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Concrete is often treated as the climate villain holding up the modern world. Every new flyover, housing block, metro tunnel, and dam carries a carbon cost that begins in a kiln, where limestone is heated until it releases carbon dioxide as part of the chemistry of making clinker. The cement sector has drawn global attention because this is not a niche problem. It is one of the largest industrial sources of carbon pollution, and demand is expected to keep rising in many developing economies as they build homes and infrastructure. Yet cement has another story running quietly in the background, inside the walls and roads we already live with. Over time, cement-based materials slowly absorb carbon dioxide from the air through carbonation, a natural chemical process. A study published in PNAS argues that this uptake is large enough to matter for how we think about the carbon cycle and how we count emissions.
Why Cement Became a Climate Flashpoint
The climate problem of cement begins with scale and chemistry. Cement production involves both fuel emissions from generating extreme heat and process emissions from the breakdown of limestone during clinker formation. That second part is especially hard to avoid because it is built into the material itself. Reporting and analysis often cite cement as responsible for roughly 7% to 8% of global carbon dioxide emissions, which is why it sits in the “hard to abate” category alongside steel and chemicals. That is also why climate policy has increasingly focused on cement, from cleaner fuels and efficiency upgrades to deeper shifts like cutting clinker content and changing building codes to reward lower carbon mixes.
There is also a politics to cement, because it is tied to development. Most countries cannot simply stop using it without rethinking housing, transport, and urban growth. The International Energy Agency notes that progress has been slow, with emissions intensity broadly flat in recent years, and the sector “not on track” for a pathway consistent with net zero by mid century. That tension, between what society wants to build and what the atmosphere can take, is exactly why any credible new piece of evidence about cement’s full climate footprint draws attention.
When Buildings Interact With the Atmosphere
Concrete does not stop changing once it hardens. Carbonation is the slow reaction between carbon dioxide in the air and calcium-rich compounds in cement paste, forming stable carbonates over time. It happens from the surface inward, and it depends on moisture, temperature, porosity, and how exposed the material is. A sheltered indoor column carbonates differently from a bridge pier facing rain and wind, and demolished concrete rubble can carbonate faster because more surface area is exposed. This is basic chemistry, but in climate accounting, it has often been treated as background noise, partly because it unfolds over decades and is difficult to measure directly at the national scale.
The PNAS study adds an important layer by linking this chemistry to how cement is produced, sold, used, and replaced in the real economy. In other words, it treats carbon uptake as something shaped by market structure and material flows, not just lab behaviour. The authors estimate that in 2024, carbon uptake by cement-based materials could sequester about 13% of the process emissions linked to cement consumption, around 6.7 million tonnes of CO2. The number is not presented as a magic eraser for cement’s climate damage, but as evidence that the built environment is actively exchanging carbon with the atmosphere in a way inventories often fail to capture.
How Much Carbon Is Actually Being Reabsorbed
Zooming out beyond one country or one year, multiple research groups have tried to quantify carbon sequestration as a global carbon sink. A study estimated that carbonating cement materials stored a cumulative 4.5 gigatonnes of carbon between 1930 and 2013, and argued that this uptake is substantial and largely missing from emissions inventories. More recent work has continued refining the picture with better datasets and uncertainty ranges. Research published in Earth System Science Data estimated that the accumulated global CO2 uptake by cement produced from 1930 to 2021 was about 22.9 gigatonnes of CO2, with a reported uncertainty range of 19.6 to 26.6 gigatonnes.
These headline figures can sound dramatic, so the careful point is what they do and do not mean. Carbon uptake happens gradually, and it does not cancel out the large and immediate emissions released when cement is manufactured, transported, and used. The uptake also depends on local conditions and on what happens at the end of life. A city that demolishes and crushes concrete, exposing it to air, can see more uptake than a city that leaves reinforced concrete sealed and coated for decades. That is why scientists keep circling back to the same point. Carbonation is not a theoretical idea or a laboratory curiosity. It happens in the real world, across cities and infrastructure, but it does not happen evenly. To understand its climate impact, estimates need to follow how buildings age, how often they are demolished, what happens to concrete after it is torn down, and how local climate shapes exposure to air. Looking only at cement sales tells only a small part of the story.
What This Means for Climate Accounting and Policy
The accounting question is where the story becomes uncomfortable. Many national inventories count cement-related emissions, including process emissions, but do not consistently account for the carbon dioxide that cement-based materials absorb over their lifetimes. Industry and researchers have argued for years that this can distort the real net impact of cement, especially in regions where construction booms from previous decades are now aging and being replaced. The PNAS paper adds weight to this argument by offering a clearer link between market dynamics and carbon uptake, which could make it easier for governments to estimate uptake in a transparent, repeatable way.
Policy-wise, there is a tightrope. Overcounting uptake could be used to excuse continued high emissions, while ignoring uptake can push countries toward less accurate inventories and weaker climate planning. The practical path forward is to treat carbonation as part of lifecycle carbon accounting, alongside proven mitigation options like lowering the clinker to cement ratio, improving efficiency, switching fuels, and scaling carbon capture for the remaining emissions. The IEA has been clear that without faster progress, including technology deployment and policy support, the cement sector will not align with net-zero pathways. Uptake is a piece of the puzzle, but it is not a substitute for cutting emissions at the source.
Cement’s climate reputation was built on solid evidence: the sector emits a vast amount of carbon dioxide, and much of it is difficult to avoid. What the newer science is showing is that the story does not end at the factory gate. The material we have poured into cities over the last century is still reacting with the atmosphere, taking in carbon slowly, unevenly, and in ways that depend on how societies build, maintain, and demolish. Recognising that does not let cement off the hook. It does something else that is just as important. It forces climate reporting and climate policy to grow up a little, to follow carbon through the full life of the materials that shape everyday life, from kiln to city block to rubble, and sometimes back into the air again.
References:
https://doi.org/10.1073/pnas.2515116122
https://www.mdpi.com/2571-8797/7/4/85
https://www.irena.org/Digital-Report/World-Energy-Transitions-Outlook-2022
Concrete Experts International
https://essd.copernicus.org/articles/15/4947/
Banner image: Photo byAlex Sherstnev onUnsplash
