Phase diagrams of ASR products from thermodynamic modeling

A comprehensive thermodynamic database for Na/K ASR products in the CaO–SiO2–Na2O–K2O system is developed by integrating experimental solubility data and data from molecular simulations, enabling the determination of standard thermodynamic properties. Equilibrium thermodynamic calculations are then used to construct binary and ternary phase diagrams and to analyze the effects of temperature, calcium content, and alkali composition in ASR. The thermodynamic competition between ASR products and alkali-bearing C–S–H is quantified, providing insight into alkali uptake mechanisms and phase coexistence. We predict the formation of Na-shlykovite and Na-magadiite (or alternatively Na-kanemite at temperatures below 25°C) in CaO–SiO2–Na2O systems, and of K-shlykovite together with ASR-P1 in CaO–SiO2–K2O systems. The formation of these ASR products is intimately linked to the formation of alkali-bearing C–S–H. The phase diagrams corroborate the thermodynamic incompatibility of portlandite with ASR products. The diagram built for various temperatures provides a way of evaluating the representativeness of accelerated ASR testing under increased temperature conditions. Finally, the propensity to generate crystallization pressure is evaluated according to the phase diagrams.