Alteration of Si-B-Na-Al model glass in water at 90°C: experiments and thermodynamic modelling

Abstract

ABSTRACTAn attempt to model the alteration process of a model glass (Si-B-Al-Na), similar to the French reference SON68 glass, by using the geochemical code KINDIS, was developed. This simulation study was compared with experiments carried out in parallel: glass samples were placed at 90°C in pure water for 30 to 180 days, at two different S/V ratios (1 and 80 cm-1).The formation of the alteration layer was simulated by the precipitation of an ideal solid solution. The cases of oxides, hydroxides and metasilicates as end-members were tested. The considered solid solution always contains a siliceous end-member, which consists of amorphous silica, chalcedony or quartz.The relative thermodynamic stability of the chosen end-members (especially siliceous ones) influences directly the composition of the predicted gel and thus the composition of the solution. The results obtained by using chalcedony and hydroxides as end-members show a good agreement between the experimental and modelled silicon contents in solutions and in alteration gels, whatever the S/V ratio.The gel layer of the glass contains significant amounts of Na and Al, which are equimolar on average, what would probably correspond to the compensation of AlO4- charges by Na+.