• Experimental DTA/TG analysis of three subsystems • DSC measurements on selected mixtures in Na 2 SO 4 -CaSO 4 and NaCl-CaSO 4 systems • HT-XRD investigation of selected mixtures in Na 2 SO 4 -CaSO 4 system • Development of a robust and internally consistent thermodynamic database • Modeling of the constituent subsystems as well as the reciprocal system This study aims to provide a comprehensive investigation of the reciprocal system Na + , Ca 2+ // Cl - , SO 4 2- by integrating experimental analysis with thermodynamic modelling. The experimental approach involved the complementary use of Differential Thermal Analysis/Thermogravimetry (DTA/TG), Differential Scanning Calorimetry (DSC), and High-Temperature X-ray Diffraction (HT-XRD). These techniques provided essential experimental evidence for the phase equilibria description and served as a foundation for thermodynamic modelling. The reciprocal system was examined within the framework of phase change material (PCM) screening and identification for thermal energy storage applications. Based on the results obtained in this work, the thermodynamic database was developed for the aforementioned salt system. Notably, experimental findings highlighted significant modifications in the description of the binary system Na 2 SO 4 -CaSO 4 , namely the formation of the solid solution based on the high temperature modification of Na 2 SO 4 and the identification of intermediate compounds Na 4 Ca(SO 4 ) 3 , which replaces the previously assumed Na 6 Ca(SO 4 ) 4 compound. Additionally, while Na 2 Ca(SO 4 ) 2 had already been reported in prior studies, this work has provided a more precise definition of its thermal stability window, based on new experimental evidence. Furthermore, the experimental study of the NaCl-CaSO 4 and CaCl 2 -Na 2 SO 4 systems, representing the ionic exchange reaction, enabled a more detailed investigation of the reciprocal system, contributing to an improved thermodynamic description within the current database. This study reports, for the first time, the enthalpy of fusion of the eutectic composition in the NaCl-CaSO 4 system, determined to be 39.3 ± 1.7 kJ/mol, with a corresponding melting temperature of 724 °C (997 K). Through the thermodynamic assessment of the sub-systems investigated both in this study and in previous works, an updated thermodynamic database has been developed, providing a comprehensive description of the entire reciprocal system and enabling a more accurate estimation of eutectic compositions within it. These findings enhance the accuracy of phase equilibria modelling and provide valuable insights into the thermal properties of potential PCM candidates.
Morsa et al. (Wed,) studied this question.