Novel approach to thermal degradation kinetics of gypsum: application of peak deconvolution and Model-Free isoconversional method

Statistical approaches in the analysis of in situ thermo‐Raman spectroscopic data for gypsum as a basis for studying dehydration and phase transformations in crystalline hydrates

“…), thermal analysis, in situ thermo‐X‐ray powder and single crystal diffraction, including excitation by synchrotron radiation and neutrons. [ 1–12 ] These numerous studies are stimulated both by the interest in fundamental crystal chemistry and mineralogy of crystalline hydrates and by the issues arising from the widespread practical use of these phases. In practice, the study of CaSO 4 ·2H 2 O gypsum—a typical natural crystalline hydrate, multipurpose raw material with a wide range of product varieties, primarily constructive—is of the greatest interest.…”

“…Water molecules localized between these layers form hydrogen bonds with the oxygen of sulfate groups, resulting in each Ca cation being coordinated by six oxygen atoms of sulfate groups and two water molecules. [ 4 ] Gypsum thermal transformations have been repeatedly investigated using various methods (see, for example, the literature [ 5–14 ] ). The first endothermic peak on the DTA curves associated with gypsum degradation is observed at 393–413 K [ 29,30 ] or at 413 K. [ 7 ] It should be noted that according to Raman spectroscopic (RS) data, it is fixed at 388 K [ 10,31 ] or at 360–425 K [ 8 ] and at 382–413 K according to SR‐PXD.…”

“…[ 4 ] Gypsum thermal transformations have been repeatedly investigated using various methods (see, for example, the literature [ 5–14 ] ). The first endothermic peak on the DTA curves associated with gypsum degradation is observed at 393–413 K [ 29,30 ] or at 413 K. [ 7 ] It should be noted that according to Raman spectroscopic (RS) data, it is fixed at 388 K [ 10,31 ] or at 360–425 K [ 8 ] and at 382–413 K according to SR‐PXD. [ 1 ] The initial temperature of the effect, as a rule, is about 373 K [ 10 ] and varies depending on the content of impurities of alkali halides in gypsum, the thermal history of sample, the size, shape and compactness of the starting gypsum (crystal or powder), as well as experimental conditions, in particular, heating rate, calcination time, and air exchange.…”

“…The second peak in the thermal curves, associated with bassanite dehydration, is recorded at 428 K [ 7 ] or at 449–506 K. [ 13 ] According to Raman spectroscopic data, the structural transformation corresponding to this thermal process occurs at 448 K [ 10 ] and at 438–445 K according to SR‐PXD data. [ 1 ] In the indicated temperature range, hemihydrate transforms into soluble γ‐anhydrite, which is accompanied by the transformation of the orthorhombic lattice into a hexagonal one.…”

“…According to the ESR spectroscopy data, [ 4 ] the distortions caused by the rearrangement of crystal structure of phases in CaSO 4 ·2H 2 O → CaSO 4 ·0.5H 2 O → γ‐CaSO 4 → β‐CaSO 4 series create numerous microstresses in the bulk of newly formed phases. According to conventional X‐ray diffraction, in situ high‐temperature X‐ray diffraction, infrared spectroscopy, simultaneous thermal gravimetry, and differential thermal technique, [ 7 ] the mechanism of gypsum degradation was found to be complex and included the series of superimposed processes: the possible coexistence of bassanite and CaSO 4 ‐subhydrates. It was shown in Kilic and Kilic [ 11 ] that, depending on the microstructure of the initial gypsum sample, the lattice of the intermediate product of CaSO 4 hemihydrate varied.…”