Calcium-Based Binders in Concrete or Soil Stabilization: Challenges, Problems, and Calcined Clay as Partial Replacement to Produce Low-Carbon Cement

Calcium-based binders, such as ordinary Portland cement (OPC) and lime (CaO), are the most common artificial cementitious materials used worldwide for concrete and soil improvement. However, using cement and lime has become one of the main concerns for engineers because they negatively affect the environment and economy, prompting research into alternative materials. The energy consumption involved in producing cementitious materials is high, and the subsequent CO2 emissions account for 8% of the total CO2 emissions. In recent years, an investigation into cement concrete’s sustainable and low-carbon characteristics has become the industry’s focus, achieved by using supplementary cementitious materials. This paper aims to review the problems and challenges encountered when using cement and lime. Calcined clay (natural pozzolana) has been used as a possible supplement or partial substitute to produce low-carbon cement or lime from 2012–2022. These materials can improve the concrete mixture’s performance, durability, and sustainability. Calcined clay has been utilized widely in concrete mixtures because it produces a low-carbon cement-based material. Owing to the large amount of calcined clay used, the clinker content of cement can be lowered by as much as 50% compared with traditional OPC. It helps conserve the limestone resources used in cement manufacture and helps reduce the carbon footprint associated with the cement industry. Its application is gradually growing in places such as Latin America and South Asia.


Introduction
Soil stabilization is a very useful technique for civil engineering work. Soil stabilization is the modification of one or more soil characteristics through chemical or mechanical methods to generate better soil material containing the desired engineering properties. Soils may also be stabilized to enhance durability and strength or to limit dust production and erosion.
Regardless of the aim for the stabilization, the desired result is the formation of a soil system or soil material that will remain and sustain in place under the design use conditions for the design life of the construction [1]. Traditional stabilization methods include the use of cement, lime, and waste materials. By enhancing various soil engineering properties, these stabilization methods generate an improved construction material [2]. Limestone is one of the most common crushed rocks and is an important part of building materials such as cement, lime, and building stones [3]. Using hydraulic binders in soil improvement is a widespread practice in foundation work [4]. binders in soft soil stabilization, to propose solutions to the problems by using calcined clay as a partial replacement, and to compare the efficiency of the stabilization performance achieved by calcium-based binders alone as stabilizers and calcium-based binders with calcined clay as a primary stabilizer.

Cement-Soil Stabilization
One of the most important and common techniques of chemical stabilization is mixing the soft soil with cement material to produce a soil-cement mixture, which contains soil, water, and measured amounts of cement and is compacted to the required density [31]. Many geotechnical problems are encountered when construction activities are carried out in soft soil deposits due to their high compressibility characteristics and low shear strength. Therefore, cement-soil stabilization has become a popular soft soil modification and stabilization technique in cement slurry or dry cement powder [32,33].
The modification of the soil-cement mixture occurs when Ca ions released from the cement during hydration and hydrolysis occupy the positions of exchangeable ions on the surface of the clay minerals, increasing stability and strength, controlling deformability, and reducing plasticity. At the same time, the stabilization of the soil-cement mixture occurs when cement is added to a reactive soil to generate long-term strength gain through cement reaction. This reaction generates stable cementitious products (calcium aluminate hydrates and calcium silicate hydrates) as the Ca from the cement reacts with the silicates and aluminates that are solubilized from the clay. As a result, cement treatment produces high and long-lasting strength gains [13]. Many researchers [9,12] have conducted experimental studies to identify how cement stabilization procedures can increase strength and compressibility in soft ground. Cement-soil stabilization has many benefits, such as decreased swelling and shrinking, increased strength and elastic modulus, and resistance to the damaging effects of moisture, freezing, and thawing. The cement additive decreased the maximum dry density and increased the optimum water content of sandy soils [34,35].Cement-treated soils are more brittle than untreated soils [36].

Lime-Soil Stabilization
Lime is made by heating limestone to very high temperatures. Three different forms of lime can be used to improve soil: hydrated lime (calcium hydroxide, Ca [OH] 2 ), quicklime (calcium oxide, CaO), and hydrated lime slurry [37,38]. Lime is known to raise the soil's shrinkage limit, optimum moisture content, and strength, while decreasing its liquid limit, plasticity index, swelling potential, and maximum dry density [4,5,19]. Lime also enhances the compatibility and workability of subgrade soils [39][40][41]. Soft soils benefit from lime stabilization in several ways, particularly in terms of its enhanced engineering properties, such as increased strength, less swelling, increased resilience, and resistance to the damaging effects of moisture. Clays with a range of plasticity, from medium to high, show the most improvement in these characteristics [42].
The optimum percentage of lime that increases the MDD, bearing capacity ratio, and strength, and decreases the plasticity indices is 5% lime by the dry mass of soil [43]. A total of 5% of lime is sufficient for cation exchange and pozzolanic reactions in soil and produces a new mineral (calcium aluminate hydrates) [44]. The optimum lime content for high strength has been shown to be 4-6% [45]. Lime-soil samples were prepared by many methods, such as injecting into the deep soil layers or mixing with soil in a dry state and adding water [46] (jet grouting, hydraulic, and deep soil mixing [47]), while lime columns were used for the shallow layers. Another method was lime slurry: A typical lime slurry for the stabilization of soil is made by mixing 1 kg of lime with 2.5 L of water, resulting in a 31% lime solution [48]. For example, according to the preceding guideline, 600 g of lime was combined with 1500 mL of tap water to make the lime slurry match the 6% dry soil weight previously compacted in the test mold.

Problems of Calcium-Based Binders
Environmental concerns pose a major threat to most countries worldwide, especially with the increasing infrastructure size. Cement is one of the most widely used construction materials. As a result, cement production and use have grown worldwide over time. In 1994, 1370 million tons (Mt) of cement was made worldwide, according to the United States Geological Survey (USGS) [49]. The USGS reports that global cement production has risen from 1370 Mt in 1994 to 4100 Mt in 2017, a more-than-threefold increase [50]. At the same time, the production of cement has received worldwide attention as one of the main sources of anthropogenic carbon dioxide emissions. The cement industry is a major cause of global warming [51]. It is considered the third largest industrial source of pollution, emitting more than 500,000 tons of nitrogen oxide, sulfur dioxide, and carbon monoxide, per year.
The associated CO 2 emissions for clinker are between 849 and 868 kg CO 2 /ton. For OPC, the related CO 2 emissions are between 802 and 855 kg CO 2 /ton [52]. The global process emissions in 2018 were 1.50 ± 0.12 Gt CO 2 , equivalent to approximately 4% of emissions from fossil fuels [22]. The cumulative carbon dioxide emissions from 1928 and 2018 were 38.3 ± 2.4 Gt CO 2 , 71% of which have occurred since 1990. Cement, hollow concrete blocks, and reinforcing bars (rebars) were the highest energy consumers and CO 2 emitters in the study. They were accountable for 94% of the total embodied energy and 98% of the total CO 2 emissions [53]. More information on CO 2 emissions in specific regions was found by researchers of [54]. They studied the non-fossil fuel CO 2 emissions from industrial processes in China between 2003-2018 for the production of lime, calcium carbide, plate glass, ethylene, aluminum, ferroalloys, soda ash, lead, and zinc. They showed that these industrial processes are equivalent to approximately 5% of China's total CO 2 emissions from cement production processes and fossil fuel combustion. In addition, a study on the CO 2 emission factors for Chinese cement production based on organic and inorganic carbon from 2011 to 2015 [41] showed that the CO 2 emission factor is 785.53-796.17 kgCO 2 /t cl .
Researchers [55] studied the CO 2 emission factors of cement production in China and showed that the median values for the process, fuel, and direct emission factors are 525, 369, and 919 kg CO 2 /t clinker, respectively. However, the factor for electricity emissions is 74.9 kg CO 2 /t clinker. The final emission factor calculated from cement products is 761 kg CO 2 /t cement. Moreover, carbon dioxide emissions from 1574 cement factories in China were between 500 and 600 kg CO 2 /t clinker [56]. The carbon dioxide emissions from power plants varied among different enterprises, with an average level of 348 kg CO 2 /t clinker and a standard deviation of 233 kg CO 2 /t clinker. China's cement companies, on average, emit 806 kg CO 2 /t clinker into the atmosphere. In 2009, the amounts of CO 2 reached approximately 14.8% of the national CO 2 emissions created by the cement industry in China [57]. Based on the current energy-related and emission-control policies, the researchers in [24] assessed the direct emissions of air pollutants from China's cement industry, beginning in 1990, and forecasted future emissions through to 2020. The study showed that the cement industry produces approximately one-eighth of China's total anthropogenic carbon dioxide emissions; emissions of CO 2 increased 5.8 times in 2008.
Therefore, China is considered the largest producer and consumer of cement worldwide [23]. In 2010, China produced 1.87 billion metric tons of cement, about 57% of the total cement made worldwide. CO 2 is released into the air in large amounts when fossil fuels are burned, and limestone is heated to create cement. In 2009, the cement industry released 1073 Mt of carbon dioxide into the air, representing 15% of China's total greenhouse gas emissions. One study [58] compared different ways of calculating CO 2 emissions from cement production to determine the uncertainties, finding that China's cement-related CO 2 emissions have a relative uncertainty of between 10% and 18%.
A study on the construction phase of a residential tower in Tehran Metropolitan City [59] found that the CO 2 emissions were 6%, 78%, and 10% from cement mortar, concrete, and rebar, respectively. A study on the CO 2 emissions in China [60] also showed that the CO 2 emissions increased as cement production increased. Based on clinker output, raw material consumption (primarily limestone), fuel consumption (i.e., coal), and C/CR, the study displayed China's cement CO 2 emissions, by province, between 2005 and 2014. In 2005, cement production in China produced 641.31 Mt of CO 2 ; in 2014, that number increased to 1246.04 Mt.
In Malaysia, the cement production is approximately 20 million tons per year [61]. The combustion of fossil fuels in pyro-handling units produces approximately 40% of the total emanations, while another 10% results from transporting crude materials and electricity. Finally, about 50% of carbon emissions are discharged in the decomposition of MgCO 3 and CaCO 3 to produce MgO and CaO and as the core chemical responses in the process. This study showed that, in 2006, Malaysia consumed 20 Mt of cement and had a clinker ratio of 0.89 t/t CO 2 , which is higher than the world average. Another study [25] analyzed the environmental impacts of the Brazilian cement industry, finding that fossil fuel production and clinkering together account for more than 70% of CO 2 equivalent. Research on CO 2 emissions in Poland's cement industry [26] showed that the branch emissions index for Poland's cement sector is 0.662 tons of CO 2 per ton of produced cement. The concentrations of carbon dioxide emissions in Nigeria were recorded in the range of 2440-2600 mg/m 3 [27].
The carbon dioxide (CO 2 ) emission factor from lime applied in temperate upland soil was 0.026 mg C per mg of the CaCO 3 emitted annually [62]. Furthermore, more than three billion metric tons of carbon dioxide are released into the earth's atmosphere annually through cement, lime, and gypsum manufacturing enterprises [63]. The maximum global cumulative CO 2 emissions related to the cement process would be 45.45 billion tons under the SSP3 scenario [64]. India, China, the United States (US), Nigeria, and Pakistan are responsible for the majority of the global total CO 2 emissions from cement production processes between 2015 and 2000. In a new analysis of global process emissions in 2016 [18], the global CO 2 emissions from cement production were demonstrated to be 1.45 0.20 Gt CO 2 , equivalent to about 4% of the emissions from fossil fuels. A total of 35% of the CO 2 emissions come from fuel combustion to decompose and heat limestone to produce lime or clinker in an open atmosphere, and the remaining 65% comes from limestone rock itself [65]. An investigation of CO 2 emissions [66] found that carbon emission levels in the cement industry range between 5% and 8%, with the plant producing approximately 900 kg of CO 2 for every ton of OPC manufactured. This is similar to another study [67], where the manufacturing and use of the OPC used in concrete produced 810 kg CO 2 /ton of cement. Researchers [68] have shown that the carbon dioxide emissions and energy use from the worldwide cement industry were calculated to be 633 kg CO 2 /ton of cementitious product.
The researchers in [69][70][71][72] confirmed that 6-8% of the world's ever-increasing anthropogenic CO 2 emissions originate from the OPC industry. Approximately half of the 1435 Mt/ emissions are caused by the energy required to prepare the cement. The other half is unavoidable because it is a byproduct of converting CaO from CaCO 3 and is intrinsic to binder chemistry.
Although lime is the second-largest source of CO 2 emissions from industrial processes, after cement production in China's lime industry, in general, about 800-850 kg of CO 2 is released per ton of cement clinker. This represents about 5-8% of all CO 2 emissions [73,74]. In total, the lime and cement industries were responsible for 8% of global carbon dioxide emissions between 2010 and 2011 [75]. Another study [76] showed that the process of emitting increased rapidly between 2001 and 2012, from 88.79 Mt to 141.72 Mt. The study's emission factor and activity data have a relative uncertainty of 2.83 and 3.34 percent. Similar range of CO 2 emissions from cement is given by [77]; cement plants account for approximately 5-7% of global CO 2 emissions, with 900 kg of CO 2 emitted into the atmosphere to produce one ton of cement. The cement industry produces about 5% of the global artificial carbon dioxide emissions, of which 40% is from burning fuel and 50% is from the chemical process [78]. The amount of carbon dioxide emitted by the cement industry is nearly 900 kg for every 1000 kg of cement produced. The high percentage of carbon dioxide produced in the chemical reaction leads to a large decrease in mass conversion from limestone to cement.
Carbon dioxide is emitted during the production process of non-metallic minerals, such as cement, plaster, lime, glass, and ceramics [79], which, in 2015, increased in the European Union (EU) by 2.5% compared with 2014. This is the result of widely varying trends in EU member states, with increases for the top seven emitters (with the exception of the decrease in Germany) being Germany (−0.2%), France (+2%), Italy, and Spain (+3.9%), Poland (+5.5%), Romania (+3.3%), and the United Kingdom (+3.1%). Carbon dioxide emissions are generated by the oxidation of carbonate in the cement clinker production process, which is considered the main constituent of cement and the highest of the noncombustion sources of carbon dioxide from industrial manufacturing, contributing to about 4.0% of the total global emissions in 2015. Fuel combustion related to cement production has a similar level for the emissions of CO 2 . Therefore, cement production accounts for about 8% of global CO 2 emissions. Furthermore, in 2015, China produced 58% of the world's cement, with India coming in second with 6.8% and the US coming in third with 2.7%. The EU is responsible for around 4.1% of the global CO 2 output [80]. The carbon dioxide emissions for producing one ton of NaOH, lime, slag, and limestone used as raw materials in OPC were 2.987, 2.975, and 2.987 kg/CO 2 -e from limestone, slag, and various energies, respectively [81]. The emission factor for the carbon dioxide produced by cement manufacture is 0.82 kg CO 2 -e/kg [82].
Another major problem with calcium-based binders is the sulfate attack. When sulfates are in the soil or groundwater around a concrete structure, they seriously threaten its longterm durability. External sulfate can enter and cause a sulfate attack, one of the most well-known and studied chemical attacks. In OPC systems, sulfate attacks generally cause ettringite formation, accompanied by cracking, expansion, and a loss of strength. According to new theories, this expansion is driven by crystallization pressure when ettringite forms from an oversaturated solution in small pores. Near the material's surface, gypsum and ettringite have been seen to form when a higher sulfate concentration is present. The temperature affected the durability of cement-based materials to sulfate attack, where the uniform surface crumbled at 5 • C and edges and corners scaled at 20 • C, and the damage was sharper at 20 • C than at 5 • C [83]. The researchers in [84] conducted a study to evaluate the effect of sodium and magnesium sulfate attacking OPC paste while an electric field was present. The results showed that ettringite was formed initially but broke down later to make gypsum. Thermodynamic modeling shows that the pore solution's alkalinity dropped drastically during this process, causing ettringite to decompose. In addition to the sulfate access, decalcification occurred in this area, shown by the breakdown of portlandite and C-S-H. When the sample was exposed to MgSO 4 , the access to sulfate and decalcification occurred later and in a deeper area than when it was exposed to Na 2 SO 4 .
The results of thaumasite production by sulfosilicate clinker hydration [85] showed that the clinker's belite and ternesite prefer turning into C-S-H gels, and the sulfate ions from the ternesite turn into gypsum. The chemical reaction between gypsum, carbon dioxide, and silica forms thaumasite. Thaumasite was clearly visible after 28 days of hydration, and its content in one sample reached approximately 34% in weight. The compressive strength first increased, and then decreased, within 56 days due to the sulfate attack [86,87]. When larger pores are filled with the products of erosion and develop into small pores in the early stage of erosion, in the later stage of erosion, the proportion of larger pores increases, and the cracks occur inside the specimen. Another study [88] showed that the acidic curing environment has a negative effect on the properties of concrete, where the strength decreased with an increase in the duration of the curing age and the proportion concentration of acid due to the sulfate attack.
Researchers [89] have investigated the degradation and mechanism of cast-in-situ concrete when immersed in sulfate-rich corrosive environments and found that corrosion in cast-in-situ concrete is much faster than the degradation of precast concrete due to the faster development of cracks in the cast-in-situ concrete. Sulfate attack leads to weight loss and great expansion in the later corrosion, putting concrete structures in great danger, particularly for cast-in-situ construction. The main products of corrosion induced by sulfate attack are gypsum and ettringite. A study [90] investigated the internal and external effects of sodium sulfate on the strength of soil-cement specimens and found that the internal attack decreased the strength of the soil-cement specimens by up to 70% compared with the prefabricated (about 40%) and cast-in-place (about 20%) samples.
This decrease in the trend of strength gain was seen after 28 days. The temperature and sulfate ion concentration in cement concrete sulfate attacks are highly significant, and the primary products of the erosion of sulfate attack on cement concrete are plate-like gypsum, rod-like ettringite with a larger slenderness ratio, incompletely corroded calcium hydroxide, and granular sulfate salt [91]. Under a seawater attack, the interaction between chloride ions and hydrates can form Kuzel's and Friedel's salts. Magnesium ion can replace the ion of Ca in portlandite, lowers the alkalinity of pore solution, and destabilizes the C-S-H gel. The change of phase primarily occurs on the surface of the concrete, weakening the structure and leading to delamination and spalling under the physical attack of the wave [92]. The production of gypsum and ettringite on the surface of the concrete causes a large tensile strength loss, which is the greatest threat to concrete structures in the field when subjected to a sulfate attack, particularly for OPC with a high C 3 A component [93]. Experimental studies on the paste of cement exposed to external sulfate attack (sodium sulfate solution Na 2 SO 4 ) [94] found that the ettringite first precipitates in the largest pores without causing any expansion and then penetrates the gel and capillary pores, leading to accelerated swelling.
A further study [95] assessed the durability of a soil-cement mixture subjected to external sulfate attacks (a huge amount of sodium sulfates (25 g/L Na 2 SO 4 ) was used to accelerate the degradation process). The results showed that for the most porous soilcement specimens, an external sulfate attack could cause peeling on the surface of the test samples. Given the morphology of the needles seen by SEM analysis, this might be clarified through ettringite crystallization. On the one hand, the precipitation of these minerals results in a mass gain of up to 2.25% for some specimens after two years of sulfate exposure. The researchers in [96] studied the effect of environmental conditions on OPC structures and found that the cement with a higher amount of tricalcium aluminate (C 3 A) showed a more obvious deterioration. Visual changes, such as the crystallization of expansive products, cracking, and complete disintegration, were also observed. Furthermore, resistance was lost in specimens with low slag content. The loss of strength is a direct outcome of the sulfate attack because it causes a loss of cohesion due to the C-S-H decalcification. The cement with a lower percentage of CaO showed a better performance in resisting a sulfate attack. The generated stresses and the expansions in sulfate exposure conditions increased continuously with the increasing immersion time [97]. The effects of sulfate exposure on the pore network formation of different OPC matrices after two days of casting were investigated in [98]. The results indicate that the patterns of expansive product precipitation are related to the degree of refinement of the pore network. Large pores concentrate a greater proportion of the expanding product generated during the early phases of exposure. Later stages of precipitation result in finer pore sizes.
The production of gypsum and ettringite in the presence of sulfate ions had a beneficial effect on the evolution of the characteristics of OPC during the initial stage of a sodium sulfate attack [99]. Subsequently, these samples displayed a decline in characteristics due to the growth of expansive products and the development of microcracks. The mortar's compressive strength and static elastic modulus grew during the initial immersion phase, and then plateaued as the immersion time increased. After an initial period of immersion (150 and 120 days), the static elastic modulus and compressive strength decreased as the immersion time increased. Similar behavior for strength was shown by another study [100]. The sodium sulfate solution-soaked mortar expanded and hardened to a higher density than the water-soaked type. The results also demonstrated that increasing the concentration of the sodium sulfate solution shortened the time required to achieve peak properties and accelerated the deterioration of the properties in the late stage.
A sulfate attack can generally be categorized into four phases [101]. In the first phase, ettringite forms at the expense of monosulfate (monosulfate dissolves continuously and is replaced by a greater volume of ettringite). The ettringite grows in stage two, at the expense of the carboaluminate phases (monocarbonate is destabilized and starts dissolving when the hemicarbonate is consumed, leading the ettringite growth to continue). Ettringite grows in stage three, at the expense of some hydrotalcite (hydrotalcite gradually dissolves in stages one and two, but in stage three it will be the only remaining solid that can contribute the aluminates to the solution). In the fourth phase, gypsum replaces portlandite. In addition, the calcium sulfoaluminate products form cement-stabilized clay as a result of the sulfate attack [102]. These products cause volumetric expansion and lead to the formation of microcracks in the specimens. Another study [103] found a sudden reduction in the small-strain shear modulus and a gradual increase in the hydraulic conductivity of a cement-admixed clay due to the degradation of the interparticle cementation in the specimens as a result of the sulfate attack. The damage is produced by ettringite production in pores as small as 10-50 nm, creating stresses of up to 8 MPa, exceeding the tensile strength of the binder matrix. Stresses increase with the C3A level and sulfate concentration [104]. In addition, the production of ettringite in micropores causes expansion due to the interaction of sulfate solutions with mortar, with the expansion reaching the crystallization pressure [97]. Crystals expand due to the pressure created by their growth within limited pores. The mortar bars exposed to a sulfate solution generated gypsum and ettringite, causing some expansion [105]. However, the bars eventually expanded and decomposed due to the development of thaumasite.
Cement clinker samples submerged in a sodium sulfate solution primarily revealed dicalcium silicate, tricalcium silicate, brownmillerite, and tricalcium aluminate as the primary mineral components. Ettringite is primarily formed when calcium aluminate reacts with sulfate ions. A sulfate attack, caused by ettringite expansion, manifests as microscopic fissures in the concrete. Sulfate ions can rapidly enter a solution at larger concentrations [106]. Gypsum forms in both high-and low-concentration solutions of MgSO 4 and Na 2 SO 4 , and these compounds damage the C-S-H gel [107]. Under partial soaking conditions, cement mortar can be separated into four zones: the soaking zone, the wet zone, the crystallization zone, and the dry zone [108]. Corrosion products were studied in each zone. In the wet and soaking zones, ettringite is the most common by-product of corrosion. Gypsum and crystals of Na 2 SO 4 .10H 2 O and Na 2 SO 4 are the corrosion products in the crystallization zone. Table 1 explains the major problems of calcium-based binders (cement and lime).  After several years in a specific case study, lime-treated sulfate-bearing clay swelled and disintegrated when used for road building. Abundant thaumasite, a complex mineral of calcium-silicate-hydrates, is found in heavy areas.
Cement [125] (Sulfate attack) The results show no direct correlation between the degree of expansion of cement on sulfate attack and the amount of crystalline calcium sulphoaluminate present. Other factors, such as its stability under prevalent conditions and the influence of other ions, particularly magnesium and chloride ions, may predominate. In addition, protective surface films also play a significant part.
Lime and cement [126] (Sulfate attack) In samples of 10% lime-treated heavy clay and at constant moisture content for 1 week, swelling and cracking were observed when immersed in magnesium sulfate or sodium sulfate solutions at levels less than 1.5% as SO 3 .

Alternatives and Partial Replacement by Calcined Clay
Cement in concrete is the most common artificial cementitious material worldwide, greatly impacting the world economy and environment. First, the energy consumption in producing cementitious material is high. An essential constituent of concrete releases a significant amount of CO 2 , a greenhouse gas. As illustrated above, 8% of the total carbon emissions come from cement material. Therefore, enhancing the sustainability of cement concrete has become a significant issue in recent decades to improve sustainable development. Globally, improving cement concrete's sustainable characteristics and lowcarbon content has been the focus of industry attention in recent decades.
Many methods are available to enhance cement concrete's sustainability and reduce cement material's impact on the economy or environment, such as using chemical and mineral admixtures in the concrete. One of these methods is the use of calcined clay (natural pozzolana) in concrete, which has developed rapidly in recent years, as demonstrated by the researchers reviewed in Table 2. The table summarizes the most recent research on using calcined clay as a partial replacement for cement or lime in cement and soil improvement. Figure 1 shows the effectiveness of calcined clay on compressive strength as an alternative for lime or cement in concrete. This calcined clay material can be found both artificially and naturally. The aluminate and silicate clinker reactions are affected and accelerated by the SCMs, but in varying ways and to varying degrees, such as enhanced initial ettringite formation and initial dissolution of C3A.
The partial replacement of cement with CC has the greatest promise as a worldwide short-term solution to substantially reduce cement producers' greenhouse gas emissions. Ettringite (Wt. %) 9.2 6.8 Cement + 33% CC +16.67% LS [128] Compressive strength MPa at 28 d. 68 55 Although the compressive strength is not visibly enhanced by adding calcined clay and limestone powder as a 50-70% substitution for cement, these additives considerably increase the toughness, densify the microstructure, and refine the pore structure of cementitious materials.
Reduced clinker use may benefit the cement industry both environmentally and economically. In underdeveloped nations, cheaper cement mixes will help infrastructure development and reduce greenhouse gas emissions.
Flexural strength MPa at 28 d. 9.4 9.7  Bound water (g/100 g anh, binder ) 25 23 • Significant pozzolanic activity and synergy between LS filler and CC were seen in composite CC containing a minor amount of metakaolinite. • In a ternary system, the products and degree of hydration are expected to be the same for CC with a low metakaolin content as for CC with a high metakaolin content.
It has been verified that the combination of Portland cement, calcined clay, and lime-stone filler is a promising way to maximize the potential usage of composite clays in cement-based composites.

•
The increase in CC led to an increase in the specific gravity and maximum dry density in each mixture.

•
At the early stages, the pozzolanic reaction was dominated considerably by the hydration of calcium hydroxide with alkali-exchanged clinoptilolite, carbolaluminate hydrate groups were the preliminary product of this hydration.
Adding zeolite and lime to fine sand engineering is a unique method for changing the grain size distribution of poorly graded soils by adding fine filler content. At the same time, zeolite, as a natural pozzolan in combination with calcium hydroxide, may also induce artificial cementation. Strength when using CC with OPC is closer to control OPC CC can be used as a viable alternative to replacing cement and produce a low-carbon and sustainable concrete Bound water (%) 14 11 Compressive strength (MPa) 54 46 Rapid chloride penetration (coulombs) 4700 6500 Portlandite contents (CH), % 11.4 8. 6 • The latter enhancement in the cement mortars blended with CC was due to the refinement of the pore structure, compared to the Portland cement mortar.

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The mixed cement mortars had lower carbonation resistance than the ordinary Portland cement mortar.
CO 2 emissions from cement and concrete production can be reduced by replacing some Portland cement with these SCMs.

•
As LS and CC replacement levels were raised, there was a corresponding rise in electrical resistivity.
Hwangtoh calcined clay is a type of kaolin clay that is used in construction as an eco-friendly material. In contrast to other SCMs, it can be used as an environment-purification material.
Bound water per gr binder 26.5 23 Electrical resistivity (kΩ.cm) 75 180 OPC + 50% clinker + 30% CC [142] Compressive strength (MPa) 62 62 The results explained the impact of CC on increased superplasticizer demand and show the difficulties in retaining the workability for extended durations.
-Viscosity (Pa s) 22  • The CC pozzolanic reaction in cement mortars is similarly developed in aggressive and non-aggressive curing conditions, where pore size refinement, consumption of CH, and prevention of the sulfate ingress. • Cement with 30% CC greatly resisted ESA, while a worse performance was presented for limestone cement.

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The mechanical properties of concrete with the blended binder are lower than those of pure cement mix in the first three days but increase faster and become comparable from 7 days onwards.
Comparable mechanical properties and higher endurance indicate that it is possible to produce high-performance concrete with a significant proportion of clay and limestone.

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The strength development of this mixture was slightly below the control.

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The use of CC improved the durability of concrete concerning alkali-silica reaction, chloride penetration, and drying shrinkage compared with the control mixture.
Limestone-calcined clay-cement and slag -calcined clay-cement mortar mixes exhibited great strength development after substituting about 50 percent of the Portland cement. SCMs are essential components of modern concrete and are used to increase workability and durability (e.g., embodied energy and CO 2 reduction).  The compositional analysis showed that CC gave a strongly bonded structure and a low calcium/silica ratio.
This work will pave the way for future research on using CC in oil and gas cementing and its durability over time.   OPC + 21% CC-LS -exposed to a 0.11 M Na 2 SO 4 [162] Compressive Strength (MPa) 69 72 The findings indicate that all mortars with CC/(CC + L) 0. 5  This cement paste research has proven that the LC3 cementitious system can produce more durable concrete than either OPC or the widely-used fly ash-based PPC.
The key to improving the environmental friendliness of cement is using mixes such as these, which have a low clinker content but significant performance implications.  Portland Cement + 30%CC [169] Compressive strength MPa

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The Pozzolanic reactivity is related to the specific surface area of CC.

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The specific surface area of CC is related to the calcination degree.

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The addition of CC produces the acceleration of cement hydration, improves the volume and structure of the pore system in the paste Using the suitable blended cement mix with CC makes it possible to reduce carbon dioxide emissions and improve mechanical and durability performance.  Using calcined clay (CC) offers significant advantages as a cement replacement material and a low-cost alternative binder. Using CC could enhance the strength, which reached an approximate 10% increase in compressive strength relative to the control at 90 days [168]. Its use also reduces water adsorption [166], reduces the coefficients of chloride ions [161], and increases the durability of concrete [134,140]. Importantly, using CC is considered a good solution to reduce carbon dioxide emissions and produce an eco-friendly CC at a low cost [151]. The replacement proportion in cement clinker can reach as low as 50% [132,138]. Limestone-CC cement has exceptional durability and mechanical properties, such as a higher final compressive strength. In addition, it increases the C-S-H amount at 28 days of hydration [167].
As a result, CC can be considered a global alternative to a variety of traditional lowcarbon OPC materials. This "pozzolanic calcined clay," as it is also known, in addition to reducing CO2 emissions in industries, also brings several economic advantages for cement production. Its application in South Asia and Latin America is gradually growing.

Conclusions
This paper has focused on two main parts. The first deals with the problems of using cement and lime as supplementary cementitious materials (SCMs) cause for soil, concrete, and the environment. For example, where the energy consumption involved in producing cementitious materials is high, carbon dioxide emissions account for 8% of the total carbon emissions. The presence of sulfates in the soils or groundwater surrounding a concrete structure may also cause a serious threat to the long-term durability of concrete and soil due to the effect of a sulfate attack, which is considered one of the most widely admitted and well-studied chemical attacks.
Therefore, improving the sustainability of cement concrete has become a major issue in improving socially sustainable development. Internationally, in recent decades, improving cement concrete's sustainable and low-carbon characteristics has become the focus of industry attention by using SCMs, which are industrial by-products or natural materials used to improve the durability, performance, and sustainability of concrete mixtures. One of these SCMs is calcined clay; therefore, the second part of this review paper focuses on using this material to reduce the use of cement or lime by comparing cement or lime alone and using calcined clay with cement or lime.
Based on the results of using calcined clay in concrete, which has developed rapidly in recent years, this material, can produces a low-carbon cement-based material, and it is recommended for use in concrete. Compared with traditional OPC, the clinker content in its cement can be as low as 50% because it uses a large amount of calcined clay and limestone as key components. It will conserve the limestone resources used in cement manufacturing and help to reduce the carbon footprint associated with the cement industry. Its application in places such as Latin America and South Asia is gradually growing. Data Availability Statement: All the data reported in this study was originally generated and can be requested from the corresponding author.