Aqueous alteration of five-oxide silicate glasses: Experimental approach and Monte Carlo modeling

The alteration behavior of glass comprising five oxides (61 - x)SiO2-17B2O3-18Na2O-4CaO-xZrO2 was studied during static leach tests in a buffer solution at 90 degrees C and with a glass-surface-area-to-solution-volume (SA/V) ratio of 15 cm-1. The morphological evolution of altered glasses investigated by small-angle X-ray scattering (SAXS) exhibits a strong dependence with the zirconium content in the glass. The experiments were compared with modeling results using Monte Carlo simulation. The model has been improved to simulate the alteration kinetics and alteration layer morphology, considering zirconium atoms at coordination number 6. The simulations exhibit very good agreement with experimental results, showing relations between the alteration rate and the restructuring altered layer. The model is used to interpret experimental observations by proposing a porosity closure mechanism in the altered layer to account for the diminishing alteration rate. For high zirconium concentrations, the simulation highlights the existence of percolation pathways responsible for a complete alteration of the glass. Zirconium has a hardening effect that limits the dissolution of neighboring atoms; this effect is favorable in terms of the glass alteration kinetics, but by inhibiting silicon recondensation it prevents complete closure of the porosity and the glass is completely altered. (authors)