Bassanite (CaSO4•0.5H2O) dissolution and gypsum (CaSO4•2H2O) precipitation in the presence of cellulose ethers

The setting of gypsum plaster in the presence of cellulose ether was studied by monitoring the kinetics of bassanite (calcium sulfate-hemihydrate, HH) dissolution and of gypsum (calcium sulfate dihydrate, DH) precipitation. Dissolution rates of various industrial HH vary between 4 and 9 x 10(-5) mol/m(2) s when normalized to the BET surface area and the content of soluble CaSO4. An apparent activation energy of about 10-20 kJ/mol indicates a diffusion controlled dissolution mechanism in deionized water as well as in cellulose ether (CE) containing solution. The presence of CE at industrially relevant conditions of 0.25 wt% retards the dissolution rate by 50%. The incubation time for DH nucleation is extended by 100% in presence of 0.25 wt% CE. The subsequent DH growth is significantly affected up to 80% by the presence of CE depending on the degree of supersaturation. Close to equilibrium, a surface reaction controls the growth of DH in pure CaSO4 solution as well as in the presence of CE. At high degrees of supersaturation, DH growth is controlled by volume diffusion. Since 90% of the HH-DH transformation occurs at high degrees of supersaturation, we conclude that the interaction between CE and HH as well as DH surfaces is weak and the dissolution and precipitation processes are affected primarly due to the diffusion properties of dissolved Ca2+ and SO42 species in CE solution. (C) 2001 Elsevier Science B.V. All rights reserved.