The hydration kinetics of tricalcium silicate (C3S) has been the subject of much study, yet the experimentally observed effects of the water-to-cement (w/c) ratio and particle size distribution have been difficult to explain with models. Here, we propose a simple hypothesis that provides an explanation of the lack of any significant effect of w/c on the kinetics and for the strong effect of the particle size distribution on the amount of early hydration associated with the main hydration peak. The hypothesis is that during the early hydration period the calcium-silicate-hydrate product forms only in a reaction zone close to the surface of the C3S particles. To test the hypothesis, a new microstructure-based kinetics (MBK) model has been developed. The MBK model treats the C3S particle size distribution in a statistical way to save computation time and treats the early hydration as essentially a boundary nucleation and growth process. The MBK model is used to fit kinetic data from two published studies for C3S with different size distributions, one for alite (impure C3S) pastes and one for stirred C3S suspensions. The model is able to fit all the data sets with parameters that show no significant trend with particle size, providing support for the reaction zone hypothesis.