The mineral bentonite can be found all over the world. It is formed from weathering volcanic ash. It has some exceptional properties: when stirred into water, it demonstrates a so-called thixotropic reaction. It reacts like a fluid when mechanically stressed, for example, shaken or stirred. However, it hardens in quiescent condition because its viscosity increases.

The most common use of bentonite is in drilling fluids. The bentonite in the flush fluid lubricates and cools the cutting tools while protecting against corrosion. As the drilling fluid generates hydrostatic pressure in the borehole, it hinders fluid and gas penetration. The drilling liquid conveys the drill cuttings to the surface. When the purge pumps have idle periods, the thixotropy of the bentonite hinders the stone material from dropping back into the drilled shaft.

As a flush fluid, bentonite seals the drilled shaft from water ingress downwards and at the sides of the shaft. The mineral forms a firm sludge cake on the bore wall which provides the borehole with additional stability. The fine bentonite particles enter into the bore wall, swell and harden. Depending on the constitution of the ground structure, fine and coarse particles are needed.