A General Mechanism for Gel Layer Formation on Borosilicate Glass under Aqueous Corrosion

Mineral and glass dissolution is a scientific topic that has been deeply investigated but is not understood completely; it continues to be of great interest in the geochemical and materials science communities. If the interfacial dissolution/reprecipitation mechanism seems to be applicable to most silicate minerals, the debate remains open concerning glass. Here, we studied two model glasses, a ternary borosilicate (CJ1) and the same glass doped with 4.1 mol % of Al2O3 (CJ2). The two glasses were altered at 90 °C, pH 9, and in conditions far and close to saturation with respect to amorphous silica to determine the initial and residual rates. Moreover, a specific experiment was conducted for a short duration with a solution highly enriched with 18O and 29Si isotopes to understand how passivating gels form. SEM, TEM, and ToF-SIMS characterizations along with Monte Carlo simulations were used to understand the rate limiting reactions at play and infer the role of Al. We show that Al yields a slower matrix dissolution in dilute conditions. However, it slows down the formation and the maturation of the passivating gel and favors alteration by partial hydrolysis of Si and Al entities followed by in situ reorganization/relaxation into a porous network. Unexpectedly, CJ1 experienced both interfacial dissolution/reprecipitation and partial hydrolysis followed by in situ reorganization of the silicate network during the course of a single experiment. This study offers a unified concept that can pave the way for the future development of a predictive kinetic model based on a detailed description of bond breaking and bond forming as a function of glass composition and alteration conditions