A Review of Strategies to Achieve Net Zero Targets in the Cement and Concrete Sectors

Abstract

The cement and concrete industries face significant challenges in decarbonization due to escalating demand driven by rapid urbanization, population growth, and infrastructure restoration needs. Cement production alone accounts for 8% of global anthropogenic CO₂ emissions, underscoring the urgency of exploring pathways to achieve net-zero emissions in these sectors. With over 120 nations committing to net-zero targets by 2050, a comprehensive examination of emerging carbon-saving technologies is imperative. While several promising innovations are in nascent stages, rigorous life cycle assessments are essential to determine their potential for carbon reduction. Practical strategies for achieving net-zero objectives and UN sustainability goals involve embracing circular economy principles, harnessing diverse by-product sources, fostering stakeholder engagement, and fostering technological innovation. An efficiency approach that integrates advancements in materials science, alternative fuels, and sector-wide efficiencies is crucial for success. This assessment identifies promising technologies, highlights knowledge gaps, underscores the importance of further research, and offers recommendations for implementing best practices on the path to net zero.

1. Introduction

There is global agreement that the biggest threat to the environment and the economy in our time is global warming. Research by Mahlia [1] and Zhang et al. [2] indicates that greenhouse gas (GHG) emissions associated with human activity cause global warming, which can have disastrous consequences if left unchecked. The frequency of extreme natural occurrences such as hurricanes, droughts, heavy rains, and wildfires has increased globally in recent decades [3]. Climate change continues to worsen these natural disasters, making them more frequent and more intense [4]. Numerous studies indicate that human-caused carbon dioxide emissions are primarily responsible for climate change and global warming [5]. Nevertheless, to mitigate potential but severe environmental, social, and economic losses, climate change scientists have recommended keeping the increase in global temperature to 1.5°C [6]. To achieve this aim, carbon emissions in the current century must be strictly reduced through a rapidly shrinking “carbon budget” method (i.e., the maximum amount of CO₂ that can be emitted to the atmosphere while limiting global warming to a given degree) [7].

Population growth, urbanization, and built environment expansion have led to global warming. Concrete, the most used artificial material, is needed for infrastructure expansion, with 30 billion tons used annually, surpassing per capita steel and wood consumption [8]. The main ingredient in concrete, cement, has an annual production of over 4 billion tons [9]. Cement manufacturing has raised major environmental concerns because it accounts for 5–8% of global anthropogenic CO₂ emissions. Global process-related CO₂ emissions from the cement sector were 1.57 Gt in 2019 [10], whereas total carbon emissions from cement manufacturing were about 2.9 Gt in 2021 [11].

The cement and concrete industry must decarbonize to meet climate change targets by 2050. Research papers focus on mitigation measures, providing stakeholders with information for net zero roadmaps. This review paper examines the potential of emerging technologies to reduce carbon emissions in the cement sector and suggests research areas for further reduction.

2. Decarbonization strategies

This section reviews major solutions for decarbonizing cement manufacturing, including alternative clinker formulations, fuels, and replacements for clinker in cement production. It also suggests possible research areas to reduce CO₂ emissions in the cement and concrete industries. Below is a diagram of the strategies the cement industry could provide as pathways to net zero by 2050. A top-down strategy was used for the collecting and filtering of publication data, and a mixture of broad keywords was used to extract the preliminary publications. The search was conducted in 2024 and was restricted to English-language peer-reviewed academic papers. “Alternative cement,” “alternative clinker,” “alternative fuel,” “alternative concrete,” “carbon emission,” “cement carbon sequestration”, and “machine learning and AI in cement manufacturing” are some of the broad keywords used in the search process. The methodical approach of the sectors thought to be options for lowering CO₂ emissions in the cement and concrete industries is depicted in Figure 1.

Figure 1.

Shows the major areas considered for possible reduction in CO₂ emission for the cement manufacturing and concrete industry.

2.1 Nanoscale calcium carbonate for gigatonne-scale cement industry decarbonization

Nanoscale calcium carbonate (NCC) are calcium carbonate particles smaller than a micron, ranging from 15–35 to 400 nm [12]. Typically produced as precipitated calcium carbonate (PCC), they present diverse applications across various industries such as adhesives, sealants, food, pharmaceuticals, paints, coatings, paper, and construction materials. Producing higher-value polymorphs of calcium carbonate, like calcite, instead of aragonite or vaterite, can help offset the costs associated with carbon capture, utilization, and storage (CCUS) [13]. Lastly, PCC can also be generated from Ca (OH)₂ using ultrasound [14]. Boyjoo et al. [15] offer additional synthesis approaches for both micro and nano-sized calcium carbonate, organizing these first into the CO₂ bubbling method, which sees the most industrial use. Calcite is the primary synthesized polymorph, with NCC more likely than micro and nano-sized particles. Biomimetic approaches involve precipitation and reverse emulsion, separated into spontaneous precipitation and slow carbonation reactions.

NCC in the form of PCC can be made with surfactants with concomitant changes in size, shape, and other properties [12]. These particles can also be sourced from carbon mineralization activities designed to remove CO₂ from point sources or the atmosphere. PCC can also be made with nano-silica using microbes [16] and by using CaCl₂ from Solvay process wastewater to remove CO₂ [17]. Liu et al. [16] describe the precautions for managing adverse health outcomes such as silicosis from breathing in nano-silica [18]. The Solvay wastewater carbonation route’s scalability depends on wastewater calcium concentration, flow rate, and CO₂ concentration, and has been commercially advanced for carbonating subsurface mineral brines. Displacing clinker is how NCC offers a significant decarbonization potential. According to Batuecas et al. [19], 2% of NCC promises to displace enough clinker to eliminate 69% of global greenhouse gas emissions from the global cement industry. This would be without altering the performance of the resulting cement in terms of its compressive strength or other properties. This potential may be lowered in the case of NCC made from nano-silica and microbes, as described by Liu et al. [16], due to the nutrient input required for microbial growth. Nutrient production in microbial NCC requires energy, increasing carbon footprint. Obtaining nutrients from food waste reduces LCA impact but introduces contamination concerns. Sterilization reduces LCA but requires energy. McDonald et al. [20] describe their carbon-negative manufacturing process for their admixture, PCC-A, as sequestering between 100 and 350 kg CO₂ per tonne of their PCC-A produced. Compressive strength peaks at 10 wt% PCC-A, but workability declines, requiring more water. Still, a significant emissions reduction is the result, which is 27–30%, depending on the alkali used.

Improved performance characteristics suggest value in the use of NCC. These include compressive strength and other performance properties of cement containing NCC, as well as studies demonstrating the viability of NCC in cement production [19]. NCC offers additional routes to decarbonization beyond use in the cement industry. One of those is in improving soil nutrient availability, as demonstrated in fields that grow winter wheat [21]. Another involves its deposition onto nanocellulose to make a flexible material that can make up most paper [22]. NCC is already a bright white compound ideal for the color of paper, and its use would reduce the number of new trees needed. It may even enable the use of more recycled paper materials that could be coated with CC to remove non-uniformities in color, thus providing higher value. Market acceptance and regulatory considerations offer significant hurdles to more widespread adoption. Changing the cement composition carrier has an even higher risk of successful hurdles to adoption. Even with tests showing improvements, representation in databases used by structural engineers allowing for selection by architects on new construction is needed for structural use. Using any new composition can carry significant risks if testing is improperly done—structural failures due to poor quality cement have happened. It is crucial to identify these as project risks that have more to do with the validation and verification of proper manufacture and use of the NCC-containing cement rather than arising from the NCC itself.

2.2 Harnessing structural and non-structural infrastructure materials as carbon sinks

Another promising avenue for mitigating the impact of climate change is using carbon capture strategies, with a particular focus on structural and non-structural infrastructure materials as effective carbon sinks. The potential of these materials to sequester carbon dioxide offers an additional innovative approach, contributing to the reduction of greenhouse gas emissions and the overall fight against climate change. In a transformative materials report, Kreigh [23] advocated for the in-depth examination of certain construction materials for their high carbon-storing potential. These materials include algae-grown bricks, purpose-grown fiber, earthen slabs, non-Portland cement concrete slabs, mycelium structural tubes, and agricultural waste panels. Early-stage lab development of promising materials for building applications, evaluated for durability, fire performance, structural capacity, and thermal conductivity, could be accelerated by policies and legislation.

Structural infrastructure materials can be carbon sinks. Concrete and steel, common in construction, can act as long-term carbon sinks through carbonation, a mineralization process where CO₂ reacts with alkaline components to form stable carbonates. Forced carbonation, increasing CO₂ during early hydration, improves concrete strength, resistance to chloride permeability, freeze-thaw performance, and reduces water absorption [24]. Large structures like dams can be used for carbon sinking without compromising mechanical properties. Carbonation can continue over decades, locking away carbon. Geopolymers offer lower carbon footprints as alternative binders. Bio-based panels integrate seamlessly into construction, storing carbon, offering non-toxicity, supporting local economies, and simplifying production. Clay plaster/panels and algae cement provide fire resistance. Optimizing material formulations is crucial to maximize carbon capture potential.

Non-structural infrastructure materials can also be carbon sinks. The carbon storage capacity of buildings is primarily determined by the quantity and volume of wooden elements incorporated into the structural and non-structural components rather than being significantly impacted by factors such as the building type, wood type, or building size [25]. Beyond traditional construction materials, non-structural infrastructure elements present unique opportunities for carbon capture. Pavements, for instance, can sequester carbon through the absorption of CO₂ during curing. Incorporating recycled materials and exploring alternative pavement designs can enhance the carbon capture potential of roadways. Concrete tetrapods can also be mass-produced for the control of beach erosion, serving as an extensive carbon sink.

Bio-based materials offer versatile solutions for constructing prefabricated panels used in wall and roof enclosure systems, as well as in framing, insulation, and sheathing. These panels seamlessly integrate into current construction practices, store significant carbon, and are non-toxic. Utilizing locally available fiber residues and employing low-tech manufacturing processes enhance their appeal. Specific bio-based panels like clay plaster/panels and algae cement offer fire-resistant properties, enhancing their versatility. Thatch cladding, modernized through mechanized processes, provides durable, affordable, and esthetically pleasing roofing solutions. Carbon-negative products made from natural materials like jute and sisal offer sustainable alternatives for building infrastructures, contributing to environmental and economic development. Research indicates that when treated with alkali solution and compounded with epoxy, these fibers exhibit better mechanical properties suitable for load-bearing composites in construction [26]. Incorporating vegetation into urban landscapes, such as through green roofs and walls, serves as a means of carbon sequestration, offering both environmental and esthetic benefits. Research aimed at identifying optimal plant species, soil conditions, and maintenance practices can enhance the carbon sequestration potential of these features. Alternative materials face durability, economic feasibility, and energy requirements challenges. Life cycle assessments are needed to minimize environmental impact. Infrastructure as carbon sinks may help combat climate change, but ongoing research will be needed to maximize carbon capture and storage potential.

2.3 Use of artificial intelligence and analytics for process and formulation improvement

The emergence of artificial neural networks and machine learning algorithms has enabled developers to incorporate programming that allows decisions on cement formulation alternatives and manufacturing process analysis, similar to how the human brain would process the information. One of the primary benefits of an artificial neural network (ANN) is that it can be trained based on samples (e.g., formulation alternatives, compressive strength evaluations, etc.) and used for classification and measuring correlation. Neural networks can classify information using probability calculations and regression models, with feedback loops for further processing until sufficient certainty is reached. Before performing classification, ANNs require training to develop machine learning capabilities which involve the mapping of various inputs effectively to their respective outputs. The system processes data through the input layer. The classification methodology is based on predictions and decisions, with the number of hidden layers and neurons used to determine the complexity of the task, with careful consideration to avoid over-training and decreasing processing efficiency. Measures of correctness can then be obtained depending on how well the classification has occurred. Weights can then be applied to the confidence level in a specific feature in correctly classifying the object. With each iteration, the ANN learns as weights are modified based on the results of previous classification attempts. This methodology can be applied to analyzing the cement manufacturing process at specific facilities [27] and the contributions of each stage to overall carbon dioxide emissions. As an example, using over 31,000 sensor-based data points and daily CO₂ generation calculations, the cyclone gas outlet temperature and the kiln main drive speed control were identified during one study as the two variables with the most significant correlation with CO₂ generation. Such analytics provide valuable process data on correlations that may not be apparent.

Similarly, various artificial intelligence applications have been applied to the cement manufacturing process for potential improvements. For example, in calculating the apparent degree of calcination, Gang and Hui [28] created a model utilizing a least squares support vector radial basis function (RBF) kerneled machine called the least-squares support-vector machine (LS-SVM). The model’s inputs included the furnace’s temperature and pressure, the calciner’s outlet temperature and pressure, the tertiary air’s temperature, and the amount of cement raw that was laid off. To obtain the required level of precalcination of the raw feedstock and low level of carbon monoxide, while stabilizing the precalcination process considering the multi-variable dependencies in the precalciner system, Griparis et al. [29] developed an adaptive, resilient, and fuzzy control design approach To estimate the kiln temperature and oxygen content based on five variables, which are the coal flow to the kiln, coal flow to the precalciner, raw meal flow, rotary speed of the kiln, and negative pressure of the preheater exit, Yang et al. [30] developed a back-propagation neural network (BPNN) and radial basis function neural network (RBFNN).

Machine learning and deep learning have also been used for structural engineering, assessing characteristics such as compressive strength, tensile strength, seismic performance, vibration control, and more. A thorough review of the use of predictive models for concrete properties using machine learning and deep learning was conducted by Moein et al. [31] and depicts how learning models that respond in nonlinear space can be used for analysis when the relationships are nonlinear. The study consolidated test results from various learning methods (deep learning, support vector machine, decision trees, ANN) to evaluate performance metrics such as shear strength, flexural strength, tensile strength, and compressive strength and shared the pros and cons with each methodology used. Artificial intelligence in cement manufacturing is expected to enhance performance and promote sustainable production methods, with further research and development of new variables expected to drive growth.

2.4 Alternative clinker technology (ACT)

The production of one ton of conventional clinker releases about 0.83 tons of carbon [32, 33]. The calcination of limestone and the combustion of fossil fuels are the two primary sources of carbon emissions during clinker production. 60–65% of the carbon emissions connected to the production of clinker are caused by calcination, a high temperature chemical process in which calcium carbonate (CaCO₃) is decomposed into calcium oxide (CaO) and carbon dioxide (CO₂). One workable method for achieving decarbonization in the cement sector for medium- to long-term strategic planning is the implementation of alternative clinker technologies (ACT), involving a partial replacement of cement in producing concrete and mortar [34, 35]. In a comprehensive study, Alaloul et al. [36] investigated the addition of oil shale ash (OSA) to cement and geopolymer concretes. More carbonate minerals are often found in oil shale with lower organic content. One oil shale type with major oxide ratios resembling those of OPC clinker is calcareous oil shale [37, 38]. Goncharov and Zhutovsky [38] have demonstrated that calcareous oil shale can substitute more than 76% of the raw materials required to manufacture belite cement clinker. Belite (C₂S) can save carbon emissions by up to 10% since it has a lower CaO content and can be produced at lower calcination temperatures than alite (C₃S).

Oil shale can replace conventional clinker combustion fuel in a rotary kiln, reducing CO₂ emissions [38]. However, current research focuses on technical aspects, requiring further study on embodied carbon analysis and technical performance for accurate scale-up potential. Nehdi et al. [39] offer a thorough review of the literature on alternative clinker technology. Cement with lower carbon emissions has been developed using clinker methods, including magnesium hydroxy-carbonate cement. In 2009, Vlasopoulos and Cheeseman invented this cement, which sequesters CO₂ in hydration products but cannot be low-CO₂ due to its magnesium oxide origin [40, 41]. Therefore, basic research on the energy-efficient industrial process of manufacturing magnesium oxide (MgO) from magnesium silicate rocks is essential [42]. Solidia cement is a different non-hydraulic binder granted a patent in 2016 [43]. A 30% decrease in carbon emissions is possible because of the cement’s clinker, which has a composition like OPC clinker but less CaCO₃ and a kiln temperature of roughly 1200°C [44]. Furthermore, the curing process of Solidia cement has the potential to absorb 300 kg of CO₂ for every 1000 kg of binder; this process can be expedited at higher temperatures [44, 45]. Solidia cement can only be utilized in precast concrete plants due to its restricted carbon curing technology requirements. Karlsruhe Institute of Technology and SCHWENK Zement KG are the patent holders of the hydraulic binder Celitement [46]. This cement is made from stabilized and synthesized short-lived precursors of CSH, requiring less energy and releasing less carbon dioxide into the atmosphere [45]. Tíecnico-Lisbon and CIMPOR developed and patented X-Clinker, a distinct hydraulic binder [47]. Compared to ordinary OPC, this cement’s raw mix has 33% less CaCO₃, a lower C/S ratio, and 25% less processing carbon emissions. To create this kind of clinker, the raw combination must be melted at 1550°C during the pyroprocessing phase. The difference in temperature between this and OPC’s processing temperature is about 100°C. Consequently, technical modifications to the industrial plants are required to facilitate the development of a 100% liquid phase in the clinker manufacturing process. Refer to Refs. [44, 48, 49] for further technical information on different kinds of clinkers. Literature suggests different ACT phase compositions, but further investigation is needed to understand cement types’ engineering performance and contribute to decarbonization through the life cycle assessment of embodied carbon emissions.

2.5 Alternative fuel technology (AFT)

The mean CO₂ emissions from the manufacture of cement range from 563 to 831 kg CO₂/tonne clk worldwide. While energy emissions release 168–476 kg CO₂/tonne clk, calcining limestone releases around 365–560 kg CO₂/tonne clk [45, 50]. According to Chatterjee and Sui [51], the cement industry’s global energy consumption and CO₂ emissions in 2016 were predicted to be roughly 11 EJ and 2.2 Gt, respectively. Studies show that switching to zero-emission fuel sources in cement mills might reduce carbon emissions from cement manufacturing by 25–40% [50, 52]. Many alternative fuel technologies, or AFTs, have been implemented to lower carbon emissions associated with the energy required to produce clinker. Three main categories can be used to group these alternative technologies.

Alternative fuels derived from biomass residues from biogenic and non-biogenic processes have been widely used in the cement industry due to their range of resources. Agriculture, manufacturing, packaging, construction and fabrication, food processing and animal husbandry, community and home, transportation, and automotive resources were the categories Chatterjee and Sui [51] divided waste and biomass fuels into. Research studies explore waste and biomass energy compositions, including sewage sludge, which can be combined with Portland cement to create cement or co-combust in a kiln. The latter technology (2016) allows for energy recovery from sewage sludge. Used tires and waste plastic are thought to be two of the readily available energy sources to produce cement; the energy content of plastics is 28–40 MJ/kg. Numerous tire kinds have a chemical composition like that produced in fossil fuel kilns that could be utilized as an alternative fuel for clinker manufacture in cement kilns. Industrial oils and solvents are another important energy source for cement manufacturing, with LHVs ranging from 29 to 36 MJ/kg. Rahman et al. [53] evaluated the heating value and fundamental makeup of waste solvents and heavy fuel oil that can be burned in a cement kiln.

Hydrogen is a desirable clean alternative fuel for future energy-intensive materials research and industrial operations due to its unique properties, which include its complete storage capacity, renewability, zero emissions, adaptability, and quick recovery [54]. The cement and hydrogen fuel industries can collaborate through two methods: burning H₂ for clinker thermal energy and recovering heat from cement manufacturing facilities. Juangsa et al. [55] demonstrated by thermodynamic analysis that, compared to a coal-based plant, the carbon emissions from the carbon-free combustion of H₂ fuel in a cement manufacturing system combined with NH₃ dehydrogenation may be reduced by 44%. El-Emam and Gabriel [56] investigated seven alternative energy mix scenarios where hydrogen fuel partially supplied heat in the kiln. When these hydrogen-based scenarios are used instead of coal-based cement manufacture, they find that carbon emissions can be reduced by 15.0–19.6%. Several notable cement production sites, including Heidelberg Cement in the UK and Cemex in Spain, have conducted hydrogen fuel mix trials. The creation of hydrogen in cement plants is another decarbonization tactic suggested in several studies [57]. Weil et al. [58] proposed a plant design that uses waste gasification to produce hydrogen within a cement production facility. They stated several benefits to the proposed hybrid system, such as reduced manufacturing costs, gasification products instead of primary fuels, and clinker production using fuel ash as a feedstock [58]. Ozturk and Dincer [59] developed an integrated system using waste heat from cement slag and natural gas to produce hydrogen. The cement plant achieved 55% and 22% energy efficiency in its waste heat recovery system, indicating the feasibility of decarbonizing the industry using hydrogen fuel. In their study, Nehdi et al. [39] offer an incredibly comprehensive literature review on this topic.

2.6 Alternative cement technologies (ACMTs)

First developed in the late 1980s, Portland Lime Cement (PLC) technology steadily progressed in the 2000s [60]. Clinker, gypsum, and limestone are interground to create PLC; limestone typically substitutes 10–20% of the OPC clinker [61]. As per the European standard EN 197 (EN BSm, 2011), CEM Type II/B-L PLCs are allowed to have higher replacement levels of up to 35%. Most early studies on PLCs evaluated the material’s mechanical and durability properties; hence, the environmental benefits were largely disregarded [62, 63]. As more people realized that blended cement had less influence on the environment, PLCs started to appear in the early 2000s [64, 65]. According to reports, PLC emits 10% less carbon dioxide than OPC [66]. PLCs’ clinker factor and carbon emissions can be further reduced using supplemental cementitious materials (SCMs) [67]. Nevertheless, few publicly available research studies contrast how different kinds of PLCs contribute to the cement industry’s short-term decarbonization strategies. Nevertheless, the environmental benefits of PCL might be increased even further by replacing a portion of the clinker with calcined clay.

The clinker factor in PLC manufacturing may be further reduced with additional SCMs. Limestone calcined clay cement (LC3) is a potential technique for employing calcined clays as an effective SCM to partially replace clinker in cement production [68]. Consequently, clay is calcined by heating it to 700–850°C. Clays containing kaolinite can be calcined in roller hearth kilns, flash calcination units, and conventional rotary kilns due to their lower calcination temperature than OPC clinker [69]. A blend of limestone and calcined clay might replace a significant amount of the OPC clinker in blended cement. Half of the OPC clinker can usually be replaced with 30% calcined clay, 15% limestone, and 5% gypsum to provide an engineering performance like that of regular OPC. The mechanical, durability, and hydration dynamics of concrete built using LC3 have all been the subject of numerous studies [70]. LC3 has been shown to dramatically reduce carbon emissions from the cement industry [71]. Thus, by replacing 50% of the clinker with calcined clay and limestone, it is possible to reduce carbon emissions by up to 25–40% while maintaining excellent mechanical and durability performances [72, 73]. Clinker-free hydraulic binders are created from various waste types, reducing environmental impact and carbon emissions. Early research substituted essential elements, but studies show it is possible to create hydraulic binders entirely from waste.

2.7 Carbon capture, utilization, and storage (CCUS)

To decarbonize the cement and concrete sectors by 2050, carbon capture, utilization, and storage (CCUS) is an up-and-coming and essential long-term solution. Cement plants [74], concrete production during the curing process [75], and the recycling of cement-based composites that have reached the end of their useful life [76] are all applications of CCUS technology. This section covers the most recent CCUS technologies that apply to cement plants. The IPCC special report on CCUS [77] lists industrial separation, post-combustion, pre-combustion, and oxyfuel combustion as the four primary categories of CO₂ capture methods. The pre-combustion CCUS techniques are mostly coupled with gasification technology to produce hydrogen fuel. However, as noted by Plaza et al. [74], there is a great deal of opportunity for CCUS in the cement industry with direct CO₂ capture [78], oxyfuel combustion [79], and post-combustion [80]. Consequently, the annual CO₂ storage capacity varies from 25,000 to 2 million tons for different methods. For instance, the Holcim Portland Cement Plant in Colorado, USA, intends to operate a trial system capable of capturing two million tons of CO₂ yearly [74]. Cement companies can reduce the linked impact of climate change by 74–91% by switching to oxyfuel combustion technology, claim [81]. It was demonstrated that for each kilogram of clinker generated, using oxyfuel in conjunction with biomass fuels can produce harmful CO₂ emissions of 24 to 169 gCO₂, or equivalent [81]. Since many different technologies are suitable for the cement industry, more research is needed to evaluate the GHG emissions and potential carbon savings for different CCUS systems. For example, Izumi et al. [82] proposed GHG emission calculations for mineral CCUS technology that may be used in LCA inventories. Even though studies have shown that CCUS has a promising future for decarbonizing the cement industry, more thorough life cycle assessment (LCA) research focusing on different technical systems is needed to forecast reductions in carbon emissions more precisely. The US CCUS market could reach USD 4.3 billion–8.5 billion by 2027. However, it is essential to note that this is still an ongoing technology being developed and optimized to be applied in cement manufacturing and even in electrical power generation plants.

2.8 Supplementary cementitious materials (SCMs)

Innovative low- or zero-carbon concrete technologies are being developed to reduce the construction industry’s carbon emissions. These technologies focus on enhancing the binder system and waste valorization techniques. SCMs, extracted from various materials, are widely used in the concrete industry as a partial replacement or cement substitute, resulting in significant carbon emissions reduction. Furthermore, using SCMs derived from industrial wastes promotes CE through waste valorization [83]. The main barrier to SCM use is the scarcity of classic SCMs, such as FA, GGBFS, and SF (silica fume). Therefore, it is critical to pinpoint the expanding sources of new SCMs. Juenger et al. [84] assessed the research on innovative SCM resources and their long-term effects on the performance and durability of concrete. The technical aspects of using SCMs in concrete technology have also been the subject of various studies [85]. However, a comprehensive examination of the contributions that various SCM types make to carbon reduction strategies has not yet been carried out.

This section discusses the potential carbon reductions of SCMs made from different resources. Industrial waste is the source of the SCMs most frequently used in concrete production. SF, GGBFS, and FA are the additives that are most frequently utilized in cement and concrete production worldwide. The effects of using these SCMs in cement and concrete production have been the subject of numerous investigations [85, 86]. SCMs improve the engineering properties of concrete while also having a positive impact on the environment [87]. For instance, Tushar et al. [88] discovered that substituting 50% of the world’s cement output with FA and GGBFS can save 209 billion dollars and reduce 2745 million tons of CO₂ emissions. Using various traditional SCMs can lower greenhouse gas emissions by 10–28% and the global warming potential (GWP) by 20–38%, according to Hossain et al. [89]. Comparable results were also found in similar experiments [90]. Conventional supply chain management (SCM)’s integration into net-zero carbon goals remains a contentious issue due to constraints like the absence of popular industrial leftover-derived solutions. Miller [9] asserts that the distance and mode of transportation of SCMs may overshadow their contribution to greenhouse gas emissions, making the strategic deployment of SCMs particularly crucial. Arrigoni et al. [91] indicated that SCMs with an unfettered market can minimize GHG emissions in most circumstances, highlighting the significance of SCMs’ market limits in LCA research. Inefficient use of restricted SCM alternatives, particularly when transported over long distances, can potentially exceed the GHG emissions of standard OPC concrete. Additionally, they discovered, in keeping with Miller’s [9] findings, that lowering SCMs does not always translate into a decrease in concrete’s greenhouse gas emissions.

2.9 Alkali-activated materials (AAMs)

Geopolymers and other alkali-activated materials (AAMs) are recognized as environmentally friendly building materials that drastically reduce carbon emissions. Though they might only partially replace OPC concrete globally, AAMs have a strong chance of serving as an alternative concrete technology in regions with locally available raw materials. An aqueous solution of an alkali hydroxide, silicate, carbonate, or sulfate activator is required for AAMs to function. These AAMs are typically divided into two categories: one-part (also called “just add water”) and two-part (sometimes called “traditional”) AAMs. A dry combination comprising a solid aluminosilicate precursor and a solid alkali-activator is added to water to create the binder matrix in one-part mixes [92]. A range of aluminosilicate binders, including FA, GGBFS, metakaolin, and calcined clay, can be the precursor in AAMs.

Although numerous studies in the literature [93, 94] assessed the technical aspects of developing AAMs, this section concentrates on the potential of AAMs in lowering carbon emissions. In contrast to the other alternative technologies investigated in this study, life cycle assessments (LCAs) in the public domain have been used to investigate the prospect of lowering AAM carbon emissions in detail [95]. AAMs can save CO₂ emissions by more than 50% compared to OPC concrete, making them a good option for short-term decarbonization initiatives. The findings of this study, however, emphasize the critical role that activators play in determining the amount of carbon that AAMs can preserve. According to Robayo-Salazar et al. [96] and Fernando et al. [97], alkali activators such as sodium silicate and sodium hydroxide are responsible for most carbon emissions in AAMs.

Therefore, while creating low-carbon AAMs, it is crucial to carefully consider using chemical admixtures, especially when the target strength is high. A further important factor in producing AAMs with significantly lower carbon footprints is the availability of local raw materials for use as precursors. The transportation of raw materials and the use of low-reactivity precursors may increase the carbon footprint of AAMs. One potential replacement for carbon-intensive activators in one-part AAMs is using suitable industrial wastes. Adesanya et al. [98] used desulfurization dust instead of store-bought sodium hydroxide activators. They offered evidence that using these types of industrial wastes can create AAMs that are cleaner and have a lower carbon footprint. Similar outcomes were also seen when one-part AAMs were activated with RGP, RHA, and lime kiln dust [99]. More research is required to determine how newly developed AAMs that substitute various types of waste for chemical activators would affect the environment.

2.10 Recycled concrete

Reusing leftover concrete in environmentally friendly buildings is a common practice. Recent studies explore the use of leftover cement paste to create recycled cement. Recycling construction and demolition waste (CDW) is crucial for sustainability. Recycled aggregate concrete (RAC) is made by substituting RA for NA, with progress in mechanical and durability characteristics reviewed [100, 101]. Furthermore, fundamental research has been conducted to assess the extent to which RAC technology lessens the environmental impact of the concrete industry [90, 102]. RAC technology can decrease concrete production’s carbon footprint, but factors like transportation distance to landfills and waste disposal facilities are crucial for determining success [103]. RA instead of NA may not significantly reduce carbon emissions but eliminating long transit of CWD or NA can.

Carbonated RAC technology can reduce emissions and improve RA quality through carbon mineralization. According to Tan et al. [104], RA carbon-conditioning is a practical method that provides a feasible usage for CRAC. Xiao et al. [105] found that concrete made with carbonated RA had 7.1–13.3% lower carbon emissions than concrete made with NA or non-carbonated RA, respectively, due to the possibility of carbon absorption in carbonated RA. An interesting option to increase carbon savings is to combine RAC and CRAC with another waste valorization technique, such as employing by-products as SCMs or precursors for AAMs [106, 107]. Research shows recycled concrete can decarbonize the concrete industry, with innovative applications like reusing it as filler in tetrapods, promoting sustainable waste utilization.

2.11 Carbon sequestration in concrete

Carbon sequestration in concrete curing can reduce carbon footprint by capturing CO₂ and forming stable calcium carbonate. Early applications show promising results [108]. Several innovative CCUS technologies have been proposed for the concrete industry, including carbonation of recycled aggregate concrete (CRAC) and CO₂ mineralization in synthetic cementitious materials (SCMs) [109]. While contemporary CCUS technology holds potential for carbon sequestration, challenges remain regarding the environmental impact and effectiveness of these processes. For example, while adding ground granulated blast furnace slag (GGBFS) to cement can result in significant carbon absorption without compromising compressive strength, concerns exist regarding water usage and achieving full carbonation throughout the concrete’s depth [110]. Some studies have even questioned the net CO₂ benefit of CCU concrete, emphasizing the need for further research to optimize electricity usage and enhance compressive strength during carbon curing [75, 111]. Thorough experimental and life cycle assessment (LCA) studies are essential to accurately evaluate the carbon-saving potential of various CCUS technologies in the concrete sector. Biochar, derived from biomass pyrolysis, has the potential to sequester up to 2.6 tons of CO₂ per ton [112].

Studies indicate that adding biochar to cement at low concentrations enhances hydration and compressive strength [113]. While its impact on concrete properties has been extensively studied, research on its carbon-sequestering potential is limited. Nevertheless, biochar incorporation can support circular economy initiatives and reduce concrete’s carbon footprint [114]. Factors influencing its carbon capture effectiveness include activation methods and manufacturing characteristics [115]. Recent studies suggest that concrete with biochar and supplementary cementitious materials can achieve carbon-negative status [10]. However, further research is needed to explore composite properties and optimize formulations [116]. Overall, biochar shows promise in reducing concrete’s carbon footprint, but comprehensive life cycle assessments are necessary for a thorough understanding of its efficacy.

2.12 Biomineralized cement and living building materials (LBMs)

Microbial biomineralization is the basis for developing bio-cement technology, which aims to improve the sustainability of newly produced building materials. The utilization of calcium carbonate biomineralization processes in construction materials was reviewed by Beatty et al. [117]. Among biomineralization methods, engineered living building materials (LBM) provide a novel technology that adds various functionalities to building materials using biology [118]. Sand-gelatin structural scaffolds are biomineralized by photosynthetic bacteria that can produce microbially induced calcium carbonate precipitation (MICP), such as Synechococcus sp. [119] and Escherichia coli, to create LBMs [120]. As a result, LBMs can acquire strength without the use of cement. This method has garnered attention because CO₂ sequestration during cell growth has such a promising potential. During photosynthesis, sequestered CO₂ is liberated and reacts with water to produce biomineral calcium carbonate. According to Heveran et al. [119], LBM is a technology that gives building materials multifunctionality sensing, responsiveness, and regeneration through biology. However, since LBM is still in its infancy, more study is needed to determine its scalability and other possible uses, like carbon sequestration [121]. Additional uses for biomineralization include better self-healing cement-based composites and bacterial treatment of waste-based cementitious materials. According to Sharma et al. [122], adding up to 40% sandstone powder to mortars and bacterial treatment can enhance mechanical and durability while reducing carbon emissions. The ability of LBMs to self-heal was investigated by Delesky et al. [123] using biomineralizing Synechococcus sp. PCC 7002. Biomineralization, a new technology, can lower cement consumption in the building sector by promoting calcium biological carbonate precipitation for self-healing, repair, and rehabilitation of cement-based composites or creating cement-free LBMs.

2.13 Public awareness and incentivization

Equal to the level of effort taken to improve cement manufacturing processes and formulations is the need for public awareness to enforce these changes through the procurement of more eco-friendly products. Colorado and California have recently incorporated legislation for environmental product declarations to include the amount of CO₂ produced while manufacturing construction materials such as cement and asphalt. Incentivization efforts such as product rating systems can help consumers share in the efforts for increased sustainable production. The Concrete Sustainability Council is one such organization offering a certification system designed to provide concrete, cement, and aggregate companies insight into the levels at which source ingredient companies operate in an environmentally, socially, and economically responsible way [124]. Rating systems like LEED and Greenroads promote environmentally friendly construction materials, while EnergyStar certifications assess cement efficiency. Promoting sustainable manufacturing requires differentiators and public awareness.

3. Discussion

In Section 2, 13 different approaches were described together with ranges of CO₂ emissions reduction percentages; these are listed in Table 1. Table 1 shows the ongoing exploration of CO₂ emission reduction solutions in the cement and concrete sector. However, most technologies are in the early stages, and it’s unclear how much CO₂ will be reduced. CCUS technology promises a 91% reduction. None of the CCUS systems are currently operational or under study with promising funding, while other solutions aim to reduce CO₂ emissions by over 50%, but this does not necessarily mean a significant reduction. A 50% reduction for the entire cement sector may be less than the 69% estimated for the use of NCC, but 50% of 6 Gt yields a much larger amount of carbon emissions saved than 50% of a million tonnes. Reporting values that share a common point of reference, such as the annual emissions of a cement kiln per tonne of clinker made, can be very helpful. The second item refers to the values of the emissions reduction percentages above. As a very rough approximation, one can add the percentages to determine a range of emissions reductions achievable from 57% to 161%.

Section	Review findings	CO2 emissions reduction percentage
2.1 Nano CaCO3—Clinker displacement	Ref. [15]. The CO2 bubbling method is widely used in industry for creating microsizes and nanosizes of calcium carbonate.	Up to 69%
2.2 Structural and non-structural infrastructure materials as carbon sinks	Ref. [24]. Forced carbonation, is a process that increases CO2 during early hydration, enhances concrete strength, resistance to chloride permeability, freeze-thaw performance, and reduces water absorption.	Not quantified (new technology)
2.3 AI and analytics for process and formulation improvement	Ref. [31]. The study examined the utilization of machine learning and deep learning in predicting concrete properties.	Not quantified (new technology)
2.4 Alternative clinker technology (ACT)	Ref. [44]. Solidia cement’s curing process can absorb 300 kg of CO2 for 1000 kg of binder and can be expedited at higher temperatures.	Up to 30%
2.5 Alternative fuel technology (AFT)	Ref. [50]. Studies indicate that transitioning to zero-emission fuel sources in cement mills could potentially decrease carbon emissions by 25–40%.	Up to 25–40%
2.6 Alternative cement technologies (ACMTs)	Ref. [67]. PLCs’ clinker factor and carbon emissions can be further reduced using supplemental cementitious materials (SCMs)	Up to 25–40%
2.7 Carbon capture, utilization, and storage (CCUS)	Ref. [81]. Cement companies can reduce the linked impact of climate change by 74–91% by switching to oxyfuel combustion technology, claim.	The current promise is up to 91%
2.8 Supplementary cementitious materials (SCMs)	Ref. [89]. Using various traditional SCMs can lower greenhouse gas emissions by 10–28% and the global warming potential (GWP) by 20–38%.	Up to 10–38%
2.9 Alkali-activated materials (AAMs)	Refs. [95, 96, 97]. AAMs can save CO2 emissions by more than 50% compared to OPC concrete, making them a good option for short-term decarbonization initiatives.	>50%
2.10 Recycled concrete	Ref. [105] concrete made with carbonated RA had 7.1–13.3% lower carbon emissions than concrete made with NA or non-carbonated RA, respectively, due to the possibility of carbon absorption in carbonated RA	Between 7.1% and 13.3%
2.11 Carbon sequestration in concrete	Ref. [112]. Biochar, derived from biomass pyrolysis, has the potential to sequester up to 2.6 tons of CO2 per ton	Up to 2.6 tons of CO2 per ton of biochar
2.12 Biomineralized cement and living building materials (LBMs)	Refs. [119, 121]. LBM technology offers multifunctional building materials sensing, responsiveness, and regeneration through biology, but further study is needed to determine scalability and potential uses like carbon sequestration.	Not quantified (new research area)
2.13 Public awareness and incentivization	Ref. [124] companies in the concrete, cement, and aggregate industries insights into their environmental, social, and economic responsibility.	N/A

Table 1.

Shows the summary percentage of CO₂ emission reduction in the 13 areas presented.

In recent times, the cement and concrete industry has heavily focused on Alternative Fuel Technology (AFT), Supplementary Cementitious Materials (SCMs), and Recycled Concrete as solutions for the reduction of emission of CO₂. However as clearly shown in Table 1 these options of solution alone will not help deal with the CO₂ emission problem. To reach the goal of NetZero by 2050 the industry must heavily invest in more alternative solutions and look at which ones are viable. CCUS is the most promising since it captures most of the CO₂ emission at the manufacturing stage. The other alternative for concrete can be used as CO₂ sequestration to remove CO₂ from the atmosphere.

4. Conclusion

This methodical, in-depth, and critical review looks at the emerging alternative technologies to decarbonize the cement and concrete sectors. Most of the recommended technologies are still in their early stages of development. Therefore, further experimental and LCA research is still needed to expand their applicability. Since most research on low- and zero-carbon technologies has focused on technological advancements, a concerted effort is still required to ascertain the specific contribution of each technology to carbon neutrality in the cement and concrete industries. The cement and concrete industries can achieve carbon neutrality by integrating a circular economy (CCUS) with low-carbon technology, utilizing waste appreciation, and upcycling unused concrete to minimize their carbon footprint. At this point, it seems impossible to imagine achieving net-zero cement and concrete by 2050. Given the grave threat posed by climate change, there is great hope that the cement and concrete sector will be able to fulfill its commitments and decarbonize. Strict regulatory rules, innovative design, production advancement, innovative technologies, new scientific breakthroughs, and unwavering dedication from all stakeholders can accomplish this.

Acknowledgments

The authors would like to thank the researchers at the Systems Engineering Department of Colorado State University for their support.

Conflict of interest

The authors have no conflict of interest in the publication of this work.