For the first time, researchers, including those from the University of Tokyo, found out how to determine how much carbon dioxide (CO2) from either natural or anthropogenic sources can be absorbed by special concrete production methods. This can be useful for economic tools such as carbon trading and carbon accounting, and the work could lead to similar measurement techniques for other gases of concern.

Concrete production is a huge source of the CO2 driving climate change, so naturally, the industry is under significant pressure to improve the situation. One key advancement has been to coax concrete into actually absorbing CO2 with some success. When this was discovered, attempts were made to use industrial sources of CO2 in order to reduce a particular company’s or sector’s emissions. But there was an open question in this matter: How much of the absorbed CO2 is from exhaust gases and how much came from environmental sources? Professor Ippei Maruyama, from the Building Material Engineering Lab at the University of Tokyo, and his team set out to find the answer.

“In the context of carbon neutrality, the concept of carbon accounting recognizes that different CO₂ sources vary in their value. From this perspective, distinguishing the origin of CO₂ gas in, for example concrete, as performed in this study, is highly meaningful,” said Maruyama. “We successfully identified and quantified the proportion of CO₂ absorbed by cementitious materials that originated from specific sources, such as industrial exhaust gases, and that originating from the atmosphere. The trick was to look at different forms, or isotopes, of carbon that vary depending on the carbon source.”

An isotope is essentially just an atom, but one where the number of constituent neutrons is different from that of its most common, or most stable, state. In the case of carbon, the most common type is carbon-12, and the most common isotope is carbon-13 — so called because although it has 12 protons, positively charged particles in its nucleus, it also has 13 neutrons, which have no charge. But there is a less common form called carbon-14, which has 14 neutrons. As carbon-14 decays over time, there is almost none at all in CO2 from burning fossil fuels, but it is far more common in the atmosphere, where it forms naturally as part of a process involving cosmic rays from space. You can use the ratio of carbon-14 to carbon-12 to determine the age of things from recent history, and Maruyama and his team used a similar technique in their approach, albeit using the ratio of carbon-13 to carbon-12.

“A key focus was the need for an evaluation method in situations where atmospheric air becomes mixed into the exhaust gas during the CO₂ fixation process. In addition, when we considered using carbon isotope ratio measurements to distinguish the origin of emitted gases, we realized that conventional correction methods used in standard radiocarbon dating incorrectly evaluate the ratio of carbon-13 to carbon-12 under such conditions. This led us to recognize the necessity for, and to develop, a new correction approach,” Maruyama said. “In mixed-gas environments, where conventional standard calculation methods are prone to significant errors, we introduced a new calculation model that appropriately corrects for variations in stable isotope ratios caused by isotope fractionation, when mixtures separate, thereby dramatically improving measurement accuracy.”

The researchers achieved this by grinding up concrete into powder before exposing it to different gas conditions. Under laboratory conditions, exhaust gases can be incorporated into concrete with very high efficiency. But in the real world, conditions vary and so do things like the carbon isotope ratios and humidity, which can have a huge impact on the way carbon can be incorporated into concrete. So, the team plans to validate its analytical methodology in a broader range of situations to make it more robust and useful in industrial settings.