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Global cement production accounts for 7% of annual greenhouse gas emissions, largely due to the burning of quarry limestone. Now, a research team led by CU Boulder has found a way to make cement production carbon neutral — and even carbon negative — by extracting carbon dioxide from the air using microalgae.

CU Boulder engineers and colleagues from the Algal Resources Collection at the University of North Carolina at Wilmington (UNCW) and the National Renewable Energy Laboratory (NREL) have been recognized for their innovative work with a $3.2 million grant dollars from the US Department of Energy (DOE) Advanced Research Projects Agency – Energy (ARPA-E). The research team recently selected by the HESTIA program (Harnessing Emissions into Structures Taking Inputs from the Atmosphere) to expand and scale up the manufacture of limestone-based biogenic portland cement and help build a carbon-free future.

“It’s a really exciting time for our team,” said Wil SrubarProject Senior Principal Investigator and Associate Professor of Civil, Environmental, and Architectural Engineering and CU Boulder’s Materials Science and Engineering Program. “For the industry, now is the time to address this very thorny issue. We believe we have one of the best solutions, if not the best solution, for the cement and concrete industry to solve their carbon problem.

Concrete is one of the most ubiquitous materials on the planet, a staple in construction around the world. It starts with a mixture of water and Portland cement, which forms a paste to which materials such as sand, gravel or crushed stone are added. The paste binds the aggregates together and the mixture hardens into concrete.

To make Portland cement, the most common type of cement, limestone is quarried and burned at high temperatures, releasing large amounts of carbon dioxide. The research team found that replacing quarried limestone with organically grown limestone, a natural process that some species of calcareous microalgae supplement through photosynthesis (much like coral reef growth), creates a carbon-neutral way to make portland cement. In short, the carbon dioxide released into the atmosphere is equivalent to what the microalgae have already captured.

Crushed limestone is also often used as a filler material in Portland cement, typically replacing 15% of the mix. By using biogenic limestone instead of quarried limestone as a filler, Portland cement could become not only net neutral, but also carbon negative by drawing carbon dioxide from the atmosphere and storing it permanently in the concrete.

If all cement-based construction in the world were replaced with biogenic limestone cement, every year 2 gigatonnes of carbon dioxide would no longer be released into the atmosphere and more than 250 million additional tonnes of carbon dioxide would be extracted of the atmosphere. atmosphere and stored in these materials.

This could theoretically happen overnight, as biogenic limestone can “plug and play” with modern cement production processes, Srubar said.

“We see a world in which using concrete as we know it is a mechanism to heal the planet,” Srubar said. “We have the tools and the technology to do that today.”

Real-time limestone

Srubar, who directs the Living Materials Lab at CU Boulder, received a National Science Foundation Career Award in 2020 to explore how to grow limestone particles using microalgae to produce concrete with positive environmental benefits. The idea came to him while snorkeling on his honeymoon in Thailand in 2017.

He has seen with his own eyes in coral reefs how nature develops its own durable structures from calcium carbonate, a main component of limestone. If nature can grow limestone, why can’t we? he was thinking.

“There was a lot of clarity in what I needed to pursue at that time. And everything I’ve done since then has really contributed to that,” Srubar said.

He and his team began cultivating coccolithophores, cloudy white microalgae that sequester and store carbon dioxide in mineral form through photosynthesis. The only difference between limestone and what these organisms create in real time is a few million years.

With only sunlight, seawater and dissolved carbon dioxide, these tiny organisms produce the greatest amounts of new calcium carbonate on the planet, and at a faster rate than coral reefs. Coccolithophore blooms in the world’s oceans are so large that they can be seen from space.

“On the surface, they create these beautiful, very complex calcium carbonate shells. It’s basically an armor of limestone that surrounds the cells,” Srubar said.

Commercialization of coccolithophores

These microalgae are hardy little creatures, living in both warm and cold, salty and fresh waters around the world, making them excellent candidates for cultivation almost anywhere, in cities, on land or at sea. The team estimates that only 1-2 million acres of open ponds would be needed to produce all the cement the United States needs – 0.5% of the entire land area of ​​the United States and only 1 % of land used to grow maize.

And limestone is not the only product that microalgae can create: the lipids, proteins, sugars and carbohydrates of microalgae can be used to produce biofuels, food and cosmetics, which means that these microalgae could also be a source of other more expensive co-products, helping to offset limestone production costs.

To create these co-products from algal biomass and increase limestone production as quickly as possible, UNCW’s Algal Resource Collection assists in strain selection and optimization of microalgae growth. NREL provides state-of-the-art molecular and analytical tools to perform the biochemical conversion of algal biomass into biofuels and biobased products.

Some companies are interested in purchasing these materials and limestone is already available in limited quantities.

Minus Materials, Inc.a 2021-founded CU startup and team’s commercialization partner, is pushing the team’s research into the commercial space with financial backing from investors and corporate partnerships, according to Srubar, co-founder and CEO by interim. Minus Materials has already won the university-wide Lab Venture Challenge pitch competition and secured $125,000 in seed funding for the company.

The current pace of global construction is staggering, on track to build a new New York City every month for the next 40 years. For Srubar, this global growth is not just an opportunity to convert buildings into carbon sinks, but to clean up the construction industry. He hopes that replacing quarried limestone with a local version can also improve air quality, reduce environmental damage and increase equitable access to building materials worldwide.

“We make more concrete than any other material on the planet, and that means it touches everyone’s life,” Srubar said. “It’s really important for us to remember that this material needs to be affordable and easy to produce, and the benefits need to be shared globally.”

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