Governments around the world are aiming for net-zero carbon emissions by 2050. To drive broader decarbonization efforts, the building industry is challenged by the mission to reduce embodied carbon emissions, which stem from building materials and systems. Most efforts in buildings are focused on operational energy, but up to 80% of a building’s emissions occur before use and occupancy, from extraction to construction phases. These emissions are irreversible and contribute around 11% of global carbon emissions. This study emphasizes the need to prioritize the decarbonization of conventional building materials during production using low carbon constituents to mitigate associated environmental impact. Quantitative data analysis and reviews have identified alternative low-carbon options for possible application in mix design. Examples include substituting general use (GU) Portland cement with Portland limestone cement, using supplementary cementitious materials (SCMs) to reduce cement content, and utilizing low-carbon concrete masonry units. Achieving net-zero embodied carbon requires promoting circular bio-based products and reducing conventional materials’ carbon throughout their life cycle. At material level analysis, this is possible not only by promoting the use of circular bio-based materials but also by purposefully reducing the embodied carbon of conventional building materials throughout their life cycle, as well as communicating the best practices as lessons learnt to promote broader adoption by the architecture, construction and engineering (ACE) industry. However, collaboration among designers, contractors, and manufacturers is essential. This study provides a preliminary pathway to overall decarbonization efforts, understanding that conventional, existing building materials will play a significant role in attaining the net-zero commitments of the future built environment.
1 Introduction
According to the World Green Building Council, the building sector contributes about 40% of global emissions, 75% of which comes from building operations and 25% from carbon embodied in building materials [1]. As buildings are becoming more energy-efficient and moving toward reduced operational carbon, embodied carbon emissions will constitute a higher proportion of the whole-life carbon in the future [2]. Yet, efforts to mitigate embodied carbon from the design, manufacturing, and deployment of building materials like cement, steel, and aluminum are significantly lagging behind [3]. The majority of material economies remain linear rather than circular, relying heavily on energy-intensive production of virgin and non-renewable materials, applying the take–make–use–throwaway approach. Consequently, there is a shortage of recyclables in terms of both quantity and quality to meet the current material demands. Adopting the product circularity concept that aims at minimizing the use of virgin resources, energy, and waste flows has the potential to reduce embodied carbon emissions [4].
Structural systems such as concrete, steel, and masonry contribute the most to the total embodied impact associated with buildings. Adapting building codes and educating construction professionals can result in reduction in associated emissions, up to 25% from cement and concrete. Currently, concrete accounts for 7% of the global carbon emissions, thus emphasizing the need for immediate decarbonization through low-carbon mix and other design innovations [5]. With steel ranking as the second most used material in the architecture, construction, and engineering (ACE) industry and contributing 7.2% to global greenhouse gas (GHG) emissions, prioritizing its reuse and recycling has become crucial as part of the decarbonization effort [8]. This joint effort is necessary to avoid an increase in raw material extraction as the world’s population continues to increase, given that manufacturing steel products from scrap can save between 60% and 80% of the embodied energy. However, a widening gap between the demand for reusable steel components and the limited quantity of available steel scraps remains a key challenge to the massive adoption of recycled steel in the industry.
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According to reviews in the World Green Building Council report, global cement consumption is expected to rise by up to 23% by 2050 and global steel production is projected to increase by 30% during the same period. It is, however, believed that recycled secondary steel will be a critical component of this increase, outpacing the virgin steel content used in the manufacturing process [1]. Beyond structural materials, the building envelope, particularly components like window systems (e.g., aluminum), and insulating materials also contribute significantly to embodied carbon emissions, accounting for 48–50% of the associated emissions in a typical house [1, 5, 6]. For example, the research findings by Azari and Abbasabadi show that windows designed with aluminum framing with no recycled content have the highest impact from building envelopes [7].
This chapter presents preliminary research on the low-carbon options available for conventional building materials in the Canadian ACE industry. It investigates the performance of structural concrete and steel, as the two key materials having relatively high carbon intensity and are expected to remain the most used construction materials in the future. The chapter summarizes opportunities to reduce embodied carbon emissions using strategies to decrease the material quantities of these high-impact constituents and replace them with low-carbon options and alternatives with higher recycled content. The results from this investigation can perhaps serve in developing innovative design concepts and guidelines for the creation of low-carbon building materials, applicable in the (ACE) industry and the built environment at large.
2 Methodology
Embodied carbon represents the global warming potential (GWP), measured in kilograms of carbon dioxide, CO2 equivalent (kgCO2e), of a material or product. GWP quantifies the heat absorbed by the atmosphere due to greenhouse gas emissions. For a whole life cycle approach to evaluating the carbon performance of materials, the International Organization for Standardization (ISO) considers embodied carbon life cycle assessment (LCA) as consisting of the: (i) product stage (A1–A3) from raw material extraction to manufacturing; (ii) construction process stage from transportation to installation (A4–A5); (iii) use stage (B2–B5) from maintenance and repair to replacement and refurbishment; and (iv) end-of-life stage from deconstruction and waste transportation to waste disposal (C1–C4) [9]. Embodied carbon emissions are irreversible and primarily stem from upfront emissions, occurring before construction. Geographical location significantly influences the embodied energy and carbon impact from construction due to variations in manufacturing and production practices, energy supply grid systems, construction methods, local energy infrastructures, transportation modes, distance to local or international suppliers, and other region-specific economic factors.
This chapter provides preliminary information on low-carbon structural materials applicable to the construction of high rise commercial buildings in Canada, utilizing mainly concrete and steel frame design. It summarizes alternative strategies for material selection, design specifications, and construction approaches to reduce embodied carbon impact, particularly for concrete and steel. However, specific design specifications information will need to be directed toward individual project details and design team expectations during the design development (DD) stage before actual construction of the project.
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This research employs quantitative data analysis by reviewing manufacturers’ and suppliers’ websites to gather data on the low-carbon material options available in the market. This includes reviewing online resources as third-party information available to the general public, utilizing embodied carbon information available from environmental product declarations (EPDs), identifying the key stages in the life cycle of products at which carbon emissions are most substantial or finding evidence of potential reduction in impacts throughout the life cycle stages. An environmental product declaration (EPD) is built upon a comprehensive product life cycle assessment and is thus a verifiable key research tool for understanding the life cycle embodied impact of products and manufacturers’ commitment to product circularity. Further analysis includes telephone interviews and site visits to obtain specific information on manufacturing procedures and industry techniques not made available to the public.
3 Research Findings and Analysis
3.1 Carbon Reduction in Concrete
Concrete mix comprises of crushed stone, sand, water, and Portland cement. Cement in concrete is the primary contributor to embodied impact due to the high carbon intensity during its production. Thus, to attain low carbon in concrete, there is a need to target reducing the quantity of cement content in the mix, as it is primarily responsible for 75% of the entire CO2 emissions [8]. Mix aggregates such as crushed stone and sand contribute less than 20%, which is often linked to electricity usage and, to a lesser extent, to excavation, hauling, blasting, and transportation. The contemporary approach to low-carbon concrete involves incorporating by-products (e.g. fly ash, slag) into the concrete mix. By minimizing the use of virgin materials and maximizing the utilization of by-products, a substantial reduction in the carbon footprint can be achieved.
This subsection discusses the strategies to reduce the embodied carbon emissions associated with concrete, focusing on ready mix concrete products, manufacturer and suppliers.
Manufacturer: CarbonCure Ready-Mix Solution
The CarbonCure mix solution highlights the effective use of carbon dioxide (CO2) to mitigate the environmental impact of concrete through carbon removal technology. The process involves injecting a precise dosage of CO2 into the concrete during mixing, leading to a chemical reaction that converts it into a mineral, specifically calcium carbonate, permanently sequestering CO2 in a stable form. CarbonCure collaborates with concrete mix plant locations across provinces in Canada, seamlessly integrating the CarbonCure Valve Box and Control Box for precise CO2 injection during mixing. This sustainable concrete technology significantly enhances performance and compressive strength by reacting with calcium ions, enabling adjustments in cementitious content, reducing carbon footprints, and yielding cost savings. CarbonCure Ready Mix, applicable to various concrete production methods, demonstrates substantial carbon savings per unit, thus fostering environmental sustainability without compromising either quality or performance of the concrete mix. The observed improvements, up to a 10% increase in compressive strength beyond 28 days, highlight the technology’s potential for delivering low carbon concrete while maintaining strength in performance [10]. Research findings show that by integrating CarbonCure Ready Mix with CarbonCure Reclaimed Water, the cementitious content and water in the slurry tank can be revitalized, converting them into valuable upcycled resources. This innovative approach results in additional CO2 savings of 10–25 lb per cubic yard (5–11 kilograms per cubic meter), contributing to an overall carbon reduction of approximately 10% compared to traditional concrete practices [11] (Fig. 1).
Fig. 1
The CarbonCure Ready Mix process. (Adopted from CarbonCure Technologies Inc, 2024)
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Supplier: LafargeHolcim and Lehigh Hanson
LafargeHolcim and Lehigh Hanson are the two main local suppliers of low carbon ready-mix concrete in across Canadian provinces. These mixes include supplementary cementitious materials (SCMs) and admixture ingredients. ECOPact by Lafarge offers a significant reduction in embodied carbon emissions (scope A1–A3) compared to standard ready-mix concrete without cement substitution [12]. ECOPact is available at various plants across six provinces in Canada, including British Columbia, Alberta, Manitoba, Saskatchewan, Ontario, and Quebec.
Low-carbon product range:
ECOPact: 30–50% CO2 reduction from a baseline mix with 0% SCMs, utilizing blended cement
ECOPact Prime: 50–70% CO2 reduction, engineered with higher blends and supplementary materials
ECOPact Max: 70–90% CO2 reduction; the lowest carbon range manufactured with a cement alternative technology like alkali activators
When comparing concrete alternatives, Lafarge ECOPact demonstrates an advantage over other conventional designs with a high SCM content, ensuring stability in various weather conditions and enabling year-round pouring without compromising performance. When regulatory conditions allow, ECOPact integrates upcycled construction and demolition materials, thus contributing to a circular design. The ECOPact Prime variant emphasizes specifying a 56-day strength for effective design and construction. The carbon reduction achieved with ECOPact depends on concrete strength, requiring early collaboration with concrete suppliers to determine optimal mixes for different structural elements due to potential performance implications.
Lehigh Hanson, on the other hand, has introduced low-carbon concrete mixes in their EvoBuild Bronze and EvoBuild Silver series. Both series come with verified environmental product declarations (EPDs) that support reductions in global warming potential (GWP). The Bronze series generally achieves a reduction of 30–50%, whereas the Silver series attains a more substantial 50–70% reduction compared to the industry average mix. Leigh Hanson’s EvoBuild places a priority on carbon optimization without compromising on mix performance, striving to minimize any design and construction implications [13]. It is worth noting that certain Silver mixes may have elevated supplementary cementitious material (SCM) content and necessitate a 56-day design strength, as indicated in the relevant EPDs.
Product Ingredient: General Use Limestone (GUL) Cement
General use limestone (GUL) cement can be a viable alternative to general use (GU) Portland cement in various concrete structural constructions, providing approximately 10% reduction in GWP compared to GU cement. However, it is important to recognize that GUL cement is susceptible to “sulfate exposure,” posing potential risks in specific elements like footings, piles, and slab-on-grade applications. Some concrete suppliers have developed specialized GUL cement formulations to address these risks, emphasizing the need for verification with the supplier before design specifications [14] (Table 1).
Table 1
Average material content for 1 metric ton (1000 kg) of the GU and GUL types are converted to percentages.
The positive synergy between GUL cement and SCMs has been the focal point of contemporary research studies. This interaction enhances concrete sustainability and reduces embodied carbon impact compared to standard GU cement. Utilizing GUL cement requires a nuanced approach, considering both its environmental benefits and potential challenges in specific structural applications. Further research and development could refine the understanding and application of GUL cement in sustainable construction practices.
Supplementary cementitious materials (SCMs) are natural or industrial by-products that exhibit cementitious properties when mixed with water or other compounds. Examples include fly ash, slag cement, and silica fume from coal power stations of steel and silicon metal production. Natural SCMs, like calcined shale, calcined clay, and metakaolin, undergo controlled heating and purifying processes. These materials are sustainable, promoting recycling and reducing cement in concrete mixes, thereby lowering GWP. Hydraulic SCMs, like ground granulated blast furnace slag (GGBFS), react directly with water, whereas pozzolanic SCMs, such as fly ash and silica fume, chemically react with calcium hydroxide.
SCMs significantly influence the concrete properties affecting water requirements, workability, and setting time during the fresh stage as well as strength, permeability, and other characteristics once hardened. Their positive effects encompass reduced bleeding, enhanced strength, and lower permeability. However, their drawbacks include extended set time and potential construction cost implications. General recommendations include specifying performance criteria over SCM limits, utilizing maximum percentages under freeze–thaw conditions, and close collaboration with stakeholders for optimal low-carbon mix design, considering the potential impacts on construction schedules.
SCMs Availablity in Canada by Provinces
SCMs have a long history in Canada. Fly ash and slag are extensively used, with the former being predominantly and widely adopted in most provinces. Currently, 60% of concrete manufacturers in Ontario incorporate GGBFS into cast-in-place concrete production. In the prairies and western Canada (e.g. Alberta), approximately 90% of concrete manufacturers integrate fly ash as cement replacement Fly ash, a coal combustion by-product, is classified based on its calcium oxide (CaO) content, with high-calcium fly ash acting as a sole cementing material, providing moderate strength, and mitigating heat of hydration. Silica fume, on the other hand, mainly serves for specific purposes, primarily used to enhance specific mix properties. Silica fume, derived from silicon metal manufacturing, enhances the durability of structures subjected to corrosion, abrasion, and chemical attacks. Natural pozzolans, like volcanic ashes, have limited applications in Canada.
Regulating Supply Market for SCMs in Canada
The supply chain for SCMs, such as slag and fly ash, is not managed by the ready-mix industry, posing a potential risk to interested end users such as contractors and design team. For instance, slag, a by-product of steel manufacturing, is supplied by steel manufacturers in Ontario and sourced from the United States. Lafarge offers products like NewCem and NewCem Plus, blending slag and fly ash and allowing for high replacement rates. Several suppliers, including Carbon Upcycling Technologies, Lehigh Hanson, and Holcim, contribute to the SCM supply chain in different regions of Canada.
3.2 Carbon Reduction in Steel Products
Steel is produced using a blend of raw primary materials (ore) and recycled materials (scrap steel). The majority of the embodied carbon in fabricated structural steel, approximately 90%, occurs during the cradle-to-mill-gate stage. Opting for electric arc furnace (EAF) structural products, boasting up to 97% recycled content, can significantly reduce embodied carbon by up to half (50%) compared to basic oxygen furnace (BOF) steel, which relies on virgin iron. Table 2 presents characteristics of EAF and BOF (Table 2). The choice between EAF and BOF depends on factors such as scale, raw material availability, energy sources, and environmental considerations. Despite EAF’s sustainability advantages, BOF remains crucial in the steel industry, balancing cost efficiency and production scale, as approximately 71% of global steel production relies on the BOF process [15‐17]. Scrap metals originate from pre- and post-consumer steel, car scraps, household steel wastes, etc., from recycling centers (Table 2).
Table 2
Comparison of electric arc furnace (EAF) and basic oxygen furnace (BOF)
Factors
Electric arc furnace (EAF)
Basic oxygen furnace (BOF)
Raw materials
Utilizes scrap steel as the primary raw material, making it more environmentally friendly by recycling existing steel
Relies on iron ore, which requires mining, and metallurgical coal, contributing to a higher carbon footprint
Energy consumption
Consumes less energy, particularly when powered by electricity from renewable sources
Requires significant energy input, mainly from burning fossil fuels during iron ore smelting
Carbon emissions and environmental impact
Tends to have lower carbon emissions, especially when powered by cleaner energy sources. Generally considered more environmentally sustainable due to its use of recycled materials and lower emissions
Emits more carbon dioxide due to the use of coal in the reduction of iron ore. Involves more environmental concerns, including deforestation for iron ore extraction and higher greenhouse gas emissions
Cost
Initially more expensive to set up but can be cost-effective in the long run, especially with the availability of affordable scrap
May have lower initial setup costs but is more susceptible to fluctuations in raw material prices, impacting long-term profitability
Technology and innovation
Allows for easier integration of advanced technologies, including enhanced process control and automation
Traditional technology with limitations on rapid integration of cutting-edge advancements
Flexibility in design
Offers flexibility in adjusting production levels and can quickly switch between different steel grades
Typically designed for continuous production of large batches, making it less flexible for smaller-scale operations
Manufacturer: Structural Steel Products
Numerous manufacturers of structural steel, steel deck, and cold-formed steel have released EPDs tailored to their products, enabling the direct specification of maximum CO2 for these elements. The ArcelorMittal XCarb™ initiative, for example, dedicated to achieving carbon-neutral steel, integrates reduced carbon products, steelmaking activities, and low-carbon innovations. XCarb™ structural steel, produced using EAF technology with 97% recycled content and powered by renewable energy, exhibits a 60% lower CO2 than does conventional steel and a 73% reduction compared to the North American average. This initiative aligns with ArcelorMittal North America’s commitment to a 25% CO2 reduction by 2030, contributing to the broader goal of carbon neutrality by 2050. The use of “recycled and renewably” produced steel, powered by 100% renewable energy in the EAF process, results in an impressively low CO2 footprint, potentially reaching 300 kg of CO2 per ton of finished steel with 100% scrap metallics [14].
Product Ingredient: High-Recycled-Content
Scrap-based steels offer lower embodied carbon than most virgin steel manufacturing [16, 18]. High-recycled-content steel from EAFs, with more recycled content than BOFs, can effectively reduce embodied carbon in construction projects. However, challenges in the supply chain may impact the desired reductions. It is recommended for design consulting team to engage early with steel manufacturers to identify feasible low-carbon options within project locations. The recycled content in structural steel varies based on factors like scrap availability, product type, chemical composition, and production route. Importing steel to regions without shape mills, such as the west coast of North America, may increase carbon impact.
ArcelorMittal manufactures rolled sections for the North American market in Luxembourg, Europe, using EAF with 51–98% recycled iron content [19]. Given that GHG emissions of overseas transportation are associated with steel manufacturing in Luxembourg, conducting a comprehensive embodied carbon study comparing ArcelorMittal’s steel to North American EAF-produced steel is advisable. North American EAFs may have higher manufacturing emissions, especially when compared to potentially less environmentally friendly electrical grids. However, ArcelorMittal’s EPD for structural steel from Luxembourg (GWP = 0.84 kg CO2e/kg for A1–A3) demonstrates a 31% lower GWP than does North America’s national average according to the American Institute of Steel Construction (AISC). In Canada, ArcelorMittal has four tubular facilities producing mechanical steel tubing and seamless/welded precision tubes with 30–60% recycled content. ArcelorMittal Contrecoeur (Quebec) is Canada’s largest rebar manufacturer, using locally extracted iron from northern Quebec mines [20]. The flat mill in Hamilton, Ontario, employs a blast oxygen furnace. To minimize embodied carbon, rolled sections should be prioritized over hollow structural sections.
Promoting the Use of Locally Produced Steel
Specifying domestic steel can mitigate embodied carbon in steel production and transportation, depending on project and source locations. Domestic hot-rolled sections are EAF-produced, whereas structural plate and coils forming high strength steel (HSS) may come from BOF or EAF furnaces. Imported structural products are more likely BOF-produced, but the global BOF to EAF ratio is 70:30. Not all EAF steel is equal; Canadian BOF-produced steel may have lower embodied carbon than foreign EAF steel, depending on the mill’s input mix of scrap, direct reduced iron (DRI) or pig iron, and energy grid source. Checking producer-specific EPDs is recommended [21].
Examples of Canadian concrete rebar suppliers include AltaSteel in Edmonton, Alberta, using EAF technology with 100% scrap [22]. ArcelorMittal Montreal in Contrecoeur, Quebec, offers a wide range of long products, including concrete reinforcement bars, with a published EPD showing 1.29 kg CO2e/kg GWP [23]. Due to complexities in the steel supply chain, reducing carbon emissions in steel involves minimizing tonnage, favoring high-strength steel for efficient designs, smaller foundations, and replacing complicated sections with rolled sections. However, confirming the carbon emissions of higher-strength steel with the producer is advisable. The greatest challenge remains the lack of publicly available EPDs for steel products [21].
4 Conclusions
This research provides valuable insights into the challenges and opportunities associated with reducing embodied carbon in the construction industry, focusing on key materials such as concrete and steel. Here is a summary of the key points:
1.
Structural concrete: There is evidence of low-carbon options for concrete, emphasizing the use of CarbonCure Ready Mix and Lafarge ECOPact. The findings highlight the potential for substantial carbon reduction through innovative technologies and replacing cement content with SCMs as an alternative mix design. Improving strength in concrete is considered a key priority for decarbonization, such as reducing the clinker-to-cement ratio, embracing renewable energy for production, and incorporating carbon capture technologies. Decarbonizing cement requires a shift to alternative binders, prefabrication of reusable circular units, and transitioning to electric kilns powered by a decarbonized electric grid [3].
2.
Structural steel and steel reinforcement: In primary steel production, transitioning from blast furnace to direct reduced iron technology, utilizing electric arc furnaces powered by renewable energy, and integrating high recycled content in manufacturing product ingredients show potential for substantial reduction in carbon emissions.
Initiating a prompt and collaborative process during the design development stage is crucial to the overall decarbonization effort. This design iterative approach should be supported by both the “supply and demand sides of stakeholders” [1]. Exploring hybrid alternatives and adopting systemized manufacturing further optimizes construction techniques [24]. Emphasizing carbon reduction strategies from project inception creates an environment conducive to achieving systemic reductions, offering significant opportunities for substantial reductions throughout the life cycle of the project. This chapter emphasizes the importance of transitioning to sustainable practices in material production, construction techniques, and overall project planning. Overall, this study provides a foundation for developing innovative design concepts and guidelines to create low-carbon building materials’ specifications, contributing to sustainability in the architecture, construction, and engineering (ACE) industry.
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