Concrete slowly absorbs small amounts of carbon dioxide from the air over its lifetime, a natural process sometimes proposed as a solution to the cement industry's heavy carbon footprint. But a new UCLA-led study finds that this passive absorption process is far too slow and too small in scale to meaningfully offset the emissions released when cement is made.
Published in Communications Sustainability , a Nature Portfolio journal, the study was led by researchers at the UCLA Samueli School of Engineering.
Cement is the binding ingredient in concrete — the most widely used building material in the world — but its production is responsible for roughly 10% of global carbon dioxide emissions, driven largely by the chemical process of heating limestone to make the key ingredient in cement.
Using thermodynamic and diffusion-based modeling, UCLA researchers studied the rate and extent to which concrete used in buildings, roads and other structures slowly absorbs carbon dioxide from the air over time. The gas reacts with alkaline compounds inside the concrete to form calcium carbonate — the same mineral found in limestone and seashells. But this natural process, known as ambient carbonation, offsets less than 10% of the cement industry's own annual carbon dioxide emissions — far below previous estimates, which reached as high as 57%. A typical concrete beam, slab or pavement, fully exposed to the atmosphere, would take roughly 1,000 years to reach even 50% carbonation under normal outdoor conditions.
As the built environment continues to expand, global cement production is expected to approach 4.83 billion metric tons annually by 2030. By then, the researchers project that concrete in service around the world will passively absorb roughly 230 million metric tons of carbon dioxide each year. While this, in itself, represents a large amount of carbon dioxide absorption, it is less than 10% of the estimated 3 billion metric tons of carbon dioxide emissions 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%."
Much of the optimism for passive carbonation stems from assumptions about what happens to concrete structures at the end of their service life. While demolition can accelerate the rate of carbonation, the researchers note that crushed concrete is typically landfilled, stockpiled, or reused in low-exposure applications like road base — conditions that may restrict air contact and further limit additional carbon dioxide uptake.
The researchers argue that the industry should instead prioritize active technological measures that reduce or eliminate emissions at the point of production, including reducing the amount of cement used per structure, substituting lower-carbon materials for a portion of cement in concrete, adopting alternative fuels, deploying carbon capture technologies and reimagining ways to make cement.
Findings from this paper are aligned with other research studies that have taken a critical eye toward cement industry projections of ambient carbonation. The UCLA team's analysis estimates median, upper and lower bounds for global carbon absorption in concrete over a 50-year lifespan, incorporating a wider range of variables, including concrete's mixture design, cement content, concrete porosity and the surface-to-volume ratio of concrete elements that affect absorption rates.
At UCLA, Sant directs the Institute for Carbon Management , which has developed a range of carbon emission mitigation technologies, including the low-carbon CarbonBuilt technology for concrete production, which won the grand prize in the NRG COSIA Carbon XPRIZE in 2021, and the Equatic process that removes carbon dioxide from the atmosphere by expanding the ocean's natural ability to absorb more of the greenhouse gas while producing green hydrogen as a byproduct. Time magazine named the technology one of the best inventions in 2023 .
"Following a net-present value framing, emissions mitigated today matter far more than those slowly reabsorbed decades from now," said Sant, who also holds a joint appointment in materials science and engineering and is a member of the California NanoSystems Institute at UCLA. "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."
According to the research team, the findings are particularly relevant to how countries characterize their national climate inventories, helping ensure they do not overemphasize the benefits of long-duration abatement processes, which may be marginal.
UCLA Samueli postdoctoral scholars Rui Xiao and Dale Prentice are co-first authors on the paper. Other authors include UCLA postdoctoral scholar Manas Sarkar and Fabian Rosner, an assistant professor of civil and environmental engineering; Aditya Kumar, an associate professor at Missouri University of Science and Technology; Narayanan Neithalath, a professor at Arizona State University; and Erika La Plante, an associate professor at UC Davis.
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.
