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Characteristics of variably saturated granular bentonite after long-term storage at near-field relevant temperatures

Published online by Cambridge University Press:  09 July 2018

M. Valter  and
M. Plötze
M. Valter*
Affiliation:
ClayLab, Institute for Geotechnical Engineering, ETH Zürich, Schafmattstrasse 6, 8093 Zürich, Switzerland
M. Plötze
Affiliation:
ClayLab, Institute for Geotechnical Engineering, ETH Zürich, Schafmattstrasse 6, 8093 Zürich, Switzerland
*
*E-mail: martin.valter@igt.baug.ethz.ch
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Abstract

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Bentonite is a potential material for use in the engineered barrier of radioactive waste repositories because of its low hydraulic permeability, self-sealing capability and retention capacity. It is expected that bentonite would react at the elevated temperatures accompanying the radioactive decay in the nuclear waste. The presented study was started in order to improve understanding of the coupled influence of temperature and (pore) water on the physicochemical and mineralogical properties of bentonite during thermal treatment under near-field relevant conditions. Granular Na-bentonite MX-80 was differently saturated (Sr = 1–0.05) and stored at different temperatures (50–150°C) in a closed system. Upon dismantling after different periods of time (3 to 18 months), mineralogical characteristics, cation exchange capacity and content of leachable cations, as well as physicochemical properties such as surface area and water adsorption were investigated.

The results showed a high mineralogical stability. A slight conversion from the sodium to an earth alkali form of the bentonite was observed. However, considerable changes in the physicochemical properties of the bentonite were observed, particularly by treatment above the critical temperature of 120°C. The cation exchange capacity decreased during heating at 150°C by approximately. 10%. The specific surface area dropped by more than 50%. The water uptake capacity under free swelling conditions showed a slight tendency to lower values especially for samples heated for more than 12 months. The water vapour adsorption ability in contrast drops by 25% already within three months at T = 120°C. These changes are mostly related to the variations in the interlayer cation composition and to smectite aggregation processes. The observed alterations are rather subtle. However, temperatures ⩾ 120°C had a remarkable negative influence on different properties of MX-80.

Keywords

radioactive waste repositorythermal treatmentNa-bentonitebentonite alteration
Type
Research Article
Information
Clay Minerals , Volume 48 , Issue 2 , May 2013 , pp. 343 - 361
Creative Commons
Creative Common License - CCCreative Common License - BY
Copyright © The Mineralogical Society of Great Britain and Ireland 2013 This is an Open Access article, distributed under the terms of the Creative Commons Attribution license. (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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