Tricalcium aluminate (C3A) is one of the main constituents of Portland cement. Even though it represents less than 10% of the total composition, its strong reaction with water can lead to a rapid setting, called flash set. Gypsum is added to regulate this reaction and preserves the workability of the cement paste at early ages. The understanding of the C3A-gypsum reaction is therefore crucial for the comprehension of the early hydration of cement. The role of the amount of C3A and the sulfate balance of cement hydration are of major interest since two important routes for the development of new cementitious materials are the increasing rate of substitution materials and the increasing level of aluminate in clinker. This thesis aimed to investigate the C3A-gypsum reaction alone and in the presence of alite in order to provide basic knowledge on the C3A-gypsum reactions and study the interactions that occur between the cement phases when hydration occurs in alite-C3A-gypsum systems. Alite and C3A as well as clinkers of controlled composition were synthesized. Model systems composed of C3A with different gypsum additions and alite-C3A-gypsum systems were studied in terms of hydration kinetics, phase assemblage and microstructural development. This work confirmed the findings of previous works on the mechanism that controls C3A-gypsum hydration when sulfate ions are present in solution and gave new results on the reaction when gypsum is depleted. It was shown that AFm phases do not crystallize only as platelets that fill the space between the C3A grains but also form an "inner" product within the original C3A grain boundaries and that hydrogarnet (for which the presence depends on the gypsum addition) crystallizes as a rim around C3A grains. Moreover the influence of the gypsum addition on the morphology of the AFm platelets and the role of their morphology on the hydration rate were highlighted. In the presence of alite the hydration kinetics of C3A-gypsum systems was subject to change due to the adsorption of sulfate ions on C-S-H and the reduction of the space available for the reaction. In addition, with the correlation of calorimetric, XRD and SEM analyses it was possible to observe a second formation of ettringite from C3A and sulfate ions released from C-S-H after the depletion of gypsum. Finally, the rate of alite hydration related to the growth of C-S-H was shown to be modified in the presence of C3A and gypsum.