Analysis of the structural changes of a pellet/powder bentonite mixture upon wetting by X-ray computed microtomography

• Published in: Applied clay science Vol. 165; pp. 164 - 169
• Main Authors: Molinero Guerra, Agustín, Aimedieu, Patrick, Bornert, Michel, Cui, Yu-Jun, Tang, Anh Minh, Sun, Zhao, Mokni, Nadia, Delage, Pierre, Bernier, Frédéric
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2018; Elsevier
• Subjects: Civil Engineering; Digital volume correlation; Engineering Sciences; Génie civil nucléaire; Géotechnique; Homogenization; Initial heterogeneity; Mechanics; Mechanics of materials; Pellet/powder bentonite mixture; Water saturation; X-ray computed microtomography; Digital volume correlation; Water saturation; Homogenization; Initial heterogeneity; X-ray computed microtomography; Pellet/powder bentonite mixture; Homogenizatio
• ISSN: 0169-1317; 1872-9053
• DOI: 10.1016/j.clay.2018.07.043

Pellet/powder bentonite mixture is one of the candidate materials for sealing plugs in deep geological high-level radioactive waste disposal. This note presents an investigation on the structure changes of this mixture occurring during the saturation process by means of X-ray computed micro-tomography. The test was performed in an infiltration column (60 mm in inner diameter and 120 mm in height). Water was supplied to the two ends of the column and the changes of the sample morphology were observed during a period of 100 days of hydration. Digital Volume Correlation (DVC) technique was used to determine the vertical displacement field of the bentonite powder. A pressure transducer was used to measure the axial swelling pressure during the hydration. The results show that the initial distribution of powder in the inter-pellet pores was not homogeneous; the powder filled almost completely the pores in the zones close to the two ends while air-filled inter-pellet voids were observed in the middle of the column specimen. When water started to infiltrate inside the specimen from both ends, the pellets and the powder grains started to swell (because of the swelling properties of smectite, the principal mineral of bentonite) and filled the voids. That induced at the same time increase of swelling pressure and downward movement of powder grains. The results allowed a better understanding on the hydro-mechanical couplings, at the pellet scale, in the pellet/powder bentonite mixture upon wetting. •3D observation of pellet/powder bentonite mixture upon wetting•The initial structure of the mixture is heterogeneous and granular•Hydration creates a homogeneous material in a column of 120 mm length after 100 days of hydration from both ends.•The bentonite powder grains tend to move downward during the first day of hydration when the bentonite pellets are rearranged due to their wetting-induced swelling.