New study shows recycled cements drive down emissions

Engineers at the University of São Paulo and Princeton have developed an approach to recycling cement waste into a sustainable, low-carbon alternative comparable in performance to the industry standard.

Researchers believe the process could enable new uses for construction and demolition waste, of which concrete is a significant component.

“Construction waste typically ends up either in a landfill, or, if it’s recycled, will be used in low-grade applications such as in pavements or soils,” says research leader Sérgio Angulo, a professor of civil and urban construction engineering at the University of São Paulo. “It’s exciting to show that we can … recycle this recovered cement waste into a high-quality application.”

In their paper, published in ACS Sustainable Chemistry & Engineering, the researchers demonstrated that mixtures containing up to 80 percent of this recycled cement were just as strong as conventional Portland cement while generating a fraction of the carbon emissions. Portland cement is the most common binder used to create concrete, but its high carbon intensity is the main reason the cement and concrete industry is responsible for around 8 percent of global emissions.

If fully realized and deployed in coordination with other emerging technologies, the researchers estimated that emissions from the cement industry could be cut by up to 61 percent. The estimated reductions are higher than the 9 percent emissions cuts that the Global Cement and Concrete Association projected to be possible with clinker replacement approaches.

After pulverizing or crushing the concrete into a fine powder, the team heated it to 500 degrees Celsius, which was a high enough temperature to dehydrate the cement powder and restore its properties as a binder but low enough to prevent the decomposition of carbonate components in the material.

While this “thermoactivated” cement could be used to make concrete, the researchers found that its high surface area and water demand during the mixing process led to a final material with high porosity and diminished strength. However, by combining the recycled cement with small amounts of finely ground Portland cement or limestone, the resulting cement binder demonstrated strength gains and workability on par with industry standards.

The strength gain occurs because the finely ground Portland cement or limestone fills the pores in the recycled cement with a material other than water, reducing the overall water demand and even forming new products, called hydration products, that increase the material’s strength.

“Previously, if you only used thermoactivated recycled cement, it didn’t perform well enough to be an acceptable replacement,” says co-author Claire White, a professor of civil and environmental engineering and the Andlinger Center for Energy and the Environment. “But by lowering the surface area and optimizing the packing of particles in the material’s microstructure, we get something that behaves quite comparably to Portland cement.”

Since the process repurposes construction waste, the researchers said the process could move the world toward a more circular carbon economy while generating fewer carbon emissions than other emerging low-carbon cement alternatives. In the paper, for instance, the team estimates their cement emits between 198 to 320 kilograms of carbon dioxide per metric ton, up to 40 percent fewer emissions than a commercially available low-carbon alternative known as limestone calcined clay cement. 

For more information, read the full report here: Recycled cements drive down emissions without slacking on strength - Princeton Engineering.