Portland cement is a mixture of solid phases which all react with water. Tricalcium silicate (Ca3SiO5) is its main component and is often used in model systems to study cement hydration. It is generally recognized that setting and hardening of cement are due to the formation, by a dissolution-precipitation process, of a calcium silicate hydrate (C-S-H) on anhydrous grains during Ca3SiO5 hydration. The purpose of this paper is to study the effect of Ca3SiO5 particle size on the nucleation-growth process of C-S-H. An experimental study of the rate of hydration by using different grain sizes under controlled conditions has been performed. The experimental data have been compared with results obtained by numerical simulation based on a particles-aggregation model. The thickness of the C-S-H layer when the reaction becomes limited by diffusion has been then estimated.