The hydration of tricalcium silicate (C3S)-the major phase in cement-is effectively arrested when the activity of water (a(H)) decreases below the critical value of 0.70. While it is implicitly understood that the reduction in a(H) suppresses the hydration of tricalcium aluminate (C(3)A: the most reactive phase in cement), the dependence of kinetics of C(3)A hydration on a(H) and the critical a(H) at which hydration of C(3)A is arrested are not known. This study employs isothermal microcalorimetry and complementary material characterization techniques to elucidate the influence of a(H) on the hydration of C(3)A in [C(3)A + calcium sulfate (C$) + water] pastes. Reductions in water activity are achieved by partially replacing the water in the pastes with isopropanol. The results show that with decreasing a(H), the kinetics of all reactions associated with C(3)A (eg, with C$, resulting in ettringite formation; and with ettringite, resulting in monosulfoaluminate formation) are proportionately suppressed. When a(H) <= 0.45, the hydration of C(3)A and the precipitation of all resultant hydrates are arrested; even in liquid saturated systems. In addition to-and separate from-the experiments, a thermodynamic analysis also indicates that the hydration of C(3)A does not commence or advance when a(H) <= 0.45. On the basis of this critical a(H), the solubility product of C(3)A (K-C3A) was estimated as 10(-20.65). The outcomes of this work articulate the dependency of C(3)A hydration and its kinetics on water activity, and establish-for the first time-significant thermodynamic parameters (ie, critical a(H) and K-C3A) that are prerequisites for numerical modeling of C(3)A hydration.