Chloride ion migration in natural bentonite

The disposal of nuclear wastes in geological formations demands the construction of engineering barriers. Bentonite clay rock is frequently used both as natural and engineering barrier. The natural bentonite rock in its original form is considered as compacted bentonite if the density is higher than 1.2 g/cm³. In this paper, the risk of the extrapolation of the laboratory experiments to field conditions and the high differences of the natural samples are emphasized: as much as 52 % standard deviation was obtained in the migration coefficients characterizing bentonite samples collected from the same site with a very small extent of sampling. Moreover, the bulk densities (1.18–1.87 g/cm³) and montmorillonite content are also rather different (45–71 m/m %).The contradictions of the effects of the swelling clay mineral (montmorillonite) content and the bulk density of bentonite are illustrated: it is shown that the migration rate of chloride anion is determined by the ratio of the different water types (interlayer water of montmorillonite to free pore water of bentonite, including the electric double layer water). The apparent migration coefficients of bentonite clay and concrete (natural and artificial engineering barrier) are compared.