Hydration of calcium sulfate hemihydrate (CaSO4 center dot 1/2H2O) into gypsum (CaSO4 center dot 2H(2)O). The influence of the sodium poly(acrylate)/surface interaction and molecular weight

The retarding influence of sodium poly(acrylate) (PANa) on the hydration of calcium sulfate hemihydrate (CaSO(4)1/2H(2)O) was investigated. This study reports the influence of sodium poly(acrylate) on hemihydrate dissolution, on homogenous and heterogeneous gypsum (CaSO4. 2H(2)O) nucleation as well as on gypsum growth. It is shown that adsorption of PANa does not hinder th e dissolution of hemihydrate in the present experimental conditions. The sp ecific interaction of PANa with gypsum can explain the oriented growth of g ypsum crystal. The gypsum growth is slowed down but cannot be blocked by th e adsorption of PANa. On the other hand, PANa can block the heterogeneous a nd homogenous gypsum nucleation. As soon Its a critical surface density of PANa onto the hemihydrate surface is reached, the heterogeneous gypsum nucl eation is prevented and hemihydrate hydration is indefinitely blocked. The interaction between PANa and the hemihydrate surface is of prime importance to control hydration. Also, the influence of the molecular weight of PANa on homogenous nucleation has been investigated. The precipitation of calciu m polyacrylate can explain the differences between the two molecular weight s used (2100 and 20 000). This work leads to the conclusion that heterogene ous nucleation is the key process that controls hydration of a system in wh ich hemihydrate dissolution, gypsum nucleation and growth are all occurring at the same time in a continuous manner. (C) 2000 Published by Elsevier Sc ience B.V.