Toward a better understanding of the mechanisms that control dissolution of calcium aluminosilicate glass

We investigated the dissolution behaviour of calcium alumininosilicate (CAS) glass at different pH values and temperature in batch and flow-through experiments. The temperature dependence of the dissolution rate over a wide pH range was measured under flow-through conditions at ambient temperature and at 50 and 75 °C, using crushed CAS glass powder (25 µm < x < 63 µm). CAS powder and single glass chips were exposed to deionised water (initial pH 5.6), an acetate buffer (pH 4.4–4.5) and a 0.1 M NaOH (pH 13) solution for 28 and 150 days in static batch mode at ambient temperature. Flow-through dissolution experiments demonstrated close to congruent dissolution of CAS powder with a minimum dissolution rate at neutral pH and an insignificant temperature dependence. Batch dissolution experiments resulted in slower initial dissolution rates than in flow-through tests, a dissolution rate drop over time, and incongruent CAS dissolution, with specific behaviour highly dependent on solid surface area-to-solution volume (S/V) and pH. Monitoring of the microstructural changes of CAS chips showed that CAS developed a complex calcium depleted layer in slightly acidic batch dissolution media. The thickness of this layer ranged from a few nanometres to several micrometres depending on S/V. At neutral pH, the thickness of the Ca depleted layer was only a few nanometres and Al enrichment (adsorption or secondary phase) of the top glass chip surface was observed. Samples leached in 0.1 M NaOH solution showed formation of secondary phases with various morphologies and stoichiometries, depending on initial S/V. The small size (≤ 1 µm) of these secondary phases made it difficult to identify them. The results highlight the complexity of CAS glass dissolution, which can involve congruent dissolution, leached layer formation or dissolution-precipitation mechanisms, depending on pH, temperature, and S/V.