This paper focuses on the interactions that occur between cement phases during early hydration, especially between the hydration of alite and C(3)A-gypsum. This study shows that alite reaction is accelerated in the presence of gypsum. This acceleration was attributed to the interaction of gypsum with Aluminium ions present in alite. Three exothermic peaks attributable to the aluminate reaction could be observed in model cements instead of one in C(3)A-gypsum systems. The first corresponds to the C(3)A dissolution when the sulfate ions run out, but the product formed is ettringite. The second corresponds to the formation of calcium monosulfoaluminate, but the third could not be assigned to any specific reaction. The effect of alite/C(3)A ratio, the dispersion of the phases in the grains and the temperature were also investigated: With respect to temperature, it was found that the activation energy for the C(3)A-gypsum reaction was higher than that for alite. (c) 2012 Elsevier Ltd. All rights reserved.