Ceramics-Silikáty 38, (3) 173 - 177 (1994)

The thermal treatment of gypsum in boiling 1M, 2M, 3M, 4M and 4.5M solutions of magnesium chloride, at atmoshperic pressure, in order to obtain α-CaSO₄.0.5H₂O was performed. Three mechanism of α-hemihydrate formation were identified in solutions of different concentrations. Gypsum transformation into α-hemihydrate operated in solid phase, through an intermediate product, β-hemihydrate, in solutions of low concentrations (1 and 2M), direct in solutions of high concentrations (3 and 4M) and in solid phase in the solution of the highest concentration (4.5M). Specific surface areas of the obtained products indicated that slow transformation processes (regardless ol the mechanism involved) gave products of small surface areas, or high usability.