Making cement is one of the single largest drivers of climate change, accounting for almost a tenth of global carbon dioxide emissions. Ground-up limestone is typically cooked together with sand, clay, and other materials in kilns that are heated to around 1,500 ˚C (2,700 ˚F).
The limestone releases carbon dioxide as it breaks down, as do the fossil fuels that are burned to achieve those temperatures. For every resulting pound of cement, roughly a pound of carbon dioxide escapes into the atmosphere.
Leah Ellis came up with a better way. Sublime Systems, a startup she cofounded in March 2020, dissolves pulverized limestone in water and then applies an electric current to trigger a series of chemical reactions.
The general idea of using electricity rather than heat to break down limestone has been around for a while, though earlier attempts worked at higher temperatures. Sublime’s apparatus operates at room temperature. Lots of carbon dioxide is still released from the limestone, but it’s much easier to capture and reuse—the gas comes out of one end of the device, mixed with oxygen, while hydrogen gas is released from the other end.
This electrochemical reaction produces pure lime, a white powder made of calcium, oxygen, and hydrogen. It can then be cleanly cooked in a kiln with silicon and oxygen to make cement. Ellis and her colleagues are still considering a variety of potential business models. Because they can rely on increasingly cheap electricity from solar or wind farms, Ellis says, they’ll be able to match the prices of standard cement.