Heterogeneous reactions of dioctahedral smectites in illite-smectite and kaolinite-smectite mixed-layers : applications to clay materials for engineered barriers

Abstract The clay materials selected for use in the engineered barriers of the French nuclear waste isolation programme are mainly composed of dioctahedral smectite, either bentonite of Wyoming type or kaolinite-smectite mixed-layered minerals from the Sparnacian deposit of the Paris Basin. These dioctahedral smectites most often consist of randomly stacked layers with low and high charges. In the case of the Wyoming-type bentonite, these two differently charged layers do not react in the same way when subjected to hydrothermal alteration. Overall, the low-charge smectite layers react to form high-charge smectite layers + quartz + kaolinite. Then, fixing K ions, the high-charge smectite layers are transformed into iilite-smectite mixed-layers (I/S) when the temperature conditions increase (diagenetic formations, geothermal fields or metasomatic environments). A symmetrical process is observed in natural or experimental hydrothermal conditions when the high-charge smectite layers of I/S minerals react with quartz and/or kaolinite to produce low-charge smectite layers. The chemical properties (cation exchange capacity, irreversible K fixation) of the bentonite-engineered barriers clearly depend on the low charge/high charge smectite layer proportion, which is in turn controlled by the temperature-dependent reactions in the vicinity of the waste disposal. Although there are fewer published data on the kaolinite-smectite mixed-layered minerals (K/S), a similar low charge-high charge reaction appears to affect their smectite component. The experimental alteration of K/S leads to the formation of a low-charge beidellite with an increase in the cation-exchange capacity and in the expandability of the clay material. Thus, the properties of the engineered barrier seems to be improved after hydrothermal alteration.