Modelling the interaction of bentonite with hyperalkaline fluids |
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| Institution: | 1. Glenn T. Seaborg Institute, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA;2. Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany;3. Biosciences and Biotechnology Division, Physical & Life Sciences Directorate, Lawrence Livermore National Laboratory, L-231, P.O. Box 808, Livermore, CA 94550, USA;1. Department of Engineering, Huzhou University, Huzhou 313000, PR China;2. College of Nuclear Science and Engineering, East China Institute of Technology, 330013 Nanchang, PR China;3. Paul Scherrer Institut, Laboratory for Waste Management, CH-5232 Villigen PSI, Switzerland;1. Key Laboratory of Geotechnical & Underground Engineering of Ministry of Education, Department of Geotechnical Engineering, Tongji University, Shanghai 200092, PR China;2. Laboratoire Navier/CERMES, Ecole des Ponts - ParisTech, 6 - 8 av. Blaise Pascal, Cité Descartes, 77455 Marne - la – Vallée, France;3. School of Chemical Science and Engineering, Tongji University, Shanghai 200092, PR China |
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| Abstract: | Many designs for geological disposal facilities for radioactive and toxic wastes envisage the use of cement together with bentonite clay as engineered barriers. However, there are concerns that the mineralogical composition of the bentonite will not be stable under the hyperalkaline pore fluid conditions (pH > 12) typical of cement and its properties will degrade over long time periods. The possible extent of reaction between bentonite and cement pore fluids was simulated using the reaction-transport model, PRECIP. Key minerals in the bentonite (Na-montmorillonite, analcite, chalcedony, quartz, calcite) were allowed to dissolve and precipitate using kinetic (time-dependent) reaction mechanisms. Simulations were carried out with different model variants investigating the effects of: temperature (25 and 70 °C); cement pore fluid composition; dissolution mechanism of montmorillonite; rates of growth of product minerals; solubilities of product minerals; and aqueous speciation of Si at high pH. Simulations were run for a maximum of 3.2 ka. The results of all simulations showed complex fronts of mineral dissolution and growth, driven by the relative rates of these processes for different minerals. Calcium silicate hydrate (CSH) minerals formed closest to the cement-bentonite boundary, whereas zeolites and sheet silicates formed further away. Some growth of primary bentonite minerals (analcite, chalcedony, calcite and montmorillonite) was observed under certain conditions. Most alteration was associated with the fluid of highest pH, which showed total removal of primary bentonite minerals up to 60 cm from the contact with cement after ～1 ka. The maximum porosity increase observed was up to 80–90% over a narrow zone 1–2 cm wide, close to the cement pore fluid- bentonite contact. All simulations (except that with alternative aqueous speciation data for Si) showed total filling of porosity a few cms beyond this interface with the cement, which occurred after a maximum of 3.2 ka. Porosity occlusion was principally a function of the growth of CSH minerals such as tobermorite. There was very little difference in the alteration attained using different model variants, suggesting that bentonite alteration was not sensitive to the changes in parameters under the conditions studied, so that transport of pore fluid through the bentonite governed the amount of alteration predicted. Principal remaining uncertainties associated with the modelling relate to assumptions concerning the evolution of surface areas of minerals with time, and the synergy between changing porosity and fluid flow/diffusion. |
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