Ind. Eng. Chem. Res. 2008, 47, 2991-2999
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MATERIALS AND INTERFACES Structural Evolution of Fly Ash Based Geopolymers in Alkaline Environments Sindhunata, John L. Provis, Grant C. Lukey, Hua Xu, and Jannie S. J. van Deventer* Department of Chemical and Biomolecular Engineering, UniVersity of Melbourne, Victoria 3010, Australia
The effects of immersion in alkaline solutions on the gel structure and pore network of fly ash based geopolymers are investigated. Immersion in carbonate or hydroxide solutions of up to pH 14 results in very little leaching of framework components (Si or Al) from the geopolymer gel, and a largely unchanged mesoporous gel structure. Higher concentrations, up to 8 M NaOH, cause more damage to the gel framework as species are leached into solution and the pore network collapses. Crystallization of small quantities of zeolites from the initially X-ray amorphous gel is also observed. The zeolitic products formed are in general the same products that are observed in geopolymers cured at elevated temperatures for extended periods of time, suggesting that the reactions taking place during alkaline immersion are to some extent a continuation of the initial geopolymerization process. Introduction Geopolymers are a class of alkali-activated aluminosilicate materials that are currently finding increasing use worldwide either as a substitute for Portland-based cements or as a roomtemperature means of synthesizing aluminosilicate ceramics.1-4 The binder phase in geopolymers is generally an aluminosilicate gel, with Si and Al in tetrahedral coordination and with the negative framework charge balanced by alkali metal cations. Geopolymers can be considered to be a mesoporous material, where the pore structure is dependent on the reaction temperature, water content, alkali concentration, and silicate concentration of the activating solution. Leaching in acid has been shown to have some effects on geopolymer network structure, although their acid resistance is in most cases significantly superior to that of ordinary Portland cements.5-8 It is also known that the geopolymer gel structure can be modified by immersing geopolymers in alkaline and/or salt solutions.9 However, this alkali-induced structural modification in fly ash based geopolymers has not previously been studied in detail. There are significant applications that have been proposed for geopolymer concrete involving exposure to relatively high alkali concentrations for extended periods, in particular in the context of CO2 separation and geosequestration, where the availability of a concrete that is resistant to attack by concentrated alkaline carbonate solutions will be of immense value. Such solutions occur as process fluids in these and other chemical processes, and the ability to contain these solutions using alkali-resistant concretes rather than steel vessels will give marked economic benefits. Concretes that can withstand exposure to alkaline soil conditions will also be of great value in construction applications in many parts of the world, where high levels of sodium carbonate in soils can increase the soil pH to around 9 or higher. In the synthesis of aluminosilicate gels and glasses, the porosity and other physicochemical properties can be tailored * To whom correspondence should be addressed. Tel: +61-383449737. Fax: +61-3-83444153. E-mail:
[email protected].
by immersing wet gels in various alkaline solutions,10-12 salt solutions,13,14 and organic solvents.14,15 Regarding the use of alkaline solutions, it is found that the alteration of pore structure in wet or dry silica gels is largely dependent on the immersion time, concentration, and pH.13,16-18 A longer immersion time results in an increase in pore size and decrease in specific surface area by eliminating narrower pores.10,11 Aging or washing at a near-neutral pH results in a porous gel with a higher pore volume and lower surface area than at lower pH. The alteration of pore structure in porous silica gels follows an Ostwald ripening mechanism.19 Along with a higher extent of aluminosilicate condensation, the growth of necks occurs between larger particles at the expense of dissolution of smaller particles. As a result, the pore volume and mean pore size increase, and the specific surface area decreases. In this study, fly ash based geopolymers were immersed in various alkali metal hydroxide and carbonate solutions for varying periods. The transport behavior of network-forming elements, porosity, crystallinity, and morphologies of these “aged” geopolymer gels are investigated. Recent work has focused on the structural evolution of fly ash based geopolymers under extended curing in sealed vessels20,21 and at different temperatures.22 The current investigation complements this work by showing the effects of alkaline conditions on structure development. The primary purpose of this paper is therefore to address the following questions: (a) How does aging under alkaline conditions affect the porosity and crystallinity of geopolymers? (b) What are the mechanisms underlying these observed changes? These factors will also be important in determining the longterm durability of geopolymers exposed to moist but not necessarily alkaline environments, as the pore solution within the geopolymeric gel is in fact highly alkaline, and may also be subject to carbonation by atmospheric CO2. The trends observed during immersion in alkaline solutions may therefore be considered to be at least somewhat indicative of the structural changes that will be induced by interactions between the binder
10.1021/ie0707671 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008
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Table 1. Oxide Composition of the Fly Ash Used component
composition (mass %)
SiO2 Al2O3 Fe2O3 TiO2 MnO CaO MgO K2O Na2O P2O5 SO3 loss on ignition
50.01 27.97 11.95 1.30 0.19 3.48 1.32 0.69 0.19 1.01 0.50 1.39
total
100.00
and the pore solution while a geopolymeric mortar or concrete is in service. Experimental Section Materials and Sample Synthesis. The fly ash (FA) used in this study was Class F according to ASTM C618, generated at Gladstone power station, Queensland, Australia, and obtained from Queensland Cement Ltd. The oxide composition of FA was determined using a Siemens SRS3000 X-ray fluorescence (XRF) spectrometer (Table 1). FA also contains crystalline phases, such as quartz, mullite, hematite, and magnetite, which are here assumed to be nonreactive upon alkali activation.23 Quantitative X-ray diffraction (XRD) analysis using SiroQuant software showed that FA contains 17 mass % amorphous Al2O3 and 39 mass % amorphous SiO2. Activating solutions were prepared by mixing potassium hydroxide or sodium hydroxide (Orica Australia) with water and commercial silicate solutions (PQ Australia). The concentration of alkali and silicate in the activating solution is expressed in terms of the H2O/M2O and SiO2/M2O ratios, where M ) Na or K. The H2O/M2O ratio was kept constant at 14.85, while the SiO2/M2O ratio was 0.0, 0.2, or 0.79. Samples are named according to activating solution SiO2/M2O ratio and alkali metal cation; for example, 0.2Na was the sample whose activating solution had SiO2/Na2O ) 0.2. Samples synthesized at higher silicate contents, e.g., 2.0Na,22 were subjected to preliminary testing but showed quite limited resistance to immersion in highly alkaline solutions, and so were not investigated in detail. The water to fly ash mass ratio was kept constant at 0.3, resulting in a constant Alamorphous/M+ ratio of 1.5. Geopolymer mix formulations, on a mass basis, are given in Table 2. Geopolymers were synthesized by mixing fly ash and activating solutions by hand in a plastic bucket or beaker until a consistent mixture was obtained. The geopolymer slurry was then cast in 70 cm3 cylindrical molds, sealed, and cured at 50 °C for 24 h. A longer curing time of 48 h was applied to samples 0.0K and 0.2K in order to allow them to become sufficiently hard to be demolded. Immersion Method. Geopolymers were demolded and immersed at room temperature in various alkali and carbonate solutions, namely NaOH, KOH, Na2CO3, and K2CO3, as well as distilled water (Table 3). Each 70 cm3 sample was immersed (as a monolith) in 100 cm3 of solution, giving a solid/liquid ratio of 0.70, which is very high for a standard leaching test but is designed to replicate service conditions of geopolymeric materials being used in moist, enclosed environments. Aliquots of 3 mL were taken after different time intervals. Each aliquot solution was diluted before being subjected to elemental analysis (Al3+, Si4+) using inductively coupled plasma optical emission spectroscopy (ICP-OES). At time intervals up to 90 days,
geopolymer samples were crushed, dried at 105 °C for 24 h, and subjected to N2 sorption (Tristar, adsorption and desorption branches measured at 77 K) and X-ray diffractometry (XRD) (Philips PW 1800, Cu KR radiation). Results and Discussion Si and Al Leaching from Na-Geopolymers. The amounts of Si and Al dissolved from sodium-containing fly ash based geopolymers during immersion in water, alkali hydroxide, and carbonate solutions for different periods of time are presented in Figure 1. It is seen from these plots that there are two clear groupings among the alkaline solutions tested: 5 and 8 M NaOH and 5 M KOH all behave relatively similarly, whereas 1 M NaOH, saturated Na2CO3, and water also show trends similar to each other and show much less aggressive leaching behavior. This is not unexpected, taking into account the differences in pH between the highly concentrated alkali hydroxides (>14) and the Na2CO3 solution (∼11). The water used for leaching will also have become significantly alkaline due to the release of free alkali from the geopolymer samples during immersion at the high solid/liquid ratio used, although the pH was not specifically monitored. Perera et al.24 showed that the release rate of alkali metal cations from both fly ash based and metakaolin-based geopolymers during leaching in distilled water (PCT-B test) was much higher than the release of Si or Al. If hydroxide is released along with these cations, the final pH of the leach solution may be estimated to be around 12-13, and this corresponds very well with the fact that it is observed to leach Si and Al from geopolymers at a rate that is generally intermediate between saturated Na2CO3 (pH ∼11) and 1 M NaOH. A 1 M NaOH solution has pH ∼14, and so would be expected to behave more similarly to the other hydroxide solutions than the carbonate and water. The fact that it does not suggests that there are more complex effects occurring than just simple hydroxide activity-dependent dissolution of the geopolymer gel, and this will be discussed in detail later. Reprecipitation of aluminosilicate species is observed in all systems plotted in Figure 1. This is seen from the change from an initially clear to a cloudy aging solution, which is filled with opaque flakes, indicative of aluminosilicate gel reprecipitation. Apart from Si and Al, other elements such as Fe and Ca will also dissolve from the geopolymer and/or remnant ash phases. The dissolution of Fe in particular is evident from the formation of brownish precipitates (possibly Fe(OH)3) after 28 days (672 h) in solutions of high alkali concentrations (5 and 8 M NaOH). It is not clear whether this Fe is leaching from unreacted fly ash particles or from the iron (oxy)hydroxide phases that are believed to form during geopolymerization,25 but in either case its presence indicates that significant damage to the geopolymer structure has occurred, as discussed in detail below. At lower alkalinity, reprecipitation dominates to the extent that the leached ion concentrations decrease from 48 to 168 h in all systems in Figure 1, for leaching in water, 1 M NaOH, or saturated Na2CO3. Leaching beyond 28 days (not shown in Figure 1) leads to further reprecipitation in most of the high-alkalinity systems, as zeolites begin to crystallize in significant quantities. Effects of Na-K Substitution. Figure 2 presents data for the leaching of Si and Al from potassium-containing geopolymers (K-geopolymers). The observed behavior is, as expected, broadly similar to that of sodium-containing geopolymers (Nageopolymers), with the same groupings of high- and loweralkalinity solutions each displaying similar behavior. The overall leachability of framework species from K-geopolymers is in general higher than from Na-geopolymers, and more so for Al than Si.
Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 2993 Table 2. Compositions of All Samples Tested (mass %) sample
fly ash
Na2O or K2O
SiO2
H2O
0.0Na 0.2Na 0.5Na 0.79Na 0.0K 0.2K 0.5K 0.79K
50.25 49.37 48.11 46.95 47.92 47.12 45.97 44.91
9.36 9.19 8.96 8.74 13.56 13.33 13.01 12.71
0 1.75 4.26 6.57 0 1.67 4.07 6.29
40.40 39.69 38.67 37.74 38.52 37.88 36.95 36.10
Table 3. Solutions Used for Aging Tests solution
concentration (M)
NaOH KOH Na2CO3 K2CO3 distilled water
1, 5, 8 1, 5, 8 2.2 (saturated) 2.5 -
It is also seen from Figures 1 and 2 that the Si and Al release rates at high alkalinity are dependent on the silicate ratios of original geopolymers, but follow slightly different trends for Na- and K-geopolymers. Sample 0.79Na generally shows the highest Si and Al leach rates of the Na-geopolymers studied. 0.0Na releases more Si than 0.2Na, but the trend in Al release is much less obvious. In K-geopolymers, Si leaching from 0.79K at high alkalinity is initially slower than from 0.2K or 0.0K, but reaches a higher extent after 28 days. There is little difference in the Al leaching from the three K-geopolymer compositions. There are not such clear trends observed at low alkalinity, due mainly to the complicating effects of reprecipitation reactions and the participation of the alkaline solution components (particularly carbonate) in these reactions. An interesting trend is observed as Na-geopolymers are aged in 5 M KOH solution. In these systems, the initial rate of release of both Si and Al is higher than in the same concentration of NaOH. However, the final concentrations observed after 28 days of immersion in 5 M KOH are uniformly lower than in 5 M NaOH. This may be correlated to some extent with the differences in silicate speciation equilibria induced by the two cations, where Na+ tends to associate with smaller silicate units while K+ promotes condensation of silicate or aluminosilicate species.26-28 The initial faster rate of dissolution in KOH solution compared to NaOH may be attributed to the differences in alkalinity of the two solutions, where KOH is a stronger base than NaOH. However, the strength of the effect, as well as the fact that it takes place only in Na-geopolymers and not in K-geopolymers, suggests that more subtle influences are also significant. It is possible that, during geopolymerization in Nabased systems, some glassy phases in the fly ash remained unreacted in the presence of Na+, but remained susceptible to attack by K+. The immersion of the Na-geopolymers in KOH solution then enabled attack on and dissolution of these remnant phases, giving a rapid initial release of Si and Al. A similar explanation may then be applied to the initial rapid release of Si during immersion of K-geopolymers (0.0K and 0.2K) in 5 M NaOH, where it appears that the phases that were susceptible to Na+ attack but not K+ attack were Si-rich. This would then be responsible for the differences in trends in Si and Al release between NaOH and KOH immersion in K-geopolymers, where immersion in 5 M NaOH released more Si but less Al than 5 M KOH. It has also been previously observed that sample 0.79K shows a significantly higher extent of geopolymer gel formation than 0.0K or 0.2K.22 This may then be responsible for the fact that the trend of higher Si release from K-geopolymers in NaOH than KOH is not followed by sample 0.79K, as the increased
gel volume would coat more of the fly ash particles and prevent the hydroxide solution from penetrating to their surfaces. It is also of interest to note that, in the K-geopolymer samples, immersion in 5 M NaOH uniformly and unexpectedly led to higher leached Si concentrations than did immersion in 8 M NaOH, but lower Al concentrations. It is possible that the greater zeolite-forming tendencies of the more alkaline solution led to a depression of Si solubility in the most alkaline system. Alternatively, attack by 8 M NaOH may have initially released more non-aluminosilicate species such as Fe from the remnant fly ash particles, which would then have precipitated as hydroxides and lowered the alkalinity of the solution. It is not clear why this occurred, or why a similar phenomenon was not observed in Na-geopolymers, where the difference between immersion in 5 M NaOH and in 8 M NaOH is generally minimal in terms of leaching rate and extent. Other than the differences in alkalinity, there will be significant differences in transport behavior in saturated Na2CO3 solutions compared to the other solutions tested. In most systems, diffusion of leached species into the surrounding solution will occur relatively freely as the leaching solution is well below saturation. However, the saturated Na2CO3 solution prevents the generation of any driving force for ionic transport from geopolymer pores into the solution. Release of Na from the geopolymer matrix and pore solution will therefore be significantly hindered by the common ion effect, which reduces the structural disruption to the geopolymer gel and hence the leaching of Si and Al. Similar to Na2CO3 immersion of Nageopolymers, the concentrations of Si and Al released from K-geopolymers into K2CO3 solutions are relatively low. However, K-geopolymers that are immersed in 2.5 M K2CO3 solution show a higher extent of dissolution of Si and Al than their Nageopolymer counterparts, because 2.5 M K2CO3 is not a saturated solution and so the driving force for dissolution is much stronger. The water-immersed samples also show a certain extent of transport control in the initial stages of leaching, as the generation of an alkaline environment for leaching of Si and Al from the geopolymer matrix depends on the diffusion of alkali metal and hydroxide ions from the pores of the monolithic sample to increase the pH of the solution. Pore Structure Evolution of Geopolymers. The BarrettJoyner-Halenda (BJH) pore size distributions calculated from nitrogen porosimetry of nonimmersed Na-geopolymers after 1, 14, and 28 days (sealed samples stored at room temperature) are presented in Figure 3. While it is known that the BJH method has significant and well-known drawbacks in quantitative analysis of complex pore shapes such as those expected to be observed in geopolymers, it can nonetheless provide valuable data for semiquantitative comparison of pore size distribution evolution during aging and/or immersion. Representative N2 sorption-desorption isotherms are supplied as Supporting Information. Zeolites have been observed to form in Na-geopolymers activated at low silicate concentrations (e.g., SiO2/Na2O ) 0.0 and 0.2).29,30 In Figure 3, the porosity of zeolites in Na-based
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Figure 1. Leached concentrations of Si and Al for samples (a, b) 0.0Na, (c, d) 0.2Na, and (e, f) 0.79Na, cured at 50 °C, in alkaline solutions as marked.
geopolymers can be seen from the significant pore volume in the submesopore region (2-3.6 nm, visible in Figure 3a but particularly in Figure 3b). In addition, there is also a significant pore volume in the mesopore region (3.6-50 nm), indicating the presence of geopolymer gel. As the aging time is prolonged to 28 days at low silica content, there is a shift of pore size distribution to be predominantly in the submesopore region, which suggests a greater extent of structural ordering accompanied by pore network refinement as the gel evolves toward a more zeolitic structure. Sample 0.2Na (Figure 3b) provides key indications of the mechanism by which this process occurs: the higher silica
content of this system reduces the rate of pore network refinement compared to 0.0Na, so the shift of the larger pore radius peak is clearly observable as it moves from around 20 to 10 to 6 nm at 1, 14, and 28 days, respectively. This corresponds to a gradual gel rearrangement, and also shows the increase in binder content as the geopolymerization reaction continues in these sealed (and therefore moist) samples. However, it is also observed from Figure 3a that part of a peak corresponding to micropores or very small mesopores (
