Hydration of tricalcium silicate (Ca3SiO5), the pure phase used as a model of the portland cements, is the chemical process leading to the formation of hydrates, while setting is a definite time event corresponding to the change of the paste from the soft to the hard state. Setting results from interactions between anhydrous or very partially hydrated particles. The analysis of these interactions leads to the identification of two fundamental steps : the coagulation of cement grains during the first minutes following the mixing and the rigidification of the coagulated structure which arises simultaneously with the acceleration of the calcium silicate hydrates (C-S-H) formation. The chemical evolution of the system, which controls the lime concentration in solution, determines the nature of particle interactions and the physical evolution of the suspension or paste : there exists a critical range of lime concentration required to initiate the coagulation of cement particles and the ability of C-S-H to make rigid the coagulated structure depends on the solubility of these particles.