Humans produce more concrete than any other material on the planet. It’s the literal foundation of modern civilization – and for good reason.
Concrete is strong, durable, affordable and available to almost every community on the planet. However, the global concrete industry has a dirty little secret: it alone is responsible for more than 8% of global carbon dioxide emissions – more than three times the emissions from aviation. Those emissions have doubled over the past two decades as Asian cities have grown, and demand continues to grow at an unprecedented rate.
It’s also one of the hardest industries to decarbonise, in part because manufacturers tend to be hyper-local and operate on small margins, leaving little to invest in technologies that can lower emissions.
However, difficult does not necessarily mean impossible.
Architects, engineers, scientists and cement and concrete manufacturers around the world are researching and testing several new strategies and technologies that can significantly reduce the carbon footprint of cement and concrete. Here are a few, including one my University of Colorado team is working on: figuring out ways to use all-natural microalgae to solve concrete’s biggest emission problem: cement.
It doesn’t have to be 100% cement
The main culprit behind concrete’s climate impact is the production of Portland cement, the powder used to make concrete.
Cement is made by heating limestone, rich in calcium carbonate, to over 2,640 degrees Fahrenheit. The calcium carbonate breaks down into calcium oxide, or quicklime, and carbon dioxide – a climate-warming greenhouse gas. This chemical reaction, what the Portland Cement Association calls a “chemical fact of life,” is responsible for as much as 60% of cement-related emissions. The rest comes from energy to heat the oven.
One of the most promising short-term strategies to reduce the carbon footprint of concrete uses materials such as fly ash from coal-fired power plants, ironmaking slag and calcined clay to replace some of the Portland cement in concrete mixes. These are known as supplementary cementitious materials.
Using 20% to 50% fly ash, slag or calcined clay can reduce the embodied carbon of concrete mixes by approximately the same percentages.
Another method uses small amounts of crushed limestone to replace some of the cement and is becoming a best practice. After thorough testing, the California Department of Transportation recently announced it would allow Portland limestone cement mixtures, known as PLC, in its projects. Using 5% to 15% crushed limestone to replace cement, PLC can reduce emissions by about the same amount. California’s decision soon led other states to approve the use of PLC.
Many researchers are now advocating the use of limestone cement from calcined clay, which contains approximately 55% Portland cement, 15% crushed limestone, and 30% calcined clay. It could reduce emissions by more than 45%.
What electrification and carbon capture can do?
Cement plants have also started testing carbon capture technologies and electric furnaces to reduce emissions. But carbon capture is expensive, and scaling the technology to meet the demands of the cement and concrete industry is no small feat.
Electrification of furnaces faces the same barriers. New technologies and major capital investments are needed to electrify one of the world’s most energy-intensive processes. However, the promise of zero combustion-related emissions is attractive enough to some entrepreneurs and cement companies – including those interested in using 100% solar energy for cement production – who are racing to find solutions that are both technologically and economically viable at scale.
The Inflation Reduction Act, which Congress passed in August 2022, could help broaden the use of some of these technologies. It includes funding to upgrade equipment and add carbon capture capabilities, as well as tax incentives for manufacturers to reduce their emissions.
Go cement-free, possibly with algae
Another strategy is to produce functionally equivalent materials that contain no Portland cement at all.
My team at the University of Colorado Boulder and I are investigating the use of algae-derived limestone for Portland cement production, which could help eliminate 60% of the emissions associated with cement production. This technology is attractive because it is plug and play with conventional cement production.
Using concrete to trap captured CO2
Engineers are also experimenting with injecting captured carbon dioxide into concrete and using aggregates made from carbon dioxide instead of gravel or sand mixed into concrete.
It’s an exciting concept, but so far injection has yielded limited carbon dioxide reduction, and production of carbon dioxide-storing aggregates has yet to scale up.
A growing problem
Ultimately, time will tell whether these and other technologies will deliver on their promise.
What is certain is that within the cement and concrete industry there has been a worldwide reckoning that it has a problem to be solved and not a panacea. A range of solutions tailored to both local and global markets may be required to address the short and long-term challenges of keeping pace with an ever-growing population and rapidly changing climate.
Wil Srubar is an assistant professor of engineering and materials science at the University of Colorado Boulder.