In a process similar to coral reef growth, microalgae could create limestone to make cement production carbon neutral – and even carbon negative

Coccolithophore bloom off Brittany, France. Nasa’s Aqua satellite captured the image on June 15, 2004. Source: <a href="https://commons.wikimedia.org/"target="_blank">Wikimedia Commons</a>, <a href=" https://en.wikipedia.org/wiki/Creative_Commons_license"target="_blank">Creative Commons License</a>

A team of scientists led by the University of Colorado Boulder (CU Boulder) was rewarded with a $3.2 million grant to develop the technology

Cement is an important material to create concrete. A team of scientists led by the University of Colorado Boulder (CU Boulder) has now found a way to make cement production carbon neutral – and even carbon negative – by pulling carbon dioxide out of the air with the help of microalgae. The process is similar to how microorganisms grow coral reefs using solar energy through photosynthesis.

In this way, buildings could be converted into carbon sinks.

There are two ways to reduce C02 emissions from cement production: One is replacing the quarried limestone as raw material with biologically grown limestone. In short, the carbon dioxide released into the atmosphere (from the ovens needed to burn the quarried limestone) equals what the microalgae already captured.

And more: ground limestone is also often used as a filler material in ready portland cement, typically replacing 15% of the mixture. Using biogenic limestone instead of quarried limestone as the filler, portland cement could become not only net neutral but also carbon negative by pulling carbon dioxide out of the atmosphere and storing it permanently in the concrete.

“We see a future in which using concrete as we know it is a mechanism to heal the planet,” said Will Srubar, lead principal investigator on the project and associate professor at CU Boulder. “We have the tools and the technology to do this today.

A scanning electron micrograph of a single coccolithophore cell, Emiliania huxleyi. Source: <a href="https://commons.wikimedia.org/"target="_blank">Wikimedia Commons</a>, <a href=" https://en.wikipedia.org/wiki/Creative_Commons_license"target="_blank">Creative Commons License</a> / Alison R. Taylor, University of North Carolina Wilmington Microscopy Facility

The idea for the research came to him while snorkeling on his honeymoon in Thailand in 2017: He saw firsthand in coral reefs how nature grows its own durable, long-lasting structures from calcium carbonate, a main component of limestone. If nature can grow limestone, why can’t we? he thought.

He and his team began to cultivate 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 largest amounts of new calcium carbonate on the planet, and at a faster pace than coral reefs.

Will Srubar holds a sample cube of white biogenic limestone produced by calcifying microalgae, known as coccolithophores. Photo: Glenn Asakawa / CU Boulder

Coccolithophore blooms in the world’s oceans are so big, they can be seen from space.

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

These microalgae are hardy little creatures, living in both warm and cold, salt and fresh waters around the world, making them great candidates for cultivation almost anywhere—in cities, on land, or at sea. According to the team’s estimates, only 1 to 2 million acres of open ponds would be required to produce all of the cement that the U.S. needs—0.5% of all land area in the U.S. and only 1% of the land used to grow corn.

And limestone isn’t the only product microalgae can create: microalgae’s lipids, proteins, sugars and carbohydrates can be used to produce biofuels, food and cosmetics, meaning these microalgae could also be a source of other, more expensive co-products—helping to offset the costs of limestone production.

Castle Rock chalk formation, south of Quinter, Kansas, USA composed of numerous coccolith microfossils. Source: <a href="https://commons.wikimedia.org/"target="_blank">Wikimedia Commons</a>, <a href=" https://en.wikipedia.org/wiki/Creative_Commons_license"target="_blank">Creative Commons License</a>

There are companies interested in buying these materials, and the limestone from algae production is already available in limited quantities.

The CU Boulder engineers and their colleagues at the Algal Resources Collection at the University of North Carolina Wilmington (UNCW) and the National Renewable Energy Laboratory (NREL) have been rewarded for their innovative work with a $3.2 million grant from the U.S. Department of Energy’s (DOE) Advanced Research Projects Agency – Energy (ARPA-E). The research team was recently selected by the HESTIA program (Harnessing Emissions into Structures Taking Inputs from the Atmosphere).

Source: University of Colorado Boulder (CU Boulder)

(29.06.2022, USA: 06.29.2022)