The sustainability of concrete can improve by utilizing pozzolanic materials as a partial replacement of Portland cement . Natural pozzolan is one of the types of pozzolanic materials when used as a part of the cementitious binder; it develops some of the fresh properties, mechanical properties, and/or durability of concrete , . Given the absence of naturally formed pozzolans in Iraq and the rarity in industries producing pozzolanic materials as waste, the available alternative is to utilize mineral resources in obtaining natural pozzolan. Iraqi bentonite represents a promising source for natural pozzolan production; its validity for this purpose has not previously been investigated.
Bentonite is a clay that consists dominantly of smectite minerals . In addition to smectite minerals, bentonite may contain multiple species of clay minerals such as palygorskite, kaolinite, illite, or others, and may contain non-clay minerals such as quartzite, gypsum, feldspar, or others , . The unique property of smectite minerals is the potentiality to swell due to water absorption . In most cases, bentonite contains a high percentage of montmorillonite mineral, a member of the smectite group. Therefore, bentonite may define as clay consists mostly of montmorillonite mineral .
There are numerous allegations about the pozzolanicity of raw bentonite –, but these allegations are not supported by conclusive evidence of the occurrence of the pozzolanic reaction between bentonite components and calcium hydroxide resulting from Portland cement hydration . On the contrary, precise examinations have shown that only minor changes in montmorillonite structure occur in highly alkaline environments , and a small fraction of montmorillonite may contribute to pozzolanic reaction . On the other hand, the pozzolanicity of both calcined montmorillonite (CM) and calcined bentonite (CB) has been confirmed in several studies –.
CB modifies the concrete properties in plastic and hardened states due to both pozzolanic and physical actions. In plastic state, CB incorporation causes a linearly increase in water demand to achieve standard consistency of the paste , , extends both initial and final setting time –, reduces workability of concrete , and increases the dosage of superplasticizer required for self-compacted mortar  and concrete . In a hardened state, CB reduces the rate of strength gain due to the low pozzolanic reactivity of CM , –. Therefore, CB incorporation leads to a reduction in compressive strength of concrete at early ages, i.e., 21 days or less , , ,  but the reductions in concrete strength tend to disappear at later ages, i.e., 28–90 days , , . The most important effect of CB is enhancing the durability of concrete against alkalis-silica reaction , sulfates attack , acids attack , and chlorides penetration , .
In Iraq, the petroleum industries are the largest consumer of the bentonite. These industries require high swelling bentonite, i.e., sodium bentonite while the bentonite deposits in Iraq contain only low swelling bentonite, i.e., calcium bentonite , . Therefore, Iraqi calcium bentonite shall be processed to convert it to high swelling bentonite before using it in the extraction of crude oil. Calcium content is the criterion in determining the validity of calcium bentonite to process and thus converting to high swelling bentonite . On this basis, Iraqi bentonite is classified into two classes –: high grade or low-calcium bentonite in which CaO ≤ 5%, and low grade or high-calcium bentonite in which CaO > 5%. High-calcium bentonite requires complex processes to be converted into high swelling bentonite , , . Because of this, the producing of the high swelling bentonite from the high-calcium bentonite is not economically feasible, and the utilization of Iraqi bentonite is limited to the low-calcium bentonite only, while high-calcium bentonite considered as useless material. Therefore, the production of pozzolanic material from Iraqi high-calcium bentonite opens up prospects for exploiting mineral resources that are not yet exploited.