New Insight into the Residual Rate of Borosilicate Glasses: Effect of S/V and Glass Composition

We report and discuss initial and residual rate measurements performed on 6 borosilicate glasses to highlight (i) some glass composition effects (namely Zn and Fe), (ii) the role of dissolved silica, and (iii) the importance of the glass-surface-area-to-solution-volume (S/V) ratio. We show that there is no apparent relation between the initial and the residual rate. As an example, the well-known positive effect of Zn on short-term glass dissolution becomes adverse under residual rate conditions. Dissolved silica as well as S/V strongly impact the transient rate drop regime, but have no or minor effects on the magnitude of the residual rate of the SON68 glass. Increasing S/V is indeed a good mean to explore higher reaction progresses. Additional simulations performed with the GRAAL model show that, in the case of SON68 glass altered in deionized water, the residual rate continuously diminishes despite the formation of secondary crystalline phases. This result can be used to derive conservative rate values. More generally, it is shown that the residual rate can fluctuate depending on the relative importance of two coupled mechanisms: the formation of a passivating layer and the transformation of this layer into more stable compounds.