Fig. 7: Illustration of the competition between passivating gels and secondary phases.

a TEM cross section of SON68 glass altered in Mg-rich clayey groundwater at 70 °C for 224 days (Adapted from De Echave et al.⁹⁷ with permission, copyright Elsevier). The growth of the Mg-bearing phyllosilicates on the gel surface consumes Si and Al in the gel, resulting in bigger pores near the precipitates. b SEM image of the inner part of the alteration layer formed on a 9-oxide borosilicate glass altered at 150 °C, pH 1 for 96 h (adapted from Geisler et al.²⁵⁹ with permission, copyright Elsevier). The oscillating banding pattern (ptz) forms an inner amorphous zone resulting from glass dissolution followed by interfacial reprecipitation. This example illustrates the fact that the alteration layer can be formed by different mechanisms. c TEM cross section of ISG altered at 70 °C for 385 days in cement water (adapted from Ferrand et al.²⁶⁰ with permission, copyright MDPI). High pH (pH^70°C ~ 10) favors the precipitation of calcium-silicate-hydrates at the expense of the passivating gel.