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

Published online by Cambridge University Press:  01 January 2024

Lukas M. Keller ,
Ali Seiphoori ,
Philippe Gasser ,
Falk Lucas ,
Lorenz Holzer  and
Alessio Ferrari
Lukas M. Keller*
Affiliation:
Zürich University of Applied Sciences, Winterthur, Switzerland
Ali Seiphoori
Affiliation:
Ecole Polytechnique Fédérale de Lausanne (EPFL), School of Architecture, Civil and Environmental Engineering (ENAC), Laboratory for Soil Mechanics (LMS), Switzerland
Philippe Gasser
Affiliation:
Swiss Federal Institute of Technology, Centre for Imaging Science and Technology, Zürich, Switzerland
Falk Lucas
Affiliation:
Swiss Federal Institute of Technology, Centre for Imaging Science and Technology, Zürich, Switzerland
Lorenz Holzer
Affiliation:
Zürich University of Applied Sciences, Winterthur, Switzerland
Alessio Ferrari
Affiliation:
Ecole Polytechnique Fédérale de Lausanne (EPFL), School of Architecture, Civil and Environmental Engineering (ENAC), Laboratory for Soil Mechanics (LMS), Switzerland
*
*E-mail address of corresponding author: [email protected]
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Abstract

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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.

Keywords

Clay GelsCryo-sample PreparationMX-80 BentoniteNanotomography
Type
Article
Information
Clays and Clay Minerals , Volume 62 , Issue 3 , June 2014 , pp. 174 - 187
Copyright
Copyright © Clay Minerals Society 2014

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