Investigation of alkali-silica reaction in alkali activated cements by thermodynamic modelling

The likelihood and consequences of the alkali-silica reaction (ASR) in mortars and concretes based on alkali-activated materials (AAMs) are still under discussion, due largely to the characteristically high alkali levels of this class of cements. In this study, applying thermodynamic modelling to the study of ASR provides a new insight into ASR mechanisms as a function of binder chemistry. By considering different activators (NaOH, Na2SO4, Na2CO3, Na2O.nSiO2 with various values of the modulus n) at different dosages, the volume of shlykovite-type ASR products that can potentially form in AAMs with partially reactive siliceous aggregates was calculated. The solution chemistry and phase assemblage after hydration provide further information to aid in explaining the observed trends. Although high concentrations of Na-bearing activators were used in the AAM formulations, much less Na-shlykovite and no K-shlykovite are formed, compared to Portland cement. The volume of Na-shlykovite formed decreases with an increase in the dosage of activators (for all activators tested), and with a decrease in the modulus of sodium silicate when this is the activator used. A high concentration of Ca after hydration, rather than the concentration of alkalis in the pore solution, is the controlling factor in shlykovite formation, which represents ASR in these simulations.