**1. Introduction**

Sustainable development, understood as progress that meet the needs of the present without compromising the ability of future generations to meet their needs, is now the basic idea behind the current paradigm of technological progress. The greatest threat to the achievement of the objectives of sustainable development is the excessive emission of greenhouse gases, including, above all, carbon dioxide, which results in global warming with many serious negative consequences for future development and its prospects.

The building industry makes a significant contribution to increasing the carbon dioxide content in the atmosphere, with cement production accounting for 5% of global carbon dioxide emissions [1] being its largest source. This is due, on the one hand, to the high energy intensity of the Portland clinker production process. The production of one ton of clinker requires the supply of 3.1–3.8 GJ of heat, whereas in the older generation wet method kilns the demand may reach even 6 GJ/t [1]. On the other hand, during cement production, carbon dioxide is emitted from raw materials, mainly limestone. The amount of this emission is about 0.53 kg/kg of clinker [1].

The possibilities of reducing the energy intensity of cement production are limited, although it is already possible to reduce heat consumption to 2.9 GJ/t of clinker. The use of alternative fuels further reduces carbon dioxide emissions from fossil fuels. However, the use of limestone in clinker production cannot be eliminated or reduced. Therefore, solutions to further reduce CO2 emissions to the atmosphere by the cement and concrete industry are the production of blended cements that reduce the amount of clinker and the use of supplementary cementitious materials (SCM), mainly pozzolans, in the concrete technology. A pozzolan, according to ASTM C125, is "a siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided

form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties" [2].

The most commonly used SCM so far is fly ash from coal power plants. However, this additive, which is used in the production of both cement and concrete, is slowly losing its primary importance due to the progressive decommissioning of coal and lignite power plants. As a result, the supply of fly ash will be significantly reduced in the near future. Another cement additive used to reduce clinker content is a granulated blast furnace slag. However, it has no potential to replace fly ash, not least because its global supply is far below potential demand. Additionally, it is only available in countries where the steel industry exists. However, even where it is available, its share in blended cements production is not large. In India, for example, Portland slag cement accounts for only about 8% of total cement production, while ordinary Portland cement accounts for 24% of production, and Portland pozzolana cement accounts for 65%, where the main addition is fly ash [3,4].

The decreasing supply of fly ash from the power industry is encouraging the search for new sources of pozzolan additives for cement and concrete production. One of the directions of this search is ashes obtained from biomass combustion, [5]. Another is the use of natural and artificial pozzolana. The former include, among others, volcanic tuffs already known in ancient times or zeolites. The best known representative of the second group is the metakaolin formed by the calcination of kaolinite from clays with its significant content.

Metakaolin was shown to be the best clay raw material for SCMs production, but in its pure form it is only found in a limited number of deposits, so its availability is not sufficient to meet the needs of the building materials industry. It is also in the focus of interest of other industries [6,7]. For this reason, there has been interest in the possibility of producing SCMs from other locally available natural clays containing, in addition to kaolinite, other minerals which have the potential to develop pozzolanic activity upon appropriate activation. Research conducted in this direction led to the separation of a new group of pozzolanic materials—calcined clay.

Clay is a widely spread material in the world, cheap and easily accessible [8]. At the same time, it is a material with a great diversity in terms of mineralogical composition, hence numerous literature items devoted to the analysis of the possibility of using clays from specific deposits for the production of SCM in the calcination process [9–11].

In general, the literature on the activation of clay minerals and their use in civil engineering is very rich and covers a wide range of issues. It would be very difficult to discuss this vast area of research in one review article, so certain assumptions on the scope of this review were made First of all, it focuses on calcined clays used as SCM in binders, where cement is the dominant component, and in the cement-based concrete. Outside the scope of interest remain cement-free binders based on calcined clays, which are the basis of geopolymers, as well as other alkaline-activated binders (including clay minerals). Issues that do not fall within the assumed topics of the article are mentioned where it was indicated for various reasons.

The paper is divided into eight thematic sections. The first section describes a fundamental knowledge about clay minerals and publications on clays from various deposits in the world. In the next section the topic of clay minerals activation with emphasis on calcination process and research of hydration mechanisms and pozzolanic activity with factors influencing them are presented. A separate section is devoted to the role of calcined clays in concrete technology (as SCM) and in blended cements production. Due to the wealth of literature devoted to lime calcined clay cement (LC3), a separate section is also devoted to it. The next chapter discusses the results of research on the impact of calcined clays on concrete durability. Numerous publications on calcined clay, which cannot be clearly classified as one of the above thematic areas, have been placed in a separate section devoted to suitable aspects of the application of these materials. The summary presents the conclusion of the literature review
