A novel method for storing carbon dioxide (CO2) in concrete has been discovered by a team of engineers led by Northwestern University, utilizing a carbonated water-based solution instead of still water during the concrete manufacturing process. This innovative technique not only sequesters CO2 from the atmosphere but also produces concrete with uncompromised strength and durability.

Concrete, the second most consumed material worldwide after water, is made by combining water, fine aggregates (such as sand), coarse aggregates (like gravel), and cement. Cement binds all the ingredients together, but its production is a significant source of human-caused CO2 emissions. The cement and concrete industries contribute to 8% of global greenhouse gas emissions.

Alessandro Rotta Loria, the Louis Berger Assistant Professor of Civil and Environmental Engineering at Northwestern’s McCormick School of Engineering, explained the motivation behind the study: “We are trying to develop approaches that lower CO2 emissions associated with those industries and, eventually, could turn cement and concrete into massive ‘carbon sinks.’ We are not there yet, but we now have a new method to reuse some of the CO2 emitted as a result of concrete manufacturing in this very same material. And our solution is so simple technologically that it should be relatively easy for industry to implement.”

Previous attempts to store CO2 in concrete have fallen into two main categories: hardened concrete carbonation and fresh concrete carbonation. The hardened approach involves placing solid concrete blocks into chambers where CO2 gas is injected at high pressures, while the fresh version involves injecting CO2 gas into the mixture of water, cement, and aggregates during concrete production. Both methods result in the formation of solid calcium carbonate crystals but suffer from low CO2 capture efficiency, high energy consumption, and weakened concrete.

In the new approach developed by Rotta Loria's team, CO2 is injected into water mixed with a small amount of cement powder to create a carbonated suspension. This suspension is then mixed with the rest of the cement and aggregates, resulting in concrete that absorbs CO2 during its manufacturing. “The cement suspension carbonated in our approach is a much lower viscosity fluid compared to the mix of water, cement and aggregates that is customarily employed in present approaches to carbonate fresh concrete,” Rotta Loria said. “So, we can mix it very quickly and leverage a very fast kinetics of the chemical reactions that result in calcium carbonate minerals. The result is a concrete product with a significant concentration of calcium carbonate minerals compared to when CO2 is injected into the fresh concrete mix.”

Lab experiments demonstrated a CO2 sequestration efficiency of up to 45%, meaning nearly half of the CO2 injected during concrete manufacturing was captured and stored. This process could help offset CO2 emissions from the cement and concrete industries. “More interestingly, this approach to accelerate and accentuate the carbonation of cement-based materials provides an opportunity to engineer new clinker-based products where CO2 becomes a key ingredient,” said study coauthor Davide Zampini, vice president of global research and development at CEMEX, a global building materials company dedicated to sustainable construction.

The research, published in Communications Materials, was a collaboration between Rotta Loria’s laboratory and CEMEX. After analyzing the carbonated concrete, the researchers found its strength to be comparable to regular concrete. “A typical limitation of carbonation approaches is that strength is often affected by the chemical reactions,” Rotta Loria noted. “But, based on our experiments, we show the strength might actually be even higher. We still need to test this further, but, at the very least, we can say that it’s uncompromised. Because the strength is unchanged, the applications also don’t change. It could be used in beams, slabs, columns, foundations — everything we currently use concrete for.”