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Alteration of MX-80 bentonite backfill material by high-pH cementitious fluids under lithostatic conditions – an experimental approach using core infiltration techniques

By
F. Dolder
F. Dolder
RWI, Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland
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U. Mäder
U. Mäder
RWI, Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland
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A. Jenni
A. Jenni
RWI, Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland
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B. Münch
B. Münch
EMPA, Swiss Federal Institute for Materials Testing and Research, 8600 Dübendorf, Switzerland
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Published:
January 01, 2017

Abstract

We characterize and quantify processes at a cement–bentonite interface spatially and temporally during a long-term core infiltration experiment.

A young ordinary Portland cement pore fluid (K+–Na+–OH: pH 13.4) was infiltrated into a MX-80 bentonite core with an initial saturated density of 1920 kg m−3 that shifted to 1890–1930 kg m−3 after 761 days. A hydrostatic external pressure of 4.1 MPa and an infiltration pressure of 2.1 MPa were applied in a triaxial-type apparatus. A decrease in hydraulic conductivity from approximately 2.2×10−13 to approximately 4.2×10−15 m s−1 was observed passing from advection-dominated flow to a diffusion-dominated regime. Sulphate replaced chloride in the outflow during the high-pH infiltration period controlled by the dissolution of gypsum, the uptake of K+ by ion exchange, and complex mineral reactions occurred near the inlet. X-ray computed tomography (CT) scans performed repeatedly during the experiment tracked a progressing hemispherical reaction plume in the first millimetres of the bentonite, revealing a zone of bulk density increase. This zone consisted of two distinct, but overlapping, zones of Mg- and Ca-enrichment related to precipitation of saponite and calcite. The experiment attested an effective chemical buffering capacity for bentonite, a progressing coupled hydraulic–chemical sealing process and also the preservation of the physical integrity of the interface region in this set-up with a total pressure boundary condition on the core sample.

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Geological Society, London, Special Publications

Radioactive Waste Confinement: Clays in Natural and Engineered Barriers

S. Norris
S. Norris
Radioactive Waste Management, UK
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J. Bruno
J. Bruno
Amphos 21, Spain
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M. van Geet
M. van Geet
ONDRAF/NIRAS, Belgium
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E. Verhoef
E. Verhoef
COVRA, The Netherlands
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Geological Society of London
Volume
443
ISBN electronic:
9781786203267
Publication date:
January 01, 2017

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