Bentonite
Bentonite is a naturally-occurring material created by the alteration of volcanic ash in marine environments. It consists mainly of the clay mineral smectite, usually montmorillonite, which occur as layers which are compressed between other rock types.
Bentonite is usually obtained by quarrying, with a solid extracted form with a typical moisture content of around 30%. This moisture content is reduced to approximately 15% by air and/or forced drying after being crushed. Once this has been done, bentonite is either sieved in a granular form, or milled into powder form.
In civil engineering, bentonite is used as a thixotropic, support and lubricant agent. Typical applications for this agent include the construction of diaphragm walls, foundations, pipe jacking, tunnelling, and so on. Its viscosity and plasticity also make it suitable for use in Portland cement and mortars.
Bentonite’s thixotropic properties mean it forms a highly water-resistant gel which, when mixed with additives, can create a permanent barrier to water flow. This is commonly used in situations where soil particles are too small for cement grouting to be suitable, typically to combat seepage in alluvial soils beneath the foundations of dams or other water-bound structures.
For drilling purposes, bentonite can be used as a mud constituent, to seal borehole walls, remove drill cuttings, and so on.
It can also be used in varnishes and paints to act as a thickening and/or suspension agent.
NB High Speed Rail (Crewe – Manchester) Environmental Statement, Glossary, abbreviations and references, published by the Department for Transport in 2022, defines bentonite as: ‘A natural clay that, when mixed with water, swells and forms a thixotropic gel (a particular type of viscous behaviour exhibited by some liquids). It can be used temporarily to support trenches or retaining walls, and helps to prevent collapse when they are being backfilled (i.e. refilled) or concreted.’
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