Effect of Alkali Cations on Aluminum Incorporation in Geopolymeric Gels P. Duxson, † G. C. Lukey, † F. Separovic, ‡ and J. S. J. van Deventer* ,† Department of Chemical and Biomolecular Engineering and School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia In geopolymeric gels, aluminum is known to have tetrahedral coordination and to be associated with an alkali cation to satisfy electronic neutrality. However, the mechanism of aluminum incorporation and the effects of different synthesis parameters are not well understood. 2 H, 27 Al, and 23 Na MAS NMR techniques have been used to elucidate the structural properties and mechanism of aluminum incorporation in metakaolin-based geopolymers. Na + (aq) and Al(OH) 4 - (aq) species were detected in the pore solution of geopolymers with Si/Al ratios of e1.40 and found to be directly related to the concentration of soluble silicate in the activating solution used for synthesis. The relative concentration of Na + (aq) in sodium and mixed-alkali geopolymers indicated a preferential incorporation of potassium into geopolymeric gels during curing. This work demonstrates a direct link between the composition of the silicate activator and aluminum incorporation in geopolymeric gels. Introduction Magic angle spinning (MAS) NMR spectroscopy was used by Davidovits 1 to determine the coordination of silicon and aluminum within geopolymeric binders. This work was seen as a major breakthrough in the under- standing of generic geopolymer structure. 2 The results of NMR experiments demonstrated that metakaolin- based geopolymers contain predominantly Al(IV) with trace amounts of Al(VI). Metakaolin contains Al(IV), Al- (V), and Al(VI); thus, during the course of geopolymer- ization, Al(V) and Al(VI) are converted to tetrahedral geometry, and an alkali cation is associated to maintain electronic neutrality. 1 Because of the highly reactive geometry, all Al(V) is believed to be consumed during reaction with small amounts of Al(VI), originating from unreacted metakaolin, remaining to form part of the hardened material. Until recently, there has been little expansion of Davidovits’ initial work. 29 Si MAS NMR spectra from Davidovits’ initial in- vestigations exhibited a predominantly featureless broad resonance at -94.5 ppm, similar to that of silicon in tetrahedral geometry in zeolite gels prior to crystalliza- tion. 2 The observed resonance was thought to consist of the five silicon Q 4 (mAl) centers as seen in previous investigations of aluminosilicates. 3 NMR experiments have since been conducted on geopolymers derived from different raw materials, and these confirm that the original results are also representative of these geopoly- mers. 4 Singh et al. 5 attempted to determine the chemical reaction mechanism for geopolymerization at ambient conditions by the study of two samples utilizing 27 Al and 29 Si MAS NMR spectroscopy. Phair and van Deventer investigated the link between the pH of the alkali silicate activating solution and the mechanical proper- ties of geopolymers using 29 Si solution NMR spectros- copy to determine speciation in the alkaline silicate solutions. 6 These studies, however, were unable to identify links between the NMR results and structural characteristics. In addition to 27 Al and 29 Si, 23 Na and other nuclei such as 39 K have been investigated, but the focus of these studies has not been on the effect of the alkali on the generic geopolymer structure. 4,7,8 Conflicting results in the literature suggest that both bound and unbound components of water exist in the pore and framework structure of a geopolymer after hardening, 4,9 which greatly affects the mechanistic understanding of aluminum incorporation. Xu and van Deventer 4,9 utilized differential scanning calorimetry (DSC) to observe an increase in the temperature at which a large endothermic peak occurred in geopolymer specimens synthesized using a mixture of feldspars (Na,K-AlSi 3 O 8 ) and kaolinite [Al 2 Si 2 O 5 (OH) 4 ]. 4 The results show that the temperature for the endothermic peak associated with dehydration varies slightly with composition, but is too low to suggest that strong bonds exist between water and geopolymeric gel. The work of Xu and van Deventer 4 correlates with the earlier work of Rahier et al., 9 who combined DSC and Thermo- gravimetry (TG) to measure the weight loss and energy flux associated with the heating of geopolymers. Both hydroxyl groups and bound H 2 O can be clearly identified using 1 H and 2 H MAS NMR spectra as peaks off the main isotropic water resonance. NMR spectroscopy therefore presents a powerful means for further elucida- tion of the environment of water residing in geopoly- meric gels after hardening. The current work represents a systematic investiga- tion of the role of the alkali cation and soluble silicate concentration on the incorporation of aluminum in metakaolin-based geopolymers, using 2 H, 27 Al, and 23 Na MAS NMR spectroscopy. Further understanding of the molecular structure of geopolymeric gels is elucidated by investigating changes observed in NMR spectra of geopolymers with different compositions. Because changes in molecular structure and aluminum incorpo- ration should be most easily identified in specimens with * To whom correspondence should be addressed. E-mail: jannie@unimelb.edu.au. Fax: +61-3-8344-7707. Tel.: +61-3- 8344-6619. † Department of Chemical and Biomolecular Engineering. ‡ School of Chemistry. 832 Ind. Eng. Chem. Res. 2005, 44, 832-839 10.1021/ie0494216 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005