Crystallization of gypsum from hemihydrate in presence of additives

The crystallization of gypsum (CaSO4 . 2H(2)O) from a suspension of calcium sulphate hemihydrate is well-known as the setting of plaster. In this paper we present results concerning the crystallization of gypsum in the presence of different acids. The influence of additives on the crystallization kinetics and on the morphology of gypsum crystal have been studied. The rate retarding effect was determined by recording, versus time, the conductivity of suspension of hemihydrate. The amount of additives adsorbed was measured by capillary electrophoresis. The results are well-related to the amount of adsorbed and incorporated additives during the crystallization. The influence of different additives on the morphology of a crystal of gypsum was also investigated and discussed. Their efficiency is related to the presence of calcium on the faces (1 2 0) and ((1) over bar 1 1) and to the correspondence between distances between the carboxylic groups of the additives and between the adsorption sites at the surface of the crystal. (C) 1999 Elsevier Science B.V. All rights reserved.