Brimstone raises $55 million for CO₂-negative cement

Brimstone Energy has raised $55 million in series A funding for its carbon-negative route to Portland cement. Lime (CaO) is the key chemical in Portland cement, the dominant cement type for concrete. It is normally made by heating limestone (CaCO₃), a process that consumes large amounts of energy and releases CO₂, resulting in 5.5% of global greenhouse gas emissions, the firm says. Brimstone instead starts with calcium silicate, which doesn’t release CO₂ when converted to CaO. The process also yields magnesium minerals that go on to absorb CO₂ from ambient air, making the overall process carbon negative, the firm says. Bill Gates’s Breakthrough Energy Ventures and Amazon are among the investors.