Many designs for geological disposal facilities for radioactive and toxic wastes envisage the use of cement together with bentonite (mostly montmorillonite clay, with traces of quartz, feldspar, zeolites and carbonates) as engineered barriers. In addition, bentonite-based drilling muds and cement are often used together in boreholes drilled for hydrocarbon exploration. Often, the use of these two materials together takes advantage of the chemical buffering properties of cement and the low permeability and swelling capacity of the bentonite (so-called 'belt and braces' approach). However, there are concerns that the mineralogical composition of 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.
To this end, Quintessa (in conjunction with the British Geological Survey) has recently completed an experimental and modelling study financially supported by the Japan Nuclear Cycle Development Institute (JNC). The project investigated the likely solid and fluid products of cement-bentonite interaction at temperatures ranging up to 90C. Framework silicates such as analcite and leucite, together with calcium silicate hydrate (CSH) gels are typical solid products of the reactions. The study has also highlighted the need to incorporate realistic kinetic data for mineral dissolution and growth at elevated pH, but dissolution reactions are best modelled by assumptions of far-from-equilibrium chemical conditions. Alteration of bentonite is predicted to be extensive over 1000-year timescales, but reactions are likely to be self-prohibiting due to the occlusion of porosity by neo-formed solids.
We conclude that although cement encourages the mineralogical alteration of bentonite, its porosity and permeability will decrease with time due to cementation by the growth of zeolites and CSH gels and solids.
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