Concrete's natural CO₂ absorption offsets less than 10% of cement industry emissions, UCLA study finds

A UCLA-led study published in Communications Sustainability, a Nature Portfolio journal, has found that concrete's natural absorption of carbon dioxide from the atmosphere is insufficient in both rate and scale to meaningfully offset the emissions generated during cement production. The research was led by scientists at the UCLA Samueli School of Engineering.

Cement, the binding ingredient in concrete and the most widely used building material in the world, is responsible for approximately 10% of global carbon dioxide emissions. This is driven largely by the chemical process of heating limestone to produce the key ingredient in cement. To examine the extent to which concrete structures passively recapture some of these emissions over time, UCLA researchers applied thermodynamic and diffusion-based modeling to study the rate and extent of carbon dioxide absorption in concrete used in buildings, roads, and other infrastructure.

The process under examination, known as ambient carbonation, occurs when carbon dioxide from the air reacts with alkaline compounds inside concrete to form calcium carbonate, the same mineral found in limestone and seashells. The study finds that this natural process offsets less than 10% of the cement industry's own annual carbon dioxide emissions, a figure substantially below previous estimates, which had reached as high as 57%. Under normal outdoor conditions, a typical concrete beam, slab, or pavement fully exposed to the atmosphere would take approximately 1,000 years to reach even 50% carbonation.

As global construction activity continues to expand, cement production is expected to approach 4.83 billion metric tons annually by 2030. The researchers project that concrete in service worldwide will passively absorb roughly 230 million metric tons of carbon dioxide per year by that point. While large in absolute terms, this figure represents less than 10% of the estimated 3 billion metric tons of carbon dioxide emissions projected from the cement industry by 2030.

"Ambient carbonation cannot be relied upon as a meaningful tool for reducing atmospheric carbon dioxide accumulations," said study leader Gaurav Sant, a professor of civil and environmental engineering and the Pritzker Professor in Sustainability at UCLA Samueli. "The effect is real and substantial when viewed in isolation, but it's trivial at the gigatonne scale that matters — and it is far too slow to help the cement industry meet its 2030 goal of cutting carbon emissions by 40%."

The study also addresses assumptions that have fueled optimism about concrete's carbon absorption potential, particularly those related to what happens at the end of a structure's service life. Although demolition can accelerate the rate of carbonation, the researchers note that crushed concrete is typically landfilled, stockpiled, or reused in low-exposure applications such as road base — conditions that may restrict air contact and further limit additional carbon dioxide uptake.

The UCLA team's analysis estimates median, upper, and lower bounds for global carbon absorption in concrete over a 50-year lifespan. The modeling incorporates a range of variables including concrete mixture design, cement content, porosity, and the surface-to-volume ratio of concrete elements, all of which affect absorption rates.

"Following a net-present value framing, emissions mitigated today matter far more than those slowly reabsorbed decades from now," Sant said. "Thus, the most effective pathways are those that abate emissions at the time of emission, rather than smearing the emissions reduction benefit over extended periods of time."

The researchers argue that the industry should instead prioritize active technological measures targeting emissions at the point of production. These include reducing the quantity of cement used per structure, substituting lower-carbon materials for a portion of cement in concrete mixes, adopting alternative fuels, deploying carbon capture technologies, and pursuing new approaches to cement manufacturing. The research team also states that the findings are relevant to how countries characterize their national climate inventories, as they may help ensure that long-duration, marginal abatement processes are not overemphasized in national accounting frameworks.

UCLA Samueli postdoctoral scholars Rui Xiao and Dale Prentice are co-first authors on the paper. Additional authors include UCLA postdoctoral scholar Manas Sarkar, Fabian Rosner (assistant professor of civil and environmental engineering at UCLA), Aditya Kumar (associate professor at Missouri University of Science and Technology), Narayanan Neithalath (professor at Arizona State University), and Erika La Plante (associate professor at UC Davis). The research received support from 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 Pritzker Family Foundation.