Electrical conductivity of bentonites

• Water content–electrical conductivity (WC–EC) curves of several bentonites recorded.
• Two types of WC–EC curves were found depending on the type of interlayer cation.
• Microstructure affected by sample pretreatment also affects the WC–EC curve.
• Improvement of existing models possible based on new conceptual model.

Clay electrical properties are known to depend on cation exchange capacity (CEC) and the swelling clay minerals have the highest CEC. Despite the use of advanced models to calculate dispersed media electrical properties, reported differences in bentonite (Ben) and smectite (Sm) electrical conductivities (EC) can rarely be explained. The aim of this study, therefore, was to examine the EC of 40 samples, which were mostly well-characterized bentonites, and systematically investigate the reason for the different EC. Comparing single EC values for different samples is not sufficient because of the effect of different water contents (WC). Results shown in the present study suggest that electrical properties of bentonites have to be compared considering the water content (ideally measuring WC–EC curves) or, even better, comparing the WC–EC curve slopes after decreasing the WC. For the first time WC–EC curves of a significant set of bentonites were compared which revealed that two different types (shapes) of WC–EC curves exist which can be attributed to the type of interlayer cation. The EC of Na+-saturated materials started to increase at larger water contents (> 80%) compared to Ca2 +/Mg2 +-dominated samples (< 80%). This can be explained by the delamination of Na+-Sm in which layer–layer contacts remain despite a large WC. The water content of Ca2 +/Mg2 +-dominated samples has to reach a critical before the EC starts to increase, but the WC–EC slopes of both types are similar. Geoelectrical data are commonly interpreted based on models. These, however, could not be used for bentonites. For the improvement of the existing models the microstructure which depends on both sample pretreatment/state and type of exchangeable cation has to be taken into account. The existing models have to be improved in a way that they at least can predict the effect of different interlayer population on the electrical properties.