Modeling Si/Al Ordering in Metakaolin‐Based Geopolymers

A statistical thermodynamic model for the ordering of aluminum- and silicon-centered tetrahedra in the X-ray amorphous network structure of metakaolin-based geopolymers in the presence of different charge-balancing cations and mixtures of cations is presented. The model utilizes Gibbs energy minimization calculations, based on the energetic preference for Si-O-Al linkages over Si-O-Si and Al-O-Al, to calculate the concentrations of each type of bond. 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 found to predict the observed behavior satisfactorily. A correction to account for the presence of unreacted metakaolin in geopolymers is found to improve the model fit at low silica content. Next-nearest-neighbor effects are evident in deconvoluted NMR peak positions. The calculated energy penalty parameters closely match those obtained from the literature, and the trends in these parameters allow analysis of the effects of the different cations on geopolymerisation of metakaolin. The applicability of this model to geopolymeric systems indicates the potential for wider utilization in description of other amorphous aluminosilicate systems.