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The role of a polymer as a retarder additive is to enhance workability of the gypsum paste by modifying its setting process and to improve its properties. The study focuses on the influence of water-soluble cellulose derivative hydroxypropyl methylcellulose (HPMC) on the setting kinetics as well as on the properties and the microstructure of gypsum. The influence of HPMC (used in mass fractions of up to 1.5%) on the kinetics of crystallization/hydration of hemihydrate calcium sulfate was investigated by heat flow measurements using differential scanning calorimetry (DSC). To describe the experimental results, the Avrami equation was used and its parameters n and K were determined. The hydration/crystallization process was modified by the presence of the polymer. As a result, the K value decreased, but n ≈ 2.5 pointed to the three-dimensional diffusion-controlled mechanism. The morphology studies involved porosity measurements. Mechanical properties of the gypsum plaster composites were examined using the bending strength test and correlated with the sample microstructure.

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