RETARDING ACTION OF VARIOUS CHEMICALS ON SETTING AND HARDENING CHARACTERISTICS OF GYPSUM PLASTER AT DIFFERENT PH

Gypsum plaster sets quickly due to its natural process of crystallizat ion, For commercial applications, the retardation of plaster to a desi red level is required. The effect of various chemicals as retarders on the setting time, compressive strength and microstructure of the gyps um plaster was investigated at pH 4 to 12 adjusted by the addition of Ca(OH)(2) or HCl to gypsum plaster. It was found that retardation fact or has no direct relation with the compressive strength of gypsum plas ter but pH is certainly related to the strength factor. Maximum compre ssive strength of the plaster was obtained at pH 7.0. The morphology o f the hardened gypsum plaster was found to change according to variati on in the pH of the aqueous phase as well as the nature of retarder ad ded.