Building blocks for the global energy future
Concrete is the world’s most-used building material and one of its most carbon-intensive. In the technological race to reduce emissions, solving the problem of concrete’s massive carbon footprint is a question that some engineers are tackling by experimenting with methods that will turn a major carbon emitter into a material that sequesters carbon.
The production of concrete accounts for about 8 percent of CO2 emissions globally, and the demand for concrete is increasing. “It’s already the most popular building material and the second most consumed material,” said Lori Tunstall, an assistant professor of civil and environmental engineering who is an expert in concrete. “The first is water.”
Tunstall, who is one of dozens of researchers involved in Mines’ Carbon Capture, Utilization and Storage Initiative, is working with National Renewable Energy Laboratory researcher Brennan Pecha to develop an innovative way of sequestering carbon in concrete that emits less carbon dioxide as the cement is made and, in their preliminary research, has properties that even improve upon the industry-standard Portland cement.
“What I really like about it as far as potential for innovation is, we’re not using a new material,” she said. “That’s huge in a conservative industry.”
Why is concrete so carbon-intensive?
Concrete is made by mixing three components: cement, an aggregate and water. The cement is what makes it a carbon hog: carbon dioxide is emitted twice in the production of cement, said Michael McGuirk, assistant professor of chemistry. The first comes from the fuel needed to heat the raw materials, but the second is the result of a chemical process. “Every single molecule, when you produce the raw material for cement, releases a molecule of CO2,” he said.
The process of mining and transporting materials for concrete also comes with emissions, but the cement itself is the biggest chunk of concrete’s overall carbon footprint.
“We heat up the calcium carbonate to really high temperatures to get rid of the CO2, so we’re just left with the lime, the calcium oxide,” Tunstall said. “But that means the process itself releases a lot of CO2. The heat involved, the grinding process—it’s a high-energy process. Cement is the heavy hitter.”
Around the globe, researchers are working on lower-emissions alternatives, such as hydrogen power, to generate the heat needed for cement and other high-temperature industrial applications.
“We are as a society so dependent on upstream and downstream processes that require the burning of fossil fuels and the immense energy they can provide in an instantaneous fashion,” McGuirk said. “You can’t just flip a switch and change the way we produce energy in our entire society. The development of these technologies is necessary to offset the inevitable energy transition—it’s a gradual transition.”
But for cement, a lower-emissions heat source will only solve part of the carbon equation, since the chemical process of creating lime also releases carbon dioxide. “We can’t really optimize that any further, because it’s the chemistry involved in making calcium carbonate into lime,” Tunstall said.
“It’s an extremely simple chemical process, and in theory, it’s a process you could reverse,” McGuirk said. For chemists, this makes it an intriguing problem. So far, no one has figured out how to make this reversible process efficient and fast enough for industrial application. The alternatives, Tunstall said, are to come up with different materials or capture or sequester carbon in the ones they’re using. The construction industry prefers to forge ahead with materials that they know how to work with to make safe structures.
Sequestering carbon in cement
In her research with NREL’s Pecha, who is a biochar expert, Tunstall is mixing biochar into cement to sequester carbon. With biochar, the tree has already done the work of capturing the carbon: it’s made from the pyrolysis of wood, in which the wood is heated in a low- to no-oxygen environment. Biochar is one of many additives researchers have tried adding to concrete to sequester carbon. “Other biomass sources can also be used, which is something we will be exploring further in the future, but so far, we have focused on pine,” Tunstall said.
“In this case, we still use Portland cement, but we use this biochar material that has really high carbon sequestration power, and we use that as an additive in the mix,” she said. “Instead of it being a totally different material, it’s an additive, and that’s an area where the concrete industry has been open to change.”
So far, their research into concrete with added biochar is promising. “What we’ve found so far from initial testing is that not only are we able to replace about 10 percent of the cement, which is huge, because the cement is high-carbon, and the char itself holds carbon in a really stable form,” she said.
Tunstall said their goal was to make a concrete that didn’t lose any of the properties engineers and builders count on in the building material. In their initial tests, the opposite is happening—they’re improving it.
“From initial testing, we have been able to replace at least 15 percent of the cement with biochar, all while improving the strength of the mix,” Tunstall said. “This is a huge win, because not only are we replacing the ingredient associated with the most CO2 emissions, but we are also replacing it with a carbon-rich material that holds CO2 in a stable form. If the wood were left to decompose, it would release this CO2, but the biochar carbon is stable for centuries.
“And on top of that, considering only carbon sequestration and the reduction in cement usage, we estimate a nearly 50 percent reduction in the carbon emissions associated with concrete. This doesn’t even consider other potential benefits, like the cogeneration of biofuels during pyrolysis of the biomass.”
One of the most exciting things about this, Tunstall said, is that she can realistically see biochar sequestering carbon in concrete in the near future.
“A lot of research efforts are so far out, so the fact that this seems like it could be implemented in a few years is exciting,” she said. “We already have a viable solution—it’s just a matter of making it as cheap as possible.”