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Calcium aluminate cements (CAC) are commonly used in building chemistry applications like self-levelling overlayment or underlayment, tile adhesive, special mortars In these applications specific properties like rapid hardening, rapid drying or shrinkage compensation are delivered through hydration of CAC in combination with calcium sulphates leading to rapid precipitation of ettringite, alumina hydrates and possibly AFm hydrates in some cases (monosulfo or monocarbo aluminates).Historically the main hydraulic phase of CAC has been monocalcium aluminate CaO.Al2O3 or CA.This paper investigates the impact of a mineralogical evolution from CA to C12A7 on early hydration and properties in combination with calcium sulphate.Hydration kinetics is measured by micro calorimetry.Solid and liquid phase evolution are characterised and their evolution is linked with macroscopic properties.Although CA or C12A7 globally leads to the same family of hydrated compounds (mainly ettringite and alumina hydrate), important differences are observed in the kinetics of precipitation.C12A7 leads to a more efficient hydration occurring in one step whereas it is usually divided in two steps with CA as shown by micro calorimetry.Early age properties are enhanced in the case of C12A7: quicker hardening and drying, better shrinkage compensation.Globally the system appears to be more robust to chemistry or temperature changes.These differences are interpreted through a mechanism based on solubility rate and stoichiometry of the predominant calcium aluminate phase.