Concrete is a ubiquitous building material; it’s in our roads, our sidewalks, our buildings, our bridges, and our tunnels. In fact, concrete is the second-most utilized substance in the world, second only to water, and “twice as much concrete is used in construction as all other building materials combined” (1). Currently, the world produces 30 billion tons of concrete each year, and global demand for concrete is only increasing, particularly as industrialization accelerates in many countries in the Global South (2).
However, concrete has a significant carbon footprint. In fact, one ton of cement releases 0.85 tons of carbon dioxide (CO 2), and the concrete industry produces 4-8% of the world’s carbon emissions (3). Currently, no other building material can match concrete’s versatility, low cost, and ease of production, but is there any way to decrease concrete’s carbon footprint while also potentially cutting costs for the industry?
As it turns out, although it emits CO2 during the mixing process, hardened concrete also absorbs carbon through a passive chemical reaction, known as weathering carbonation. During this process, calcium hydroxide in the concrete reacts with CO2 in the air to form calcium carbonate. This is a form of carbon sequestration — carbon from the air is “sequestered” in mineral form within the concrete’s molecular structure.
Unfortunately, this process happens very slowly; one ton of concrete absorbs up to 0.9 kg of CO2 per year via weathering carbonation, although that value is highly dependent on environmental conditions, such as humidity and temperature (2). This means that the carbon uptake which occurs during weathering carbonation is far less than the carbon emitted by the industry. However, we can use the basic chemical reactions, which occur during weathering carbonation as a blueprint for designing a technique for sequestering carbon in concrete that is fast-paced and active, as opposed to slow and passive.
As it turns out, the industry currently has two methods. The first is called mineral carbonation, a “fast-paced imitation of rock weathering” (2). Mineral carbonation targets binder compounds; normally, cement is simply mixed with water to form the hydration products which bind concrete’s aggregates together. However, if CO2 is first dissolved in water to form carbonic acid, the hydronium ions from the acid can react with those hydration products to form stable carbonates – in other words, carbon is sequestered through a chemical reaction that stores carbon in concrete’s binding compounds. According to one study, commercial mineral carbonation could sequester up to 3 Gt of carbon per year.
The second method targets the concrete itself. Concrete is often cured after being mixed to ensure rapid hydration reactions, which has beneficial effects for the concrete’s long-term durability and strength. Steam is the typical medium through which concrete is cured (4), but CO2 can also be used to have the same effect. This process, where CO2 gas is injected into early-age concrete (i.e., at most a few days after mixing) is known as carbonation curing. Similar to mineral carbonation, carbonation curing involves the reaction of silicate oxides with water and CO2 to form stable carbonates.
Of course, obtaining pure CO2 gas, as well as designing and maintaining the closed reaction chambers necessary for curing, means an additional cost for the manufacturer. However, it is possible to cut these costs by substituting concrete’s traditional binding and aggregate materials with recycled alternatives to make what is known in the industry as “green concrete.” For example, Portland cement, the typical choice for concrete’s cement component, can be partially or completely replaced with fly ash (a byproduct of the coal industry) or steel slag.
In addition, gravel and crushed rock, the typical choices for coarse aggregates, can be replaced with demolition byproducts (crushed bricks, concrete, etc.). Mineral carbonation can also be performed on these aggregates to increase their strength and durability, as well as their ability to sequester carbon. Finally, fine aggregates can be replaced with biochar, which has the added benefit of being a product of carbon sequestration itself (biochar is formed by burning organic matter at very high temperatures in a low-oxygen environment, leading to the formation of stable carbon structures). Some studies have found that biochar can accelerate hydration reactions during early-age carbonation curing, leading to higher compressive strength (5).
Unlike some more well-known carbon sequestration methods, green concrete does not require large-scale change from the existing industry or the creation of a new one altogether; all it takes is incorporating recycled byproducts and using carbonation curing technologies instead of steam. Cement manufacturers currently have the means to invest in a more climate-adaptive future — one where we build our cities out of sequestered CO2.
References
(1) Gagg, C. R. Cement and Concrete as an Engineering Material: An Historic Appraisal and Case Study Analysis. Engineering Failure Analysis 2014, 40, 114–140. https://doi.org/10.1016/j.engfailanal.2014.02.004.
(2) Kazemian, M.; Shafei, B. Carbon Sequestration and Storage in Concrete: A State-of-The-Art Review of Compositions, Methods, and Developments. Journal of CO2 Utilization 2023, 70, 102443. https://doi.org/10.1016/j.jcou.2023.102443
(3) Jessa, E.; Ajidahun, A. Sustainable Practices in Cement and Concrete Production: Reducing CO2 Emissions and Enhancing Carbon Sequestration. World Journal of Advanced Research and Reviews 2024 (02), 2301–2310. https://doi.org/10.30574/wjarr.2024.22.2.1412.
(4) Zhang, D.; Ghouleh, Z.; Shao, Y. Review on Carbonation Curing of Cement-Based Materials. Journal of CO2 Utilization 2017, 21, 119–131. https://doi.org/10.1016/j.jcou.2017.07.003.
(5) Roychand, R.; Li, J.; Kilmartin-Lynch, S.; Saberian, M.; Zhu, J.; Youssf, O.; Ngo, T. Carbon Sequestration from Waste and Carbon Dioxide Mineralisation in Concrete – a Stronger, Sustainable and Eco-Friendly Solution to Support Circular Economy. Construction and Building Materials 2023, 379, 131221. https://doi.org/10.1016/j.conbuildmat.2023.131221.f