Abstract
Plaster is primarily used as a building material obtained by the calcination of gypsum. Its rapid setting time (time for the mixture to solidify) and the low quality of labor generate a large amount of nonused material. Due to its solubility in water, wasted gypsum cannot be disposed of in the environment, and its recycling process is encouraged. In this work, quantitative phase analyses (QPA) using X-ray powder diffraction (XRPD) data and the Rietveld method were carried out to determine the amounts of each compound present in commercial, hydrated, and laboratory-recycled plasters, and the results compared with those obtained by thermogravimetric analysis (TGA). It was inferred that the Rietveld method associated with XRPD data is quite efficient since it identifies compounds not seen in the TGA. Furthermore, the amount of water used in the preparation of hydrated samples influences the proper hydration of the material and, consequently, the recycled composition of the samples.
Highlights
Plaster, in the form of hemihydrate (CaSO4 ·0.5H2 O), undergoes an exothermic reaction when hydrated and returns to its natural chemical formula, forming a plaster paste commonly used in construction as internal coatings, decorative elements, and plasterboards [1], constituting the construction sector responsible for the consumption of 95% of the total production of gypsum [2]
When analyzing the results obtained with the thermogravimetric analysis (TGA) and X-ray powder diffraction (XRPD) techniques, the bassanite content slightly increased after recycling, as the anhydrite content was reduced with the increase in water amount (4.1% for commercial plaster (CP), 3.1% for RP_40%, 1.9% for RP_50%, and 1.7% for RP_60%), indicating adequate material hydration
This difference may be because only the presences of hemihydrate and dolomite were considered in the TGA analyses, whereas anhydrite, calcite, olivine, and quartz were detected in the XRPD data
Summary
In the form of hemihydrate (CaSO4 ·0.5H2 O), undergoes an exothermic reaction when hydrated and returns to its natural chemical formula (gypsum, CaSO4 ·2H2 O), forming a plaster paste commonly used in construction as internal coatings, decorative elements, and plasterboards [1], constituting the construction sector responsible for the consumption of 95% of the total production of gypsum [2]. Bezou et al [26] used synchrotron and neutron diffraction data and the Rietveld method to determine the fractional coordinates of water molecules within the soluble anhydrite structure and two forms of subhydrated calcium hydroxide (CaSO4 ·0.5H2 O and CaSO4 ·0.6H2 O); they did not present results regarding the quantitative phase analyses of the materials. The present work brings the main contribution of using X-ray powder diffraction data and the Rietveld method to perform a microstructural study of plaster to analyze eventual phase changes generated by the recycling process by comparing the results obtained with a commercial plaster after hydration and after calcination. A correlation was made between the results and those obtained by using thermogravimetric analysis
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