. Sodium bentonite Sodium bentonite expands when wet, possibly absorbing several times its dry mass in water. Because of its excellent colloidal properties (see Odom ref below) it's often used in drilling mud for oil and gas wells and for geotechnical and environmental investigations. The property of swelling also makes sodium bentonite useful as a sealant, especially for the sealing of subsurface disposal systems for spent nuclear fuel [http://www.skb.se/templates/SKBPage 8806.aspx] (External L… (More on Bentonite)