**6. Lime Calcined Clay Cement (LC3)**

The cement consisting of Portland clinker, calcined clay (preferably rich in kaolinite), calcium carbonate and gypsum is described in literature as LC3 [148]. It is a solution for a demand for cement that is more environmentally friendly, the production of which takes place with lower CO2 emissions to the atmosphere, and which at the same time is not inferior to ordinary Portland cement with a clinker content of at least 90%. The abundance of raw clays and limestone is also an important factor in the development of LC3 production, in contrast to the shrinking resources of good quality fly ash or even their unavailability in some countries.

The most common composition for the LC<sup>3</sup> is 50% of ground Portland clinker, 30% of ground calcined clay, 15% of ground limestone and 5% of ground gypsum. Such a mixture is sometimes referred to in the literature as LC3-50. Other proportions of components are also possible [149], but it is known that cement prepared according to the above composition, according to the research [113,150], reaches mechanical parameters corresponding to OPC already after seven days of hydration, provided that the clay contains at least 40% kaolin.

An important aspect of LC<sup>3</sup> production is its cost and profitability. Papers [151,152] present the results of economic analysis of production of this type of ternary blend in India. The authors concluded that the production of LC<sup>3</sup> is economically viable if the following conditions are met: the cost of fly ash will be high, the quality of fly ash will be low, the acquisition of fly ash will require longer transport than the acquisition of clay and the quality of the extracted clay will reduce the amount of clinker in cement. Most of these conditions are already met, for example, in countries which do not have sources of good quality fly ash. Given the restrictions on the use of fossil fuels in energy production, these conditions may soon be met automatically.

Apart from the economic aspect, the durability and performance of concrete with LC<sup>3</sup> is no less important. Several publications have been devoted to this issue. In papers [153,154] the authors assessed the pore structure in concrete made of three types of cement: OPC, Portland pozzolana cement with 30% of Class F fly ash and LC3. The results of the study showed a much finer pore structure in concrete with LC3, which was proved in the mercury intrusion porosimetry study. Additionally, the conductivity of the concrete obtained in this way turned out to be lower, which allows one to assume that it will have higher resistance to the penetration of harmful ions into its structure. In short, its durability can be predicted to be significantly higher than that of other concrete series compared in the work.

Khan et al. [155] discussed carbonation of LC3. The study covered investigation of concrete with cement, in which 15%, 30% and 45% of the mass was replaced by a mixture of calcined clay (containing about 50% metakaolin and 50% quartz) and limestone in a ratio of 2:1. For comparison, two series of concrete with OPC were used. One of them had the same aggregate, water and binder ratio as the LC3 concrete series, and the other modified proportions and quantities. The results indicate that concrete with LC<sup>3</sup> and 15% cement exchange rate showed higher carbonation resistance than concrete with OPC. At 30% a slight advantage was gained by concrete with OPC, and at a 45% cement exchange rate to a mixture of calcined clay and limestone, concrete with LC3 showed a high carbonation rate. The same authors [156] presented the results of the same concretes according to their fresh properties, strength, porosity and drying shrinkage. They showed that the concrete with LC<sup>3</sup> was characterized by worse workability. As far as strength was concerned, the highest was reached by concrete with a 15% exchange rate of cement, which also had the lowest porosity. As far as drying shrinkage is concerned, all series of LC<sup>3</sup> concrete were lower than OPC concrete.

An interesting approach to the issue of carbonation is presented in paper of Joseph et al. [157]. The authors have been tempted to create the protocol of testing the durability of LC<sup>3</sup> (although not only) with respect to carbonation. This protocol included the determination of indicators of the carbonation process, such as diffusivity of CO2, hydration and carbonation products, total possible carbonation, and pH change due to the carbonation and rate of carbonation. The paper also contains a suggestion to determine the above mentioned elements and the possibility to apply the presented approach to other issues concerning concrete durability.

Nguyen and Castel [158] evaluated the resistance of concrete with LC3 to chloride diffusion. Three series of concrete were made for research purposes. One contained only general purpose cement corresponding to Type I cement according to ASTM C150/C150M [159]. In the remaining two 15% and 20% of the cement was replaced by a mixture of calcined clay and limestone in the ratio 2:1. The results indicate that the use of LC<sup>3</sup> in concrete significantly increased its resistance to chloride diffusion. The continuation of the research on chloride diffusion resistance Nguyen et al. presented in [160], where they compared concrete made with OPC with concrete containing calcined clay obtained in two technological processes: flash calcination and calcination in rotary kiln. Due to differences in activity of both types of calcined clay, the authors decided to apply different proportions of general purpose cement (as per Australian Standard AS 3972 [161]) to calcined clay (20% for flash calcined clay and 44% for clay calcined in rotary kiln). This variation was due to the intention to obtain the appropriate compressive strength of the hardened concrete (>45 MPa). The results indicated four times higher chloride diffusion resistance of concrete with LC3. The difference between the results of concrete made with different types of calcined clay turned out to be insignificant, which led the authors to conclude that it is crucial in this case to properly select the proportion of cement exchanged for calcined clay to meet the requirements of specific strength of concrete exposed to chloride ingressives.

A different approach to chloride diffusion research was applied by Yang et al. [162]; they discussed the results of simulations of this phenomenon in LC3 concrete and concrete with fly ash compared to it. The authors conducted extensive simulations taking into account many elements affecting the course of chloride diffusion in hardened concrete. The results showed that both concretes had comparable service life even though the LC<sup>3</sup> concrete contained less clinker. The conclusions also indicated important parameters that should be introduced to the simulation in order for the results to be reliable.

Nguyen and Castel [163] presented the results of research on corrosion of reinforcing steel in concrete made using LC3. This issue is important because of the lower alkalinity of this type of concrete due to the lower amount of portlandite, which is consumed by calcined clay in the pozzolanic reaction. The research, carried out over 500 days, allowed the conclusion to be drawn that performance of reinforced concrete prepared with LC<sup>3</sup> was comparable to that for concrete with OPC in the propagation phase.

Rengaraju et al. [164] also investigated corrosion of steel in concrete with LC3. They presented the results of corrosion testing of steel in three concrete mixes prepared with the use of: OPC, a blend of 70% OPC with 30% of fly ash and a blend of 50% OPC with a 50% mixture of limestone and calcined clay. In their previous work [165] they carried out tests according to AASHTO T 358 standard [166], which showed that concrete with LC3 showed very high resistivity, which is a good predictor of resistance to chloride ion penetration and, consequently, to corrosion of reinforcing steel. As a continuation of the tests, corrosion tests of steel in concrete were carried out using the method included in ASTM G109 [167] and the impressed current corrosion test method. In the conclusions, apart from the statement that LC3 cement composites showed higher resistance to chloride ingress and better protection of steel against corrosion, the authors also claimed that the corrosion products of steel in concrete with LC3 were less expansive and thus less destructive to concrete.

The study of corrosion of reinforcement in concrete made with LC<sup>3</sup> was also conducted by Pillai et al. [168]. They determined such concrete parameters as chloride diffusion coefficient, ageing coefficient and chloride threshold for seven mixtures containing OPC and blends of OPC with pulverised fuel ash (PFA) or with calcined clay and limestone (i.e., LC3). Using these parameters they determined the service life of the two construction elements. The results showed that a construction made of LC<sup>3</sup> or blended cement with PFA will had a significantly longer service life with a much smaller carbon footprint.

The issue of carbon footprint is focusing our attention on the eco-efficiency of LC3. In [169] the authors have undertaken the assessment of the sustainability of LC<sup>3</sup> using two methods: life cycle analysis and eco-efficiency. The first method was used to assess the environmental impact of LC3 production compared to OPC and Portland pozzolana cement (with 20% zeolite content). The second method assessed how the use of LC3 in the construction of a model residential building will increase its eco-efficiency. The calculations led to the conclusion that the use of LC3 may lead to a reduction of cement production costs by 4–40% and CO2 emissions by 15–30%.

No less important are the technological aspects related to LC3 production. The authors of paper [170] undertook the assessment of suitability of four clays from deposits located in Cuba for LC3 production. The results turned out to be promising as they indicated that all examined clays had the potential to be used in ternary blend cement production. Additionally, the authors showed that the pozzolan activity was directly proportional to kaolinite equivalent (KEQ), and that the specific surface area of the obtained cements depended mainly on the calcination temperature of the clays and their mineralogical composition.

Nair et al. [171] presented the results of rheological properties test of cement paste and mortar prepared using LC3 in comparison with OPC and Portland-fly ash cement (containing 30% Class F fly ash). The effectiveness of five superplasticising admixtures based on polycarboxylic ethers (PCE) differing in chemical composition and one admixture based on sulphonated naphthalene formaldehyde (SNF) were also compared. The studies carried out have led to the conclusion that larger quantities of superplasticisers were required for blends with LC3, with PCE providing a lower viscosity than SNF and the latter additive had to be dosed in larger quantities to not allow to reduce the w/b ratio below 0.4.

Li, X. and Scrivener, K.L [172] devoted their paper to the comparison of three methods of determination of reacted metakaolin in LC3. The authors compared: mass balance [173], thermodynamic modelling with Gibbs Free Energy Minimization Software [174] and the Partial or not Known Crystalline Structure method (PONKCS) [175]. Of the methods analysed, the mass balance approach yielded the most reliable results, although the use of GEMS software was, on the other hand, less time-consuming and required less labour-intensive input. The PONKCS method proved to be reliable in the case of LC3-50 mixtures containing clays with metakaolin content above 60%.

Papers [176,177] present the results of two approaches to the pilot production of LC3 in India. The composition of blends in both cases was almost the same: 50% Portland clinker, 30% or 31% calcined clay, 15% crushed limestone and 4% or 5% gypsum. In the first approach [176] several problems of technological nature emerged, such as too little fineness of the cement due to the use of ground limestone and calcination of only part of the used clay. This resulted in a large scattering of results and lower strength values of LC<sup>3</sup> concretes. In the second approach [177], the authors did not report any technological complications concluding that it is possible to produce good quality LC3 even without the need to change technologies in existing cement plants.

The influence of the degree of fineness of LC<sup>3</sup> components on its selected physical and mechanical parameters was examined and presented in [178]. It was assumed that each of the three binder components (Portland cement—55%, calcined clay—30% and limestone—15%) can be ground to two degrees: coarse and fine. In addition, in part of the series, clay or limestone was replaced by finely ground quartz. The results indicate that the greatest influence on the strength parameters of concrete with LC3 had the degree of clinker fineness, and a slightly lesser calcined clay fineness. Limestone fineness was important only in the initial period of concrete strength development (up to three days).

Examples of practical application of LC3 were presented by Maity et al. in [179]. Four different LC<sup>3</sup> blends with two types of clay and two types of limestone and, for comparison, Portland Pozzolan Cement were used to produce concrete and various structural elements. All these products have undergone quality control and have been incorporated into the construction of residential buildings. The obtained results showed that the mechanical parameters of LC3 concrete can be even higher than those of PPC concrete. The resulting buildings are tangible proof that LC3 had moved from the concept and research phase to the first practical application tests.

Dhandapani and Santhanam [180] analysed the impact of the mutual ratio of limestone and calcined clay on the binding and strength development of LC<sup>3</sup> and blends in which the calcined clay was replaced by Class F fly ash. Binary blends without added limestone were used as a reference. The research showed a clear influence of limestone on the setting of the concrete and much less on the further development of its strength. In addition, concrete using calcined clay achieved a very significantly higher strength after seven and 28 days of hardening.

Ferreiro et al. [181] analysed the influence of the clay calcination temperature in the flash calcination process and the fineness of raw clay on the workability and strength performance of LC3. They used two types of clay, which were calcined in two different installations (gas suspension calciner and flash calciner) and at two different temperatures. The clays used consisted mainly of minerals of group 2:1, which was an additional value of the work, as most of the research on LC<sup>3</sup> was based mainly on clays containing kaolin. The results obtained allowed to conclude that both the calcination temperature and the degree of fragmentation of the material subjected to this process had a significant impact on the workability of fresh LC<sup>3</sup> mortars and their strength development.

The influence of the method of constituent grinding on rheology and early strength of LC<sup>3</sup> tested on mortars was examined by Perez et al. [182]. The constituent were ground both separately and jointly. Clinker, limestone and calcined clay ground separately were divided into three fractions, from which LC<sup>3</sup> was then composed using different combinations of the degree of grinding of the components. In the case of components ground jointly the grinding time varied (25, 45 and 65 min). As far as the latter case is concerned, the results indicate that the longer the grinding time, the higher the strength of the LC3 later reached, while the change of mini slump radius over 45 min of grinding time was no longer significant. In the case of separated milling, the degree of grinding of the clinker was crucial and the degree of grinding of limestone was of secondary importance. The influence of the degree of milling calcined clay was not studied by the authors.

The suitability of LC3 as a material for 3D printing process has been investigated by Chen et al. [183]. They prepared four mixtures containing different proportions of two calcined clay with lower (49%) and higher (90%) metakaolinite content, and then conducted extrudability and earlier strength tests on them. The results showed that as the amount of metakaolinite in LC<sup>3</sup> blend increased, its extrudability decreased but at the same time the early strength increased. Therefore, it is crucial to find the optimal content of metakaolinite in LC3 used for blends in 3D printing. A referral to the same topic is in [184], in which the authors analyse the influence of viscosity modifying admixture on extrudability of LC3 based materials.

Avet and Scrivener [185,186] focused on the issue of hydration of LC3 depending on the content of calcined kaolinite in the blend. The research was carried out on LC3-50 blends with different metakaolinite content. The results showed that with calcined kaolinite above 65% hydration of clinker was slowed down after three days due to refinement of the pore structure. However, despite this, the strength was still increasing, which is connected with further reaction of metakaolinite and increase of the C-A-S-H amount.

Zunino and Scrivener [187] dealt with reactivity and mechanical performance of three mixtures containing Portland cement, Portland cement-limestone blend and LC3. The distinction of this publication is that the prepared mixtures were cured at a lowered temperature of 10 ◦C. The same mixtures cured at 20 ◦C were used as a reference. The research included determination of compressive strength after 1, 7 and 28 days, isothermal calorimetry as well as XRD phase assemblage assessment and MIP pore refinement assessment. They found that LC3 cured at lower temperature achieved significantly higher compressive strength than the same cement paste cured at higher temperature. The authors claim that this was an effect of, among others, a slightly different course of hydration of this cement.

In the summary of this chapter, it would be appropriate to repeat the concluding statements of the previous section which indicate the need to extend the interest in clays which may be active after calcination and which do not contain kaolinite or contain small amounts of kaolinite. Another interesting aspect that comes to mind after reading the above article on the practical application of LC3 is the possibility to test the suitability of this type of binder as a base for concrete for road construction.

#### **7. Influence of Calcined Clay on the Durability of Concrete**

Durability issues are important for each of the cement-based materials used nowadays. A studies related to the durability of cement-based composites prepared with calcined clays are presented in this chapter.

Trümer and Ludwig [188] put forward the thesis that one of the obstacles to the wider use of calcined clays in cement and concrete technology is the lack of information on the long-time behaviour of the concrete. They used clays of various mineralogical composition, which they subjected to the process of thermal activation and used in concrete mixtures as a substitute for 30% of the cement. The investigated concretes focused on the resistance to sulphate attack, alkali-silica reaction, chloride ingress as well as freezing-thawing resistance and carbonation. They revealed that calcined clays in concrete showed significantly lower pozzolanic activity than in other cement systems, which had a direct impact on the durability of concrete. As far as resistance to chloride ingress is concerned, all clays have passed the test. At the other extreme is freeze-thaw resistance, where only concrete with metakaolin showed satisfactory performance. Carbonation of concrete with calcined clay, due to the reduction of Ca(OH)2 consumed in the pozzolanic reaction, progressed faster than in concrete with OPC. The confirmation of pozzolanic activity should not be synonymous with the recognition that the tested clay is suitable for the production of cement or concrete, because depending on its composition and quality its influence on durability parameters may be diametrically different.

Slightly more optimistic conclusions about the durability of concrete with cement blended with calcined clay were formulated by Pierkes et al. [189]. They prepared a series of concretes using cements with 20% (CEM II/A-Q) and 40% (CEM IV/A-Q) of various calcined clays and determined their durability parameters, i.e., resistance to carbonation, chloride migration, freezing-thawing and frost-deicing salts. The first three tests were carried out on non-air-entrained concrete and the fourth on air-entrained concrete. The results indicated that concrete with the addition of calcined clays was able to achieve comparable values of durability parameters as concrete with the addition of other pozzolanic additives, such as fly ash or silica fume.

Shah et al. [190] prepared a short literature review on the durability of concrete with low clinker content, which is replaced by various mineral additives: fly ash, calcined clay, limestone and slag. This review covered chloride migration/penetration, carbonation, sulphate attack and alkali-silica reaction. The authors concluded that significant chloride resistance can be achieved by using a combination of mineral additives and, after that, the resistance was higher than blended cements. The influence of mineral additions on carbonation was summarised by the statement that above a certain level of exchange rate resistance of cement with mineral additions this resistance started to decrease. Calcined clays were found to cause the faster front of carbonation in the material. ASR (alkali-silica reaction) could be mitigated by SCMs, of which metakaolin is directly named. When it comes to resistance to sulphate attack, the authors are of the opinion that a combination of calcined clay and limestone appears to deteriorate in a sulphate environment.

Castillo Lara et al. [191] have conducted tests of physical-mechanical and durability properties on micro-concrete. From this work only the part concerning the tests of durability, which in this case were water absorption and sorptivity, was discussed. The tests were carried out on a cement system defined as micro-concrete, whose characteristic feature was a maximum aggregate grain size of 5 mm, i.e., more than for mortars and less than for ordinary concrete. Two kinds of calcined clay obtained from raw clay soil were used as a substitute for 30% cement. The results showed a decrease in both water absorption and sorptivity of the micro-concrete tested after using a cement blend with calcined clay compared to the material with OPC only.

The issue of carbonation of binders with metakaolin has been investigated by Bucher et al. [192]. The authors have carried out the research using the cements of type CEM I, CEM II/A-LL and CEM II/A-V as a reference. The depth of carbonation was determined on concrete specimens made with cements without additives and with partial replacement of cement with metakaolin. The results confirmed that the use of calcined clay accelerated the carbonation of concrete except when the component of cement blend was limestone (i.e., in the case of CEM II/A-L cement). Concrete with this blend containing 15% of metakaolin showed higher resistance to carbonation than concrete with Portland cement itself.

Studies [193,194] were devoted to resistance of cement mortars, containing, e.g., calcined clays, to chloride ingress. The first one discusses the results of tests carried out on mortar specimens prepared using ten different binders. These binders were composed by replacing 35% of the cement with pozzolanic active materials, including calcined clay. The research concerned concretes subjected to an artificial marine environment for 270 days. The use of alternative binders allowed the obtaining of a material with adequate resistance to chloride ingress while reducing CO2 emissions by 15%. In the other paper the authors investigated, among others, the resistance to chloride ingress of mortar prepared from cement with 35% content of calcined clay. In one of the mortar series, water was replaced with 0.5 M Na2SO4 solution, which served as a chemical activator and precursor of ettringite formation. The results of strength and resistance to chloride ingress revealed that the use of such a chemical activator positively affected both parameters.

Calcined clay is a pozzolanic material which makes it possible to reduce the negative effects of alkali-silica reaction, which has been demonstrated in a number of studies [188,195,196]. This is one of the methods of preventing ASR, besides the use of low-reactivity aggregate [197]. Paper [195] presents results of tests of mortars made with high alkali Portland limestone cement, in which 0%–30% of cement binder was replaced with calcined clay. Cement used in the research was characterized by 4.32% Na2Oeq. As the amount of calcined clay in cement increased, the expansion of mortar bars decreased. The lowest expansion value was achieved by mortar with 25% content. The authors attributed this to the formation of more CaSi2O5 and the associated reduction of sodium silicate, which is the main product of the ASR reaction responsible for the damage caused by it. In [196] the authors presented the results of research on effectiveness of calcined clays in ASR mitigation. Mortars containing highly reactive aggregate and four different types of calcined clays (replacing cement in the amount of 5%, 10%, 15% and 20%) were used in the research. The research showed that the chemical and mineral composition of the clay was crucial for the effectiveness of ASR mitigation. Trümer and Ludwig [188] tested the resistance of concrete with cement mixed with calcined clay to both ASR and sulphate attack. Three calcined clays containing as basic minerals, respectively, kaolinite, illite and montmorillonite after mixing with CEM I cement in a ratio of 30:70 significantly reduced the expansion of mortars and increased their resistance to sulphate attack.

Studies [198–200] were devoted to research on the resistance of cement composites to sulphate attack. Aramburo et al. [198] tested cement with a high content of calcined clay to show that it was possible to produce CEM IV/A-SR and CEM IV/B-SR type cements, which complied with the requirements of the current European standard UNE EN 197-1:2011 [201]. The specificity of this study was the very high degree of cement exchange for calcined clay, which was 40%, 50%, 60% and 70%. Apart from calcined clay, two different Portland cements were used in cement mixes, one with high C3A content and the other with high C3S content. The research showed the required increase in resistance to sulphate cements with calcined clay was accompanied by a decrease in compressive strength. Al-Akhras [199] presented the results of a wide range of research, in which the influence of the exchange of 5%, 10% and 15% of cement to metakaolin on the durability of concrete to sulphate attack was analysed. Apart from various degrees of cement exchange to calcined kaolin, the w/c ratio, initial moist curing period, curing type and air content were also variable parameters. The results allowed to formulate a conclusion that 10% and 15% of the exchange of cement to metakaolin allowed to achieve excellent durability to sulphate attack. Concrete with a lower w/c ratio achieved better durability performances, the time of moist curing proved to be insignificant, autoclaving increased the resistance to moist curing ratio as well as offer a higher air content in concrete. Shi et al. [200] presented the results of a study on the influence of 35% of cement exchange on calcined clay (metakaolin or calcined montmorillonite) or calcined clay and limestone in different proportions on sulphate resistance of mortars. White Portland cement and ordinary Portland cement were used as base cements. The results showed that all mixtures in which the ratio of calcined clay to the sum of calcined clay and limestone was greater than 0.5 showed excellent resistance to sulphate attack.

Quite a large group of papers on durability are focused on cement systems with calcined kaolin. Saillio et al. [202], as well as Yazıcı [203], have analysed various aspects of the durability of concrete and mortars produced with metakaolin addition. Tafraoui et al. [204] gave a brief literature review on the durability of ultra-high performance concrete containing metakaolin. Badogiannis and Tsivilis [205] dedicated their article to the study of the effect of Greek low kaolinite on the durability of concrete, and Shekarchi et al. [206] examined the transport properties (i.e., water penetration, gas permeability, water absorption, electrical resistivity and chloride ingress) of metakaolin-blended cement.

Shi et al. [207] analysed the results of a durability study of Portland cement blends containing, apart from cement, the following additives: pure limestone, pure metakaolin, metakaolin and limestone in 3:1 mass proportion, metakaolin and silica fume, and the three additions simultaneously. The obtained results allowed to state that mortar with pure limestone showed the worst durability parameters in all tests. Mortar with pure cement had the highest resistance to carbonation, but it did not perform well in resistance tests to sulphate attack and chloride ingress, and mortar with pure metakaolinite obtained exactly the opposite resistance parameters.

The issues of concrete durability with calcined clay have also been addressed in two review articles on calcined clays [47] and SCMs in general [58].

A review of studies on the durability of concrete with calcined clay makes it clear that the subject is by no means exhausted. There are relatively few studies on frost resistance and resistance to surface scaling. These issues are important for countries where, during the colder seasons, the temperature can repeatedly pass through zero, while relatively fewer articles come from these countries. When studying these durability properties, it also seems appropriate to determine the compatibility of binders containing calcined clay with air-entraining agents.

Due to the confirmed effectiveness of the addition of calcined clay in limiting the alkali-silica reaction, it is worth considering the simultaneous use of these co-binders with waste glass, either as an aggregate in concrete or as an additional material exhibiting pozzolanic activity. The use of waste glass has so far been limited by concerns about the durability of concrete. Among the studies on the durability aspects of concrete with calcined clays, it is also difficult to find those concerning air permeability. It can be assumed that in this aspect of concrete with the addition of calcined clay will be superior to concrete based on traditional cements, but there is a great deal of room for research between the conjecture and hard evidence.

#### **8. Conclusions**

This review paper presents the idea of the replacement of Portland cement by addition of calcined clays. Such application of this kind of pozzolanic materials, as clinker/cement replacement or as supplementary cementitious materials, can be an ecological and economically justified way to meet the global need to reduce CO2 emissions in concrete technology. Although clay is a material with a very diverse mineralogical composition, its low price, availability and, above all, its distribution in the world make it a valuable supplementary cementitious material in concrete technology.

Several studies have shown that the thermal treatment is necessary to activate the clay's minerals or to increase their pozzolanic activity. To determine the conditions of success of the calcination process, the effects of time, temperature and particle size were investigated.

It was shown that mixing various clays in appropriate proportions with cement allowed to obtain a concrete with satisfactory mechanical parameters, much better than the reference one without calcined clay. It was described that the use of calcined clay increased both early age and long-term mechanical properties of cement paste, mortar and concrete. Moreover, it was found that the replacement of cement content by calcined clay influenced the reduction of the bleeding and shrinkage. The simultaneous use of calcined clays as binder components with other types of pozzolan was also shown.

Research on various aspects of a new binder type—lime calcined clay cement, LC3—was thoroughly described. Its composition (Portland clinker, calcined clay, calcium carbonate and gypsum) and as

well as fresh mix properties and concrete resistance against aggressive liquid and gaseous media were presented.

It is confirmed that cement systems containing calcined clay, also with the addition of limestone, showed better durability and increased resistance to most of the aggressive actions to which concrete was exposed. The exception was carbonation, but satisfactory results have also been achieved in this area.

Most of the discussed papers were carried out on mortar or cement paste, and clearly less on concrete. It seems that the application of calcined clay as a supplementary cementitious material in concrete technology requires more research, as not all the findings performed on a smaller scale (i.e., on mortars and cement paste) can be directly transferred to the parameters and performance of concrete.

Future research aimed at improving the long-term durability of cement-based composites containing calcined clays is needed. It is suggested to optimize the composition of calcined clay cements by selecting the appropriate chemical and mineral components of raw materials to obtain the required mechanical parameters. Additionally, more research should be carried out on the practical application in the cement and concrete production of calcined clays of group 2:1. While there is a great deal of basic research on clays and minerals themselves, application research in this field is dominated by clays containing mainly kaolinite.

**Author Contributions:** Conceptualization: R.J. and D.J.-N.; methodology: R.J., D.J.-N. and Y.Y.; formal analysis: R.J. and D.J.-N.; resources: R.J. and Y.Y.; writing—original draft preparation: R.J. and D.J.-N.; writing—review and editing: R.J. and D.J.-N.; supervision: D.J.-N. and R.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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