Clay-water interactions in swelling claystones: The case of the Callovo-Oxfordian claystone

摘要

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