Research examines ability of concrete to capture carbon dioxide

By measuring the ratio of certain carbon isotopes in concrete that had been exposed to the air and concrete that had not, the team from the University of Tokyo and Nagoya University could verify that direct air carbon capture had occurred. The tam’s findings are detailed in the Journal of Advanced Concrete Technology. 

Despite international agreements, greenhouse gas emissions from fossil fuels have continued to increase. According to the United Nations’ Intergovernmental Panel on Climate Change, reducing and preventing further emissions should be complimented with the removal of carbon dioxide from the atmosphere.

“As much as 800kg of CO₂ is emitted per ton of cement during its production, so reducing emissions has become a significant issue in the concrete industry,” said 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.”

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 CO₂ identified in concrete has been freshly extracted from the air or comes from rocks,” Maruyama said in a statement. “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.”

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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 non-exposed 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 accelerator mass spectrometry analysed the ratio of several carbon isotopes, 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.