Microstructure of montmorillonite/silica sand mixture and its effects on the diffusion of strontium ions

Abstract A mixture of bentonite and silica sand is expected to be an important engineered barrier in a high-level radioactive waste repository. Therefore, the diffusion behavior of radionuclides in this mixture is a key issue for the safety assessment of geological disposal. This study determined the apparent diffusion coefficients of strontium ions in montmorillonite and silica sand mixtures under different temperatures at a total dry density of 1.8 Mg m−3, while the mixing ratios of montmorillonite and silica sand were adjusted to obtain partial dry densities of Na-montmorillonite from 1.0 to 1.7 Mg m−3. When the total dry density of the pure montmorillonite sample was identical to the partial dry density of the montmorillonite in the mixture samples, the apparent diffusion coefficients in the mixture samples at 298 K were almost the same as those in pure montmorillonite. However, the activation energy in the mixture sample was found to be higher than that in the pure sample having a partial dry density of 1.2 Mg m−3, whereas it was lower in the sample having a partial dry density of 1.7 Mg m−3. This suggests that the diffusion behaviour of strontium ions was affected by the addition of silica sand at these partial dry densities, although there was no significant effect of the silica sand on the apparent diffusion coefficient. The microstructures of the compacted montmorillonite samples were observed with microfocus X-ray computerized tomograph (micro-CT). Three-dimensional images of the microstructures were obtained for the pure and the mixture samples under the dry and wet states. The micro-CT observations suggest the addition of silica sand to the montmorillonite samples could affect the degree of water-saturation and/or swelling of montmorillonite particles. This agrees with the results of the XRD measurements that determined the effects of silica sand on the basal spacing of montmorillonite.