The National Academies
10/08/2024 | News release | Archived content
Pathways to Decarbonize Society’s Most Popular Material
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NAE Perspectives
Sabbie A. Miller is an associate professor in the Department of Civil and Environmental Engineering at UC Davis and a faculty scientist at Lawrence Berkeley National Laboratory
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Pathways to Decarbonize Society's Most Popular Material
Perspectives| October 8, 2024
It is widely accepted that increasing emissions of greenhouse gasses (GHG) in general, and carbon dioxide (CO2) in particular, are impacting global climate with detrimental consequences for humans and ecosystems. Since the largest contributor to emissions is use of fossil fuels (contributing an estimated 75% of GHG emissions and 90% of CO2 emissions), the clearest path to lowering emissions is reducing use of coal, oil, and gas. However, one ubiquitous material, concrete, produces over 7% of anthropogenic CO2 (IEA 2018), with less than half of these emissions attributable to fuel (Chen et al. 2022) and, as such, requires other decarbonization strategies.
The bulk of concrete is made up of aggregates (e.g., sand and crushed rocks), which are commonly sourced locally. Cement is mixed with water to "glue" the aggregates together and form rock-like materials - including concrete, mortar, and other materials, but referred to herein as "concrete" for simplicity - that can be cast into any shape, gain strength as hydration reactions take place, and are both inexpensive and remarkably durable. This class of material products supports our ability to build roadways, water and wastewater systems, and foundations and structures, as well as other applications. Due to our societal need for such systems, concrete is one of the most consumed materials on Earth. Unfortunately, the production of cement requires a chemical reaction called calcination that emits CO2. The production of cement alone accounts for 70% to 90% of the emissions from concrete, with the calcination reaction being responsible for what is estimated as over 60% of these emissions (Chen et al. 2022).
Sabbie A. Miller
Concrete is not per se an environmentally unfavorable material; rather, we are consuming so much of this one material that it is causing substantial impacts. Building materials are our most-consumed resources after water. We currently produce almost 2 billion metric tons (Gt) of steel each year (Kelly et al. 2014), with some of that steel being used in appliances, automobiles, and packaging, and the remaining approximately 55% used in construction applications (Cullen et al. 2012). Notably, while steel is used in several forms in construction, it is also often embedded in concrete as reinforcement (although, we note, this form of steel tends to have lower emissions from production than some other steels). Beyond steel, about 1500 billion baked-clay bricks are produced annually (ILO et al. 2017). And we require myriad other materials including lumber, plastics for piping and flooring, asphalt pavements, and glass, to name a few. Yet, by mass, production of each of these materials is dwarfed by the production of concrete. Annual current production of cement is approximately 4.36 Gt (Kelly et al. 2014), and annual production of concrete is about 30 Gt. The widespread use of concrete worldwide is largely due to its remarkably low cost, which results from the abundance of materials needed to produce it. This affordability, however, makes it challenging to introduce more expensive innovations unless they offer significant benefits, such as improved performance, that can justify the higher price in the market.
GHG emissions from concrete production are driven by the production of clinker in cement. Our modern cement is composed of clinker, a material produced from limestone and clays that are kilned (at approximately 1450 degrees Celsius) and then quenched (rapidly cooled), with other mineral additives. While the mineral additives are commonly directly quarried or are byproducts of other industries, the production of clinker relies on: (1) the calcination process, an energy-driven chemical reaction in which limestone is broken down to provide reactive calcium compounds, directly off-gassing CO2; and (2) energy-derived emissions from both the calcination and kilning stages, where kilning is used to form desired reactive calcium silicates. Currently, the energy resources commonly used in cement production are fossil fuels (e.g., coal), so GHG emissions can potentially be lowered up to about 25% through energy transitions. Use of alternative fuels, such as biomass (e.g., agricultural waste, wood chips), or other mechanisms, such as electrification, in the kilns can support a reduction in reliance on high GHG emissions fuels. Improving kiln efficiency and/or the efficiency of other production processes can also reduce energy demands, thus offering lower energy-related emissions.
Decarbonization Strategies
For the non-fuel emissions, there are several methods that can reduce the carbon footprint of cement and concrete production, often including some (1) material-focused or structure-focused strategies, (2) process-focused strategies, and/or (3) distribution-focused strategies.
Material-focused and structure-focused strategies include replacing a portion of the clinker with supplementary cementitious materials that have lower emissions intensity. Also, some impacts fromconventional cement production can be avoided through the use of alternative binders. There are several forms of alternative binders, including those that still require a kilning process but offer lower calcination-related emissions and/or require less energy, thus lowering energy-derived GHG emissions. There are also cements that do not require kilning, such as alkali-activated binders in which aluminosilicate precursors react with alkaline solutions to form a binder that can behave in similar ways to a hydrated cement. Among the most readily implementable strategies to reduce emissions is improved material efficiency - a strategy in which one reduces material demand through measures such as improved design or increased reuse of resources at their end-of-life to lower impacts while achieving equivalent performance. In the production stages of concrete systems, material efficiency strategies often include using clinker more efficiently in cement, cement more efficiently in concrete, or concrete more efficiently in structures.
Process-focused strategies and distribution-focused strategies include altering production and acquisition methods to reduce emissions. Carbon capture, utilization, and storage (CCUS), in which CO₂ emissions are captured directly before they are released into the atmosphere and either utilized (e.g., to make carbon-based products) or stored (e.g., in geologic reservoirs), can be explored. Notably, CCUS is a key mechanism to potentially capture the chemical-reaction-derived emissions in cement production, but this method will take decades and unknown amounts of money to implement. Another process-focused strategy is the use of non-fossil energy sources, which have included exploration of using renewable resources for energy and electrification to leverage low-emissions electricity generation methods, as noted before. Distribution-focused strategies include improving supply chain logistics to lower GHG emissions from transportation where feasible. It should be mentioned that the logistics of transporting and storing CO₂ after capture is a commonly discussed factor for CCUS implementation.
We Cannot Wait for CCUS to Act as a Silver Bullet
Urgent action to reduce GHG emissions is essential to prevent the worst impacts of climate change, and delays in tackling this challenge by waiting for the installation of carbon capture systems or the adoption of carbon-utilizing materials can have negative repercussions. GHG emissions have a cumulative impact on the atmosphere, and prolonging emissions-reduction strategies will continue to exacerbate climate damages, making future efforts to mitigate damages more challenging. Large-scale deployment of CCUS will take time due to the need for technological refinement, infrastructure development, regulatory approvals, and substantial investment. It is unclear the extent to which low- and middle-income economies would be able to scale such implementation. Further, even if CCUS were fully deployed, it is expected that it would need to be combined with other aggressive emission-reduction measures, and due to the energy and infrastructure needed for CCUS, it could contribute to other environmental burdens. Early action with other emissions-reduction levers, particularly those that have co-benefits of reducing other environmental impacts (such as increased material efficiency), could support a more gradual and equitable transition, which could spread costs over time, potentially enabling economic and social adaptation.
Stakeholder Barriers
Implementation of decarbonization efforts in the cement and concrete industry involves the perspectives of multiple key stakeholders, each facing unique challenges, which can further influence how rapidly decarbonization measures are implemented. Regulatory bodies may lack an in-depth understanding of required material performance and the complexities of how manufacturing alterations can shift materials' behavior. Yet regulations and non-governmental certifications, such as LEED, as well as the desire for positive public relations coverage, can play a crucial role in driving industry transitions through "carrot and stick" approaches, and they can incentivize emissions reductions and support large-scale initiatives like CO₂ sequestration infrastructure, alternative fuel adoption, and participation in carbon markets. On the industry side, decisions are often influenced by economic factors, such as the costs of material storage, resource acquisition, and thorough internal testing and validation. They can also experience drivers through large markets, such as government procurement. Owners can have high emissions-reduction aspirations, but they must be vigilant against greenwashing, as low levels of clinker reduction are insufficient in addressing the climate emergency despite being marketed as low-carbon solutions. Engineers and contractors could require education on material alternatives and how to compare these alternatives effectively. However, notably, given that we rely on concrete for its longevity and for the safe use of buildings and infrastructure, this group of stakeholders can have substantial concern about risks and liabilities associated with using unproven materials, which can dampen their enthusiasm for innovation. Further, the development and adoption of building codes and standards is often slow and consensus-driven, potentially limiting the aggressiveness of emissions-reduction goals.
Tiered Approach to GHG Emission Reduction
While rapidly addressing the GHG emissions from such a widely consumed material with so many stakeholders without causing unintended additional impacts is challenging, there are several pathways forward that should be further investigated. By structuring the emission-reduction strategy into tiers, stakeholders can effectively manage the risks, costs, and timeframes associated with transitioning to lower-carbon concrete, ensuring a steady and scalable reduction in CO₂ emissions from the construction sector.
Focus can initially be placed on implementation of existing, validated low-carbon technologies and practices through policy mandates, incentives, and public procurement. These measures could include increased use of established supplementary cementitious materials and improved material efficiency measures.
For the deployment of emerging materials and technologies, such as still-to-be-proven cement alternatives or partial replacements for conventional cement, efforts can concentrate more on independent validation, pilot projects, and industry collaboration, ensuring these innovations can survive and scale in the market. For next-generation low-carbon technologies with the potential to drastically reduce emissions, support of early-stage research and development through methods such as funding mechanisms and tech-to-market efforts can encourage the advancement of novel methods.
It has been argued that the very long use period of building materials can contribute to carbon storage in the built environment for carbon-utilizing materials (Churkina et al. 2020), and significant efforts are ongoing to better understand how locally available resources or improved concrete and concrete system design can lower GHG emissions. By approaching the technologies to lower emissions based on their maturity and applicability for a given region, we can begin to rapidly address the deleterious emissions from cement production while supporting the societal need for concrete.
Disclaimer
The views expressed in this perspective are those of the author and not necessarily of the author's organization, the National Academy of Engineering (NAE), or the National Academies of Sciences, Engineering, and Medicine (the National Academies). This perspective is intended to help inform and stimulate discussion. It is not a report of the NAE or the National Academies.
References
Chen C, Xu R, Tong D, Qin X, Cheng J, Liu J, Zheng B, Yan L, Zhang Q. 2022. A striking growth of CO2 emissions from the global cement industry driven by new facilities in emerging countries. Environ. Res. Lett. 17(4):044007.
Churkina G, Organschi A, Reyer CPO, Ruff A, Vinke K, Liu Z, Reck BK, Graedel TE, Schellnhuber HJ. 2020. Buildings as a global carbon sink. Nat Sustain 3:269-276.
Cullen JM, Allwood JM, Bambach MD. 2012. Mapping the global flow of steel: From steelmaking to end-use goods. Environ. Sci. Technol. 46(24):13048-13055.
IEA (International Energy Agency). 2018. Technology Roadmap - Low-Carbon Transition in the Cement Industry. IEA. Paris.
ILO (International Labour Organization), the Brooke Hospital for Animals, the Donkey Sanctuary. 2017. Environment, Human Labour, and Animal Welfare: Unveiling the Full Picture of South Asia's Brick Kilns and Building the Blocks for Change. ILO. Geneva.
Kelly TD, Matos GR, Buckingham DA, DiFrancesco CA, Porter KE, Berry C, Crane M, Goonan T, Sznopek J. 2014. Historical statistics for mineral and material commodities in the United States (2016 version), U.S. Geological Survey Data Series 140.
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