Effect of fluid chemistry on the microstructure of light backfill: An X-ray CT investigation

Bentonite-sand mixtures are widely accepted as the candidate sealing materials of radioactive waste repository due to their high swelling capacity and low hydraulic conductivity. These properties of the material depend largely on its microstructure, particularly, the pore space geometry. The aim of this investigation is to explore the effect of the pore fluid (NaCl and CaCl2) on the microstructure, pore geometry and hydraulic conductivity of a barrier material. Representative samples of light backfill (LBF) prepared with a 50-50 bentonite-sand mixture at a compacted dry density of 1.24 Mg/m(3) were used. As a first attempt, X-ray computed tomography (X-ray CT) was used to study the LBF under the distilled water (DW) and two other pore fluid conditions. In order to acquire a good quality image with high resolution, X-ray source, detector, and a small LBF specimen (5.5 mm in diameter) were placed close together and scanned with Xradia Micro XCT-400. The voxel of the scanned images were (1.15 x 1.15 x 1.15) mu m(3), indicating that the particles with a diameter greater than 2 mu m could be easily observed. The porosity value estimated at the end of the consolidation test showed significantly higher values compared to the X-ray CT analysis. The interconnected pore components and absolute permeability of distilled water or salt solution saturated LBF samples were analyzed using Avizo software. This analysis showed that the volume of interconnected pore increased due to the presence of salt solutions and resulted higher hydraulic conductivity. The salt solution increased porosity, pore size, volume of interconnected pore and hydraulic conductivity of the LBF. (c) 2016 Elsevier B.V. All rights reserved.