Geotechnical properties indicating environmental uses for an unusual Australian bentonite

Samples of bentonite from a large deposit at Arumpo in southern New South Wales were characterised chemically and mineralogically. The samples of highly pure clay mineral, representing a large part of the deposit, together with a sodium-saturated sample of Wyoming bentonite were analysed for their rheological and free swell properties. Their permeability under a range of applied vertical stresses was also determined using a modified oedometer. As mined, the Arumpo bentonite showed little viscosity and swelling when suspended in water. Surprisingly, however, its permeability at all applied vertical stresses was as low as that of the high swelling Na-bentonite from Wyoming. Although the permeability of the Arumpo bentonite decreased with an increase in applied vertical stress, its void ratio hardly changed. On the other hand, the void ratio of the Wyoming bentonite showed an almost four-fold decrease with the same change in applied vertical stress. Since Arumpo bentonite has a high concentration of very small particles, it had a high external surface area (230 m2 g−1), whereas the external surface area of the Wyoming bentonite sample was only 14 m2 g−1. The permeability of Arumpo bentonite is intrinsically low because of the close packing of its particles. By contrast, the literature shows that Wyoming bentonite requires extensive swelling in order to achieve a low permeability, and needs to be in the Na-exchanged form for this purpose. As the Arumpo bentonite has a low permeability that is not dependent on swelling, or on the nature of the exchangeable cation, the material can be used as a barrier to many contaminants that are more stable than swelling bentonites. Barriers made from the Arumpo bentonite should also show minimal cracking on drying.