Interpretation of silica-disk-tests including the effect of limited mass transfer at the surface

The concentration of water penetrated into silica depends strongly on the stress state in the surface region. In the water diffusion zone hydroxyl water is generated by the water/silica reaction that is at temperatures <450°C a first-order reaction. When [2S]=[2 SiOH] is the concentration of the immovable hydroxyl, the reaction equation reads

≡Si-O-Si≡ +H$_2$O ↔ [2 SiOH]

When evaluating the deformation measurements on water-infiltrated silica disks, the question remained unanswered as to why the tests can lead to increased diffusion constants or whether these are accidental (possibly dependent on the special glass composition).
The result of this Report is:
• The activation volume for stress affected hydroxyl S is ΔV≅58 cm$^3$/mol,
• the mass transfer coefficient of water into silica is h/√D = 0.266/√h,
• due to the mass transfer coefficient, the effective diffusivity isincreased by a factor of roughly 1.6.