Mechanism of dissolution reactivity and reactions of various calcium borate glasses and glass-ceramics

This study investigated the effect of calcium addition on boron coordination and ion release in deionized water (DIW) of five melt-quench-derived compositions: xCaO-(100-x)B2O3 (x=25, 27, 33, 38 and 48 mol%) and one sol-gel-derived 48CaO-52B2O3 glass. The mechanism of hydration and hydrolysis reactions was elaborated based on ionic release and structural change pre- and post- ageing in DIW. For melt-quench-derived glasses, within 8 h ageing, the hydration reactions were more rapid than the hydrolysis reactions, resulting in solution pH increase, where ionic release was more dependent on composition than boron coordination. At longer ageing times, hydration and hydrolysis reactions occurred simultaneously, leading to solution pH stabilization, and calcium and boron release decreased with an increase in the boron tetrahedral fraction (N4) in glasses. Non-bridging-oxygens in the melt-quench-derived 48CaO-52B2O3 glass-ceramic and hydroxyl groups in the equivalent sol-gel-derived glass fostered calcium and boron release. All glass formulations demonstrated higher N4 post DIW exposure.