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Humans produce more concrete than any other material on the planet. It is 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 three times the emissions . Those emissions doubled in the past two decades as Asian cities grew, and demand is continuing to expand at an unprecedented rate.

Concrete is responsible for more greenhouse gas emissions than aviation.

It’s also one of the most difficult industries to decarbonize, in part because manufacturers are typically hyperlocal and operate on slim margins, leaving little to invest in technologies that could lower emissions.

However, difficult does not necessarily mean impossible.

Architects, engineers, scientists and cement and concrete manufacturers around the world are investigating and piloting several new strategies and technologies that can significantly reduce the carbon footprint of cement and concrete. Here are a few of them, including one at the University of Colorado is working on: figuring out ways to use all-natural microalgae to solve concrete’s biggest emissions problem — cement.

It doesn’t have to be 100% cement

The primary 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 (1,450 Celsius). The calcium carbonate decomposes into calcium oxide, or quicklime, and carbon dioxide — a climate-warming greenhouse gas. This chemical reaction, what the Portland Cement Association calls a “,” is responsible for a whopping 60% or so of cement-related emissions. The remainder comes from energy to heat the kiln.

One of the most promising short-term strategies for reducing concrete’s carbon footprint uses materials like fly ash from coal plants, slag from iron production, and calcined clay to replace some of the portland cement in concrete mixtures. These are known as .

Using 20% to 50% fly ash, slag or calcined clay can reduce the embodied carbon of concrete mixtures .

Another method uses small amounts of ground limestone to replace some of the cement and is becoming a best practice. After rigorous testing, the California Department of Transportation recently announced it would . With 5% to 15% ground limestone replacing cement, PLC can reduce emissions by about the same amount. California’s decision quickly .

Many researchers are now advocating for the adoption of , which contains about 55% portland cement, 15% ground limestone and 30% calcined clay. It could cut emissions by more than 45%.

What electrification and carbon capture can do

Cement plants have also started testing technologies and to slash emissions. But carbon capture is expensive, and scaling the technology to meet the demand of the cement and concrete industry is no easy feat.

Kiln electrification faces the same barriers. New technologies and large capital investments are required to electrify one of the world’s most energy-intensive processes. However, the promise of zero combustion-related emissions is enticing enough for some entrepreneurs and cement companies — including those interested in using — who are racing to find solutions that are both technologically and economically viable at scale.

The , which Congress passed in August 2022, could help put some of these technologies to wider use. It includes funding for modernizing equipment and adding carbon capture capabilities, as well as tax credit incentives for manufacturers to cut their emissions.

Going cement-free, possibly with algae

Another strategy is to produce functionally equivalent materials that contain no portland cement whatsoever.

Materials like alkali-activated slag or fly ash cement concrete are produced by combining slag, fly ash or both with a very strong base. These materials have been shown to , and they might meet scale and cost criteria, but they still face technical and regulatory challenges.

Some examples of low-carbon, portland cement-free concrete products that have gained market traction include , and – including those produced .

Algae have also been used as an alternative biofuel for heating cement kilns, and algae cultivation systems have also been .

My team at the University of Colorado Boulder and I are looking into the use of , which could help eliminate 60% of the emissions associated with cement manufacturing. This technology is appealing because it is plug-and-play with conventional cement production.

Using concrete to lock captured CO₂ away

Engineers are also experimenting with injecting captured carbon dioxide into concrete as well as using aggregates made of carbon dioxide in place of gravel or sand that is mixed into concrete.

It’s an exciting concept, but so far injection has yielded , and production of carbon-dioxide-storing aggregates has .

A growing problem

Ultimately, time will tell whether these and other technologies will live up to their promise.

What is certain is that there has been a worldwide reckoning within the cement and concrete industry that it has a problem to solve and no silver bullet solution. It may take a suite of solutions tailored to both local and global markets to address the immediate and long-term challenges of keeping up with an ever-growing population and rapidly changing climate.

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