Water/clay ratio, clay porosity models and impacts upon clay transformations

• Alteration in lab experiments with dispersed clays may not be typical of that in compacted clays.
• Modelling shows that water/clay ratio impacts upon the magnitude and nature of alteration.
• The most relevant water/clay ratio should be used in modelling and experimental systems.

The performance of bentonite used in geological repositories for radioactive waste may be impaired by long-term clay transformations to non-swelling minerals. Intrinsic to alteration processes is the role of water/clay ratio, defined in a bentonite-pore fluid system by (the inverse of) porosity. Water/(water + clay) mass ratios are low for both ‘total’ (≤ 0.25) and ‘free’ (≤ 0.05) porosities in compacted bentonite at the dry density envisaged for waste package buffers (≥ 1500 kg m− 3). A survey of laboratory experimental studies of clay alteration has shown that they have tended to focus on systems with dispersed clays at high water/(water + clay) mass ratios (≥ 0.75) because of experimental practicalities and a desire to accelerate reactions.New thermodynamic calculations have illustrated that the fluid/clay ratio can have an important impact not only upon the magnitude of alteration, but also upon the nature of the reaction path. Reaction of a pure Na-montmorillonite with cement pore fluids, a Fe-rich fluid and a KCl solution to attempt to simulate reaction of clay with cement/concrete, iron/steel, and potassium-rich fluids (to investigate the smectite to illite reaction path), respectively has shown that under fluid-dominated conditions (high water/clay ratio), clay alteration consisted of C–S–H solids, low-Si zeolites, and chlorite. Under clay-dominated conditions (low water/clay ratios), alteration typically consisted of high-Si zeolites, feldspar and Mg-corrensite. Consequently, it is of key importance that the most relevant water/clay ratio (‘porosity’) is used not only in geochemical calculations, but also in experimental systems.