Analysis of microstructural images of dry and water-saturated compacted bentonite samples observed with X-ray micro CT

Abstract

Compacted bentonite, of which the major clay mineral is montmorillonite, is a candidate buffer material for geological disposal of high-level radioactive waste.In this study, a microfocus X-ray computed tomography (micro-CT, X-ray microscope), which enables non-destructive, three-dimensional observation of the interior microstructure of a sample with high resolution (several microns), examined compacted montmorillonite samples under dry and water-saturated states. The images thus obtained were analyzed by a computer code developed for this study to obtain Preprint submitted to Elsevier 22 December 2007the information on the size and shape of montmorillonite grains in the samples before and after the water saturation. From the results of the image analysis, it can be supposed that the outer montmorillonite sheets of grains swelled and formed a gel, whereas the inner montmorillonite sheets did not change significantly in the water-saturation process.

Self-Diffusion of Water and Ions in Clay Barriers

“…These methods, however, sometimes disagree significantly on important features such as the fraction of the clay mineral surface charge that is screened in the Stern layer (by surface complexes) versus the diffuse ion swarm (by long-range electrostatic interactions). Secondly, the microstructure of water-saturated clay material is not precisely known at the microscopic (pore network) scale despite advances in X-ray diffraction (Liu et al, 2003a;Wilson et al, 2004;Ferrage et al, 2005;Holmboe et al, 2010Holmboe et al, , 2012, X-ray computed tomography (Liu et al, 2003b;Tomioka et al, 2010;Keller et al, 2013), small angle neutron scattering (Swift et al, 2014), NMR (Montavon et al, 2009), and electron microscopy (Hicher et al, 2000;Melkior et al, 2009), because none of these techniques can probe the full range of length scales characteristic of clay layers and their mesoscale assemblages (from 10 À9 to 10 À6 m) in conditions that exist in compacted, water-saturated swelling clays.…”

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

“…At larger length scales, other studies used X-ray computed microtomography to study the microstructure of bentonites (e.g. Tomioka et al, 2010).…”

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

“…In water-saturated compacted bentonite, mesopores that contain free pore water may be distinguished from nanoscale pores that contain water molecules bound by surface forces. The multi-scale pore structure continuously changes during hydration, swelling and compaction processes, which complicates its characterization (Holzer et al, 2010;Tomioka et al, 2010). This explains the scarcity of microstructural data available to quantify the pore size distribution and pore connectivity in compacted bentonite.…”

“…The effective conductivity was then approximated by an analytical formula combining the individual cells in parallel or in series depending on their orientation with respect to the macroscopic flow direction. Tomioka et al (2010) used Xray microtomography for examining the morphological evolution of compacted Na-montmorillonite before and after saturation. 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.…”