Two types of gypsum raw materials, selenite and alabaster, were used to prepare dental stone (α-hemihydrate).Gypsum lumps (0.8～2.5 cm) were hydrothermally treated at 135, 160 and 180°C for 6, 2 and 1 h respectively.The physicochemical properties and composition characteristics of the α-hemihydrates prepared from selenite and alabaster raw gypsum were determined. The results indicated that both the selenite and alabaster raw gypsum show the same chemical and mineralogical composition of calcium sulphate dihydrate. They differs only in their microstructure, selenite raw gypsum exhibits perfect regular crystals while alabaster gypsum, on the other hand,exhibits irregular, large size and interlocked crystals. Selenite is more accessible to dehydration than alabaster raw gypsum. The α-hemihydrate samples prepared from alabaster raw gypsum showed a very short setting time and a lower compressive strength values in relation to that prepared from selenite gypsum. The compressive strength values of the all prepared samples were higher than that specified by A.D.A specification for dental use. To optimize the setting time of both products further studies are needed.
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