Year in and year out, cement production is one of the biggest contributors to humanity’s carbon emissions.
Making portland cement, the most frequently used cement around the world and a key component in concrete, releases about three billion metric tons of carbon dioxide per year. That’s roughly 8% of all emissions that arise from human activity.
A team led by University of Wisconsin-Madison researchers is embarking on a project to not only slow that environmental impact—but, in fact, reverse it. The project is led by Rob Annex, a professor of biological systems engineering in the College of Agricultural and Life Sciences, and Bu Wang, an assistant professor of civil and environmental engineering. They are collaborating with Farshad Rajabipour and Ali Memari from Pennsylvania State University and Meneesh Singh at the University of Illinois Chicago.
Bu Wang
The team’s work builds upon a direct-capture system Wang and Anex pioneered for the 2022 XPRIZE carbon removal competition. In that system, they pull carbon dioxide from the air through a simple chemical reaction with hydroxide. This creates carbonate ions, which, when combined with coal ash, can be turned into limestone and activated silica particles for use in cement.
Through its new process, Wang says the team is hoping to replace portland cement with a new, more environmentally friendly mixture that combines this innovative air capture system with upcycled industrial waste products.
“Portland cement today is produced using raw materials like limestone, clay and sand,” Wang says. “In our new cement, one of the key ingredients is waste materials—like coal ash, slag from steel and iron production, or cement kiln dust.”
Processing raw materials for use in cement is energy-intensive, involves heating them to very high temperatures, and produces a lot of carbon dioxide. Processing raw limestone accounts for about half of cement’s emissions impact—or, a whopping 4% of global carbon emissions.
While the process could work with raw materials, focusing on using waste also alleviates the need to extract those raw materials from the earth and allows the team tap into materials that are already available for use, thanks to earlier production. It’s another potential way to reduce cement’s impact on the environment. “There are huge amounts of ash everywhere,” Wang says. “We’re not even actively focused on ash that’s currently being produced from coal plants—we’re focused on ash produced decades ago that’s stored in landfills. So by doing this, we can also pull waste out of landfills and put it to use.”
The team’s technique incorporates water-based reactions (Wang says the specific type will vary, based on the materials used) for processing waste materials for use. Most importantly, it could provide a drastic reduction in carbon emissions compared to today’s standard practices, which involve heating ingredients up to roughly 1,500 degrees Celsius.
Wang, who is also a fellow of the Grainger Institute for Engineering at UW-Madison, has long researched sustainable concrete and cement production. He says one of the biggest hurdles to reaching that goal is the way we currently use concrete, which often is transported to a job site as a mix, then poured and set during construction. While there have been efforts to make concrete that captures carbon, such mixtures haven’t been suitable for casting and setting on-site—nor have they captured much carbon. Wang hopes this new mixture can buck those trends.
“With this new capture technology, we may finally have a way to put the carbon dioxide into the material before it’s manufactured,” he says. “When we make the concrete, the carbon dioxide is already there, so all we’d have to do is pour it like normal and allow it to harden by itself.”
The researchers’ project is one of 18 funded by the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) through the Harnessing Emissions into Structures Taking Inputs from the Atmosphere (HESTIA) program. The project, which will last three years, secured $2.3 million in grant funding through HESTIA.
Featured image photo caption: The Sustainable Materials Innovation Lab in the Civil and Environmental Engineering Department at UW-Madison is creating a carbon-negative cement using by-products from a carbon capture process that removes carbon dioxide directly from the air. Payam Hosseini, research scientist in Professor Bu Wang’s group at UW-Madison, conducts reactivity testing on cementitious materials.