1991 Volume 40 Issue 457 Pages 1372-1378
The effects of three types of polymer modification on both hydration characteristics and physical properties of the gypsum pastes and mortars originated from anhydrite plaster have been studied at various polymer-gypsum ratios. The polymer dispersions used were ethylene-vinyl acetate (EVA), polyacrylic ester and styrene-butadiene rubber. The degree of hydration of the gypsum pastes and mortars was examined by X-ray diffraction, scanning electron microscopy, differential scanning calorimetry and porosity measurement. The setting time of fresh gypsum mortars (polymer-modified and unmodified) as well as the physical properties of hardened gypsum mortars were also measured. The hydration was complete in about 3 days, and the degree of hydration of the polymer-modified gypsum mortars was smaller than that of unmodified mortar. The setting time of gypsum mortars was retarded by the addition of polymer, and was affected by the chemical structure of polymer. The flexural and tensile strengths of the EVA-modified mortar with a polymer-gypsum ratio of 5% were higher than those of unmodified mortar. Its elastic modulus was the smallest of all, and further EVA-modified mortars showed small linear expansion. From the above test results, the EVA-modified gypsum mortar with a polymer-gypsum ratio of 5% was shown to be one of the candidates of the better plastering materials for polystyrene and polyurethane foams.
