Characterization of the hydration process of calcium sulphate hemihydrate by simultaneous evaluation of chemical and physical parameters

A new approach to follow the hydration reaction of calcium sulphate hemihydrate is described, based on two automated-computerized systems. The results of the study show that the hydration process can be divided into four stages: induction period, nucleation, crystal growth and diffusion-controlled reaction. It was found that the initial setting time obtained by the ‘knife’ test coincides with the end of the induction period, during which the degree of hydration is zero. Initial setting time obtained by the Vicat test coincided with the time when the degree of hydration was 10%. The length change curves could be interpreted in terms of four stages. Shrinkage occurred when water was consumed in the hydration reaction during the time of the Vicat initial setting; a high rate of expansion was initiated when the degree of hydration was about 20% and it was accompanied by a high rate of heat evolution. These two processes terminated at about the same time.