BariteSodium bentonite is most widely known for its ability to swell. It can absorb nearly 5 times its weight in water and at full saturation may occupy a volume 12 to 15 times its dry bulk.  The high water absorption capacity of bentonite also makes it very plastic and resistant to fracturing or cracking. Interestingly, bentonite can be hydrated and dried an infinite number of times without losing its original swelling capacity. It can similarly be frozen and thawed repeatedly without losing its ability to swell. Because the platelets are uniformly broad and flat, sodium bentonite has an exceptionally high surface area of 600 to 800 square meters per gram.  Less than 10 grams of bentonite, if fully dispersed, could cover a football field.

It is a combination of these physical properties that make sodium bentonite an ideal waterproofing material. A layer of hydrated bentonite provides a dense, low porosity barrier to fluid flow that exhibits a typical hydraulic conductivity of 1 x 10-9 cm/sec. The property of swelling makes sodium bentonite useful as a sealant, since it provides a self-sealing, low permeability barrier. It is used to line the base of landfills for example.