Corrosion and crystallization at the inner surfaces of glass bricks

Glass bricks are important transparent building materials. They are produced by joining two halves of glass pressings at 600-700 °C. During this production process alkali oxides evaporate and are redeposited at the cooler inner front surfaces of the bricks. This surface layer reacts with H 2 O and CO 2 from the residual brick atmosphere, leading to the formation of an alkali-rich silicate-hydrate layer of ≥ 50 nm thickness, which could be evidenced leading to a reduced nano-hardness of similar thickness, and from which NaHCO 3 crystals can finally grow. Climate chamber experiments (repeated cooling between at -8 and -14 °C and reheating to 0 to 15 °C) resulted in reversible NaHCO 3 crystallization and redissolution, presumably influenced by water evaporation or condensation and driven by the NaHCO 3 supersaturation of the silicate-hydrate layer. Depending on the time-temperature schedule, different crystal morphologies became visible in this closed system, e.g. isolated spherical crystals, crystals arranged in chains and in double-chains, respectively, which can limit already the transmittance of the glass bricks. When a crack occurs or the brick is opened, the hygroscopic NaHCO 3 crystals take up more H 2 O from the ambient, react irreversibly with the glass surface, finally leading to a total loss of transmittance.