The 30 billion tons of concrete that the world makes each year have a massive carbon footprint. Cement, the main ingredient, is responsible for around 8% of global emissions—meaning that if it were a country, it would be the third-biggest climate polluter on the planet.
A California-based startup called Brimstone Energy has patented a new process that reduces the emissions in Portland cement (the most commonly used type) to zero. Used with renewable energy, the process is actually carbon negative, meaning that cement could go from being a climate problem to a solution.
Energy is one part of the problem in traditional cement manufacturing, since it runs on fossil fuels and involves heating up a kiln to more than 2,500 degrees Fahrenheit. But even if cement companies powered the entire process with renewable energy, there’s still a fundamental challenge because of the chemistry. “You start with a rock called limestone, and limestone is solidified CO2, basically,” says Cody Finke, cofounder and CEO of Brimstone, whose financial backers include Bill Gates’s Breakthrough Energy Ventures. When limestone, or calcium carbonate, is heated up to high temperatures, it creates lime, an ingredient in cement. But a huge amount of CO2 is released at the same time.
Brimstone instead uses another type of rock, calcium silicate, which doesn’t produce CO2 when it’s heated to make lime. It does create silica, a by-product that can be used to replace fly ash, a waste product that comes from coal-fired power plants that is another ingredient in typical cement. “That process was invented around peak coal, to match the energy system,” Finke says. With coal plants disappearing, the material is less available, and it’s one more source of emissions in the standard product (though carbon accounting usually doesn’t include fly ash in its calculation of the total footprint of cement).
While a handful of other startups are working on making new types of cement that also shrink emissions, Brimstone wanted to make Portland cement, the industry standard, which has a specific chemical composition. Finke says that engineers aren’t inclined to take risks on new materials, for obvious reasons. “You can either build your building out of the exact same material that every building on the planet is built, or you can build your building out of new material that no skyscraper has ever been built out of before,” he says. Because the risk is so huge if something were to go wrong—potential deaths, and hundreds of millions of dollars in costs for large buildings—engineers want to keep choosing something that’s proven.
“Entrepreneurs have struggled for decades to develop cement formulas that sequester more CO2, or provide equal strength with less mass, or mix the ingredients of cement in less-polluting proportions,” says Zachary Bogue, cofounder and managing partner of DCVC, which just invested in a seed round of funding for Brimstone along with Breakthrough Energy Ventures. “None of the alternatives worked very well: Sequestration captured just a fraction of the CO2, and messing with cement’s recipe worried regulators concerned with structural integrity.”
When Brimstone processes the calcium silicate, the startup will also end up with piles of magnesium, a rock that naturally absorbs CO2 from the air. That means, Finke says, that even if the cement is made with fossil fuels for now, it will still be carbon neutral. If it’s made with renewable energy, it “would be highly carbon negative,” he says.
The company is designing its first pilot plant now, and will partner with existing cement and concrete manufacturers. When the product comes to market, it’s expected to cost the same as traditional Portland cement, or even less. That’s the only way it will get adoption quickly, Finke says, noting, “My theory of change is that things have never gone from less expensive to more expensive. They always go from more expensive to less expensive.”