Bentonite is high swelling clay generated casually from the alteration of volcanic ash and consisting predominantly of smectite minerals. It may form when volcanic ash weathers over long periods of time. Smectites are swelling clay minerals, of which montmorillonite is the most common type. For lining and containment applications, bentonite with high smectite content is desirable. Its name was derived from the Fort Benton series of cretaceous rocks in Wyoming, where it was first found. There are three commercial types of bentonite, each named after respective dominant element, such as sodium (Na) bentonite, calcium (Ca) bentonite, and activated sodium (Na2CO3-Na) bentonite. The swelling capacity of sodium bentonite is the most excellent among them: sodium bentonite has high affinity for water swelling up to 15~20 times its original volume when hydrated. Bentonite attracts water particles charged positive; thus, it rapidly hydrates when exposed to liquid, such as water or leachate. When hydrated but constrained not to swell free, it forms layer impermeable to water as well as renders any tiny hole to be sealed. Good-quality sodium bentonite has been produced at Wyoming in USA, supplying 95 percent of the natural sodium bentonite in the world. The natural sodium bentonite is often replaced by the calcium bentonite treated chemically with soda ash: artificially activated sodium bentonite. However, there have been various tests as well as papers that proved poor hydraulic conductivity of the activated sodium bentonite due to degradation within 24 or 28 months. Products of Korea Paramount resort to the natural sodium bentonite.

Mineral characteristics of Bentonite
1. Swelling
Sodium swells 15 to 20 times of its original volume when hydrated and it absorbs water 5 times of its own weight. The crystal of bentonite consists of three layers: a silicon tetrahedron, an aluminum octahedron and another silicon tetrahedron. The unit structure is a very thin platelet about 10A [1 nm] thick and 0.1 to 1μm wide. When bentonite comes in contact with water, it absorbs the water and swells up. This swelling is caused by an interaction between interlayer cations and water molecules. The gap between bentonite layers is widened as the interlayer cations attract the water molecules. When bentonite is dispersed in water, the platelets bond together electrostatically to form a house-of-cards structure and the liquid becomes viscous. When the house-of-cards structure develops further, the bentonite-dispersed liquid becomes a gel.

2. Adhesion
Sodium bentonite not only has its own adhesion but also adheres to other materials. It does not give any chemical effect to other material. Bentonite is used as binder of molding sand, stabilizing agent of underground diaphragm wall construction, and admix material for impermeable barrier mixed with soil and cement.

3. Cohesion
When bentonite particle is dispersed in water, it does no longer precipitate but maintain colloidal status. By its own cohesion and bonding characteristics, bentonite is used for paints, chemicals, soil binders, cleaners, and coagulants of waste water.

4. Gelation
bentonite has a superior property of gelation with water. It reacts with water and forms gel to impede water get through it.

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Waterproofing characteristics of Bentonite
1. Impermeable
Sodium Bentonite layer having thickness from 4mm to 9mm builds up barrier impermeable 100 times more than 30 cm thick clay layer.

2. Permanent
Bentonite waterproofing is known to be least susceptible to chemical, provided that electrical balance of its particles is not disturbed. Hydration-dehydration cycle or freezing-thawing cycle that is repeated infinitely does not affect permanent performance of bentonite waterproofing system.

3. Self-Sealing
Hydrated sodium bentonite layer, when confined not to swell free, is directed into any cracks or holes that may exist around the layer and provides sealant. This self-healing effect is unique to bentonite waterproofing.