Studies on Manufacture of Gypsum Plaster by the Special Hydrothermal Method, (III)

The further study, in continuing the previous studies [This Journal, 56, (631), 43 (1948) and 57, (635), 6 (1949)], was carried out, as following: (1) Powder of two kind sof natural gypsum crystal and waste mould gynsum from pottery industry was treated by the hydrothermal heating of about 2 atm. pressure and about 123°C steam in the dilute 0.05-0.1% citric acid solution, and, dried up to the special gypsum plaster of the type of α-gypsum. (2) Waste mould gypsum powder was treated by the same way of the autoclave heating but in water, cooled naturally to the room temperature. The crystal gypsum thus refined and recrystallized to CaSO4⋅2H2O, and then calcined to the common gypsum plaster by heating about at 180°C by the common gypsum calcining process. (3) These gypsum samples were comparatively tested for their physical and chemical properties. The results of testing show the superiority of the gypsum, obtained by the hydrothermal treatment in the autoclave with the dilute (0.05-0.1%) citric acid solution. (4) The citric acid solution of 0.05-0.1% was observed to be most suitable concentration for making the best calcined gypsum as α-gypsum for pottery industry, dental use, etc.