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The structure and behaviour of two gypsum composites after exposition to elevated temperatures were investigated. The silica sand and fine basalt aggregate were used as solid fillers. The changes in structure and composition at temperatures from 50 to 1,000°C were investigated by scanning electron microscopy and X-ray diffraction together with the size and strength of the samples and their pore size distribution. The structure of gypsum matrix changed significantly at 1,000°C in both composites, while the aggregate particles were not changed. It was found that even if the silica sand is considered as less suitable filler at high temperatures because of its volume changes, the gypsum with sand performed better than gypsum with basalt at the highest temperatures, because the shrinkage of the gypsum matrix was compensated by the increase in the volume of aggregate. The final volume change at 1,000°C was 3.5% in composite with silica sand and 6.8% in composite with basalt. The residual compressive strength of both composites was about 9.4%. No cracks appeared in the samples and no spalling was observed.

A method for lightweight-gypsum material design using waste stone dust as the foaming agent is described. The main objective is to reach several physical properties which are inversely related in a certain way. Therefore, a linear optimization method is applied to handle this task systematically. The optimization process is based on sequential measurement of physical properties. The results are subsequently point-awarded according to a complex point criterion and new composition is proposed. After 17 trials the final mixture is obtained, having the bulk density equal to (586 ± 19) kg/m3 and compressive strength (1.10 ± 0.07) MPa. According to a detailed comparative analysis with reference gypsum, the newly developed material can be used as excellent thermally insulating interior plaster with the thermal conductivity of (0.082 ± 0.005) W/(m·K). In addition, its practical application can bring substantial economic and environmental benefits as the material contains 25 % of waste stone dust.