During a trip to Thailand on his honeymoon, while swimming above a coral reef, Wil Srubar was thinking about the beauty of nature—but he was also thinking about cement. In the reef, microalgae were growing calcium carbonate, a key material used to make cement, the glue that holds together concrete. Srubar, a materials scientist who teaches at the University of Colorado Boulder, realized that this type of algae could help lower the concrete industry’s massive carbon footprint.

“I had been doing a lot of work with alternative cements,” he says. “But it really wasn’t until I was snorkeling on my honeymoon that I started to really think about how these organisms, these macro- and microscopic algae, grow these structures. All they need is sunlight, seawater, and CO2.” Grown at a large scale in ponds, the microalgae, called coccolithophores, could begin to supply the cement industry with another source of calcium carbonate.Right now, cement production is responsible for around 8% of global emissions, more than double the emissions from the airline industry. Some of those emissions come from energy use. But the basic chemistry of the process is also an environmental problem, since making cement involves heating up limestone (composed of calcium carbonate), which triggers a chemical reaction, releasing huge amounts of CO2. If the limestone could be made with algae—and capture CO2 as it grows—Srubar knew that part of the process could become carbon neutral.

If this type of “biogenic” limestone is used along with other changes in cement production, including a switch to clean energy and carbon capture, making cement could actually be carbon negative, meaning it could capture more CO2 than it produces.

Several startups are working to reduce emissions in cement and concrete in different ways. Brimstone Energy replaces limestone with a different rock that doesn’t emit CO2. Another company, Biomason, uses bacteria and other materials to form calcium carbonate. Others partially replace cement with different materials, or embed captured CO2 into concrete.

Srubar, who spun out his research on microalgae into the startup Minus Materials, maintains that using microalgae to make limestone has unique advantages. First, unlike some cement alternatives, the final product is Portland cement, the industry standard, which has a specific chemical composition. “We don’t have to change a thing about Portland cement production,” he says. No new equipment is needed, and the product already meets existing standards. It can cut emissions 60%, or if combined with other changes, by more than 100%. And it can compete on cost. “We have a pathway to cost parity of traditional limestone,” Srubar says.

Like other microorganisms, the algae grow quickly, doubling within hours. The startup estimates that it will be able to grow between 25 and 50 tons of limestone per acre, per year. The cost has the potential to be low because the algae can also create other products; the algae itself can be turned into ingredients for food or cosmetics or into biofuel. “They’re very rich in fatty acids and lipids and proteins and sugars that are precursors to some high-value chemicals,” he says.

Minus Materials, which has received an undisclosed seed investment from the global venture capital firm SOSV, plans to launch more fundraising this fall, and hopes to begin pilot-scale production within 12 to 24 months. It’s already beginning to supply small samples to partners in the cement industry and companies, such as Microsoft, that are searching for new ways to decarbonize. Srubar envisions eventually forming a network of global production sites that can supply the biogenic limestone locally, around the world. “What we want to demonstrate with our pilot at-scale cultivation is that this is a replicable model anywhere in the world,” he says.