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Reaction mechanisms for the early stages of hydration of tricalcium silicate (Ca3SiO5) have not been agreed upon, although theories have appeared in the literature. In this paper, a mechanistic description is proposed that is consistent with a wide range of reported experimental observations, and which is supported quantitatively by simulations using HydratiCA, a new three-dimensional microstructure model of chemical kinetics. Rate processes are quantitatively modeled using probabilistic cellular automaton algorithms that are based on the principles of transition state theory. The model can test alternate assumptions about the reaction paths and rate-controlling steps, making it a kind of experimental tool for investigating kinetics and interpreting experimental observations. It is used here to show that hydration of Ca3SiO5 is most likely controlled by nucleation and growth of a compositionally variable calcium silicate hydrate solid, mediated at very early times by a transient, thermodynamically metastable solid that rapidly covers and sharply reduces the dissolution rate of Ca3SiO5. This proposed mechanism involves important elements of two leading theories of Ca3SiO5 hydration, neither of which alone has been able to capture the full range of experimental data when tested by the model.