Geopolymer Gel Research Articles

High-Resolution Nanoprobe X-ray Fluorescence Characterization of Heterogeneous Calcium and Heavy Metal Distributions in Alkali-Activated Fly Ash

The nanoscale distribution of elements within fly ash and the aluminosilicate gel products of its alkaline activation ("fly ash geopolymers") are analyzed by means of synchrotron X-ray fluorescence using a hard X-ray Nanoprobe instrument. The distribution of calcium within a hydroxide-activated (fly ash/KOH solution) geopolymer gel is seen to be highly heterogeneous, with these data providing for the first time direct evidence of the formation of discrete high-calcium particles within the binder structure of a geopolymer synthesized from a low-calcium (<2 wt % as oxides) fly ash. The silicate-activated (fly ash/potassium silicate solution) sample, by contrast, shows a much more homogeneous geopolymer gel binder structure surrounding the unreacted fly ash particles. This has important implications for the understanding of calcium chemistry within aluminosilicate geopolymer gel phases. Additionally, chromium and iron are seen to be very closely correlated within the structures of both fly ash and the geopolymer product and remain within the regions of the geopolymer which can be identified as unreacted fly ash particles. Given that the potential for chromium release has been one of the queries surrounding the widespread utilization of construction materials derived from fly ash, the observation that this element appears to be localized within the fly ash rather than dispersed throughout the gel binder indicates that it is unlikely to be released problematically into the environment.

Correlating mechanical and thermal properties of sodium silicate-fly ash geopolymers

The correlation between mechanical and dilatometric properties of aluminosilicate geopolymer binders is highlighted by analysis of a set of samples synthesised from a single ash source using different activating solution compositions and liquid/solid ratios. The geopolymers which display the best strength performance also show a small expansion in the temperature range 700–800 °C, which is identified as corresponding to the swelling of a high-silica phase present as pockets within the geopolymeric gel structure. Systems in which this phase is absent (made using hydroxide activating solutions or very low liquid/solid ratios) generally show a low extent of binder formation and do not achieve high strength, while systems in which the expansive phase dominates have a sub-optimally structured geopolymer phase and also correspondingly reduced strength.

Carbonate mineral addition to metakaolin-based geopolymers

The effect of adding significant percentages of alkaline earth carbonate minerals (calcite and dolomite) to metakaolin-based geopolymers is investigated by compressive strength testing, X-ray diffractometry and electron microscopy, with the aim of better understanding the role played by calcium ions, carbonate ions and mineral surfaces in geopolymers. Addition of around 20% calcite or dolomite is seen to improve the compressive strength of the geopolymeric material, although does induce additional shrinkage during the first 90 days of aging. More than 20% mineral additive has a deleterious effect on strength due to significant disruption of the geopolymer gel network and the reduced reactive aluminosilicate content. No distinct calcium silicate hydrate phase formation is observed in any of the systems studied. Some dissolution of the mineral particles is observed, however this is not a major effect in most instances. The mineral particles interact with the geopolymer gel predominantly via surface binding, which appears to be somewhat stronger in the case of calcite than dolomite.

Geopolymerisation kinetics. 3. Effects of Cs and Sr salts

Alternating current impedance spectroscopy (ACIS) is used to monitor in situ the kinetics of geopolymer formation by alkali silicate activation of metakaolin at 40 °C. The evolution of conductivity and resistivity as a function of time are found to correlate very well with existing models of geopolymerisation kinetics. The dissolution of metakaolin gives a rapid increase in conductivity, with a subsequent decrease due to gelation and gel ageing. After a period of curing, the resistivity of the gel network is seen to be a sensitive probe of the development of a well-formed gel structure. This enables investigation of the changes in geopolymer formation kinetics and gel structure caused by addition of various salts of interest in nuclear waste management, in particular the nitrate, sulphate and hydroxide salts of caesium and strontium. Each of the six salts studied is shown to have specific effects on the kinetics of geopolymer formation and the pore structure of the final geopolymer gel. Caesium nitrate and caesium sulphate have very similar effects, with a slight retardation of gelation followed by significant disruption of gel structure while caesium hydroxide accelerates gel formation markedly. Strontium nitrate at high concentrations interferes significantly with the pore network development, while strontium sulphate is converted gradually to carbonate salts by atmospheric carbonation.

Structural Evolution of Fly Ash Based Geopolymers in Alkaline Environments

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.

Geopolymers for immobilization of Cr 6+, Cd 2+, and Pb 2+

Alkali activation of fly ash by sodium silicate solutions, forming geopolymeric binders, provides a potential means of treating wastes containing heavy metals. Here, the effects on geopolymer structure of contamination of geopolymers by Cr(VI), Cd(II) and Pb(II) in the forms of various nitrate and chromate salts are investigated. The addition of soluble salts results in a high extent of dispersal of contaminant ions throughout the geopolymer matrix, however very little change in geopolymer structure is observed when these materials are compared to their uncontaminated counterparts. Successful immobilization of these species will rely on chemical binding either into the geopolymer gel or into other low-solubility (silicate or aluminosilicate) phases. In the case of Pb, the results of this work tentatively support a previous identification of Pb 3SiO 5 as a potential candidate phase for hosting Pb(II) within the geopolymer structure, although the data are not entirely conclusive. The addition of relatively low levels of heavy metal salts is seen to have little effect on the compressive strength of the geopolymeric material, and in some cases actually gives an increase in strength. Sparingly soluble salts may undergo some chemical conversion due to the highly alkaline conditions prevalent during geopolymerization, and in general are trapped in the geopolymer matrix by a simple physical encapsulation mechanism. Lead is in general very effectively immobilized in geopolymers, as is cadmium in all except the most acidic leaching environments. Hexavalent chromium is problematic, whether added as a highly soluble salt or in sparingly soluble form.

The mechanism of geopolymer gel formation investigated through seeded nucleation

Seeding of hydroxide-activated geopolymer syntheses with high surface area Al2O3 nanoparticles causes significant changes both in the kinetics of the reaction and in the structures of the products formed, as shown by in situ and ex situ ATR-FTIR spectroscopy. Nucleation at the seed surfaces leads to the formation of phase-separated gels from very early in the reaction process, eliminating the induction period prior to geopolymer gel formation that is normally observed with hydroxide activation. However, other than the formation of small regions of very high-silica gel within the seeded system, the nature of the dominant geopolymer gel phase remains largely unchanged. After an extended period of curing, both seeded and unseeded geopolymers display the formation of zeolitic phases. However, while the unseeded system contains small amounts of faujasite as the favoured crystalline product, seeding leads to the formation of zeolite Na–F, a synthetic zeolite with the edingtonite structure type that has not previously been observed in geopolymers. These observations are explained mechanistically in terms of the catalytic effects of the seed particles on gel phase formation and separation.

Effect of calcium silicate sources on geopolymerisation

Seven different calcium silicate materials were used to investigate the role of calcium in geopolymerisation. At low alkalinity, the compressive strength of matrices prepared with predominantly amorphous calcium silicates (blast furnace slag) or containing crystalline phases specifically manufactured for reactivity (cement) is much higher than when the calcium is supplied as crystalline silicate minerals. The compressive strength of matrices containing natural (crystalline) calcium silicates improves with increasing alkalinity, however the opposite trend is observed in matrices synthesised with processed calcium silicate sources. The difference in compressive strength between matrices synthesised using different calcium silicate sources is significantly reduced at high alkalinity. An insufficient amount of calcium is dissolved from crystalline calcium silicates at relatively low alkalinity to enable formation of calcium silicate hydrate in coexistence with the aluminosilicate geopolymeric gel, and this leads to the poor mechanical properties of such matrices. At high alkalinity, calcium plays a lesser role in affecting the nature of the final binder, as it forms precipitates rather than hydrated gels. Thus, the different calcium silicate sources will not have a major impact on the mechanical properties of these matrices. The effects of different calcium silicates on geopolymerisation are therefore seen to depend most significantly on two factors: the crystallinity of the calcium silicate source, and the alkalinity of the activating solution used.

In Situ ATR-FTIR Study of the Early Stages of Fly Ash Geopolymer Gel Formation

The kinetics of geopolymer formation are monitored using a novel in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic technique. Reaction rates are determined from the intensity variation of the bands related to the geopolymer gel network and the unreacted fly ash particles. Comparison with deuterated geopolymer samples provides critical information regarding peak assignments. An initial induction (lag) period is observed to occur for hydroxide-activated geopolymers, followed by gel evolution according to an approximately linear reaction profile. The length of the lag period is reduced by increasing the concentration of NaOH. An increase in the rate of network formation also occurs with increasing NaOH concentration up to a maximum point, beyond which an increased NaOH concentration leads to a reduced rate of network formation. This trend is attributed to the competing effects of increased alkalinity and stronger ion pairing with an increase in NaOH concentration. In situ analysis also shows that the rate of fly ash dissolution is similar for all moderate- to high-alkali geopolymer slurries, which is attributed to the very highly water-deficient nature of these systems and is contrary to predictions from classical glass dissolution chemistry. This provides for the first time detailed kinetic information describing fly ash geopolymer formation kinetics.

Chemical characterisation of the steel–geopolymeric gel interface

Geopolymers could be used as non-combustible adhesives for steel, so that it is important to understand the interaction of stainless steel and pure iron with geopolymeric gel. Different artificial gel forming solutions with varied concentrations of alkali and Si/Al ratio were used to investigate the effect of the Al–Si precursors on the interactions between the gel and substrates. The gelation process and the interactions at the gel/substrate interface were studied by Fourier transform infrared spectroscopy-specular reflectance (FTIR), X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy (AES). It was found that the different gel forming solutions formed different structures, which reacted to yield aluminosilicate gels of different composition and structure. Artificial gel forming solutions with high Si/Al ratios formed structures in which the Si/Al ratio decreased with increasing polymerisation, whereas at low Si/Al ratios, more ordered structures with almost a constant Si/Al ratio were formed. The aluminosilicate gel growth was more rapid on an iron than a stainless steel substrate. At the interface, there was a transition zone in which the Si/Al ratio differed from the bulk gel. At the gel/metal interface, the Si/Al ratio was less than 1 for both iron and stainless steel substrates. FTIR and XPS results indicated that the chemical interactions between gel polymer and iron-based substrates were likely attributed to the formation of the Al–O–Fe bond. The gel has reacted with the oxide layer, forming the transition zone.

Kinetics of geopolymerization: Role of Al 2O 3 and SiO 2

The early-stage reaction kinetics of metakaolin/sodium silicate/sodium hydroxide geopolymer system have been investigated. The setting and early strength development characteristics, and associated mineral and microstructural phase development of mixtures containing varying SiO 2/Al 2O 3 ratios, cured at 40 °C for up to 72 h, were carefully studied. It was observed that setting time of the geopolymer systems was mainly controlled by the alumina content. Essentially, the setting time increased with increasing SiO 2/Al 2O 3 ratio of the initial mixture. Up to a certain limit, the SiO 2/Al 2O 3 ratio was also found to be responsible for observed high-strength gains at later stages. An increase in the Al 2O 3 content, i.e. for low SiO 2/Al 2O 3 ratio, led to products of low strength, accompanied by microstructures with increased amounts of Na–Al–Si-containing “massive” phases (grains). EDAX analyses showed that the SiO 2/Al 2O 3 ratios of geopolymer gel phases were quite similar to those of the starting mixtures, but with an overall lower Na content. Most importantly, this study clearly demonstrates that the properties of resulting geopolymer systems can be drastically affected by minor changes in the available Si and Al concentrations during synthesis.

Direct measurement of the kinetics of geopolymerisation by in-situ energy dispersive X-ray diffractometry

In-situ energy dispersive X-ray diffractometry (EDXRD) using synchrotron radiation has been used to directly observe the kinetics of formation of a geopolymeric gel from a metakaolin precursor. The use of a purpose-built hydrothermal cell with polychromatic radiation from a wiggler source enables collection of a full diffraction pattern approximately every 150 s. This provides sufficient time resolution to observe the collapse of the metakaolin structure as it dissolves in the activating solution, accompanied by the reprecipitation of the geopolymeric gel binder phase from the now-supersaturated solution. Measurements taken on a limited set of samples of different composition (Si/Al ratio) show a clear trend in the rate of reaction with composition, and also a distinctly different mechanism of reaction in the most highly alkaline systems compared to those containing higher levels of dissolved silica in the activating solution. This corresponds to the results of previous microscopic observations showing significantly different microstructures in these systems, and confirms the value of this technique in analysis of the kinetics of geopolymerisation.

39K NMR of Free Potassium in Geopolymers

Nuclear magnetic resonance (NMR) studies of geopolymer gels have shown directly that free Na cations are present in the pore solution of some specimens. Furthermore, it has been suggested previously but not directly proven that potassium is incorporated into these materials, in preference to sodium. Here, the presence of free potassium in the pore solution of geopolymeric gels prepared without sodium hydroxide was confirmed by 39K NMR. The preferential incorporation of potassium was directly shown by the absence of free potassium in the mixed-alkali geopolymer gel.

Synthesis and heavy metal immobilization behaviors of slag based geopolymer

In this paper, two aspects of studies are carried out: (1) synthesis of geopolymer by using slag and metakaolin; (2) immobilization behaviors of slag based geopolymer in a presence of Pb and Cu ions. As for the synthesis of slag based geopolymer, four different slag content (10%, 30%, 50%, 70%) and three types of curing regimes (standard curing, steam curing and autoclave curing) are investigated to obtain the optimum synthesis condition based on the compressive and flexural strength. The testing results showed that geopolymer mortar containing 50% slag that is synthesized at steam curing (80 °C for 8 h), exhibits higher mechanical strengths. The compressive and flexural strengths of slag based geopolymer mortar are 75.2 MPa and 10.1 MPa, respectively. Additionally, Infrared (IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques are used to characterize the microstructure of the slag based geopolymer paste. IR spectra show that the absorptive band at 1086 cm −1 shifts to lower wave number around 1007 cm −1, and some six-coordinated Als transforms into four-coordination during the synthesis of slag based geopolymer paste. The resulting slag based geopolymeric products are X-ray amorphous materials. SEM observation shows that it is possible to have geopolymeric gel and calcium silicate hydrate (C–S–H) gel forming simultaneously within slag based geopolymer paste. As for immobilization of heavy metals, the leaching tests are employed to investigate the immobilization behaviors of the slag based geopolymer mortar synthesized under the above optimum condition. The leaching tests show that slag based geopolymer mortar can effectively immobilize Cu and Pb heavy metal ions, and the immobilization efficiency reach 98.5% greater when heavy metals are incorporated in the slag geopolymeric matrix in the range of 0.1–0.3%. The Pb exhibits better immobilization efficiency than the Cu in the case of large dosages of heavy metals.

Nano- and Microporosity in Geopolymer Gels

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Formation of Nanocrystalline Zeolites in Geopolymer Gels

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Modeling Speciation in Highly Concentrated Alkaline Silicate Solutions

A model describing the distribution of silicate sites in concentrated alkali metal silicate solutions used in the synthesis of geopolymeric gels is presented. The model is based on a free energy minimization algorithm, using free energy of formation data obtained from the literature. A simplified form of the Pitzer method (Pitzer, K. S. J. Phys. Chem. 1973, 77, 26) is used to calculate activity coefficients, with interaction parameters calculated from known values for ions of similar size and charge type. Ion pairing effects are incorporated into the model formulation by a simple approximation, whereby larger cyclic or cagelike ions form strong ion pairs with dissolved alkali metal cations, and smaller monomeric (Q0) or end-group (Q1) sites pair less strongly with the cations. The model is able to describe accurately the speciation data obtained from a systematic 29Si NMR investigation of sodium, potassium, and mixed (1:1) sodium/potassium silicate solutions. The model is also tested against a large data set from the literature on sodium silicate solutions with a wide range of compositions. The model provides understanding of the speciation of silicate solutions and a basis for further understanding and modeling of the geopolymerization process.

Understanding the relationship between geopolymer composition, microstructure and mechanical properties

A mechanistic model accounting for reduced structural reorganization and densification in the microstructure of geopolymer gels with high concentrations of soluble silicon in the activating solution has been proposed. The mechanical strength and Young's modulus of geopolymers synthesized by the alkali activation of metakaolin with Si/Al ratio between 1.15 and 2.15 are correlated with their respective microstructures through SEM analysis. The microstructure of specimens is observed to be highly porous for Si/Al ratios ≤1.40 but largely homogeneous for Si/Al ≥1.65, and mechanistic arguments explaining the change in microstructure based on speciation of the alkali silicate activating solutions are presented. All specimens with a homogeneous microstructure exhibit an almost identical Young's modulus, suggesting that the Young's modulus of geopolymers is determined largely by the microstructure rather than simply through compositional effects as has been previously assumed. The strength of geopolymers is maximized at Si/Al = 1.90. Specimens with higher Si/Al ratio exhibit reduced strength, contrary to predictions based on compositional arguments alone. The decrease in strength with higher silica content has been linked to the amount of unreacted material in the specimens, which act as defect sites. This work demonstrates that the microstructures of geopolymers can be tailored for specific applications.

The coexistence of geopolymeric gel and calcium silicate hydrate at the early stage of alkaline activation

Scanning electron microscopy was used to study the effects of the addition of ground granulated blast furnace slag (GGBFS) on the microstructure and mechanical properties of metakaolin (MK) based geopolymers. It was found that it is possible to have geopolymeric gel and calcium silicate hydrate (CSH) gel forming simultaneously within a single binder. The coexistence of these two phases is dependent on the alkalinity of the alkali activator and the MK / GGBFS mass ratio. It has been found that the formation of CSH gel together with the geopolymeric gel occurs only in a system at low alkalinity. In the presence of high concentrations of NaOH (> 7.5 M), the geopolymeric gel is the predominant phase formed with small calcium precipitates scattered within the binder. The coexistence of the two phases is not observed unless a substantial amount of a reactive calcium source is present initially. It is thought that voids and pores within the geopolymeric binder become filled with the CSH gel. This helps to bridge the gaps between the different hydrated phases and unreacted particles; thereby resulting in the observed increase in mechanical strength for these binders.

29Si NMR Study of Structural Ordering in Aluminosilicate Geopolymer Gels

A systematic series of aluminosilicate geopolymer gels was synthesized and then analyzed using 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) in combination with Gaussian peak deconvolution to characterize the short-range ordering in terms of T-O-T bonds (where T is Al or Si). The effect of nominal Na2O/(Na2O + K2O) and Si/Al ratios on short-range network ordering was quantified by deconvolution of the 29Si MAS NMR spectra into individual Gaussian peaks representing different Q4(mAl) silicon centers. The deconvolution procedure developed in this work is applicable to other aluminosilicate gel systems. The short-range ordering observed here indicates that Loewenstein's Rule of perfect aluminum avoidance may not apply strictly to geopolymeric gels, although further analyses are required to quantify the degree of aluminum avoidance. Potassium geopolymers appeared to exhibit a more random Si/Al distribution compared to that of mixed-alkali and sodium systems. This work provides a quantitative account of the silicon and aluminum ordering in geopolymers, which is essential for extending our understanding of the mechanical strength, chemical and thermal stability, and fundamental structure of these systems.