Concrete may absorb carbon dioxide from the atmosphere, but the process is far too slow and limited to counteract the emissions generated by cement production, according to a new UCLA-led study. The research challenges widely circulated estimates that natural carbonation inside aging concrete could offset as much as 57% of the cement industry’s carbon dioxide emissions. Instead, the researchers conclude that ambient carbonation accounts for less than 10% of the industry’s annual emissions and cannot be treated as a meaningful substitute for direct emissions reductions.
The study, published in Communications Sustainability, examined how concrete structures absorb carbon dioxide over their service lives. Concrete is made primarily from cement, water and aggregates such as sand and crushed stone. Cement acts as the binding agent, but manufacturing it is highly carbon-intensive. Limestone is heated to extremely high temperatures to produce clinker, the reactive material at the heart of cement. During this process, carbon dioxide is released both from the fuel used to generate heat and from the limestone itself as it chemically decomposes. Together, these emissions make cement production responsible for roughly 10% of global carbon dioxide emissions.
After concrete is placed in buildings, bridges, roads and other infrastructure, carbon dioxide from the surrounding air can gradually penetrate its pores. The gas reacts with alkaline compounds produced during cement hydration, particularly calcium hydroxide, forming calcium carbonate. This reaction is essentially the reverse of part of the cement-making process, in which calcium carbonate is heated to produce clinker. Because of that chemistry, carbonation has often been presented as a natural carbon sink embedded within the built environment. The UCLA researchers say the reaction is real, but its speed and overall scale have been substantially overstated.
Using thermodynamic calculations and diffusion-based modeling, the team evaluated how carbonation progresses through concrete under a wide range of conditions. Carbon dioxide must first move from the atmosphere into the material, then diffuse through the concrete’s pore network before reacting with available alkaline compounds. Dense, low-porosity concrete can be especially resistant to penetration. The researchers also considered cement content, mixture design, surface-to-volume ratio, exposure conditions and the way concrete elements are used. A thin pavement surface exposed on multiple sides may carbonate more rapidly than a massive structural column, but neither scenario produces an immediate or complete climate benefit.
The analysis indicates that a typical concrete beam, slab or pavement fully exposed to outdoor air may require approximately 1,000 years to reach even 50% carbonation under normal conditions. In many structures, only the outer layers are exposed, while the interior remains protected from atmospheric gases. Coatings, weather barriers, soil contact and dense construction can slow the process further. As a result, the quantity of carbon dioxide absorbed during the first several decades of a structure’s life is much smaller than the amount released during the production of the cement used to build it.
The researchers projected that global cement production could approach 4.83 billion metric tons annually by 2030. Under those conditions, concrete in service around the world might absorb approximately 230 million metric tons of carbon dioxide each year. That figure is substantial when viewed in isolation, but it is small compared with the estimated 3 billion metric tons of annual carbon dioxide emissions associated with cement production. The projected uptake therefore represents less than one-tenth of the industry’s yearly emissions, leaving the overwhelming majority of the carbon burden unaddressed.
“Ambient carbonation cannot be relied upon as a meaningful tool for reducing atmospheric carbon dioxide accumulations,” said Gaurav Sant, the study’s leader, a professor of civil and environmental engineering at the UCLA Samueli School of Engineering and the Pritzker Professor in Sustainability. Sant said the process is significant when examined on its own, but “trivial at the gigatonne scale that matters.” He also noted that carbonation occurs gradually, while climate policy requires emissions to be avoided or removed quickly enough to influence atmospheric carbon dioxide concentrations and near-term warming.
End-of-life treatment can change the rate of carbonation, but the researchers warn that demolition does not automatically unlock a large additional carbon sink. Breaking concrete into smaller pieces increases its exposed surface area and can allow carbon dioxide to reach fresh material. However, demolished concrete is often buried in landfills, stored in stockpiles or reused as road base and other low-exposure fill. These applications may restrict air movement and reduce contact between atmospheric carbon dioxide and the reactive interior of the material. Even when crushing accelerates the chemistry, the resulting uptake still occurs after the original manufacturing emissions have already entered the atmosphere.
The findings have implications for national greenhouse-gas inventories and for the cement industry’s plans to reach climate targets. Counting long-term carbonation is scientifically appropriate when its contribution is measured accurately, but the UCLA team argues that it should not be used to create the impression that cement emissions are being neutralized naturally. The researchers say the most effective strategies must reduce emissions at the point of production. These include using less cement through more efficient structural design, replacing part of the cement with lower-carbon supplementary materials, improving energy efficiency, switching to alternative fuels, deploying carbon capture and storage, and developing fundamentally different cement chemistries. “Emissions mitigated today matter far more than those slowly reabsorbed decades from now,” Sant said, emphasizing that immediate reductions provide greater climate value than benefits spread across centuries.
The study’s authors describe their work as a broad assessment of the variables that control carbonation across a 50-year concrete lifespan, including lower, median and upper estimates of global carbon dioxide absorption. Rui Xiao and Dale Prentice, postdoctoral scholars at UCLA, are co-first authors. The research was supported by the Chan-Zuckerberg Initiative, the Grantham Foundation for the Protection of the Environment, the U.S. Department of Energy, the U.S. National Science Foundation, the University of California Office of the President’s Carbon Neutrality Initiative and the Anthony and Jeanne Pritzker Family Foundation. The team’s conclusion is not that concrete carbonation is irrelevant, but that its slow, diffuse and incomplete nature makes it incapable of carrying the cement industry’s climate burden. For an industry emitting billions of tons of carbon dioxide each year, the decisive solutions must begin before concrete ever reaches the construction site.
Subject of Research: Not applicable
Article Title: Ambient concrete carbonation is a trivial contributor in mitigating carbon dioxide emissions from cement production
News Publication Date: 25-Jul-2026
Web References: Communications Sustainability: https://www.nature.com/articles/s44458-026-00116-9
References: DOI: 10.1038/s44458-026-00116-9
Image Credits: Institute for Carbon Management/UCLA
Keywords
Concrete carbonation, cement production, carbon dioxide emissions, climate change, construction materials, carbon capture, sustainable construction, UCLA, ambient carbonation, cement industry
