Through a simple approach, using carbonated (rather than still) water during concrete manufacturing, researchers have found a new way to sequester CO₂ in concrete. Given how much of this material the world is using, this could be a much-needed boon for sustainability.

Concrete problems

Concrete is one of the most widely used building materials globally, yet its production is a major contributor to greenhouse gas emissions. Ordinary Portland Cement (OPC), the key ingredient in concrete, is responsible for about 8% of the world’s anthropogenic CO₂ emissions. If the concrete industry were a country, its emissions would only be surpassed by the US and China.

However, the new approach promises to turn this environmental liability into an opportunity by sequestering CO₂ in concrete while maintaining or enhancing its strength.

Traditional methods of carbonating concrete often result in weakened material and involve high energy consumption due to the need for pressurized and CO₂-rich environments. These processes typically achieve a CO₂ uptake rate of only 5-20%. Researchers Xiaoxu Fu, Alexandre Guerini, Davide Zampini, and Alessandro F. Rotta Loria, decided this is not enough.

The cement and concrete industries significantly contribute to human-caused CO₂ emissions,” said Northwestern’s Alessandro Rotta Loria, who led 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.”

“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 co-author Davide Zampini, vice president of global research and development at CEMEX.

Sparkling solutions

The new method involves injecting CO₂ into a concrete suspension (a mix of concrete and water) rather than the concrete directly. In traditional concrete carbonation, CO₂ is introduced to hardened or fresh concrete. Here, it slowly diffuses and reacts with the cementitious materials to form calcium carbonate (CaCO₃). This process is slow and energy-intensive.

Meanwhile, the new approach allows for a much greater uptake of 45% without decreasing the concrete’s strength in any way. This suspension undergoes rapid carbonation, forming CaCO₃ crystals more efficiently.

“The idea is that cement already reacts with CO₂,” Rotta Loria explained. “That’s why concrete structures naturally absorb CO₂. But, of course, the absorbed CO₂ is a small fraction of the CO₂ emitted from producing the cement needed to create concrete.”

The cement suspension is then mixed with additional cement and aggregates to form concrete. This multi-stage preparation not only accelerates the carbonation reaction but also ensures that the resulting concrete retains its structural integrity.

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.”

The team tested the concrete formed thusly in the lab, finding no significant difference between “regular” concrete and the concrete formed with the new method.

Strength and sustainability

“A typical limitation of carbonation approaches is that strength is often affected by the chemical reactions,” he said. “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.”

“The findings of this research underline that although carbonation of cement-based materials is a well-known reaction, there is still room to further optimize the CO₂ uptake through a better understanding of the mechanisms tied to materials processing,” Zampini said.

The potential environmental benefits of this method are substantial. By converting CO₂ into stable carbonate crystals within the concrete, this approach not only reduces the carbon footprint of concrete production but also creates long-term carbon storage. The crystals formed are stable and have a longer lifespan than the concrete structures themselves, effectively sequestering CO₂ for centuries.

With further optimization and scaling, this method could mark a turning point in reducing the cement industry’s carbon emissions. The approach also opens avenues for further research into optimizing the carbonation process and exploring its application in other cementitious materials.