Further insight into the microstructure of compacted bentonite-sand mixture

Compacted bricks of bentonite/sand mixture are planned to be used as sealing plugs in deep radioactive waste disposal repositories because of their low permeability, high swelling capacity and favourable properties with respect to radionuclide retention. The isolating capacity of compacted bentonite/sand mixture is closely related to microstructure features that have been often investigated, in particular by using scanning electron microscopy (SEM or ESEM) and mercury intrusion porosimetry (MIP).In this work, microfocus X-ray computed tomography (μCT) observations were used in parallel with MIP measurements to further investigate at larger scale the microstructure of a laboratory compacted bentonite/sand disc (65/35% in mass). Qualitative observation of μCT images showed that sand grains were inter-connected with some large pores between them that were clearly identified in the bimodal pore distribution obtained from MIP measurements. Due to gravitational and to frictional effects along the specimen periphery, a higher density was observed in the centre of the specimen with bentonite grains more closely compacted together. This porosity heterogeneity was qualitatively estimated by means of image analysis that also allowed the definition of the representative elementary volume. Image analysis also provided an estimation of the large porosity in good agreement with MIP measurements.