According to recent research from the MIT Concrete Sustainability Hub, cement, the binding ingredient used in concrete, gradually absorbs carbon dioxide from the air over decades. This process is known as cement carbon uptake.

The study offers the first bottom-up, nationwide estimate of CO₂ uptake in infrastructure and buildings. It demonstrates that carbon sequestration in concrete has a quantifiable impact on reducing emissions from the production of cement.

Researchers discovered that about 6.5 million metric tonnes of CO₂ are absorbed annually by cement carbon absorption in American infrastructure and buildings. That amounts to around 13% of the emissions produced during the manufacture of cement in the United States.

Every year, structures in Mexico absorb roughly 5 million tonnes. Even if less cement is used, about 25% of the emissions from cement manufacturing are offset by carbon sequestration in cement. Concrete carbonation has been understood by scientists for many years. Through microscopic holes, carbon dioxide enters cement-based materials and combines with chemicals that are rich in calcium.

The gas is permanently stored in calcium carbonate, which is created during that process. This chemical process facilitates the uptake of carbon dioxide in mortar and concrete.

However, large-scale cement carbon uptake estimation has proven challenging. Across the built environment emissions profile, uptake varies greatly.

Concrete masonry units in apartment complexes absorb CO₂ in a different way than a concrete motorway. Concrete carbon sequestration rates are influenced by climate, exposure, and material selection.

Hessam AzariJafari, the lead author and a research scientist at the MIT Concrete Sustainability Hub, stated that context has a significant impact on carbon uptake.

According to him, the type of cement, the product form, the structural geometry, and the exposure conditions are the four parameters that influence carbon dioxide uptake. The carbonation of concrete can vary fivefold even within a single structure. Numerous slabs, walls, and pavements would need to be simulated in order to mimic cement carbon uptake countrywide because every construction behaves differently.

Rather, hundreds of sample archetypes were created by the researchers. These designs depict common infrastructure and buildings in several geographical areas.

The team modelled carbon sequestration in cement across construction patterns and climates using these archetypes. They then used national and state statistics to scale the results.

By using this method, researchers were able to calculate the cement emissions in the United States that are offset by absorption and compare them to those in Mexico. Cement carbon uptake was found to be significantly influenced by two parameters. The first was current trends in construction. More material is actively undergoing concrete carbonation in areas where cement is added quickly.

The usage of mortar was the second element. Mortars increase carbon dioxide uptake because they are more porous and absorb carbon dioxide roughly 10 times faster than concrete. In comparison to process emissions, states with higher mortar use demonstrated higher absorption. “There is something special about Mexico,” Azari Jafari remarked. “The country has three-quarters of the uptake even though it uses half as much cement as the United States.”

“The difference is explained by construction practices,” he stated. Mexico is more dependent on bagged cement mixed on-site, mortars, and lower-strength concrete. Higher carbon sequestration in cement is caused by these mechanisms.