Elucidating the Effects of Water Activity on the Hydration Kinetics and Thermodynamics of Ye’elimite–Calcium Sulfate Hydrate Systems

Relative humidity and water activity (aH) reductions have been known to suppress the hydration of anhydrous cement clinker phases. However, the relationship between ye’elimite (C4A3$) hydration kinetics and water activity remain unclear. This study employs experimental and thermodynamic modeling approaches to investigate the influence of water activity on ye’elimite hydration in the “C4A3$ + water”, and “C4A3$ + gypsum + water” systems. Experimental findings indicate that as water activity decreases, C4A3$ hydration diminishes until the reaction is brought to a halt at a threshold aH. The critical aH and corresponding solubility constant of C4A3$ (KC4A3S¯) estimated from thermodynamic analysis are 0.46 and 10–24.554, respectively. Critical aH and KC4A3S¯ are fundamental data for the numerical modeling of hydration in C4A3$-based cementitious systems. Additionally, thermodynamic modeling was employed to reveal the effects of isopropanol ingress on the precipitation-dissolution of hydrated phase assemblage and the speciation in the aqueous pore solution.