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 pape r we present results concerning the crystallization of gypsum in the presen ce of different acids. The influence of additives on the crystallization ki netics 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 measure d by capillary electrophoresis. The results are well-related to the amount of adsorbed and incorporated additives during the crystallization. The infl uence of different additives on the morphology of a crystal of gypsum was a lso 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 correspo ndence between distances between the carboxylic groups of the additives and between the adsorption sites at the surface of the crystal. (C) 1999 Elsev ier Science B.V. All rights reserved.