studied by comparing the XRD patterns of different specimens.

The XRD patterns are compared based on the identified minerals, the intensity of the

17

crystalline peaks as a measure of concentration of the crystalline minerals, and the

amorphous phase characteristics consisting of intensity, width, and location in the XRD

pattern. A Scintag XDS 2000 PTS Diffractometer using Cu K radiation, at 2.00

degree/min ranging from 10.00 to 70.00 degrees with 0.600 second count time is used to

perform the XRD analysis.

The FTIR spectroscopy is performed to study the effect of geopolymerization on the

materials’ chemical bonds at different conditions. The spectra are obtained using Thermo

Nicolet 370 FTIR / EZ Omnic using a smart performance ATR ZnSe crystal. The

spectrometer covers wavelengths from 600 to 4000 cm-1

.

1.4. Dissertation Layout

This dissertation is organized following the University of Arizona Graduate College’s

Manual for Theses and Dissertations and includes two chapters followed by six appendices.

This first chapter describes the background, research objectives, research approach and

the layout of the dissertation. The second chapter summarizes the main findings of this

research presented in the appendices. Appendix A is a published paper regarding the

geopolymerization of fly ash added mine tailings, which studies the effect of fly ash

content (Si/Al ratio), NaOH concentration and curing duration on the mechanical

properties of geopolymerized mine tailings. Appendix B is a manuscript already accepted

for publication investigating the effect of major factors on the geopolymerization of

(pure) mine tailings. The optimum curing temperature is first obtained and then the effect

18

of activator type and composition on geopolymerization is studied. Furthermore, the

kinetics of geopolymerization at different curing temperatures is investigated. In

Appendix C which a paper already published, the feasibility of producing mine tailings-

based geopolymer brick is studied. Appendix D evaluates the environmental performance

of the mine tailings-based geopolymer bricks discussed in Appendix C. The leaching

kinetics of the immobilized heavy metals is also studied using an analytical model. In

Appendix E, the enhancement of the mine tailings-based geopolymer bricks by adding a

small amount of CKD is discussed. Finally in Appendix F which is a manuscript already

accepted for publication, the feasibility of using waste concrete fines together with fly ash

as the source material for geopolymer binder production is studied. The focus of this

study is on the contribution of calcium to geopolymerization.

19

CHAPTER 2

PRESENT STUDY

2.1. Research Performed

The methods, results, and conclusions of this research are presented in the appendices.

Appendices A, B, C, and F are the important outcome of this research, which have been

published or accepted for publication. Appendices D and E present the most recent

findings of this research and the results are not submitted for publication yet.

As mentioned in Chapter 1, recycling and reuse of mine tailings (MT) and waste concrete

(WC) through geopolymerization is an important contribution to sustainable

infrastructural materials. Since MT are mainly crystalline materials and exhibit slow

reaction to alkali activators at room temperature, it is important to systematically study

the effect of different factors in order to optimize the geopolymerization of MT. In

Appendix A, the geopolymerization of MT was maximized by adding fly ash to optimize

the initial Si/Al ratio. In this study, fly ash content (Si/Al ratio), NaOH concentration, and

curing time were investigated at a curing temperature of 60 °C. In a further study

presented in Appendix B, in order to maximize the usage of MT, only MT were used as

the source material and the effect of different factors such as activator type and

composition and curing temperature were investigated. This study led to determination of

20

optimum activator composition and curing temperature and better understanding of how

different factors influence the geopolymerization. In Appendix C, MT-based geopolymer

bricks were produced by using different forming pressures, initial water contents, curing

temperatures, and NaOH concentrations. The UCS and water absorption test results show

that by properly selecting these four parameters, MT-based geopolymer bricks meeting

the ASTM standard requirements for almost all applications can be produced. Since MT

contain heavy metal contaminants, it is important to ensure that the MT-based

geopolymer bricks be environmentally safe. Therefore, leaching analysis was performed

in Appendix D on MT-based geopolymer specimens. The results indicate effective

immobilization of heavy metals through chemical and/or physical encapsulation in the

geopolymer structure. The leaching kinetics of the immobilized heavy metals is also

studied using an analytical model. Appendix E studies the enhancement of MT-based

geopolymer bricks by adding a small amount of CKD so that the usage of NaOH can be

reduced and the production of MT-based geopolymer bricks can be more economical.

Finally in Appendix F, the feasibility of using waste concrete fines together with fly ash

as the source material for geopolymer binder production is studied. The study identifies

the optimum combination of waste concrete fines and fly ash for producing the best

performance geopolymer binder.

2.2. Conclusions

The significant conclusions from this research are summarized below.

21

A) (Appendix A) Using a hybrid system in geopolymerization is a convenient way to

achieve required properties. By doing this, the advantages offered by individual

systems are amalgamated into an improved whole system. Due to its higher reactivity

and lower Si/Al ratio, FA can be added to adjust the reactivity and Si/Al ratio of MT

and result in an FA added MT-based geopolymer system with improved mechanical

properties. Regardless of the FA content, the MT-based geopolymer binder shows

fast setting and most of the ultimate strength is gained within 7 days. NaOH

concentration plays an important role in geopolymerization since at higher NaOH

concentration, higher amount of Na+ will be available to react with the solid

aluminosilicates.

B) (Appendix B) The optimum curing temperature for MT-based geopolymer is around

90 °C and further increase in curing temperature will have adverse effects on

geopolymerization. The leaching kinetics of aluminosilicates at different temperatures

indicates the dominance of curing temperature in determining the dissolution process.

The effect of other factors such as NaOH concentration and activator type and

composition also depends on the curing temperature. Increase of NaOH concentration

from 10 to 15 M does not have an improving effect at temperatures lower than 75 °C

but significantly increases the strength at 90 °C. Addition of sodium silicate (SS) to

the NaOH solution leads to strength improvement even at low temperature say 60 °C

but addition of sodium aluminate (SA) profoundly delays the setting at 60 °C

although it improves the UCS at 90 °C. This is due to the difference in the leaching

kinetics of Si and Al at different temperatures. The curing temperature also affects the

22

microstructure of MT-based geopolymer, higher temperature leading to denser matrix

due to the formation of larger amount of geopolymer gel.

C) (Appendix C) NaOH concentration, initial water content, forming pressure, and

curing temperature are four major factors in determining the physical and mechanical

properties of MT-based geopolymer bricks. The geopolymer bricks prepared at higher

NaOH concentration (15 M) have greater UCS than those at lower NaOH concentration

(10 M). High initial water content means larger amount of NaOH at a constant NaOH

concentration and thus increases the strength of the geopolymer brick specimens. Higher

forming pressure leads to larger degree of compaction and thus higher UCS if no water

is squeezed out during the molding process. When the forming pressure is too high,

some water and thus NaOH is lost and the UCS decreases. As concluded in Appendix

B, the optimum curing temperature is around 90 C for MT-based geopolymer. By

properly selecting these four factors, MT-based geopolymer bricks can be produced to

meet the ASTM requirements for most applications.

D) (Appendix D) The produced MT-based geopolymer bricks exhibit effective

immobilization of heavy metals meeting the USEPA standard limits while the

original MT release large amount of Mg, Ca, Mn, Cu, and Zn some of which

exceeding the USEPA standard limits. The analysis based on the first order

reaction/diffusion model indicates that the solubility of ions and chemical retardation

are important factors affecting the leaching of heavy metals in MT. The SEM/EDX

analysis indicates that the heavy metals are effectively incorporated in the

geopolymer structure.

23

E) (Appendix E) A second type of hybrid system was studied by introducing CKD into

MT. Addition of up to 10% CKD results in significant improvement of the physical and

mechanical properties and durability of MT-based geopolymer bricks. Adding 10%

CKD to MT at 10 M NaOH can lead to UCS higher than that at 15 M NaOH without

CKD. The addition of CKD also decreases the loss of weight and UCS of specimens

after immersing in water. Utilization of CKD can reduce the usage of NaOH and makes

the production of MT-based geopolymer bricks more economical.

F) (Appendix F) A third type of hybrid system was studied by using waste concrete fines

together with fly ash to produce geopolymer binder so that the waste concrete can be

completely recycled. The results indicate that utilization of waste concrete fines

together with fly ash can increase the UCS of the geopolymeric binder up to 50%

waste concrete fines content. Further increase of waste concrete fines decreases the

UCS of the geopolymeric binder. With proper combination of waste concrete fines

and fly ash, the geopolymeric binder with required strength can be produced.

2.3. Recommendations for Future Research

The research shows promising results on successful recycling of different types of wastes

through geopolymerization. The research also contributes to the knowledge of

geopolymerization of MT and WC, which are only studied by few researchers. For full-

scale commercial application of the research results, further work is required. Future

research can be done in the following areas:

24

One important requirement for bricks is the freeze and thaw resistance. Therefore,

freeze and thaw tests should be conducted on the CKD-added MT-based

geopolymer bricks at different conditions. In addition, the durability of the CKD-

added MT-based geopolymer bricks in strong acids should also be evaluated.

The study on Ca-added geopolymer came up with promising results. Although the

microscopic and spectroscopic techniques were employed to shed light on the

underlying mechanism of Ca contribution to geopolymerization, MAS-NMR as a

robust tool is recommended to study the evolution in the Ca environment.

The geopolymerization technology can also be applied to geotechnical

engineering as a sustainable soil improvement method. Cellcrete is a new OPC-

based porous material that has been used as a soil improvement technique by

replacing in-situ weak soil with it. The advantage of Cellcrete is its high strength

and light weight. The lightweight is achieved by introducing chemical admixtures

into OPC paste to create evenly distributed pores that are not connected to each

another. A similar study can be performed to produce porous geopolymer

concrete.

The research so far has focused on the experimental study. Future work can be

done based on computational chemistry to study the effect of different factors at

atomistic and molecular levels. Understanding the behavior of MT particles on the

surface and how they react to the activating solution is the key to improve the

reactivity of MT, especially considering their crystalline nature. Researchers have

used practically difficult or energy intensive methods such as calcination and

25

chemical pre-treatment to enhance the reactivity of crystalline materials. Surface

chemistry can help better understand how the surface properties can be

improved.

26

REFERENCES

[1] Collins RJ, Ciesielski SK. Recycling and use of waste materials and by-products

in highway construction. National Cooperative Highway Research Program

Synthesis of Highway Practice 199, Washington DC: Transportation Research

Board; 1994.

[2] TFHRC. Mineral Processing Wastes - Material Description. TFHRC (Turner-

Fairbank Highway Research Center); 2009.

http://www.tfhrc.gov.hnr20/recycle/waste/mwst1.htm.

[3] Sultan HA. Utilization of copper mill tailings for highway construction . Final

Tech. Report, Washington DC: National Science Foundation; 1978, p. 235.

[4] Sultan, HA. Stabilized copper mill tailings for highway construction.

Transportation Research Record, Transportation Res; 1979, p. 1-7.

[5] Poon CS, Peters CJ, Perry R, Barnes P, Baker AP. Mechanism of metal

stabilization by cement based fixation processes. Science of the Total

Environment 1985;41:55-71.

[6] Whinney HG, Cocke DL, Balke K, Ortego JD. An investigation of mercury

solidification and stabilization in Portland cement using X-ray photoelectron

spectroscopy and energy dispersivescopy. Cement and Concrete Research

1990;20:79-91.

`

[7] Fernandez-Jimenez A, Garcı´a-Lodeiro A, Palomo A. Durability of alkali-

activated fly ash cementitious materials. Journal of Materials Science

2007;42:3055-65.

[8] Al Bakri1 MM, Mohammed H, Kamarudin H,. Niza IK, Zarina Y. Review on fly

ash-based geopolymer concrete without Portland cement. Journal of Engineering

and Technology Research 2011;3(1):1-4.

27

[9] Bakharev T. Durability of geopolymer materials in sodium and magnesium

sulfate solutions. Cement and Concrete Research 2005;35:1233-46.

[10] Song XJ, Marosszeky M, Brungs M, Munn R. Durability of fly ash based

geopolymer concrete against sulphuric acid attack. 10DBMC International

Conference On Durability of Building Materials and Components, Lyon, France;

2005.

[11] Van Deventer JSJ, Provis J, Duxson P, Lukey GC. Technological environmental and

commercial drivers for the use of geopolymers in a sustainable material industry.

Intenatinal Symposium of Advanced Processing of Metals and Materials; 2006. p.

241-52.

[12] Drechsler M, Graham A. Innovative Materials Technologies: Bringing Resources

Sustainability to Construction and Mine Industries. 48th Institute of Quarrying

Conference. Adelaide SA; 2005.

[13] Zheng L, Wang W, Shi Y. The effects of alkaline dosage and Si/Al ratio on the

immobilization of heavy metals in municipal solid waste incineration fly ash-

based geopolymer. Chemosphere 2010;79:665–71.

[14] Minarikova M, Škvara F. Fixation of heavy metals in geopolymeric materials

based on brown coal fly ash. Proceedings of theWorld Geopolymer; 2005, p. 45–

50.

[15] Olivia M, Sarker P, Nikraz H. Water penetrability of low calcium fly ash

geopolymer concrete. International Conference on Construction and Building

Technology (ICCBT); 2008, A(46), p. 517-30.

[16] Palomoa A, Grutzeckb MW, Blancoa MT. Alkali-activated fly ashes: A cement

for the future. Cement and Concrete Research 1999;22:1323-9.

28

[17] Van Jaarsveld JGS, Van Deventer JSJ. The effect of metal contaminants on the

formation and properties of waste-based geopolymers. Cement and Concrete

Research 1999;29:1189-200.

[18] Xu H, Van Deventer JSJ. Effect of source materials on geopolymerization.

Industrial Engineering Chem. Res. 2003;42:1698-706.

[19] Rattanasak, U, Chindaprasirt P. Influence of NaOH solution on synthesis of fly

ash geopolymer. Mineral Engineering 2009;22:1073-8.

[20] Yip CK, Lukey GC, Van Deventer JSJ. The coexistence of geopolymeric gel and

calcium silicate hydrate at the early stage of alkaline activation. Cement and

Concrete Research 2005;35:1688-97.

[21] Buchwald A, Hilbig H, Kaps CH. Alkali-activated metakaolin-slag blends-

performance and structure in dependence of their composition. Journal of

Materials Science 2007;42:3024-32.

[22] Zhang Z, Yao X, Zhu H. Potential application of geopolymers as protection

coatings for marine concrete I. Basic properties. Applied Clay Science 2010;49:1-

6.

[23] Temuujin J, Van Riessen A, Williams R. Influence of calcium compounds on the

mechanical properties of fly ash geopolymer pastes. Journal of Hazardous

Materials 2009;167:82-8.

29

APPENDIX A

Paper is published in the Journal of Construction and Building Materials

SYNTHESIS AND CHARACTERIZATION OF FLY ASH MODIFIED MINE

TAILINGS-BASED GEOPOLYMERS

Lianyang Zhang1*

, Saeed Ahmari1, and Jinhong Zhang

2

1Department of Civil Engineering and Engineering Mechanics, University of Arizona,

Tucson, Arizona, USA

2Department of Mining and Geological Engineering, University of Arizona, Tucson,

Arizona, USA

* Corresponding author: Tel.: 1 520 6260532; fax: 1 520 6212550.

E-mail address: lyzhang@email.arizona.edu.

30

ABSTRACT

Each year, the mining industry generates a significant amount of mine tailings. Storage of

these tailings occupies large areas of land and leads to high monetary, environmental and

ecological costs. In this research, a feasibility study is performed on geopolymerization of

mine tailings so that they can be recycled and utilized as construction material.

Considering the extremely high silicon to aluminum (Si/Al) ratio for the mine tailings,

class F fly ash is used to adjust the Si/Al ratio. Sodium hydroxide (NaOH) solution is

used as the alkaline reaction agent. The research consists of unconfined compression tests

to evaluate the mechanical properties, scanning electron microscopy (SEM) imaging to

investigate the microstructure, and the X-ray diffraction (XRD) analysis to study the

phase compositions. The effects of fly ash content (which affects the Si/Al ratio),

alkalinity (NaOH concentration), and curing time on the geopolymerization of mine

tailings are studied in a systematic way. The results show that the Si/Al ratio and the

alkalinity have profound effects on the mechanical and micro-structural properties of the

mine tailings-based geopolymers. The curing time affects the mechanical and micro-

structural properties of the mine tailings-based geopolymers mainly during the first 7

days. Based on the research, it can be concluded that mine tailings are a viable and

promising construction material if the geopolymerization technology is utilized.

31

Key words: Mine Tailings, fly ash, Geopolymer, Microstructure, Uniaxial Compressive

Strength

1. Introduction and Background

Mine tailings are waste materials produced from mining and screening operations [1].

There has been a rapid growth in generation of mine tailings during last decades [2,3].

Disposal of mine tailings have been a major concern in mining industry. In current

practice, the generated mine tailings are collected and transported in paste or slurry form

and disposed in large tailings impoundments, which is costly and utilizes large areas of

land. Disposal of mine tailings in impoundments may also cause environmental and

safety problems, including contamination of surface water, groundwater and soils, and

failure of tailings dams [4-9]. Therefore, researchers have studied the stabilization of

mine tailings so that they can be utilized as construction materials. For example, Sultan

[10,11] investigated the feasibility of using stabilized copper mine tailings in road

construction. He studied the engineering properties of untreated, cement-stabilized and

asphalt-stabilized mine tailings, including compaction characteristics, compressive,

tensile and shear strength, compressibility, permeability, and erodibility by rainfall. The

results demonstrate that copper mine tailings have good engineering properties and can

be easily adapted for use in road construction. Teredesai [12] conducted laboratory

experiments to assess the potential of pile run chat (i.e., mine tailings from abandoned

mines) as a roadway base material. The pile run chat was stabilized with 10% class C fly

ash (CFA) and 10% cement kiln dust (CKD), separately. The laboratory results show that

the unconfined compressive strength of the pile run chat increased significantly due to

stabilization using CFA and CKD as stabilizing agents. The elastic modulus of the pile

run chat also exhibited increase due to the stabilization. The literature review indicates

that the current research on stabilization of mine tailings is limited and focuses on the

stabilization using cement, lime, or material containing sufficient amount of calcium. The

stabilization of mine tailings based on reaction with calcium, however, has a number of

32

limitations, such as low acid resistance, poor immobilization of contaminants, and high

energy usage and greenhouse gas emissions related to ordinary Portland cement (OPC).

To produce 1 ton of OPC, about 1.5 tons of raw materials is needed and 1 ton of CO2 is

released to the atmosphere [1,13]. Even today, only a very small percentage of mine

tailings are recycled and used as construction material.

Recently, a new type of “cement”, called geopolymer or inorganic polymer, has attracted

the attention of many research groups. Geopolymer not only provides performance

comparable to OPC in many applications, but also shows additional advantages such as

rapid development of mechanical strength, high acid resistance, excellent adherence to

aggregates, immobilization of toxic and hazardous materials, and significant reduction in

greenhouse gas emissions. These characteristics have made geopolymers of great

research interest as “an ideal material for sustainable development” [1,14]. Geopolymer

is essentially a synthetic alkali aluminosilicate material produced from the reaction of a

solid aluminosilicate with a highly concentrated alkaline hydroxide and/or silicate

solution. The reaction or geopolymerization can happen at either ambient or elevated

temperatures [15]. In geopolymerization, silica and alumina compounds dissolve in the

alkaline solution and then bond to each other to form a polymeric structure. According to

Duxson et al. [16] and Dimas et al. [17], the geopolymerization process includes

dissolution of solid aluminosilicate materials in a strong alkaline solution, formation of

silica-alumina oligomers, polycondensation of oligomeric species to form inorganic

polymeric material, and bonding of un-dissolved solid particles in the final geopolymeric

structure. Geopolymers, in general, have a chemical formula of

Mn[-(Si-O2-)z-Al-O2-]n · wH2O (1)

where M is the cation (sodium or potassium), n is the degree of polymerization, z is the

quantifying factor for amount of SiO2 monomer units (typically 1, 2, or 3), and w is the

amount of binding water, which can be up to 7 [18]. Based on the value of z, three types

of oligomers can be formed: poly sialate (PS) (-Si-O-Al-O-), poly sialate-siloxo (PSS)

(Si-O-Al-O-Si-O), and poly sialate-disiloxo (PSDS) (Si-O-Al-O-Si-O-Si-O) which has

33

the highest density and the lowest porosity and is obtained by packing SiO2 in the

polymeric network of PSS [19]. Each type of the oligomers has certain Si/Al ratio

meaning that the type of the oligomer can be identified by the ratio of constituting silica

and alumina species.

So far the research on geopolymers has been focused on utilization of kaolinite, fly ash,

and blast furnace slag as the aluminosilicate source material [20-23]. Very few

researchers have studied the geopolymerization of mine tailings [24-27]. Since mine

tailings are rich in silica and alumina, they can be used as a potential source material for

production of geopolymers. This paper studies the feasibility of geopolymerization of

copper mine tailings. Specifically, mine tailings with different amount of fly ash added

are mixed with sodium hydroxide (NaOH) solution to produce geopolymers. Different

factors such as silica to alumina (Si/Al) ratio, NaOH concentration, and curing time are

examined. Unconfined compression tests are conducted to investigate the mechanical

properties of geopolymers produced at different conditions. Scanning electron

microscopy (SEM) imaging and X-ray diffraction (XRD) analyses are also performed to

investigate the microstructure and material phases of geopolymers at different conditions.

2. Materials and Methods

2.1. Materials

The materials used in this investigation include copper mine tailings, class F fly ash,

reagent grade 98% sodium hydroxide (NaOH), and de-ionized water. The mine tailings

were received in the form of dry powder from the Mission Mine Operations of ASARCO

LLC in Tucson, Arizona. The fly ash was provided by Boral Materials Technologies Inc.

in Phoenix, Arizona. The fly ash is originated from the coal at Lee Ranch and El Segundo

mines in New Mexico. Table 1 shows the chemical composition of the mine tailings and

the fly ash. It can be seen that both the mine tailings and the fly ash consist mainly of

silica and alumina with substantial amount of calcium and iron. Grain size distribution

analysis was performed for both the mine tailings and the fly ash by mechanical sieving

34

and hydrometer analysis following ASTM D6913 and ASTM D422. Fig. 1 shows the

particle size distribution curves. The mine tailings has a mean particle size around 120

m with 36% particles passing No. 200 (75 m) sieve while the fly ash is much finer

with a mean particle size around 12.7 m and 90% particles passing No. 200 (75 m)

sieve. SEM imaging was performed on mine tailings and fly ash powders. Fig. 2 shows the

SEM micrographs of the mine tailings and fly ash powders. The mine tailings particles have

irregular shapes while the fly ash particles are smooth spheres. Some of the large mine

tailings particles are formed by flocculation of finer particles. The fly ash contains some

broken hollow spheres with finer particles inside. The specific gravity of the mine tailings

and fly ash particles are respectively 2.83 and 2.18.

The sodium hydroxide pellets were obtained from Alfa Aesar Company, in Ward Hill,

Massachusetts. The sodium hydroxide solution is prepared by dissolving the sodium

hydroxide pellets in de-ionized water.

2.2. Methods

To produce geopolymer paste, first dry mine tailings (MT) and dry fly ash (FA) at a

selected FA content are fully mixed and then the MT/FA mixture was mixed with the

sodium hydroxide solution of a selected concentration. Considering the generated heat,

enough time was allowed for the sodium hydroxide solution to cool down before it was

used. The sodium hydroxide (NaOH) solution was slowly added to the MT/FA mixture

and the resulted mixture was stirred by a mixer for at least ten minutes to ensure

sufficient dissolution of silica and alumina in the alkaline solution. The resulted

geopolymeric paste was then poured in cylindrical Plexiglas molds of 34.50 mm inner

diameter and 86.25 mm length (i.e., an aspect ratio of 2.5). To ensure consistent condition

for all specimens and study the effect of only desired factors, a constant water to solid

(both MT and FA) ratio was used for all specimens. The viscosity of produced pastes

increased slightly at higher NaOH concentration and lower FA content; but the slight

change in viscosity did not significantly affect the workability and all produced pastes

35

could be easily poured into the mold. After the mold was filled with the geopolymeric

paste, it was shaken for 2 minutes to release the trapped air bubbles. Then, the mold was

covered with a Plexiglas cap and the specimen was placed in a 60C oven for curing. The

specimens containing FA were de-molded after 3 hours and placed back in the oven for

prolonged curing while the specimens containing no FA (100% MT) were de-molded

after 24 hours due to slow setting.

Totally, five FA contents (i.e., 0, 25, 50, 75 and 100% by total weight of the dry MT/FA

mixture) for the source material and three NaOH concentrations (i.e., 5, 10 and 15 M) for

the sodium hydroxide solution were used. The water to the total solid material (MT and

FA) ratio of 27% was used for all the specimens. Different curing durations were

considered.

Unconfined compression tests were performed on the cured cylindrical samples with an

ELE Tri Flex 2 loading machine at a constant loading rate of 0.1 mm/min. The tests were

performed to measure the unconfined compressive strength (UCS) of geopolymers

produced at different conditions. For each condition, considering the relatively small

variance of measurements, three specimens were tested and the average of the measured

values was used. Before conducting the compression test, the end surfaces of the

specimens were polished to make sure they are accurately flat and parallel. In addition,

the end surfaces were lubricated to minimize the friction between the specimen and the

steel platens.

To investigate the microstructure and phase composition of the geopolymers, scanning

electron microscope (SEM) imaging and XRD analysis were also performed. The SEM

imaging of geopolymer specimens was performed in SE conventional mode using the FEI

INSPEC-S50/Thermo-Fisher Noran 6 microscope. Totally, seven specimens including

two MT and FA powder specimens and five geopolymer specimens with varying NaOH

concentrations and curing durations were studied. The freshly failed surfaces from the

36

unconfined compression tests, without polishing to keep the fractured surface “un-

contaminated”, were used for the SEM imaging. Carbon tape was used to ground the

specimens. The geopolymer specimens were not coated; but the powder specimens were

coated by Denton Vacuum Desk II gold/palladium sputter coater at 50 millitorr for one

minute.

The XRD analysis was performed to characterize the phase compositions of the mine

tailings, the fly ash and the geopolymers. The XRD analysis was performed with a

Scintag XDS 2000 PTS Diffractometer using Cu K radiation, at 2.00 degree/min

ranging from 10.00 to 70.00 degrees with 0.600 second count time.

Table 2 summarizes the combination of variables studied and the different types of tests

conducted. Three specimens were tested for each combination of variables.

3. Analysis of Results

3.1. Unconfined Compression Test

The unconfined compression test results are shown in Figs. 3 to 6. These figures show the

effect of FA content, NaOH concentration and curing time on the mechanical properties

of mine tailings-based geopolymers.

Fig. 3 shows the unconfined compression strength (UCS) of specimens after 7 days’

curing, with different FA contents and at different NaOH concentrations. Both the FA

content and the NaOH concentration have a significant effect on the UCS. Increased FA

content and NaOH concentration result in higher UCS.

The effect of curing time on UCS was investigated by measuring the compressive strength

of specimens after different curing time. Specimen synthesized with three FA contents (25,

50 and 75%), at 10 M NaOH and cured for 2, 7, 14 and 28 days were tested. Fig. 4 shows

the test results. A large portion of the ultimate compressive strength is gained within 2 days

37

and the specimens reach their ultimate compressive strength in 7 days. After 7 days, the

ultimate compressive strength is essentially unchanged.

Fig. 5 shows the stress-strain curves of the geopolymer specimens synthesized with three

different FA contents, at 10 M NaOH and after different curing time. These curves are the

middle one of the three stress-strain curve obtained at each condition. Due to the small

variations in the measurements, using either the upper or the lower curves instead will not

affect the comparison between different stress-strain curves. It can be seen that the failure

strain (the strain corresponding to the stress equal to UCS) of the geopolymer specimens

tends to increase with FA content. The specimens with low FA content fail with no distinct

failure point on the stress-strain curve while high FA content specimens collapse abruptly

with a sharp peak on the stress-strain curve. Higher NaOH concentration also leads to larger

failure strain, which can be clearly seen from the stress-strain curves in Fig. 6.

3.2. SEM Imaging

The SEM imaging was performed to study the effect of NaOH concentration and curing

time on the microstructure of geopolymers. The SEM micrographs are shown in Figs. 7 to 9.

To investigate the effect of NaOH concentration on the microstructure of the geopolymeric

matrix, SEM imaging was performed on specimens synthesized respectively at 5, 10 and 15

M NaOH (see Figs. 7 and 8). Fig. 7 shows the comparison between SEM micrographs

corresponding to different NaOH concentrations at low magnifications. Fewer individual

FA particles can be seen as the NaOH concentration increases, clearly indicating higher

degree of geopolymerization at higher NaOH concentration. Fig. 8 shows the micrographs

of the same specimens but at higher magnifications. It can be seen that as the concentration

of NaOH increases, more compact structure with larger amount of geopolymer gels is

resulted. The geopolymer gels at 15 M NaOH also seem thicker than those at 5 or 10 M

NaOH, which is due to higher concentration of NaOH and thus larger degree of dissolution

of FA and MT particles. Another important feature, which can be seen from Fig. 8 (c), is the

38

large number of broken FA particles. These broken FA particles are likely generated due to

the strong dissolution at high NaOH concentration and the passing of the failure surface

through the FA particles.

The SEM micrographs of the geopolymer specimens synthesized with 50% FA, at 10 M

NaOH and cured at 2, 7 and 28 days are shown in Fig. 9. Comparison of these micrographs

shows that there is no significant change in the micro-structure of the geopolymer specimens

after 2 days, supporting the unconfined compression test results that a large portion of the

UCS is gained within 2 days.

3.3. XRD Analysis

The XRD patterns of the mine tailings powder, the fly ash powder, and the geopolymer

specimens are shown in Fig. 10 in two scales so that both the crystalline and amorphous

phases can be clearly seen. The XRD pattern of the mine tailings shows that they mainly

consist of crystalline minerals, in which crystalline silica or quartz (SiO2) is the main

component. The crystalline alumina exists in feldspar minerals such as anorthite

(CaAl2Si2O8) and labradorite ((Ca,Na)(Al,Si)4O8). Gypsum (CaSO4·2H2O) and cuprite

(Cu2O) are also detected as the crystalline peaks. The XRD pattern of the fly ash powder

shows that it mainly consists of crystalline materials including silica and mullite. The

amorphous phase of the fly ash powder appears as a broad hump extending from 16 to 44

degrees. This phase can also be seen in the geopolymer specimens. The XRD patterns of the

50% FA geopolymer specimens show that the crystalline peaks drop after

geopolymerization indicating dissolution of the crystalline phase. The XRD patterns of the 2

days’ and 28 days’ cured specimens show slight reduction in the intensity of the crystalline

peaks indicating further dissolution of the silica and alumina components after 2 days.

4. Discussion

Effect of Si/Al Ratio on UCS

39

The increase of UCS with FA content can be explained by the Si/Al ratio of the MT/FA

mixture and the difference between the reactivity of MT and FA particles. Si/Al ratio is one

of the most important factors that affect geopolymerization. The research indicates that in

order to obtain a geopolymer with maximum strength, the Si/Al ratio, in general, should

be in the range of 1 to 3, the specific value depending on the source material used

[19,22,28-32]. The chemical formula proposed by Davidavits [18] for geopolymers

shows that generally the Si/Al ratio can take values of 1, 2, or 3. Cheng and Chiu [28]

reported that a Si/Al ratio of 1.58-1.73 (SiO2/Al2O3 = 3.16-3.46) results in the highest

strength values for the mixture of metakaolin and blast furnace slags. Xu and Van

Deventer [22] tested different combinations of kaolinite, albite and fly ash. Their test

results showed that the geopolymers synthesized with the mixture of all three materials

and the mixture of kaolinite and fly ash with the corresponding Si/Al ratios respectively

of 2.1 and 2, have the highest compressive strength. Duxson et al. [29] studied the effect

of Si/Al ratio on the microstructure and mechanical properties of metakaolin based

geopolymer. They varied the Si/Al ratio from 1.15 to 2.15 and found 1.9 as the ratio

corresponding to the maximum compressive strength. However, Stevenson and Sagoe-

Crentsil [30] reported the optimum Si/Al ratio of 1.75-1.9 (SiO2/Al2O3 = 3.5-3.8) for

metakaolin based geopolymers. Similarly, Silva et al. [31] came up with the optimum

Si/Al ratio of 1.7-1.9 (SiO2/Al2O3 = 3.4-3.8) for metakaolin derived geopolymers when

the Al2O3/Na2O ratio equals 0.8-1.0. Zheng et al. [32] showed that a Si/Al ratio close to

2.0 is the optimum one for municipal solid waste incinerator (MSWI) fly ash based

geopolymer. The optimum ranges of the Si/Al ratio are summarized in Fig. 11.

The MT and FA have a Si/Al ratio respectively of 7.8 (SiO2/Al2O3 = 15.6) and 1.89

(SiO2/Al2O3 = 3.78). By adding the FA into the MT, the Si/Al ratio is decreased and gets

closer to the optimum ranges shown in Fig. 11. Based on the content of FA, the Si/Al ratio

of the MT/FA mixture can be simply calculated and Fig. 12 can be produced from Fig. 3 by

replacing the FA content with the corresponding Si/Al ratio. The UCS is the highest at Si/Al

= 1.89 and decreases with larger Si/Al value. Fig. 13 compares the results from the current

40

experiment with those in the literature. In the figure, the normalized UCS is the UCS

divided by the corresponding maximum UCS at the same condition. For example, for the

current experiment, the normalized UCS is the UCS divided by the maximum UCS obtained

at the same NaOH concentration and after the same curing time. The general trend of the

current experimental results is in good agreement with those from the literature. The

Si/Al ratio of 1.89 at the highest UCS is very close to the average of the optimum Si/Al

ratios obtained by other researchers.

In addition to the Si/Al ratio, the higher reactivity of FA particles than MT particles may

also account for the increase of UCS with addition of FA. For materials with higher

reactivity, more silica and alumina species are dissolved and incorporated in

geopolymerization. Reactivity of materials can be linked to the fineness of particles. Since

the reaction occurs at the particle-liquid interface, the finer particles have higher specific

surface area and are thus more reactive than the larger ones [33]. The FA particles have a

mean size of about 12.7 m while the MT particles have a mean size of approximately

120 m. Due to the smaller size and consequently higher specific surface area, the FA

particles have higher reactivity that the MT particles to the NaOH solution.

Effect of NaOH Concentration

Alkaline solution plays two important roles during geopolymerization, dissolution of

silica and alumina species and charge-balancing of alumina species by providing metal

cations [30]. The current experimental results show increase in UCS with NaOH

concentration, which is also reported by other researchers [34-36]. The increase in UCS

with higher NaOH concentration is mainly due to the dissolution of more silica and

alumina components and thus incorporation of larger quantities of silica and alumina

components in geopolymerization. However, it has been reported that this effect is true

only up to a certain level, after which the increase in NaOH concentration will not result

in higher strength [36-38]. This might be due to the presence of excess Na (or K if KOH

solution is used) ions in the geopolymer framework [37]. Panagiotopoulou et al. [38]

41

showed that silica leaching in NaOH solution for different source materials increases up

to 10 M and further increase in the alkalinity does not lead to further dissolution of silica.

However, the current experiment shows that the UCS increases with alkalinity up to at

least 15 M, which is likely due to the high degree of dissolution of crystalline silica and

alumina and the pozzolanic reactions to form CSH gels. The former possibility can be

clearly seen from the SEM micrographs in Fig. 8 which show larger content of

geopolymer gels and more broken FA particles at higher alkalinity conditions. Formation

of CSH gels as a result of elevated alkalinity concentration and presence of sufficient

calcium has been reported by different researchers [36,39-43,45], which may help explain

the improving effect of high alkalinity on the UCS. The formed CSH gels fill the voids in

the geopolymeric matrix and lead to a stronger microstructure [26,27,44,45]. Since both

the MT and the FA contain a substantial amount of calcium, CSH gels might have formed

contributing to higher UCS.

Contribution of CSH gels to the final product of alkali activated calcium-containing

aluminosilicates at both low and high alkali concentrations (as high as 24 M) and

coexistence of CSH and geopolymer gels have been reported by many researchers

[26,27,36,39-46]. However, the formation and contribution of CSH gels are different at

low and high alkali activator concentrations [39,40,46,47]. At low alkaline concentrations

(low pH values), calcium can be relatively easily dissolved for formation of CSH gels. At

high alkaline concentrations (high pH values), the dissolution of calcium into the

activator solution is hindered by OH- ions but the formation of geopolymeric gels and the

participation of OH- in the geopolymeric network decrease the pH values and thus lead to

dissolution of calcium and formation of CSH gels [39-40]. The measured percentage of

silica species fixed as CSH gels shows increase with alkalinity although the percentage of

silica fixed as geopolymer gels grows even faster [39,40].

In addition to the dissolution of silica and alumina species, the second role of alkaline

solution is to provide metal cations to charge-balance the alumina groups. The Na/Al

42

ratio is an important factor affecting the structure of geopolymeric gels. The Si/Al and

Na/Al ratios used in this investigation are listed in Table 2. Depending on the NaOH

concentration and the FA content, the Na/Al ratio varies from 0.26 to 2.88. Researchers

have used different Na/Al ratios ranging from 0.38 to 2.06 [18,23,30,31,48-50]. The

results show that the optimum Na/Al ratio is around 1 [51]. Too high a Na/Al ratio will

lead to excess metal cations in the polymeric network and adversely affect the

mechanical properties of geopolymers [30], which may explain the very little increase of

UCS at 15 M NaOH concentration and 0% FA (corresponding to a Na/Al ratio of 2.88).

Curing Time Effect

The test results show that all geopolymer specimens reach their ultimate strength within 7

days and a major portion of the ultimate strength (about 80%) is gained within only 2

days. Although the FA particles are more reactive than the MT particles, the rate of

change in the mechanical properties with curing time is almost the same for specimens

with different FA content. Van Jaarsveld et al. [52] showed that only the amorphous

phase is responsible for the strength development with time as no change in the

crystalline phase takes place with time. The XRD patterns of the specimens after different

curing time (Fig. 10) show no significant change either in the amorphous or crystalline

phase after 2 days, although the magnitude of the crystalline peaks drops slightly due to

further dissolution. This may account for the small change of UCS after 2 days.

5. Summary and Conclusions

This paper presents the results of a feasibility study on utilization of mine tailings as

construction material through geopolymerization. Considering the extremely high Si/Al

ratio of the mine tailings, class F fly ash is used to modify the Si/Al ratio. Based on the

study, the following conclusions can be drawn.

1) Si/Al ratio, NaOH concentration, and curing time are three major factors affecting

the behavior of FA modified MT-based geopolymers:

43

Addition of FA to MT results in higher UCS and larger failure strain of the MT-

based geopolymer. The SEM imaging shows formation of more compact

microstructure by increasing the FA content. The improving effect of adding FA to

MT is mainly due to the decrease of Si/Al ratio reaching the range of the optimum

Si/Al ratio.

Higher NaOH concentration contributes to dissolution of more silica and alumina

and formation of larger amount of geopolymeric gels and consequently results in

higher UCS and larger failure strain of the MT-based geopolymer.

The MT-based geopolymers reach their ultimate strength within 7 days and a

major portion of the ultimate strength (about 80%) is gained within only 2 days.

No obvious strength gain and microstructure evolution happen after 7 days.

2) As a result, the compressive strength of the MT-based geopolymer can be controlled

by adjusting the FA content and NaOH concentration. The MT-based geopolymers

synthesized by varying these factors show UCS ranging from 1.37 to 21.2 MPa. So,

the MT-based geopolymers are a viable and promising construction material which

can be tailored for different applications.

Acknowledgements

This work is supported by the National Science Foundation under Grant No. CMMI-

0969385 and the University of Arizona Faculty Seed Grants Program. The authors

gratefully acknowledge the Mission Mine Operations of ASARCO LLC (Dr. Krishna

Parameswaran) and Boral Materials Technologies Inc. (Mr. David Allen) for providing

mine tailings and fly ash used in this investigation.

References

[1] Drechsler M, Graham A. Innovative material technologies: bringing resources

sustainability to construction and mining industries. 48th Institute of Quarrying

Conference, Adelide SA, Australia; 2005.

44

[2] Jakubick AT, McKenna G, Robertson AM. Stabilisation of tailings deposits.

International Experience in proceedings of Mining and the Environment III, Canada;

2003. p. 25-8.

[3] MAC (Mines and Communities). www.minesandcommunities.org, Environment

News Service; 2007.

[4] Skousen JG, Ziemkiewicz P. (eds.) Acid mine drainage control and treatment. 2nd ed.

Morgantown: West Virginia University and the National Mine Land Reclamation

Center; 1995.

[5] Morin KA, Hutt NM. Environmental geochemistry of minesite drainage: Practical and

case studies. Minesite Drainage Assessment Group (MDAG) Publishing, Vancouver,

B.C.; 1997.

[6] Zeng X, Wu J, Rohlf RA. Seismic stability of coal-waste tailings dams. Geotechnical

Earthquake Engineering and Soil Dynamics III, Proceedings of a Specialty

Conference, ASCE; 1998. p. 950-61.

[7] Spooner J. The tailings spill in southern Spain: shearing the experience. CIM Calgary

99, 101st Annual General Meeting; 1999.

[8] De Souza E, Dirige AP. Assessment of the reliability of tailings dam structures by

centrifuge modeling. Environmental Issues and Management of Waste in Energy and

Mineral Production, Singhal and Mehrotra eds.; 2000. p. 257-64.

[9] Mendez-Ortiz BA, Carrillo-Chavez A, Monroy-Fernandez MG. Acid rock drainage

and metal leaching from mine waste material (tailings) of a Pb-Zn-Ag skarn deposit:

45

environmental assessment through static and kinetic laboratory tests. Revista

Mexicana de Ciencias Geológicas 2007;24(2): 161-169.

[10] Sultan HA. Utilization of copper mill tailings for highway construction. Final Tech.

Report, National Science Foundation, Washington DC; 1978. 235 p.

[11] Sultan, HA Stabilized copper mill tailings for highway construction. Transportation

Research Record; 1979. p. 1-7.

[12] Teredesai RV. Stabilization of pile run chat for roadway base application. MS Thesis,

University of Oklahoma; 2005.

[13] Sato T. Nanoscience for greener concrete. NRCC (National Research Council Canada)

Research Report NRCC-51339; 2009. p. 1-5.

[14] Majidi B. Geopolymer technology, from fundamentals to advanced applications: a

review. Materials Technology 2009;24(2):79-87.

[15] Van Deventer JSJ, Provis J, Duxson P, Lukey GC. Technological environmental and

commercial drivers for the use of geopolymers in a sustainable material industry.

Intenatinal Symposium of Advanced Processing of Metals and Materials; 2006. p.

241-52.

[16] Duxson,P, Mallicoat SW, Lukey GC, Kriven WM, Van Deventer JSJ. The effect of

alkali and Si/Al ratio on the development of mechanical properties of metakaolin-

based geopolymers. Colloids and Surfaces A: Physicochem. Eng. Aspects

2007;292:8–20.

46

[17] Dimas D, Giannopoulou I, Panias D. Polymerization in sodium silicate solutions: a

fundamental process in geopolymerization technology. Journal of Materials Science

2009;44:3719-30.

[18] Davidovits J. Mineral polymers and methods of making them. US Patent 4349386;

14th Sept. 1982.

[19] Davidovits J. Geopolymers: inorganic polymeric new materials. Journal of Thermal

Analysis 1991;37(8): 1633-56.

[20] Palomo A, Grutzeck MW, Blanco MT. Alkali-activated fly ashes A cement for the

future. Cement and Concrete Research 1999;29(18):1323–29.

[21] Van Jaarsveld JGS, Van Devente JSJ. The effect of metal contaminants on the

formation and properties of waste-based geopolymers. Cement and Concrete Research

1999;29(8):1189–200.

[22] Xu H, Van Deventer JSJ. Effect of source materials on geopolymerization. Industrial

and Engineerng Chemistry Research 2003;42:1698-706.

[23] Rattanasak U, Chindaprasirt P. Influence of NaOH solution on the synthesis of fly ash

geopolymer. Mineral Engineering 2009;22:1073-78.

[24] Giannopoulou IP, Panias D. Development of geopolymeric materials from industrial

solid wastes. 2nd International Conference on Advances in Mineral Resources

Management and Environmental Geotechnology, Greece; 2006.

47

[25] Southam DC, Brent GF, Felipe F, Carr C, Hart RD, Wright K. Towards more

sustainable mine fills - replacement of ordinary portland cement with geopolymer

cements. World Gold Conference, Australia; 2007.

[26] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Properties of tungsten mine waste

geopolymeric binder. Construction and Building Materials 2008;22(6):1201-11.

[27] Pacheo-Torgal F, Castro-Gomes J, Jalili S. Durability and environmental performance

of alkali-activated tungsten mine waste mud mortars. Journal of Materials in Civil

Engineering 2010;22:897-904.

[28] Cheng TW, Chiu JP. Fire resistant geopolymer produced by granulated blast furnace

slag. Mineral Engineering 2003;16:205-10.

[29] Duxson P, Provis JL, Lukey GC, Mallicoat SW, Kriven WM, Van Deventer JSJ.

Understanding the relationship between geopolymer composition, microstructure and

mechanical properties. Colloids and Surfaces A: Physicochem. Eng. Aspects

2005;269(1-3):47–58.

[30] Stevenson M, Sagoe-Crentsil K. Relationships between composition, structure and

strength of inorganic polymers, Part I Metakaolin-derived inorganic polymers. Journal

of Materials Science 2005;40:2023-36.

[31] Silva PD, Sagoe-crenstil K, Sirivivatnanon V. Kinetics of geopolymerization: Role of

Al2O3 and SiO2. Cement and Concrete Research; 2007;37:512–8.

[32] Zheng L, Wang W, Shi Y. The effects of alkaline dosage and Si/Al ratio on the

immobilization of heavy metals in municipal solid waste incineration fly ash-based

geopolymer. Chemosphere 2010;79:665–71.

48

[33] Diaz EI, Allouche EN, Eklund S. Factors affecting the suitability of fly ash as source

material for geopolymers. Fuel 2010;89(5):992-6.

[34] Hardjito D, Wallah SE, Sumajouw DMJ, Rangan BV. On the development of fly ash-

based geopolymer concrete. ACI Material Journal 2004;101(6):467-72.

[35] Yunfen H, Dongmin W, Wenjuan Z, Hongbo L, Lin W. Effect of activator and curing

mode on fly ash-based geopolymer. Journal of Wuhan University of Technology-

Mater. Sci. Ed. 2009;24(5):711-5.

[36] Guo X, Shi H, Dick WA. Compressive strength and microstructural charachteristics of

class C fly ash geopolymer. Cement & Concrete Composites 2010;32:142-7.

[37] Khale D, Chaudhary R. Mechanism of geopolymerization and factors influencing its

development: a review. Journal of Materials Science 2007;42:729-46.

[38] Panagiotopoulou CH, Kontori E, Perraki TH, Kakali G. Dissolution of aluminosilicate

minerals and by-products in alkaline media. Journal of Materials Science

2007;42:2967-73.

[39] Granizo ML, Alonso S, Blanco-Varela MT, Palomo A. Alkaline activation of

metakaolin: Effect of calcium hydroxide in the products of reaction. Journal of the

American Ceramic Society 2002;85(1):225-31.

[40] Alonso S, Palomo A. Calorimetric study of alkaline activation of calcium hydroxide-

metakaolin solid mixtures. Cement and Concrete Research 2001;31(1):25-30.

49

[41] Yip CK, Lukey GC, Van Deventer JSJ. The coexistence of geopolymeric gel and

calcium silicate hydrate at the early stage of alkaline activation. Cement and Concrete

Research 2005;35(9):1688-97.

[42] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Tungsten mine waste geopolymeric

binder: Preliminary hydration products investigations. Construction and Building

Materials 2009;23(1):200-9.

[43] Komnitsas K, Zaharaki D. Utilisation of low-calcium slags to improve the strength and

durability of geopolymers. Geopolymers Structure, Processing, Properties, and

Industurial Applications, Provis JL and Van Deventer JSJ eds. Woodhead Publishing

Limited and CRC Press LLC; 2009. p. 343-75.

[44] Alonso S, Palomo A. Alkaline activation of metakaolin and calcium hydroxide

mixtures: Influence of temperature, activator concentration and solid ratio. Materials

Letters 2001;47(1-2):55-62.

[45] Temuujin J, Van Riessen A, Williams R. Influence of calcium compounds on the

mechanical properties of fly ash geopolymer paste. Journal of Hazardous Materials

2009;167(1-3):82-8.

[46] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Investigations of tungsten mine waste

geopolymeric binder: Strength and microstructure. Construction and Building

Materials 2008;22(11):2212–9.

[47] Duxson P, Fernandez-Jimenez A, Provis JL, Lukey GC, Palomo A, Van Deventer JSJ.

Geopolymer technology: the current state of the art. Journal of Materials Science

2007;42(9):2917-33.

50

[48] Muniz-Villarreal MS, Manzano-Ramirez A, Sampieri-Bulbarela S, Gasca-Tirado JR,

Reyes-Araiza JL, Rubio-Avalos JC, Perez-Bueno JJ, Apatiga LM, Zaldivar-Cadena

A, Amigo-Borras V. The effect of temperature on the geopolymerization process of a

metakaolin-based geopolymer. Materials Letters 2011;65(6):995-8.

[49] Rowles M, O’Connor B. Chemical optimisation of the compressive strength of

aluminosilicate geopolymers synthesised by sodium silicate activation of

metakaolinite. Journal of Materials Chemistry 2003;13(5):1161-5.

[50] Tippayasam C, Boonsalee S, Sajjavanich S. Geopolymer development by powders of

metakaolin and wastes in Thailand. Advances in Science and Technology 2010;

69:63-8.

[51] Provis JL. Modelling the formation of geopolymers. PhD Thesis, The University of

Melbourne; 2006.

[52] Van Jaarsveld JGS, Van Devente JSJ, Lukey GC. The effect of composition and

temperature on the properties of fly ash- and kaolinite-based geopolymers. Chemical

Engineering Journal 2002;89:63-73.

51

Table 1. Chemical composition (weight %) of mine tailings and fly ash.

Chemical Compound Mine Tailings (%) Fly Ash (%)

SiO2 64.81 58.94

Al2O3 7.08 26.45

Fe2O3 4.33 5.31

CaO 7.52 4.20

MgO 4.06 1.15

SO3 1.66 0.25

Na2O 0.90 0.84

K2O 3.26 1.09

52

Table 2. Combination of variables studied and different types of tests conducted.

Specimen

Label

Fly Ash

Content

(%)

NaOH

Concentration

(M)

Si/Al Na/Al

Curing

Time

(days)

UCS

Test XRD

SEM

/EDS

Fly ash 100 0 X X

Mine tailings 0 0 X X

5-7-0-60 0 5 7.78 0.96 7 X

5-7-25-60 25 5 4.52 0.57 7 X

5-7-50-60 50 5 3.14 0.41 7 X X

5-7-75-60 75 5 2.38 0.31 7 X

5-7-100-60 100 5 1.89 0.26 7 X

10-7-0-60 0 10 7.78 1.92 7 X

10-2-25-60 25 10 4.52 1.14 2 X

10-7-25-60 25 10 4.52 1.14 7 X

10-14-25-60 25 10 4.52 1.14 14 X

10-28-25-60 25 10 4.52 1.14 28 X

10-2-50-60 50 10 3.14 0.81 2 X X X

10-7-50-60 50 10 3.14 0.81 7 X X X

10-14-50-60 50 10 3.14 0.81 14 X

10-28-50-60 50 10 3.14 0.81 28 X X X

10-2-75-60 75 10 2.38 0.63 2 X

10-7-75-60 75 10 2.38 0.63 7 X

10-14-75-60 75 10 2.38 0.63 14 X

10-28-75-60 75 10 2.38 0.63 28 X

10-7-100-60 100 10 1.89 0.51 7 X

15-7-0-60 0 15 7.78 2.88 7 X

15-7-25-60 25 15 4.52 1.71 7 X

15-7-50-60 50 15 3.14 1.22 7 X X X

15-7-75-60 75 15 2.38 0.94 7 X

15-7-100-60 100 15 1.89 0.77 7 X

53

Fig. 1. Grain size distribution of mine tailings and fly ash.

0

10

20

30

40

50

60

70

80

90

100

1101001000

Particle size (m)

FA

MT

Pe

rce

nt p

assin

g (%

)

54

Fig. 2. SEM micrograph of a) MT powder; and b) FA powder.

a b

55

Fig. 3. (a) 7 day unconfined compressive strength (UCS) vs. FA content at different

NaOH concentrations; and b) 7 day UCS vs. NaOH concentration with different FA

contents.

b

0

5

10

15

20

0 20 40 60 80 100

FA Content (%)

15

10

5

UC

S (M

Pa

)NaOH (M)

a

0

5

10

15

20

5 7 9 11 13 15

NaOH Concentration (M)

100

75

50

25

0

UC

S (M

Pa

)U

CS

(M

Pa

)

FA Content (%)

56

Fig. 4. Unconfined compressive strength (UCS) vs. curing time for geopolymers

synthesized with respectively 25, 50 and 75% FA and at 10 M NaOH concentration.

0

1

2

3

4

5

6

7

8

9

2 7 12 17 22 27

Curing Time (days)

75

50

25

UC

S (M

Pa

)

FA Content (%)

57

Fig. 5. Stress-strain curves of geopolymers synthesized with different FA contents and at 10 M

NaOH concentration: (a) 2 days’ curing, (b) 7 days’ curing, and (c) 28 days’ curing.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 0.5 1 1.5 2 2.5

Str

ess (

kP

a)

Strain (%)

75

50

25

FA Content (%)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 0.5 1 1.5 2 2.5

Str

ess (

kP

a)

Strain (%)

75

50

25

FA Content (%)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 0.5 1 1.5 2 2.5

Str

ess (

kP

a)

Strain (%)

75

50

25

FA Content (%)

a

c

b

58

Fig. 6. Stress-strain curves of geopolymers synthesized with 50% FA, at 5, 10 and 15 M

NaOH concentration, and after 7 days’ curing.

0

2000

4000

6000

8000

10000

12000

0 1 2 3 4

Str

ess (

kP

a)

Strain (%)

15

10

5

NaOH Concentration (M)

59

Fig. 7. Low magnification SEM micrographs of geopolymers synthesized with 50% FA,

after 7 days’ curing, and at a) 5 M, b) 10 M, and c) 15 M NaOH.

a b

c

60

Fig. 8. High magnification SEM micrographs of geopolymers synthesized with 50% FA,

after 7 days’ curing, and at a) 5 M, b) 10 M, and c) 15 M NaOH (FA = fly ash, MT =

mine tailings, and GP = geopolymer).

FA

MT

GP

a

c

b

FA

MT

GP

FA

MT FA

GP

GP

61

Fig. 9. SEM micrographs of geopolymers synthesized with 50% FA, at 10 M NaOH and

cured for a) 2 days, b) 7 days, and c) 28 days (FA = fly ash, MT = mine tailings, and GP

= geopolymer).

MT

FA

GP

c

FA

MT

GP

a

MT

FA GP

b

62

Fig. 10. XRD patterns of un-reacted mine tailings powder, fly ash powder and the

geopolymer specimens synthesized with 50% FA, at different NaOH concentrations and

after different curing durations: a) scaled up; and b) scaled down (the maximum intensity

on vertical axis is limited to 2500) (A: Anorthite, C: Cupryte, G: Gypsum, L: Labradorite,

M: mullite, R: Calcium Carbonate, S: SiO2).

a

b

63

Fig. 11. Optimum Si/Al ratios from different researchers for different source material

types.

A: Fly ash, kaolinite and albite [22]

B: Metakaolin and blast furnace slags [28]

C: Metakaolin [29]

D: Metakaolin [30]

E: Metakaolin [31]

F: Municipal solid waste incinerator (MSWI) fly ash [32]

1.0

1.5

2.0

2.5

3.0

A

B

C

D

E

F

Op

tim

um

Si/A

l ra

tio

64

Fig. 12. UCS vs. nominal Si/Al ratio for geopolymers at 5, 10 and 15 M NaOH

concentrations and after 7 days’ curing.

0

5

10

15

20

1.5 2.5 3.5 4.5 5.5 6.5 7.5

Nominal Si/Al

15

10

5

UC

S (M

Pa

)

NaOH Concentration (M)

65

Fig. 13. Comparison of normalized UCS vs. Si/Al relationship between current

experiment and experiments in the literature.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Nominal Si/Al

Current experminet

[22]

[24]

[29]

[31]

No

rma

lize

d U

CS

66

APPENDIX B

Paper is pending publication to the Journal of Materials Science

EFFECTS OF ACTIVATOR TYPE/CONCENTRATION AND

CURING TEMPERATURE ON ALKALI-ACTIVATED BINDER

BASED ON COPPER MINE TAILINGS

Saeed Ahmari1, Lianyang Zhang

1,*, and Jinhong Zhang

2

1Department of Civil Engineering and Engineering Mechanics, University of Arizona,

Tucson, Arizona, USA

2Department of Mining and Geological Engineering, University of Arizona, Tucson,

Arizona, USA

* Corresponding author: Tel.: 1 520 6260532; fax: 1 520 6212550.

E-mail address: lyzhang@email.arizona.edu.

67

ABSTRACT

This paper investigates the effects of activator type/concentration and curing temperature

on alkali-activated binder based on copper mine tailings (MT). Different alkaline

activators including sodium hydroxide (NaOH), sodium silicate (SS), and sodium

aluminate (SA) at different compositions and concentrations were used and four different

curing temperatures, 60, 75, 90 and 120 °C, were considered. Scanning electron

microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), and X-ray diffraction

(XRD) were conducted to investigate the effect of these factors on the unconfined

compressive strength (UCS), microstructure, and phase composition of the binder. The

results indicate that NaOH concentration and curing temperature are two important

factors that affect the UCS and micro-structural properties of the alkali-activated MT

binder. The optimum curing temperature, i.e. the curing temperature at the maximum

UCS, depends on the NaOH concentration, lower optimum curing temperature at smaller

NaOH concentration. Addition of aqueous SS to the NaOH solution can lead to strength

improvement, with the highest UCS obtained at a SiO2/Na2O ratio of 1.0-1.26. Addition

of powder SA to the NaOH solution profoundly delays the setting at 60 °C but improves

the UCS at 90 °C. The SEM/EDX results show highly heterogeneous microstructure for

the alkali-activated MT binder as evidenced by the variable Si/Al ratios in different

phases. The XRD patterns indicate a newly formed crystalline phase, zeolite, in the 90

°C-cured specimens. The results of the present study provide useful information for

recycling and utilization of copper mine tailings as construction material through the

geopolymerization technology.

Key words: Copper mine tailings; Geopolymer; Alkaline activator; Curing temperature;

Unconfined compressive strength

68

1. Introduction

Concrete is by far the most widely used construction material. Each year, more than 10

billion tons of concrete is produced in the world [1]. However, the popularity of concrete

also carries with it an enormous impact on the environment. Ordinary Portland cement

(OPC) is a major component of concrete. The manufacturing of OPC not only consumes

significant amount of natural materials and energy but also releases substantial quantity of

green house gases. To produce 1 ton of OPC, about 1.5 tons of raw materials are needed

and 0.7 ton of carbon dioxide (CO2) is released to the atmosphere [2]. Worldwide, the

cement industry alone is estimated to be responsible for about 7% of all CO2 generated

[3-6]. Another drawback for OPC is that it may not provide the required properties for

many types of structures, such as rapid development of mechanical strength and high

resistance to chemical attack.

Growing environmental awareness and the need to ensure sustainability of construction

materials have led to efforts to look for alternative materials for OPC [6,7]. Recently, a

new type of “cement”, called geopolymer or inorganic polymer, has attracted the attention

of many researchers. Geopolymer is a synthetic material produced from the reaction of

aluminosilicates with a highly concentrated alkaline hydroxide or silicate solution, having

an amorphous polymeric structure with interconnected Si–O–Al–O–Si bonds [3,8-12].

Geopolymer not only provides performance comparable to OPC in many applications, but

also shows additional advantages such as rapid development of mechanical strength, high

acid resistance, no/low alkali-silica reaction (ASR) related expansion, excellent

adherence to aggregates, immobilization of toxic and hazardous materials and

significantly reduced greenhouse emissions. These characteristics have made geopolymer

of great research interest as “an ideal material for sustainable development” [12-16].

However, very few researchers have studied the geopolymerization of mine tailings [17-

20] despite of their abundance [14,21,22] and suitability for geopolymerization

considering the high content of silica and alumina [14,19,23-24].

69

The main objective of this study is to investigate the effects of activator

type/concentration and curing temperature on the mechanical properties and

microstructure of copper mine tailings-based geopolymer. Different activators including

sodium hydroxide, sodium silicate, and sodium aluminate at different compositions and

concentrations were used and four different curing temperatures, 60, 75, 90 and 120 °C,

were considered. The effects of these factors on the mechanical properties of copper mine

tailings-based geopolymer binders and on the kinetics of dissolution of copper mine

tailings were investigated using unconfined compression tests and leaching analyses,

respectively. Scanning electron microscopy/energy-dispersive X-ray spectroscopy

(SEM/EDX) and X-ray diffraction (XRD) were also performed to investigate the

microstructure and the elemental and phase composition of the copper mine tailings-

based geopolymer specimens prepared at different conditions.

2. Experimental Study

2.1. Materials

The materials used in this investigation include copper mine tailings (MT), reagent grade

98% sodium hydroxide (NaOH), aqueous sodium silicate (SS), powder sodium aluminate

(SA), and de-ionized water. The mine tailings were received in the form of dry powder

from a local mine company in Tucson, Arizona. Table 1 shows the chemical composition

of the mine tailings. It can be seen that the mine tailings consist mainly of silica and

alumina with substantial amount of calcium and iron. Grain size distribution analysis was

performed on the mine tailings by mechanical sieving and hydrometer tests following

ASTM D6913 and ASTM D422. Fig. 1 shows the particle size distribution curve of the

mine tailings after hand crushing to break the agglomeration. The mean particle size is

around 120 m with 36% particles passing No. 200 (75 m) sieve. The specific gravity of

the MT particles is 2.83. Fig. 2 shows the SEM micrographs of the MT powder. The MT

particles have irregular shapes and some of the small particles are attached to each other or

70

to the large particles. The XRD pattern of the MT powder is shown in Fig. 3. It can be seen

that the mine tailings are mainly crystalline materials consisting of quartz (SiO2), albite

(NaAlSi3O8), sanidine [(K0.831N0.169)(AlSi3O8)], and gypsum (CaSO4). A weak amorphous

phase, centered at about 28 °, can also be seen from the XRD pattern. The amorphous phase

is the main reactive phase for geopolymerization but, as will be seen later, the crystalline

phase also partially reacts to the alkaline solution.

The sodium hydroxide flakes were obtained from Alfa Aesar Company in Ward Hill,

Massachusetts. The sodium hydroxide solution is prepared by dissolving the sodium

hydroxide flakes in de-ionized water.

Aqueous SS (SiO2 = 29%, Na2O = 9%, and H2O = 62%) with modulus (SiO2/Na2O) of

3.22 and powder SA were obtained from Fisher Scientific in Pittsburgh, Pennsylvania.

2.2. Methods

Initially, the agglomerated particles of dry mine tailings were crushed by hand to ensure

that all particles pass No. 10 (2.0 mm) sieve. Three types of activator solutions were used

in this experiment: NaOH, mixture of NaOH and SS (NaOH/SS), and mixture of NaOH

and SA (NaOH/SA). The NaOH solution was prepared by dissolving sodium hydroxide

flakes in de-ionized water and stirring for about five minutes. Considering the generated

heat, enough time was allowed for the NaOH solution to cool down before it was used.

Aqueous SS or powder SA, if used, was added to the NaOH solution and stirred for

another five minutes to prepare the NaOH/SS or NaOH/SA solution. Then the activator

solution was slowly added to the mine tailings and the resulted mixture was stirred by a

mixer for about ten minutes to ensure sufficient dissolution of silica and alumina in the

alkaline solution. The viscosity of produced pastes increased at higher NaOH and SS (or

SA) concentrations. To prepare the specimens at consistent workability, the water content

was varied from 27 to 33%, the higher percentage corresponding to larger amount of SA

71

or SS. The resulted paste was then placed in cylindrical Plexiglas molds of 34.5 mm inner

diameter and 86.3 mm length (i.e., an aspect ratio of 2.5). The mold was shaken by a

vibrator during the casting to release the trapped air bubbles. Then, the mold was capped

and placed in oven for curing at a specified temperature. The specimens were de-molded

after 3 hours (24 hours at 60 °C due to slow setting) and then placed back in the oven for

7-day curing. At the 7th

day, the specimens are removed from oven, left in room

temperature for 6 hours, and then tested.

Totally, three sodium hydroxide concentrations, 5, 10 and 15 M, and four curing

temperatures, 60, 75, 90 and 120 °C, were used. The SS and SA added specimens were

studied only at 60 and 90 °C, respectively. For the SS and SA added specimens, the SS to

NaOH solution and SA to solid NaOH mass ratios were in the range of 0.5-2.5 and 0.4-

3.1, respectively.

Unconfined compression tests were performed on the cured cylindrical samples with an

ELE Tri Flex 2 loading machine at a constant loading rate of 0.1 mm/min. The tests were

performed to measure the unconfined compressive strength (UCS) of the geopolymer

specimens produced at different conditions. For each condition, at least three specimens

were tested and the average of the measured UCS values was used. Before conducting the

compression test, the end surfaces of the specimens were polished to make sure that they

are accurately flat and parallel. In addition, the end surfaces were lubricated to minimize

the friction between the specimen and the steel platens.

Leaching analysis was performed to investigate the effects of temperature and NaOH

concentration on the dissolution of silica and alumina species from the mine tailings. 20 g

of mine tailings powder was soaked in 5, 10 or 15 M NaOH solution with a liquid to solid

mass ratio of 5. The specimens were kept in 60 or 90 °C oven for 24 hours. After 24

hours, the specimens were filtered with a 0.2 micron filter. Finally, a Perkin Elmer Elan

72

DRC-II ICP-MS was used to measure the concentration of silicon and aluminum in the

filtrate based on the ICP-MS (inductively coupled plasma mass spectrometry) technique.

To investigate the effect of activator type/concentration and curing temperature on the

microstructure and the elemental and phase composition of the geopolymer, SEM/EDX

characterization and XRD analysis were also performed. The SEM imaging was

performed in SE conventional mode using the FEI INSPEC-S50/Thermo-Fisher Noran 6

microscope. The freshly failed surfaces from the unconfined compression tests, without

polishing to keep the fractured surface “un-contaminated” [24], were used for the SEM

imaging. The XRD analysis was performed with a Scintag XDS 2000 PTS diffractometer

using Cu K radiation, at 2.00 degree/min ranging from 10.00 to 70.00 degrees with

0.600 second count time.

Table 2 summarizes the combination of variables studied and the different types of tests

conducted.

3. Results and discussion

3.1. Effect of activator type/composition

Effect of Aqueous Sodium Silicate

Mixture of NaOH solution and aqueous SS was used as activator to investigate the effect of

addition of SS on geopolymerization of MT. Fig. 4 shows the variation of UCS with the

SiO2/Na2O ratio for specimens prepared at 10M NaOH concentration and cured at 60 °C for

7 days. The UCS increases with the SiO2/Na2O ratio up to 1.0-1.26 and then starts to

decrease with higher SiO2/Na2O. So, the optimum SiO2/Na2O is about 1.0-1.26.

Different researchers have studied the effect of activator composition for the NaOH and SS

mixture on the compressive strength of geopolymers [23,25-30]. Table 3 summarizes the

optimum SiO2/Na2O ratios reported in the literature. For comparison, the optimum

73

SiO2/Na2O ratio from the current study is also listed in the table. It can be seen that it is in

good agreement with the optimum SiO2/Na2O ratios obtained by other reserachers.

The addition of SS to NaOH improves the strength of the binder because additional silica is

provided. It is known that the aluminum component of an aluminosilicate source material

tends to dissolve more easily than the silicon component at the early stage. In this case, the

dissolved alumina needs more disolved silica for geopolymerixzation. The added SS simply

provides such required silica. However, the improvement due to the addition of SS is only

up to a certain level [36]. This is possibly because too much sodium silicate hinders

evaporation of water and fomation of polymeric structure by preventing the contact between

the solid material and the activating solution through precipitation of Si-Al phase [30,37].

Fig. 5 shows the SEM micrographs of the specimen synthesized with a mixture of 10 M

NaOH solution and aqueous SS at SiO2/Na2O = 1 and cured at 60 °C for 7 days. The

geopolymeric matrix is mainly particulate [see Fig. 5(a)], in contrast to metakaolin-based

geopolymers [11,38], and exhibits heterogeneous microstructure indicative of varying

degree of reaction [see Fig. 5(b)]. Some particles are partially reacted on their surface and

bonded to each other by the flaky shape layers which are geopolymer gels [see Fig. 5(c)]

while others remain un-reacted [see Fig. 5(d)]. The EDX analysis results show that Si, Al

and Na are the main components in both reacted and un-reacted areas, but there is a

noticeable difference between the Si/Al and Na/Al ratios in the two areas. The Si/Al in the

reacted area is lower than that in the un-reacted one while it is the opposite for the Na/Al.

The initial Si/Al and Na/Al ratios are respectively 8.5 and 2.35 (See Table 2) while they are

respectively 6.8 and 1.7 in the un-reacted area and 4.9 and 2.1 in the reacted area. So, the

Si/Al and Na/Al ratios decrease in both the un-reacted and reacted areas from the initial

values. The Si/Al in the un-reacted area is slightly lower than the initial one possibly

because some of silica from the un-reacted area is dissolved in the alkaline solution and

migrated to the reacted area [38]. The Si/Al ratio in the reacted area is much lower than the

74

initial one possibly because the initial Si/Al ratio (for the whole material) is much higher

than that of the amorphous phase (the reactive silica and alumina) which is the main source

for the geopolymer [38- 40].

The dissolution of alumina has a major role in the kinetics of gel formation [40]. Even if

large amount of amorphous silica were available, further geopolymerization would not take

place if the dissolution of alumina stops. In other words, the dissolution of silica depends on

the dissolution of alumina [40]. This may also explain why after leaching of MT in NaOH

solution at 60 °C for 24 hours, the Si/Al ratio of the leachate is only 1.85 – 2.44 despite the

initial high Si/Al ratio of 7.78 for the MT (see Table 4). As for the Na/Al ratio, its decrease

indicates that not all of the available Na cations participate in the reaction and some of them

might remain un-reacted and appear as precipitate on the surface. The decrease of Na/Al is

also noted by other researchers [35,41,42].

Fig. 6 shows the XRD patterns of different specimens including the one described above.

After reaction, the XRD pattern remains mainly crystalline. This is consistent with the

SEM micrographs in Fig. 5. However, the crystalline silica exhibits less intense peaks

after reaction indicating that some crystalline silica has participated in geopolymerization.

The decrease in the intensity of crystalline peaks can be also due to the addition of soluble

silica [28,31]. Gypsum as a crystalline peak, which was detected in the MT, disappears

after geopolymerization. This is most likely because gypsum is locked in the solution

pore. The amorphous hump in the 90 °C-cured specimens becomes broader and slightly

higher. This change in the 60 °C-cured specimen is less evident due to less reactivity at

lower temperature. This is also consistent with the SEM micrographs (see Figs. 5, 8, and

11), as will be discussed in detail later.

Effect of Powder Sodium Aluminate

75

Since the Si/Al ratio of the MT is high, the feasibility of using sodium aluminate (SA) as the

activating agent was studied. The powder SA was mixed with the NaOH solution at

different mass ratios. The specimens synthesized with the NaOH/SA solution did not show

significant hardening at 60 °C even after a few days. However, the addition of SA increased

the UCS of specimens cured at 90 °C. Fig. 7 shows the measured UCS of specimens

synthesized with a mixture of 10 M NaOH and varying amount of SA and cured at 90° C

for 7 days. The UCS increases with the SA to NaOH mass ratio (A/N) up to about 1.25 and

then starts to decrease. Since the initial Na/Al does not vary significantly but the Si/Al does

(Table 2), the Si/Al ratio may be responsible for the variation of the UCS.

Fig. 8 shows the SEM micrographs of a specimen synthesized with10 M NaOH and SA at

A/N = 1.25 and cured at 90 °C for 7 days. The low magnification micrograph [Fig. 8(a)]

shows voids of varying sizes, which may be generated due to the introduction of air bubbles

into the matrix or the evaporation of extra water. Introduction of air bubbles can be

pronounced in the SA added specimens because SA significantly raises the viscosity of the

solution and makes it more difficult to release the air bubbles. The higher magnification

micrographs show that the microstructure is quite different from what observed in the SS

added specimens (see Fig. 5). Unlike the SS added specimen, the SA added one mainly

consists of densely packed fine particles which cover the MT particles. This microstructure

looks similar to the one obtained from geopolymerization of geothermal silica by SA [43].

The morphology of the geopolymeric gel in the SA added specimen also seems different

from that of the SS added one. In the SS added specimen, the geopolymeric gel looks like

tiny flakes and acts as a binder between the MT particles but in the SA added specimen, the

geopolymeric gel is formed in two ways . First, the geopolymeric gel looks like a monolithic

thin layer and covers the fine MT or SA particles [see Fig. 8(c)]. The SA fine particles can

be seen attached on the geopolymeric gel layer. Second, the geopolymeric gel forms on the

SA and MT particle surfaces due to partial dissolution and acts as a binder between them. In

the case of partial dissolution, zeolite is likely to coexist with the geopolymeric gel as will

76

be discussed later. Since the fine SA particles are dispersed evenly within the specimen,

they may role as a bridging agent between MT particles as well [see Fig. 8(d)]. The

geopolymeric gel was seen to have the first type morphology only in a few spots, but the

second type was dominant. The EDX results show that the Si/Al ratio in the first type

geopolymer is 2.6 but in the second type is 1.9.

Fig. 9 shows the SEM micrographs of the SA added specimen at A/N = 2.5 and also cured

at 90 °C for 7 days. The specimen at A/N = 2.5 contains more and larger pores than the

specimen at A/N = 1.25 [see Figs. 8(a) and 9(a)]. This is because the viscosity of the

specimen at A/N = 2.5 is much higher than that at A/N = 1.25 and thus more and larger air

bubbles are expected to be generated. Similar to the micrographs in Fig. 8, the MT particles

are also surrounded by fine SA particles. However, as the EDX analysis results indicate, the

concentration of Na at A/N = 2.5 is much higher than that at A/N = 1.25, leading to a Na/Al

ratio of 6.5. The high final Na/Al ratio indicates precipitation of un-reacted Na cations as

seen in Fig. 9(c).

The specimen synthesized with 10 M NaOH and SA at A/N = 1.25 and cured at 90 °C for 7

days also shows broader amorphous hump and lower and fewer crystalline sharps than the

MT (see Fig. 6). In addition, a cancrinite (CAN) type of zeolite [(Na7Ca0.9(CO3)1.4(H2O)2.1

[Si6Al6O24]] is formed in the SA added specimen, as identified by the sharp peaks at 13.96°,

18.88°, 24.28°, and 27.4° in the XRD pattern. Formation of zeolite as a co-product of

geopolymerization has also been reported by other researchers [35,44-49]. The literature

indicates that the Si/Al and Na/Al ratios, pH, type of activating solution, liquid to solid

ratio, and curing temperature are the main factors affecting the formation of zeolite [50].

Alkaline silicates (if used in proper dosage) improve the strength at lower temperature (60

C) (see Fig. 4) but alkaline aluminates contribute to the strength at higher temperature (90

C). This can be explained by the Si/Al ratio and its dependence on temperature. Although

77

the Si/Al ratio of the original MT is 7.78, the available silica and alumina for

geopolymerization can be very different depending on the activator alkalinity and

temperature. The leaching tests on MT at 60 C for 24 hours result in a Si/Al ratio of 1.85 to

2.44 for the leachate at different alkalinity levels; but the corresponding Si/Al ratio at 90 C

ranges from 5.93 to 11.9 (see Table 4). Therefore, the Si/Al ratio of the leachate at 60 C is

only slightly smaller than the optimum ratio for geopolymerization, which is 2.0-3.0, but the

Si/Al ratio of the leachate at 90 C is significantly larger. This explains why the addition of

SA does not help improve the strength at 60 C but contributes to the strength at 90 C. On

the other hand, the addition of SS at 60 C results in increase of the Si/Al ratio and makes it

closer to the optimum Si/Al ratio and thus improves the UCS.

3.2. Effect of NaOH concentration/curing temperature

Fig. 10 shows the variation of UCS with curing temperature at different NaOH

concentrations. The effect of NaOH concentration on the UCS depends on the curing

temperature. At curing temperature of 60 °C, the UCS slightly increases with higher NaOH

concentration. At curing temperature of 75 °C, the UCS first increases with the NaOH

concentration from 5 to 10 M and then decreases from 10 to 15 M. At curing temperature

of 90 °C, there is a significant jump for the UCS when the NaOH concentration increases

from 10 to 15 M. At 120 °C, however, the rate of increase for the UCS with the NaOH

concentration becomes slower. The effect of NaOH concentration on the UCS of alkali-

activated mine tailings has been discussed in more detail in [51,52].

It can also be seen from Fig. 10 that elevated curing temperature results in increase of UCS

up to a certain level and then decrease of UCS. The improving effect of curing temperature

depends on the NaOH concentration. At lower alkalinity, geopolymerization is less

sensitive to curing temperature [53]. The optimum curing temperature for 5 and 10 M

NaOH is about 75 °C while for 15 M NaOH it is around 90 °C. The effect of curing

temperature on the UCS of geopolymer has been studied by many researchers [26-

78

28,39,54-56]. The optimum curing temperature depends on both the alkaline

concentration and the source material. In general, higher alkaline concentration and lower

source material reactivity will have higher optimum curing temperature. Table 5 shows

the optimum curing temperatures reported in the literature and obtained from the current

study.

The improving effect of curing temperature below the optimum one is mainly due to the

higher solubility of aluminosilicate minerals in the alkaline solution. At higher

temperature, the silica and alumina species are more likely to dissolve and larger amount

of Si and Al will be available for geopolymerization. The effect of curing temperature on

the solubility of aluminosilicates can be clearly seen in Table 4. The weakened strength of

geopolymer above the optimum temperature is mainly due to the fast polycondensation

and rapid formation of geopolymeric gel which hinders further dissolution of silica and

alumina species [57,59].

Fig. 11 shows the SEM micrographs and EDX analysis results of the specimen synthesized

at 15 M NaOH and cured at 90 °C for 7 days. The lower magnification micrograph [see Fig.

11(a)] shows varying size voids, which might be formed due to entrapped air bubbles or

evaporated extra water. As seen in Fig. 11(b), there are both partially reacted and un-reacted

MT particles. The morphology of the geopolymeric gel in this specimen [Fig. 11(c)] looks

different from that of the SS or SA added specimens although the SA added one is cured at

the same temperature. In the NaOH only specimen, a thicker layer of geopolymeric gel

covers the MT particles and the geopolymeric matrix has a denser structure [39,59].

The EDX analysis results indicate a noticeable difference between the Si/Al values of the

reacted and un-reacted areas. In the reacted area, the Si/Al ratio (5.4) is lower than the

initial one (7.8). As discussed earlier, this may be due to the difference between the initial

79

Si/Al ratio and that of the amorphous phase. The high Si/Al ratio in the un-reacted area

(12.4) may be caused by the dissolution and migration of Al to the reacted area.

The XRD pattern of the above-mentioned specimen (also shown in Fig. 6) looks similar

to that of the SA added specimen. The amorphous peak broadening and emergence of

zeolitic peaks are the XRD characteristics of NaOH activated MT at 90 °C. However, the

main difference in the XRD characteristics between the specimens activated with only

NaOH and those with NaOH/SA is the size of zeolitic peaks, which are smaller in the

NaOH only activated one. This is consistent with the above discussion about the

dependence of the formation of zeolite on both temperature and Si/Al ratio. For this

specimen, due to the higher Si/Al ratio, less amount of zeolite is generated.

Conclusions

The effect of activator type/composition and curing temperature on the mechanical

properties, microstructure, and elemental and phase composition of alkali-activated

copper mine tailings (MT) is studied in this paper. Based on the experimental results, the

following conclusions can be drawn:

1) NaOH concentration and curing temperature are two important factors that affect the

UCS and micro-structural properties of alkali-activated MT. The optimum curing

temperature, i.e. the curing temperature at the maximum UCS, depends on the NaOH

concentration, lower optimum curing temperature at smaller NaOH concentration.

2) Addition of aqueous SS to the NaOH solution can lead to strength improvement. The

highest UCS is obtained at a SiO2/Na2O ratio of about 1.0-1.26.

3) Addition of powder SA to the NaOH solution profoundly delays the setting at 60 °C

but improves the UCS at 90 °C. The SA to NaOH ratio (A/N) corresponding to the

highest UCS is about 1.25 for specimens cured at 90 °C.

80

4) The SEM/EDX results indicate a heterogeneous matrix for the alkali-activated MT.

The matrix is denser at higher curing temperature due to formation of larger amount

of geopolymer gel.

5) The XRD patterns at the optimum conditions show change in both amorphous and

crystalline phases. Formation of both zeolite and geopolymer improves the UCS at

elevated curing temperatures.

Acknowledgements

This work is partially supported by the National Science Foundation under Grant No.

CMMI-0969385, the University of Arizona Faculty Seed Grants Program, and a local mine

company in Tucson, AZ.

References

[1] Meyer C. The greening of the concrete industry. Cement and Concrete Composites

2009;31: 601-5.

[2] World Business Council for Sustainable Development. Cement Sustainability

Initiative; 2010. http://wbcsdcement.org.

[3] Davidovits J. High-alkali cements for 21st century concrete. Proceedings of V. Mohan

Malhortra Symposium: Concrete Technology, Past, Present and Future, P. K. Metha

(ed), ACI SP-144, 1994. p. 383-97.

[4] Malhotra VM. Role of supplementary cementing materials in reducing greenhouse gas

emissions. Concrete Technology for a Sustainable Development in the 21st Century.

O. E. Gjorv and K. Sakai (ed), London, E&FN Spon, 2000. p. 226–35.

[5] McCaffrey, R. Climate change and the cement industry. Global Cement and Lime

Magazine 2002;(Environmental Special Issue):15-9.

81

[6] Arm M. Mechanical properties of residues as unbound road materials - Experimental

tests on MSWI bottom ash, crushed concrete and blast furnace slag. KTH Land and

Water Resources Engineering, Stockholm, 2003.

[7] USEPA. Wastes-Resource Conservation-Reduce, Reuse, Recycle-Construction &

Demolition Materials, 2009.

http://www.epa.gov/epawaste/conserve/rrr/imr/cdm/index.htm.

[8] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Investigations of tungsten mine waste

geopolymeric binder: Strength and microstructure. Construction and Building

Materials 2008;22 (11):2212–9.

[9] Rattanasak U, Chindaprasirt P. Influence of NaOH solution on the synthesis of fly ash

geopolymer. Mineral Engineering 2009;22:1073-78.

[10] Alonso S, Palomo A. Calorimetric study of alkaline activation of calcium hydroxide-

metakaolin solid mixtures. Cement and Concrete Research 2001;31(1):25-30.

[11] Duxson P, Provis JL, Lukey GC, Mallicoat SW, Kriven WM, Van Deventer JSJ.

Understanding the relationship between geopolymer composition, microstructure and

mechanical properties. Colloids and Surfaces A: Physicochemical and Engineering

Aspects 2005;269:47–58.

[12] Duxson P, Fernandez-Jimenez A, Provis JL, Lukey GC, Palomo A, Van Deventer

JSJ. Geopolymer technology: the current state of the art. Journal of Materials Science

2007;42:2917-33.

[13] Li Z, Ding Z, Zhang Y. Development of sustainable cementitious materials.

Proceedings of International Workshop on Sustainable Development and Concrete

Technology, Beijing, China; 2004. p. 55-76.

82

[14] Drechsler M, Graham A. Innovative material technologies: bringing resources

sustainability to construction and mining industries. 48th Institute of Quarrying

Conference, Adelaide SA, Australia; 2005.

[15] Shi C, Fernandez-Jimenez A. Stabilization/solidification of hazardous and radioactive

wastes with alkali-activated cements. Journal of Hazardous Materials

2006;B137:1656-63.

[16] Majidi B. Geopolymer technology, from fundamentals to advanced applications: a

review. Materials Technology 2009;24(2):79-87.

[17] Giannopoulou IP, Panias D. Development of geopolymeric materials from industrial

solid wastes. 2nd International Conference on Advances in Mineral Resources

Management and Environmental Geotechnology, Greece; 2006.

[18] Southam DC, Brent GF, Felipe F, Carr C, Hart RD, Wright K. Towards more

sustainable mine fills - replacement of ordinary Portland cement with geopolymer

cements. World Gold Conference, Australia; 2007.

[19] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Properties of tungsten mine waste

geopolymeric binder. Construction and Building Materials 2008;22(6):1201-11.

[20] Pacheco-Torgal F, Castro-Gomes J, Jalili S. Durability and environmental performance

of alkali-activated tungsten mine waste mud mortars. Journal of Materials in Civil

Engineering 2010;22:897-904.

[21] Collins RJ, Ciesielski SK. Recycling and use of waste materials and by-products in

highway construction. National Cooperative Highway Research Program Synthesis of

Highway Practice 199, Transportation Research Board, Washington, DC; 1994.

83

[22] FHWA (Federal Highway Administration) User Guidelines for Byproduct and

Secondary Use Materials in Pavement Construction. Report No. FHWA-RD-97-

148;2008.

[23] Pacheco-Torgal F, Castro-Gomes JP, Jalali S. Investigations on mix design of tungsten

mine waste geopolymeric binder. Construction and Building Materials 2008;22:1939-

49.

[24] Xu H, Van Deventer JSJ. The geopolymerisation of alumino-silicate minerals.

International Journal of Mineral Processing 2000;59(3):247-66.

[25] Chindaprasirt P, Chareerat T, Siricicatnanon V. Workability and strength of coarse

high calcium fly ash geopolymer. Cement and Concrete Composites 2007;29(3):224-

9.

[26] Guo X, Shi H, Dick WA. Compressive strength and microstructural charachteristics of

class C fly ash geopolymer. Cement & Concrete Composites 2010;32:142-7.

[27] Yunfen H, Dongmin W, Wenjuan Z, Hongbo L, Lin W. Effect of activator and curing

mode on fly ash-based geopolymer. Journal of Wuhan University of Technology-

Mater. Sci. Ed. 2009;24(5):711-5.

[28] Villa C, Pecina ET, Torres R, Gómez L. Geopolymer synthesis using alkaline

activation of natural zeolite. Construction and Building Materials 2010;24:2084-90.

[29] Chindaprasirt P, Rattanasak U. Utilization of coal ash in geopolymeric material.

Technology and Innovation for Sustainable Development Conference (TISD2008);

2008. p. 77-81.

[30] Cheng TW, Chiu JP. Fire-resistant geopolymer produced by granulated blast furnace

slag. Minerals Engineering 2003;16:205-10.

84

[31] Provis JL, Yong CZ, Duxson P, Van Deventer JSJ. Correlating mechanical and

thermal properties of sodium silicate-fly ash geopolymers. Colloids and Surfaces A:

Physicochemical and Engineering Aspects 2009;336:57-63.

[32] Detphan S, Chindaprasirt P. Initial study on synthetics open-field burning rice husk

ash: making geopolymer mortar for sustainable development in the rural area.

Technology and Innovation for Sustainable Development Conference (TISD2008);

2008. p. 111-6.

[33] Ma Y, Hu J, Ye G. The effect of activating solution on the mechanical strength,

reaction rate, mineralogy, and microstructure of alkali-activated fly ash. Journal of

Materials Science 2012; DOI: 10.1007/s10853-012-6316-3.

[34] Law DW, Adam A, Molyneaux TK, Patnaikuni I. Durability assessment of alkali

activated slag (AAS) concrete. Journal of Materials Science 2012; DOI

10.1617/s11527-012-9842-1.

[35] Silva PD, Sagoe-Crenstil K, SirivivatnanonV. Kinetics of geopolymerization: Role of

Al2O3 and SiO2. Cement and Concrete Research 2007;37:512-8.

[36] Bernal SA, Rodriguez ED, de Gutierrez RM, Provis JL, Delvasto S. Activation of

metakaolin/slag blends using alkaline solutions based on chemically modified silica

fume and rice husk ash. Journal of Materials Science 2012;3(1):99-108.

[37] Lee WKW, Van Deventer JSJ. The effects of inorganic salt contamination on the

strength and durability of geopolymers. Colloid Surface 2002;211:115–26.

[38] Duxson P, Lukey GC, Separovic F, Van Deventer JSJ. Effect of alkali cations on

aluminum incorporation in geopolymeric gels. Industrial and Engineering Chemistry

Research 2005;44:832-9.

85

[39] Sindhunata, Van Deventer JSJ, Lukey GC, Xu H. Effect of curing temperature and

silicate concentration on fly-ash-based geopolymerization. Industrial & Engineering

Chemsitry Reserach 2006;45(10):3559-68.

[40] Fernandez-Jimenez A, Palomo A, Sobrados I, Sanz J. The role played by the reactive

alumina content in the alkaline activation of fly ashes. Microporous and Mesoporous

Materials 2006;91(1-3):111-9.

[41] Rowles M, O’Connor B. Chemical optimisation of the compressive strength of

aluminosilicate geopolymer synthesized by sodium silicate activation of metakaolinite.

Journal of Materials Chemistry 2003;13:1161-5.

[42] Schmucker M, MacKenzie KJD. Microstructure of sodium polysialate siloxo

geopolymer. Ceramics International 2005;31(3):433-7.

[43] Hajimohammadi A, Provis JL, Van Deventer JSJ. One-part geopolymer mixes from

geothermal silica and sodium aluminate. Industrial and Engineering Chemistry

Research 2008;47(23):9396-405.

[44] Krivenko PV, Kovalchuk GY. Heat resistant fly ash based geocements. In:

International Conference on Geopolymer-2002 – Turn potential into profit, Melbourne,

Australia, October 28–29; 2002.

[45] Fletcher RA, MacKenzie KJD, Nicholson CL, Shimada S. The composition range of

aluminosilicate polymers. Journal of the European Ceramic Society 2005;25(9):1471-

7.

[46] Phair JW, Van Deventer JSJ. Characterization of fly-ash-based geopolymeric binders

activated with sodium aluminate. Industrial and Engineering Chemistry Research

2002;41:4242-51.

86

[47] Brew DRM, MacKenzie KJD. Geopolymer synthesis using silica fume and sodium

aluminate. Journal of Materials Science 2007;42:3990-3.

[48] Verdolotti L, Iannace S, Lavorgna M, Lamanna R. Geopolymerization reaction to

consolidate incoherent pozzolanic soil. Journal of Materials Science 2008;43:865-73.

[49] Temuujin J, Minjigmaa A, Rickard W, Lee M, Williams I, Van Riessen A. Preparation

of metakaolin based geopolymer coating on metal substrates as thermal barries.

Applied Clay Science 2009;46:265-70.

[50] Kawano M, Tomita K. Experimental study on the formation of zeolites from obsidian

by interaction with NaOH and KOH solutions at 150 and 200 °C. Clays and Clay

Minerals 1997;45(3):365-77.

[51] Ahmari S, Zhang L. Production of eco-friendly bricks from copper mine tailings

through geopolymerization. Construction and Building Materials 2012;29:323-31.

[52] Zhang L, Ahmari S, Zhang S. Synthesis and characterization of fly ash modified mine

tailings-based geopolymers. Construction and Building Materials 2011;25(9):3773-81.

[53] Chen C, Gong W, Lutze W, Pegg IL. Kinetics of fly ash geopolymerization. Journal of

Materials Science 2011;46(9):3073-83.

[54] Khale D, Chaudhary R. Mechanism of geopolymerization and factors influencing its

development: a review. Journal of Materials Science 2007;42:729-46.

[55] Panagiotopoulou CH, Kontori E, Perraki TH, Kakali G. Dissolution of aluminosilicate

minerals and by-products in alkaline media. Journal of Materials Science

2007;42:2967-73.

87

[56] Thakur RN, Ghosh S. Effect of mix composition on compressive strength and

microstructure of fly ash based geopolymer composites. ARPN Journal of Engineering

and Applied Sciences 2009;4(4):68-74.

[57] Yao X, Zhang Z, Zhua H, Chen Y. Geopolymerization process of alkali–metakaolinite

characterized by isothermal calorimetry. Thermochimica Acta 2009;493(1-2):49-54.

[58] Cyr M, Idir R, Poinot T. Properties of inorganic polymer (geopolymer) mortars made

of glass cullet. Journal of Materials Science 2012;47(6):2782-97.

[59] Muñiz-Villarreal MS, Manzano-Ramírez A, Sampieri-Bulbarela S, Gasca-Tirado JR,

Reyes-Araiza JL, Rubio-Ávalos JC, Pérez-Bueno JJ, Apatiga LM, Zaldivar-Cadena A,

Amigó-Borrás V. The effect of temperature on the geopolymerization process of a

metakaolin-based geopolymer. Materials Letters 2010(6);65:995-8.

88

Table 1. Chemical composition (weight %) of mine tailings.

Chemical Compound Content* (%)

Standard Deviation

(%)

SiO2 64.8 2.08

Al2O3 7.08 0.70

Fe2O3 4.33 0.71

CaO 7.52 1.06

MgO 4.06 0.93

SO3 1.66 0.31

Na2O 0.90 0.23

K2O 3.26 0.42

* The values are the average of 7 tailings samples.

89

Table 2. Summary of studied variables and conducted tests.

Specimen NH

*

( M)

SS*

(%)

SA*

(%)

Water

(%) Si/Al Na/Al

Curing

Temp.

(°C)

Curing

time

(days)

UCS

Test XRD

SEM

/EDS

5-7-60 5 0 0 27 7.78 0.96 60 7 X

10-7-60 10 0 0 27 7.78 1.92 60 7 X

15-7-60 15 0 0 27 7.78 2.88 60 7 X

5-7-75 5 0 0 27 7.78 0.96 75 7 X

10-7-75 10 0 0 27 7.78 1.92 75 7 X

15-7-75 15 0 0 27 7.78 2.88 75 7 X

5-7-90 5 0 0 27 7.78 0.96 90 7 X

10-7-90 10 0 0 27 7.78 1.92 90 7 X

15-7-90 15 0 0 27 7.78 2.88 90 7 X X X

5-7-120 5 0 0 27 7.78 0.96 120 7 X

10-7-120 10 0 0 27 7.78 1.92 120 7 X

15-7-120 15 0 0 27 7.78 2.88 120 7 X

SS1 10 21.2 0 27 8.82 2.55 60 7 X

SS2 10 19.3 0 27 8.73 2.49 60 7 X

SS3 10 17.3 0 27 8.63 2.43 60 7 X

SS4 10 14.7 0 27 8.50 2.35 60 7 X X X

SS5 10 9.6 0 27 8.24 2.20 60 7 X

10SA1 10 0 4.6 27.0 4.91 1.58 90 7 X

10SA2 10 0 8.8 27.0 3.58 1.42 90 7 X X X

10SA3 10 0 12.6 28.0 2.82 1.33 90 7 X

10SA4 10 0 17.8 31.0 2.08 1.25 90 7 X X X

10SA5 10 0 22.1 33.0 1.67 1.20 90 7 X

* NH, SS, and SA stand respectively for NaOH, aqueous sodium silicate, and solid sodium

aluminate.

90

Table 3. Summary of optimum composition of alkaline solution reported in the literature.

No. Source Material Opimum

SiO2/Na2O Reference

1 Fly ash 0.5-0.89 [25]

2 Fly ash 1.5 [26]

3 Fly ash 1.0-1.5 [31]

4 Metakaolin 1.5 [11]

5 Fly ash 1.0-1.5 [33]

6 Granulate blast furnace slag 1.0-1.25 [34]

7 Copper mine tailings 1.0-1.26 Current study

91

Table 4. Results of MT leaching tests at 60 and 90 °C and different NaOH concentrations.

Temperature (°C) 60 90

NaOH (M) 5 10 15 5 10 15

Si (ppm) 71 171 233 1,846 3,970 4,570

Al (ppm) 28 76 121 299 319 550

Si/Al 2.44 2.16 1.85 5.93 11.9 7.98

92

Table 5. Summary of optimum curing temperature reported in the literature.

Source Material

Optimum Curing

Temperature (°C)

NaOH Concentration

(M)*

Reference

Metakaolin 35 4.3 [57]

Natural zeolite 40 7 [28]

Glass cullet 40 5-10 [58]

Class C fly ash 60 8.1 [26]

Class F fly ash 75 7.5 [39]

Class F fly ash 80 7 [27]

MT 75 5 & 10 Current study

MT 90 15 Current study

* Equivalent NaOH concentration is presented in the case that stoichiometric molar ratio of the

alkaline cation or the mixture of NaOH and SS is used in the literature

93

Fig. 1. Particle size distribution of mine tailings.

0

10

20

30

40

50

60

70

80

90

100

1 10 100 1000

Particle size (m)

Perc

ent passin

g (

%)

94

Fig. 2. SEM micrographs of mine tailings powder.

95

Fig. 3. XRD pattern of mine tailings powder (A: albite, G: gypsum, P: sanidine, S:

quartz).

10 20 30 40 50 60 70

2q

G

A

P

A

S SS

S

S

SA

G

P P

P S

96

Fig. 4. UCS versus SiO2/Na2O ratio for specimens activated with a mixture of 10 M

NaOH and SS and cured at 60 °C for 7 days.

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

UC

S (M

Pa

)

SiO2/Na2O

97

Fig.5. SEM micrographs and EDX analysis results of aqueous SS added specimen at

SiO2/Na2O = 1 and cured at 60 °C for 7 days: (a) low magnification image of whole area;

(b) higher magnification image of area shown by the square in (a); and (c) and (d) higher

b c

d

a b

c d

0

2000

4000

6000

8000

10000

12000

14000

0 1 2 3 4 5

Inte

nsity (cp

s)

keV

C

O

Fe

Na

Al

Si

Mg K CaS

Si/Al = 4.9

Na/Al = 2.1

0

1000

2000

3000

4000

5000

6000

7000

8000

0 1 2 3 4 5

Inte

nsity (cp

s)

keV

C

O

FeMg

Al

Na

Si

K Ca

Si/Al = 6.8

Na/Al = 1.7

S

98

magnification images respectively of reacted and un-reacted areas shown by squares in

(b). The EDX spectra are for (c) and (d).

Fig. 6. XRD patterns of MT powder; SS4: binder synthesized with 10 M NaOH and

aqueous SS at SiO2/Na2O = 1 and cured at 60 °C for 7 days; 10SA2: binder synthesized at

10 M NaOH and powder SA at A/N = 1.25 and cured at 90 °C for 7 days; 15-7-90: binder

synthesized at 15 M NaOH and cured at 90 °C for 7 days (A: sodium aluminum silicate

(albite), G: gypsum, N: sodium aluminum silicate hydrate (zeolite), P: potassium

aluminum silicate (sanidine), S: quartz).

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70

2q

P

15-7-90

10SA2

MT Powder

S

SS4

N AS

S P

P S

P SS S

P

NN

N

SS

SSS

N

N

N A

P

P

P

S

P

P

PP

ASS

S

SS

S

P P

N

P

A

SS

SS

SP

PP

PGG

A

AP

A

A

A

A P

S

SS

S

S

S

S

P

P

P

S

S

S

P

P

P

P

N

N

P

S

99

Fig. 7. UCS versus SA to NaOH ratio (A/N) for specimens synthesized with a mixture of

10 M NaOH and SA and cured at 90 °C for 7 days.

0

2

4

6

8

10

12

14

16

18

0.0 0.5 1.0 1.5 2.0 2.5 3.0

A/N

UC

S (M

Pa

)

100

Fig. 8. SEM micrographs and EDX analysis results of specimen synthesized with a

mixture of 10 M NaOH and SA at A/N = 1.25 and cured at 90 °C for 7 days: (a) low

magnification image of whole area; (b) higher magnification image of the area shown by

the square in (a); (c) higher magnification image of the geopolymeric gel shown by the

arrow in (a); and (d) high magnification image of the area shown by the square in (b). The

EDX spectra are for (c) and (d).

a b

c d

c

b

d

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 1 2 3 4 5

Inte

nsity (cp

s)

keV

C

O

FeAs

Al

Na

Si

K Ca

Si/Al = 2.6

Na/Al = 2.7

S

0

2000

4000

6000

8000

10000

12000

14000

0 1 2 3 4 5

Inte

nsity (cp

s)

keV

C

O

Fe As

Al

NaSi

K Ca

Si/Al = 1.9

Na/Al = 1.6

S

101

Fig. 9. SEM micrographs and EDX analysis results of specimens synthesized with a

mixture of 10 M NaOH and SA at A/N = 2.5 and cured at 90 °C for 7 days: (a) low

magnification image of whole area; (b) higher magnification image of area shown by

square in (a); and (c) higher magnification image of area shown by square in (b). The

EDX spectrum is for (c).

0

2000

4000

6000

8000

10000

12000

0 1 2 3 4 5

Inte

nsity (cp

s)

keV

C

O

Al

Na

Si

K

Si/Al = 1

Na/Al = 6.5

S

a b

c

b

c

102

Fig. 10. UCS versus curing temperature for specimens synthesized at different NaOH

concentrations and cured for 7 days.

0

5

10

15

20

25

30

60 75 90 105 120

UC

S (M

Pa

)

Temperature ( C)

5

10

15

NaOH (M)

103

Fig. 11. SEM micrographs and EDX analysis results of specimen synthesized at 15 M

NaOH and cured at 90 °C for 7 days: (a) low magnification image of whole area; (b)

higher magnification image of area shown by square in (a); and (c) and (d) higher

magnification images respectively of the reacted and un-reacted areas shown by squares

in (b). The EDX spectra are for (c) and (d).

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 1 2 3 4 5

Inte

nsity (cp

s)

keV

C

O

Fe As

Al

Na

Si

KCa

Si/Al = 5.4

Na/Al = 2.7

S

0

2000

4000

6000

8000

10000

12000

14000

0 1 2 3 4 5

Inte

nsity (cp

s)

keV

C

O

As

Al

Na

Si

K Ca

Si/Al = 12.4

Na/Al = 3.1

S

b

c d

a b

c d

104

104

APPENDIX C

Paper is published in the Journal of Construction and Building Materials

PRODUCTION OF ECO-FRIENDLY BRICKS FROM COPPER

MINE TAILINGS THROUGH GEOPOLYMERIZATION

Saeed Ahmari, Lianyang Zhang*

Department of Civil Engineering and Engineering Mechanics, University of Arizona,

Tucson, Arizona 85721, USA

* Corresponding author: Tel.: 1 520 6260532; fax: 1 520 6212550.

E-mail address: lyzhang@email.arizona.edu.

105

105

ABSTRACT

This paper studies the feasibility of utilizing copper mine tailings for production of eco-

friendly bricks based on the geopolymerization technology. The procedure for producing

the bricks simply includes mixing the tailings with an alkaline solution, forming the brick

by compressing the mixture within a mold under a specified pressure, and curing the

brick at a slightly elevated temperature. Unlike the conventional method for producing

bricks, the new procedure neither uses clay and shale nor requires high temperature kiln

firing, having significant environmental and ecological benefits. In this study, the effects

of four major factors, sodium hydroxide (NaOH) solution concentration (10 and 15 M),

water content (8 to 18%), forming pressure (0 to 35 MPa), and curing temperature (60 to

120 C), on the physical and mechanical properties of copper mine tailings-based

geopolymer bricks are investigated using water absorption and unconfined compression

tests. Scanning electron microscopy (SEM) imaging and X-ray diffraction (XRD)

analysis are also performed to investigate the microstructure and phase composition of

the mine tailings-based geopolymer bricks prepared at different conditions. The results

show that copper mine tailings can be used to produce eco-friendly bricks based on the

geopolymerization technology to meet the ASTM requirements.

Key words: Mine tailings; Bricks; Geopolymer; Forming pressure; Curing temperature;

Compressive strength; Water absorption; Microstructure

106

106

1. Introduction

Bricks are a widely used construction and building material. For example, in the United

States, about 9 billion bricks are used a year [1,2]. Conventional production of bricks

usually utilizes clay and shale as the source material and requires high temperature (900 –

1,000 C) kiln firing. Quarrying operations for producing the clay and shale are energy

intensive, adversely affect the landscape, and can release high level of waste materials.

The high temperature kiln firing not only consumes significant amount of energy, but

also releases substantial quantity of greenhouse gases. It is also noted that there is a

shortage of clay and shale in many parts of the world. To protect the clay and shale

resource and protect the environment, some countries such as China have started to limit

the use of bricks made from clay and shale [3,4].

Researchers have studied the utilization of different types of wastes to produce

construction and building bricks [4-10]. Chen et al. [4] studied the feasibility of utilizing

hematite tailings together with clay and Class F fly ash to produce bricks and found that

the percentage of tailings used could be up to 84% of the total weight. Based on the test

results, they recommended a tailings:clay:fly ash ratio of 84:10:6, with a forming water

content of 12.5-15%, a forming pressure of 20-25 MPa, and a firing temperature of 980 –

1,030 C for 2 hours, to produce good quality bricks. Chou et al. [5] investigated the

utilization of Class F fly ash to replace part of the clay and shale in production of bricks

using the conventional procedure. Bricks with up to 40% of fly ash were successfully

produced in commercial-scale production test runs, with the properties exceeding the

ASTM commercial specifications. Morchhale et al. [6] studied the production of bricks

by mixing copper mine tailings with different amount of ordinary Portland cement (OPC)

and then compressing the mixture in a mold. The results show that the bricks have higher

compressive strength and lower water absorption when the OPC content increases. Roy et

al. [7] used gold mill tailings to make bricks by mixing them with OPC, black cotton soils

or red soils. The OPC-tailings bricks were just cured by immersing them in water but the

soil-tailings bricks were sun-dried and then fired at high temperatures (750, 850, and 950

107

107

C). Liu et al. [8] explored the feasibility of using the sludge derived from dyestuff-

making wastewater coagulation for producing unfired bricks. They tried four typical

cements, OPC, ground clinker of silicate cement, alumina cement, and slag cement, as the

binder. The experimental results showed that the cement solidified sludge could meet all

performance criteria for unfired bricks at a cement/dry sludge/water ratio of 1:0.5–

0.8:0.5–0.8. The compressive strength of alumina cement solidified sludge was the

highest and exceeded 40 MPa. Algin and Turgut [9] tried to use cotton wastes (CW) and

limestone powder wastes together with OPC to produce bricks and found that the amount

of CW used affect both the density and the mechanical properties of bricks. Bricks with

30% of CW had a compressive strength of 7 MPa and a flexural strength of 2.2 MPa.

Shon et al. [10] studied the use of stockpiled circulating fluidized bed combustion ash

(SCFBCA) with Type I cement, lime, Class F fly ash, and/or calcium chloride to

manufacture compressed bricks. They used a compaction pressure of 55.2 MPa and

placed the specimens at 23 C and 100% relative humidity room for 1 day before air

curing at room temperature. It is noted that these different methods for utilizing wastes to

make bricks either require high temperature kiln firing or use cement as the binder.

Therefore, they still have the drawbacks of high-energy consumption and large quantity

of greenhouse gas emissions.

Recently, researchers have started to use the geopolymerization technology to produce

bricks from wastes. Geopolymerization is the reaction undergone by aluminosilicates in a

highly concentrated alkali hydroxide or silicate solution, forming a very stable material

called geopolymer having amorphous polymeric structures with interconnected Si–O–Al–

O–Si bonds [11-17]. According to Duxson et al. [13] and Dimas et al. [14], the

geopolymerization process includes dissolution of solid aluminosilicate materials in a

strong alkaline solution, formation of silica-alumina oligomers, polycondensation of the

oligomeric species to form inorganic polymeric material, and bonding of un-dissolved

solid particles in the final geopolymeric structure. Geopolymer not only provides

performance comparable to OPC in many applications, but shows additional advantages

108

108

such as rapid development of mechanical strength, high acid resistance, no/low alkali-

silica reaction (ASR) related expansion, excellent adherence to aggregates,

immobilization of toxic and hazardous materials, and significantly reduced greenhouse

gas emissions [11-13,18-20]. Freidin [21] used geopolymerization of Class F fly ash (FA)

or a combination of FA and bottom ash (BA) to produce cementless bricks. He used

water glass with a silica module of 2.3 as the alkali activator and applied different

forming pressures to prepare the test specimens. The results showed that the cementless

bricks based on geopolymerization could meet the requirements of Israeli Standard for

conventional cement concrete blocks. Diop and Grutzeck [22] investigated the feasibility

of utilizing an aluminosilicate-rich tuff to produce bricks based on the geopolymerization

technology. They used sodium hydroxide (NaOH) solution as the alkali activator and

prepared the test specimens by compressing the tuff-NaOH solution mixture in a cylinder

with a pressure of about 10 MPa. They studied the effect of both the NaOH concentration

(4, 8, and 12 M) and the curing temperature (40, 80, and 120 C). The results showed that

the strength increases with the NaOH concentration and the curing temperature. Mohsen

and Mostafa [23] studied the utilization of low kaolinitic clays (white clay, grey clay, and

red clay) to produce geopolymer bricks. The clay raw materials were activated by

calcination at 700 C for 2 hours and ground in an alumina ball mill and sieved to < 120

m before being used. Both NaOH solution and NaOH + sodium silicate solution were

used as the alkali activator. The test specimens were molded using a forming pressure of

15 MPa in a special steel mold. The molded specimens were allowed to mature at room

temperature for 24 hours and then cured at different temperature for different time (room

temperature for 3 days, 75 C for 24 hours, or 150 C for 24 hours) before being tested.

The results showed that the type of alkali activator and the curing temperature are two

major factors affecting the behavior of geopolymer bricks. With the right alkali activator

and the appropriate curing temperature, all of the three studied low kaolinitic clays are

suitable for producing geopolymer bricks.

109

109

Considering the fact that a large amount of copper mine tailings are generated each year

[24-26] and that copper mine tailings are rich in silica and alumina and can be used as a

potential source material for production of geopolymers [19,27-30], this paper studies the

feasibility of utilizing copper mine tailings to produce eco-friendly geopolymer bricks.

The geopolymer bricks are produced simply by mixing the tailings with an alkaline

solution, forming the brick by compressing the mixture within a mold under a specified

pressure, and curing the brick at a slightly elevated temperature. Unlike the conventional

method for producing bricks, the new procedure neither uses clay and shale nor requires

high temperature kiln firing, thus having significant environmental and ecological

benefits.

2. Experimental Study

2.1. Materials

The materials used in this investigation include copper mine tailings (MT), reagent grade

98% sodium hydroxide (NaOH), and de-ionized water. The mine tailings were received

in the form of dry powder from Mission Mine Operations of ASARCO LLC in Tucson,

Arizona. Table 1 shows the chemical composition of the mine tailings. It can be seen that

the mine tailings consist mainly of silica and alumina with substantial amount of calcium

and iron. Grain size distribution analysis was performed on the mine tailings using

mechanical sieving and hydrometer analysis following ASTM D6913 and ASTM D422.

Fig. 1 shows the particle size distribution curve. The mean particle size is around 120 m

with 36% particles passing No. 200 (75 m) sieve. The specific gravity of the MT

particles is 2.83. The XRD pattern of the mine tailings powder is shown in Fig. 2. The mine

tailings are mainly crystalline materials consisting of quartz (SiO2) as the main constituent,

albite (NaAlSi3O8), sanidine (K,Na)(Si,Al)4O8, and gypsum (CaSO4·2H2O).

The sodium hydroxide (NaOH) flakes were obtained from Alfa Aesar Company in Ward

Hill, Massachusetts. The sodium hydroxide solution is prepared by dissolving the sodium

hydroxide flakes in de-ionized water.

110

110

2.2. Preparation of geopolymer brick samples

First, the mine tailings were mixed with sodium hydroxide solution. The sodium

hydroxide solution was prepared by adding sodium hydroxide flakes to de-ionized water

and stirring for at least five minutes. Due to the generated heat, enough time was allowed

for the solution to cool down to room temperature before it was used. The NaOH solution

was slowly added to the dry mine tailings and mixed for 10 minutes to ensure the

homogeneity of the mixture. The generated mine tailings and NaOH solution mixture

exhibits varying consistency depending on the initial water content. The mixture’s

consistency varies from semi-dry to semi-paste as the water content changes from 8% to

18%. The mixture was placed in the Harvard Miniature Compaction cylindrical molds of

33.4 mm diameter and 72.5 mm height with minor compaction. The compacted

specimens were then compressed with a Geotest compression machine at different

loading rates to ensure that the duration of forming pressure was about 10 minutes for all

the specimens. Fig. 3 shows the typical load-displacement curves for different forming

pressures. At low forming pressures and high water contents substantial amount of elastic

deformations can be seen. At high forming pressures and low water contents, however,

the elastic deformation seems negligible indicating that the occurred deformations are

mainly plastic, which leads to volume decrease of voids within the granular matrix. After

the compression, the specimens were de-molded and placed uncovered in an oven for

curing at a specified temperature for 7 days before tested. The specimens were weighed

before and after the curing to measure the final water content.

2.3. Methodology

Unconfined compression tests were performed to measure the 7 days’ unconfined

compressive strength (UCS) of geopolymer bricks produced at different conditions. The

effects of NaOH concentration, curing temperature, water content, and forming pressure

on the UCS were investigated. Specimens were prepared at two NaOH concentrations of

10 and 15 M, curing temperature ranging from 60 to 120 °C, water content from 8 to

18%, and forming pressure from 0 to 35 MPa. Water content indicates the mass ratio

111

111

between the water in the activating solution and the solid part of the mixture. The mass

ratio between the activator, NaOH, and MT varies from 4.8 to 10.8% depending on the

NaOH concentration and water content. For each condition, at least three specimens were

tested and the average of the measured UCS values was used. Totally, about 150 tests

were performed for the UCS measurements. The cylindrical specimens were polished at

the end surfaces to ensure that they are accurately flat and parallel. The Geotest loading

machine was used for the compression test at a constant loading rate of 0.1 mm/min.

Water absorption tests were conducted according to ASTM C67-07 [31] to study the

capability of specimens in absorbing water, which depends on the microstructure and

porosity of the specimens. Besides that, water absorption can be an indicator of the

degree of geopolymeric reaction. The geopolymer brick specimens prepared at 16%

initial water content, 15 M NaOH concentration, and different forming pressures and

cured at 90 C for 7 days were soaked in water and weighed every 24 hours for 6 days. 5

specimens were tested for each forming pressure and the average was used for the plot.

Before weighing the soaked specimens, the wet surface was dried with a damp cloth. The

percentage absorption was calculated as follows

Absorption (%) = [(W2 – W1)/ W1] ×100 (1)

where W1 = weight of specimen after complete drying at 105°C, and W2 = weight of

specimen after soaking.

To investigate the effect of moisture content and forming pressure on the microstructure

and phase composition of the geopolymer bricks, SEM imaging and XRD analysis were

also performed. The SEM imaging of geopolymer specimens was performed in the SE

conventional mode using the FEI INSPEC-S50/Thermo-Fisher Noran 6 microscope. The

freshly failed surfaces from the unconfined compression tests, without polishing to keep

the fractured surface “un-contaminated”, were used for the SEM imaging. The XRD

analysis was performed with a Scintag XDS 2000 PTS diffractometer using Cu K

112

112

radiation, at 2.00 degree/min ranging from 10.00 to 70.00 degrees with 0.600 second

count time.

Table 2 summarizes the tests conducted on the brick specimens at different conditions.

3. Results and discussion

3.1. UCS

3.1.1. Effect of Curing Temperature and NaOH Concentration

Fig. 4 shows the variation of UCS with curing temperature for specimens prepared at 12%

initial water content, 25 MPa forming pressure, and respectively at 10 and 15 M NaOH

concentrations. At both 10 and 15 M NaOH, UCS increases with the curing temperature up

to about 90 °C and then decreases. The change of UCS with curing temperature can be

explained by the underlying mechanism in geopolymerization. As stated earlier, dissolution

and polycondensation are the two main steps in geopolymerization. Increasing the curing

temperature helps accelerate the dissolution of silica and alumina species and then

polycondensation. However, when the temperature is above a certain level, the fast

polycondensation and rapid formation of geopolymeric gel will hinder further dissolution of

silica and alumina species and thus affect the strength adversely [32,33]. Besides that, since

the brick specimens are cured in the oven without any coverage, too high a temperature

causes fast evaporation of water and may lead to incomplete geopolymerization. A similar

relationship between UCS and curing temperature is also reported by other researchers

[22,23,34]. Diop and Grutzeck [22] tested tuff-based geopolymer bricks and came up with

40 °C and 80 °C as the optimum temperatures, respectively for 8-12 M and 4 M NaOH

concentrations. Mohsen and Mostafa [23] studied the curing temperature effect on calcined

clay-based geopolymer bricks and reported an optimum temperature of 75 °C. Arioz et al.

[34] tested fly ash-based geopolymer bricks cured between 40 and 100 °C and obtained the

highest UCS at about 60 °C.

113

113

The UCS at 15 M NaOH is higher than that at 10 M NaOH for all curing temperatures

considered, which can be simply explained by the fact that at higher NaOH concentration,

higher NaOH/MT ratio and consequently higher Na/Al and Na/Si ratios were obtained (see

Table 2). The higher Na/Al and Na/Si ratios indicate that a larger amount of Na+ cation is

available to dissolve silica and alumina and consequently thicker geopolymeric binder is

produced. The geopolymeric binder serves as a link between the un-reacted or partially

reacted particles and contributes directly to the strength of the geopolymer material. The

improving effect of alkalinity on geopolymerization is reported by a number of researchers

[35-38]. In particular, Wang et al. [37] studied the effect of NaOH concentration on

metakaolin-based geopolymer specimens prepared at a water content of about 30% and a

forming pressure of 4 MPa. The results show that when the NaOH concentration was

increased from 4 to 12 M, higher UCS, flexural strength, and apparent density were

obtained.

3.1.2. Effect of water content and forming pressure

Considering the effect of curing temperature and NaOH concentration on UCS as discussed

in the previous subsection, 90 °C and 15 M NaOH were selected to study the effects of

water content and forming pressure. Fig. 5 shows the unconfined compression test results at

different initial water contents and forming pressures. Higher initial water content, which

means higher amount of NaOH (or higher NaOH/MT ratio) at constant NaOH

concentration, results in higher UCS. The highest UCS of 33.7 MPa was obtained at 18%

initial water content and 0.2 MPa forming pressure. The increase of UCS with the initial

water content may be explained from two aspects. First, water itself acts as a medium for the

geopolymeric reaction. After dissolution, the liberated monomers diffuse in the liquid

medium and form oligomers. It is important that sufficient amount of water is available for

the formation of geopolymeric binder linking the un-reacted or partially reacted particles.

However, too much water will cause the formation of large pores, which weakens the

geopolymeric specimens. Too high a water content may also adversely affect the brick

forming process. The forming pressure causes the MT particles to rearrange to a denser

114

114

configuration by pushing the air out of the matrix. This leads to a degree of saturation close

to 100% when the forming pressure is sufficiently high. At higher water content, the

saturation state will be achieved at a lower pressure and a less dense structure will be

obtained. Further increase in forming pressure will lead to squeezing out of water from the

matrix.

The other aspect is related to the availability of sufficient amount of NaOH in the liquid

phase for geopolymerization. The availability of the activating agent (or NaOH/MT) can be

expressed in two different ratios, Na/Al and Na/Si, to differentiate the role of the activating

agent in dissolving Al and Si. Higher Na/Al ratio leads to dissolution of more Al and

therefore sufficient amount of Na+ cation must be available for charge balancing the alumina

ions. For charge balancing, the Na/Al ratio has to be in a certain range. To produce

geopolymer concrete, different Na/Al ratios ranging from 0.38 to 2.06 have been used by

researchers [15,32,35,39-42]. Zhang et al. [30] showed that for geopolymerization of fly ash

added mine tailings, the increase in the Na/Al ratio up to 2.0 results in higher UCS. In the

current study, the Na/Al ratios vary from 0.86 to 1.94 corresponding to the 8% to 18% initial

water contents (see Table 2). By increasing the initial water content at a constant NaOH

concentration, the Na/Al ratio increases and thus higher strength is resulted.

Increased Na/Si ratio due to the increase in NaOH is another reason for the improving effect

of water content. In addition to Al, NaOH also acts as a dissolving agent for Si. Increasing

water content at constant NaOH concentration requires more NaOH, which results in

dissolution of more Si. The amorphous phase of MT is the primary source of Si and Al

species; however, the crystalline phase is also likely to provide additional Si and Al. The Si

source in the crystalline phase can be quartz, albite, and sanidine while the Al source is

albite and sanidine. Since Si is harder than Al to dissolve and quartz is more stable than the

other minerals, increasing alkalinity may help incorporate more Si in geopolymerization.

The Na/Si ratio varies between 0.11 and 0.25 corresponding to water content of 8 to 18%

(see Table 2).

115

115

The forming pressure has an improving effect on UCS but only up to a certain level. Fig. 5

shows that when the initial water content is 10% or lower, UCS tends to increase with the

forming pressure. However, when the initial water content is higher than 10%, UCS

increases with the forming pressure up to a certain level and then decreases. This can be

explained by the counteracting effect of water content and forming pressure at high water

content levels. When the initial water content is low, higher forming pressure leads to higher

degree of compaction of the specimen but no NaOH solution is squeezed out from the

specimen during the forming process. The sole compaction effect leads to increase of UCS

with higher forming pressure. When the initial water content is high, however, the NaOH

solution will be squeezed out from the specimen after the forming pressure exceeds a certain

limit. As sated earlier, the amount of NaOH solution (or MT/NaOH ratio) affects the degree

of geopolymerization and thus the strength of the geopolymer specimen. The loss of NaOH

solution due to the higher forming pressure will lead the decrease of UCS. So, at high initial

water content, the combined effects of compaction and NaOH solution loss due to the

forming pressure will control the final strength of the geopolymer specimen. Fig. 5 shows

that the highest UCS is obtained at 25, 10, 0.5, and 0.2 MPa forming pressure respectively

for the initial water content of 12, 14, 16, and 18%. Fig. 6 shows the initial water content

and forming pressure used by different researchers. In general, the forming pressure is

related to the initial water content, higher forming pressure corresponding to lower initial

water content. At the lowest initial water content of 8%, a very high forming pressure of 300

MPa is used [43].

SEM imaging and XRD analysis were also performed to further investigate the effect of

initial water content and forming pressure on the microstructure and phase composition of

the geopolymer brick specimens. Two initial water content/forming pressure combinations,

12% /25 MPa and 16% /0.5 MPa, were selected for the comparison. Fig. 7 shows the SEM

micrographs of the original MT and the geopolymer brick at both low and high

magnifications. The original MT particles have irregular shapes and the fine particles are

attached to each other and to the surface of the coarse particles (see Fig. 7a and b). As can be

116

116

seen in the micrographs of the geopolymer brick at low magnifications, at the lower initial

water content, the particles and particle aggregates are more isolated with large voids and

gaps (see Fig. 7c) while at the higher initial water content, the distribution of particles and

particle aggregates is more pervasive with only tiny voids (see Fig. 7e). The micrographs at

higher magnifications clearly indicate the degree of geopolymerization affected by the initial

water content. At the lower initial water content, which means lower NaOH amount (or

NaOH/MT ratio) at constant NaOH concentration, only limited amount of geopolymeric gel

is generated, leaving a large portion of the mine tailings particle surface un-reacted (see Fig.

7d). At the higher initial water content, however, a much larger amount of geopolymeric gel

is generated, covering essentially the surface of all mine tailings particles (see Fig. 7f).

Fig. 8 shows the XRD patterns of the mine tailings powder and the two geopolymer brick

specimens prepared respectively at the initial water content/forming pressure combinations

of 12%/25 MPa and 16%/0.5 MPa. The mine tailings are mainly crystalline material with a

large amount of silica, which agrees with Table 1. After geopolymerization, although the

intensity of the crystalline peaks decreases, the patterns are still crystalline. This is due to

only partial dissolution of the mine tailings particles. As shown in the SEM micrographs,

most particles react only on their surface and dissolve partially in the alkaline solution. The

main change in the XRD patterns due to geopolymerization is the reduction in the

crystalline peaks indicating the partial dissolution and formation of the amorphous and semi-

crystalline phases as shown in Fig. 8. The crystalline peak corresponding to gypsum does

not appear after geopolymerization. It might have been encapsulated or incorporated in the

geopolymeric gel. The amorphous phase in the original MT is a weak broad hump, which

extends from about 22° to 32°. After geopolymerization, the broad hump, which is also

superimposed with less intense crystalline peaks, covers a wider range from 22 to 38°. The

broad hump is slightly higher for the 16%/0.5 MPa specimen indicating formation of more

geopolymer gel. Another change in the XRD patterns is the transition of the sharp

crystalline peaks at 26.70° and 34.82° to less featured broad humps. They do not match with

any type of zeolitic materials. According to [46], zeolite is more likely to form at high water

117

117

contents. Fig. 8b shows the difference between the intensities of the 16%/0.5 MPa specimen

and those of the 12%/25 MPa specimen. A negative value means that the intensity at

16%/0.5 MPa is lower than that at 12%/25 MPa. The large negative peaks indicate that more

crystalline silica is dissolved in the 16%/0.5 MPa specimen than in the 12%/25 MPa

specimen, which agrees with the SEM micrographs that show the generation of more

geopolymer gel in the 16%/0.5 MPa specimen.

Due to the water loss during the molding process, the initial water content cannot represent

the true one during geopolymerization. Therefore, we determined the final water content

based on the weights of the molded specimen before and after curing. Fig. 9 shows the

variation of UCS with the final water content at different forming pressures. As expected,

UCS increases with both the forming pressure and the final water content. Increasing the

forming pressure physically improves the granular matrix by decreasing the volume of voids

and forcing the particles to be closer to each other while increasing the final water content,

which means larger amount of NaOH (or larger NaOH/MT ratio) at constant NaOH

concentration, chemically improves the microstructure by generating larger amount of

geopolymeric gel providing a stronger bond between the particles. The effect of the final

water content is much greater than that of the forming pressure in increasing the UCS,

particularly when the forming pressure is low. This can be seen in Fig. 9 that a single trend

line is fitted well to all of the data points corresponding to the forming pressures of 0 to 5

MPa.

The limited improving effect of the forming pressure has been observed by other researchers

as well [4,21]. Freidin [21] tested fly ash-based geopolymer bricks formed with a pressure

up to 20 MPa. The results indicated that the rate of increase in UCS with the forming

pressure decreases as the forming pressure is higher.

118

118

3.2. Water absorption

Water absorption is an important parameter for bricks. It indicates the permeability of bricks

and shows the degree of reaction for fired bricks. This is also true for geopolymer bricks

because higher degree of geopolymerization results in a less porous and permeable matrix.

Fig. 10 shows the results of water absorption tests on the specimens prepared at 16% initial

water content and different forming pressures and cured at 90 °C for 7 days. The water

absorption increases with the time of soaking, the rate of increase becoming lower as the

time of soaking increases. After 4 days, the change in water absorption is essentially

negligible. The water absorption after 4 days’ soaking varies from 2.26 to 4.73%

corresponding to forming pressure from 0.5 to 15 MPa. Freidin [21] showed that for fly ash-

based geopolymer bricks without hydrophobic additives, the water absorption reached its

ultimate value, about 25%, within just 1 day. He also showed that the addition of

hydrophobic agent decreased the ultimate water absorption to less than 10%, which was

reached after about one week.

The underlying mechanism responsible for the effects of the initial water content and the

forming pressure on UCS also explains the effect of the forming pressure on the water

absorption as shown in Fig. 10. At a lower forming pressure, the final water content and thus

the NaOH amount (or NaOH/MT ratio) are higher and a larger amount of geopolymeric gel

is generated, leading to lower porosity and permeability. As the forming pressure increases,

although the particles are compacted tighter to each other, less amount of geopolymeric gel

is generated due to water and thus NaOH loss, leading to higher porosity and permeability.

3.3. Bulk unit weight

Fig. 11 shows the variation of the bulk unit weight with the forming pressure for

geopolymer brick specimens prepared at 15 M NaOH concentration and different initial

water contents and cured at 90 C for 7 days. As expected, the unit weight increases with

both the initial water content and the forming pressure. The increase of the unit weight with

the initial water content is simply due to the larger amount of NaOH. The unit weight

119

119

increases with the forming pressure up to a certain level and then the rate of increase drops.

This is possibly because of the loss of water and thus NaOH beyond these levels of forming

pressure. These levels of forming pressures are close to the forming pressures corresponding

to the maximum UCS’s as shown in Fig. 5.

3.4. ASTM standards

Since no specification is available for geopolymer bricks, the ASTM specifications for

different types of bricks are used here to evaluate the quality of the mine tailings-based

geopolymer brick specimens. Table 3 summarizes the minimum compressive strengths, the

maximum water absorptions, and the maximum abrasion indices required for different types

of bricks [47-51]. The minimum compressive strength required by the ASTM standards

varies from 4.8 to 55.2 MPa depending on the application of the bricks. The compressive

strength of the geopolymer brick specimens in the current study varies from 3.69 to 33.7

MPa depending on the NaOH concentration, initial water content, forming pressure and

curing temperature. By selecting appropriate preparation conditions, a geopolymer brick can

be produced to meet all the ASTM strength requirements except for the SX grade pedestrian

and light traffic paving bricks, which requires at least 55.2 MPa. For example, to prepare a

building brick with a minimum strength of 20.7 MPa at severe weathering condition, a 15

NaOH concentration, an initial water content/forming pressure combination of 16%/0.5

MPa, and 90 °C curing temperature can be selected.

Water absorption tests were conducted only on the 16% initial water content specimens. The

24-hour water absorption varies from 0.5% to 3.45% depending on the forming pressure,

which are far below the ASTM limits.

In addition to the compressive strength and the water absorption, ASTM C902-07 requires

pedestrian and light traffic paving bricks to be abrasion resistant. To evaluate the abrasion

resistance, an abrasion index can be determined:

120

120

(psi)UCS

(%)absorption100IndexAbrasion

(2)

The calculated abrasion indices for the 16% initial water content specimens are shown in

Table 4. They are below the maximum limits shown in Table 3 indicating that the produced

geopolymer bricks are resistant to extensive abrasion.

4. Summary and Conclusions

The feasibility of using copper mine tailings to produce geopolymer bricks was studied by

conducting unconfined compression tests, water absorption tests, SEM imaging, and XRD

analysis. The study investigated the effect of four major factors, NaOH concentration,

initial water content, forming pressure, and curing temperature, on the physical and

mechanical properties, composition, and microstructure of the produced geopolymer brick

specimens. Based on the experimental results, the following conclusions can be drawn.

a) The geopolymer brick specimens prepared at 15 M NaOH concentration have higher

UCS than those at 10 M. This is because higher NaOH concentration provides larger

amount of NaOH at a certain initial water content required for the geopolymerization.

b) Higher initial water content means larger amount of NaOH at a constant NaOH

concentration and thus increases the strength of the geopolymer brick specimens.

c) Higher forming pressure leads to larger degree of compaction and thus higher UCS if no

water is squeezed out during the molding process. When the forming pressure is too

high, some water and thus NaOH will be lost and the UCS will decrease.

d) Curing temperature is an important factor affecting the geopolymerization and thus the

strength of geopolymer brick specimens. The UCS increases with the curing temperature

up to a certain level and then decreases with the curing temperature. For the copper mine

tailings studied in this paper, the optimum curing temperature is around 90 C.

e) By selecting appropriate preparation conditions (NaOH concentration, initial water

content, forming pressure, and curing temperature), eco-friendly geopolymer bricks

can be produced from the copper mine tailings to meet the ASTM requirements.

121

121

5. Acknowledgements

This work is supported by the National Science Foundation under Grant No. CMMI-

0969385 and the University of Arizona Faculty Seed Grants Program. The authors

gratefully acknowledge the Mission Mine Operations of ASARCO LLC for providing

mine tailings used in this investigation.

6. References

[1] Grahl C. Brick market overview: Steady growth continues in the brick industry. Ceramic

Industry 2004; October Issue.

[2] The Brick Industry Association. Overview of the American Brick Industry.

http://www.gobrick.com/Resources/AmericanBrickIndustry/tabid/7644/Default.aspx.

[3] China Econimic Trade Committee. Tenth five-year program of building materials

industry. China Building Materials 2001;7:7-10.

[4] Chen Y, Zhang Y, Chen T, Zhao Y, Bao S. Preparation of eco-friendly construction

bricks from hematite tailings. Construction and Building Materials 2011;25:2107-11.

[5] Chou MI, Chou SF, Patel V, Pickering MD, Stucki JW. Manufacturing fired bricks with

class F fly ash from Illinois Basin Coals. Combustion Byproduct Recycling Consortium,

Project Number 02-CBRC-M12, Final Report; 2006.

[6] Morchhale RK, Ramakrishnan N, Dindorkar N. Utilization of copper mine tailings in

production of bricks. Journal of the Institution of Engineers, Indian Civil Engineering

Division 2006;87:13-6.

122

122

[7] Roy S, Adhikari GR, Gupta, RN. Use of gold mill tailings in making bricks: a feasibility

study. Waste management & Research 2007;25;475-82.

[8] Liu Z, Chen Q, Xie X, Xue G, Du F, Ning Q, Huang L. Utilization of the sludge

derived from dyestuff-making wastewater coagulation for unfired bricks.

Construction and Building Materials 2011;25(4):1699-706.

[9] Algin HM, Turgut P. Cotton and limestone powder wastes as brick material.

Construction and Building Materials 2008;22(6):1074–80.

[10] Shon CS, Saylak D, Zollinger DG. Potential use of stockpiled circulating fluidized

bed combustion ashes in manufacturing compressed earth bricks. Construction and

Building Materials 2009;23(5):2062–71.

[11] Majidi B. Geopolymer technology, from fundamentals to advanced applications: a

review. Materials Technology 2009;24(2):79-87.

[12] Van Deventer JSJ, Provis J, Duxson P, Lukey GC. Technological environmental and

commercial drivers for the use of geopolymers in a sustainable material industry.

Intenatinal Symposium of Advanced Processing of Metals and Materials; 2006. p. 241-

52.

[13] Duxson P, Mallicoat SW, Lukey GC, Kriven WM, Van Deventer JSJ. The effect of

alkali and Si/Al ratio on the development of mechanical properties of metakaolin-based

geopolymers. Colloids and Surfaces A: Physicochemical and Engineering Aspects

2007;292(1):8–20.

123

123

[14] Dimas D, Giannopoulou I, Panias D. Polymerization in sodium silicate solutions: a

fundamental process in geopolymerization technology. Journal of Materials Science

2009;44:3719-30.

[15] Davidovits J. Mineral polymers and methods of making them. US Patent 4349386;

14th Sept. 1982.

[16] Davidovits J. Geopolymers: inorganic polymeric new materials. Journal of Thermal

Analysis 1991;37(8): 1633-56.

[17] Palomo A, Grutzeck MW, Blanco MT. Alkali-activated fly ashes A cement for the

future. Cement and Concrete Research 1999;29(18):1323–29.

[18] Li Z, Ding Z, Zhang Y. Development of sustainable cementitious materials.

Proceedings of International Workshop on Sustainable Development and Concrete

Technology, Beijing, China; 2004. p. 55-76.

[19] Drechsler M, Graham A. Innovative material technologies: bringing resources

sustainability to construction and mining industries. 48th Institute of Quarrying

Conference, Adelide SA, Australia; 2005.

[20] Shi C, Fernandez-Jimenez A. Stabilization/solidification of hazardous and radioactive

wastes with alkali-activated cements. Journal of Hazardous Materials 2006;137(3):1656-

63.

[21] Freidin C. Cementless pressed blocks from waste products of coal-firing power

station. Construction and Building Materials 2007;21:12–18.

124

124

[22] Diop MB, Grutzeck MW. Low temperature process to create brick. Construction and

Building Materials 2008;22(6):1114–21.

[23] Mohsen Q, Mostafa NY. Investigating the possibility of utilizing low kaolinitic clays

in production of geopolymer bricks. Ceramics – Silikaty 2010;54(2):160-8.

[24] Sultan HA. Stabilized copper mill tailings for highway construction. Transportation

Research Record; 1979. p. 1-7.

[25] EPA (Environmental Prtotection Agency). Copper mining and production wastes.

http://www.epa.gov/radiation/tenorm/copper.html.

[26] FHWA (Federal Highway Administration). User Guidelines for Byproduct and

Secondary Use Materials in Pavement Construction. Report No. FHWA-RD-97-148;

2008.

[27] Pacheco-Torgal F, Castro-Gomes JP, Jalali S. Investigations on mix design of tungsten

mine waste geopolymeric binder. Construction and Building Materials 2008;22(9):1939-

49.

[28] Pacheco-Torgal F, Castro-Gomes JP, Jalali S. Properties of tungsten mine waste

geopolymeric binder. Construction and Building Materials 2008;22:1201-11.

[29] Pacheco-Torgal F, Jalali S. Influence of sodium carbonate addition on the thermal

reactivity of tungsten mine waste mud based binders. Construction and Building

Materials 2010;24:56-60.

125

125

[30] Zhang L, Ahmari S, Zhang S. Synthesis and characterization of fly ash modified

mine tailings-based geopolymers. Construction and Building Materials

2011;25(9):3773-81.

[31] ASTM Standard C67-07a. Standard test methods for sampling and testing brick and

structural clay tile. ASTM International, West Conshohocken, PA, 2007, DOI:

10.1520/C0067-07, www.astm.org.

[32] Muñiz-Villarreal MS, Manzano-Ramírez A, Sampieri-Bulbarela S, Gasca-Tirado JR,

Reyes-Araiza JL, Rubio-Ávalos JC, Pérez-Bueno JJ, Apatiga LM, Zaldivar-Cadena A,

Amigó-Borrás V. The effect of temperature on the geopolymerization process of a

metakaolin-based geopolymer. Materials Letters 2011;65(6):995-8.

[33] Yao X, Zhang Z, Zhua H, Chen Y. Geopolymerization process of alkali–metakaolinite

characterized by isothermal calorimetry. Thermochimica Acta 2009;493(1-2):49-54.

[34] Arioz O, Kilinc K, Tuncan M, Tuncan A, Kavas T. Physical, mechanical and micro-

structural properties of F type fly-ash based geopolymeric bricks produced by pressure

forming process. Advances in Science and Technology 2010;69:69-74.

[35] Rattanasak U, Chindaprasirt P. Influence of NaOH solution on the synthesis of fly ash

geopolymer. Mineral Engineering 2009;22:1073-78.

[36] Somna K, Jaturapitakkul C, Kajitvichyanukul P, Chindaprasirt P. NaOH-activated

ground fly ash geopolymer cured at ambient temperature. Fuel 2011;90(6):2118-24.

[37] Wang H, Li H, Yan F. Synthesis and mechanical properties of metakaolinite-based

geopolymer. Colloids and Surfaces A: Physicochemical and Engineering Aspects

2005;268(1-3):1-6.

126

126

[38] Khale D, Chaudhary R. Mechanism of geopolymerization and factors influencing its

development: a review. Journal of Materials Science 2007;42:729-46.

[39] Steveson M, Sagoe-Crentsil K. Relationships between composition, structure and

strength of inorganic polymers, Part I Metakaolin-derived inorganic polymers. Journal

of Materials Science 2005;40:2023-36.

[40] De Silva P, Sagoe-Crenstil K, Sirivivatnanon V. Kinetics of geopolymerization: Role

of Al2O3 and SiO2. Cement and Concrete Research; 2007;37(4):512–8.

[41] Rowles M, O’Connor B. Chemical optimisation of the compressive strength of

aluminosilicate geopolymers synthesised by sodium silicate activation of metakaolinite.

Journal of Materials Chemistry 2003;13(5):1161-5.

[42] Tippayasam C, Boonsalee S, Sajjavanich S, Ponzoni C, Kamseu E, Chaysuwan D.

Geopolymer development by powders of metakaolin and wastes in Thailand. Advances

in Science and Technology 2010; 69:63-8.

[43] Zivica V, Balkovic S, Drabik M. Properties of metakaolin geopolymer hardened paste

prepared by high-pressure compaction. Construction and Building Materials

2011;25(5):2206-13.

[44] Majkrzak II GL, Watson JP, Bryant MM, Clayton K. Effect of cenospheres on fly ash

brick properties. World of Coal Ash (WOCA), Kentucky; 2007.

[45] Liu H, Van Engelenhoven J. Use of ASTM standards for testing freeze-thaw resistance

of fly ash bricks. World of Coal Ash (WOCA) Conference, KY, USA; 2009.

127

127

[46] Hawkings DB. Kinetics of glass dissolution and zeolite formation under hydrothermal

conditions. Clays and Clay Minerals 1981;29(5):331-40.

[47] ASTM Standard C34-03. Standard specification for structural clay load-bearing wall

tile. ASTM International, West Conshohocken, PA, 2003, DOI: 10.1520/C0034-03,

www.astm.org.

[48] ASTM Standard C62-10. Standard specification for building brick (solid masonry units

made from clay or shale). ASTM International, West Conshohocken, PA, 2010, DOI:

10.1520/C0062-10, www.astm.org.

[49] ASTM Standard C126-99. Standard specification for ceramic glazed structural clay

facing tile, facing brick, and solid masonry units. ASTM International, West

Conshohocken, PA, 1999, DOI: 10.1520/C0126-99, www.astm.org.

[50] ASTM Standard C216-07a. Standard specification for facing brick (solid masonry units

made from clay or shale). ASTM International, West Conshohocken, PA, 2007, DOI:

10.1520/C0216-07A, www.astm.org.

[51] ASTM Standard C902-07. Standard specification for pedestrian and light traffic paving

brick. ASTM International, West Conshohocken, PA, 2007, DOI:10.1520/C0902-07,

www.astm.org.

128

128

Table 1. Chemical composition of mine tailings.

Chemical Compound Weight %

SiO2 64.8

Al2O3 7.08

Fe2O3 4.33

CaO 7.52

MgO 4.06

SO3 1.66

Na2O 0.90

K2O 3.26

129

129

Table 2. Specimen properties and different types of tests conducted.

Specimen Label

NaOH

Conc.

(M)

Water

content

(%)

Forming

Pressure

(MPa)

NaOH/

MT

(%) Na/Al Na/Si

Curing

Temp.

(C)

UCS

Test Absorption

Test XRD SEM

10-12-25-60 10 12 25 4.8 0.86 0.11 60 X

10-12-25-90 10 12 25 4.8 0.86 0.11 90 X

10-12-25-120 10 12 25 4.8 0.86 0.11 120 X

15-12-25-60 15 12 25 7.2 1.30 0.17 60 X

15-12-25-75 15 12 25 7.2 1.30 0.17 75 X

15-12-25-90 15 12 25 7.2 1.30 0.17 90 X X X

15-12-25-105 15 12 25 7.2 1.30 0.17 105 X

15-12-25-120 15 12 25 7.2 1.30 0.17 120 X

15-8-5-90 15 8 5 4.8 0.86 0.11 90 X

15-8-15-90 15 8 15 4.8 0.86 0.11 90 X

15-8-25-90 15 8 25 4.8 0.86 0.11 90 X

15-8-35-90 15 8 35 4.8 0.86 0.11 90 X

15-10-5-90 15 10 5 6 1.08 0.14 90 X

15-10-15-90 15 10 15 6 1.08 0.14 90 X

15-10-25-90 15 10 25 6 1.08 0.14 90 X

15-10-35-90 15 10 35 6 1.08 0.14 90 X

15-12-5-90 15 12 5 7.2 1.30 0.17 90 X

15-12-15-90 15 12 15 7.2 1.30 0.17 90 X

15-12-35-90 15 12 35 7.2 1.30 0.17 90 X

15-14-5-90 15 14 5 8.4 1.51 0.19 90 X

15-14-10-90 15 14 10 8.4 1.51 0.19 90 X

15-14-15-90 15 14 15 8.4 1.51 0.19 90 X

15-14-25-90 15 14 25 8.4 1.51 0.19 90 X

15-16-0-90 15 16 0 9.6 1.73 0.22 90 X

15-16-05-90 15 16 0.5 9.6 1.73 0.22 90 X X X X

15-16-105-90 15 16 1.5 9.6 1.73 0.22 90 X X

15-16-3-90 15 16 3 9.6 1.73 0.22 90 X X

15-16-5-90 15 16 5 9.6 1.73 0.22 90 X X

15-16-15-90 15 16 15 9.6 1.73 0.22 90 X X

15-18-0-90 15 18 0 9.6 1.94 0.22 90 X

15-18-02-90 15 18 0.2 10.8 1.94 0.25 90 X

15-18-04-90 15 18 0.4 10.8 1.94 0.25 90 X

15-18-05-90 15 18 0.5 10.8 1.94 0.25 90 X

15-18-105-90 15 18 1.5 10.8 1.94 0.25 90 X

130

130

Table 3. ASTM specifications for different applications of bricks.

Notes: ALBX = load bearing exposed;

BLB = load bearing non-exposed;

Cend

construction use; Dside construction use;

Ebased on 1 hour boiling water absorption;

Fsevere weathering;

Gmoderate weathering;

Hnegligible weathering;

Ibased on 5 hour

boiling water absorption; and JType I, II, and III are respectively subjected to extensive,

intermediate, and low abrasion.

Title of specification ASTM

Designation Type/Grade

Minimum

UCS (MPa)

Maximum

water

absorption

(%)

Abrasion Index

Structural clay load

bearing wall tile C34-03

LBX A

9.6 C 16

E NA

LBX 4.8 D 16

E NA

LB B

6.8 C 25

E NA

LB 4.8 D 25

E NA

Building brick C62-10

SW F

20.7 17 NA

MW G

17.2 22 NA

NW H

10.3 No limit NA

Solid masonry unit C126-99

Vertical coring 20.7 NA NA

Horizontal

coring 13.8 NA NA

Facing brick C216-07a

SW 20.7 17 I

NA

MW 17.2 22 I

NA

Pedestrian and light

traffic paving brick

C902-07

SX 55.2 8 Type I J

0.11

MX 20.7 14 Type II J

0.25

NX 20.7 No limit Type III J

0.50

131

131

Table 4. Abrasion indices for geopolymer brick specimens prepared at 16% initial

content and cured at 90 °C for 7 days.

Forming pressure

(MPa)

UCS

(MPa)/(psi)

24 hour water

absorption (%) Abrasion Index

0.5 28/4,040 0.93 0.02

1.5 25/3,591 2.18 0.06

3.0 22/3,250 2.92 0.09

5.0 21/3,086 3.45 0.11

15.0 21/3,059 3.15 0.10

132

132

Fig. 1. Particle size distribution of mine tailings.

0

10

20

30

40

50

60

70

80

90

100

1101001000

Particle size (m)

Pe

rce

nt p

assin

g (%

)

133

133

Fig. 2. XRD pattern of un-reacted mine tailings (A: albite, G: gypsum, P: sanidine, S:

quartz).

134

134

Fig. 2. Load-displacement curves at forming stage for different initial water contents and

forming pressures.

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Lo

ad

(kN

)

Displacement (mm)

0.5/16

1.5/16

5/14

15/14

25/12

Forming Pressure (MPa) / water content (%)

0

0.3

0.6

0.9

1.2

1.5

0 1 2 3 4

Lo

ad

(kN

)

Displacement (mm)

0.5/16

1.5/16

135

135

Fig. 3. UCS vs. curing temperature for specimens prepared at 12% initial water content,

25 MPa forming pressure, and respectively 10 and 15 M NaOH concentrations.

6

11

16

21

60 75 90 105 120

UC

S (M

Pa

)

Temperature ( C)

15

10

NaOH (M)

136

136

Fig. 4. UCS vs. forming pressure for specimens prepared at different initial water

contents and 15 M NaOH concentration and cured for 7 days at 90 °C.

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35

UC

S (M

Pa

)

Forming Pressure (MPa)

8

10

12

14

16

18

Initial Water Content (%)

20

25

30

35

0 1 2 3U

CS

(M

Pa)

Forming Pressure (MPa)

137

137

Fig. 5. The initial water contents and optimum forming pressures used in the current

study and by other researchers.

0

1

10

100

1,000

6 8 10 12 14 16 18 20

Op

tim

um

Fo

rmin

g P

ressu

re (

MP

a)

Initial Water Content (%)

A

DC

BE

F

A: metakaolin-based geopolymer [43] B: steam-cured fly ash [44]

C: fired hematite tailings, clay, and fly ash [4] D: room-cured class C fly ash [45]

E: calcined clay-based geopolymer [23] F: tuff-based geopolymer [22] O: current study

138

138

Fig. 6. SEM micrographs of MT powder – a) and b), and geopolymer brick at initial

water content/forming pressure combinations of 12%/25 MPa - c) and d), and 16%/0.5

MPa - e) and f), for the specimens cured at 90 °C for 7 days (GP: geopolymer, MT: mine

tailings particle).

a b

c d

e

MT

GP

MT

GP

f

139

139

Fig. 7. XRD patterns: a) mine tailings powder and geopolymer brick specimens prepared

at initial water content/forming pressure respectively of 12%/25 MPa and 16%/0.5 MPa,

and cured at 90 °C for 7 days; and b) differential XRD between the two brick specimens

(A: albite, G: gypsum, P: sanidine, S: quartz).

b

a

140

140

Fig. 8. UCS vs. final water content for specimens prepared at 15 M NaOH and different

forming pressures and cured for 7 days at 90 °C.

15: R² = 0.80

25: R² = 0.80

35: R² = 0.54

0-5: R² = 0.90

0

5

10

15

20

25

30

35

40

45

50

6 8 10 12 14 16 18 20

UC

S (M

Pa

)

Final Water Content (%)

0

0.2

0.4

0.5

1.5

3

5

10

15

25

35

15

25

35

0-5

Forming Pressure (MPa)Coefficient of Regression

141

141

Fig. 9. Water absorption versus soaking time for specimens prepared at 16% initial water

content and different forming pressures and cured at 90 °C for 7 days.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1 2 3 4 5 6 7 8

Wa

ter

Ab

so

rptio

n (%

)

Soaking Time (day)

0.5

1.5

3

5

15

FormingPressure (MPa)

142

142

Fig. 10. Bulk unit weight versus forming pressure for specimens prepared at different

initial water contents and 15 M NaOH and cured at 90 °C for 7 days.

17.0

17.5

18.0

18.5

19.0

19.5

20.0

0 5 10 15 20 25 30 35

Un

it W

eig

ht (

kN

/m3)

Forming Pressure (MPa)

8

10

12

14

16

Initial Water Content (%)

b

143

APPENDIX D

Paper was prepared with the intent to publish to a journal

LEACHING BEHAVIOR OF MINE TAILINGS-BASED

GEOPOLYMER BRICKS

1. Introduction

In Appendix C, construction bricks based on alkali activation of mine tailings were studied.

It was shown that by properly selecting the preparation conditions (NaOH concentration,

initial water content, forming pressure, and curing temperature), geopolymer bricks can be

produced from the mine tailings to meet the ASTM requirements. In this section, the

leaching behavior of the MT-based geopolymer bricks produced at initial moisture

content/forming pressure respectively of 16%/0.5 MPa and 12%/25 MPa is studied. The

leaching behavior of different metals is investigated by fitting first order reaction/diffusion

model to the experimental data. Then the back calculated parameters are used to study the

effect of micro-structural properties on the leaching kinetics of Al, Cu, Fe, and Zn. The

elemental composition analysis by SEM/EDX on the surface of geopolymer gel is

performed to study the incorporation of heavy metals in the microstructure of geopolymer.

2. Materials and Methods

In addition to the materials listed in Appendix C, nitric acid was used in this study. The

nitric acid (HNO3) was manufactured by BDH and supplied by VWR.

144

The experiments performed in this study consist of SEM/EDX and ICP-MS. The details

on sample preparation are explained in Appendix C. The geopolymer bricks prepared at

optimum initial water content/forming pressure respectively of 16%/0.5 MPa and 12%/25

MPa were selected to study the environmental performance. The specimens were soaked

in pH = 4 and 7 solutions for four months.

In order to study the leaching behavior and particularly the effect of geopolymeric

microstructure on the leaching kinetics of heavy metals, static leaching test was

performed by soaking the MT powder and geopolymer brick samples in two different

pH-solutions. The MT powder is used for comparison and studying the effect of

geopolymerization on immobilization of heavy metals. The pH was monitored during the

experiment at least two times a day and was adjusted by adding nitric acid to the solution.

A solid to liquid mass ratio of 1:15 was used for all the specimens throughout the

experiment. After specified immersion times, 1, 3, 5, 7, 14, 21, 28, 90, and 105 days,

solution sample less than 5 ml was taken and then filtered with a 0.45 m membrane

filter. The filtrate was diluted with 1% nitric acid and then the concentration of metals in

the diluted extracted sample was measured based on the ICP-MS (inductively coupled

plasma mass spectrometry) technique. The total amount of extraction from the solutions

was less than 5% of the total solution to ensure that the solid to liquid ratio does not

substantially change during the experiment.

3. Results and Discussion

3.1. Efficiency of Immobilization

Table 2 shows the concentration of different cations leached from MT powder and 12/25

and 16/0.5 brick specimens respectively at pH = 4 and 7 and after 90 days. The threshold

concentrations regulated by different standards are also shown in the table. The

concentration of released cations is consistent with the chemical composition and the

content of trace elements shown in Table 1. The MT powder contains substantial amount

of Fe, Ca, Mg, K, and Na but only trace amount of Mn, Cu, Zn, and Mo. The MT exhibit

145

high leachability for Ca, Mg, K, Na and trace elements Mn, Cu, and Zn in acidic

condition. However, Fe did not show considerable leaching despite its high content in the

MT powder. In copper MT, Fe mainly exists as pyrite (FeS) or chalcopyrite (CuFeS2) and

during AMD process, it may oxidize and yield FeO or Fe2O3. Considering the low

solubility of Fe in current experiment, Fe2+

is most likely to be the dominant valence

since Fe2+

is less soluble. Table 3 shows the percentage and immobilization efficiency of

the cations fixed in the geopolymer bricks. The immobilization efficiency is obtained by

dividing the mass of the released elements by its initial mass in the solid specimen. The

percentage of un-dissolved portion of the contaminant is calculated by deducting the

obtained ratio from 1. The geopolymer brick shows effective immobilization in most of

the heavy metals. However, the immobilization efficiency is not satisfactory for K,

especially in the 16/0.5 specimen. The high release of K could be possibly due to

dissolution during alkali-activation and incorporation in the polymeric structure. After

acid attack, K along with Ca and Na leaches out. Since the 16/0.5 specimen contains

larger amount of geopolymer gel due to the higher amount of initial water content and

NaOH, this phenomenon is more pronounced. The efficiency of immobilization of heavy

metals in geopolymeric gel except for Fe shows correlation with their ionic radius [2].

The ionic radius increases in the following order

Fe2+

< Cu2+

< Zn2+

< Mn2+

It can be seen from Table 3 that except for Fe, the immobilization efficiency of the heavy

metals follows the same order. The likely reasons for the immobilization efficiency are

further discussed later.

Immobilization of heavy metals can take place chemically through incorporation into the

polymeric structure as charge-balancing cation or physical encapsulation [3]. The EDX

analysis results in Fig. 1 indicate presence of Fe, Zn, Mg, K, and Ca on the surface of

geopolymer gels. As these elements were also detected in geopolymer gels before

immersion, they possibly take part in the geopolymeric reaction and act as charge

balancing ion. Ca is most likely released from the un-reacted MT particles as they contain

146

large amount of Ca. The leaching results indicate that chemical stabilization dominates

the physical encapsulation because the structural breakdown of the geopolymer gel does

not result in significant release of all the mentioned elements.

The concentration of cations at pH = 4 is higher than that at pH = 7 because most metals

have higher solubility at acidic condition [4]. At pH = 4, the concentrations of Mn, Cu,

and Zn for MT exceed the DIN or Greek standard limits, but these heavy metals are

effectively immobilized in the brick specimens and exhibit concentrations significantly

lower than the standard limits.

3.2. Kinetics of leaching

The leaching behavior of Al, Fe, Cu, and Zn is studied by back-calculation of the first-

order reaction/ diffusion model parameters. This model explains leaching of species out

of solid specimens through a simplified mechanism, which is shown to be sufficiently

accurate for solid wastes [5,6]. The model consists of the first order reaction model

(FRM) which involves dissolution of the species at the solid-liquid interface and the bulk

diffusion model (BDM) which accounts for transportation of the dissolved species

through the porous medium. The governing differential equations for the FRM and the

one-dimensional BDM are as follows:

2

2

x

CD

t

C

kQdt

dQ

(1)

where Q, k, C, and D denote the amount of soluble contaminant in the solid waste,

reaction rate, concentration of the contaminant at time t and position x, and coefficient of

diffusion. The obtained concentrations from both equations can be superimposed to

account for the dissolution/diffusion phenomenon as in many systems the leaching

behavior is dominated by both dissolution and diffusion. The combined solution to the

FRM and BDM models is called the first order reaction/diffusion model (FRDM). Suzuki

147

et al. [6] and Zheng et al. [5] successfully predicted leaching kinetics by using the FRDM

model as following:

2

1

00 )(2)]exp(1[

tDSCktQM

obs

(2)

where M, Q0, S, C0, Dobs

are respectively the cumulative concentration of the

contaminant, initial amount of the soluble contaminant, surface area, total concentration

of the contaminant in the solid specimen, and observed diffusivity. Dobs

represents the

effect of physical barrier due to transport through the tortuous pores and chemical

retardation due to sorption on the solid phase. Effective diffusivity (Deff) represents only

the effect of tortuous pores and their connectivity on the transport of contaminants. In

case of no sorption, Dobs

and Deff are identical. However, in the case of linear sorption,

Dobs

can be obtained by multiplying Deff by a factor, which indicates chemical retardation

[7]. Zheng et al. [5] employed FRDM using Deff and introducing two factors accounting

for chemical and physical retardation. In this study, the chemical and physical

contribution to Dobs

is separated by introducing a physical retardation factor (fp) and

chemical diffusion (Dc) and only depends on the diffusing contaminant. Thus, Eq. (2) is

reduced to the following equation:

2

1

00 )(2)]exp(1[

tfDSCktQM

pc (3)

The non-linear regression method using Microsoft Excel solver was applied to fit Eq. (3)

to the measured concentration vs. time curves (see Fig. 2). First, Dc is back-calculated by

fitting Eq.(3) to the measured concentration in the MT powder specimen and assuming fp

= 1. The obtained Dc is then substituted in the equation and fp is back-calculated by fitting

Eq. (3) to the measured concentrations of the contaminants from the solid specimens. The

back-predicted parameters are summarized in Table 4. Dobs

varies significantly with the

diffusing contaminant indicating that chemical retardation is an important factor in

leaching behavior of the specimens. Except for Al, Dobs

does not change largely with the

specimen type. In other words, fp does not exhibit significant variation with the specimen

type although it is slightly smaller for the 16/0.5 specimen. Although 16/0.5 has a denser

148

microstructure than 12/25 (see Appendix C), the pores are still large enough to let the

contaminants migrate.

Dc is assumed dependant only on the contaminant type because it represents the chemical

barrier for diffusion, which is due to sorption. Sorption depends on the diffusing ions not

the porosity of the specimen. The lowest and highest retardation factors are respectively

obtained for Cu and Fe. “k” for Fe has the lowest value indicating that it has low reaction

rate with the dissolving solution. Therefore, the low leachability of Fe is attributed to its

low solubility but not fixation in the geopolymer gel because it also shows low

leachability in the MT powder. On the other hand, Cu exhibits a lower immobilization

efficiency than Fe because it has a higher reaction rate. Therefore, in terms of leaching

behavior, reaction rate dominates the chemical retardation. The Back-predictions of the

measured concentrations of Zn show that the contribution from the diffusion part of eq.

(3) is zero. In other words, BDM or reaction part of the equation fits into the measured

curve. This means that leaching of Zn from MT-based matrix is controlled by high-rate

chemical reaction. Zn might be coming from outer surface of the specimen so that it does

not face any physical or chemical barrier.

4. Summary and Conclusions

In this section, the leaching behavior of MT-based geopolymer bricks was investigated

based on leaching analysis. The results indicate that the heavy metals are effectively

immobilized in the MT-based geopolymer bricks. The effective immobilization is mainly

attributed to the incorporation of heavy metals in the geopolymeric network. The analysis

based on the first-order reaction/diffusion model indicates that solubility or reaction rate

is an important factor controlling the leaching behavior of heavy metals. From the study,

it can be concluded that geopolymerization is an effective way in immobilizing heavy

metals in MT.

149

5. References

[1] http://www.chemicool.com/elements.

[2] Van Jaarsveld JGS, Van Deventer JSJ. The effect of metal contaminants on the

formation and properties of waste-based geopolymers. Cement and Concrete Research

1999;29:1189-1200.

[3] Van Jaarsveld JGS, Van Deventer JSJ, Lorenzen L. Factors affecting the

immobilization of metals in geopolymerized fly ash. Metallurgical and Materials

Transactions B 1998;29:283–91.

[4] LaGrega M, Buckingham P, Evans J, Environmental Resources Management.

Hazardous Waste Management. 2nd

Edition, McGraw-Hill; 1994.

[5] Zheng L, Wang W, Shi Y. The effects of alkaline dosage and Si/Al ratio on the

immobilization of heavy metals in municipal solid waste incineration fly ash-based

geopolymer. Chemosphere 2010;79:665–71.

[6] Suzuki K, Ono Y. Leaching characteristics of stabilized/solidified fly ash generated from

ash-melting plant. Chemosphere 2008;71(5):922-32.

[7] Park JY, Batchelor B. A multi-component numerical leach model coupled with a

general chemical speciation code, Water Research 2002;36:156-66.

150

Table 1. Chemical composition (weight %) of mine tailings.

Chemical Compound Content* (%) Standard Deviation (%)

SiO2 64.8 2.08

Al2O3 7.08 0.70

Fe2O3 4.33 0.71

CaO 7.52 1.06

MgO 4.06 0.93

SO3 1.66 0.31

Na2O 0.90 0.23

K2O 3.26 0.42

Trace Elements

Pb 0.000286 0.0007

Zr 0.012 0.001

Mo 0.022 0.003

Zn 0.068 0.009

Cu 0.076 0.009

Mn 0.163 0.034

Ti 0.213 0.006

* The values are the average of 7 tailings samples.

151

Table 2. Concentration of leached metals from MT powder and 12/25 and 16/0.5 brick specimen immersed in pH = 4 and 7 solution

for 90 days.

pH Na Mg Al K Ca Cr Mn Fe Co Ni Cu Zn As Se Mo Cd Pb

MT

7 5.9 36.9 0.2 7.1 358.9 0.1 1.0 0.0 0.1 0.1 0.0 0.0 0.1 0.0 0.13 0.0 0.0

4 11.1 497.3 1.2 28.6 2998 0.0 8.8 1.4 0.0 0.0 3.9 1.9 0.0 0.1 0.0 0.0 0.0

12/25

7 2952 1.2 0.4 101.2 78.0 0.0 0.2 0.0 0.1 0.0 0.1 0.1 0.7 0.0 0.7 0.0 0.0

4 3740 8.8 1.3 123.5 69.3 0.0 0.1 0.9 0.0 0.0 0.3 0.1 0.1 0.0 0.7 0.0 0.0

16/0.5

7 4135 1.6 0.6 132.2 87.3 0.0 0.5 0.0 0.1 0.0 0.0 0.0 0.8 0.0 0.8 0.0 0.0

4 4858 0.6 0.6 592.0 46.7 0.0 0.0 1.4 0.0 0.0 0.2 0.2 0.1 0.1 0.8 0.0 0.0

EPA Limit NA NA NA NA NA 5 NA NA NA 5 NA NA 5 1 NA 1 5.0

DIN NA NA NA NA NA NA NA NA NA NA 2-5 2-5 NA NA NA NA NA

GREEK NA NA 10 NA NA NA 2 NA NA NA 0.5 0.5 NA NA NA NA NA

152

Table 3. Percentage of retained elements in MT and immobilization efficiency of the brick

specimens immersed in pH = 4 solution for four months.

Element Retained in

MT (%)

Immobilization

Efficiency (%)

Ionic radius [1]

12/25 16/0.5

Al 99.95 99.94 99.97

Fe 99.93 99.95 99.92 0.63/0.77

Ca 16.33 97.92 98.57

Mg 69.53 99.46 99.96

K 98.42 92.65 63.98

Zn 95.79 99.86 99.71 0.88

Cu 92.26 99.70 99.48 0.87/.91

Mn 91.93 99.92 99.90 0.89/0.75

153

Table 4. Summary of the back-predicted FRDM parameters.

Element Specimen

Label

Q0

(mg/Kg)

k

(1/hr)

fp

Dc

(m2/hr)

Dobs

R2

Al 12/25 0.140 0.053 8.154E-01 2.846E-05 2.32E-05 96.2

16/.5 0.462 0.012 3.810E-03 2.846E-05 1.08E-07 94.2

Cu 12/25 0.209 0.017 9.703E-04 1.851E-07 1.80E-10 99.3

16/.5 0.075 0.122 1.204E-03 1.851E-07 2.23E-10 98.3

Fe 12/25 0.383 0.004 8.244E-04 3.580E-01 2.95E-04 98.1

16/.5 1.553 0.004 1.825E-03 3.181E-01 5.80E-04 99.6

Zn 12/25 0.060 0.011 NA NA NA 95.1

16/.5 0.189 0.004 NA NA NA 93.8

154

Fig. 1. SEM micrographs and EDX analysis results of 12/25 and 16/0.5 specimens immersed in

pH = 4 and 7 solutions for four months.

0 1 2 3 4

Fe NaMgAl

Si

K CaC

N

O

0 1 2 3 4

NaMg

Al

Si

KCa

C

O

S

0 1 2 3 4

Fe NaMg Al

Si

K Ca

O

NC

0 1 2 3 4

Fe

NaZnMg

Al

Si

SK CaC

0 1 2 3 4

Fe

Na

Mg

Al

Si

S K CaCN

O

a)12/25, pH = 4

b)16/0.5, pH = 4

c)16/0.5, pH = 7

155

Fig. 2. Measured and predicted concentrations of heavy metals at pH = 4.0 by FRDM.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80 100 120

Co

nce

ntr

atio

n (p

pm

)

Time (hr)

Cu (pH=4)

12/25 Measured

12/25 predicted

16/0.5 measured

16/0.5 predicted

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 20 40 60 80 100 120

Co

nce

ntr

atio

n (p

pm

)

Time (hr)

Fe (pH=4)

12/25 Measured

12/25 predicted

16/0.5 measured

16/0.5 predicted

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 20 40 60 80 100 120

Co

nce

ntr

atio

n (p

pm

)

Time (hr)

Zn (pH=4)

12/25 Measured

12/25 predicted

16/0.5 measured

16/0.5 predicted

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

0 20 40 60 80 100 120

Co

nce

ntr

atio

n (p

pm

)

Time (days)

Al (pH=4)

12/25 Measured

12/25 predicted

16/0.5 measured

16/0.5 predicted

156

APPENDIX E

Paper was prepared with the intent to publish to a journal

UTILIZATION OF CKD TO ENHANCE MINE TAILINGS-

BASED GEOPOLYMER BRICKS

1. Introduction

In Appendix C, the physical and mechanical properties and microstructure of mine

tailings (MT)-based geopolymer bricks was investigated. It was shown that the bricks

produced at properly selected conditions could meet the ASTM requirements on strength,

water absorption, and abrasion resistance for nearly all types of applications. This

Appendix presents the feasibility study of further improvement of the physical and

mechanical properties of MT-based geopolymer bricks by adding a small amount of

cement kiln dust (CKD). Addition of CKD is also expected to reduce the required amount

of NaOH and make the production of (MT)-based geopolymer bricks more economical.

CKD is a by-product of the ordinary Portland cement (OPC) manufacturing process. To

produce OPC, calcium carbonate and clay are ground, mixed and calcined by heating at

very high temperatures. During this process, which is called kiln process, calcium silicate

is produced and dust, called “cement kiln dust (CKD)”, is generated. In current practice,

CKD is collected and then landfilled or fed back into the kiln process for further

calcination. CKD contains very fine particles of un-reacted or partially calcined minerals,

and clinker dust [1]. Silica and calcium compounds constitute a major portion of CKD

and minor amount of alumina and other types of metals such as Fe, K, Mg, and Na are

also present [2]. Due to presence of Ca, Na, and K, CKD’s natural pH is alkaline. CKD

imposes costs to cement plants due to material loss, usage of energy for collecting and

157

reprocessing, and landfilling [3]. If CKD contains a high content of alkali, it has to be

landfilled because of the difficulty in reprocessing [3,4]. Therefore, researchers have

attempted to utilize CKD for different applications including as additive to OPC along

with fly ash (FA), soil stabilization, daily liner for landfills, stabilization/solidification of

wastes, mine reclamation, and well-stabilizing binder in oil and gas industry [2,5-10].

Incorporation of CKD in geopolymer binder has been studied by few researchers

[3,4,11]. For example, Buchwald and Schultz [4] and Konsta-Gdoutos and Shah [11]

used CKD as an activator for furnace slag due to its high alkali content. The results show

that the contribution of CKD to geopolymerization significantly depends on its physical

and chemical composition. To the best knowledge of the authors, so far no research has

been done on the usage of CKD in MT-based geopolymer.

This Appendix presents the research on utilization of CKD in MT-based geopolymer

bricks. The research systematically investigates the effect of CKD content on the

unconfined compressive strength (UCS), water absorption, and durability (weight and

UCS loss due to immersion in water) at different conditions. SEM, XRD, and FTIR are

also performed to study the underlying mechanism for the contribution of CKD to

geopolymerization.

2. Materials and Methods

The materials used in this study and the experimental procedure are the same as those in

Appendix C. In addition, CKD provided by CalPortland Company in Tucson, AZ was

also used. Table 1 shows the chemical composition of CKD. CaO has the highest

concentration and there is only small amount of alkali. Loss on ignition (LOI) at 950 °C

is 36% because calcium carbonate and magnesium carbonate constitute a major portion

of the material. Fig. 1 shows the SEM micrograph and EDX analysis results on the CKD

powder. It can be seen that the CKD particles are very fine(finer than 20 m) and have

irregular shapes. The EDX analysis results also indicate that Ca is the major constituent

158

and there are substantial amount of Si, Al, and Mg. The XRD pattern of the CKD powder

(see Fig. 2) shows that the dominant crystalline phases are CaO, CaCO3, and quartz. The

peaks corresponding to Ca(OH)2 are broad indicative of semi-crystalline structure of this

phase. MgCO3 is likely to be present, but it is difficult to detect since its characteristic

reflections are very close to those of CaCO3.

For the preparation of MT-based geopolymer specimens, CKD was first dry mixed with

mine tailings at a specified content, 0, 2.5, 5, and 10% (by total solid mass). Then the

NaOH solution was slowly added to the mixture while mixing. Addition of CKD resulted

in less workable paste due to two reasons. First, CKD is much finer than MT and requires

higher water content to reach the same consistency. Second, calcium may leach out from

the reactive phase, hydrate, and precipitate as Ca(OH)2. The resulted paste was placed in

the Harvard Miniature Compaction cylindrical molds of 33.4 mm diameter and 72.5 mm

height with minor compaction. The compacted specimens were then compressed for

about 10 minutes. After the compression, the specimens were de-molded and placed

uncovered in an oven for curing at 90 °C for 7 days before tested. The specimens are

prepared respectively at 12, 16, and 20% water content with corresponding forming

pressure of 25, 0.5, and 0 MPa. These are the optimum forming pressures corresponding

to the mentioned water contents, as discussed in Appendix C. For the 20% water content,

the specimens were not pre-compressed since even at a small forming pressure the NaOH

solution would be squeezed out. Six specimens are prepared for each preparation

condition; three are used for dry UCS and the rest are soaked in water for durability

evaluation. After soaking in water for 7 days, the specimens are taken out, surface-dried,

and weighed. The wet specimens are then dried for 24 hours at 100 °C, weighed, and

tested.

To investigate the effect of CKD on the microstructure and the elemental and phase

composition of the MT-based geopolymer, SEM/EDX, XRD and FTIR were also

performed. The SEM imaging/EDX analysis was performed in SE conventional mode

159

using the FEI INSPEC-S50/Thermo-Fisher Noran 6 microscope. The XRD analysis was

performed with a Scintag XDS 2000 PTS diffractometer using Cu K radiation, at 2.00

degree/min ranging from 10.00 to 70.00 degrees with 0.600 second count time. The FTIR

analysis was performed using a Thermo Nicolet 370 FTIR / EZ Omnic with a smart

performance ATR ZnSe crystal and covering wavelengths from 600 to 4000 cm-1

.

Table 2 summarizes the combination of variables studied and the different types of tests

conducted.

3. Results and Discussion

3.1. Macro-scale Properties

Fig. 3 shows the relationship between UCS and CKD content at 16% initial water content

and respectively 10 and 15 M NaOH concentrations. The dry UCS significantly increases

with the CKD content at both 10 and 15 M NaOH. Addition of 10% CKD at 10 and 15 M

NaOH respectively results in about 200 and 90 % increase in UCS. The 10% CKD-added

brick at 10 M NaOH exhibits higher strength than the no CKD-added one at 15 M NaOH,

meaning that more than 30% NaOH can be saved by adding 10% CKD. This will further

reduce the cost of MT-based geopolymer bricks.

The improvement of UCS due to addition of CKD is possibly due to five reasons. First, it

provides additional activating agent, which results in dissolution of larger amount of Si

and Al from the MT. Although the used CKD does not contain a large amount of Na or

K, the dissolved Ca raises the alkalinity and contributes to further dissolution of Si and Al

from the MT. Second, during geopolymerization, Ca can act as charge balancing of

alumina species by in integration into the geopolymer structure. This is important

considering durability of geopolymer because ion exchange is one of the reasons for

deterioration of geopolymer. Since Ca has a larger valance than Na, it has less affinity to

be replaced by attacking cations. Third, the silica and alumina in the CKD are additional

source of aluminosilicates and contribute to formation of geopolymer gels. Third,

160

because the CKD particles are significantly fine, they can fill the small pores, react to the

alkaline solution, and help form a denser microstructure. Fourth, the hydration of calcium

and pozzolanic reactions yield calcium hydroxide [Ca(OH)2], C-S-H gel, and CaCO3

which coexist with the geopolymer gel.

Fig. 3 also shows the strength of MT-based geopolymer specimens after immersion in

water versus the CKD content. It can be seen that the addition of CKD improves the

durability of MT-based geopolymer bricks in water. The results, in fact, indicate the

durability in an alkaline solution since after immersion of the brick specimens, the water

turns into an alkaline solution due to the release of un-reacted Ca and Na. Therefore, it

can be considered the durability in a more severe condition than neutral solution.

Bakharev [12] studied the durability of geopolymer in acidic condition and reported

strength loss due to depolymerization of geopolymer gel. Deterioration of geopolymeric

structure in alkaline solution is also observed by others [13,14]. For instance, Temuujin et

al. [13] reported approximately 80 and 90% weight loss and 20 and 30% reduction in

UCS for FA-based geopolymer immersed respectively in 18% HCl and 14 M NaOH for 5

days. The current study shows that 10% CKD-added MT-based geopolymer bricks in

alkaline condition, which is certainly less aggressive than 14 M NaOH, undergo 18 –

26% reduction in UCS. It should be noted that FA is more reactive than MT, thus, a

better performance is expected from FA-based geopolymers.

Fig. 4 shows the effect of initial water content on the UCS of CKD-added MT-based

geopolymer bricks prepared at 10 M NaOH with 10% CKD. The UCS increases with

higher initial water content in both dry and wet conditions. The effect of initial water

content on the UCS of MT-based geopolymer bricks has been discussed in detail in

Appendix C. In addition to the factors discussed in Appendix C, hydration and fineness

are two other factors related to CKD. At low water contents, some CKD remains un-

reacted due to lack of water; by increasing the water content, more CKD is likely to

161

hydrate and contribute the strength. Since CKD is much finer than MT, higher water

content is required to reach at the same level of consistency.

Figs. 5 shows water absorption versus CKD content for specimens prepared respectively

at 10 and 15 M NaOH concentrations and with 16 % initial water content. Water

absorption increases with CKD content, which confirms the above discussion about

hydration of CKD. The rate of increase in water absorption significantly decreases with

CKD content possibly because when CKD content goes beyond 5%, it acts as filler in

pores and results in smaller pore size. Except for the specimens prepared at 10 M NaOH

and with 5 and 10% CKD, the water absorption for all specimens meets the ASTM

requirements for different applications. The water absorptions of these two types of

specimens are respectively 8.5 and 9.4%, which are only slightly higher the most critical

water absorption limit of 8% for pedestrian and light traffic paving brick at severe

weathering condition. Since the amount of absorbed water is not entirely indicative of

porosity and might be due to further hydration of Ca, the slightly higher water absorption

than the standard limit should not be an concern, especially considering the good

durability at these two conditions.

Fig. 6 shows the weight loss versus CKD content at 16% initial water content and

respectively 10 and 15 M NaOH concentrations. Addition of more CKD results in less

weight loss for the MT-based geopolymer bricks. This is possibly because of the

formation of more durable geopolymer as a result of incorporation of Ca in the

geopolymer structure.

3.2. Micro/nano-scale Properties

Fig. 7 shows the SEM micrographs of specimens prepared at 15 M NaOH, 16% initial water

content, and respectively 0 and 10% CKD. In both specimens, the partially reacted or un-

reacted particles (shown by B) and the glassy binders (shown by B or C) are the distinct

phases. The binder phase in the no-CKD specimen looks less compact and has a particulate

162

morphology. Fig. 8 shows higher magnification SEM micrographs of specimens at

respectively 0, 5, and 10% CKD. The morphologies of the binder phase in 0 and 5% CKD

specimens are similar to each another; but the 10% CKD specimen contain two different

binder phases: a monolithic binder between the particles (shown by C) and a binder at the

interface between the monolithic binder and the un-reacted particles (shown by B). The

morphology of the latter binder is similar to that of the binder in the 0 and 5% CKD

specimens. Table 3 shows the chemical composition of the three phases, as Si/Al, Na/Al,

and Ca/Si ratios, determined from the EDX analysis. It can be seen that the Si/Al ratio of the

binder phase (A) increases with CKD content. This indicates that the increase of CKD

content results in formation of a more rigid geopolymer gel. The Ca/Si ratio of the binder

phase obviously increases with CKD content due to the availability of more Ca. Both 5 and

10% CKD-added specimens exhibit integration of Ca into the binder phase. Fig. 8(d)

indicates that after soaking the specimen in water for 7 days, no significant change in the

microstructure occurs, but the Si/Al and Ca/Si ratios of the binder phase slightly decrease.

Fig. 9 shows the IR spectra of the MT and CKD powders and the geopolymer specimens

prepared respectively 0, 5, and 10% CKD. The identified IR characteristics are

summarized in Table 4. For the MT powder, the band centers at around 1000 cm-1

corresponding to the stretching vibrations of Si-O bonds. After geopolymerization, this

band shifts toward lower wave numbers indicative of depolymerization of the original

aluminosilicates, and a new band, centered at 1400 cm-1

, appears, which is attributed to

polymerization of the dissolved silicates.

The CKD powder exhibits noticeable characteristic bands at 870, 1400, and 3640 cm-1

,

corresponding respectively to CO3 vibrations in CaCO3, Si-O vibrations, and O-H

vibration of Ca(OH)2 [15-17]. There is also a weak band centered around 1000 cm-1

,

which is attributed to the Si-O stretching vibrations of SiO4 [15,18,19]. After

geopolymerization the band corresponding to the vibration of Ca(OH)2 disappears and

the band centered at 1400 cm-1

shifts toward a slightly higher wave number. The

163

vanishing of Ca(OH)2 is attributed to incorporation of Ca in the geopolymer structure.

The corresponding wide band of CaCO3 in the CKD powder changes into a weak

crystalline band at the same wave number. The shift of Si-O vibration at around 1400 cm-

1 toward a higher wave number is indicative of transition into stronger atomic bonds,

which is associated with formation of geopolymer gel following dissolution of the

aluminosilicates from both CKD and MT. The geopolymer gel in the no CKD specimen

shows a band around 1420 cm-1

, while in the 5 and 10% CKD added specimens, it

exhibits a band respectively near 1440 and 1460 cm-1

, indicating that increasing CKD

content leads to larger shifts of the Si-O band toward greater wave numbers. This is

consistent with the EDX analysis results implying that the Si/Al ratio of the binder

increases with CKD content. This is possibly due to the extra silica provided by CKD.

Therefore, the enhancement of the MT-based geopolymer due to the addition of CKD is

attributed to the incorporation of silica and Ca of CKD in the geopolymerization. The Si-

O band of the CKD powder is wider than that of the geopolymer specimen indicating that

the silica in CKD is more amorphous and after dissolution and incorporation in the

geopolymer, it constitutes a more ordered structure. The weak bands centered at 2350 cm-

1 for the 10% CKD added specimens correspond to the C-O vibrations of CO2

constrained in the amorphous phase. The CO2 is resulted from dissociation of CaCO3 into

CaO and CO2. There is no significant difference between the IR spectra of CKD added

specimens before and after immersion in water, which is consistent with the good

durability exhibited by the CKD added specimens. However, immersion of no-CKD

specimens in water results in vanishing of the band at 1400 cm-1

due to dissolution of a

major part of the geopolymer gel in water. The band at around 1650 cm-1

is ascribed to

the bending mode of H-O-H, which becomes slightly larger in the CKD added specimen

after immersion in water due to the hydration of CKD.

Fig. 10 shows the XRD patterns of the source materials and the geopolymer specimen of

15 M NaOH, 16% initial water content, and 10% CKD, before and after immersion in

water. After geopolymerization, the XRD spectrum of the solid specimen takes over the

164

low-angle reflections of the CKD powder. The crystalline phases that appear in the MT

powder are also present in the brick specimens. CaO and Ca(OH)2 that are present in the

CKD powder completely disappear after geopolymerization. On the other hand, the peaks

corresponding to CaCO3 increases, indicating that part of the dissolved Ca reacts to air

and yields CaCO3. Since CaCO3 has very low solubility in water and alkaline solutions,

its formation contributes to the enhancement of durability of the MT-based geopolymer

bricks. It can be seen that there is not any noticeable difference between the XRD

patterns of the specimens before and after immersion in water.

4. Conclusions

The feasibility of improving the physical and mechanical properties and durability of MT-

based geopolymer bricks and reducing the usage of NaOH by adding CKD was studied.

Based on the experimental results, the following conclusions can be drawn:

Addition of up to 10 % CKD results in significant improvement of the physical and

mechanical properties and durability of MT-based geopolymer bricks. Adding 10%

CKD to MT at10 M NaOH can lead to UCS higher than that at 15 M NaOH without

CKD. The addition of CKD decreases the loss of weight and UCS of specimens after

immersing in water. Although water absorption increases with CKD content, it is not

a critical issue because even the highest water absorption is only slightly higher than

the lowest ASTM standard limit and the increase is attributed to the hydration of

CKD.

The enhancement of MT-based geopolymer bricks with addition of CKD can be

attributed to the following reasons.

a) The silica and alumina from the CKD provide additional source of

aluminosilicates and contribute to formation of geopolymer gels.

b) Addition of CKD to MT elevates the alkalinity and improves the dissolution

of silica and alumina in MT for geopolymer formation.

c) Ca from the added CKD can act as a charge balancing cation and be integrated

into the geopolymer network.

165

d) Addition of CKD helps formation of CaCO3 which can coexist with

geopolymer gel. Due to its low solubility in water and alkaline solution, the

formation of CaCO3 contributes to the durability of MT-based geopolymer

bricks.

e) CKD particles are very fine and can act as a filler in the pores and

consequently result in a denser structure.

5. References

[1] IEEE-IAS Cement Industry Committee, Wayne S, Taubert DH. Beneficial uses of

cement kiln dust. 2008 IEEE/PCA 50th Cement Industry Technical Conference,

Miami, FL; 2008.

[2] Haynes BW, Kramer GW. Characterization of U.S. cement kiln dust. Bureau of

Mines, United States Department of Interior, Washington, DC, USA, Information

Circular #8885; 1982.

[3] Khater HM, Zedane SR. Geopolymerization of industrial by-products and study of

their stability upon firing treatment. Greener Journal of Physical Sciences.

2012;2(1):1-9.

[4] Buchwald A, Schultz M. Alkali-activated binders by use of industrial by-products.

Cement and Concrete Research 2005;35:968-73.

[5] Abo-El-Enein SA, Hekal EE, Gabr NA, El-Barbary MI. Blended cements

containing cement kiln dust. Silicates Industrials 1994;59(9-10).

[6] Zaman M, Laguros JG, Sayah AI. Soil stabilization using cement kiln dust.

Proceedings of the 7th International Conference on Expansive Soils, Dallas, Texas,

USA; 1992.

[7] Sayah A I. Stabilization of a highly expansive clay using cement kiln dust, M.Sc.

Thesis, University of Oklahoma, Graduate College, Norman, Oklahoma, USA;

1993.

[8] Crosby TW, Duffey AM, Hullings DE, Kulesza EM. Cement kiln dust: from

mining by-product to valuable cover material. Proceedings of the Air & Waste

Management Association’s 93rd Annual Conference & Exhibition, Salt Lake City,

UT, AWMA, Philadelphia, Pennsylvania, USA; 2000.

166

[9] Ballivy G, Rouis J, Breton D. Use of cement residual kiln dust as landfill liner.

Cement Industry Solutions to Waste Management, LT168, Canadian Portland

Cement Association, Toronto, Ontario, Canada; 1992.

[10] Nehdi M, Tariq A. Stabilization of sulphidic mine tailings for prevention of metal

release and acid drainage using cementitious materials: A review. Journal of

Environmental Engineering and Science 2007;6(4):423-36.

[11] Konsta-Gdoutos MS, Shah SP. Hydration and properties of novel blended cements

based on cement kiln dust and blast furnace slag Cement and Concrete Research

2003;33:1269-76.

[12] Bakharev T. Durability of geopolymer materials in sodium and magnesium sulfate

solutions. Cement and Concrete Research 2005;35:1233-46.

[13] Temuujin J, Minjigmaa A, Lee M, Chen-Tan N, van Riessen A. Characterisation of

class F fly ash geopolymer pastes immersed in acid and alkaline solutions. Cement

and Concrete Composites 2011;33:1086-91.

[14] Sindhunata, Provis JL, Lukey GC, Xu H, van Deventer JSJ. Structural evolution of

fly ash based geopolymers in alkaline environments. Industrial and Engineering

Chemistry Research 2008;47:2991-9.

[15] Buchwald A, Hilbig H, Kaps CH. Alkali-activated metakaolin-slag blends-

performance and structure in dependence of their composition. Journal of Materials

Science 2007;42:3024-32.

[16] Guo X, Shi H, Dick WA. Compressive strength and microstructural characteristics

of class C fly ash geopolymer. Cement and Concrete Composites 2010;32:142-7.

[17] Trezza MA. Hydration study of ordinary Portland cement in the presence of zinc

ions. Materials Research 2007;10(4):331-4.

[18] Allahverdi A, Khani EN. Construction wastes as raw materials for geopolymer

binders. International Journal of Civil Engineering 2009;7(3):154-60.

[19] Lee WKW, Van Deventer JSJ. Use of infrared spectroscopy to study

geopolymerization of heterogeneous amorphous aluminosilicates. Langmuir

2003;19:8726-34.

[20] Mingyu H, Xiaomin Z, Fumei L. Alkali-activated fly ash-based geopolymers with

zeolite or bentonite as additives. Cement and Concrete Composites 2009;31:762-8.

167

[21] Zhang Y, Sun W, Li Z. Infrared spectroscopy study of structural nature of

geopolymeric products. Journal of Wuhan University of Technology-Materials

Science Ed. 2008;23(4):522-7.

[22] Fernandez L, Alonso C, Hidalgo A, Andrade C. The role of magnesium during the

hydration of C3S and C-S-H formation. Scanning electron microscopy and mid-

infrared studies. Advances in Cement Research 2005;17(1):9-21.

[23] Sakulich AR. Characterization of Environmentally-Friendly Alkali Activated Slag

Cements and Ancient Building Materials. Ph.D. Thesis, Drexel University. 2009.

[24] Treadwell DR, Dabbs DM, Aksay IA. Mullite (3Al2O3-2SiO2) Synthesis with

Aluminosiloxanes. Chemistry of Materials 1996;8:2056-60.

168

Table 1. Chemical composition of the CKD used in this study

Chemical compound Concentration (%)

CaO 42.0

SiO2 11.0

Al2O3 3.9

MgO 3.6

Fe2O3 2.0

K2O 0.6

169

Table 2. Specimen properties and mechanical and micro/nano-structural experiments

conducted in this study.

NaOH

(M)

Water

(%)

CKD

(%)

Forming

Pressure

(Mpa)

NaOH/

Solid

(%)

Na/Al Na/Si Ca/Si UCS

Test*

Absorption

Test

Weight

loss SEM FTIR XRD

0 0 0 0 0 10.46 1.35 0.12 X X X

0 0 100 0 0 0.00 0.00 4.09 X X X

10 12 10 25 4.8 0.90 0.12 0.20 X X X

10 16 0 0.5 6.4 1.15 0.15 0.12 X X X

10 16 2.5 0.5 6.4 1.17 0.15 0.14 X X X

10 16 5 0.5 6.4 1.18 0.15 0.16 X X X

10 16 10 0.5 6.4 1.21 0.16 0.20 X X X

10 20 10 0 8 1.51 0.20 0.20 X X X

15 16 0 0.5 9.6 1.73 0.22 0.12 X X X X X X

15 16 2.5 0.5 9.6 1.75 0.23 0.14 X X X

15 16 5 0.5 9.6 1.77 0.23 0.16 X X X X X

15 16 10 0.5 9.6 1.81 0.24 0.20 X X X X X X

* Both dry and wet UCS

170

Table 3. EDX analysis results on the phases shown in Fig. 8 (A and C are the binder

phase and B is the crystalline phase).

* The results are for the specimen immersed in water for 7 days.

Phase CKD (%) Si/Al Na/Al Ca/Si

A

0 3.50 1.61 0.32

5 4.63 2.78 0.41

10 4.77 5.69 0.60

B

0 3.44 0.80 0.10

5 14.34 3.09 0.09

10 3.14 1.62 0.82

10* 5.56 1.74 0.42

C 10 8.00 0.81 0.81

10*

6.23 2.26 0.65

171

Table 4. Infrared (IR) characteristic bands identified in MT and CKD powder and the

geopolymer specimens shown in Fig. 9.

Wave

Number

(cm-1

)

Characteristic bands References

800-1,200 Si-O stretching vibrations of SiO4 [15,18-21]

872 -CO3 vibrations in CaCO3 [15,17]

970 stretching vibration mode of Si-O (3) in CSH gel [15,17,22]

1,400 Si-O vibrations [16]

1,650 bending (2) mode of H-O-H [15,17]

2,350

C-O vibrations in CO2 constrained in amorphous

phase [23,24]

2,920

C-O vibrations in CO2 constrained in amorphous

phase [23,24]

3,645 O-H stretching vibration of portlandite [15,17]

172

Fig. 1. SEM micrograph and EDX analysis result of CKD powder.

Element

Line

Concentration

(%)

Error

(%)

C K 4.63 +/- 0.08

O K 33.13 +/- 0.37

Mg K 2.06 +/- 0.05

Al K 2.01 +/- 0.06

Si K 3.8 +/- 0.06

S K 0.26 +/- 0.04

Cl K 0.74 +/- 0.06

K K 0.76 +/- 0.04

Ca K 50.94 +/- 0.28

Fe K 1.67 +/- 0.22

Total 100

173

Fig. 2. XRD pattern of CKD powder [C: CaO, O: CaCO3, S: SiO2, T:Ca(OH)2]

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70

2q

S

C

C

C

C CTT

T

O

O

S

SS

174

Fig. 3. Dry and wet UCS (before and after immersion in water) vs. CKD content for

geopolymer brick specimens prepared with 16% initial water content and cured at 90 °C

for 7 days (The numbers in the parenthesis show the percentage loss of strength after

immersion in water).

78%

45%

14%

23%

0

10

20

30

40

50

60

0 5 10

UC

S (

MP

a)

CKD Content (%)

15 M NaOH 16% Water

Dry

Wet

54%

31%

25%

19%

0

5

10

15

20

25

30

35

40

0 5 10

UC

S (

MP

a)

CKD Content (%)

10 M NaOH 16% Water

Dry

Wet

175

Fig. 4. Dry and wet UCS (before and after immersion in water) vs. initial water content

for geopolymer brick specimens prepared with 10 M NaOH and 10% CKD and cured at

90 °C for 7 days (The numbers in the parenthesis show the percentage loss of strength

after immersion in water).

35%

19%

26%

0

10

20

30

40

50

60

12 16 20

UC

S (

MP

a)

Initial Water Content (%)

10 M NaOH 10% CKD

Dry

Wet

176

Fig. 5. Water absorption vs. CKD content for geopolymer brick specimens prepared

respectively at 10 and 15 M NaOH, and with 16% water content and cured at 90°C for 7

days.

2

4

6

8

10

0 5 10

Wa

ter

Ab

so

rptio

n (%

)

CKD Content (%)

15

10

NaOH (M)

177

Fig. 6. Weight loss vs. CKD content for geopolymer brick specimens prepared

respectively at 10 and 15 M NaOH, and with 16% water content and cured at 90°C for 7

days.

0

2

4

6

8

10

12

14

0 5 10

We

igh

t lo

ss (%

)

CKD Content (%)

15

10

NaOH (M)

178

Fig. 7. SEM micrographs of geopolymer brick specimens prepared at 15 M NaOH, 16%

initial water content, and cured at 90 ° C for 7 days: a) no CKD added, and b) 10% CKD

added.

a b

B C

A

B

A

179

Fig. 8. Higher magnification SEM micrographs of geopolymer brick specimens prepared

at 15 M NaOH, 16% initial water content, and cured at 90 ° C for 7 days: a) no CKD

added, b) 5% CKD added, c) 10% CKD added, and d) 10% CKD added and after 7 days

of immersion in water. (A and C denote the binder phase, and B denotes the un-reacted

phase).

a b

c

A

B

A

B

B

C

A

C

B

d

180

Fig. 9. IR spectra of MT and CKD powders and geopolymer brick specimens prepared at

15 M NaOH and 16% water content and with respectively a) 0, b) 5%, and c) 10 % CKD

(dry and wet denote the specimens before and after immersion in water).

6001,1001,6002,1002,6003,1003,600

wave number (cm-1)

Dry 15-16-10

CKD

Wet 15-16-10

6001,1001,6002,1002,6003,1003,600

wave number (cm-1)

MT Powder

Dry 15-16-0

Wet 15-16-0

6001,1001,6002,1002,6003,1003,600

wave number (cm-1)

CKD

Dry 15-16-5

Wet 15-16-5

a b

c

181

Fig. 10. XRD patterns of MT and CKD powder and geopolymer brick specimens

prepared at 15 M NaOH, 16% initial water content, 10% CKD, and cured at 90 °C for 7

days and before and after immersion in water

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70

2q

15-16-10 (wet)

15-16-10 (dry)

MT Powder

S

CKD

SS

SS

SP

PP

PGG

A

AP

C

C

C

C CT TT

OO

S

182

APPENDIX F

Paper is pending publication to the Journal of Construction and Building Materials

PRODUCTION OF GEOPOLYMERIC BINDER FROM

BLENDED WASTE CONCRETE POWDER AND FLY ASH

Saeed Ahmari, Xin Ren, Vahab Toufigh, Lianyang Zhang*

Department of Civil Engineering and Engineering Mechanics, University of Arizona,

Tucson, Arizona, USA

* Corresponding author: Tel.: 1 520 6260532; fax: 1 520 6212550.

E-mail address: lyzhang@email.arizona.edu.

183

ABSTRACT

Recycling and utilization of waste concrete is a significant contribution to environment

and sustainable development. In current practice, the recycling of waste concrete is

mainly limited to the use of crushed aggregates in low-specification applications. Few

researchers have investigated complete recycling of waste concrete. These complete

recycling methods, however, need to re-clinker the hydrated cement using the standard

cement kiln procedures and thus consume significant amount of energy and release large

quantity of CO2. To completely recycle and utilize waste concrete in a sustainable and

environmentally-friendly way, a method that does not need re-clinkering at high

temperature should be used. This paper studies the production of geopolymeric binder

from ground waste concrete (GWC) powder mixed with class F fly ash (FA), which can

then be used with recycled concrete aggregates to produce new concrete. Specifically, the

effect of composition and concentration of the alkaline solution and the content of GWC

on the unconfined compressive strength (UCS) of the produced geopolymeric binder is

investigated. SEM/EDX, XRD, and FTIR analyses are also performed to investigate the

micro/nano-structure, morphology and phase/surface elemental compositions of the

produced geopolymeric binder and the effect of calcium (Ca) on them. The results

indicate that utilization of GWC together with FA can increase the UCS of the

geopolymeric binder up to 50% GWC content. Further increase of GWC decreases the

UCS of the geopolymeric binder. So with proper combination of GWC and FA, the

geopolymeric binder with required strength can be produced.

184

Key words: Waste concrete, Fly ash, Geopolymer, CSH gel, Micro/nano structure,

Unconfined compressive strength

185

1. Introduction

Concrete is the major construction material of infrastructure. Repairing and upgrading the

deteriorating infrastructure systems will utilize large quantity of new concrete and in the

meantime generate significant amount of waste concrete. Ordinary Portland cement

(OPC) is commonly used as the binder in the manufacture of concrete. It is well known

that the production of OPC not only consumes significant amount of natural resources

and energy but also releases substantial quantity of greenhouse gases. To produce 1 ton

of OPC, about 1.5 tons of raw materials is needed and 1 ton of CO2 is released to the

atmosphere [1-3]. Production of concrete also utilizes sand and aggregate. Quarrying

operations for producing the sand and aggregate are energy intensive and can release high

level of waste materials. The shortage of natural resources for construction materials in

many regions has led to long-distance haulage and significantly increased costs [4-6].

Growing environmental awareness, the need to ensure sustainability of construction

materials, and public concern to safeguard the countryside limit the use of quarrying sites

and encourage the construction industry to look for alternative materials [6-10].

On the other hand, it is a great challenge to handle the significant amount of waste

concrete to be generated from repairing and upgrading the deteriorating infrastructure

systems [11-14]. For example, in the United States, concrete waste occupies one third of

the volume of waste materials in landfills [14]. Besides, finding areas suitable for

landfilling is getting harder and disposing is getting more expensive. Therefore, recycling

of waste concrete is encouraged by different agencies and sought by various institutions.

Although extensive research has been conducted [15-22], current recycling of waste

concrete is still predominately limited to the use of concrete aggregates in low-

specification applications such as base course and non-structural fill with the remainder

still being landfilled [23,24]. When waste concrete is crushed, a certain amount of cement

paste/mortar from the original concrete remains attached to stone particles in the concrete

aggregate. The attached paste/mortar is the main reason for the lower quality of the

concrete aggregate than the natural aggregate. Compared to natural aggregates, the

186

concrete aggregate has increased water absorption, decreased bulk density, decreased

specific gravity, increased abrasion loss, increased crushability, and increased quantity of

dust particles. The low quality of the concrete aggregate generally leads to new OPC

concrete with inferior strength, durability, and shrinkage properties. Therefore, the

utilization of concrete aggregates in structural concrete is very limited. In cases that the

concrete aggregate is used together with natural aggregate for production of structural

concrete, a limit of 30% of concrete aggregate is usually recommended [23,25].

Aggregate refining methods such as “heating and rubbing” [26] and “mechanical

grinding” [27] have been developed for refining the quality of concrete aggregates by

removing the attached paste/mortar; but these methods are energy intensive and produce

additional fines which need to be disposed of. It is also noted that utilization of concrete

aggregates as base course and non-structural fill may cause environmental problems such

as contaminant leaching and pH changes in the surrounding soil and water [28-30].

Few researchers have investigated complete recycling of waste concrete [31,32]. These

complete recycling methods, however, need to re-clinker the hydrated cement using the

standard cement kiln procedures and thus consume significant amount of energy and

release large quantity of CO2. To completely recycle and utilize waste concrete in a

sustainable and environmentally-friendly way, a method that does not need re-clinkering

at high temperature should be used.

Recently, a new type of “cement”, called geopolymer or inorganic polymer, has been

investigated by different research groups. Geopolymer is a synthetic alkali

aluminosilicate material produced from the reaction of a solid aluminosilicate with a

highly concentrated aqueous alkaline hydroxide and/or silicate solution, having an

amorphous to semi-crystalline polymeric structure. Geopolymer has many advantages

over OPC and has been of great research interest as an ideal material for sustainable

development. Different raw materials, which provide the silica and alumina source, have

been used to produce geopolymers. The research has shown that the presence of calcium

187

compounds in the raw material can improve the mechanical properties of geopolymer

products due to the coexistence of the geopolymer gel and the calcium silicate hydrate

(CSH) and calcium aluminum hydrate (CASH) gels [33-36].

Very limited research has been conducted on recycling waste concrete via

geopolymerization. Yang et al. [37,38] produced geopolymer concrete by using recycled

aggregates as partial replacement for the fresh ones and mixture of waste concrete

powder and metakaolin along with silica fume as the source materials for the

geopolymeric binder. Their study indicated that the content of metakaolin and silica fume

and the raise in alkalinity lead to increase in compressive strength. Allahverdi and Kani

[39] investigated geopolymerization of mixture of finely ground waste brick and concrete

in different mix proportions. They demonstrated that higher brick content and alkalinity

resulted in stronger geopolymeric binder and the final setting time reduced when higher

alkalinity.

In order for waste concrete to be completely recycled, both the crushed aggregates and

the fine powder fraction need to be utilized. This can be achieved by utilizing the crushed

aggregates as the filler and the fine powder fraction (together with fly ash) as the

geopolymer binder. In this method, the adherence between the crushed aggregates and the

binder is not of concern anymore, since the old hardened cement attached to the crushed

aggregates will participate in geopolymerization as a source of silicon (Si) and calcium

(Ca) and result in a good bond between the new binder and the old aggregates. This paper

mainly focuses on the feasibility of producing geopolymeric binder using the mixture of

ground waste concrete (GWC) powder and fly ash (FA) activated by sodium hydroxide

(NaOH) and sodium silicate (SS) solution. The produced binder was investigated at

different NaOH concentrations, SS to NaOH ratios (SS/N), and GWC contents to study

the mechanical properties, micro/nano-structure, and phase/elemental composition, based

on unconfined compression tests, scanning electron microscopy/energy-dispersive X-ray

188

spectroscopy (SEM/EDX) characterization, and X-ray diffraction (XRD) and Fourier

Transform Infra Red (FTIR) analyses.

2. Materials and Methods

2.1. Materials

The materials used in this investigation include class F FA, GWC powder, reagent grade

98% NaOH, aqueous SS, and de-ionized water. The class F FA was received from Salt

River Materials Group in Phoenix, Arizona. The GWC powder was obtained by crushing

and grinding the tested OPC concrete specimens in the structural laboratory at the

University of Arizona. Table 1 shows the elemental composition of the FA and the GWC

powder based on the XRF analysis. It can be seen that the major constituents of the GWC

powder are silica and calcium compounds with minor amount of alumina and iron oxide

while in the FA, silica and alumina are the major and calcium and iron oxide are the minor

constituents. The original concrete specimens were prepared with 14.5% (weight) type II

Portland cement, 31.2% sand, 44.8% coarse aggregate, and 9.5% water. They were tested

after 28 day curing in moisture room and the tested specimens were left for another 3

months in the air before crushed and mill ground (see next subsection for more details).

The particle size distribution of the GWC powder and the FA was determined using a

Beckman Coulter LS 13 320 Laser Diffraction Particle Size Analyzer. Fig. 1 shows the

particle size distribution curves. The mean particle size of the GWC powder and the FA are

respectively 18.0 and 13.5 m with respectively about 80 % and 90% particles finer than 75

m. The specific gravity of the GWC and FA particles are respectively 2.53 and 1.97. Fig. 2

shows the SEM micrographs of the GWC powder and the FA. The GWC powder contains

irregularly shaped particles with a rough surface while the FA particles are spheres with a

smooth surface.

The NaOH flakes were obtained from Alfa Aesar Company in Ward Hill, Massachusetts.

The NaOH solution was prepared by dissolving the NaOH flakes in de-ionized water.

189

Aqueous SS (SiO2 = 29%, Na2O = 9%, and H2O = 62%) with modulus (SiO2/Na2O) of 3.22

was obtained from Fisher Scientific in Pittsburgh, Pennsylvania.

2.2. Experimental methods

After the 28 day-cured concrete specimens were tested and left in the air for another 3

months, they were crushed by a jaw crusher and separated into different parts based on

the particle size. The part passing mesh 20 (0.853 mm) was subject to further grinding

using a grinding mill to ensure that most particles pass mesh 200 (75 m). The obtained

GWC powder was mixed with FA at different proportions, 0, 25, 50, 75, and 100% (by

the total mass of GWC powder and FA), to produce the geopolymer binder source

material. The mixture of NaOH and SS solution was used as the alkaline activator. First,

the NaOH solution was prepared by dissolving NaOH flakes in de-ionized water and

stirred for at least five minutes. Considering the generated heat, the solution was allowed

to cool down to room temperature and then mixed with the SS solution at a specified

SS/N weight ratio and stirred for another five minutes. The resulted alkaline solution was

kept in room temperature for half an hour and then slowly added to the GWC/FA

mixture. The generated paste was stirred by a mixer for about five minutes to ensure

sufficient dissolution of silica, alumina, and calcium (Ca) in the alkaline solution. The

viscosity of the produced pastes increased at higher NaOH and SS concentrations and

greater GWC content. Depending on the alkaline solution concentration and GWC content,

different water contents were used to reach consistent workability for the pastes. Table 2

shows the water contents for the specimens at different alkaline concentrations and GWC

contents. The water content increases with higher NaOH concentration and greater GWC

content. The resulted paste was placed in cylindrical Plexiglas molds of 34.5 mm inner

diameter and 86.3 mm length (i.e., an aspect ratio of 2.5). The mold was shaken by a

vibrator during the casting to release the trapped air bubbles. The mold was capped and left

in room temperature for curing. The specimens were de-molded after 24 hours and then

placed in a plastic bag for 6 days’ curing before tested.

190

The NaOH solution was prepared at two concentrations, 5 and 10 M, and mixed with SS at

SS/N ratios respectively of 1 and 2. At NaOH concentration of 5 M and SS/N = 2, the

geopolymer paste was not sufficiently workable to be molded due to quick setting.

Therefore, only the results for 10 M NaOH are reported at SS/N = 2.

Unconfined compression tests were performed on the cured cylindrical specimens with an

ELE Tri Flex 2 loading machine at a constant loading rate of 0.1 mm/min to measure the

unconfined compressive strength (UCS). The UCS was studied at different GWC contents,

NaOH concentrations, and SS/N ratios. For each condition, at least three samples were

tested and the average of the measured values was shown. Before conducting the

compression test, the end surfaces of the specimens were polished to make sure they are

accurately flat and parallel.

SEM/EDX was used to investigate the microstructure of the specimens and the morphology

and elemental composition of the constituting matrix components. The SEM imaging/EDX

analysis was performed in SE conventional mode using the FEI INSPEC-S50/Thermo-

Fisher Noran 6 microscope. The freshly failed surfaces from the unconfined compression

tests, without polishing to keep the fractured surface “un-contaminated”, were used for the

SEM imaging/EDX analysis. XRD was used to study the phase composition of the original

material and the change due to the combined effect of geopolymerization and pozzolanic

reactions. The XRD analysis was performed with a Scintag XDS 2000 PTS diffractometer

using Cu K radiation, at 2.00 degree/min ranging from 10.00 to 70.00 degrees with 0.600

second count time. FTIR analysis was also performed to study the effect of

geopolymerization and pozzolanic reactions on the materials’ chemical bonds before and

after reaction. Spectra were obtained using Thermo Nicolet 370 FTIR / EZ Omnic using a

smart performance ATR ZnSe crystal. The spectrometer covers wavelengths from 600 to

4000 cm-1

.

3. Results and Discussion

3.1. UCS

191

The study investigated the effects of the composition and concentration of the alkaline

solution and the content of the GWC powder on the UCS of the geopolymer binder.

3.1.1. Effect of NaOH concentration

Fig. 3 shows UCS versus GWC content at NaOH concentration respectively of 5 and 10

M with SS/N = 1. The UCS values at 10 M NaOH concentration are higher than those at

5 M NaOH concentration. The increase of UCS with NaOH concentration can be

explained by the fact that at higher NaOH concentration, larger amount of Na+ cations

attack the surface of the solid phase leading to dissolution of more Si and Al and thus

higher concentration of Si and Al in the liquid phase. Alkali activation of the raw

aluminosilicate involves the chemical reaction between NaOH and the raw

aluminosilicate by which the bridging oxygen (BO) atoms in the raw aluminosilicate

structure transform into non-bridging oxygen (NBO) atoms, which leads to isolation of

the Si and Al atoms via the following scheme [40]:

According to this scheme, Na+ acts as a modifying (activating) and dissolving agent on

both Si and Al, and thus the Na/Al and Na/Si ratios are important to display the

availability of Na+ for Si and Al atoms, respectively. The Na/Al and Na/Si ratios in the

current study range respectively from 0.21 to 1.39 and 0.10 to 0.26 (see Table 2). The

two highest Na/Al and Na/Si ratios correspond to specimens 10-100-1 and 10-100-2,

which were prepared with 100% GWC and synthesized with 10 M NaOH at SS/N ratios

respectively of 1 and 2. Despite the highest ratios of Na/Al and Na/Si, these specimens

exhibited the lowest UCS values indicating that in addition to the alkalinity, other factors

such as Si/Al and Ca/Si ratios also significantly affect the reaction, which will be

discussed later. However, at constant Si/Al and Ca/Si ratios, higher NaOH concentration

will lead to dissolution of larger amount of Si and Al and result in higher UCS [41]. In

addition to the dissolution function, Na+ also functions as charge balancing for 4-

coordinated Al3+

. A Na/Al ratio of about 1 is commonly recommended [42-44]. It is

Na

+ - (Al

- - O - Si)raw- + 2Na

+OH

- Na

+ - (Al

- - O

- Na

+) + (Na

+O

- - Si)- + H2O (1)

BO

NB O

NB O

192

noted that the Na/Al ratios listed in Table 2 are for the initial materials and are not

necessarily the same as the final ratios in the geopolymer gels.

The increase of UCS with NaOH concentration in the geopolymeric system was also

noted by other researchers [39,45-47]. For example, Allahverdi and Khani [39] studied

the production of geopolymer material from ground waste brick and concrete powders.

They used NaOH at three different levels, 6, 7, and 8% (by weight of the dry binder), and

concluded that the increase of alkalinity resulted in shorter final setting time and higher

strength. They also reported that the strength improvement with alkalinity was higher

when the specimens contained more waste brick than concrete since the waste brick is

more amorphous than the waste concrete. Similarly, in the present study, the elevated

alkalinity resulted in larger increase of UCS when FA is the dominant fraction of the

paste, i.e. when less than 50% GWC was used. This is because the FA particles are more

amorphous than the GWC powder and thus more reactive to the alkaline solution (see

Section 3.2 for more detailed discussion).

3.1.2. Effect of soluble silicate

Fig. 3 depicts the effect of SS/N ratio on UCS at different GWC contents with 10 M

NaOH concentration. The increase of SS/N ratio results in higher UCS at all GWC

contents. Delayed setting and increased SiO2/Na2O ratio account for the increase of UCS

with SS/N ratio. Delayed setting by addition of SS in the Ca-added geopolymer system

was also noted by Pacheco-Torgal et al. [48]. They reported flash setting in tungsten mine

tailings-based geopolymer at high NaOH concentrations when Ca(OH)2 was added. This

problem was resolved by increasing the SS/N ratio. Due to the high alkalinity and the fast

dissolution of Ca-silicates, Ca(OH)2 precipitates on the particle surface and prevents

further dissolution of Si and Al. Addition of soluble silicates helps delay setting and thus

more time is allowed for the dissolution of Si and Al. The other reason for the improving

effect of SS on UCS is the availability of soluble silicates. Dissolution and hydrolysis of

silica and alumina from the solid aluminosilicate source is the first step in

193

geopolymerization. Hydrolysis of silica and alumina species involves formation of Si-OH

and Al-OH bonds, which is followed by condensation. In a system where soluble silicate

exists in the alkaline solution, the hydrolysis process is already accomplished and thus the

geopolymerization will take place faster. According to Silva et al. [49], hydrolysis of Si

and Al from solid aluminosilicates takes place as the following:

2

222

322

4232

](OH)[SiOOH2SiO

][SiO(OH)OHOHSiO

][Al(OH)2OH2OH3OAl

(2)

The hydrolysis of Si and Al is then followed by oligomerization and polycondensation.

Polycondensation at low Si/Al ratios mainly happens between silica and alumina species

leading to formation of poly-sialate (PS) while at high Si/Al ratios, first the silica species

condense between themselves and then the formed silicate polymer condenses with the

alumina species, which eventually result in formation of a 3D rigid geopolymeric

network of PSS or PSD [49]. Addition of SS to the geopolymer mixture helps increase

the Si/Al ratio in the reactive phase and thus form more rigid polymeric network.

Fig. 4a shows the variation of UCS with the initial Si/Al ratio. In order to display the

effect of only the Si/Al ratio, the results corresponding to a limited range of Na/Al and

Ca/Si ratios are shown in the figure. It can be seen that the UCS increases with the Si/Al

ratio up to Si/Al about 3.38 and then decreases. Fig. 4b compares the results from the

current study with those reported by other researchers. The general agreement between

them is good, although the optimum initial Si/Al ratio (i.e. the Si/Al ratio at the largest

UCS) from the current study is slightly higher than those reported by the other

researchers. This is possibly because different source materials are used for the

geopolymer. The results from other researchers in Fig. 4b are mainly related to

metakaolin-based geopolymers. Since metakaolin is highly reactive, the final Si/Al ratio

is likely close to the initial one. If the aluminosilicate source material contains

considerable amount of un-reactive phase, there might be a notable difference between

the initial Si/Al ratio and the final one which takes a value from 1 to 3, depending on the

194

oligomer types. For example, Klabprasit et al. [52] reported Si/Al = 8.0 as the optimum

initial ratio for the rice husk-bark ash added FA-based geopolymer.

3.1.3. Effect of calcium

The addition of GWC, at both 5 and 10 M NaOH and SS/N = 1 and 2, improves UCS up

to 50% GWC content and then results in UCS declination (see Fig. 3). The improving

effect of GWC is greater at the higher SS/N ratio. The improving effect of GWC is

mainly due to the provision of Ca and Si for the formation of CSH and geopolymeric

gels. The presence of Ca in a geopolymeric system also accelerates dissolution and

hardening due to the extra nucleation sites provided by Ca [53,54].

The improving effect of Ca on the geopolymeric system has been noted by different

researchers [33,55-57]. Two mechanisms have been used to explain the contribution of

Ca to the geopolymeric system. The first states that Ca2+

acts as a charge-balancing agent

and is integrated into the geopolymeric network via the following scheme [57]:

The second states that Ca contributes to the formation of CSH gel which can coexist with

the geopolymeric gel. In this case, the geopolymeric and CSH gels act as independent

phases as also shown by Yip et al. [33] with the metakaolin-slag based geopolymeric

system. The CSH gel as a product of geopolymeric or pozzolanic reaction can have

different types depending on the concentration of Ca in the system. The CSH gel resulted

from the hydration of Portland cement generally has Ca/Si = 1.7 – 1.8 although Ca/Si =

1.5 – 2 is also reported [57]. The CSH gel as a co-product to the geopolymeric gel,

however, has much lower Ca/Si ratios, generally smaller than 0.7 [33,57,58,59].

It is noted that the improving effect of Ca is limited to a certain amount of Ca in the

geopolymeric system [33,36,60]. In the current study, the improving effect of GWC is

limited to 50% added GWC. Further increase results in decrease of UCS. The adverse

≡Si-O- + Ca

2+ ≡Si-O

-Ca

++ H2O

≡Si-O-Ca

+ + O

--Al + H2O [≡Si-O-Ca-O-Al≡] ≡Si-O-Al≡ + Ca

2++ 2OH

-

(3)

195

effect of GWC is mainly due to the precipitation of Ca(OH)2 and too high a water content

required for the sample preparation. Since the GWC particles are fine with high surface

area, the Ca-silicate based coating layer dissolves quickly in the alkaline solution. The

quick dissolution of calcium cations in the alkaline solution leads to fast elevation of Ca

concentration in the solution until it exceeds the threshold concentration at the high pH

and subsequently causes precipitation of Ca in the form of Ca(OH)2. Too much

precipitated Ca(OH)2 increases the viscosity and makes it difficult to mold the specimen.

In order to adjust the viscosity and achieve desired workability, extra water is required.

To reach a consistent workability at higher GWC content, more water is required. Table 2

lists the water content used in this study which in general increases with the GWC

content. Too much water in the paste results in porous microstructure due to evaporation

of extra water and thus the decrease of UCS [61]. Furthermore, at high GWC content, the

Ca-based precipitates create a coating film on both FA and GWC particle surfaces and

consequently inhibit further dissolution of silica and alumina and hinder

geopolymerization. This is similar to the phenomenon observed when too much SS is

used in the alkaline solution.

Fig. 5 is reproduced from Fig. 3 to show the relationship between UCS and Ca/Si ratio.

The data is presented at three different narrow ranges of Si/Al ratios in order to clearly

show the effect of Ca/Si on UCS. The average Si/Al ratio of each range is respectively

2.37, 3.30, and 5.33. The first two averages are close respectively to the Si/Al ratio of

PSS (poly sialate-siloxo) and PSD (poly sialate-disiloxo) which have theoretical Si/Al

ratio respectively of 2 and 3. It can be seen from Fig. 5 that the highest UCS’s are

obtained at Si/Al = 2.80 - 3.81, which are close to the Si/Al ratio of PSD. The optimum

Ca/Si ratios (corresponding to the highest UCS) are respectively 0.15, 0.18, and 0.25 for

the average Si/Al ratios of 2.37, 3.30, and 5.33. This indicates that in Ca-added

geopolymeric systems, the optimum Ca/Si ratio slightly increases with the Si/Al ratio,

which is in agreement with the results from other researchers (see Fig. 6).

196

3.2. Micro/nano-scale investigation

The micro/nano structure, morphology and phase/surface elemental compositions of the

geopolymer binders are investigated by SEM/EDX, XRD, and FTIR analyses.

3.2.1. SEM imaging /EDX analysis

Fig. 7 shows the SEM micrographs of the 0% GWC (100% FA) specimen synthesized at

10 M NaOH concentration and SS/N = 2 and cured at room temperature for 7 days. Three

distinct phases can be clearly seen: the partially reacted FA particles, the glassy phase or

the geopolymer gel which surrounds the FA particles, and the un-reacted FA particles

which are embedded in the glassy gel.

Fig. 8 shows the SEM micrographs of the 50% GWC (50% FA) specimen synthesized at

10 M NaOH concentration and SS/N = 2 and cured at room temperature for 7 days. In

addition to the phases seen in Fig. 7, there are three more phases: the CSH gel, the very

little crystalline phase deposited on the FA particle surface, and the partially reacted

GWC particles. Both the GWC and FA particles are attached by the glassy phase or the

geopolymeric gel. The needle-shaped crystalline phase on the FA particle surface is less

likely to be calcium compounds as the EDX analysis indicates that Si and Al are the

major constituents. Therefore, the crystalline phase is possibly due to minor

crystallization of the aluminosilicate species. Cristelo et al. [64] also reported formation

of the same type of crystalline structure on class F FA particle surfaces with similar

elemental composition.

Table 3 compares the final Si/Al, Na/Al, and Ca/Si ratios obtained from the EDX analysis

on the phases described above with the initial ones. In both the 0% and 50% GWC

specimens, (Si/Al)RF < (Si/Al)GP < (Si/Al)UF, where RF, GP and UF stand respectively for

(partially) reacted fly ash, geopolymer and un-reacted fly ash. The fact that (Si/Al)UF is

greater than the initial Si/Al ratio of the FA powder indicates that Al is likely to dissolve

and migrate to the other phases. (Si/Al)UF is also greater than (Si/Al)RF possibly because

197

only Al dissolves from the un-reacted FA particle surface while both Si and Al dissolve

from the reacted FA particle surface. (Si/Al)RF < (Si/Al)GP indicates that the concentration

of Si in the geopolymer is higher than that in the interface between the geopolymer and

FA particle. In other words, the transition phase in the FA-GP interface is weaker than

the geopolymeric gel. The 50% GWC specimen has higher (Si/Al)GP than the 0% GWC

specimen. (Si/Al)GP = 2.21 and 3.74 respectively for the 0% and 50% GWC specimens.

So the chemical composition of the geopolymer in the 0% and 50% GWC specimens is

respectively close to the chemical composition of the PSS and PSD type geopolymer. In

other words, the addition of calcium to the geopolymer system leads to formation of more

rigid geopolymer. The 50% GWC specimen also has much higher Ca/Si ratio in the

geopolymeric gel than the 0% GWC specimen, which indicates the incorporation of Ca in

the geopolymeric network, as a charge-balancing cation as discussed earlier. As stated

earlier, Ca is provided by dissolution of the coating layer on the GWC particles (see Fig.

2).

Fig. 9 shows the high magnification SEM micrograph and EDX analysis results on the

labeled area in Fig. 2d. The GWC mainly contains crystalline particles, which are coated

with a thin rough layer. This layer is possibly the product of hydrated OPC along with

some fine aggregates, which disappears after geopolymerization (see Fig. 8). This

indicates the dissolution of the coating layer and incorporation in the geopolymerization.

The EDX results confirm that the main constituent of the coating layer is Ca while the

XRF analysis on the whole GWC indicates that Si and Ca are the main constituting

elements. So the coating layer is mainly Ca(OH)2 and CSH gel.

3.2.2. XRD analysis

Fig. 10 shows the XRD analysis results for the FA and GWC powders and the

geopolymer specimens prepared with respectively 0% and 50% GWC, synthesized at 10

M NaOH and SS/N = 2, and cured at room temperature for 7 days.

198

The XRD pattern of the FA powder represents amorphous material with a diffused halo

peak centered at about 22° along with a crystalline phase which consists of quartz (SiO2)

and mullite (Al4.984 Si1.016 O9.508). The GWC powder is mainly crystalline material

consisting of anorthite [Ca (Al2Si2O8)], CSH gel [(CaO)x.SiO2.(H2O)y], portlandite

[Ca(OH)2], and quartz. In the GWC powder, anorthite and quartz originate from the

powdered aggregates of the original concrete and the CSH gel and portlandite are the

hydration products of the Portland cement. The CSH gel has characteristics reflections at

3.04, 2.79, and 1.82 Å, respectively corresponding to (2 2 0), (4 0 0), and (0 4 0) planes

of 1.1-nm tobermorite [65-68]. Beside the crystalline phase in the GWC powder, there is

a weak amorphous phase which extends from about 25° to 40°.

The geopolymerization has two main effects on the XRD patterns. First, the broad hump

in both the 0% and 50% GWC specimens becomes wider and its center shifts toward

larger angles, indicating formation of new amorphous material. The amorphous hump

extending from about 20 to 40° is characteristic of the geopolymeric gel. The second

effect is the decrease of the intensity of the crystalline peaks, which indicates partial

dissolution of the crystalline phase, especially for the FA particles, as also evidenced by

the SEM micrographs (see Figs. 7 and 8). In addition to the partial dissolution, the

decrease of the intensity of crystalline peaks can be due to the addition of sodium silicate

and consequently the increase of the SiO2/Na2O ratio [69,70].

Portilandite is the only crystalline phase in the GWC powder which undergoes full

dissolution after geopolymerization as the corresponding peaks in the GWC powder

disappear in the 50% GWC specimen. This is also confirmed by the SEM/EDX analysis

as discussed earlier. However, the CSH gel only undergoes partial dissolution. This

indicates that the CSH gel and portlandite, which are the main hydration products of

Portland cement, do not exhibit the same reactivity to the alkaline solution.

199

In the 50% GWC specimen a semi-crystalline phase is observed corresponding to the

humps centered at approximately 30 and 50°. This is due to the formation of low calcium

CSH gel as also demonstrated by Buchwald et al [34] with the slag added metakaolin

based-geopolymeric system. The formation of CSH gel in geopolymeric systems have

been reported by many researchers [33,34,63,65,71] although some researchers did not

find any CSH gel in geopolymeric systems even with Ca added [36,61,72,73]. For

example, Mackenzie et al. [72] reported that the added Ca was incorporated into the

geopolymeric network. In the current study, based on the SEM/EDX and XRD results,

the added Ca can simultaneously be incorporated into the geopolymeric network and

contribute to the formation of the low calcium CSH gel. However, due to the high

alkalinity, the geopolymeric gel is the main product of the reaction.

3.2.3. FTIR Analysis

Fig. 11 shows the IR spectra of the FA and GWC powders and the geopolymer specimens

prepared with respectively 0 and 50% GWC, synthesized at 10 M NaOH and SS/N = 2,

and cured at room temperature for 7 days. The IR characteristic bands are summarized in

Table 4.

All the powder and geopolymer specimens exhibit strong wide bands centered around

1000 cm-1

, which is attributed to the Si-O stretching vibrations of SiO4 tetrahedra in the

aluminosilicates [39,40,57,74,75]. For the GWC powder, the wide band centered around

970 cm-1

is also attributed to the Si-O stretching vibrations of SiO4 tetrahedra in the CSH

gel [34,76,77]. The GWC spectra represent sharper bands than the FA spectra meaning

that FA is more amorphous than GWC. This is also confirmed by the XRD analysis (see

Fig. 10).

The main change in the IR spectra of FA and GWC after geopolymerization is related to

the Si-O vibration bands, which undergo broadening and shifting toward a lower wave

number. This is also noted by other researchers [73,75,80-82]. The peak broadening

200

means transition to a less ordered structure due to formation of randomly distributed Si-

Al bonds [75]. The transition of the Si-O related bands near 1000 cm-1

to lower wave

numbers is due to the transition of symmetric to asymmetric stretching mode of Si-O

bonds, which follows depolymerization of silicates and substitution of some Si with Al

[40,57,75]. Some researchers have also reported the shift of Si-O peaks toward greater

wave numbers due to geopolymerization [61,78,82]. For example, Giannopoulou et al.

[82] reported the shift toward higher wave numbers for geopolymerization of ferronickel

slag but the shift toward lower wave numbers for red mud/metakaolin-based geopolymer.

The Si-O stretching vibration of SiQn units corresponds to the bands centered around 850,

900, 950, 1100, and 1200 cm-1

, respectively for n = 0, 1, 2, 3, and 4 [57]. For example,

the Si-O vibration of SiQ3 corresponds to 1100 cm

-1, which has the second highest degree

of polymerization, but due to alkali activation, it shifts toward a lower wave number and

then after poly-condensation shifts toward a higher wave number. In the current study,

both the 0% and 50% GWC geopolymer specimens exhibit shift toward lower wave

numbers. The larger shifting in the 50% GWC specimen indicates higher extent of alkali

activation in the calcium added geopolymer systems. This is also consistent with the final

Na/Al ratios shown in Table 3, as larger number of NBO’s (non bridging oxygens) exist

at Na/Al ratios larger than 1 [40], which is the case in the 50% GWC specimen. Besides

that, a new weak and broad peak appears at 1400 cm-1

, which is due to the formation of

new aluminosilicate phase related to geopolymerization [47]. This band is also stronger

in the 50% GWC specimen. The decrease in the height of the band related to silicates

after geopolymerization is due to the formation of CSH gel, which is more crystalline

than geopolymer [83].

The other major change in the IR spectra of the original materials is related to the full

dissolution of portlandite in the GWC powder, which disappear in the 50% GWC

spectrum. The dissolution of portlandite is also confirmed with the XRD analysis as

presented in the previous subsection.

201

4. Summary and Conclusions

The feasibility of utilizing GWC (together with class F FA) to produce geopolymer

binder was studied. Specifically, the effect of GWC content, NaOH concentration, and

SS/N on UCS was investigated. And the micro/nano-structure, morphology and

phase/surface elemental compositions of the geopolymer binder were also studied by

SEM/EDX, XRD, and FTIR analyses. Based on the experimental results, the following

major conclusions can be drawn.

1- Inclusion of GWC helps improve the UCS of geopolymer binder up to a certain GWC

content and further increase of GWC content leads to decrease of UCS. In the current

experiment, 50% was found as the optimum GWC content at 5 and 10 M NaOH and

with SS/N =1 and 2. The optimum initial Ca/Si ratio (the Ca/Si ratio at the highest

UCS) is low (0.15 to 0.25) for the GWC/FA geopolymer binder, which suggests

formation of low-Ca CSH gel in the geopolymer system.

2- Increased NaOH concentration results in higher UCS, especially at GWC content less

than 50%. Addition of SS also improves UCS due to provision of additional SiO2 and

delayed setting. The optimum initial Si/Al (the Si/Al ratio at the highest UCS) for the

GWC/FA geopolymer binder is around 3.38.

3- The SEM/EDX, XRD and FTIR analyses confirm that the Ca in GWC enhances the

strength mainly due to the formation of low Ca semi-crystalline CSH gel which

coexists with the geopolymer gel and the incorporation of Ca+ into the geopolymer

network as charge balancing cation.

4- The geopolymer in the GWC/FA geopolymer binder is close to PSD and thus

stronger than the geopolymer in the pure FA geopolymer binder which is close to

PSS.

5. Acknowledgements

This work is supported by the Environmental Research and Education Foundation (EREF).

The authors gratefully acknowledge the Salt River Materials Group in Phoenix, Arizona for

providing the fly ash used in this investigation.

202

6. References

[1] Davidovits, J. High-alkali cements for 21st century concrete. Proceedings of V.

Mohan Malhortra Symposium: Concrete Technology, Past, Present and Future, P.

K. Metha (ed), ACI SP-144; 1994. p. 383-97.

[2] McCaffrey R. Climate change and the cement industry. Global Cement and Lime

Magazine (Environmental Special Issue); 2002. p. 15-9.

[3] Arm M. Mechanical Properties of Residues as Unbound Road Materials –

Experimental Tests on MSWI Bottom Ash, Crushed Concrete and Blast Furnace

Slag. KTH Land and Water Resources Engineering, Stockholm; 2003.

[4] TCEQ (Texas Commission of Environmental Quality). Recycling urban resources:

Reclaimed and reused. Natural Outlook, Winter 2004; 2009,

http://www.tceq.state.tx.us/comm_exec/forms_pubs/ pubs/pd/020/04-01/index.html.

[5] Cochran K, Villamizar N. Recycling construction materials: An important part of

the construction process. Construction Business Owner; June 2007.

[6] USEPA. Wastes-Resource Conservation-Reduce, Reuse, Recycle-Construction &

Demolition Materials; 2009,

http://www.epa.gov/epawaste/conserve/rrr/imr/cdm/index. htm.

[7] McKelvey D, Sivakumar V, Bell A, McLaverty G. Shear strength of recycled

construction materials intended for use in vibro ground improvement. Ground

Improvement 2002;6(2),59-68.

[8] Drechsler M, Graham A. Innovative materials technologies: Bringing resources

sustainability to construction and mine industries. 48th Institute of Quarrying

Conference, Adelaide SA; 2005.

[9] EEA (European Environment Agency). Effectiveness of environmental taxes and

charges for managing sand, gravel and rock extraction in selected EU countries.

EEA Report No 2/2008; 2008.

[10] Schneider M, Romer M, Tschudin M, Bolio H. Sustainable cement production-

present and future. Cement and Concrete Research 2011;41:642-50.

203

[11] Stokoe MJ, Kwong PY, Lau MM. 1999, Waste reduction: a tool for sustainable

waste management for Hong Kong, In: A. Barrage and Y. Edelmann (ed.),

Proceedings of R’99 World Congress, Geneva; 1999. p. 165-70.

[12] Formoso CT, Soibelman L, De Cesare C, Isatto EL. Material waste in building

industry: main causes and prevention. Journal of Construction Engineering and

Management 2002;128(4):316-25.

[13] Craven DJ, Okraglik HM, Eilenberg IM. Construction waste and a new design

methodology, In: C.J. Kibert (ed.), Proceedings of the First Conference of CIB TG

16 on Sustainable Construction, Tampa; 1994. p. 89-98.

[14] Kibert CJ. Deconstruction as an essential component of sustainable construction, In:

Proceedings of the Second Southern African Conference on Sustainable

Development in the Built Environment, Pretoria; 2000. p. 1-5.

[15] Hansen TC. Recycling of Demolished Concrete and Masonry. Taylor and Francis.

Oxfordshire, UK; 1992.

[16] Tavakoli M, Soroushian P. Strengths of recycled aggregate concrete made using

field-demolished concrete as aggregate. ACI Materials Journal 1996;93(2):182-90.

[17] Sagoe-Crentsil KK, Brown T, Taylor AH. Performance of concrete made with

commercially produced coarse recycled concrete aggregate. Cement and Concrete

Research 2001;31(5):707-12.

[18] Shayan A, Xu A. Performance and properties of structural concrete made with

recycled concrete aggregate. ACI Materials Journal 2003;100(5):371-80.

[19] Tam VWY, Go X F, Tam CM. Microstructural analysis of recycled aggregate

concrete produced from two-stage mixing approach. Cement and Concrete

Research 2005;35:1195-203.

[20] Xiao JZ, Li JB, Zhang C. On relationships between the mechanical properties of

recycled aggregate concrete: An overview. Materials and Structures 2006;39:655-

64.

[21] Poon CS, Lam CS. The effect of aggregate-to-cement ratio and types of aggregates

on properties of precast concrete blocks. Cement and Concrete Composites

2008;30:283-9.

[22] Malešev M, Radonjanin V, Marinković S. Recycled concrete as aggregate for

structural concrete production. Sustainability 2010;2:1204-25.

204

[23] Hack DR, Bryan DP (2006). Aggregates. Industrial Minerals and Rocks, Kogel EK,

Trivedi NC, Barker JM, Krukowski ST (eds), 7th Edition, Littleton, Colorado,

Society for Mining, Metallurgy and Exploration; 2006. p. 1105-19.

[24] Langer W. Sustainability of aggregates in construction. Sustainability of

Construction Materials, Khatib JM (ed), CRC Press; 2009. p. 1-30.

[25] Australian Standard. Guide to the use of recycled concrete and masonry materials.

HB155-2002; 2002.

[26] Kuroda Y, Hashida H. A closed-loop concrete system on a construction site.

Proceedings CANMET/ACI/JCI, Three-Day International Symposium on

Sustainable Development of Cement, Concrete and Concrete Structures, Toronto,

Canada; 2005. p. 371-88.

[27] Yanagibashi K, Inoue, K, Seko S, Tsuji D. A study of cyclic use of aggregate for

structural concrete. SB05: The 2005 World Sustainable Building Conference,

Tokyo; 2005. p. 2585-92.

[28] Kanare HK, West PB. Leachability of selected chemical elements from concrete.

Proceedings of the Symposium on Cement and Concrete in the Global

Environment. SP114, Portland Cement Association, Chicago, IL; 1993. p. 366.

[29] Sangha CM, Hillier SR, Plunkett BA, Walden PJ. Long-term leaching of toxic trace

metals from Portland cement concrete. Cement and Concrete Research

1999;29:515-21.

[30] Mulligan S. Recycled Concrete Materials Report. Ohio Department of

Transportation. Columbus. OH; 2002.

[31] Tomosawa F, Noguchi T. Towards completely recyclable concrete. Integrated

Design and Environmental Issues in Concrete Technology. Sakai K(ed.), E & FN

Spon, London, UK; 1996. p. 263-72.

[32] Costes JR, Majcherczyk C, Binkhorst IP. Total Recycling of Concrete; 2007,

http://omogine.blogspot.com/.

[33] Yip CK, Lukey GC, Van Deventer JSJ. The coexistence of geopolymeric gel and

calcium silicate hydrate at the early stage of alkaline activation. Cement and

Concrete Research 2005;35:1688-97.

[34] Buchwald A, Hilbig H, Kaps CH. Alkali-activated metakaolin-slag blends-

performance and structure in dependence of their composition. Journal of Materials

Science 2007;42:3024-32.

205

[35] Zhang Z, Yao X, Zhu H. Potential application of geopolymers as protection coatings

for marine concrete I. Basic properties. Applied Clay Science 2010;49:1-6

[36] Temuujin J, Van Riessen A, Williams R. Influence of calcium compounds on the

mechanical properties of fly ash geopolymer pastes. Journal of Hazardous Materials

2009;167:82-8.

[37] Yang ZX, Ha NR, Jang MS, Hwang KH. Geopolymer concrete fabricated by waste

concrete sludge with silica fume. Material Science Forum 2009;620-622:791-4.

[38] Yang ZX, Ha NR, Jang MS, Hwang KH, Jun BS, Lee JK. The performance of

geopolymer based on recycled concrete sludge. Ceramic Materials and Components

for Energy and Environmental Applications: a collection of papers presented at the

9th International Symposium on Ceramic Materials for Energy and Environmental

Applications and the Fourth Laser Ceramics Symposium, Shanghai, China; 2008.

[39] Allahverdi A, Khani EN. Construction wastes as raw materials for geopolymer

binders. International Journal of Civil Engineering 2009;7(3):154-60.

[40] Lee WKW, Van Deventer JSJ. Use of infrared spectroscopy to study

geopolymerization of heterogeneous amorphous aluminosilicates. Langmuir

2003;19:8726-34.

[41] Panias D, Giannopoulou IP, Perraki T. Effect of synthesis parameters on the

mechanical properties of fly ash-based geopolymers. Colloids and Surfaces A:

Physicochemical Engineering Aspects 2007;301:246-54.

[42] Davidovits J. Mineral polymers and methods of making them. US Patent 4349386;

14th Sept. 1982.

[43] Duxson P, Provis JL, Lukey GC, Mallicoat SW, Kriven WM, Van Deventer JSJ.

Understanding the relationship between geopolymer composition, microstructure

and mechanical properties. Colloids and Surfaces A: Physicochem. Eng. Aspects

2005;269(1-3):47-58.

[44] Lukey GC, Van Deventer JSJ, Provis JL, Duxson P. Design of geopolymeric

materials based on nanostructural characterization and modeling. Project Report

AOARD-054025, University of Melbourne; 2006.

[45] Hardjito D, Wallah SE, Sumajouw DMJ, Rangan BV. On the development of fly

ash-based geopolymer concrete. ACI Materials Journal 2004;101(6):467-72.

206

[46] Yunfen H, Dongmin W, Wenjuan Z, Hongbo L, Lin W. Effect of activator and

curing mode on fly ash-based geopolymer. Journal of Wuhan University of

Technology-Mater. Science Ed. 2009;24(5):711-5.

[47] Guo X, Shi H, Dick WA. Compressive strength and microstructural characteristics

of class C fly ash geopolymer. Cement and Concrete Composites 2010;32:142-7.

[48] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Properties of tungsten mine waste

geopolymeric binder. Construction and Building Materials 2008;22:1201-11.

[49] Silva PD, Sagoe-crenstil K, Sirivivatnanon V. Kinetics of geopolymerization: Role

of Al2O3 and SiO2. Cement and Concrete Research; 2007;37:512–8.

[50] Xu H, Van Deventer JSJ. Effect of source materials on geopolymerization.

Industrial and Engineering Chemistry Research 2003;42:1698-706.

[51] Giannopoulou IP, Panias D. Development of geopolymeric materials from industrial

solid wastes. 2nd International Conference on Advances in Mineral Resources

Management and Environmental Geotechnology, Greece; 2006.

[52] Klabprasit T, Jaturapitakkul C, Songpiriyakij S. Influence of Si/Al ratio on rice

husk-bark ash and fly ash-based geopolymer properties. International Conference

on the Role of Universities in Hands-On Education Rajamangala University of

Technology Lanna, Chiang-Mai, Thailand; August 2009, p. 445-51.

[53] Van Deventer JSJ, Provis JL, Duxson P, Luckey GC. Reaction mechanisms in the

geopolymeric conversion of inorganic waste to useful products, Journal of

Hazardous Materials 2007;A139:506-13.

[54] Lee WKW, Van Deventer JSJ. The effect of ionic contaminants on the early age

properties of alkali-aktivated fly ash based cements. Cement and Concrete Research

2002;32:577-84.

[55] Lee WKW, Van Deventer JSJ. The effects of inorganic salt contamination on the

strength and durability of geopolymers. Colloids and Surfaces A: Physicochemical

and Engineering Aspects 2002;211:115-26.

[56] Phair JW, Van Deventer JSJ. Effect of silicate activator pH on the leaching and

material characteristics of waste-based inorganic polymers. Minerals Engineering

2001;14:289-304.

207

[57] Lecomte I, Henrist C, Liegeois M, Maseri, Rulmont A, Cloots R. (Micro)-structural

comparison between geopolymers, alkali-activated slag cement and Portland

cement. Journal of the European Ceramic Society 2006;26(16):3789-97.

[58] Fernandez-Jimenez A, Puertas F, Sobrados I, Sanz J. Structure of calcium silicate

hydrates formed in alkaline-activated slag: influence of the type of alkaline

activator. Journal of the American Ceramic Society 2003;86(8):1389-94.

[59] Davidovits J. Geopolymer chemistry and applications. Institute Geopolymere. Saint-

Quentin; 2008.

[60] Pacheco-Torgal F, Castro-Gomes JP, Jalali S. Investigations on mix design of

tungsten mine waste geopolymeric binder. Construction and Building Materials

2008;22:1939-49.

[61] Maragkos I, Giannopoulou IP, Panias D. Synthesis of ferronickel slag-based

geopolymers. Minerals Engineering 2009;22:196-203.

[62] Bondar D, Lynsdale CJ, Milestone NB, Hassani N, Ramezanianpour AA.

Geopolymer Cement from Alkali-Activated Natural Pozzolans: Effect of Addition

of Minerals. 2nd International Conference on Sustainable Construction Materials

and Technologies, Ancona, Italy; 2010.

[63] Dombrowski K, Buchwald A, Weil M. The influence of calcium content on the

structure and thermal performance of fly ash based geopolymers. Journal of

Materials Science 2007;42:3033-43.

[64] Cristelo N, Glendinning S, Fernandes L, Pinto AT. Effect of calcium content on soil

stabilisation with alkaline activation. Construction and Building Materials

2012;29:167-74.

[65] Oh JE, Monteiro PJM, Jun SS, Choi S, Clark SM. The evolution of strength and

crystalline phases for alkali-activated ground blast furnace slag and fly ash-based

geopolymers. Cement and Concrete Research 2010;40:189-96

[66] Taylor HFW. Cement Chemistry. Academic Press. London; 1990.

[67] Cong X, Kirkpatrick RJ. 29Si MAS NMR study of the structure of calcium silicate

hydrate. Advanced Cement Based Materials 1996;3:144-56.

[68] Selvam RP, Subramani VJ, Murray S, Hall K. Potential application of

nanotechnology on cement based materials. Project Report MBTC DOT 2095/3004;

2009.

208

[69] Provis JL, Yong CZ, Duxson P, Van Deventer JSJ. Correlating mechanical and

thermal properties of sodium silicate-fly ash geopolymers. Colloids and Surfaces A:

Physicochemical and Engineering Aspects 2009;336:57-63.

[70] Villa C, Pecina ET, Torres R, Gómez L. Geopolymer synthesis using alkaline

activation of natural zeolite. Construction and Building Materials 2010;24:2084-90.

[71] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Investigations of tungsten mine waste

geopolymeric binder: Strength and microstructure. Construction and Building

Materials 2008;22 (11):2212–9.

[72] MacKenzie KJD, Smith ME, Wong A. A multinuclear MAS NMR study of

calcium-containing aluminosilicate inorganic polymers. Journal of Materials

Chemistry 2007;17: 5090-6.

[73] Fernandez Jimenez A, Palomo A. Characterisation of fly ashes. Potential reactivity

as alkaline cements. Fuel 2003;82:2259-65.

[74] Mingyu H, Xiaomin Z, Fumei L. Alkali-activated fly ash-based geopolymers with

zeolite or bentonite as additives. Cement and Concrete Composites 2009;31:762-8.

[75] Zhang Y, Sun W, Li Z. Infrared spectroscopy study of structural nature of

geopolymeric products. Journal of Wuhan University of Technology-Materials

Science Ed. 2008;23(4):522-7.

[76] Trezza MA. Hydration study of ordinary Portland cement in the presence of zinc

ions. Materials Research 2007;10(4):331-4.

[77] Fernandez L, Alonso C, Hidalgo A, Andrade C. The role of magnesium during the

hydration of C3S and C-S-H formation. Scanning electron microscopy and mid-

infrared studies. Advances in Cement Research 2005;17(1):9-21.

[78] Sakulich AR. Characterization of Environmentally-Friendly Alkali Activated Slag

Cements and Ancient Building Materials. Ph.D. Thesis, Drexel University. 2009.

[79] Treadwell DR, Dabbs DM, Aksay IA. Mullite (3Al2O3-2SiO2) Synthesis with

Aluminosiloxanes. Chemistry of Materials 1996;8:2056-60.

[80] Alonso S, Palomo A. Alkaline activation of metakaolin and calcium hydroxide

mixtures: Influence of temperature, activator concentration and solid ratio.

Materials Letters 2001;47(1-2):55-62.

209

[81] Bakharev T. Geopolymeric materials prepared using Class F fly ash and elevated

temperature curing. Cement and Concrete Research 2005;35:1224-1232.

[82] Giannopoulou I, Dimas D, Maragkos I, Panias D. Utilization of metallurgical solid

by-products for the development of inorganic polymeric construction materials.

Global NEST Journal 2009;11(2):127-136.

[83] Yunsheng Z, Wei S, Qianli C, Lin C. Synthesis and heavy metal immobilization

behaviors of slag based geopolymer. Journal of Hazardous Materials 2007;143(1-

2):206-12.

210

Table 1. Chemical composition of ground waste concrete (GWC) powder and fly ash

(FA) based on XRF analysis.

Chemical Compound GWC (%) FA (%)

SiO2 40.1 57.5

CaO 20.6 6.0

Al2O3 9.6 29.3

Fe2O3 3.5 2.95

K2O 2.3 NA

H2O 2.2 NA

MgO 2.1 1.36

Na2O 1.7 2.6

211

Table 2. Chemical composition and conducted tests on the ground waste concrete

(GWC)/fly ash (FA) specimens.

Specimen GWC

(%)

NaOH

Conc.

(M)

SS/N

Water

content

(%)

Si/Al Ca/Si Na/Al Na/Si UCS

Test XRD SEM FTIR

GWC 100 NA NA NA 3.56 0.55 NA NA NA X X X

FA 0 NA NA NA 1.67 0.11 NA NA NA X X X

5-0-1* 0 5 1 25.3 2.05 0.09 0.21 0.10 X

5-25-1 25 5 1 25.6 2.32 0.15 0.26 0.11 X

5-50-1 50 5 1 32.0 2.87 0.22 0.41 0.14 X

5-75-1 75 5 1 32.8 3.62 0.29 0.56 0.16 X

5-100-1 100 5 1 36.2 5.26 0.37 0.94 0.18 X

10-0-1 0 10 1 31.0 2.19 0.08 0.41 0.19 X

10-25-1 25 10 1 28.1 2.42 0.15 0.44 0.18 X

10-50-1 50 10 1 28.6 2.86 0.22 0.57 0.20 X

10-75-1 75 10 1 35.3 3.81 0.28 0.94 0.25 X

10-100-1 100 10 1 34.5 5.36 0.37 1.39 0.26 X

10-0-2 0 10 2 24.27 2.36 0.08 0.28 0.12 X X X X

10-25-2 25 10 2 27.5 2.80 0.13 0.38 0.14 X

10-50-2 50 10 2 28.9 3.38 0.18 0.50 0.15 X X X X

10-75-2 75 10 2 26.1 4.12 0.26 0.61 0.15 X

10-100-2 100 10 2 34.6 6.61 0.30 1.22 0.19 X

* 5-0-1 represents specimen at 5 M NaOH concentration, 0% GWC and SS/N = 1.

212

Table 3. Comparison of the elemental compositions of the different phases shown in Figs.

7 and 8 obtained from EDX analysis with the initial ones before reaction.

Specimen Phase Si/Al Na/Al Ca/Si

Init

ial

FA 3.33 NA 0.11

GWC 3.89 0.21 1.36

10-100-2

combination 6.61 1.22 0.30

10-50-2

Combination 3.38 0.50 0.18

10-0

-2

UF*

2.70 0.59 0.08

RF*

1.33 0.22 0.02

GP*

2.12 0.59 0.05

10-5

0-2

UF 3.96 0.55 0.08

RF 2.61 0.52 0.13

GP 3.59 2.34 0.32

* UF, RF, GP, and CR are respectively un-reacted, (partially) reacted, geopolymer, and

crystalline phase (see Figs. 7 and 8).

213

Table 4. Infrared (IR) characteristic bands identified in fly ash (FA) and ground waste

concrete (GWC) powders and the geopolymer specimens shown in Fig. 11.

Wave

Number

(cm-1

)

Characteristic bands References

800-1,200 Si-O stretching vibrations of SiO4 [34,39,40,74,75]

872 -CO3 vibrations in CaCO3 [34,76]

970 stretching vibration mode of Si-O (3) in CSH gel [34,76,77]

1,400 Si-O vibrations [47]

1,650 bending (2) mode of H-O-H [34,76]

2,350

C-O vibrations in CO2 constrained in amorphous

phase [78,79]

2,920

C-O vibrations in CO2 constrained in amorphous

phase [78,79]

3,645 O-H stretching vibration of portlandite [34,76]

214

Fig. 1. Particle size distribution of fly ash (FA) and ground waste concrete (GWC) by

laser diffraction.

0

20

40

60

80

100

0.1 1 10 100 1000

Pe

rce

nt p

assin

g (%

)

Particle size (m)

GWC

FA

215

Fig. 2. Low and high-magnification SEM micrographs of fly ash (FA) – (a), (b), and

ground waste concrete (GWC) powder – (c), (d).

a

c

a b

c d

216

Fig. 3. UCS versus ground waste concrete (GWC) content (percent of total mass of FA

and GWC) for specimens cured at room temperature for 7 days and synthesized at 5 and

10 M NaOH and SS/N = 1 and 2.

5

10

15

20

25

30

35

0 25 50 75 100

UC

S (M

Pa

)

GWC Content (%)

NaOH = 5 M, SS/N = 1

NaOH = 10 M, SS/N = 1

NaOH = 10 M, SS/N = 2

217

Fig. 4 (a) UCS versus initial Si/Al ratio from current study; and (b) Comparison of

normalized UCS versus initial Si/Al ratio from current study and other researchers.

0

5

10

15

20

25

30

35

40

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

UC

S (M

Pa

)

Initial Si/Al

Ca/Si Na/Al Na/Si

0.08 - 0.26 0.16 - 0.37 0.08 - 0.16

Si/Al = 3.38a

0.0

0.2

0.4

0.6

0.8

1.0

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Initial Si/Al

Current study

[43]

[50]

[51]

No

rma

lize

d U

CS

b

218

Fig. 5. UCS versus Ca/Si ratio at different ranges of Si/Al ratios from the current study.

0

5

10

15

20

25

30

35

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

UC

S (M

Pa

)

Ca/Si

Si/Al

2.05 - 2.42

2.80 - 3.81

4.12 - 6.61

Na/Al

0.21 - 0.44

0.38 - 0.94

0.61 - 1.22

219

Fig. 6. Comparison of initial Si/Al versus optimum Ca/Si (the Ca/Si at maximum UCS)

from current study and other researchers.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Op

tim

um

Ca

/Si

Initial Si/Al

Current study

[33]

[39]

[59]

[60]

[62]

[63]

220

Fig. 7. SEM micrographs of geopolymer specimen of 0% GWC (100% FA) synthesized

at 10 M NaOH and SS/N =2 and cured at room temperature for 7 days: (a) low

magnification micrograph, (b) higher magnification micrograph of the labeled area in (a),

(c) higher magnification micrograph of the labeled area in (b), and (d) micrograph of an

un-reacted FA (GP: geopolymer, RF: partially or fully reacted FA, UF: un-reacted FA).

UF

GP

RF

c

a

b

d c

b

221

Fig. 8. SEM micrographs of geopolymer specimen of 50% GWC synthesized at 10 M

NaOH and SS/N =2 and cured at room temperature for 7 days: (a) low magnification

micrograph, (b) higher magnification micrograph of the labeled area in (a), (c) higher

magnification micrograph of the labeled area in (b), (d) crystalline structure on un-

reacted FA, and (e) EDX analysis result on the crystalline structure in (e) (GP:

geopolymer, RF: partially or fully reacted FA, UF: un-reacted FA, CSH: calcium silicate

hydrate gel, CR: crystalline phase).

0 1 2 3 4

keV

Crystalline Structure

C

O

Fe

Na

Mg

Al

Si

KCa

UF

RF

CSH

GP

UF

RF

CR

b

c

a b

c d

e

GWC

222

Fig.9. SEM/EDX analysis of the coating layer on the ground waste concrete (GWC)

particle surface.

Element

Line

Weight

(%)

Weight

Error (%)

C K 2.96 +/- 0.07

O K 43.06 +/- 0.28

Na K 0.88 +/- 0.05

Mg K 1.35 +/- 0.04

Al K 4.51 +/- 0.07

Si K 17.55 +/- 0.11

Si L --- ---

S K 0.63 +/- 0.05

S L --- ---

K K 1.55 +/- 0.05

K L --- ---

Ca K 25.18 +/- 0.23

Ca L --- ---

Fe K --- ---

Fe L 2.32 +/- 0.34

Total 100

223

Fig. 10. XRD patterns of FA and GWC powders and geopolymer specimens prepared

with respectively 0% and 50% GWC at 10 M NaOH and SS/N = 2.0 and cured at room

temperature for 7 days (A: anorthite, M: mullite, P: portlandite, Q: quartz, T: 1.1-nm

tobermorite).

10 15 20 25 30 35 40 45 50 55 60 65 70

2q

Q

MM

M

M

FA

GWC

10-50-2

10-0-2

T

TP

P PP

CS

H

CS

H

AA

A

A

QT

Q

MM

224

Fig. 11. IR spectra of FA and GWC powders and geopolymer specimens prepared with

respectively 0% and 50% GWC at 10 M NaOH and SS/N = 2.0 and cured at room

temperature for 7 days.

6001,1001,6002,1002,6003,1003,600

wave number (cm-1)

FA

GWC

10-50-2

10-0-2