impact of various additives onto morphology of gypsum crystals

Nowadays, functional additives represented by multiple classes of substances and compounds, including polymers of different origin, are available for introduction into dry mixtures based on gypsum binders. However, their impact onto the growth and formation of calcium sulfate dihydrate (CaSO∙2HO) crystals generated in the course of hardening of gypsum binders is not quite clear. Therefore, the objective of the research was to analyze the processes of growth and formation of calcium sulfate dihydrate crystals caused by functional additives based on polymers of different origin. The analysis was composed of three stages. At the first stage, both pure and modified calcium sulfate dihydrate crystals were synthesized. Super plasticizers based on sulfonated melamine-formaldehyde resin, methylcellulose (MC) and redispersible polymer powder based on copolymer of vinyl acetate, ethylene and vinyl chloride (VAEVC) were applied as additives. At the second stage, the objective was to identify the influence of polymer additives on the shape and size of calcium sulfate dihydrate crystals; therefore, an X-ray analysis of synthesized crystals was performed. Following the X-ray analysis data and with the help of a special software programme, possible combinations of simple forms of CaSO∙2HO were simulated, and the habitus of generated crystals was also studied. At the third stage, the objective was to validate the models of calcium sulfate dihydrate crystals by means of an electron-microscopic analysis. The following conclusions were made upon completion of the research: A. It has been identified that additives based on polymers of different origin affect the processes of crystallization, the size and shape of gypsum crystals. The presence of the super plasticizer based on sulfonated melamine-formaldehyde resin, methylcellulose and redispersible polymer powder based on a copolymer of vinyl acetate, ethylene and vinyl chloride, causes reduction of the size of crystals, while the crystals turn elongated. The crystal-to-crystal contact area increases; therefore, it is likely that the strength of the hardening agent goes up. B. It has been demonstrated that the X-ray analysis can be applied to simulate the shape and habitus of crystals.