THE HYDRATION OF PLASTER OF PARIS

Abstract

The hemihydrate of calcium sulphate labelled with calcium-45 or sulphur-35 was hydrated in a supersaturated solution of calcium sulphate at 21 °C. The distribution of the radioactive isotope between the solution and the solid phases and the concentration of the solution throughout the period of hydration were measured. The exchange of calcium ions between crystalline gypsum and its solution was also studied. The experimental results are discussed from the viewpoint of the mechanism involved. They are found to be in agreement with a mechanism which postulates a single passage of the structural units of the solid hemihydrate through the solution and not consistent with one of direct hydration without temporary independent existence of the products in the liquid phase.