The tricalcium aluminate reacts very quickly with water to form calcium hydroaluminates while the addition of calcium sulphate, leads to the formation of ettringite, with a slower process. The systematic study of the systems: “C3A” and “C3A-sulphate” by isothermal microcalorimetry and a follow up of ionic concentrations of diluted system suspensions made it possible to understand the sequence of the reactions and the limiting stages in both systems. In all cases, as it is in contact with the solution, the C3A dissolves what leads to the instantaneous precipitation of a certain quantity of calcium hydroaluminate. In case of only C3A hydration, there is then a decrease of hydration rate (which remains high) imposed by C3A dissolution. In presence of gypsum, the rate of hydration is then controlled by the low rate of ettringite formation. The observed second thermal peak is associated with the continuation of C3A dissolution after the sulphates consumption.