Sodium bentonite is a non-metallic clay composed mostly of the mineral montmorillonite. It is formed from volcanic ash.

Montmorillonite is a layered clay mineral with broad, flat platelets that are ideally shaped to provide a hydraulic barrier. Sodium ions located between these platelets allows water to hydrate the bentonite in a remarkable fashion that results in the high swelling characteristic.

Dry sodium bentonite can swell to many times its original volume when exposed to water. Because of its high swelling capability bentonite is able to seal around penetrations, giving geosynthetic clay liners their self-healing characteristic. A fully hydrated layer of sodium bentonite will result in a hydraulic conductivity of less than 5 x 10-9 cm/sec. This is approximately twenty times lower than a typical compacted clay layer.