Thermal transformations of glass in anorthite composition Available to Purchase

The knowledge about thermal transformations of glass is important, especially if the glass is used as a precursor for glass-ceramic materials. In this study, thermal transformations of glass with composition near to anorthite stoichiometry (2SiO₂·Al₂O₃·CaO) are studied up to 1300 °C. Differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) were used for this purpose. The samples were heat-treated at various temperatures to reveal the nature of the exothermic and endothermic effects on DSC. The change in heat capacity related to glass transition was observed at 722 °C. Besides, one exothermic and one endothermic peak appeared at 957 °C and 1157 °C, respectively. No further thermal effects were observed up to 1300 °C. The peak centered at 957 °C was attributed to the overlapping crystallization of gehlenite (2CaO·Al₂O₃·SiO), anorthite, and CaO·Al₂O₃·2SiO polymorph (Ca-feldspar). To explain the endothermic effect, DSC experiments were performed on thermally treated glass. The glass previously heated at 1157 °C showed weak crystallization (exothermic) effect during the second DSC run. Moreover, no decrease in crystalline phase content was detected by XRD. A usual explanation of the second endothermic peak – the dissolution of crystalline phases, was questioned. Conversion of Ca-feldspar and gehlenite to anorthite is suggested as an alternative explanation.