Thermo-hydraulic gradients on bentonite: distribution of soluble salts, microstructure and modification of the hydraulic and mechanical behaviour

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In the context of an investigation of the near field for a repository of high-level radioactive waste (HLW), the FEBEX Project, a set of laboratory tests has been designed to give a better understanding of the thermo-hydro-mechanical and geochemical behaviour of the compacted bentonite. Small compacted samples of bentonite were heated during variable periods of time of up to 80 days under different thermal gradients. The hydration water was either granitic, simulating the conditions of the outer part of the barrier, or saline, which simulates the chemistry of the pore waters inside the bentonite barrier. At the end of the thermo-hydraulic (TH) treatment, a geochemical characterisation was performed in different sections, both of the soluble components and of the solid phase. The microstructure was analysed by means of optical microscopy and determination of the BET surface and pore size distribution. The swelling capacity of the whole sample and its permeability were also checked. The results of the permeability and swelling tests were compared to those obtained in experiments performed under the same conditions with nontreated samples. As a result of hydration, there exists a rapid movement of chloride towards the heater. This ion is progressively excluded of the bentonite after saturation that makes the bulk salinity of the clay decrease. Anion exclusion should prevent the anion transport once the bentonite is saturated, but the existence of preferential passages is postulated to explain the continuous salinity decrease. This phenomenon makes the salt content in the compacted bentonite very limited, with localised anomalies prior to saturation. All the physico-chemical parameters that have been determined are virtually unchanged during the TH treatment. An increase of the hydraulic conductivity after the TH treatment with saline water has been observed, while the swelling capacity of the samples treated with granitic water slightly increases after treatment.