Bassanite (CaSO4 center dot 0.5H(2)O) dissolution and gypsum (CaSO4 centerdot 2H(2)O) precipitation in the presence of cellulose ethers

The setting of gypsum plaster in the presence of cellulose ether was studie d by monitoring the kinetics of bassanite (calcium sulfate-hemihydrate, HH) dissolution and of gypsum (calcium sulfate dihydrate, DH) precipitation. D issolution 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 diff usion controlled dissolution mechanism in deionized water as well as in cel lulose ether (CE) containing solution. The presence of CE at industrially r elevant conditions of 0.25 wt% retards the dissolution rate by 50%. The inc ubation 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 pre sence of CE depending on the degree of supersaturation. Close to equilibriu m, a surface reaction controls the growth of DH in pure CaSO4 solution as w ell 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 o ccurs 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 pr ecipitation processes are affected primarly due to the diffusion properties of dissolved Ca2+ and SO42 species in CE solution. (C) 2001 Elsevier Scien ce B.V. All rights reserved.