Non-Linear Effects of Glass Composition on Chemical Durability: Physical Stability of Surface Layers

Abstract

ABSTRACTPresented here are results relating to the possible causes for the very strong nonlinearity in the relationship between glass composition and glass durability, as determined by MCC 3 or Product Consistency (PCT) tests. We have previously found a rapid increase in leachate concentrations with small changes in composition (as little as 1 weight percent SiO2 or Al2O3) over some composition ranges near those of interest for nuclear waste disposal. This rapid rise occurs over a narrow transition region in composition space that provides a natural division between more durable and less durable glasses. Our new data on glasses of interest to the West Valley Demonstration Project (WVDP) suggest that this effect is not due to phase separation. Results from leach tests conducted with and without constant agitation are compared in an attempt to determine whether the physical integrity of alteration layers plays an essential role, as might be the case if formation-spallation cycles were important in this process. Microstructural data are also presented in order to compare the alteration layers on the more durable and less durable glasses leached with and without agitation.