Synergy between Ca²⁺ and high ionic field-strength cations during the corrosion of alkali aluminoborosilicate glasses in hyper-alkaline media

One major factor impeding the design of nuclear waste glasses with enhanced waste loadings is our insufficient understanding of their composition-structure-durability relationships, specifically in the environments the waste form is expected to encounter in a geological repository. In particular, the high field-strength cations (HFSCs) are an integral component of most waste streams. However, their impact on the long-term performance of the glassy waste form remains mostly undeciphered. In this context, the present study aims to understand the impact of some HFSCs (i.e., Nb5+, Zr4+, Ti4+, and La3+) on the dissolution behavior of alkali/alkaline-earth aluminoborosilicate-based model nuclear waste glasses in hyper-alkaline media. At pH = 13, the studied glasses dissolve through the dissolution-reprecipitation mechanism, with Ca precipitation being the most vital step to passivation. In Ca-free glasses, although the HFSCs slow down the forward rate, they do not seem to impact the residual rate behavior of glasses. The presence of Ca2+, however, initiates the rapid precipitation of network polymerizing HFSCs (i.e., Nb5+, Zr4+, and Ti4+) into a Ca2+/HFSCs-based passivating layer, thus suggesting a synergy between Ca2+ and HFSCs that contributes to the enhanced long-term durability of the glasses. Such synergy is not strongly evident for La3+, but instead, a potential La/Si affinity is observed upon the formation of the alteration layer.