New research from MIT’s Concrete Sustainability Hub shows that ordinary cement in buildings and infrastructure naturally absorbs carbon dioxide (CO₂) from the air over its lifetime, a process that has been known in principle but never quantified at national scale. By developing a detailed bottom-up analysis that represents typical structural elements, such as slabs, walls, pavements, and masonry, across different climates and construction types, the team estimated that U.S. cement products sequester over 6.5 million metric tons of CO₂ annually. That figure equates to roughly 13% of the CO₂ released in the cement manufacturing process, a significant but often overlooked part of the material’s lifecycle, tells MIT News.
The carbon uptake occurs through a slow chemical reaction called carbonation. Atmospheric CO₂ enters the tiny pores of concrete and mortar and reacts with calcium-rich compounds to form stable calcium carbonate. This is essentially cement “breathing in” carbon and storing it in solid form. The rate of uptake varies widely depending on several factors: the type of material (porous mortars take up CO₂ much faster than dense concrete), the geometry of the element, and local environmental conditions such as temperature and humidity. That means a concrete block in a dry climate sequesters carbon differently than a pavement in a humid city.
To estimate this effect at scale, researchers created hundreds of archetypal building and infrastructure components and modeled their CO₂ uptake over time. That allowed them to combine material characteristics with real data on construction trends across U.S. states, producing a nuanced picture of how much CO₂ is sequestered across the built environment. They also compared uptake in the United States with Mexico, where different construction practices, especially more use of mortar, result in higher relative sequestration rates despite lower overall cement use.
The study underscores that concrete’s environmental impact isn’t just about emissions from production. Structures already in place act as a distributed carbon sink, gradually capturing CO₂ throughout their lives. Recognizing and incorporating this uptake in national carbon inventories and climate policy could improve accuracy in reporting and open design strategies that increase exposure and uptake without compromising structural performance.