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A statistical thermodynamic model for the ordering of aluminum- and silicon-centered tetrahedra in fully condensed X-ray amorphous aluminosilicate network structures is presented. In particular, the model is applied to metakaolin-based geopolymers in the presence of different charge-balancing cations and mixtures of cations. The model uses Gibbs energy minimization techniques based on the energetic preference for Si−O−Al linkages over Si−O−Si and Al−O−Al to calculate the concentrations of each of the types of bonds. A random bond distribution is then applied to calculate the concentration of each possible tetrahedron coordination. The model output is compared with experimental 29 Si MAS NMR results, and is found to predict the observed behavior satisfactorily. A correction to account for the presence of unreacted metakaolin in geopolymers is found to significantly improve the model fit at low silica content. The calculated energy penalty parameters closely match those obtained from the literature, and the trends in the parameters allow discussion and analysis of the effects of the different cations on geopolymerization of metakaolin. The applicability of this model to geopolymeric systems indicates the potential for wider utilization in description of other amorphous aluminosilicate systems.
Provis et al. (Sun,) studied this question.