Bentonite is a mixture of clay and non-clay minerals. Montmorillonite clay mineral is a dominant mineral in bentonite. Silica and alumina bond in the crystal structure of montmorillonite. Therefore, they cannot contribute to the pozzolanic reaction. Heat treatment of bentonite leads to the destruction of the crystal structure of montmorillonite and converting silica and alumina to reactive phases. Thermally activated bentonite (TAB) is a relatively low reactive pozzolan when used as partial replacement of Portland cement modifies both fresh and hardened properties of cement paste, mortar, and concrete. The most desired effects of TAB are: improve segregation resistance, reduce the rate of strength gain, and enhance concrete durability against sulfates, chlorides, and acids, in addition to economic and ecological beneficiations. This paper provides information related to heat treatment of bentonite clays and montmorillonite minerals, and their effects on the paste, mortar, and concrete when used as a partial replacement of Portland cement.

Bentonite is a type of clay that consists mainly of the montmorillonite mineral when it is finely ground and used as a partial replacement of Portland cement. The result is the low-cost and eco-friendly cementitious binder. Using this binder leads to modify both fresh and hardened properties of cement paste, mortar, and concrete. The desired modifications include; reduce the heat of hydration, reduce the absorption, promote both sulphates and acids resistance; the undesired effects include: increase water demand, reduce setting time, and declination of the mechanical properties. Although raw bentonite may meet the standard requirements for natural pozzolan, it’s considering as a supplementary cementing material that still lacks conclusive evidence.

Iraqi high-calcium bentonite was used for the first time to prepare natural pozzolan. Twenty-four calcination programs were studied. The optimum calcination program is 800°C for 90min. The calcined bentonite meets the requirements of the Iraqi specification of pozzolanic materials. The incorporation of calcined bentonite as a part of cementitious binder extends both initial and final setting time in proportion to the percentage of calcined bentonite. In concrete, the calcined bentonite reduces workability and early strength in proportion to the percentage of Portland cement replacement, while promotes the later strength at 5–15% replacement. The retardation effect of calcined bentonite makes it suitable for concrete production in hot climate territories and for mass concrete. Moreover, the relatively low embodied energy and low carbon dioxide emission of calcined bentonite enhance the sustainability of concrete.