Proc. of the 12th fib International PhD Symposium in Civil Engineering Aug 29 to 31, 2018, Czech Technical University in Prague, Prague, Czech Republic

145

Strength and microstructure of tungsten mining waste-based hybrid alkaline material: effect of activators Naim Sedira and João Castro-Gomes

Centre of Materials and Building Technologies (C–MADE/UBI), Department of Civil Engineering and Architecture, University of Beira Interior (UBI), 6201–001 Covilhã, Portugal

Abstract This work experimentally investigated the effects of different alkaline activators (sodium silicate, so-dium hydroxide, and potassium hydroxide), when used with varying proportions and concentrations with tungsten mining waste mud (TMW) and grounded granulated blast furnace slag (GGBFS) as a precursors to produce hybrid alkaline materials. The compressive strength development and the micro-structure was studied. The precursors used in this study are a combination of TMW (90 vt.%) and GGBFS (10 vt.%). After preparing the mixtures and molding, the samples were placed in an oven at a temperature of 60°C for 24 hours. After the hardened specimens were cured in laboratory condition, after (7, 14 and 28 days) the samples explored by compressive strength tests, also by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) were measured to the microstructure.

1 Introduction The carbonate decomposition is the third-largest source of anthropogenic emissions of carbon dioxide to the atmosphere, and the global manufacturing of Portland cement (PC) is the main source of CO2 emissions from decomposition of carbonates [1]. In order to reduce the CO2 and greenhouse gas emissions an alternative cement to PC have been proposed [2]. Alkali-activated blended Portland cement is an innovative material which is under developing nowadays; named also as “hybrid cement”. These cementitious materials use industrial by-products wastes such as fly ash or granulated grounded blast furnace slag (GGBFS) with a minority of PC or clinker as precursors and use different alkali activation solutions [3]–[7]. By combining PC or slags with industrial by-products under the alkali activation reaction, these hybrid cementitious materials have better mechanical and proved durability properties even at early ages [6], [8], [9]. Different sources of aluminosilicate materials used as precursors, the mine tailings is waste generated from mining activities which have been used in various previous studies alkali–activated materials development. Tungsten tailing is waste generated from a tungsten mine. It can lead to environmental risk, like contamination of soil, pollution of water and air in the surrounding areas, only in Panasquera mine in Portugal approximately 37000 tonnes per year [10]. Many studies nowadays focus on the valorisation and re-use the mine tailing waste in the construction sector as technical-artistic value [11], as a binder by combining the mining waste with other mineral wastes to produce alkali-activated binders with high performances [12], [13]. In the other hand researchers use tungsten mining waste in other innovative application [14], [15]. The hydration products resulted in such cementitious composites are different in nature and composition from those that are traditionally obtained from PC and are dependent on the type of industrial by-products wastes or aluminosilicate precursors used [16]. The hybrid cementitious materials contain different gels, which formation depends on the precursors used as raw materials. The gels produced in hybrid materials include the following: calcium silicate hydrate (C-S-H) due to the normal hydration of PC or slags (calcium and silicate rich materials); sodium aluminosilicate hydrate (N-A-S-H), which is the main product of the alkaline activation of aluminosilicate (also present in industrial waste by-products); and calcium aluminosilicate hydrate (C-A-S-H) and (N, C)-A-S-H gels, which are dependent on the pH and the calcium/ silicate (Ca/Si) and silicate/alumina (Si/Al) contents of the system [3]. So, in synthesis, and hybrid alkaline material contains either PC or slags (such us blast furnace slag and phosphorous slag), blended with other aluminosilicate rich materials (such us fly-ash or similar). Garcia-Lodeiro et al. [17] made an hybrid cement by blending 70% fly ash (FA) and 30% of PC as precursors, and using different alkaline activating solutions of different pH values. The hybrid cement resulted from the study corroborated the presence of different gels, such us N-A-S-H, C-A-S-H, and (N, C)-A-S-H.

12th fib International PhD Symposium in Civil Engineering

146 Advanced materials

In this study, we evaluated the feasibility to produce hybrid materials by blending tungsten mining waste (TMW) and grounded granulated blast furnace slag (GGBFS) activated with a composition of sodium silicate, sodium hydroxide and potassium hydroxide. This research work is part of REMINE research program (H2020 RISE-Marie Curie Action) that aims to the reuse of mining waste into innovative geopolymeric-based structural panels, precast, ready mixes and in-situ applications.

2 Experimental program

2.1 Materials The materials used in this work are (non-calcinated, i.e. natural) tungsten mining waste (N-TMW) and grounded granulated blast furnace slag (GGBFS). The mud waste was collected from Panasqueira mine located in Covilhã, Portugal. The mud was dried in an oven at 60°C temperature for 24 hours. Afterwards, the dried mud was mechanically disaggregated using a crushing machine and sieved under 500 µm particle size. The GGBFS was obtained from the cement plant Ain El Kebira located in Algeria. Prior to mixing the GGBFS was sieved to obtain size particles under 250 µm.

The particle size distribution of N-TMW and GGBFS are shown in Fig. 1 were determined using laser diffraction (Mastersizer 2000, Malvern Instruments). This indicates a mean particle size of approximately 4.35 µm for N-TMW and 2.55 µm for GGBFS, a d50 (50% of volume less than this size) of 5.43 µm for N-TMW and 3.12 µm for GGBFS, a d10 value of 11.1 µm for N-TMW and 8.05 µm for GGBFS.

The bulk powder densities of N-TMW and GGBFS were determined using a helium gas displacement pycnometer (model AccuPyc 1340, Micromeritics, Norcross, Georgia) and were determined as 3.03 and 2.92 g/cm3, respectively. The Blaine fineness of the different powders was determined according to EN 196-6, by using a Blaine air permeability apparatus (model ACMEL BSA1) and were determined as 3339 and 3115 cm²/g, respectively.

The physical characteristics and the element chemical compositions of the N-TMW and GGBFS materials are presented in Table 1.

Particle size distribution curve of N-TMW and GGBFS.

Strength and microstructure of tungsten mining waste-based hybrid alkaline material: Effect of activators

Naim Sedira and João Castro-Gomes 147

Table 1 Physical characteristics and chemical composition (% mass) of N-TMW and GGBFS.

Chemical compound/Properties Materials

GGBFS N-TMW

SiO2 34.09 46.67

Al2O3 7.79 17.05

TiO2 0.37 0.6

SO3 1.27 7.9

K2O 0.51 4.9

CaO 48.71 0.69

Fe2O3 0.4 15.47

MgO 5.09 4.8

Na2O 0.5 0.85

ZnO - 1.09

MnO 1.23 –

Density [g/cm3] 2.7339 3.0319

Blaine specific area [cm²/g] 2954 3339

For the alkaline activators, the sodium hydroxide solution was prepared by dissolving sodium hydroxide pellets (98% purity, obtained from Fisher Scientific, Schwerte, Germany) in deionized water and allowed to cool before use. For the preparation of potassium hydroxide (98% purity, obtained from Quimialmel Chemical Minerals Ltd., Portugal) solution was adopted the same process used for sodium hydroxide solution.

Sodium silicate (obtained from Solvay SA, Póvoa de Santa Iria, Portugal) presented the following chemical composition: SiO3/Na2O = 3.23 (8.60% by weight Na2O, 27.79% by weight SiO2, 63.19% by weight H2O, and 0.4% by weight Al2O3).

N-TMW and GGBFS X-ray diffractograms are presented in figure 2. The X–ray diffractogram of the N-TMW indicates it has a crystalline nature and is consists mainly of muscovite and quartz, which were identified by their characteristic, as follows: muscovite (Ref. PDF#46–1409), quartz (Ref. PDF#46–1045) and clinochlore (ref. PDF#29–0701) [18].

X–ray diffractograms of a) N-TMW and b) GGBFS - phases identified are Quartz (Q),

Muscovite (M), Clinochlore (C), Calcite (Cl), Akermanite (AK).

X–ray diffractogram spectra of GGBFS reveals a mixture of poorly crystalline phases with compositions resembling gehlenite (2CaO·Al2O3·SiO2) and akermanite (2CaO·MgO·2SiO2) and depolymerized calcium silicate glasses [19]. It confirms that Blast furnace slag is a predominantly an amorphous product, containing small amounts of crystalline phases like Akermanite (Ca2MgSi2O7), or

a) N-TMW b) GGBFS

12th fib International PhD Symposium in Civil Engineering

148 Advanced materials

Quartz (SiO2). It offers hydraulic properties similar to those of Portland Cement and hence may partly substitute the clinker component in cement blends [20].

2.2 Mixing and curing conditions The N-TMW hybrid alkaline mixes were produced by mixing N-TMW and GGBFS as precursors, with different alkaline activator solutions (NaOH-sodium hydroxide 8M, KOH-potassium hydroxide 8M and 10M). For each concentration, the activating solutions were mixed with sodium silicate (SS) using a 2:1 mass ratio. For the precursors/activators (P/A) mass ratios both 2:1 and 3:1 ratios were adopted, as shown in the Table 2.

Table 2 Proportion of N-TMW hybrid alkaline material mixes.

Mixture Activator Activators combinations (A) Precursors (P) N-TMW - TM

GGBFS - S

Ratio (P/A)

SS

NaOH 8M

KOH 8M

KOH 10M

MAH1 Na-1 0.67 0.33 - - 90%TM+10%S 2:1

MAH2 Na-2 0.67 0.33 - - 90%TM+10%S 3:1

MAH3 K-1 0.67 - 0.33 - 90%TM+10%S 2:1

MAH4 K-2 0.67 0.33 90%TM+10%S 3:1

MAH5 K-3 0.67 - - 0.33 90%TM+10%S 3:1

Thus, the blended mixtures were activated with a combination; of different solutions, namely (Na group) SS+NaOH and (K group) SS+KOH. Before beginning to prepare the mixtures, the two activators were first mixed together. Afterwards, the liquid activating solution was mixed with the precursors and poured in the curing moulds. Then, the filled moulds were placed in an oven at a temperature of 60°C for 24 hours, for synthesis. To avoid water evaporation the N-TMW hybrid alkaline materials during curing, the filled moulds were wrapped with plastic film during the oven curing synthesis. After the initial 24 hours oven curing period, the specimens were demoulded and were left to cure in laboratory conditions (about 20 ºC) placed inside sealed plastic bags, for 7, 14 and 28 days.

3 Result and Discussion

3.1 Strength development using different parameters The compressive strength tests were performed on 3000 kN electro–hydraulic mechanical testing ma-chine ‘ADR Touch 3000 BS EN Compression Machine with Digital Readout and Self Centring Platens', in accordance with EN 196-1. Compressive strength data was obtained using 25 mm cubic size speci-mens. The compressive strength results of N-TMW hybrid alkaline materials (average of testing 5 spec-imens per each mixture) were obtained at the ages of 7, 14 and 28 days. Figure 3 shows the development of compressive strength in terms of the curing time for different N-TMW hybrid alkaline mixes.

3.1.1 Effect of the type of alkaline activator To study the effect of the type of alkaline activator on the development of the strength of the hybrid materials the concentration solution and P/A ratio were kept constant. Thus, the comparison was made between MHA1 and MHA3 pair, and between MAH2 and MAH4 pair. MAH1 and MAH3 mixes ob-tained the same compressive strength results. Besides, the samples with a 3:1 P/A ratio present different results at 28 days; i.e. MAH2 produced with “Na group” obtained 24.5 MPa at 7 days, and 25.5 MPa at 28 days; MAH4 prepared using “K group” obtained 20.8 MPa at 7 days and 28.3 MPa after 28 days of curing. The sample made with “K group” (MAH4) obtained higher compressive strength that the pair made with “Na group”.

3.1.2 Effect of Precursor/Activator (P/A) ratios It is clear that the ratio of precursors to activators had a great influence on the development of the compressive strength, for all samples. In the first 7 days of curing, the samples with 2:1 P/A ratio

Strength and microstructure of tungsten mining waste-based hybrid alkaline material: Effect of activators

Naim Sedira and João Castro-Gomes 149

(MAH1 and MAH3) obtained a compressive strength of 17.2 MPa and 18.3 MPa, while MAH2 and MAH4 samples (3:1 P/A ratio) obtained a compressive strength of 24.5 MPa and 20.8 MPa, respec-tively. The same difference (of about 42% and 16%) is found at the ages of 14 and 28 days.

3.1.3 Effect of activators concentration The effect of alkaline activator concentration on the compressive strength development was made by comparison between MAH4 and MAH5 samples (both made with SS+KOH alkaline activator and the same P/A ratio). MAH4 (SS+KOH 8M) specimens showed a slight compressive strength increase, of about 1%, when comparing to MAH5 (SS+KOH 10M), at all tested ages.

Compressive strength of N-TMW hybrid alkaline materials activated with different com-

bination of activators in 7,14 and 28 days.

3.1.4 Effect of curing time The effect of development of strength along time was made for different type of alkaline activators, concentration and P/A ratio. MAH1 and MAH3 specimens gave the lowest results at all the curing ages; in the first 7 days gave 17.2 MPa and 18.3 MPa.Afterwards, it slowly developed to 22 MPa and 21.8 MPa at 28 days, respectively. The MAH3 specimen presented highest results after 7 days of curing of about 24.5 MPa. Afterwards its compressive strength did not increase significantly; i.e. 25.0 MPa at 14 days and 25.5 MPa at 28 days. During the same period of study, from 7 to 28 days, the highest increase in compressive strength was found out for samples MAH4 and MAH5; i.e. about 39% increase, starting from 20,8 and 21 MPa and reaching to 28.3 and 29.2 MPa, respectively. As a synthesis, the N-TMW hybrid alkaline made with SS+NaOH 8M gave the highest result at 7 days without further development for later ages, while the materials made with SS+KOH 10M presented the highest compressive strength at 28 days of curing.

3.2 Microstructure Fig. 4 show the microstructures of the hybrid alkaline N-TMW/GGBFS 90/10 at curing age of 28 days, studied with an Hitachi S-4800 Scanning Electron Microscope (SEM) in the Optical Centre of the Uni-versity of Beira Interior. All hybrid materials produced (Na-1, Na-2, K-1, K-2 and K-3) were studied. For the SEM analysis, specimens were mounted in epoxy, polished, and Au coated.

12th fib International PhD Symposium in Civil Engineering

150 Advanced materials

(a) MAH1 (b) MAH2

(c) MAH3 (d) MAH4

(e) MAH5

SEM images of N-TMW hybrid alkaline materials: (a) MAH1;(b) MAH2; (c) MAH3; (d) MAH4; (e) MAH5.

The main synthesis products that were found in the SEM image analysis were the (N,C)–A–S–H gels and C2ASH8 hydrates, coagulated around the N-TMW and GGBS particles [see point 1]. A gel with a high silicon and aluminium content and smaller amounts of calcium and sodium, which the authors believe might be a N–A–S–H gel containing a certain amount of calcium (N,C)–A–S–H is shown in point 2 [21], [22]. Some spongy particles are observed in the N-TMW hybrid materials made with all different alkaline activator solutions, and these spongy particles are considered to be gehlenite hydrates C2ASH8 [Point 3] [23]. The mixes made with 2:1 P/A ratio present more cracks comparing with the samples made with the 3:1 P/A ratio, which could be caused by the evaporation of the excess of acti-vator solution which in turn caused an high drying-shrinkage. Thus, the precursor/activator ratio can

Strength and microstructure of tungsten mining waste-based hybrid alkaline material: Effect of activators

Naim Sedira and João Castro-Gomes 151

have a major impact on the cracks formed in N-TMW hybrid alkaline materials. Nevertheless, much more cracks were observed in SS+NaOH than in SS + KOH specimens. This last specimen only shows small cracks that can be formed due to its rapid synthesis period. However, SS+NaOH alkaline activated slag cements suffer of such cracking behaviour as in the case of this study [16]. It was also noted that the compressive strength is related to the cracking degree found by SEM studies of the N-TMW hybrid alkaline materials.

4 Conclusions This research has reported experimental results and analyses on the effect of the type of activators, the activators molarity and P/A ratios on the hybrid alkaline binders based on GGBFS and N-TMW. The following conclusions can be drawn from the strength testing and SEM analyse:

! The N-TMW+GGBFS hybrid alkaline materials made with SS+NaOH 8M with 3:1 P/A ratio present higher compressive strength results in the first curing 7 days. However, the hybrid made with SS+KOH 10M with 3:1 P/A ratio gave lowest compressive strength results in the first 7 days. However, the same hybrid material (made with SS+KOH 10M with 3:1 P/A ratio) reached the highest compressive strength, at 28 days curing.

! The N-TMW+GGBFS hybrid (made with SS+KOH 10M with 3:1 P/A ratio), cured for 28 days yielded a compressive strength of 29.2 MPa, which can be compared with commercial Portland cements.

! The scanning electron microscopy study showed that the N-TMW+GGBFS hybrid alkaline ma-terial (made with SS+KOH 10M with 3:1 P/A ratio) has a fairly compact structure due to the presence of N-A-S-H and gehlenite hydrate, which are characteristic of this type of materials. In addition, small cracks form due to the rapid hydration/synthesis that occurs in these SS+KOH alkali-activated materials; this phenomenon must be controlled.

! The overall results of this study show that the type and the molarity of alkaline activators to-gether with the precursor/activator ratio has highly beneficial influence on N-TMW+GGBFS hybrid alkaline materials strength and microstructure development.

Acknowledgements This research was partially supported by European Commission under Horizon 2020, Marie Skłodow-ska-Curie Actions, Research and Innovation Staff Exchange (RISE), by REMINE “Reuse of Mining Waste into Innovative Geopolymeric-based Structural Panels, Precast, Ready Mixes and Insitu Appli-cations”. Project no 645696 (https://reminemsca.wordpress.com).

This work was also partially financed by Portuguese national funds through FCT - Foundation for Science and Technology, IP, within the research unit C-MADE, Centre of Materials and Building Tech-nologies (CIVE-Central covilhã-4082), University of Beira Interior, Portugal.

References [1] Andrew, R. M. 2017. “Global CO2 emissions from cement production,” Earth Syst. Sci. Data

Discuss., no. August, pp. 1–52. [2] Turner, L. K., and F. G. Collins. 2013. “Carbon dioxide equivalent (CO2-e) emissions: A com-

parison between geopolymer and OPC cement concrete,” Constr. Build. Mater., vol. 43, pp. 125–130.

[3] Garcia-Lodeiro, I., O. Maltseva, A. Palomo, and A. Fernandez-Jimenez. 2012. “Hybrid Alka-line Cements. Part I: Fundamentals,” Rev. Rom. Mater. J. Mater., vol. 42, no. 4, pp. 330–335, 2012.

[4] Law, D. W., A. A. Adam, T. K. Molyneaux, and I. Patnaikuni. “Durability assessment of alkali activated slag (AAS) concrete,” Mater. Struct., vol. 45, no. 9, pp. 1425–1437.

[5] Yang, K. H., J. K. Song, A. F. Ashour, and E. T. Lee. 2008. “Properties of cementless mortars activated by sodium silicate,” Constr. Build. Mater., vol. 22, no. 9, pp. 1981–1989.

[6] Palomo, A., O. Maltseva, I. Garcia-Lodeiro, and A. Fernandez-Jimenez. 2013. “Hybrid Alka-line Cements. Part II: The Clinker Factor,” Rev. Rom. Mater. J. Mater., vol. 43, no. 1, pp. 74–80.

[7] Puertas, F., M. Palacios, and T. Vázquez. 2006. “Carbonation process of alkali-activated slag mortars,” J. Mater. Sci., vol. 41, no. 10, pp. 3071–3082.

12th fib International PhD Symposium in Civil Engineering

152 Advanced materials

[8] Puertas, F. et al. 2011. “Alkali-Activation of Slag Cements: Activation Process, Microstructure and Mechanical Properties,” 13th Int. Congr. Chem. Cem., no. July 2011, pp. 1–7.

[9] Fernandez-Jimenez, A., E. Flores, O. Maltseva, I. Garcia-Lodeiro, and A. Palomo. 2013. “Hyb-rid Alkaline Cements. Part III. Durability and Industrial Applications,” Rev. Rom. Mater. J. Mater., vol. 43, no. 2, pp. 195–200.

[10] Sedira, N., J. Castro-Gomes, G. Kastiukas, X. Zhou, and A. Vargas. 2017. “A review on mi-neral waste for chemical-activated binders: mineralogical and chemical characteristics,” Min. Sci., vol. 24, pp. 29–58.

[11] Castro-Gomes, J. P., A. P. Silva, R. P. Cano, J. Durán Suarez, and A. Albuquerque. 2012. “Po-tential for reuse of tungsten mining waste-rock in technical-artistic value added products,” J. Clean. Prod., vol. 25, pp. 34–41.

[12] Sedira, N., J. Castro-Gomes, and M. Magrinho. 2018. “Red clay brick and tungsten mud waste based alkali-activated binder: microstructural and mechanical properties,” Constr. Build. Mater. (submitted for publication).

[13] Sedira, N., and J. Castro-Gomes. 2017. “Effects of EAF-S on alkali-activation of tungsten mi-ning waste: mechanical properties,” in REMINE - International Conference & Brokerage Event (RICON17) - UBI, Covilha, Portugal.

[14] Castro-Gomes, J. et al. 2017. “Alkali-activation of tungsten mining waste mud blended with waste glass: reactivity, performance and innovative applications,” in International Congress on Engineering UBI, Covilha, Portugal.

[15] Sedira, N., and J. Castro-Gomes. 2018. “Study of an alkali-activated binder based on tungsten mining mud and brick powder waste,” in 8th Scientific-Technical Conference on Material Problems in Civil Engineering, Kraków, Poland (submitted for publication).

[16] Angulo-Ramírez, D. E., R. Mejía de Gutiérrez, and F. Puertas. 2017. “Alkali-activated Portland blast-furnace slag cement: Mechanical properties and hydration,” Constr. Build. Mater., vol. 140, pp. 119–128.

[17] Garcia-Lodeiro, I., A. Fernandez-Jimenez, and A. Palomo. 2013. “Hydration kinetics in hybrid binders: Early reaction stages,” Cem. Concr. Compos., vol. 39, pp. 82–92.

[18] Pacheco-Torgal, F., J. Castro-Gomes, and S. Jalali. 2009. “Tungsten mine waste geopolymeric binder: Preliminary hydration products investigations,” Constr. Build. Mater., vol. 23, pp. 200–209.

[19] Tsuyuki, N., and K. Koizumi. 1999. “Granularity and Surface Structure of Ground Granulated Blast-Furnace Slags,” J. Am. Ceram. Soc., vol. 82, no. 8, pp. 2188–2192.

[20] Schmidt, R., and R. Eisenhower. 2008. “Lab Report XRD 65: Quantitative phase analysis of Blast Furnace Slag Cements,” Bruker AXS Lab.

[21] Garcıa-Lodeiro, I., A. Fernandez-Jime, A. Palomo, and D. E. Macphee. 2010. “Effect of Cal-cium Additions on N–A–S–H Cementitious Gels ´,” Am. Ceram. Soc., vol. 1940, pp. 1934–1940.

[22] García-Lodeiro, I., and A. Palomo. 2013. “Variation in hybrid cements over time. Alkaline activation of fl y ash – portland cement blends,” Cem. Concr. Res., vol. 52, pp. 112–122.

[23] Yang, K. H., A. R. Cho, J. K. Song, and S. H. Nam. 2012. “Hydration products and strength development of calcium hydroxide-based alkali-activated slag mortars,” Constr. Build. Mater., vol. 29, pp. 410–419.