Led by Hessam AzariJafari, a research scientist in the Department of Civil and Environmental Engineering at the Massachusetts Institute of Technology, a team from the MIT Concrete Sustainability Hub has completed the most detailed national-scale assessment to date of how cement-based materials absorb carbon dioxide from the atmosphere. The research examines a long-known but poorly quantified process and places it into a broader context for carbon accounting in the built environment.
AzariJafari, H., Manav, I. B., Rahimi, M., Moore, E., Huet, B., Levy, C., & Kirchain, R. (2025). Carbon uptake dynamics of cement-based materials: Linking market structure, material use, and the carbon cycle. Proceedings of the National Academy of Sciences, 122(51). https://doi.org/10.1073/pnas.2515116122
Cement is the binding material that gives concrete its structural integrity, and it is also responsible for a significant share of global carbon emissions. During production, limestone is heated to high temperatures, releasing carbon dioxide as part of the chemical reaction that forms clinker. These emissions, known as process emissions, are difficult to eliminate entirely even as kilns become more energy efficient. Less widely appreciated, however, is that cement-based materials slowly reabsorb carbon dioxide after they are placed into service.
This process, known as carbon uptake or carbonation, occurs when carbon dioxide from the air diffuses into concrete or mortar through microscopic pores. Once inside, it reacts with calcium-bearing compounds in the cement paste to form calcium carbonate, a stable mineral similar to limestone. The chemistry behind this reaction has been understood for decades, but estimating how much carbon dioxide is absorbed across an entire country’s building stock has remained a challenge.
Hessam AzariJafari, at the Massachusetts Institute of Technology stated,
“There is a real opportunity to refine how carbon uptake from cement is represented in national inventories. The buildings around us and the concrete beneath our feet are constantly ‘breathing in’ millions of tons of CO2. Nevertheless, some of the simplified values in widely used reporting frameworks can lead to higher estimates than what we observe empirically.
The MIT team addressed this gap by developing a bottom-up modeling framework capable of capturing differences in material type, construction practices, geometry, age, and environmental exposure. Rather than attempting to simulate every individual structure, the researchers created hundreds of representative archetypes that reflect common forms of buildings and infrastructure such as residential walls, foundation slabs, pavements, bridges, and masonry units. Each archetype was modeled under different climatic and usage conditions and then scaled based on regional construction data.
Using this approach, the researchers estimated that cement-based materials in the United States absorb more than 6.5 million metric tons of carbon dioxide each year. This amount corresponds to roughly 13 percent of the carbon dioxide released annually through cement manufacturing process emissions in the country. In Mexico, where construction practices differ, the estimated uptake reaches approximately 5 million metric tons per year despite significantly lower overall cement consumption.
According to AzariJafari, the variability between regions highlights why previous global estimates have struggled to capture real-world behavior. Carbon uptake depends strongly on four interacting factors: the type of cement used, the form of the cement-based product, the geometry of the structure, and the environmental conditions it experiences over time. Even within a single building, different components can absorb carbon dioxide at rates that differ by several multiples.
One of the most influential variables identified in the study is the ratio of mortar to concrete within a region’s construction stock. Mortar, which is typically more porous than structural concrete, allows carbon dioxide to penetrate more easily and therefore carbonates more quickly. In regions where masonry construction and on-site mixing of bagged cement are more common, carbon uptake represents a larger fraction of manufacturing emissions.
This pattern is particularly evident in Mexico, where widespread use of concrete masonry units and lower-strength concrete contributes to higher relative uptake. The study suggests that nearly one quarter of cement manufacturing emissions in Mexico are offset over time through carbonation of cement-based materials already in use. In contrast, regions that rely heavily on dense, high-strength concrete with limited exposed surface area show slower uptake rates.
The research also examined how construction trends influence carbon uptake. Areas experiencing rapid growth add large amounts of new cement-based material to the building stock, increasing the total surface area available for carbonation. At the same time, newer structures carbonate more slowly at first, meaning that uptake reflects not only the quantity of material but also its age distribution.
While the study does not suggest that carbon uptake can fully counterbalance cement production emissions, it does indicate that this process is large enough to matter for national and international carbon inventories. Current reporting frameworks often rely on simplified assumptions that do not adequately reflect differences in materials, design, or exposure conditions. As a result, reported uptake values can diverge from what is observed when these factors are modeled in detail.
Randolph Kirchain, director of the MIT Concrete Sustainability Hub and senior author of the study, notes that understanding carbon uptake also opens opportunities for informed design choices. Increasing exposed surface area, avoiding unnecessary coatings, and selecting concrete mixtures that meet performance requirements without excessive cement content can all influence how much carbon dioxide is absorbed over a structure’s lifetime. These strategies must be applied carefully, particularly for reinforced concrete elements where carbonation can accelerate steel corrosion if not properly managed.
Beyond carbon accounting, the modeling framework developed by the MIT team provides a tool that could be applied internationally. By combining national construction data with regional climate information, similar assessments could be conducted in other countries to better understand how existing infrastructure interacts with the carbon cycle. This information could help policymakers refine emissions inventories and guide decisions about where mitigation efforts are most effective.
As efforts to decarbonize construction continue, much attention remains focused on alternative binders, carbon capture technologies, and low-emission production methods. The findings from MIT add a complementary perspective by showing that the built environment itself plays an active, ongoing role in carbon exchange. Buildings, roads, and other structures are not static sources of emissions but materials that continue to evolve chemically long after construction is complete.
By quantifying this process with greater resolution, the study provides a clearer picture of cement’s net environmental impact and highlights the importance of aligning carbon accounting practices with material science and real-world construction behavior.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
