Limestone is a key ingredient in cement, but adding it to the mix is very energy-intensive. It first needs to be extracted from the earth, then pulverized, roasted and processed at extremely high temperatures, which involves burning large amounts of fossil fuels and thus releasing stored carbon. Scientists have made promising progress on a more sustainable approach by replacing limestone with discarded clay or volcanic rock, but the latest progress came via a snorkeling trip in Thailand in 2017.
While visiting coral reefs in the region, materials scientist Wil Srubar was inspired by the way these structures naturally form from calcium carbonate, a key component of limestone. He wondered if this natural process could be used to produce the material in a more environmentally friendly way, which led him and his team to a microscopic algae called coccolithophores.
These tiny organisms naturally sequester and store carbon dioxide through photosynthesis, using sunlight and seawater to convert it into calcium carbonate faster than coral reefs can. They also live in warm, cold, brackish and fresh water, which heralds the potential to cultivate them on a global scale.
“On the surface, they create these very complex, beautiful shells of calcium carbonate,” Srubar said. “It’s basically the limestone ‘armor’ that surrounds the cells.”
The research team used coccolithophores to produce biologically grown limestone that was used to replace quarried limestone to create concrete with a lower environmental impact. These “look, feel and behave exactly like concrete,” Srubar said, but the cement used to make it is net carbon neutral, or even carbon negative, because the associated carbon dioxide emissions are lower than those captured by microalgae quantity.
“This is a very exciting time for our team,” Srubar said. “It is time for the industry to address this very evil problem. We believe we have the best solution, if not the best solution, for the cement and concrete industry to address its carbon problem. “
Srubar and his team just received a $3.2 million grant from the U.S. Department of Energy to continue developing the technology and research ways to scale up biolimestone-based cement production.
Scientists have calculated that 10-20,000 acres of open-air ponds are needed to grow enough microalgae to meet the cement needs of the United States, which they note is just one percent of the land used to grow corn. Applying the technology globally could reduce carbon dioxide emissions by 2 billion gigatons per year, they say. Perhaps most encouragingly, the bio-limestone is described as being “plug-and-play” with current cement production processes, and could theoretically be implemented rapidly overnight.
“We see a world in which the use of concrete as we know it is a mechanism to heal the planet,” Srubar said. “We have the tools and technology to do that today.”
