The Pore Structure of Compacted and Partly Saturated MX-80 Bentonite at Different Dry Densities

Abstract

Compacted MX-80 bentonite is a potential backfill material in radioactive-waste repositories. Pore space in MX-80 has been the subject of considerable debate. 3D reconstructions of the pore space based on tomographic methods could provide new insights into the nature of the pore space of compacted bentonites. To date, few such reconstructions have been done because of problems with the preparation of bentonite samples for electron microscopy. The nanoscale intergranular pore space was investigated here by cryo-Focused Ion Beam nanotomography (FIB-nt) applied to previously high-pressure frozen MX-80 bentonite samples. This approach allowed a tomographic investigation of the in situ microstructure related to different dry densities (1.24, 1.46, and 1.67 g/cm3). The FIB-nt technique is able to resolve intergranular pores with radii >10 nm. With increasing dry density (1.24 – 1.67 g/cm3) the intergranular porosity (>10 nm) decreased from ∼5 vol.% to 0.1 vol.%. At dry densities of 1.24 and 1.46 g/cm3, intergranular pores were filled with clay aggregates, which formed a mesh-like structure, similar to the honeycomb structure observed in diagenetic smectite. Unlike 'typical' clay gels, the cores of the honeycomb structure were not filled with pure water, but instead were filled with a less dense material which presumably consists of very fine clay similar to a colloid. In the low-density sample this honeycomb-structured material partly filled the intergranular pore space but some open pores were also present. In the 1.46 g/cm3 sample, the material filled the intergranular pores almost completely. At the highest densities investigated (1.67 g/cm3), the honeycomb-structured material was not present, probably because of the lack of intergranular pores which suppressed the formation of the honeycomb framework or skeleton consisting of clay aggregates

Fluid flow and effective conductivity calculations on numerical images of bentonite microstructure

“…Smectites are characterized by a heterogeneous arrangement of nanometerscale lamellae stacked together to form clay particles, micrometer-scale grains, water and air which saturate the various pore spaces that appear on different scales (Jozja, 2003;Holzer et al, 2010;Keller et al, 2014;Pusch and Yong, 2006;Tessier et al, 1992). As a result, clay microstructure controls most of their physical properties such as ion diffusion, swelling pressure and hydraulic conductivity.…”

“…This explains the scarcity of microstructural data available to quantify the pore size distribution and pore connectivity in compacted bentonite. As pointed out in (Holzer et al, 2010;Keller et al, 2014), the proportions of free and bound porewater in compacted bentonite are the subject of ongoing discussions and investigations, and tomography techniques are still under development in order to quantify precisely the mesopore distribution in compacted swelling clays. In (Pusch and Schomburg, 1999;Pusch, 2001) Transmission Electron Microscopy (TEM) micrographs of compacted and hydrated MX-80 bentonite displayed the formation of clay gels of variable density by linking of aggregates exfoliated from the expanding clay grains.…”

“…They concluded that the outer montmorillonite sheets are likely to swell and form a gel that occupies the intergranular voids, whereas the inner sheets are not affected by the water saturation process. Holzer et al (2010) and Keller et al (2014) investigated the intergranular pore space (mesopores) of compacted and hydrated MX-80 bentonite using high resolution 3D imaging with Focused Ion Beam nanotomography. They observed that clay particles' hydration in non-compacted bentonite (dry density ρ dry = 0.39 g/cm 3 ) is associated with extensive exfoliation and dispersion of thin clay layers.…”

“…According to Baltean (1999) and Wodie (1992), the effective permeability will be obtained on the macroscopic scale by upscaling from the mesoscopic scale alone, consisting here of the arrangement of clay aggregates, inter-aggregate voids and non-smectite grains. As mentioned in Section 1.1, microstructural data regarding the intergranular space (Holzer et al, 2010;Keller et al, 2014;Pusch, 2001;Pusch and Yong, 2006) allow us to distinguish between dense and soft parts of the heterogeneous clay matrix, implying regions of low permeability and highly permeable channels. Accordingly, we assume that the soft gels always contribute to the macroscopic flow, while the dense gels may be impervious to fluid flow, completely open to flow, or characterized by an effective conductivity that can be determined from a separate upscaling approach.…”

“…Based on experimental evidence (Holzer et al, 2010;Keller et al, 2014;Pusch, 2001), the general description of a bentonite microstructure where clay gels of variable density partially fill the intergranular pore space has been retained. Three distinct formulations are obtained for the hydraulic conductivity at the macroscopic scale, depending on the distribution of the mesopores.…”