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

In this work, we have reinvestigated the thermal degradation kinetics of synthetic gypsum (CaSO₄·2H₂O) using a novel approach based on peak deconvolution followed by the application of Model-Free isoconversional method. Gypsum was prepared using wet chemical route and characterized by conventional X-ray diffraction, in situ high-temperature X-ray diffraction (HT-XRD), infrared spectroscopy (IRTF-ATR), simultaneous thermal gravimetry, and differential thermal technique (TG/DTA). The physicochemical analysis showed that gypsum thermally degrades into calcium sulfate anhydrite (γ-CaSO₄; anhydrite III) via an intermediate phase formed by calcium hemihydrate (CaSO₄·0.5H₂O; bassanite). HT-XRD analyses revealed the difference between the bassanite and anhydrite III phases, although they have a similar structure. The thermal kinetics of gypsum indicated a complex behavior of overall process mechanism consisting of overlapping contributions, which were separated into two individual ones using a mathematical deconvolution of Fraser Suzuki function. The separate thermal processes were analyzed using Model-Free isoconversional and Malek's methodology. The kinetic results showed that both processes may be represented by Johnson–Mehl–Avrami [JMA(n)] equation which corresponds to nucleation and growth mechanisms, with n > 1. The first process corresponding to the partial dehydration of gypsum into bassanite was carried out by a two-dimensional JMA mechanism, while the second process, attributed to the complete dehydration of gypsum, was performed according to a three-dimensional JMA. Calculations of thermodynamic parameters have shown that the dehydration process of gypsum is accompanied by endothermic effects and requires heat, in agreement with the thermal analysis data.