Scientists hope to use algal-grown limestone for the construction of cities.
The 7% annual greenhouse gas emissions caused by cement manufacturing worldwide are largely due to burning limestone quarries. The University of Colorado, Boulder, discovered a way to use microalgae to absorb carbon dioxide from the air. This makes cement production carbon-neutral or even carbon negative.
The U.S. The U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) awarded $3.2 million to the CU Boulder engineers, their colleagues at the National Renewable Energy Laboratory NREL and the Algal Resources Collection at UNCW for their innovative work. The HESTIA program (Harnessing emissions into structures Taking inputs from the atmosphere) recently selected the research group to help advance the production of biogenic limestone-based portland concrete and to contribute to a zero-carbon future.
“This is a very exciting moment for the team,” stated Wil Srubar, principal investigator and associate professor in Civil, Environmental, and Architectural Engineering and CU Boulder’s Materials Science and Engineering Program. This is the right time for the industry to address this wicked problem. We believe we have the best if not best, solution for the cement and concrete industries to solve its carbon problem.
Concrete is one of the most widely used materials and the foundation of all buildings worldwide. Concrete is a mixture of water and portland concrete. Next, gravel, sand, or crushed rock are added. The paste holds together the particles and makes concrete.
Portland cement is the most well-known form of cement. It is made by taking limestone from large quarries and burning it at high temperatures to produce a lot of carbon dioxide. The study team discovered a net carbon neutral way to produce portland cement. This involves using biologically produced limestone instead of quarried limestone. It is a natural process that some species of calcareous microalgae use for photosynthesis, like coral reef building. The microalgae already capture more carbon dioxide than the atmospheric emissions.
Ground limestone is another common filler material in Portland cement. It usually replaces 15%. If biogenic limestone is used instead of quarried limestone, Portland cement can become net neutral and even carbon negative.
If all cement-based construction were replaced by biogenic limestone cement, a staggering 2 gigatons of carbon dioxide wouldn’t be released into the atmosphere every year. Additionally, more than 250,000,000 tons of carbon dioxide could be extracted from the atmosphere and stored in these materials.
Limestone in real-time
Subaru, the Living Materials Laboratory leader at CU Boulder, was awarded a National Science Foundation CAREER award for 2020 to study how microalgae can grow limestone particles to make concrete with positive environmental effects. He got the idea while on his honeymoon in Thailand in 2017.
He witnessed firsthand how coral reefs can grow their own durable and long-lasting structures using calcium carbonate, a major limestone component. Why can’t we grow limestone like nature? He thought.
His team started to grow coccolithophores, cloudy white microalgae which sequester carbon dioxide through photosynthesis. There is only one difference between limestone and the reality of what these organisms produce in real-time: it takes a few million years.
These tiny organisms can produce new calcium carbonate much faster than coral reefs, thanks to only sunlight and seawater. The coccolithophore flower blooms in the global oceans are so large that they can even be seen from space.
Coccolithophores can be commercialized.
These microalgae can survive in any climate, including cold and fresh water and salt and fresh water. This makes them ideal candidates for cultivation anywhere, on land or in cities. The team estimates that only 1 to 2,000,000 acres of open ponds are required to produce all the cement the U.S. requires. This is 0.5% of all U.S. land and only 1% for corn cultivation.