Abstract
The hydration of calcium sulfate hemihydrate is studied mainly by the conduction calorimeter. The hydration process can be explaind by dissolution and recrystallization as follows; calcium sulfate hemihydrate dissolves into liquid phase and the nuclei of calcium sulfate dehydrate appear and grow into crystallites. As ‘gypsum gel’ is not found, the topochemical reaction is not considered to occur in the hydration of calcium sulfate hemihydrate.A small amount of calcium sulfate dihydrate is produced on the grain surface of plaster of Paris when it is exposed to moist air. The dehydrates thus produced play a very important role in the hydration process, because it acts as the nuclei. β hemihydrate has larger specific surface area, and dissolves faster than α hemihydrate. However, the production of dihydrate on the surface is retarded by the soluble anhydrite exist in the sample. Accordingly, induction period is, sometimes, longer than that of α hemihydrate.
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