Process for making calcium sulfate alpha-hemihydrate from calcium sulfate dihydrate

In a process for making calcium sulfate hemihydrate from a finely divided calcium sulfate dihydrate by recrystallization transformation of said calcium sulfate dihydrate in the presence of saturated steam, the improvement wherein a molded body is formed by pressing said calcium sulfate dihydrate, at a pressure from 0.1 to 14 N/mm.sup.2, said dihydrate having physically bound water in a range from about 3 to 20% by weight, said pressing being sufficient to achieve a pore volume that is 15 to 60% of total volume and more than 5% by volume of said pore volume contains air, said molded body being fed at ambient temperature to an autoclave and being heated in said autoclave with saturated steam, water being formed by condensation of said saturated steam and subsequently being drawn into said molded body as a condensate by capillary action of open systems of said pores, water held in said pores at least partially coming from said finely divided calcium sulphate dihydrate from which said molded body is formed, regulating crystal growth and crystal pattern of said calcium sulfate hemihydrate which grows from an aqueous solution phase by a process temperature in a range between 110.degree. C. and 180.degree. C. and by the pressure of the process atmosphere in said autoclave said pressure being at or above that of saturated steam at said process temperature, after said growth at least partially forcing out said water present in said pore volume of said molded body by controlled release of said pressure in said autoclave, and said molded body being removed from said autoclave.