PNNL

Abstract

Silicate glasses dissolve in water at a rate dependent on the solution concentration of orthosilicic acid (H4SiO4). In general, higher [H4SiO4] leads to lower dissolution rates. It has often been observed that the precipitation of certain silica-bearing alteration products can cause the dissolution of the glass to increase, even after the rate has decreased significantly. However, it has also been observed that in the concentrations of these silica-bearing solution species do not significantly decrease while other elements continue to be released. In this study, we have used the Geochemist’s Workbench code to investigate the relationship between glass dissolution rates and the precipitation rate of a silica-bearing alteration product, analcime (Na(AlSi2O6)?H2O). In this initial study and to simplify the calculations, we suppressed all alteration products except analcime, gibbsite (Al(OH)3), and amorphous silica. The ‘cross affinity’ code option allowed us to account for the fact that glass is a thermodynamically unstable solid with respect to its alteration products in contact with water. The cross-affinity option in the Geochemist’s Workbench geochemical code allowed us to substitute the amorphous silica equilibrium-constant matrix for the glass equilibrium-constant matrix. In this article, we present the results of our calculations of the glass dissolution rate at different values for the analcime precipitation rate constant and the effects of varying the glass dissolution rate constant at a constant analcime precipitation rate constant. In all cases, our results indicate that the glass dissolution rate controls the rate of analcime precipitation in the long term. Our results, compared in general terms with experiments, show the importance of the gel layer that forms during glass alteration. The meaning of these results pertinent to long-term glass durability is discussed.