Chemical and mineralogical reactions of bentonites in geotechnical barriers at elevated temperatures: review of experimental evidence and modelling progress

Bentonites are proposed to be used as buffer in high-level radioactive waste repositories. The elevated temperatures in repositories may, however, affect bentonites desired properties. For instance, heating under dry conditions can cause cation fixation potentially affecting swelling properties. The kinetics of mineral dissolution and precipitation reactions will equally be influenced by temperature. Redistributions of Ca-sulphates and -carbonates has been observed, as well as illitization of smectite. Illitization, however, has only been observed in laboratory experiments at large solution to solid ratios, while it has not yet been unambiguously identified in large-scale experiments. In many large-scale tests, cation exchange is the first observable geochemical reaction. In addition, an enrichment of Mg close to the heater is found in many such tests. The thermal gradient and (incongruent) smectite dissolution are suspected to play a role with respect to the Mg-enrichment but the underlying mechanism has not been unravelled so far. To predict the long term performance of a bentonite buffer, numerical modelling is required to be able to simulate the reactions of all accompanying mineral phases. Smectites, which dominate the bentonite composition, are, thereby particularly difficult to characterise, as their dissolution is often observed to be non-stoichiometric. Different model approaches exist to simulate smectite reactions mostly based on kinetic rate reactions, ideally considering the effect of pH (congruent or incongruent dissolution), temperature, and the degree of saturation of the solution. Reassessing and improving the thermodynamic/kinetic data of smectites is the prerequisite for improving long term buffer performance assessment.