Abstract
When exposed to water, silicate glasses and minerals can form a hydrated gel surface layer concurrent with a decrease in their dissolution kinetics—a phenomenon known as the “passivation effect.” However, the atomic-scale origin of such passivation remains debated. Here, based on reactive molecular dynamics simulations, we investigate the hydration of a series of modified borosilicate glasses with varying compositions. We show that, upon the aging of the gel, the passivation effect manifests itself as a drop in hydrogen mobility. Nevertheless, only select glass compositions are found to exhibit some passivation. Based on these results, we demonstrate that the passivation effect cannot be solely explained by the repolymerization of the hydrated gel upon aging. Rather, we establish that the propensity for passivation is intrinsically governed by the reorganization of the medium-range order structure of the gel upon aging and, specifically, the formation of small silicate rings that hinder water mobility.
Highlights
All hydrated gels are found to exhibit some degree of repolymerization upon aging—which highlights that the passivation effect cannot be solely explained by the repolymerization of the gel
We demonstrate here that the passivation effect is controlled by the reorganization of the medium-range order structure of the gel upon aging and, the formation of small silicate rings
To investigate the effect of the composition of the parent glass on the propensity of the resulting hydrated gel to exhibit some passivation, we first simulate using molecular dynamics a series of modified borosilicate glasses (Na2O)0.3–x(CaO)0.1(B2O3)x(SiO2)0.6, where x = 0.00, 0.05, 0.10, 0.15, 0.20, and 0.30. These compositions are intended to offer a simplified model for complex multi-component nuclear waste glasses and to study the effect of the Na-to-B ratio.[8]
Summary
Silicate glasses and minerals tend to dissolve via several mechanisms, including hydration, hydrolysis, and ion-exchange.[1,2,3] In most cases, the leaching of the mobile cations initially present in the silicate phase (e.g., B and alkali), their replacement by hydrated species, and the restructuring of the leached material results in the formation of a disordered, porous, and hydrated “gel” layer on the surface of the dissolving phase.[3,4] The formation of this alteration layer is usually associated with a drop in the corrosion rate—a behavior known as the passivation effect.[4,5] This slowdown in the corrosion kinetics of silicate phases upon their passivation has important consequences in earth science and technological applications involving outdoor silicate phases.[6,7] In particular, such passivation is expected to largely control the long-term durability of glasses used as a matrix to immobilize nuclear waste.[8,9]Despite the importance of silicate corrosion, the origin of the passivation effect has far remained debated.[3]. Silicate glasses and minerals tend to dissolve via several mechanisms, including hydration, hydrolysis, and ion-exchange.[1,2,3] In most cases, the leaching of the mobile cations initially present in the silicate phase (e.g., B and alkali), their replacement by hydrated species, and the restructuring of the leached material results in the formation of a disordered, porous, and hydrated “gel” layer on the surface of the dissolving phase.[3,4] The formation of this alteration layer is usually associated with a drop in the corrosion rate—a behavior known as the passivation effect.[4,5] This slowdown in the corrosion kinetics of silicate phases upon their passivation has important consequences in earth science and technological applications involving outdoor silicate phases.[6,7] In particular, such passivation is expected to largely control the long-term durability of glasses used as a matrix to immobilize nuclear waste.[8,9]. We demonstrate here that the passivation effect is controlled by the reorganization of the medium-range order structure of the gel upon aging and, the formation of small silicate rings
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
