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Time evolution of MX-80 bentonite geochemistry under thermo-hydraulic gradients

Published online by Cambridge University Press:  02 January 2018

R. Gómez-Espina  and
M.V. Villar
R. Gómez-Espina
Affiliation:
Universidad Andres Bello, Carretera Concepción-Talcahuano, 7100 Concepción, Chile
M.V. Villar*
Affiliation:
CIEMAT, Avd. Complutense 40, 28040 Madrid, Spain
*
*E-mail: mv.villar@ciemat.es
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Abstract

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Two 20-cm long columns of MX-80 bentonite compacted at a nominal dry density of 1.7 g/cm3 with a water content of 17% were tested in thermo-hydraulic (TH) cells with the aim of simulating the conditions of a sealing material in a nuclear waste repository. On top of the columns a hydration surface simulated the host rock supplying groundwater and at the bottom a heater simulated the waste canister. The tests comprised two phases: a heating phase and a ‘heating + hydration’ phase. The temperatures at the ends of the columns were set during the last phase to 30°C at the top and 140°C at the bottom, respectively. The thermo-hydraulic treatment resulted in major changes along the bentonite columns. These changes led to significant gradients along the column with respect to the physical state (water content, dry density) and geochemistry of the bentonite. Smectite dissolution processes occurred. As a result, colloids were probably produced, particularly in the more hydrated areas. In the warmest part of the columns precipitation of carbonates took place, caused by their solubility decrease with temperature and the evaporation. The increase in water content reduced the ionic strength of the pore water in the more hydrated areas where species such as gypsum were dissolved. The solubilized ions were transported towards the bottom of the columns; Na+, Ca+, Mg2+ and SO42− moved at a similar rate and K+ and Cl− moved farther. These solubilized ions precipitated in the form of salts farther away along the columns as the test was longer. The TH treatment implied the loss of exchangeable positions in the smectite, particularly towards the heater. The cation exchange complex was also modified.

Keywords

bentoniteengineered barriersnuclear wastepore waterthermo-hydraulic treatment
Type
Research Article
Information
Clay Minerals , Volume 51 , Issue 2 , May 2016 , pp. 145 - 160
Creative Commons
Creative Common License - CCCreative Common License - BY
Copyright © The Mineralogical Society of Great Britain and Ireland 2016 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 2016

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