Making sustainable cement is hard, but likely not impossible

// Civil & Environmental Engineering

Photo of Bu Wang

Bu Wang

Cement has a carbon footprint problem. A key ingredient in concrete—one of the most widely used manmade construction materials on the planet—it’s the “paste” that binds together crushed rocks and other materials.

With more than 4.1 billion tons made globally in 2020, cement is also one of the most produced materials in the world.

In fact, cement production alone accounts for about 8% of the world’s carbon emissions each year; about half that total comes from emissions created by burning fossil fuels for the high-temperature process and by the processes associated with making cement—for example, vehicle emissions to transport materials or the finished products to their destinations.

The other 4% arises from the actual chemical process of creating cement. Cement production requires an alkaline like calcium oxide. Creating the alkaline requires a thermal chemical reaction that breaks down calcium carbonate, or limestone, which also produces carbon dioxide.

Unfortunately, it’s an indispensable part of the process, says Bu Wang, an assistant professor of civil and environmental engineering at the University of Wisconsin-Madison. He says there have been efforts to completely remake the cement production process, but they have so far been unable to get around that necessary reaction.

“To make cement, you have to fire the ingredients up to about 1,500 degrees Celsius,” Wang says. “You have to burn fuel to get to that high temperature, which of course creates emissions. During this heating process, the limestone used as an ingredient breaks down into lime and carbon dioxide, which creates further emissions. The challenge here is that the chemical reaction is a necessary part of making cement.”

There are already ways to reduce emissions in some parts of the cement production cycle. Electric vehicles can cut down on transportation-based emissions, and Wang says it may be possible to use renewable energy sources as an alternative to burning fossil fuels to heat cement’s ingredients.

“But for the reaction, it is difficult to get around the emissions,” Wang says. “If we look forward, it’s not far-fetched to think that cement’s role in global carbon emissions may continue to grow, because other sectors can get cleaner. Because of this necessary reaction, it’s much harder to do that with cement. It’s possible, that by 2050, cement production could account for 50 percent of all CO2 emissions.”

Wang is leading a multi-institution research team that aims to halt that looming trend. He received a $1.9 million grant from the National Science Foundation’s Emerging Frontiers in Research and Innovation program to find sustainable ways to make cement.

Rather than attempt to reinvent the wheel, Wang’s team is looking to take advantage of cement’s ubiquity. Because so much of it is produced every year—and has been, for decades—there’s plenty of alkaline in concrete that’s no longer used. Now, the researchers hope to pull the necessary alkaline from old concrete, and a big part of the project will involve creating reactions and processes that can recapture that old material as efficiently as possible. If their effort proves successful, it could open a new, sustainable way to make cement.

“Current cement uses calcium oxide as an alkaline,” Wang says. “After it’s made into concrete, that calcium is still in there. If we can extract the calcium from spent cement paste, we’ve solved the issue—as long as we can get an alkaline without generating additional CO2, we’re good.”

The effectiveness of the solution will depend on the availability of old cement relative to the need for new cement. For example, Wang says an area where as many old buildings are being demolished as new ones are being constructed may find a balance in the cement extracted from old concrete. But, in a growing region where there’s more construction than old buildings to pull from, that balance can shift unfavorably.

The team will create a method for big-picture regional analysis to determine if an area has enough materials to make the method viable. In doing so, they’ll account for factors such as a region’s economic activity, and the emissions generated from moving recycled materials around to determine if the method would provide an environmental benefit.

In the absence of sufficient concrete supplies, Wang says the team will also consider additional recyclable materials, such as coal ash, cement kiln dust, which is a waste material from cement production, and slag from steel and iron production.

“We’ve been generating these materials for decades and decades,” Wang says. “Even as coal plants are closing down, there are materials in landfills that we could draw from. Our hope is that combining these recycled materials will be enough to reduce overall CO2 emissions.”

Their ultimate hope is that one or more of their methods can provide an alternative to producing new cement. While there are ways to improve upon cement, Wang says if they’re not easily accessible and affordable, they won’t catch on.

“The big issue is that because this material is used so much, we need materials that are widely available and cheap,” Wang says. “You can find or design something that’s better than concrete, but if it’s not easy to get or is expensive, people aren’t going to use it. We hope that this process, because concrete is everywhere, can provide an alternative to help solve this problem.”

Wang is also working with Y. Austin Chang Assistant Professor of Materials Science and Engineering Dawei Feng and Biological Engineering Professor Robert Anex at UW-Madison. The project also includes University of Illinois-Chicago Professor of Chemical Engineering Meenesh Singh and Fort Lewis College Assistant Professor of Physics and Engineering William Nollet.

Author: Alex Holloway