Gypsum grown under pressure from dense suspension of CaSO₄.0.67H₂O

The growth of gypsum, CaSO₄.2H₂O, under pressure was studied starting from a dense suspension of CaSO₄.0.67H₂O in water. Pressures of 150, 300 or 600 MPa were applied at different times from the beginning of hydration of the initial material and were maintained constant for 15 min. The crystallized material was examined by DTA, X-ray and SEM techniques. Whether the hemihydrate or gypsum crystallizes depends on the time of initial hydration, i.e. whether it is short (less than about 15 min) or longer. When the time of hydration is less than 5 min and the pressure is low, only partially dissolved crystals of the hemihydrate are found in the sample. When the pressure increases, the hemihydrate partially recrystallizes as well formed elongated crystals. When both the time of hydration and pressure increase, more and more gypsum crystals form with typical habit. At constant hydration time, an increase in pressure favours the occurrence of gypsum. When the hydration time is about 30 min, only gypsum crystals occur. When the pressure is applied 30 min after the time of hydration, most of the gypsum crystals are broken in the solid texture.