Effects of Anions on the Formation of Aluminosilicate Gel in

Impact of sulfate and nitrate incorporation on potassium- and sodium-based geopolymers: geopolymerization and materials properties. Charlene Desbats-L...
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Ind. Eng. Chem. Res. 2002, 41, 4550-4558

APPLIED CHEMISTRY Effects of Anions on the Formation of Aluminosilicate Gel in Geopolymers W. K. W. Lee and J. S. J van Deventer* Department of Chemical Engineering, The University of Melbourne, Victoria, 3010, Australia

This work demonstrates that an increase in soluble silicate concentration over ∼200 mM in an aqueous alkaline solution (pH ) ∼13.95) can significantly increase the reactivity of a Class F fly ash at room temperature. The greatly increased dissolution of the fly ash primary phase(s) and the subsequent secondary precipitation are responsible for the formation of an aluminosilicate gel similar to the major binding phase of a geopolymer. On the basis of this observation, a reaction model was developed to simulate aluminosilicate gel formation in a real geopolymeric system. In this work, potassium salts of chloride, carbonate, oxalate, and phosphate (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) were used as solution contaminants. It was found that the anions (Cl-, CO32-, C2O42-, and PO43-) significantly affected the formation kinetics as well as the nature of the aluminosilicate gel formed in the reaction model. This observation could be successfully used to mechanistically explain the retardation effects of the various salts on the gel solidification time of a geopolymer. Aluminosilicate crystallization in the chloride-affected and the oxalateaffected reaction models implies that similar processes could also occur in geopolymers. This could affect the chemical stability and durability of geopolymers and thoroughly deserves a careful future investigation. 1. Introduction Alkali activation on recycled industrial aluminosilicate wastes, such as fly ash and blast furnace slag, can be used to synthesize geopolymers of excellent physical and chemical properties suitable for construction purposes.1-4 To achieve desirable mature product strength, high concentrations of soluble silicates are often added to the “activating” solutions of high alkalinity. This has created a major problem. The solidification time of the aluminosilicate gels (or geopolymeric gels) is too short to suit most of the practical needs. Controlling the gel solidification time, therefore, is of paramount importance for the commercialization of this new emerging material. As numerous previous studies have indicated,2,5 geopolymeric gel formation is initialized by mineral dissolution of the solid starting raw materials. Increasing solution ionic strength through inorganic salt addition, which is known to enhance or exert no effects on mineral dissolution of silicates or aluminosilicates,6-8 should be expected to accelerate or have no effects on the early geopolymeric gel formation. This, however, was not observed experimentally, as dissolved sodium or potassium chloride salts in the activating solutions were found to retard geopolymeric gel solidification.9,10 Clearly, the rate of geopolymeric gel solidification is not controlled by the kinetics of the mineral dissolution alone. * To whom correspondence should be addressed. Phone: +61-3-83446620. Fax: +61-3-83444153. E-mail: jannie@ unimelb.edu.au.

In this study, simple salts of KCl, K2CO3, K2C2O4‚H2O, and K2HPO4 were used as solution contaminants in search for an effective geopolymeric gel solidification retarder. Through the use of a reaction model, these salts could also act as reaction probes to better understand the mechanism of gel solidification processes in a geopolymer. The implications of the effects of the salt contamination on the nature of the secondary precipitated products (the aluminosilicate gel) to geopolymer synthesis will also be discussed. 2. Experimental Methods 2.1. Materials. Coal-origin fly ash as supplied by Queensland Cement Limited (QCL) was obtained from Gladstone in Queensland, Australia. Fusion analysis using a Siemens SRS3000 sequential X-ray fluorescence (XRF) spectrometer was used to determine the mass composition. The fly ash typically contains 50.01% SiO2, 27.97% Al2O3, 11.95% Fe2O3, 1.30% TiO2, 0.19% MnO, 3.48% CaO, 1.32% MgO, 0.69% K2O, 0.19% Na2O, 1.01% P2O5, 0.50% SO3, and combustibles. The major crystalline constituents of the fly ash as determined by X-ray diffraction (XRD) analysis are R-quartz (SiO2), mullite (3Al2O3‚SiO2), hematite (Fe2O3), magnetite (Fe3O4), lime (CaO), and gypsum (CaSO4‚2H2O). The Gladstone fly ash, based on the relatively low calcium content, was classified as Class F according to the American Society for Testing and Materials (ASTM) definition. The Brunauer-Emmett-Teller (BET) surface area, as determined by nitrogen adsorption on a Micromeritics ASAP2000 instrument, was 4.13 m2/g and the mean particle size (d50) was 14.1 µm according to the supplier.

10.1021/ie0109410 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/30/2002

Ind. Eng. Chem. Res., Vol. 41, No. 18, 2002 4551 Table 1. Calculated Molar Compositions of the Various Leaching and Alkali-Activating Solutions system

anion

Aa Ba Ca Da Ea Fa Ga Ha Ia Ja Ka La Ma Geo-1b Geo-2b Geo-3b Geo-4b Geo-5b

ClCO32(COO)22HPO42-

ClCO32C2O42HPO42ClCO32C2O42HPO42-

[SiO2] (mM)

SiO2/M2Oc

0 0 0 0 0 14.24 28.48 213.60 569.60 569.60 569.60 569.60 569.60 1000.00 1000.00 1000.00 1000.00 1000.00

0.0237 0.0475 0.356 0.949 0.949 0.949 0.949 0.949 0.250 0.250 0.250 0.250 0.250

a The anion concentration used in all of the leaching solutions was 80 mM with pH ) 13.95. b The anion concentration used in all the activating solutions was 400 mM with [OH-] ) ∼ 10 M. c M ) Na and K with molar Na/K ) 1.50.

Sodium silicate solution (Vitrosol N(N40), molar ratio SiO2/Na2O ) Rm ) 3.32, [SiO2] ) 6.63 M) was obtained from PQ Australia. Laboratory-grade reagents (NaOH, KOH, KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) were obtained from Ajax Chemicals, Australia. Distilled/ deionized water was used for all dilutions. 2.2. Geopolymer Synthesis. Sodium/potassium silicate solution (molar SiO2/M2O ) 0.25, M ) Na and K with molar Na/K ) 1.50, [SiO2] ) 1.00 M, [OH-] ) ∼10 M) was prepared using calculated amounts of distilled H2O, NaOH, KOH, and the sodium silicate solution (Vitrosol N(N40)). The solution was allowed to cool to room-temperature overnight. Calculated amounts of the potassium salts (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) were then dissolved in the sodium/potassium silicate solution (molar SiO2/M2O ) 0.25, molar Na/K ) 1.5, [SiO2] ) 1.00 M) (See Table 1). The resultant solutions (activating solutions) were allowed to equilibrate with rigorous stirring for 2 h before the geopolymer synthesis. After placing 500 g of fly ash in a mixing bowl, 250 g of the activating solution was added. The contents of the mixing bowl were mixed in a mechanical mixer for 5 min to produce homogeneous pastes. These pastes were then transferred to plastic containers, which were alkali resistant and were cured at 20 ( 3 °C and a humidity of 50% ( 5% under atmospheric pressure. 2.3. Geopolymer Characterization. The yield stress of the early pastes before solidification was determined on a Haake vane rheometer attached with vane C. An arbitrary gel solidification time was set at a yield stress of 5 kPa where the gel was reasonably hard and was unable to be molded thereafter. Compressive strength tests were obtained conforming to ASTM C39 on an ELE compression machine using a pace rate of 1 kN/s. The cylindrical sample dimension was 50 mm × 100 mm. All of the values collected were the averages of three separate tests, with a standard deviation of less than 5%. 2.4. Leaching Experiments. Leaching was conducted in search for a reaction model that simulated real geopolymer synthesis. Fly ash was dried at 105 °C overnight before leaching. Polyethylene bottles and

conical flasks were used for all solution preparations and leaching to avoid silicon contamination. Stock solutions of sodium/potassium silicate (molar ratio SiO2/ M2O ) 3.00, M ) Na and K with molar ratio Na/K ) 1.50, [SiO2] ) 5.00 M), 0.6 M MOH (molar ratio Na/K ) 1.50), 15 M MOH (molar ratio Na/K ) 1.50) were prepared separately, sealed to avoid carbonation, and left to cool to room-temperature overnight. Calculated amounts of the sodium/potassium silicate stock solution (molar ratio SiO2/M2O ) 3.00, [SiO2] ) 5.00 M) and the potassium salts (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) were dissolved in 800 mL of the 0.6 M MOH solution (molar ratio Na/K ) 1.50) in a 1 L conical flask with vigorous agitation and then made up to 1000 mL. The pH was adjusted to 13.95 with vigorous stirring using the 15 M MOH stock solution (molar ratio Na/K ) 1.50). The solution prepared this way is termed leaching solution whose solution composition is shown in Table 1. The leaching solution was sealed and stirred at ∼90 rpm for 2 h prior to leaching. For each leaching experiment, 100 g of fly ash was added to 1000 mL of the leaching solution. The slurries were stirred at 20 ( 0.5 °C with minimum agitation (150 rpm) to prevent sedimentation. The pH of the slurries was measured throughout the leaching experiments, using a pH meter corrected for highly alkaline solutions. It was found that the pH of all of the leaching slurries remained very stable at pH 13.93 ∼ 13.98. No additional buffering was required. At the designated time intervals, 10 mL of the suspension was collected and centrifuged at low speed for 5 min to separate about 95% of the solids from the solution. A 0.2-µm Minisart membrane filter from Sartorius AG, Germany, was used to remove the remaining solids in the supernatant solution. The solids obtained from the centrifugation and the filtration procedures were collected for analysis as described here and are termed leached solids. The filtrate, or the leached solution, was further diluted if required so that the solution of approximately 400 ppm Si was allowed to equilibrate for 2 days in sealed polyethylene tubes before analysis. No precipitates and no visible colloidal particulates were found in any of the diluted solutions, even after 2 weeks. Note that the diluted solutions analyzed in this work should include all soluble silicates of monomeric and oligomeric silicates and may contain colloidal polysilicates as well. This paper does not intend to differentiate the speciation of soluble silicates in the leaching and the leached solutions. 2.5. Elemental Composition of Leached Solutions. Inductively coupled plasma equipped with optical emission spectroscopy (ICP-OES) was used to determine Si, Al, Ca, and Mg contents in the diluted solution, using a PerkinElmer 3000 machine. The accuracy of the ICP-OES technique for determining elemental concentrations of alkaline solutions of relatively high soluble silicate contents was double-checked by making standard solutions of known compositions (pH ) 13.95, 600 mM Si, 3.50 mM Al, and 6.50 mM Ca). The error was found to be within 1% for each of the elements. 2.6. Characterization of Leached Solids. The leached solids were washed with deionized water and then filtered through a 0.2-µm Minisart filter. The filter cakes were dried in an oven of 60 °C for 48 h before analysis. Samples used for X-ray diffraction and infrared spectroscopy were pulverized in a ring mill using tungsten carbide as the crusher.

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Figure 1. Yield stress developments of the various geopolymers at 20 ( 3 °C and a humidity of 50% ( 5% under atmospheric pressure.

Figure 2. Silicon leaching characteristics of Systems A and F-I. [Si] ) concentration of Si in the solution at time t. [Si]0 ) concentration of Si at the start of leaching.

Table 2. Compressive Strengths of the Various Geopolymers at the Curing Age of 28 Days at 20 °C

exert no adverse effects toward the product early compressive strength (Table 2). Hence, the inorganic salts demonstrate a potential as effective gel solidification controllers for geopolymers. The following experiments were designed to mechanistically explain the effects of the salts on geopolymer synthesis and geopolymeric gel solidification. 3.2. Development of the Reaction Model. The results of the leaching experiments are presented in Figures 2-4 and Table 3. From Figure 2 and Table 3, it is clear that the addition of soluble silicates to the leaching solution (Systems F-I) could induce significant changes to the fly ash dissolution/precipitation characteristics as compared to the one without (System A). When less than 200 mM of soluble silicates were added in the leaching solution (Systems F and G, [Si]0 < 200 mM), although not shown clearly in Figure 2, it was observed that there was an initial precipitation of the added soluble silicates, followed by a net Si dissolution where the slope of the [Si] - [Si]0 versus time curve was positive. This suggests that the apparent fly ash dissolution was a result of the competition between the primary phase dissolution and the secondary phase precipitation. As no apparent Si dissolution was observed for Systems F and G ([Si] - [Si]0 < 0; see Figure 2 and Table 3), the extent of Si dissolution was less than that of the Si precipitation within these systems. It is therefore expected that secondary precipitates should deposit on top of the fly ash primary phases. From SEM, secondary precipitates were found on the surfaces of the leached solids of Systems F and G. They were also found on the leached solids of System A as shown in Figure 5. It seems that, within the concentration range of 0 e [Si]0 < 200 mM, the greater the soluble silicate dosage, the denser and the more complete was the surface coverage of this secondary layer. According to SEM-EDS, the secondary layer on the fly ash particles of System A was an aluminosilicate with an averaged elemental composition of 51.46% O, 20.39% Al, 22.64% Si, 3.78% Ca, 0.94% Na, and 0.66% K. The Ca content was about 10 times greater than that of the untreated fly ash due to the surface precipitation of the dissolved Ca, as shown in Figure 3.

Geo-1 Geo-2 Geo-3 Geo-4 Geo-5 compressive strength (MPa) 25.10 25.80 31.14 23.81 25.51

X-ray diffractograms were obtained using a Phillips PW 1800 diffractometer with Cu KR radiation generated at 30 mA and 40 kV (λCuKR ) 1.541 84 Å) and were calibrated against R-quartz. Specimens were stepscanned as random powder mounts from 10° to 50° 2θ at 0.05° 2θ steps and integrated at the rate of 2 s step-1. Fourier transform infrared spectra were acquired using a Bio-Rad FTS 165 FTIR spectrometer in absorbance mode using the KBr pellet technique (0.5 mg sample with 250 mg KBr). Pellets of samples were prepared using the normal procedure, dried in an oven at 60 °C overnight, and pressed again before spectra were taken. All spectra were obtained with a sensitivity of 8 cm-1 and 64 scans per spectrum taken. The baseline of each spectrum was corrected for easier peak interpretation. Scanning electron micrographs of the untreated fly ash and the leached solids after 168 h were obtained using a Phillips XL30 SEM coupled with an Oxford Instruments energy dispersive spectrometer (EDS) using secondary electron (SE) detection. Solids were gently crushed by hand for the leached solids treated under Systems A-G while uncrushed filter cakes were examined for those treated under Systems H-M. Each elemental composition determined using the scanning electron microscopy (SEM)-EDS is an average of 15 analyses taken. 3. Results and Discussion 3.1. Effects of Anions on Geopolymer Synthesis. The effects of the various potassium salts (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) on the rate of geopolymeric gel solidification are summarized in Figure 1. The phosphate salt (Geo-5) was clearly the most effective gel solidification retarder, followed by the chloride (Geo2), the oxalate (Geo-4), and then the carbonate (Geo-3). At the dosage used (400 mM), these salts were found to

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Figure 3. Leaching characteristics of Systems A-E for (a) silicon and (b) calcium. [Si] ) concentration of Si in the solution at time t. [Si]0 ) concentration of Si at the start of leaching. Same notations for Ca.

This observation is consistent with the results by Phair and van Deventer.11 They reported that the formation of an aluminosilicate layer (or polysialate) on solid surfaces was favorable under the conditions where monomeric silicate was the predominant species in a highly alkaline aluminosilicate solution. The “polysialate” product was proposed to be a result of the polymerization of the monomeric silicates, incorporating elements such as Al, Na, or K, via a heterogeneous nucleation mechanism. According to Falcone,12 the alkaline silicate solutions used in this work, with the designated SiO2/M2O ratios and 0 e [Si]0 < 200 mM concentration range, should consist predominantly of monomeric silicates. Hence, aluminosilicate precipitation was possible in Systems A, F, and G. This could contribute to the limited primary phase dissolution of the fly ash as the mass fluxes of ions or molecules became diffusion-controlled across this secondary layer.13 Both the XRD study (Figure 6a) and FTIR analysis (Figure 7a) on the leached solids also revealed that fly ash reactivity was limited in Systems A, F, and G. The

Figure 4. Dissolution characteristics of Systems I-M for (a) silicon and (b) calcium. [Si] ) concentration of Si in the solution at time t. [Si]0 ) concentration of Si at the start of leaching. Same notations for Ca.

infrared spectral bands and X-ray diffraction peaks of the Systems A, F, and G leached solids were almost identical to those of the untreated fly ash. The major IR vibrational bands were assigned and summarized in Table 4 by referring to numerous previous studies.14-23 If, however, more than 200 mM of soluble silicates were used (Systems H and I, [Si]0 > 200 mM), the maximum apparent Si dissolution was found to exceed that of the system initially free of soluble silicates (System A) (see Table 3). For [Si]0 > 200 mM, it seems the greater the soluble silicate dosage in the leaching solution, the greater was the maximum apparent Si dissolution. An increased dissolution of Ca from fly ash was also observed in the same systems (Systems H and I). In fact, both the maximum apparent dissolutions of Si and Ca in System I were about 30 times greater than those of System A. Increasing soluble silicate dosage over 200 mM, hence, was very effective in promoting significant structural breakdown of the primary phases within the fly ash. This was confirmed by the IR analysis presented in Figure 7b; the vibrational bands at 1074 and 561 cm-1 and the shoulder at 620 cm-1,

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Table 3. Maximum Apparent Dissolution Observed from Gladstone Fly Ash in the Alkaline Silicate Solutions at 20 °C maximum apparent dissolution (mM)a system

Al

Si

Ca

Mg

A B C D E F G H I J K L M theoretical max dissolution

6.89 (168) 7.52 (168) 7.08 (168) 9.93 (96) 14.12 (96) 9.56 (168) 7.75 (168) 0.92 (48) 3.65 (8) 4.08 (48) 6.12 (24) 4.71 (8) 3.85 (24) 548.7

4.49 (168) 4.31 (168) 7.01 (96) 11.93 (48) 19.47 (96) b b 6.05 (8) 154.86 (8) 323.96 (8) 601.64 (24) 205.77 (8) 277.68 (96) 832.3

0.81 (2) 0.91 (2) 0.56 (6) 0.02 (168) 0.10 (96) 0.13 (24) 0.08 (48) 3.77 (8) 23.25 (8) 23.02 (6) 30.44 (24) 18.24 (6) 20.58 (24) 62.1

0 0 0 0 0 0 0 0 0 0 0 0 0 32.8

a The parenthesis denotes the time of leaching in hours when the value of the maximum apparent dissolution was taken. The maximum apparent dissolution was obtained by subtracting the initial concentration from the elemental concentration at the designated time. b Only precipitation was observed.

Figure 6. XRD diffractograms of the untreated fly ash (Gdf) and the leached solids after 168 h of leaching under Systems (a) A-E and (b) I-M. Q ) R-quartz. M ) mullite. W ) whewellite.

Figure 5. SE-SEM micrographs of the leached solid after 168 h of leaching under System A. The block arrow indicates where typical SEM-EDS analysis was performed.

which were attributable to amorphous silicate or aluminosilicate phases,17,21-23 had become significantly weaker when the fly ash was leached with System I. Figure 8 shows that the secondary precipitation product of System I leached fly ash was markedly different from System A (Figure 5). All of the remaining fly ash particles of System I were interconnected by gellike structures, which according to SEM-EDS were aluminosilicates with an averaged composition of 60.76% O, 3.18% Al, 20.73% Si, 6.59% Ca, 3.28% Na, and 1.97% K. From Figure 4, the formation of these aluminosilicate gels in System I could be a result of the greatly enhanced primary dissolution, which was followed by the polymerization and the eventual precipitation of the solutes in the bulk solution. This newly formed aluminosilicate gel could be assigned to the new vibrational band at 1042 cm-1 in the System I leached solids (Figure 7b) and should be amorphous in nature as no new crystalline phase was detected from the XRD analysis as shown in Figure 6b. Because of the possibility of being able to synthesize aluminosilicate gel of appreciable physical strength, the

System I leaching experiment was selected as the reaction model to mechanistically explain the effects of the anions (Cl-, CO32-, C2O42-, and PO43-) on the geopolymeric gel solidification characteristics presented in Figure 1. 3.3. Applicability of the Reaction Model. From the leaching experiments summarized in Figures 3 and 4, it is clear that the retarding effects of the added salts within the geopolymeric system (Figure 1) cannot be explained by following the leaching characteristics of the fly ash with [Si]0 ) 0 mM (Systems A-E, Figure 3). On the other hand, a leaching solution containing 569.60 mM soluble silicates (System I-M, Figure 4) was demonstrated to be a good reaction model for studying the gel solidification processes within geopolymeric systems. From Figure 1, the gelation time (defined as the time required for a geopolymeric paste to start gaining sufficient strength) for Geo-1, Geo-2, Geo-3, and Geo-4 was very close to each other at ∼75 min. This corresponds well to the results of the leaching experiments for Si, if it is assumed that the start of the apparent Si precipitation indicated the start of aluminosilicate gel formation. The time for the apparent Si secondary phase precipitation to take place was almost the same at ∼12 h (Figure 4a). This suggests that the anions Cl-, CO32-, and C2O42- did not affect the initia-

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reflected in the real geopolymer as much longer gelation time was observed in Geo-5 (Figure 1). 3.4. Mechanism of Aluminosilicate Gel Formation in Geopolymers. This study reveals that increasing the soluble silicate concentration in an alkaline solution over a threshold value of ∼200 mM can greatly enhance the fly ash reactivity by first increasing the extent of the primary phase dissolution. This is followed by the polymerization and gelation of the solutes in the bulk solution as water is removed by evaporation. A high soluble silicate dosage, therefore, is a necessary prerequisite for synthesizing aluminosilicate gels that provide good interparticle bonding and good physical strength of geopolymers, as was also described by Davidovits.1 From Figure 3, when the potassium salts (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) were added to the leaching solution initially free of soluble silicates ([Si]0 ) 0 mM), the chloride-affected system (System B) was shown to exhibit the same dissolution characteristics as System A. Carbonate (System C), however, was found to accelerate Si dissolution initially and at the same time lowered the solubility of Ca. On the other hand, the oxalate (System D) and the phosphate (System E) were both effective in accelerating and increasing the extents of apparent Si dissolution. The very low solubility of Ca in Systems D and E could be due to the fact that both anions, C2O42- and HPO42-, have strong affinities for Ca via the following simplified reactions: 24-27

Ca2+ + C2O42- T CaC2O4; log(Ksp1) ) 8.59 (1) HPO42- + OH- T PO43- + H2O

(2a)

10Ca2+ + 6PO43- + 2OH- T Ca10(PO4)6(OH)2; log(Ksp2) ) 58.59 (2b)

Figure 7. FTIR spectra of the untreated fly ash (Gdf) and the leached solids after 168 h of leaching under Systems of (a) A-E and (b) I-M.

tion of aluminosilicate gel formation, either in the reaction model or in the real geopolymeric system. From leaching, the chloride salt was found to decrease the rate of the Si secondary precipitation and slowed the aluminosilicate gel formation processes. This can be translated to the smaller slope associated with the chlorideaffected yield stress curve for Geo-2 (Figure 1). The delay of the apparent Si secondary phase precipitation due to the presence of the phosphate salt in the reaction model (System M, Figure 4a) suggests that the PO43anion may participate in the delay of the initiation of aluminosilicate gel formation, as opposed to the other anions. The leaching result of System M (Figure 4) was

where Ksp1 and Ksp2 are the thermodynamic stability products of CaC2O424,25 and Ca10(PO4)6(OH)2 (hydroxyapatite)26,27 at 25 °C, respectively. These values are much higher than that of Ca(OH)2 (log(Ksp) ) 5.26);24 hence, it is expected that, when K2C2O4‚H2O or K2HPO4 was added to the leaching solution, the dissolved calcium from the fly ash was precipitated out either as calcium oxalate or as hydroxyapatite (HAP) instead of Ca(OH)2. This suggestion was confirmed by the XRD analysis. A new crystalline phase at d spacing ) 2.97, 3.65, and 5.93 Å was identified in the System D oxalate-treated solids as shown in Figure 6a. By comparing the diffractograms of this work with the literature,28-30 these peaks could be assigned to calcium oxalate monohydrate (whewellite abbreviated as W) with a chemical formula of CaC2O4‚H2O and cell parameters of a ) 12.088 Å, b ) 14.634 Å, and c ) 10.112 Å. From the IR study (Figure 7a), the presence of the infrared vibrational bands at 781, 1318, 1622, and 1650 cm-1, which were indicative of the presence of calcium oxalate monohydrate CaC2O4‚ H2O (whewellite),31 suggests that the oxalate anion was indeed included in the System D oxalate-treated solids. Figure 7a also shows that hydroxyapatite (HAP), whose IR characteristic bands are 565, 601, and 1058 cm-1,16 was also detectable in the leached solids of System E. The SEM study reveals that when the organic salts were added (K2C2O4‚H2O and K2HPO4), little polysialate-like products were observed on the fly ash

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Table 4. Characteristic IR Vibrational Bands of the Gladstone Class F Fly Asha

a

wavenumber (cm-1)

assignment

refs

950-1200 (s) 1165 (sh) 1138 (sh) 1080 (sh) a 1074 (s) 882 (s) 798 (m) 727 (sh) 620 (sh) 561 (s) 466 (s)

asymmetric stretching (Si-O-Si and Al-O-Si) asymmetric stretching (Si-O-Si) asymmetric stretching (Si-O-Si and Al-O-Si) asymmetric stretching (Si-O-Si and Al-O-Si) asymmetric stretching (Si-O-Si and Al-O-Si) Si-O stretching, OH bending (Si-OH) symmetric stretching (Si-O-Si) symmetric stretching (Si-O-Si and Al-O-Si) symmetric stretching (Si-O-Si and Al-O-Si) symmetric stretching (Al-O-Si) bending (Si-O-Si and O-Si-O)

14-16 15-19 16, 17 17 16, 17 20 16, 18 15, 21-23 15, 16, 22 16, 22 14-23

The abbreviations in parentheses are as follows: s ) strong, w ) weak, m ) medium, and sh ) shoulder.

Figure 8. SE-SEM micrographs of the leached solid after 168 h of leaching under System I. The block arrow indicates where typical SEM-EDS analysis was performed.

primary particles of Systems D and E (not shown). The surface compositions of the leached spherical particles were similar to those of the untreated particles with 55.97% O, 20.30% Al, 22.28% Si, 0.80% Ca, 0.04% Na, and 0.35% K. As the dissolved calcium, which is known to reduce the solubility of silicates in alkaline solutions,8 was taken out by C2O42- and PO43- as whewellite and hydroxyapatite, respectively, an increased Al and Si dissolution with little apparent precipitation was observed in Systems D and E (Figure 3 and Table 3). These experimental observations suggest that, if the solution was without or contained little soluble silicates ([Si]0 < 200 mM), Ca was responsible for the heterogeneous nucleation of soluble silicates at the fly ash surfaces. These surface precipitates could account for the limited fly ash reactivity observed. On the other hand, the XRD (Figure 6b) and the FTIR (Figure 7b) studies on the leached solids of the reaction model ([Si]0 ) 569.60 mM) indicate that whewellite (W) and hydroxyapatite (HAP) were absent in the leached solids of Systems L and M. From Table 1, the soluble silicate content in the leaching solutions L and M were significantly greater than that of the oxalate and the phosphate anions (80 mM). It is likely that there was competition between the soluble silicates and the oxalate/phosphate anion for the dissolved calcium. The competition favored the soluble silicates when their dosage was relatively high. As a result, most of the dissolved Ca was associated with the dissolved or soluble silicates in the solution rather than the anions added. This means that little Ca was left at the fly ash surfaces to cause surface precipitation of silicates or

Figure 9. SE-SEM micrographs of the leached solid after 168 h of leaching under System M. The block arrow indicates where typical SEM-EDS analysis was performed.

aluminosilicates. The nucleating effect of Ca could initiate in the bulk solution instead, which ultimately led to the formation of aluminosilicate gel as observed. 3.5. Implications from the Reaction Model. At the soluble silicate dosage of 569.60 mM, the effects of the various potassium salts (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) on the gel-like secondary products were varied. From the SEM micrographs, the carbonate-affected gel (System K, not shown) was the only one that resembled System I. The gel affected by K2HPO4 (System M) was shown in Figure 9. It seems that the undissolved fly ash particles were apparently covered by layers of amorphous aluminosilicates and were very different from the System I gel (Figure 8). On the other hand, the chloride salt (Figure 10) and the oxalate salt (Figure 11) had caused crystallization within the gel matrix, which otherwise resembled that of System I. Crystallization of the chloride-affected and oxalate-affected samples was found to be reproducible. According to SEM-EDS elemental analysis, the crystal in the chloride-affected matrix was calculated as 28Na2O‚K2O‚0.8CaO[NaAlO2‚ (SiO2)2]8‚xFe2O3‚0.08NaCl‚xH2O, assuming that all of the Al was in IV-coordination. Similarly, the oxalateinduced crystal was calculated as an aluminosilicate of 40Na 2 O‚0.8K 2 O‚0.4CaO[(NaAlO 2 ) 2 ‚(SiO 2 ) 3 ] 8 ‚Fe 2 O 3 ‚ yH2O. As crystallization was absent in the phosphateaffected system as compared to the chloride-affected and the oxalate-affected, it is also likely that the anions may play an important role in determining the nature of the secondary products.

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system. The changed dissolution and precipitation processes within the salt-affected reaction model were found to agree well with the observed gel solidification characteristics of the salt-affected geopolymers. Of all salts, the phosphate salt (K2HPO4) was the most effective retarder for geopolymer synthesis. No adverse effects within the time frame of this investigation were identified. Aluminosilicate crystallization in the chloride and the oxalate-affected reaction model suggests that these salts may also induce crystallization within the geopolymers. Further experiments will have to be performed to elucidate this point as well as to investigate whether this will affect the chemical stability and durability of the geopolymeric product, which is vital for the successful commercialization of this structural material. Figure 10. SE-SEM micrographs of the leached solid after 168 h of leaching under System J. The block arrow indicates where typical SEM-EDS analysis was performed.

Figure 11. SE-SEM micrographs of the leached solid after 168 h of leaching under System L. The block arrow indicates where typical SEM-EDS analysis was performed.

Because System I can be successfully applied to model the gel solidification processes of a “real” geopolymeric system, the implications of the effects of the various salts (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) on the nature of the secondary precipitation products cannot be overlooked. Crystallization as induced by KCl and K2C2O4‚H2O in the leaching slurries can also exist in the geopolymers contaminated by KCl and K2C2O4‚H2O. Whether this will affect the long-term product chemical stability and durability is certainly of paramount importance and thoroughly deserves careful future investigation. 4. Conclusions Soluble silicates were found to play a very important role in determining the processes of primary phase dissolution and secondary phase precipitation of a Class F fly ash in alkaline silicate solutions (pH ∼ 13.95) at 20 °C. They also determined how the potassium salts (KCl, K2CO3, K2C2O4‚H2O, and K2HPO4) could affect fly ash leaching characteristics. It was found that a leaching system with a soluble silicate dosage of 569.60 mM and molar SiO2/M2O ) 0.949 was suitable to model the gel solidification processes within a real geopolymeric

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Received for review November 27, 2001 Revised manuscript received May 9, 2002 Accepted June 8, 2002 IE0109410