The setting of gypsum plaster
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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 α-calcium sulphate hemihydrate set faster than the β-form, giving a higher initial and final set strength. The set α-plaster consisted of relatively short stubby dihydrate needles with a greater degree of intercrystalline bonding, resulting in greater strength. The microstructure of set α-plaster appeared to be the result of a slower precipitation rate of dihydrate during the hydration reaction of hemihydrate.
KeywordsGypsum Internal Stress Dihydrate Strength Testing Calcium Sulphate
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