Investigation of gel porosity clogging during glass leaching

A 5-oxide glass (62.5SiO2, 16.6B2O3, 13.1Na2O, 6.0CaO, 1.8ZrO2) was leached at 90 °C at a high glass-surface-area-to-solution-volume ratio (SA/V = 80 cm−1). Its dissolution rate diminished over time until it became unmeasurable. The alteration layer was characterized by 29Si isotopic tracing in the leaching solution. ToF-SIMS elemental profiles showed that glass dissolution ceased due to clogging of the gel porosity at the gel/solution interface. One of the hypotheses proposed to account for the rate drop observed during borosilicate glass alteration is based on morphological changes in the alteration gel over time. Monte Carlo modeling of glass alteration, especially with simple glasses, indicates a clogging of the porosity on the external portion of the gel (near the solution/gel interface) after densification of the layer by silicon precipitation, but this phenomenon had never previously been directly observed experimentally. The initial results obtained by isotopic tracing provide new data that appears to confirm this hypothesis.