Mechanisms and parameters controlling the tricalcium aluminate reactivity in the presence of gypsum

To understand the mechanisms and the parameters controlling the reactivity of tricalcium aluminate in the presence of gypsum at an early age, a study of the hydration of the “C₃A-sulphate” system by isothermal microcalorimetry, conductimetry and a monitoring of the ionic concentrations of diluted system suspensions have been carried out with various gypsum quantities. The role of C₃A source and its fineness were also studied. This work shows the fast initial formation of AFm phase followed by ettringite formation during the period when the sulphate is consumed. It has been highlighted that the time necessary to consume all the gypsum varies with the type of C₃A and it has been attributed to the intrinsic reactivity of each one and mainly to the change of fineness from one C₃A to another. Results are discussed alongside hypothesis from the literature to explain the slowing down of C₃A hydration process in the presence of calcium sulphate.