The hydration of tricalcium silicate (C3S) -- the major phase in cement -- is effectively arrested when the activity of water (aH) decreases below the critical value of 0.70. While it is implicitly understood that the reduction in aH suppresses the hydration of tricalcium aluminate (C3A: the most reactive phase in cement), the dependence of kinetics of C3A hydration on aH and the critical aH at which hydration of C3A is arrested are not known. This study employs isothermal microcalorimetry and complementary material characterization techniques to elucidate the influence of aH on the hydration of C3A in [C3A + calcium sulfate (C$) + water] pastes. Reductions in water activity are achieved by partially replacing the water in the pastes with isopropanol. The results show that with decreasing aH, the kinetics of all reactions associated with C3A (eg, with C$, resulting in ettringite formation; and with ettringite, resulting in monosulfoaluminate formation) are proportionately suppressed. When aH ≤ 0.45, the hydration of C3A 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 C3A does not commence or advance when aH ≤ 0.45. On the basis of this critical aH, the solubility product of C3A (KC3A) was estimated as 10-20.65. The outcomes of this work articulate the dependency of C3A hydration and its kinetics on water activity, and establish -- for the first time -- significant thermodynamic parameters (ie, critical aH and KC3A) that are prerequisites for numerical modeling of C3A hydration.