The hydration kinetics of tricalcium silicate (C3S), the main constituent of portland cement, were analyzed with a mathematical "boundary nucleation" model in which nucleation of the hydration product occurs only on internal boundaries corresponding to the C3S particle surfaces. This model more closely approximates the C3S hydration process than does the widely used Avrami nucleation and growth model. In particular, the boundary model accounts for the important effect of the C3S powder surface area on the hydration kinetics. Both models were applied to isothermal calorimetry data from hydrating C3S pastes in the temperature range of 10 degrees-40 degrees C. The boundary nucleation model provides a better fit to the early hydration rate peak than does the Avrami model, despite having one less varying parameter. The nucleation rate (per unit area) and the linear growth rate of the hydration product were calculated from the fitted values of the rate constants and the independently measured powder surface area. The growth rate follows a simple Arrhenius temperature dependence with a constant activation energy of 31.2 kJ/mol, while the activation energy associated with the nucleation rate increases with increasing temperature. The start of the nucleation and growth process coincides with the time of initial mixing, indicating that the initial slow reaction period known as the "induction period" is not a separate chemical process as has often been hypothesized.