Changes in structure and composition of gypsum paste at elevated temperatures
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Changes in structure and composition of gypsum paste at elevated temperatures A. Vimmrová1 · J. Krejsová1 · L. Scheinherrová1 · M. Doleželová1 · M. Keppert1 Received: 10 July 2019 / Accepted: 7 March 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract The structure and composition of the gypsum at elevated temperatures were studied by means of scanning electron microscopy, X-ray diffraction (XRD) and thermogravimetry (TG). The gypsum paste samples were heated to the temperatures from 50 to 1000 °C. The changes in the structure of gypsum were in good accordance with the changes in properties. The crystals of calcium sulphate dihydrate were disrupted at the temperature between 50 and 100 °C, and the strength decreased significantly; after heating to 700 °C, the crystals started to be thicker and packed closer to each other and the strength increased again. After heating to 1000 °C, the strength was the same as the original strength. The results of XRD showed that the changes of calcium sulphate forms (dihydrate to hemihydrate and then to different modifications of anhydrite) were not sudden but occurred gradually, and different forms of calcium sulphate existed in the heated gypsum paste together. It was confirmed that several parallel or subsequent reactions occurred during dehydration. The dehydration started at the temperature under 50 °C and lasted up to 500 °C. Keywords Gypsum · Elevated temperatures · Phase composition · Microstructure
Introduction Calcined gypsum is one of the oldest building binders, and seemingly, it is a thoroughly described material, whose behaviour is fully understood. Nevertheless, most of the research of gypsum chemistry and behaviour was done more than 50 years ago [1, 2], and sometimes it is impossible to find the source of the information or even the method by which the information was obtained. Another problem is that early research was made mostly with gypsum, produced from natural gypsum rock, whereas today the secondary sources (namely flue gas desulphurization gypsum) are used for the production of calcined gypsum very often. Such material can differ from the old types of calcined gypsum significantly, because it contains different impurities and also the shape and size of gypsum crystals differ [3]. One of the most appreciated properties of gypsum is its fire resistance, caused by the chemically bonded water in * A. Vimmrová [email protected] 1
Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 7, 166 29 Prague, Czech Republic
the crystals. When hardened gypsum is exposed to high temperatures, the bonded water (about 20.9% by mass) is gradually released. The dehydration is an endothermic process, and until the dehydration is finished, the temperature of the gypsum does not exceed 150 °C [4]. However, the dehydration mechanism and the phase transitions in the CaSO4–n H2O system were not fully described yet, and the literature sources differ. Several authors studied the behaviour and pro
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