Direct air capture (DAC) pulls carbon dioxide from the air using chemicals or physical processes. Increasing the use of DAC technologies is part of the International Energy Agency’s (IEA) net-zero emissions scenario, a proposed range of methods to enable the global energy sector to remove as much carbon dioxide (CO₂) as it emits by 2050. According to the IEA, industry was responsible for a quarter of global energy system CO₂ emissions in 2022. Cement is the second-most widely used industrial product (after water), and it comes with a high environmental burden.

“As much as 800 kilograms of CO₂ is emitted per ton of cement during its production, so reducing emissions has become a significant issue in the concrete industry,” explained Professor Ippei Maruyama from the Department of Architecture at the University of Tokyo Graduate School of Engineering. “Concrete has long been known to react with CO₂ in the air to form calcium carbonate, an undesirable phenomenon because it induces corrosion of the steel bars inside concrete structures. However, the concrete industry is now considering ways to make effective use of this reaction.”

Although problematic for construction, the reaction which causes calcium carbonate to form fixes or traps CO₂, removing the gas from the atmosphere. Calcium carbonate is also found naturally in rocks, such as limestone, which are used in concrete manufacturing. “This makes it difficult to distinguish whether or not CO₂identified in concrete has been freshly extracted from the air or comes from rocks,” said Maruyama. “So we developed a method to verify this, which could be used to determine whether the concrete produced can be certified as offsetting CO₂ emissions.”

Researchers carried out the study by making hydrated cement paste samples as a concrete replica. After getting sufficiently hydrated, they ground the paste sample into powder, keeping the nonexposed powder contained and leaving the exposed powder open to the air. After seven and 28 days, they dissolved the powder in acid to collect the gas and using a technique called accelerator mass spectrometry analyzed the ratio of several carbon isotopes (atoms with the same chemical properties but different physical properties), namely carbon-12, carbon-13 and carbon-14. This enabled the team to evaluate where the carbon came from, and whether it was already present in the raw materials, as the carbon ratios reflected the known proportion of carbon isotopes in the air at the time the gas was sealed.

Next the researchers want to apply this lab-based method to real-world locations and test how the varied quantities of raw materials used in local concrete production may affect results. “Fixing carbon dioxide from the air is certified as an act of offsetting CO₂ emissions, so it is economically valuable in terms of emissions trading. Digging up calcium carbonate for use in concrete is not, so the distinction is very important and this research can help to support a healthy market,” said Maruyama. “We believe that the carbon neutrality and a circular economy in the construction industry are essential to our future, particularly in Japan where this industry has a role in supporting business continuity and recovery from natural disasters.”