Sodium 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.

Because of its excellent colloidal properties, it is often used in drilling mud for oil and gas wells and boreholes for geotechnical and environmental investigations. Various surface modifications to sodium bentonite improve some rheological or sealing performance in geoenvironmental applications, for example, the addition of polymers.

Bentonite is also used in drilling fluids to lubricate and cool the cutting tools, to remove cuttings, and to help prevent blowouts. Much of bentonite's usefulness in the drilling and geotechnical engineering industry comes from its unique rheological - or “flow” properties. Relatively small quantities of bentonite suspended in water form a viscous, shear thinning material. Most often, bentonite suspensions are also thixotropic, where an increase in relative flow within the suspensions reduces its viscosity.
 
At high enough concentrations (60 grams of bentonite per litre of suspension), bentonite suspensions begin to take on the characteristics of a gel (a fluid with a minimum yield strength required to make it move). For these reasons it is a common component of drilling mud used to curtail drilling fluid invasion by its propensity for aiding in the formation of mud cake.

Sources: Wikipedia

API 13A SEC 9; Conforming to API 13A: 2010/ ISO 13500: 2009

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