Simulations of tricalcium silicate (C3S) hydration using a kinetic cellular automaton program, HydratiCA, indicate that the net rate depends both on C3S dissolution and on hydration product growth. Neither process can be considered the sole rate-controlling step because the solution remains significantly undersaturated with respect to C3S yet significantly supersaturated with respect to calcium silicate hydrate (C–S–H). The reaction rate peak is attributed to increasing coverage of C3S by C–S–H, which reduces both the dissolution rate and the supersaturation of C–S–H. This supersaturation dependence is included in a generalized boundary nucleation and growth model to describe the kinetics without requiring significant impingement of products on separate cement grains. The latter point explains the observation that paste hydration rates are insensitive to water/cement ratio. The simulations indicate that the product layer on C3S remains permeable; no transition to diffusion control is indicated, even long after the rate peak.