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What is bentonite?

Bentonite rock

Bentonite is a plastic clay generated from the alteration in situ of volcanic ash, consisting predominantly of montmorillonite. Bentonite presents strong colloidal properties and increases its volume several times when coming into contact with water, creating a gelatinous and viscous substance. Its special properties of swelling, water absorption, viscosity and thixotropy, make it a very valuable clay for a wide range of uses and applications.


Bentonite originates from the in-situ weathering of volcanic ash and consists primarily of montmorillonite. Montmorillonite is a fine particle sized hydrous aluminium silicate and is one of the three layer clay materials. In the majority of cases, bentonites are formed by the alteration of volcanic ash and rocks after intense contact with water.

Physical properties

SEM photo bentonite
  • Strong colloidal properties 
  • Swelling when in contact with water 
  • Water absorption 
  • High viscosity 
  • High plasticity 
  • Thixotropy
  • Paper
  • Paper

    This very high surface area, combined with the cationic charge of the material, makes bentonite ideal for applications requiring adsorption, flocculation and dewatering. In the papermaking process, bentonite is used as a retention aid, pitch and stickies control agent, loop water treatment agent and to dewater sludge.

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    Pile of newspapers

For other markets please visit Imerys Metallurgy.