Cement, being one of the most widely utilized materials for construction, plays a crucial role as the primary binder in concrete, leading to the formation of a durable, stone-like, hard material capable of withstanding various loads ^[1][2][3][4][1,2,3,4].

The conventional kind of cement, known as ordinary Portland cement (OPC), primarily comprises over 90% Portland cement clinker. This particular type of cement is derived from readily accessible raw materials that are widely abundant and cost-effective, making it easily obtainable in nearly all regions [5].

This inexpensive mineral binder has rapid hardening properties in nearly all livable environments, enabling the creation of diverse structures [6]. Moreover, its user-friendly nature allows untrained individuals, including those lacking literacy skills, to utilize it effectively for self-construction purposes [5].

Cement constitutes approximately 10% of the total volume of concrete on a global scale, and approximately 50% of cement is allocated to produce concrete, while the remaining portion is designated for applications such as mortars, pastes, and pre-manufactured products [7].

The global output of cement has witnessed a significant rise over the years. Specifically, it has escalated from 0.94 billion tons in 1970 to 2.284 billion tons in 2005, further increasing to 4.05 billion tons in 2017 and reaching 4.1 billion tons in 2018 [8].

In the year 2017, the countries of China and India, which are recognized as the largest global manufacturers, collectively accounted for 64% of the global cement production. This equated to a total output of 2.61 million tons of cement out of the whole global production of 4.05 million tons [6]. In the year 2019, the primary producers of cement were China, India, the European Union, and the United States ^[9][10][9,10]. These four entities collectively accounted for 56.1%, 7.8%, 4.4%, and 2.2% of the total cement production, respectively [11].

Current cement consumption is about 4.2 billion tons per year [12], which is enough to produce almost 1.6 m³ of concrete per person. This amount, which is approximately half of the volume of food produced worldwide, is expected to reach approximately 6 billion tons by the end of 2050 ^[7][13][7,13].

Cement is derived from a combination of limestone, clay, and sand, which serve as the primary sources of lime, silica, alumina, and iron [14]. Cement production by the dry manufacturing process consists of six stages ^{[6][15][16][17][18]}[6,15,16,17,18].

During the initial phase, the raw materials necessary for the process are extracted through mining operations, such as limestone, clay, laterite, bauxite, iron ore, kaolinite, sandstone, and other similar inorganic materials. All of them, properly dosed, constitute the “Portland clinker crude” (PCC), the last six being in addition the fluxes or mineralizers of the first two (those with the highest dosage), to which, despite their very considerable lower dosage, they reduce their melting point so that they can chemically react more and better and thus form Portland clinker permanently. The second stage of the process entails the characterization of diverse raw materials and their proper dosage to create PCC. During the third stage, the PCC is introduced into a preheating chamber. During the fourth stage, the pre-heated decarbonized PCC is introduced into the rotary kiln to undergo the process of clinkerization at a temperature ≥1450 °C. During the fifth stage, the clinker that emerges from the kiln undergoes a quick cooling process facilitated using pressurized air. During the concluding phase, the cooled clinker is recovered from the cooling vessels and then transferred to the mills. The clinker is ground together with the optimum amount of setting regulator [19] (natural gypsum stone) into powder using a ball mill or roller mill, or a vertical mill, and the pulverized cement is transported to storage silos using a transportation system suitable for shipping (Figure 1). Nevertheless, if the composition of the ground material consists of a combination of natural and/or artificial pozzolans and/or GGBFS along with Portland clinker, the ideal quantity of setting regulator needs to be determined utilizing the R. Talero method [20].

The six phases can be condensed into three primary stages: raw meal preparation, clinker production, and finish grinding ^[21][22][21,22].

In comparison to the year 1750, it has been observed that concentrations of CO₂ in the Earth’s atmosphere have risen from 280 to 410 parts per million by volume (ppmV) ^{[23][24][25][26][27][28][29]}[23,24,25,26,27,28,29]. This upward trajectory is projected to persist in the coming decades, potentially leading to a temperature rise of up to 5.8 °C during the present century ^[29][30][29,30].

Approximately 40% of worldwide CO₂ emissions can be attributed to four key industries: power plants, iron and steel manufacturing, cement manufacturing, and chemicals and petrochemicals [29]. The cement sector is identified as the primary contributor of process emissions ^[16][31][16,31].

Based on the available worldwide CO₂ emission data, cement plants made a substantial contribution of 2.9 billion tons of CO₂ in the year 2021 [7]. This figure represents an almost fivefold increase when compared to the emission level of 0.57 billion tons recorded in 1990 ^[29][32][29,32].

Cement production is a highly resource-intensive process that consumes significant amounts of energy and raw materials [16]. This process leads to the emission of CO₂ through two primary pathways: direct emissions from the combustion of fossil fuels in the kiln and indirect emissions from the calcination process of the primary raw material, predominantly limestone [33]. Additionally, the consumption of electricity in cement production, particularly when generated from fossil fuel combustion, contributes to overall CO₂ emissions [34].

The emission of CO₂ during the manufacturing of one metric ton of Portland cement is predicted to range from 0.73 to 0.99 metric tons throughout various geographical regions [34]. It can be asserted that the manufacturing of one kilogram of Portland cement results in the emission of about one kilogram of CO₂ into the atmosphere ^[35][36][35,36].

The global production of this product is responsible for approximately 5–9% of CO₂ emissions ^{[13][16][29][37][38][39][40][41]}[13,16,29,37,38,39,40,41]. Furthermore, it accounts for significant emissions of carbon monoxide (CO) and heavy metals [14]. In addition to CO₂, CO, and heavy metals, the use of a substantial quantity of material has led to the excessive burden on deposits of these materials and the alteration of the environment. The production of Portland cement alone entails the consumption of approximately double the quantity of raw materials required to manufacture one metric ton of Portland cement [35].

As previously stated, the process of manufacturing Portland cement results in the emission of carbon dioxide through both direct and indirect means [39]. Indirect emissions are generated because of the calcination process, wherein limestone, the principal constituent of cement, undergoes heating ^[39][42][43][39,42,43]. The process of thermal decomposition causes the calcium carbonate present in limestone to undergo a chemical transformation, resulting in the formation of calcium oxide and the liberation of CO₂ gas [39]. This procedure is responsible for approximately 50% of the total emissions generated during the manufacture of cement ^[32][41][32,41]. The production of cement involves subjecting limestone and other clay-like materials to high temperatures of approximately 1450 °C within a kiln ^[39][44][39,44]. Direct emissions arise because of the combustion of fossil fuels utilized for the purpose of heating the kiln, constituting approximately 40% of the total emissions associated with cement manufacturing ^[5][16][45][5,16,45]. The emissions associated with the quarrying of raw materials, their transportation, grinding processes [46], the electricity consumption for operating additional plant machinery, as well as the packaging and final delivery of cement, all contribute to the remaining 10% of the overall emissions ^[43][47][43,47].

Furthermore, a range of technological and managerial inefficiencies within the typical cement production process might result in additional CO₂ emissions. Geographical location, technological factors, plant and manufacturing efficiency, the energy mix utilized for electricity generation, and the choice of kiln fuels all contribute to additional carbon dioxide CO₂ emissions ^[29][38][39][29,38,39].

Construction waste results from building constructions and building renovations and consists of surplus material, unusable impaired or fractured material, cut-off pieces, processing waste, worn-out tools and accessories, dismantled shuttering, packaging, and waste produced by construction workers ^[48][49][50][48,49,50]. On the other hand, after the end of a structure’s life cycle, its demolition is crucial for the growth of cities where inadequate space is the major obstruction. CDW can also be generated in the aftermath of a natural disaster, which presents several significant challenges, such as transportation, storage in an appropriate location prior to processing, and disposal at landfill sites [51].

Overall, it can be stated that CDW is a type of solid waste generated on construction sites and during the entire or partial demolition of buildings and infrastructures ^{[52][53][54][55][56][57][58]}[52,53,54,55,56,57,58].

CDW consists primarily of inert and non-inert materials, such as gravel, concrete, sand, ceramic, tile, metal, plastic, glass, roofing materials, paper, cardboard, etc. The inert waste materials consist of soft and hard inert materials, whereas the noninert waste consists of residual waste and other materials such as metals, wood, plastic, and glass ^[53][59][60][53,59,60]. Inert fraction waste accounts for between 40 and 85 percent of total waste volume, excluding excavation soils ^[50][58][61][50,58,61].

It is estimated that the construction industry annually generates more than 3 billion tons of CDW worldwide ^[62][63][64][62,63,64]. This indicates that CDW accounts for approximately 36% of the world’s total waste production [65]. CDW in the United States rose from 50 million tons in 1980 to 600 million tons in 2018 [61]. More than 1.5 billion tons of CDW are produced annually in China ^[66][67][66,67], while in the European Union (EU), countries produce about 850 million tons/year, or 31% of the total waste generation in the EU [68].

The generation of waste results in adverse externalities on the environment, even while a significant portion of CDW consists of inert materials that may not provide as significant a risk as hazardous waste ^[69][70][69,70]. The disposal of CDW in landfills causes landslides [71], depletes limited landfill resources, exacerbates energy consumption, amplifies greenhouse gas (GHG) emissions, poses public health concerns, and contaminates the environment ^{[44][72][73][74][75]}[44,72,73,74,75].

In recent years, governmental bodies have enacted new regulations pertaining to the management of waste, encompassing responsibilities, disposal practices, and recycling efforts on a broader scale [76]. Consequently, the urban landscape is undergoing transformation through the establishment of recycling facilities, yet the current recovery rate for CDW remains very low ^[49][77][49,77]. The expansion of the worldwide population and the concurrent rise in sea levels have resulted in a reduction in the accessible land for dump sites, hence leading to an indirect escalation in the expenses associated with landfills [78].

2. Mitigation and Improvement Measures to Reduce CO₂ in Portland Cement Production

Clinker is a transitional product in the production of cement, occurring before the mineral additions (MAs) to create the final cement product. As the temperature rises, the pre-calcined materials undergo physical and chemical transformations, causing them to liquefy and combine, resulting in the formation of lumps [39]. Thus, the manufacturing of cement emits greenhouse gases through both chemical and physical processes. The thermal decomposition of limestone releases CO₂ by an endothermic chemical reaction, and the combustion of coal, fuel, or AF releases it as well (but exothermically), only that the transmission to the limestone of the heat generated at the same time, to decompose it and decarbonate it, is not carried out chemically but physically by the following ways: conduction, convection, and radiation. Although it is not possible to completely eliminate these emissions, the use of energy-saving technologies can help reduce physical emissions. Therefore, the cement industry had been actively engaged in the pursuit of techniques aimed at reducing CO₂ emissions far in advance of the emergence of global warming as a prominent concern. To address this predicament, an increasing body of research has delved into the process of decarbonization within the cement sector, as outlined in Table 1. As depicted in Table 1, the cement industry globally implements a range of mitigation techniques, with variable degrees of adoption. Some of these mitigation techniques are reviewed in the following sections.

2.1. Substitution of Alternative Fuels (AFs)

While AF substitution in the cement production process is not a novel concept ^[16][100][16,102], its prominence has grown considerably, and the utilization of AFs in cement manufacturing has received significant attention in recent years due to its efficacy in replacing the thermal energy derived from fossil fuels and mitigating pollutant emissions. The contemporary cement kiln exhibits a high degree of adaptability, enabling the cement industry to seamlessly transition between different fuel sources with moderate ease ^[5][16][5,16]. The cement rotary kiln possesses the capability to incinerate a diverse array of materials because of the extended durations spent at elevated temperatures, the inherent capacity of clinker to assimilate and confine impurities such as heavy metals within itself, and the alkaline conditions prevailing within the kiln ^[101][103]. The cement industry utilizes conventional fossil fuels, including coal, fuel oil, petroleum coke (petcoke), natural gas, and diesel, in its kilns and pre-heater systems to generate the elevated temperatures required for clinker production [46]. The aforementioned fuels account for over 94% of the thermal energy need in the worldwide cement industry ^[102][104]. The suitability of AFs is contingent upon various properties, including their physical state (solid, liquid, or gaseous), lower heating value, ash composition and content, toxicity (organic compounds, heavy metals), volatile content ^[103][105], humidity content, physical properties (scrap size, density, and homogeneity), content of circulating elements, grinding properties, storage/feeding capabilities, and calorific value ^{[15][16][43][103][104][105]}[15,16,43,105,106,107]. The utilization of AFs offers several key benefits, namely enhanced energy recovery and the preservation of finite fossil fuel resources. These advantages result in the reduction of pollutant emissions, particularly CO₂, and a projected decrease in the expenses associated with cement production ^{[16][100][101][106][107]}[16,102,103,108,109]. Nevertheless, the adoption of AFs presents numerous problems as a result of the complexities associated with integrating supplementary fuel-saving methodologies. Furthermore, it is important to note that not all AFs guarantee a reduction in CO₂ emissions due to their elevated carbon intensities ^{[45][79][100][108][109]}[45,81,102,110,111]. AFs can be broadly categorized into three primary groups ^[110][112]. The first group comprises liquid AFs, encompassing materials such as waste oil, solvents, animal fat, and sewage sludge. The second group consists of solid AFs, which include waste tires (either chipped or whole), animal and bone meal, dried sewage sludge, scrap wood, and waste materials originating from various industries, such as the pulp, paper, cardboard, plastics, packaging, and textile industries. Lastly, the third group encompasses gas AFs, which encompass landfill gases, pyrolytic gases, and biogases. Typical AFs used by the cement industry include animal meat and bone meat ^{[111][112][113][114][115][116][117][118][119]}[113,114,115,116,117,118,119,120,121], municipal solid waste ^{[108][120][121][122][123][124][125][126]}[110,122,123,124,125,126,127,128], refuse derived fuel ^{[127][128][129]}[129,130,131], waste tires ^{[108][130][131][132]}[110,132,133,134], plastic waste ^{[22][104][133]}[22,106,135], saw dust or wood ^[134][135][136,137], straw ^[136][137][138,139], agriculture and forest wastes ^{[138][139][140]}[140,141,142], almond shells ^[141][142][143,144], olive residues ^[143][145], oil palm ^[144][146], food residue ^[145][147], rice husk ash ^[146][148], natural gas ^[147][149], biogas ^[148][150], sewage sludge ^{[149][150][151]}[151,152,153], oil sludge ^[152][154], slaughterhouse residues ^[153][155], spent solvents ^[108][110], and solid recovered fuels ^[154][155][156,157]. It is projected that the global utilization of AFs will increase from 3% in 2006 to around 37% by 2050, resulting in a contribution of approximately 15% towards the intended overall reduction in CO₂ emissions ^[5][34][5,34].

2.2. Substitution of Alternative Raw Materials (ARMs)

The process of the decarbonation of commonly used raw materials, primarily limestone, results in the release of around 0.53 metric tons of CO₂ for each metric ton of clinker produced ^[151][153]. Utilizing waste and by-products that include valuable minerals, such as calcium, silica, alumina, and iron, is a viable option to substitute for traditional raw materials, including clay, shale, and limestone ^[15][156][15,158]. The incorporation of alternative materials into the clinker recipe necessitates a prudent methodology, since any modification in the chemical composition of cement will have an impact on the ultimate quality of the product ^[22][157][22,159]. Various industrial by-products and waste-derived materials have been investigated as potential substitutes for limestone and clay in the production of cement. The objective is to minimize the utilization of natural resources, decrease CO₂ emissions, and reduce heat consumption while ensuring that the manufacturing processes remain unaltered ^[157][159]. Some of the ARMs utilized in the raw meal for cement production are presented in Table 2. 82,260,267,281,282,283,284,285,286,287,288] has revealed that CC can serve as a viable substitute due to its lower carbon emissions. Specifically, LC3 technology offers advantages such as resource conservation, global scalability, cost effectiveness, high performance, and ease of implementation on standard construction sites. A comprehensive life cycle assessment (LCA) study has been conducted by Scrivener et al. ^[256][258] for the Cuban cement industry, covering the entire life cycle from production to the factory gate. Remarkably, regardless of the technological level, LC3 cement consistently achieved an approximately 30% reduction in CO₂ emissions. Moreover, it has been observed that the lowest quality LC3 cement produced during the initial industrial trial outperforms the highest quality OPC in terms of CO₂ emissions. The primary factors contributing to the large decrease in emissions were energy savings and the use of clinker substitution. Additionally, it was observed that the grinding process using LC3 resulted in a notable reduction in electricity usage compared to OPC, likely due to the softness of LC3. Researchers at the Indian Institute of Technology, Madras, have conducted another comprehensive investigation using actual data from several cement factories ^[287][289]. This investigation demonstrated a 30% reduction in CO₂ emissions for LC3 compared to OPC at the cement level. Research conducted by Pillai et al. ^[281][283] has shown that structures constructed with concrete containing LC3 have considerably longer service lives compared to those using solely OPC as the binder (which also contrasts quite a bit with the carbonation results obtained by Arámburo et al. ^[278][280]). Furthermore, it was discovered that LC3 concrete has much lower CO footprints per year of service life compared to the OPC concrete that was examined. The work by Zhang et al. ^[282][284] highlighted a new application of LC3 in the production of engineered cementitious composites (ECC) that possess exceptional tensile ductility and strain hardening properties. From an environmental perspective, the utilization of LC3 in ECC demonstrated a significant reduction in carbon emissions, with 28% less CO₂ released compared to the production of conventional concrete. However, there was only a modest decrease in energy usage and manufacturing cost. Guo et al. ^[283][285] examined recycled aggregate concrete (RAC) incorporating LC3. They stated that the utilization of both RCA and LC3 exhibits significant promise in reducing the environmental consequences associated with concrete manufacturing. In their study, Barbhuiya et al. ^[286][288] stated that LC3 exhibits a substantial capacity to diminish CO₂ emissions in comparison to conventional cement. The authors reported that research has demonstrated that LC3 has the capability to decrease CO₂ emissions by as much as 40% because of its reduced clinker concentration and the utilization of calcined clay. Additionally, LC3 exhibited reduced production cost in comparison to conventional cement due to its lower energy requirements during manufacturing and its ability to utilize locally sourced raw materials. Due to all of the above, CC has been identified as one of the most promising materials that can help the cement industry achieve its emissions objectives, but perhaps not so much in terms of the durability of the works built with its concretes, mortars, pastes, and precast components.

2.2.2. Substitution of CDW as an ARM

The chemical and mineralogical properties of CDW are sufficient to qualify it as a viable substitute raw material in the limestone–clay mixture produced during the manufacturing process of Portland clinker. The composition of CDW typically includes calcium, silicon, aluminum, iron, and several trace elements, including magnesium, potassium, titanium, and sulfur. These minor elements have the potential to contribute to the development of the primary phases of Portland cement ^{[161][167][178][288][289]}[163,169,180,290,291]. Furthermore, the substitution of CDW leads to a decrease in the generation of CO₂. This waste serves as a source of CO₂ that is separated from calcium oxide (CaO), thereby reducing the decarbonation of limestone that occurs during the flaring process in the manufacturing of clinker ^[161][178][163,180]. From the above-mentioned ARMs, CO₂ emission related studies concerning CDW are listed in Table 3. ,298]. The substitution of Portland cement with solid waste derived from various economic sectors has been extensively investigated in numerous studies as a promising alternative. These studies aim to identify optimal circumstances for such replacements, considering the necessary features for their effective application. Some of the waste, by-products, recycled materials, and natural resources used as an addition or as a partial replacement of Portland cement to produce concrete are presented in Table 4.

2.2.1. Consideration of CC as SCM: Replacement of Portland Cement by CC

The materials evaluated in Table 2 have the potential to partially substitute for Portland clinker by means of novel variations of already utilized SCMs. Among these materials, calcined clay (CC) deserves particular attention. By subjecting ordinary clay, which typically contains at least 40% kaolinite and is widely available in the earth’s crust, to moderate heat treatment (about 700 and 850 °C), it can be transformed into a pozzolanic material called CC ^[256][257][258,259]. CCs, especially when combined with limestone, are being recognized as a highly promising solution due to their excellent performance and the abundance of sufficient reserves of these materials ^[258][260]. Limestone calcined clay (LC2) and limestone calcined clay cement (LC3) systems exploit the synergistic effects of calcined clay and limestone, enabling a significant decrease of up to 50% in the utilization of clinker ^[259][261]. Nevertheless, the clays typically employed in LC3 systems consist of a minimum of 40% kaolinite ^[256][260][258,262]. Recently, there has been a significant increase in research ^{[261][262][263][264][265][266][267][268][269]}[263,264,265,266,267,268,269,270,271] focused on the potential utilization of CC as an SCM in the manufacturing of cement, with a particular emphasis on advancing its economic viability ^[270][272]. Zhu et al. ^[271][273] conducted a study on the characteristics of LC2 blended cement and compared them with fly ash (FA) and granulated blast-furnace slag (GGBS). They reported that the normal consistency of LC2 blended cement was greatly raised and the substitution of LC2 at a rate of 60% resulted in an almost twofold increase in normal consistency. Dhandapani et al. ^[272][274] reported that concrete produced with LC3 had superior compressive strengths compared to concrete with equal combination proportions at all ages up to 1 year. The investigation carried out by Vaasudevaa et al. ^[273][275] involved the substitution of cement in concrete with a combination of LC2 at a proportion of 45%. They concluded that the compressive strength of steam-cured LC3 concrete after 1 day is comparable to that of OPC concrete, exhibiting a similar strength enhancement resulting from the steam curing conditions. The study carried out by Aramburo et al. ^[274][276] aimed to evaluate the mechanical properties and sulfate resistance of blended cements containing a significant amount of CC as pozzolanic material. The objective was to demonstrate that these cements can meet the requirements of CEM type IV/A-SR and IV/B-SR cements as defined by the EN 197-1:2011 standard. The results obtained validated the increase in sulfate resistance and the decrease in the mechanical strength of PC when it was replaced by CC (whose matrix clay was kaolin doped with ≈50% quartz) in quantities greater than 40%. They also stated that the blended cements with high percentages of CC replacement successfully met the specified requirements regarding compressive and flexural strengths without prejudice to its decrease observed with the increase in its replacement by PC. The reason for both opposing behaviors, sulfatic and mechanical strengths, was the same: the very high, early, and fast pozzolanic activity of its silica and reactive alumina contents especially (38.0% and 15.0%, respectively) ^{[275][276][277]}[277,278,279], which excessively decreases the [Ca (OH)] in the liquid phase of its pastes. To verify this, the authors repeated the tests, replacing a small portion of the CC used with slaked lime powder (calcium hydroxide, Ca (OH)). Both behaviors contrasted again, but in the opposite direction; that is, the sulfate resistance decreased, and the mechanical strengths increased, as when the replacement by PC was ≤40%. This was similar to how it also increased its resistance to carbonation, which had also been significantly diminished and seriously compromised, with an increase in the replacement of CC by PC. The more impaired the material, the greater the 40% replacement was ^[278][280]. A study carried out by Yu et al. ^[279][281] investigated the practicality of creating a cost-effective and environmentally friendly cement by combining LC2 at a significant proportion of 50–80% relative to the weight of the cement. They reported that blended cements containing 50–60% LC2 exhibit satisfactory compressive strength, decreased hydration heat, reduced environmental effect, and lower material cost per unit strength but reduced workability in comparison to plain Portland cement. This contrasts quite a bit with the results of flexural and compressive strengths obtained by Arámburo et al. ^[274][276]. With regard to CO₂ emissions, a review of the existing literature ^{[80][258][265][279][280][281][282][283][284][285][286]}[ According to Gastaldi et al. ^[168][170], the utilization of HCW as a substitute for naturally mined minerals has the potential to decrease the consumption of non-renewable resources. Hydrated cement is composed of amorphous calcium silicate and calcium aluminate hydrates, as well as calcium hydroxide and a small quantity of calcium/magnesium carbonate. It was found that ordinary Portland powder and samples demonstrate weight losses of 29% and 20%, respectively. According to the authors, this implies that when 30% of HCW is utilized, it is possible to make a clinker with an equivalent mineralogical composition that emits approximately one-third less CO₂ during the combustion process. It was also reported that the substitution of regular Portland clinker with recovered samples containing HCW, Portland clinker, and gypsum results in a reduction in the emission of CO₂. Specifically, when the replacement extent reaches 40%, the amount of CO₂ released during cement manufacturing drops by more than one-fourth compared to the scenario without any replacement. The primary aim of the research conducted by Santos and Cilla ^[181][183] was to generate Portland clinker through the utilization of ACW as a mineralizer, thereby substituting a portion of the traditional combination of limestone and clay. Based on the findings derived from the experimental procedures and subsequent analyses conducted throughout the course of this study, it was reported that ACW functions as a mineralizer, expediting the reactions within the clinker formation process and augmenting the proportion of alite (C₃S) present in the resulting clinker. Furthermore, it was observed that the integration of ACW facilitated a reduction in the utilization of approximately 73.70% of limestone and 86.80% of clay in the composition of the raw material blend employed in the manufacturing process of Portland clinker. It was reported that the utilization of up to 74% ACW in the production of eco-efficient cement through experimental means offers a viable solution from both technical and environmental perspectives. This approach not only ensures the safe disposal of hazardous waste, thereby eliminating its potential to cause cancer, but also has the potential to decrease CO₂ emissions by up to 13.68% and reduce energy consumption by 10.13%. Based on the findings derived from the study conducted by Costa and Ribeiro [44], it can be inferred that the integration of the CCW technology has facilitated a reduction in the utilization of roughly 8% of limestone in the raw mix to produce Portland clinker. Consequently, its implementation has resulted in a decrease in the extraction of this natural resource. It was reported that utilizing CCW offers a potential reduction of up to 8.1% in CO₂ emissions per ton of clinker produced, solely accounting for decarbonation-related emissions. It was also stated that, when considering the entire process, including fuel combustion, the reduction amounts to 4.9% compared to clinker produced using conventional raw materials. In summary, it is important to acknowledge that the implementation of ARMs in kiln feeds has the potential to decrease specific CO₂ emissions. However, the implementation of partial raw material substitution has been limited due to several limitations. The utilization of alternate materials in partial substitution of traditional clinker leads to a reduction in initial strength and a constrained quantity of limestone ^[290][292]. Conversely, coal fire is subject to ongoing regulatory limitations in Europe, hence posing increasing challenges in terms of accessing fly ash ^[79][108][81,110].

2.3. Replacement of MAs in Portland Cement

Due to the production of GHGs, a majority of concrete mixtures use SCMs either through the use of blended cements or by individually adding them to the mixer ^[215][217]. The incorporation of low-embodied carbon and low-energy elements in the substitution of Portland cement can significantly diminish the overall environmental consequences of binders and, as a result, of concrete ^[157][291][159,293]. These materials are commonly known as MAs or SCMs. When they are included into concrete and mixed with Portland cement, they create cementitious particles. However, on their own, they do not contain any cementitious compounds ^[215][217]. The selection of MAs for substituting Portland cement is contingent upon the geographical area and the specific solid waste or byproducts produced by industries or the presence of naturally occurring minerals in these regions [37]. The utilization of MAs as substitutes for Portland cement in concrete offers various sustainability benefits. MAs typically consist of industrial waste products, natural pozzolans, and activated minerals that possess either hydraulic or pozzolanic characteristics. When MAs are used alone or in contact with water, they generally do not exhibit substantial hydraulic reactions that contribute to the cementitious properties. Nevertheless, when exposed to alkaline aqueous conditions or in the presence of calcium hydroxide, fine particles undergo a chemical process known as the pozzolanic reaction. This reaction leads to the formation of hydration products that resemble those seen in Portland cement systems ^{[198][292][293]}[200,294,295]. A wide variety of materials are available for use as MAs, including natural MAs (volcanic materials, including tuffs, ashes, pumicites, perlites, zeolites, etc.), calcined natural MAs (calcined kaolinite clay or metakaolin), LC3 materials (limestone calcined clay cement), by-product materials (agricultural wastes, CDW, ashes, glass, ferrous slags, non-ferrous slags, basic oxygen furnaces, and electric arc furnaces) ^{[198][293][294][295][296]}[200,295,296,297 Beside the studies presented in Table 5 and 6, there have been commentary research on CO₂ reduction by partly replacing Portland cement with different supplementary cementing materials. Soliman and Tagnit-Hamou ^[432][433], as well as Rajendran et al. ^[433][434], reported that the substitution of 20%w.t. glass powder can significantly reduce the cost of ultra-high-strength concrete and decrease the carbon footprint of a typical ultra-high-strength concrete. In their study, Soltanzadeh et al. ^[434][435] conducted an evaluation of the potential use of waste seashells in the manufacturing of blended cement. The findings suggest that the utilization of seashell powder as a substitute for Portland cement in the production of blended cements has the potential to improve sustainability and reduce production costs. In a study conducted by Qin et al. ^[435][436], pervious concrete samples were examined, wherein a fraction of the Portland cement was substituted with crushed biochar. Based on the results of the study, the researchers hypothesized that it is possible to reduce CO₂ emissions by making pervious concrete by the substitution of powdered biochar for up to 6.5% of the cement’s weight. For the studies presented in this section, it should be noted that a significant reduction in CO₂ emissions can be achieved by utilizing MAs as a substitute for Portland cement, which in turn leads to a decrease in cement consumption and subsequently lower cement output. Furthermore, the decrease in the disposal of non-biodegradable materials in landfills leads to the preservation of limited landfill capacity and mitigates the unsustainable consequences associated with waste disposal in open areas.

2.4. Substitution of CDW as a MA

Concrete, masonry, and brick wastes are prominent among the various waste fractions, exhibiting a significant proportion of approximately 80% in the overall global production of CDW ^{[66][436][437][438]}[66,437,438,439]. Researchers have proposed the recycling of this prominent part to serve as a viable solution to address the sustainability issues encountered by the concrete industry ^{[71][439][440][441][442][443][444][445]}[71,440,441,442,443,444,445,446]. The recycling process involves the conversion of CDW into a reduced-sized fraction through the utilization of mobile or fixed recycling plants ^[446][447]. The recycling process of CDW primarily results in the production of three distinct fractions ^{[82][447][448][449]}[84,448,449,450]. One of these fractions includes a range of 25.00–5.00 mm, which is classified as recycled coarse aggregate (RCA). Another fraction falls within the range of 5.00–0.15 mm and is referred to as recycled fine aggregates (RFA). Lastly, there is a fraction that measures less than 0.15 mm, known as recycled powder (RP). It is important to highlight that in addition to the production of recycled coarse and fine aggregates, a significant quantity of fine recycled powder (RP), comprising approximately 15–35% of the total processed CDW mass, is generated ^[447][448][448,449]. This fine powder lacks a suitable destination and is typically disposed of in landfills ^[440][450][441,451]. The particulate matter emanating from cement mortar, concrete, or bricks typically has a fine texture. The observed range of diameters for the hybrid powder obtained from the crushing and sieving location of CDW was found to vary between 45 and 150 μm ^[440][441]. Although the application of RCA has gained increasing popularity in the past years, the possible use of RP as a partial replacement for Portland cement in concrete has received significant attention due to its tiny particle size and consequential reactivity ^[451][452]. Nevertheless, the efficacy of RPs is contingent upon their primary sources, which are impeded in their practical implementation due to their intricate components. When comparing RPs to Portland cement, it is observed that RPs exhibit a greater degree of irregularity and roughness in their shapes. Additionally, the little particles tend to cluster on the larger ones, resulting in a higher water consumption requirement to obtain a desired standard consistency ^[438][451][439,452]. The primary factor impeding the utilization of untreated RP derived from CDW in cementitious materials is its inherent low activity. The untreated powder is primarily comprised of inert hydrated materials, namely quartz or calcite ^[438][444][439,445]. Several modification approaches have been devised to enhance the characteristics of untreated RP, including mechanical activation ^[452][453][453,454], CO₂ curing treatment ^{[454][455][456][457]}[455,456,457,458], thermal treatment ^{[444][458][459][460]}[445,459,460,461], tannic acid treatment ^[461][462], and chemical activators ^[462][463]. CDW-based material additions used as an addition or as a partial replacement of Portland cement to produce concrete are presented in Table 7. reduction by the partial replacement of Portland cement with CDW and chemical properties of cementitious materials used in these studies are presented in Table 8 and Table 9.

2.4.1. CO₂ Reduction by the Partial Replacement of Portland Cement with CDW

Studies conducted regarding CO₂ As presented in Table 4, MAs such as sugarcane bagasse ash, rice husk ash, palm oil fuel ash, seashell powder, recycled glass powder, ceramic waste powder, fly ash, granulated blast-furnace slag and limestone powder can be used in amounts as high as 30%, 25%, 70%, 20%, 25%, 35%, 55%, 50%, and 55% respectively as replacements for Portland cement for various types of concrete production. Nevertheless, the slow rate at which strength is developed in concrete that incorporates MAs remains a significant obstacle. The utilization of MAs in concrete is accompanied by significant quality control issues, mostly stemming from the diverse chemical and physical properties exhibited by MAs. These properties are influenced by factors such as the source and location of the materials, further complicating the task of ensuring consistent quality in concrete production ^[37][290][37,292].

CO₂ Reduction through the Partial Replacement of Portland Cement with MA

CO₂ reduction by the partial replacement of Portland cement with MAs is reviewed for two cases: first for binary blended cements in Table 5, and second for ternary blended cements in Table 6.