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The study is focused on the influence of a water-soluble polymer (in weight fraction up to 1.5%), cellulose derivativeshydroxyethyl methyl cellulose, on gypsum properties. Gypsum setting involves two processes: gypsum hydration/crystallization and probably formation of a polymer film in material pores. The processes are studied by various methods such as setting time and mechanical measurements, scanning electron microscopy and differential scanning calorimetry. The additive acts as a retarder (an increase in setting time), and it modifies the morphology of calcium sulfate dihydrate crystals, leading to the change in mechanical properties-an increase in bending stress. The mechanism of gypsum crystal growth during hemihydrate hydration is predicted to be a nucleation control process (the Avrami equation is applied). The value of nucleation rate constant decreases with an increasing additive content.

Pastes based on gypsum (calcium sulphate hemihydrate) and two chosen water-soluble polymer admixtures, i.e. hydroxyethylmethyl cellulose (HEMC) and poly(vinyl acetate) (PVAC) were prepared. The impacts of the polymers on gypsum setting, as well as thermal and mechanical properties were studied. Heat effect and the rate of setting depending on water to gypsum ratio and the presence of admixtures were measured. The results have shown a strong effect of the polymer admixture (1%) on the prolongation of gypsum setting time. The presence of polymer molecules in water solution hinders the crystallization process of gypsum dihydrate. HEMC and PVAC have also a significant influence on gypsum mechanical properties especially at low water content. Kinetics of the setting process of gypsum hemihydrate is discussed.