Factors determining the ‘water requirement’ of gypsum plaster

Abstract

Abstract An investigation has been made of the relationship between the ‘water requirement’ of gypsum plaster, the surface area of the plaster determined by gas adsorption and by permeability to air, and the bulk volume. The water requirement will undoubtedly depend upon the factors determining the surface area of the plaster, i.e. the particle size and shape, but the results show that other factors exert strong effects. The water requirement of hemihydrate prepared by still methods is shown to be considerably reduced by grinding in a mortar with a pestle, which increases the surface area. In contrast to its marked effects upon the rate of setting, recalcination does not affect the water requirement. There is a pronounced correlation between the water requirement and bulk volume. It appears that the factors that determine the bulk volume also play a large part in determining the water requirement. These factors involve the condition of the surface of the particles rather than their size and shape.