# What is bentonite?
**Bentonite is a naturally occurring clay composed primarily of the mineral montmorillonite, which has the distinctive property of swelling to many times its dry volume when it absorbs water.** This swelling capacity makes bentonite exceptionally useful in construction for waterproofing, creating impermeable barriers and stabilising boreholes during drilling and piling operations.
## Types of bentonite
There are two main types used in construction:
– **Sodium bentonite** — swells up to 15-18 times its dry volume and forms a dense, impermeable gel when hydrated. This is the type most commonly used in construction
– **Calcium bentonite** — swells less and is used more in environmental and agricultural applications
## Application
Bentonite is widely used across several construction disciplines:
### Drilling and piling
– **Drilling mud (bentonite slurry)** — mixed with water to create a viscous fluid that stabilises borehole walls during drilling, preventing collapse
– **Diaphragm wall construction** — bentonite slurry keeps the trench open while the reinforcement cage is placed and concrete is poured
– **Bored pile installation** — supports the shaft during excavation
### Waterproofing
– **Bentonite mats** — factory-made sheets of bentonite between geotextile layers, applied against foundations and basement walls
– **Bentonite grout** — injected into the ground to seal leaks or create barriers around excavations
– **Pond and landfill liners** — bentonite layers prevent water from seeping through
### Ground improvement
– **Slurry walls** — permanent bentonite-cement barriers that cut off groundwater flow
– **Annular space sealing** — bentonite fills the gap around well casings and monitoring tubes
### Key properties
– Extremely low permeability when hydrated
– Self-healing: minor punctures or cracks seal themselves as the bentonite re-swells
– Non-toxic and chemically stable in most soil conditions
– Must be kept dry during storage to prevent premature activation
## Related terms
– Bentonite mat
– Drilling mud
– Diaphragm wall
– Waterproofing
– Dewatering
Want to learn more about construction terms? Visit our [knowledge base at fredsdiyplans.com](https://fredsdiyplans.com).