Concrete’s Natural Carbon Uptake Falls Far Short of Cement’s Emissions, UCLA Study Finds

Concrete naturally absorbs carbon dioxide from the atmosphere over its lifetime, but a new UCLA-led study finds that the process is too slow and limited to significantly offset the emissions generated by cement production.

Published in Communications Sustainability, the study examines ambient carbonation, the natural process in which carbon dioxide enters concrete and reacts with alkaline compounds to form calcium carbonate. Using thermodynamic and diffusion-based modelling, researchers assessed how quickly concrete can absorb CO2 across buildings, roads and other structures.

The findings suggest that ambient carbonation offsets less than 10% of the cement industry’s annual carbon dioxide emissions, considerably below previous estimates that have suggested the process could compensate for as much as 57% of emissions.

Cement is the binding ingredient in concrete, the world’s most widely used building material. Its production generates roughly 10% of global carbon dioxide emissions, largely because limestone must be heated to produce the key ingredient in cement.

The UCLA team found that even concrete elements fully exposed to the atmosphere can take roughly 1,000 years to reach 50% carbonation under normal outdoor conditions. While carbonation does increase after concrete is demolished and crushed, the researchers note that much of this material is landfilled, stockpiled or reused in applications such as road base, where exposure to atmospheric CO2 can be limited.

The scale of the cement industry’s emissions further highlights the challenge. Global cement production is expected to approach 4.83 billion metric tons annually by 2030. Researchers estimate that concrete in service worldwide could absorb around 230 million metric tons of CO2 each year by then, compared with approximately 3 billion metric tons of annual emissions from cement production.

“Ambient carbonation cannot be relied upon as a meaningful tool for reducing atmospheric carbon dioxide accumulations,” said Gaurav Sant, professor of civil and environmental engineering and Pritzker Professor in Sustainability at UCLA Samueli. He said the process is real and substantial when considered on its own, but too small and slow to provide reductions at the scale required by the cement industry.

The researchers argue that efforts to decarbonize cement should therefore focus primarily on preventing or capturing emissions at the point of production rather than relying on carbon absorption that occurs over decades or centuries.

Potential measures include reducing the quantity of cement required in concrete, replacing portions of conventional cement with lower-carbon materials, using alternative fuels, deploying carbon capture technologies and developing fundamentally different approaches to cement production.

The study also has implications for how countries account for emissions and carbon uptake in national climate inventories. The researchers caution that long-term carbonation should not be overstated as a near-term emissions reduction strategy because the majority of its potential benefits occur over extended periods.

The work builds on broader efforts at UCLA to develop alternatives for reducing the carbon footprint of construction materials. Sant leads the Institute for Carbon Management, which has worked on technologies including CarbonBuilt, a lower-carbon concrete production approach, and Equatic, a process designed to remove carbon dioxide from the atmosphere while producing green hydrogen.

The researchers emphasize that the timing of emissions reductions matters. Cutting or capturing carbon emissions when cement is produced can deliver an immediate climate benefit, whereas relying on natural carbonation spreads a comparatively small benefit over a much longer period.

With global cement demand expected to remain substantial, the findings reinforce the need for technologies and production practices capable of addressing emissions at their source rather than treating concrete’s natural carbon uptake as a substitute for industrial decarbonization.