There are a variety of applications for glasses in alkaline environments, including glass fibers and glass-coated steel to reinforce concrete structures. To understand how a simple glass reacts in such environments, the dissolution behavior of a 25Na(2)O-25B(2)O(3)-50SiO(2) (mol%) glass, doped with and without 3 mol% P2O5, in pH 12 KOH and pH 12 KOH saturated with Ca2+ ions was studied. Ca2+ ions in the solution significantly reduce the glass dissolution rate by forming a passivating calcium silicate hydrate (C-S-H) gel layer on the glass surface. When these corroded glasses were then exposed to Ca-free KOH, the C-S-H layer redissolves into the undersaturated solution and the glass dissolution rate increases. For phosphate-doped borosilicate glass, PO43-units released from the dissolving glass react with Ca2+ ions in saturated solutions to form crystalline hydroxylapatite on the glass surface, but this layer does not protect the glass from corrosion as well as the C-S-H does. The nature of the C-S-H layer was characterized by Raman spectroscopy, which reveals a gel layer constituted mainly of silicate anions.