Leaching of nuclear waste glass in cement pore water: effect of calcium in solution

In the French geological repository concept, intermediate-level vitrified wastes could be disposed of in a cement medium. The glass dissolution mechanisms and kinetics, expected to depend strongly on the chemical composition and pH of the leaching water, were studied in various cement pore water compositions corresponding to different stages of cement aging. In this study, we focused on the effects induced by cement pore water at equilibrium with respect to Portlandite (pH[25 C] = 12.4). A decrease in the maximum glass dissolution rate due to the effect of calcium was clearly observed compared to the reference medium, i.e. at the same pH in KOH solution. At higher reaction progress, calcium in solution was almost totally consumed after a few days, probably due to the formation of Calcium Silicate Hydrate (C-S-H) phases with silicon leached from the glass. Two assumptions can be proposed to explain the effect of calcium on the initial regime: either the calcium from solution reacts with silicon released to form a C-S-H passivating layer at the glass/solution interface, or calcium compensates two non-bridging oxygen (Si-O-) in the altered layer, which could decrease the hydrolysis of silicon bonds. (authors)