Bentonite is highly colloidal, plastic clay which works as an absorbent and binder with numerous applications.

Origins and structure

Bentonite is an absorbent aluminium phyllosilicate clay derived from weathered volcanic ash composed of the mineral montmorillonite which is part of the smectite group of clays.

For industrial purposes, two main classes of bentonite exist: Sodium and Calcium bentonite. In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. Other common clay species that are sometimes dominant are montmorillonite and kaolinite. Kaolinite-dominated clays are commonly referred to as tonsteins and are typically associated with coal.

Bentonites may contain a variety of accessory minerals in addition to montmorillonite. These may include lesser amounts of other clay minerals such as kaolin, mica, illite, as well as non-clay minerals like quartz, feldspar, calcite, and gypsum. Bentonite quality, and, consequently, its applications, depends on whether it contains any of these other minerals.

Its density when dry varies depending on the quality, and may range from 2.2 to 2.8 g/cm³. Bentonite apparent density, when quarried and piled under natural moisture conditions, ranges from 1.5 to 1.8 g/cm³. The apparent density of milled bentonite products varies depending on mill fineness, ranging from 0.7 to 0.9 g/cm³.

Calcium Bentonite

Calcium bentonite is a useful adsorbent of ions in solution, as well as fats and oils. It is the main active ingredient of fuller's earth, probably one of the earliest industrial cleaning agents.

Sodium – Activated Bentonite

Calcium bentonite may be converted to sodium bentonite (termed sodium beneficiation or sodium activation) to exhibit many of sodium bentonite's properties by an ion exchange process. In common usage, this means adding 5–10% of a soluble sodium salt such as sodium carbonate to wet bentonite, mixing well, and allowing time for the ion exchange to take place and water to remove the exchanged calcium. Sodium activated bentonite expands when wet, absorbing as much as several times its dry mass in water. Because of its excellent colloidal properties, it is often used in drilling mud for oil and gas wells and boreholes for geotechnical and environmental investigations. The property of swelling also makes sodium bentonite useful as a sealant, since it provides a self-sealing, low permeability barrier. It is used to line the base of landfills, for example. Various surface modifications to sodium bentonite improve some rheological or sealing performance in geo-environmental applications, for example, the addition of polymers.