Clay-water interactions in swelling claystones: The case of the callovo-oxfordian claystone

An investigation of the microstructure changes of the Callovo-Oxfordian (COx) claystone (a possible host rock for geological radioactive waste disposal in France) due to suction changes was conducted by means of mercury intrusion porosimetry tests. A unimodal pore population around a mean diameter of 32 nm was observed at initial state, with pore size distribution (PSD) curves slightly moved towards smaller mean diameters (27-28 nm) under suctions of 150 and 331MPa, respectively, with diameter reducing to 20 nm after oven-drying.Wetting from the initial state (suction 34MPa, degree of saturation of 77.6%) to 9MPa suction led to water saturation with no significant change of the PSD curve, whereas wetting at zero suction gave rise to the appearance of cracks of several micrometers width, together with an enlargement of the initial pore population. The step hydration mechanisms of smectites by successive placement of distinct layers of water molecules within the clay platelets along the smectite faces with respect to the suction applied helped understanding these microstructure changes. An average number of 22 layers in a platelet was estimated from the dry PSD curve, based on a brick model, and it was shown that hydration at suction of various hundreds of MPa was due to the adsorption of one water layer whereas that under suction larger than 9MPa (various tens of MPa) were characterised by the adsorption of 2 water layers. Significant microstructure changes below 9MPa corresponded to the third adsorbed layer. This data evidence the significant difference in status of the strongly adsorbed intra-platelet water (estimated to 25-30% of the total water) compared to the inter-platelets water (70-75%) that behaves like free water and that is involved in water transfer and hydro-mechanical couplings.