Concrete recarbonation and the hidden CO2 absorption of cement
In the debate around the energy transition, the construction sector is often identified as one of the main contributors to global CO2 emissions. Cement production, in particular, carries a high carbon footprint, driven both by energy use and by the chemical reactions involved in the process. What is discussed far less, however, is a relevant counterbalancing effect, over its service life, concrete can reabsorb part of the CO2 it helped release, through a natural process known as concrete recarbonation.
How much CO2 cement absorbs today, new estimates from MIT
Until recently, the contribution of carbonation was generally treated as marginal in national emissions inventories.
That assumption is now being challenged by a new study from the MIT Concrete Sustainability Hub, published in the Proceedings of the National Academy of Sciences. Using a bottom up approach, researchers quantified for the first time how much CO2 is absorbed each year by cement contained in buildings and infrastructure across the United States and Mexico.
The results are far from negligible. In 2024 alone, the US building stock absorbed more than 6.5 million metric tons of CO2, equivalent to about 13 percent of the process emissions associated with cement production in the country. In Mexico, the effect is even more pronounced. Despite using roughly half as much cement as the United States, CO2 absorption reaches around 25 percent of sector emissions.
How concrete recarbonation works and what it depends on
Concrete recarbonation, often referred to simply as carbonation, is a well known chemical process. Atmospheric CO2 penetrates the pores of concrete, reacts with calcium based compounds, and forms stable calcium carbonate.
The speed and extent of this reaction depend on several variables. The type of binder, Portland cement or alternative blends, the material’s porosity, mortars versus dense concrete, the geometry of structural elements and their exposed surface area, and environmental conditions such as humidity and temperature all play a decisive role. A concrete slab in Alaska, for example, absorbs CO2 far more slowly than a mortar wall in a Mexican city.
To capture this complexity, the MIT team modeled hundreds of structural archetypes, simulating the behavior of different building elements under diverse geographic and climatic conditions. This approach allowed for a much more realistic national level estimate of concrete recarbonation.
Opportunities and limits of an underestimated resource
While concrete recarbonation cannot replace broader decarbonization strategies, it represents a tangible opportunity to improve the emissions balance of the construction sector. Design choices such as less dense mixes, geometries with higher exposed surface area, or avoiding impermeable coatings can enhance CO2 uptake without necessarily compromising durability.
There are limits, however. In reinforced concrete, increased moisture and changes in surface chemistry can raise the risk of steel corrosion. Careful trade offs are therefore required between structural design, long term performance, and sustainability goals.
The study also highlights a critical issue in current emissions accounting. Major international inventories, including those used by the IPCC, often rely on simplified models that may overestimate carbonation effects. Incorporating empirically grounded data could improve the accuracy of environmental reporting and better reflect the contribution of existing buildings.
Toward a more realistic carbon cycle accounting
The research offers one of the most detailed pictures to date of the carbon cycle associated with in use cement, addressing a gap long overlooked in emissions models.
Looking ahead, the methodology could be extended to other countries by combining building stock databases with national cement production data.
Properly accounting for concrete recarbonation in climate policy does not mean downplaying the sector’s emissions. It means recognizing that existing buildings can also play a role, over their lifetime, in reducing atmospheric CO2. A shift in perspective that could open unexpected room for progress toward low emission construction.