The main characteristic that determines the usefulness of kaolin for a large number of applications is its mineral purity taking into account the presence of impurities such as quartz, anatase, rutile, pyrite, siderite, feldspar, among others (Mitrović and Zdujić, 2014). The properties of clays vary considerably and they strongly depend on their composition and mineral structure. Therefore, based on a review of the scientific literature, this paper initially presents the relationship between the physical characteristics and the mineral composition of primary and secondary (sedimentary) kaolin clays and their pozzolanic reactivity then, it briefly describes some experiences regarding the improvement of this reactivity and, finally, the corresponding conclusions.Ĭlays containing a high percentage of kaolinite (Al 2O 3.2SiO 2.2H 2O) are commonly called kaolin. The cement industry uses mainly kaolin as a raw material (source of silica and alumina) for producing clinker for, white cement and metakaolin, used as SCM. One source of pozzolans are calcined clays and their use lies in the availability of materials and the improvement of the concrete’s durability (Sabiret al., 2001). Biogenic sedimentary rocks like diatomite and radiolarite have also been used (Krajči et al., 2013 Degirmenci and Yilmaz, 2009) or zeolite materials (Özen et al., 2016 Küçükyıldırım and Uzal, 2014).Īs for synthetic pozzolans, many types of waste containing reactive silica have been used, for example, fly ash, rice husk ash, silica fume, blast furnace slags, and others (Nurchasanah, 2013). The first ones are the natural material formed by, for example, extrusive igneous rocks formed by volcanic ash or lava containing reactive silica. In the ASTM 593-82 standard, pozzolans are classified in two types: natural pozzolans and synthetic pozzolans. ![]() Pozzolan is a composite material containing reactive silica or silica-alumina and alumina, which does not have the cementitious properties, but in the presence of water and at room temperature they react with the cement’s hydration products (especially the portlandite), in order to form new compounds that have cementitious properties. A blend of Portland cement and pozzolans has been commonly used to deal with this environmental issue and also to obtain some economic advantages in the production of concrete (Sabir et al., 2001). Likewise, it can be affected by the use of alternative raw materials, mineralizers and fluxes minerals, and Supplementary Cementitious Materials (SCMs). This value may change according to the localization, technology, production efficiency, energy sources for generating electricity and the choice of fuels for the furnace (Petek Gursel et al., 2004). The construction industry is well known as an important source of greenhouse gas emissions (GHG) when considering all the GHG emissions generated by human activities, the cement industry accounts for 5% of these emissions and it also produces approximately 8% of the global CO2 emissions (Alujas et al., 2015).Īlmost half of these emissions are from fossil fuels, since Portland cement is an energy-intensive material that requires 4-5 GJ/t in its manufacturing process (Petek Gursel et al., 2004) and the other half corresponds to the decarbonation of limestone, which releases an average of 0.84 t of CO 2 per ton of clinker to the atmosphere (CSI 2014). However, it is also widely recognized that concrete production has a significant impact on the environment. Keywords: Kaolin clays, calcined clays, pozzolan, metakaolin, supplementary cementitious materialsĬoncrete is one of the most commonly used construction material, with a global consumption rate of approximately 25 GT per year, which is almost 3.8 t per person a year (Petek Gursel et al., 2004). ![]() In the same way, and based on a review of the scientific literature, the relationship between morphologic characteristics, thermal treatments and mineral composition of clays and its pozzolanic reactivity is shown as well as with the chemical, thermal and mechanical methods for improving that reactivity. However, data reported by different authors show a very high variability that makes its modelling difficult. Calcined clays have been recognized as very good pozzolans, because they improve the durability and compressive strength of mortars and concretes. Pozzolans have demonstrated to be highly efficient mineral additions in the concrete industry, because they reduce the environmental impact generated by the cement manufacture, improve the physical, chemical and mechanical properties of blends, and reduce costs. ![]() Pozzolanic reactivity of kaolin clays, a review
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