Suppression of Surface Crystallization of CaO-Al₂O₃-SiO₂ Glass via Hydrothermal Treatment under Acidic Conditions

This study demonstrates bulk crystallization of CaO-Al₂O₃-SiO₂ (CAS) glass with suppression of surface crystallization via hydrothermal treatment under acidic conditions. It focuses on CAS glass-ceramics (GCs) with precipitated metastable CaAl₂Si₂O₈, a layered calcium aluminosilicate, because it features a single bulk crystallization phase with a relatively lower volume fraction. After hydrothermal treatment under nitric acid, the resultant glasses possessed silica-rich surface layers. When the obtained glass with the surface layer was heat-treated at 1100 °C for 6 h, the resultant surface layer was easily peeled off by hand, and the cross section of both the edge and interior of the specimen showed precipitation of only metastable CaAl₂Si₂O₈. The volume fraction of this crystalline phase was ∼50%, which was 20% larger than those reported for CAS GCs heat-treated at 1050 °C. By contrast, anorthite, the stable phase of CaAl₂Si₂O₈, grew and invaded from the surface to the interior of the glass specimen heat-treated at 1100 °C for 6 h but not subjected to the hydrothermal treatment. These results indicate that the presence of a silica-rich layer on the glass surface suppressed surface crystallization even at the temperature that promotes surface crystallization, suggesting that the range of heat treatment conditions for glass crystallization can be expanded, enabling microstructural control of CAS GCs.