Mobility of water and ions in partially water-saturated compacted MX-80 bentonite in relation with swelling pressure

Abstract

Many countries are planning to host long-lived high- and intermediate-level radioactive waste in disposal facilities sited in deep argillaceous formations. Bentonite, as a swelling material, is often chosen as engineered buffers due to its low permeability and high swelling capacity. However, its properties can be influenced by the hydraulic/gas conditions prevailing in-situ that are expected to change throughout the duration of the repository. The objective of this work is to estimate the effect of partially water-saturated conditions on the mobility of water and ions in compacted MX-80 bentonite in relation to the evolution of its swelling pressure. For that purpose, (i) diffusion of water and ions (I− and Na+) and (ii) the swelling pressure of compacted bentonite samples partially water-saturated were studied by means of the osmotic technique that allows controlling the suction from 9 to 0 MPa.Results showed that (i) water diffusivity remained unchanged independently of the degrees of saturation (70, 84 and 100%) and (ii) iodide diffusivity decreased by a factor of 12 (from 100 to 70%), while (iii) Na+ diffusion was reduced by a factor of more than 70 (from 100 to 70%), associated to a drop of the distribution factor by a factor of 16. Comparison of these results with diffusion data already acquired in clayey materials whose porous structure evolves little with suction (i.e. compacted kaolinite and intact Collovo-oxfordian clay-rocks) underlined how the bentonite porous structure whose organization evolved with the degree of saturation is a key parameter. This statement is supported by mechanical tests, which indicated a distinct evolution of the swelling pressure curves depending on the degree of saturation. At 70% saturation, mechanical tests indicate (i) a preservation of the large inter-aggregate porosity where water can diffuse in both vapor and liquid phases, and (ii) a final swelling pressure 1.5 to 2 times lower than at full water saturation, meaning that a part of swelling clay particles was not accessible to water ingress and so to 22Na+. Conversely, at 84 % and 100 % saturation, change in swelling pressure curves was explained by a collapse of bentonite inter-aggregate porosity, in favor of expanding the inter-particle and interlayer porosities where 22Na + diffusion is enhanced by surface diffusion phenomenon.