Bentonite

A Common Clay, Swelling Bentonite.

Sodium Bentonite expands when wet - it can absorb several times its dry weight in water. It is mostly used in drilling mud in the oil and gas well drilling industries. The property of swelling also makes sodium bentonite useful as a sealant, especially targeted for the sealing of subsurface disposal systems for spent nuclear fuel and for quarantining metal pollutants of groundwater. Similar uses include making slurry walls, waterproofing of below grade walls and forming other impermeable barriers (e.g. to plug old wells or as a liner in the base of landfills to prevent migration of leachate into the soil.