Reductions in relative humidity (RH) and water activity (aH) have been identified as factors that inhibit the hydration process of anhydrous cement clinker phases, particularly ye’elimite (C4A3$), which is pivotal in calcium sulfoaluminate (CSA) cement. However, the precise correlation between C4A3$ hydration kinetics, water activity, and the critical aH level at which C4A3$ hydration ceases remains unclear. This study utilizes x-ray diffraction, microcalorimetry, and thermodynamic analysis to explore how water activity affects the hydration of ye’elimite in both ye’elimite-water (C4A3$ + water = Y) and ye’elimite-gypsum-water (C4A3$ + C$H2 + water = YG) systems. Lowering water activity is achieved by replacing a portion of the water in the mixtures with isopropanol (IPA). Experimental results demonstrate that decreasing water activity proportionally reduces the rates of all reactions involving C4A3$ until hydration comes to a complete stop at approximately 90%IPA on a weight basis (%wt IPA), constituting the critical threshold. The critical aH (where C4A3$ hydration ceases) and the solubility product constant of C4A3$ (KC4A3$) are determined through thermodynamic analysis as 0.46 and 10-25.568, respectively. These critical parameters are essential for numerical modeling of hydration processes in C4A3$-based cementitious systems. Moreover, thermodynamic modeling unveils, for the first time, the effects of isopropanol infiltration on the precipitation-dissolution dynamics of hydrated phases and the speciation in the aqueous pore solution.
