Concrete is a carbon villain with a quiet redeeming habit: once in place, cement slowly reabsorbs carbon dioxide from the air through a chemical process called carbonation. Researchers at MIT's Concrete Sustainability Hub, with civil and environmental engineering colleagues and collaborators at Holcim's Innovation Center in France, have produced the first national, bottom-up estimate of that uptake for the United States and Mexico, published in the Proceedings of the National Academy of Sciences.
Rather than attempting to simulate every structure, the team constructed hundreds of representative archetypes of buildings and infrastructure, modelled how each absorbs CO2 under different climates, and weighted the results by regional prevalence. Cement type, product form, geometry and exposure conditions all shape the outcome. The totals are substantial: US buildings and infrastructure absorb over 6.5 million metric tonnes of CO2 each year, around 13 percent of US cement process emissions, while Mexico absorbs roughly 5 million tonnes despite using half as much cement, thanks largely to its porous mortars.
The findings could sharpen national emissions inventories, and they suggest some simplified IPCC default values may overstate uptake. They also hand designers a lever: choices such as greater exposed surface area and avoiding needlessly strong mixes could increase carbonation while consuming less cement in the first place, a rare case where accounting rigour and emissions reduction point the same way.