Bentonite in Civil Engineering Bentonite is a clay that’s starting to turn heads in the civil engineering and water-proofing sectors. Bentonite clay is a type of absorbent clay that is known for its ability to swell when mixed with water. It is primarily made up of montmorillonite, which gives it plasticity, and other minerals such as quartz and cristobalite. Its unique properties, such as swelling capacity, plasticity, and sealing properties, make it a valuable asset in civil engineering and waterproofing