RI 5477 Hydration-Rate Studies Of Gypsum Plasters: Effects Of Small Amounts Of Dissolved Substances ? Introduction And Summary

Gypsum plaster is calcium sulfate hemihydrate (CaS04-1/2H2O). There are two varieties, commonly designated as a and ß. The hydration setting) of this plaster involves the chemical reaction: CaS04?1/2H20(c) + 3/2H20(l) = CaSO4?2H20(c); that is, the hemihydrate combines with water to form the dihydrate. Many substances, when present as impurities, either in the hemihydrate or the water have influenced the rate of the hydration reaction. The objective of the Bureau of Mines work described in this report was to investigate the accelerating or retarding effects of small amounts of dissolved substances and to attempt to classify these effects upon a chemical basis. To eliminate competing effects, the work was conducted separately on the two varieties of hemihydrate; both were in highly pure form. (Commercial plasters usually are a mixture of the hemihydrates, with other substances present as impurities.) As the hydration reaction of each variety of hemihydrate evolves heat in amounts known from previous work, 5,6/ the authors decided to use calorimetric methods to follow the extent of the hydration reaction as a function of time. Two calorimetric methods were employed. One was designed to study the hydration reaction with a large water to plaster ratio and the other with a small water to plaster ratio (to simulate commercial practice in utilizing plaster).