In this paper, an original approach is used to study the calcium silicate hydrate (C-S-H) layer formation on the surface of grains of anhydrous silicate during tricalcium and dicalcium silicate hydration from the variation of the rate of hydration with lime concentration. The effects of C-S-H nucleation and growth on the curves for the degree of reaction against time have been separated in both experimental study of the rate of hydration in controlled conditions and numerical simulation of the growth of C-S-H on a surface from a particle aggregation model. The influence of the number of nuclei and of the different growth modes has been quantified.