Do alkalis affect the passivation of silicate glass?
Résumé
Amorphous silica-rich surface layers, also called gels, can passivate silicate glass and minerals depending on environmental conditions. However, several uncertainties remain on the mechanisms controlling the formation of these layers. Here, the influence of exogenous ions supplied by solutions is studied, both on the formation and on the properties of the gel formed on international simple glass (ISG). ISG was altered at 90 DC, pH 7, in silica-saturated solutions containing various alkaline cations separately (Li+, Na+, K+, and Cs+). The kinetics observed with Li and Na in the solution is similar to that observed with no ions, while K and Cs in the solution tend to decrease glass alteration. Furthermore, for K or Cs ions, the kinetics decreases as the ionic strength of the solution increases. The passivation layer formed in these solutions shows a selectivity toward cations following the series K > Cs > Na >> Li. These alkalis replace Ca from pristine glass in the altered structures, leading to differences in AlO4 charge compensation. Exchange between Ca and alkali also affects the free volume inside each altered structure. As a consequence, a drop in water accessibility to the pristine glass interface is observed, explaining the effect of cations on glass alteration kinetics.