The aim of this thesis is to understand the mechanisms underlying the main hydration peak and later ages of alite hydration; it proposes new mechanistic models for these two stages. Alite is the main constituent of Portland Cement (PC), and is responsible for the setting and hardening of concrete during the first reaction day. Because future cements will still be based on PC, alite remains worthy of investigation. Alite hydration can be divided into five stages according to the heat flow released during the first two days of reaction. The three last stages, the acceleration and deceleration periods (which combined are called âmain hydration peakâ and last one day) and the later age (one day to 28 days), are still under debate. As long as the mechanisms of alite hydration are not fully understood the path toward the optimization of cements is hindered. The two hypotheses referred in all major textbooks for the main hydration peak, the diffusion barrier and nucleation and growth by impingement suffer from critical pitfalls examined in chapter 2. In 2015, Bazzoni suggested a new mechanism: C-S-H needles progressively cover the alite surface while simultaneously shift from a fast to a slow growth mode. The first contribution of the thesis is the Needle model. It was built onto Bazzoniâs hypothesis and proved it to be right though with some shades. The peak time coincides with the time at which most of the needles have nucleated and are in their fast growth regime. As they gradually enter the slow growth regime, the rate decreases. The Needle model also studied for the first time an old hypothesis raised by Taylor: that the small grain dissolution may explain part of the transition from the acceleration to deceleration period. The model shows that the small grains dissolution do not affect the shape of the main hydration peak, they do not cause the transition, though they do sharpen and shift it slightly left. This effect is only visible for particle size distribution having a large fraction of grains below 10 micrometers, or alternatively for powders that reach more than 50% of degree of hydration at 24 hours. Regarding the later ages, a new model is built to test the space-filling hypothesis. The proba- bility of outer C-S-H growth is found to be not linear with the available space for precipitation. The later age kinetics are shown to be not controlled by a single overwhelming mechanism but rather by the cumulative effects of several coupled mechanisms.