Glass composition and solution speciation effects on stage III dissolution

The dissolution of glass in aqueous solution is thermodynamically driven by the non-equilibrium state of the glass itself. Under conditions of high alkalinity, the solution of glass can be highly concentrated, and near-equilibrium with respect to crystalline mineral phases. For durable glasses, the transformation of a glass into its equilibrium crystalline phases is usually kinetically limited by nucleation effects, surface alteration layers on the glass, and concentration of the solution. But in some cases, the precipitation of secondary phases can occur to drive the continued dissolution of the glass. In this study, the transformation of two simple and soluble alkali silicate glasses, Li-disilicate and Na-disilicate (LDS and NDS), was followed with solution analysis, x-ray diffraction and thermodynamic modeling using Geochemist’s Work Bench (GWB). The evolution of the concentrated glass solution, as opposed to the kinetics of dissolution, was the primary focus. It is shown that the LDS system undergoes significant supersaturation followed by precipitation of hydrated Li-disilicate crystals; this was in reasonable agreement with the GWB modeling, although the GWB outputs found the solution saturated with respect to other crystalline phases which were not observed. For the NDS system, precipitation was much less prevalent even though more than 75% of the glass was in solution. In contrast to the LDS system, the NDS system did not precipitate any of the simple binary Na-silicates, but rather formed a gel which persisted for up to 5000 hrs. The saturation indices for zeolitic phases were exceedingly high, and with the trace amounts of Al in these solutions, the crystallization of zeolite Na-P1 was observed by x-ray diffraction after 1000 hrs. These observations are relevant to the long-term stability of complex, multicomponent nuclear waste glasses.