THE SETTING OF GYPSUM PLASTER .2. THE DEVELOPMENT OF MICROSTRUCTURE AND STRENGTH

The setting process of gypsum plasters has been followed using strength testing, acoustic techniques and scanning electron microscopy. Gypsum plasters were found to set in three stages: formation of the calcium sulphate dihydrate crystal matrix followed by relief of internal stress, and finally evaporation of excess water. The alpha-calcium sulphate hemihydrate set faster than the beta-form, giving a higher initial and final set strength. The set alpha-plaster consisted of relatively short stubby dihydrate needles with a greater degree of intercrystalline bonding, resulting in greater strength. The microstructure of set alpha-plaster appeared to be the result of a slower precipitation rate of dihydrate during the hydration reaction of hemihydrate.