Characterization of Fly Ash for the Production of ... of Fly Ash for the Production of Geopolymer...

ASTM Project Grant

© 2015 ASTM International

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Characterization of Fly Ash for the Production

of Geopolymer Cements

Juan Pablo Gevaudan1* and Wil V. Srubar III, Ph.D.1

1 Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder. 1111 Engineering

Drive, ECOT 441 UCB 428, Boulder, Colorado USA 80309. *Corresponding Author: [email protected]

Abstract: Current ASTM standards can be used to provide a degree of characterization of geopolymeric materials,

yet the researcher must be aware of differences between hydraulic and geopolymer cementitious materials. The

current study assessed the applicability of available ASTM standards that analyze the insoluble content and oxide

content of cementitious materials (ASTM C114 and C311), compressive strength (ASTM C109), leaching (ASTM

C1308), and setting time (ASTM C403). It was found that the speciation of sulfur is important in fly ash precursors

and can be determined using ASTM C114’s determination of sulfur trioxide content. The speciation of sulfur,

unburned carbon content and particle size are important parameters for the characterization of a geopolymer

precursor. Furthermore, the dissolution of geopolymer precursors should be assessed at appropriate basic pH levels

to determine the content of insoluble materials. The curing conditions and resulting geopolymer microstructure must

be characterized (via FT-IR) and regulated to achieve consistency across research studies on geopolymer cements.

1. Introduction

Fly ash is a high-volume material that poses hazardous risks for human health and the

environment. Over 30 million tons of coal fly ash are disposed annually in the United States,

making it one of the largest industrial waste streams in the nation [1]. While high-quality fly ash

can be deemed usable for conventional concrete applications, low-quality ashes are not

considered for use because they do not comply with physical or chemical standards for fly ash

(set by ASTM C618) [2]. For example, a sulfur trioxide or unburned carbon content of more than

5-6% (by weight) renders fly ash unusable for concrete applications. The Clean Air Act has

promoted scrubbing and clean-burning technologies to reduce nitrogen oxide (NOx) and sulfur

oxide (SOx) emissions from coal power plants. These new technologies increase sulfur, unburned

carbon, and other chemical components, which has resulted in an increase of fly ash that is

unsuitable for use in concrete production [3].

Low-quality fly ash, which is commonly disposed in landfills and containment ponds, is a

significant health and environmental hazard. The Environmental Protection Agency (EPA)

estimates that 300 landfills and 700 ash ponds exist throughout the United States [4]. These

structures contain fly ash particles, which, due to their high concentrations of adsorbed heavy

metals, can affect the health of nearby communities. Dwivedi, et al. [5] demonstrated that

adsorbed heavy metals in fly ash (e.g., chromium, nickel, selenium) are potential carcinogens.

Studies have also shown that fly ash causes adverse health effects for birds and other wildlife

that nest around coal ash ponds [6]. Furthermore, coal ash spills or groundwater seepages have

the potential to release high levels of heavy metals into the surrounding environment, and

recovery can be costly.

mailto:[email protected]

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Geopolymer Cements are novel alternative cements to traditional hydraulic cements.

Geopolymers are formed first by the dissolution of aluminosilicate materials (i.e., fly ash) into

alumina and silica monomers and dimers [7]. These molecular structures then polycondensate to

form oligomers, or chains, of polymers. The newly formed aluminosilicate geopolymers are

characterized by a highly crosslinked three-dimensional chemical gel structure of sodium,

aluminum, silicon, and water (NASH) [8].

While existing ASTM standards are routinely employed to characterize the physical and

mechanical properties of geopolymer precursors and cements, these standards were originally

developed for hydraulic cements. Portland cement’s hydration reactions exhibit a fundamental

different chemistry than geopolymer cement’s polycondensation reaction. Thus, the standards

that govern the characterization of hydraulic cements are likely inappropriate for geopolymer

cement precursors. The lack of awareness regarding the differences in chemical reactions has

produced inconsistent research results. For example, studies have employed high-humidity

curing for geopolymer cement samples, similar to portland cement research, prior to compressive

(ASTM C39) and tensile (ASTM C496) testing [9-11]. Yet, a high-humidity environment

promotes further dissolution of the geopolymer aluminosilicate precursor, which yields a

difference in NASH gel structure and content for each geopolymer sample [8]. Such variations

produce inconsistent research data across the field of study for geopolymeric materials. To

address this inconsistency, this project assessed the appropriateness of current ASTM standards

that were formulated for hydraulic cementitious materials to characterize and evaluate

geopolymer precursors (i.e., fly ash), cements, and mortars.

2. Materials and Experimental Methods

2.1 Raw materials

Fly ash samples were collected at a local Boulder power plant operated by Xcel Energy. The

specific coal used in the power plant is Colorado bituminous coal sourced from both the

Colorado West Elk Mine and the Carbondale Mine. The fly ash was categorized as Class F or

Class C based on ASTM C618. Sodium silicate (NaSi) solution (reagent grade) was obtained

from Sigma Aldrich. The NaSi contained 10.6% (Na2O), 26.5% (SiO2), and 62.9% (water), as

reported by Sigma Aldrich and verified by Inductively Coupled Plasma Optical Emission

Spectrometry (ICP-OES). The density of the NaSi solution was 1.39 g/ml. Potassium hydroxide

(KOH) flakes (reagent grade, 90% assay) were also supplied by Sigma Aldrich. Deionized (DI)

water was used throughout the experiments.

2.2 Chemical and physical parameters of fly ash

Chemical parameters, including insoluble (and reactive) content and sulfur trioxide content, were

determined according to ASTM C114. An alternate method by Chen-Tan & Van Riessen [12]

was used to evaluate the insoluble content (and reactive content) of fly ash in a basic medium. In

the latter method, 5 g of fly ash were dissolved in a 10M KOH solution for various amounts of

time. The liquids and solids were separated through a vacuum filtration set-up with several DI

water washes, totaling approximately 800 mL of water per filtration. Chemical oxide content was

determined via ICP-OES using an ARL 3410+ with an AIM-1250 sampler attachment. A

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CETAC U-6000AT+ ultrasonic nebulizer/membrane desolvator was used to lower the detection

limits of most elements while ensuring the plasma was not extinguished from solvent vapors.

The moisture content and loss-on-ignition (unburned carbon) were measured according to ASTM

C311 using a F6000 Thermo Scientific Thermolyne high-temperature furnace (Series 1249).

Particle size was measured using a Malvern Mastersizer 3000 laser diffraction system.

2.3 Geopolymer mix design methodology

Geopolymer cements were produced via a modified method previous reported in [13]. In this

method, key parameters for the design of geopolymer cements include reactive silica, water, and

sodium oxide contents. The method was modified by replacing sodium hydroxide with KOH to

reduce thermal shrinkage in the mixes and to potentially increase strength gain, as reported in the

literature [14,15]. The calculated proportion for sodium hydroxide was used to calculate the

required amount of potassium hydroxide. The mix design parameters called for the following

percentages by weight: 64% (Fly Ash), 14% (NaSi), and 22% (KOH). Amount of water added

was dependent on fly ash to maintain a solid-to-liquid ratio of 1.13 resulting in a workable mix.

The KOH and fly ash were incorporated as solids, while the only liquids in the mixture were the

NaSi solution and water. This process allowed for harnessing the heat from the exothermic

reaction that occurs to help increase the solubility of the silica and alumina in the fly ash.

2.4 Geopolymer mixture proportioning, size and curing of samples

Geopolymer cement mortar specimens were produced for all leaching (ASTM C1308),

compressive strength (ASTM C109), and setting time (ASTM C403) experiments. Table 1

summarizes the mixture proportioning used to produce all samples. Sizes for all specimens were

determined in accordance to the aforementioned standards. Geopolymer mortar cylinders

(diameter: 27.2 mm; height: 30.4 mm) were used for the leaching tests. Samples were cured for

one day at ambient conditions (temperature of 22.5°C and low ambient relative humidity of

26%). Cubic mortar samples for compressive strength tests were 5 cm2 in dimension. All

samples were cured in a high-humidity chamber (>90%) and temperature of 25°C for 7 days

prior to testing. A geopolymer cylinder sample was cast for the setting time test with a diameter

of 15.2 cm and height of 20.3 cm. The sample was cured at ambient conditions and then used to

perform FT-IR experiments.

2.4 Geopolymer fresh- and hardened-state properties

A mortar penetrometer supplied by Hogentogler & Co. Inc, was used to perform setting time

tests. Needles of diameter 1.4 cm (0.56 in.), 0.91 cm (0.36 in.), and 0.66 cm (0.26 in.) were used.

An Instron Universal Testing Machine with a 120 kip load cell was used for compression testing.

The tests were force-controlled using a load rate of 18 kips/min. For the leaching tests, samples

were placed in 709 mL polypropylene (PP) containers, which were selected by verifying a liquid

water weight loss of less than 1% over a 24-hour period. Circular (46 mm diameter) support

screen stands for the geopolymer samples were purchased from Savillex (Part #730-0046). The

crosslinking and, thus, quality of the geopolymer NASH gel was investigated using a Nicolet

6700 Fourier Transform-Infrared (FT-IR) Spectroscopy machine. Data was collected using 32

scans, a resolution of 4 cm-1, and a data spacing value of 1.928 cm-1. No data correction was

employed in the analysis.

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3. Experimental Results

3.1 Insoluble content of raw fly ash

Figure 1 shows the results of the insoluble content analysis of raw fly ash according to the

ASTM C114 method as compared to the methodology outlined by Chen-Tan & Van Riessen

[12]. The insoluble content methodology outlined by ASTM C114 yielded an insoluble content

of 55.5%, which corresponds to a soluble content of 44.5%. The insoluble content methodology

outlined by Chen-Tan & Van Riessen [12] yielded a maximum insoluble content of 77% after 24

hours, which corresponds to a maximum soluble content of 33%. The non-linear, time-dependent

dissolution behavior of the Chen-Tan & Van Riessen methodology is shown in Figure 1, along

with the single-value solubility obtained from the ASTM C114 method.

The higher soluble content observed with the ASTM methodology can be attributed to the use of

both acid (5 mL hydrochloric acid, reagent grade) and base (100 mL sodium hydroxide solution,

0.25 M). In addition, the use of hot solutions and digestion times aids in the dissolution of silica

and other solid phases. In contrast, the Chen-Tan and Van Riessen methodology does not employ

heat additions or acids and only uses an unheated 10M KOH solution, which is more

representative of conditions in fly ash geopolymerization.

Figure 1. Dissolution of fly ash using two methodologies – ASTM C114, which yields a single-value solubility, and

a time-dependent methodology outlined in Chen-Tan & Van Riessen [12].

The insoluble content determined by ASTM C114 is mainly designed for hydraulic (portland)

cement analyses, which have high calcium contents. The procedure uses hydrochloric acid,

which aids in the dissolution of any geopolymer precursor. Yet, in geopolymerization, the system

is highly basic. Thus, the dissolution of the geopolymer precursor is highly dependent on a high

pH and the resulting complexation reactions. More plainly, in geopolymer synthesis, the

aluminosilicates are never introduced (and therefore never dissolve) in low-pH environment.

This difference can be seen in the result achieved by the ASTM standard (55.5% insolubility)

compared to a 77% insoluble content of the highly basic environment. Therefore, for geopolymer

precursors, the insoluble content should only be determined at more appropriate (basic)

conditions rather than both as prescribed by the standard.

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3.2 Oxide content of raw fly ash

Table 1 summarizes the results from the oxide content analysis obtained using an instrumental

method (ICP-OES). Results demonstrate a high sulfur oxide content (8.0% by weight) and a

significant calcium oxide content (>10% by weight). Not taking into account unreactive material,

fly ash has a silicon-to-aluminum ratio of 2.1:1. According to ASTM C114, section 6.1.2, “when

using an instrumental test method that determines total sulfur, sulfide sulfur can be determined

with sulfate and included as such.” Thus, by determining the sulfur trioxide content with ASTM

C114 reference test method, it was possible to detect the sulfide and sulfate content present.

Several laboratory trials resulted in an average sulfur trioxide content of 1.59%, which yielded a

sulfide content of 2.56% by weight. The remaining proportion is attributable to sulfates. The

high content of sulfur present in the fly ash is most likely due to the low-pollution limits set by

Xcel Energy on the Boulder power plant. The scrubbing technology used is a lime spray dryer

manufactured by Babcok & Wilcox. Such a process offers high SO2 removal (90-95% efficiency)

and reductions in SO3 emissions (>95% efficiency).

Table 1. Oxide content for fly ash as determined by ICP-OES.

SiO2 Al2O3 CaO SO Fe2O3 K2O Na2O MgO

47.5% 22.6% 12.3% 8.0% 2.8% 1.2% 1.1% 1.1%

3.3 Moisture content and loss-on-ignition of raw fly ash

Moisture content and loss-on-ignition (LOI) are important parameters for the classification of fly

ash by ASTM C618. These properties, which were determined by ASTM C311, are presented in

Figure 2. The unburned carbon content (loss-on-ignition) level of 4.25% by weight can be

attributed to the relatively low burning temperatures of the Boulder power plant, thus resulting in

remnants of carbon from coal in the fly ash. The low moisture content (1.13% by weight) can be

attributed to the lack of exposure to water of the collected fly ash.

Figure 2. Moisture and unburned carbon content as determined by ASTM C311.

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While the LOI content is lower than the limit specified by ASTM C618 (6%), any unburned

carbon absorbs alkalis which could negatively affect geopolymerization reactions by reducing

the precursor’s extent of reaction [16]. Relatedly, the moisture content – regardless if it is low or

high – is also a necessary parameter to determine the total water required for the geopolymer mix

design. Thus, in assessing the applicability of ASTM C618, it was found that this standard is

useful for determining both of these important properties of geopolymer precursors.

3.4 Particle size distribution of fly ash

Particle sizes were obtained using a Malvern Mastersizer 3000 laser diffractometer. The majority

of particles (about 66.8%) were 35 microns and below in size, while 33.2% were larger than 35

microns. High reactivity and high degree of micro-filling effect in concrete-like materials can be

expected for sub-35 micron particles [17]. Particle size affects the reactivity of fly ash and,

consequently, higher reactivity of fly ash can be expected with fine-particle ashes. This reactivity

is important whether fly ash is used as a supplementary cementitious material with hydraulic

cements or to create geopolymer cements. Particle size is also important as finer sizes (about 10

microns) can act as micro-fillers in a concrete matrix, which improves the durability of the

material by densifying the paste [18]. Given the results from this particle size analysis, in

conjunction with oxide and moisture contents, the fly ash investigated herein can be classified as

a Class F pozzolanic fly ash in accordance with ASTM C618. In terms of the standard’s

applicability to geopolymers, ASTM C618 does identify parameters of geopolymer precursors

that are useful for determining its suitability for geopolymerization. An in-depth understanding

of the oxide composition (mainly sulfur speciation and calcium content) and particle size

distribution aids in the overall determination of the precursor’s geopolymerization reactivity.

3.5 Fourier Transform Infrared spectroscopy (FTIR) analysis of fly ash and geopolymer cements

Figure 3 demonstrates the change in absorbance level and shifts in energy in fly ash before and

after geopolymerization. Geopolymer gels are inherently more amorphous than fly ash. Thus, the

structured phases in fly ash are expected to decrease as these are dissolved and polymerized to

form a geopolymer gel. In Figure 3 it can be seen that the geopolymerization of fly ash results in

a less-ordered structure as is evident by (a) a decreased absorbance of the material and (b)

diminished appearance of peaks. According to a study by M. Criado, et al. [19], a peak of 996

cm-1 corresponds to the vitreous (glassy) phase of the raw fly ash. This peak is apparent in the

original fly ash signal in Figure 3. After geopolymerization, this peak is shifted and reduced to a

broader peak located at 970 cm-1. Such shift can be attributed to the dissolution of raw fly ash

and the possible formation of a geopolymer gel. The peak at 773 cm-1 in the fly ash signal is

possibly related to a ring silicate structure that is deconstructed after geopolymerization [20].

Moreover, the peaks located between 1100-900 cm-1 are associated with the stretching vibrations

of both bridging Si-O(Si) and terminal Si-O bonds. The bands appearing at higher wavenumbers

of this range can be attributed to bridging Si-O(Si) vibrations [21]. Since the broad peak at 970

cm-1 extends from about 1100 to 900, it can be inferred that the peak at 970 cm-1 symbolizes

stretching vibrations of a terminal Si-O bond. These data signify relatively low levels of cross-

linking. The degree of cross-linking is controlled by the amount of reactive alumina, as it allows

for the formation of bridging bonds. Thus, the alumina content available in this particular sample

of fly ash may be insufficient to enable a higher degree of crosslinking.

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Figure 3. Fourier Transform Infrared (FT-IR) spectroscopy signal for both fly ash and fly ash-based geopolymer.

Currently there are no ASTM standards that characterize the degree of crosslinking (or extent of

reaction) for a geopolymer cement, where a high degree of crosslinking (approaching a

wavenumber of 1100 cm-1) corresponds to a high degree of geopolymerization. Verifying the

geopolymerization extent via FT-IR is a viable method to understand the precursor’s degree of

utilization to produce the alternative cement. Such analysis should be implemented as a common

practice to determine the extent of geopolymerization of any geopolymer sample.

3.5 Leaching tests of geopolymer cements

The disposal and landfilling of fly ash is considered an environmental hazard due to the content

of heavy metals in fly ash and their propensity to leach into the environment. Figure 4 shows the

leaching behavior of heavy metals and other elements of concern (i.e., chromium, potassium,

sulfur, arsenic). As determined from this analysis, the geopolymer material completely

immobilized metals such as lead, zinc, aluminum, magnesium, phosphorous and iron. These

compounds, although found in the geopolymer material, were not present in the leachate

(surrounding water of the leaching test). Moreover, elements such as calcium, silicon, copper,

strontium, and barium had very low leaching (with a cumulative fraction leached of less than

0.16%). Potassium and sulfur are the two most leached elements from of this geopolymer matrix.

Both elements have a cumulative fraction leached greater than 8%.

The data demonstrate that sulfur phases are highly soluble at neutral pH (DI water) and that

sulfur is not being incorporated into the geopolymer gel matrix. Potassium was the second

highest leached element in the geopolymer material. Such behavior is expected as the FT-IR data

demonstrates a low degree of crosslinking, which can possibly allow ions to have higher degrees

of mobility within the internal structure of the geopolymer sample. Overall, however, in

assessing the applicability of ASTM C1308 to geopolymer precursors and cements, it was found

that the standard is appropriate in determining the leaching properties of geopolymer samples.

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Higher degree of crosslinking in

geopolymer ~1100 cm-1

Lower degree of crosslinking in

geopolymer ~900 cm-1

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Figure 4. Leaching behavior for a fly ash geopolymer cement sample.

3.6 Setting time

Figure 5 shows the results of the setting time experiment for the selected fly ash geopolymer

mix. Due to the polycondensation reaction of geopolymer gels, setting of the material is due

primarily to water release and further condensation of the geopolymer. Collected results

demonstrate geopolymer strength gain in a low relative humidity environment.

Figure 5. Setting time of a fly ash geopolymer cement after 42 hours of testing.

Setting time of the material was delayed, and no final set (using 4000 psi as the standard) was

observed in a 42-hour period. Such behavior can be due to a low rate of water expulsion, since

the geopolymer paste was cured at ambient conditions. The setting-time test demonstrates the

importance of heat-curing of geopolymers, which may be required to achieve final set within 6.5

hours – the set-time standard of normal hydraulic (portland) cement concrete.

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As written, the standard is applicable to demonstrate the setting time of a fresh-state geopolymer

mortar sample. Yet, the authors suggest modifying the standard to avoid the covering of the

sample with a plastic cover as this affects the evaporation of water entrained in the geopolymer.

3.6 Compressive strength

Similarly to setting time, the curing (or water content) of the geopolymeric material was found to

affect the compressive strength. In trials with samples cured for 7 days in a high humidity

chamber (above 90% RH), compressive strengths reached an average of only 239 psi. For

samples that were cured for three days in a high humidity chamber and then four days at low

humidity (about 20% RH), compressive strength increased to an average of 405 psi. Thus, the

results show that curing in a low humidity environment improved the compressive strength by

allowing the material to expel a greater amount of water and contribute to the hardening

(polycondensation) of the material. Notably, this early strength gain is slower than the setting

time tests. Thus, the rate of water expulsion is a significant mechanism for early strength gain or

setting in geopolymer mortars. Regardless of the strength measurements, the current ASTM

standard, as written, adequately assesses compressive strength, yet dry curing conditions are

crucial to ensure proper early-strength gain and a consistent NASH gel formation.

4. Discussion: Applicability of ASTM Standards for Geopolymer Materials

Current ASTM standards can be used to provide a degree of characterization of geopolymeric

precursor materials, yet the researcher must be aware of differences between hydraulic and

geopolymer cementitious materials. The insoluble content determined in ASTM C114 is not

completely valid for geopolymer precursors, because the dissolution of a geopolymer precursor

is highly dependent on a high pH associated with the alkali-activating solution. In geopolymer

synthesis, the aluminosilicates never dissolve in low-pH environment. Thus, geopolymer

precursors’ insoluble content should only be determined at more appropriate (basic) conditions

rather than both as prescribed by the ASTM C114. The standard holds, however, in determining

the total sulfur content, sulfur trioxide content, and speciation of sulfur.

Another point of consideration for researchers is that ASTM standards developed for hydraulic

(portland) cements test in accordance to the hydration reaction characteristics of these hydraulic

cements. While portland cement must be cured in the presence of water (wet environments),

geopolymer cements must be cured in the absence of water (dry environments). Consequently,

the recommendations of the standards concerning curing must be discarded and modified when

dealing with geopolymer cements. For example, when calculating the setting time (ASTM C403)

of a geopolymer cement material, plastic or saturated fabric covering on the material or high-

humidity chambers should not be used, as it will delay the release of water from the material.

The results from this research also elucidated findings based on the applicability of leaching

(ASTM C1308), compressive strength testing (ASTM C109), setting time (ASTM C403), and

degree of crosslinking standards. ASTM C1308, ASTM C109, and ASTM C403 yielded valid

results for geopolymer cements, thus the authors suggest no modification to those standards other

than having them be prescribed for the characterization of both hydraulic and geopolymer

cements. However, given the nature of the geopolymerization reactions, the authors suggest that

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standards be developed to characterize extent of geopolymerization via FT-IR. The development

of a standard concerning the curing and extent of geopolymerization of geopolymer cements can

be of value for continued research and application of geopolymeric materials.

5. Conclusion

Fly ash is a high-volume waste material that poses risks to the environment and public health.

One alternative to landfilling fly ash is to create fly ash-based geopolymers – geosynthetic

material alternatives to portland cement concrete. While ASTM standards have been used to

classify cement and fly ash for portland cement concrete applications, current ASTM standards

do not consider the differences between the chemistry of geopolymerization reactions and the

chemistry of hydraulic (portland) cement hydration. This study analyzed the applicability of

current ASTM standards to adequately address the physical, chemical, and mechanical

characterization of fly ash-based geopolymer precursors and geopolymer cements. The results

show that the dissolution of geopolymer precursors should be performed at appropriate basic pH

levels. In addition, ASTM standards C1308, C109, and C403 that govern hydraulic cements are

applicable to the characterization of geopolymer precursors and cements. However, the authors

suggest that geopolymer curing conditions and analysis of the resulting geopolymer

microstructure (e.g., degree of crosslinking) be standardized to achieve consistency across

research studies and applications of geopolymer cements.

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