Modelling the alteration gel composition of simplified borosilicate glasses by precipitation of an ideal solid solution in equilibrium with the leachant

The modelling of the alteration process of two model glasses (Si–Al–B–Na and Si–Al–B–Na–Ca–Zr), having the same molar ratio as the French reference SON68 glass, with the geochemical code KINDIS was studied. The formation of the alteration layer was simulated by the precipitation of an ideal solid solution. This simulation study was compared then with the alteration experiments carried out in parallel: glasses were placed at 363 K in pure water at two different S/V ratios (1 and 80 cm−1) for duration of 30–180 days. The thermodynamic stability of the siliceous end-members of the solid solution has a fundamental influence on the simulation results. Thus, with suitable end-members (chalcedony and hydroxides), the simulation allows one to reproduce with a good agreement the evolution of the silicon content in solution as well as the chemical composition of the gel layer. The experimental results and the simulations seem to indicate a control of the gel composition over the glass dissolution process.