Bentonite moisture retaining clay
Used as an earth-electrode backfill to reduce soil resistivity by retaining moisture. The clay is a sodium activated montmorillonite, which when mixed with water swells to many times its dry volume. It has the ability to hold its moisture content for a considerable period of time and to absorb moisture from the surrounding soil (e.g. from rainfall).
Bentonite moisture retaining clay may be used to reduce the contact resistance and increase the effective size of earth electrodes, e.g. as a backfill for earth rods installed in drilled holes or as a layer encapsulating horizontal earth conductors buried in a trench.
It also has the added advantage of making intimate contact with the electrode and the surrounding soil, thus helping to protect the electrode from corrosion. The resistivity of Bentonite varies from about 3 ƿm upwards depending on its moisture content (BS7430 clause 8.5)