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A Sublime Solution to Climate’s Hardest Problem

A Massachusetts-based startup has figured out how to produce zero-carbon cement.

A cement mixer with flowers.
Heatmap Illustration/Getty Images

Over the past several months, in the sleepy city of Holyoke, Massachusetts, bulldozers have been tearing down a former paper mill. The newly leveled ground on the western banks of the Connecticut River is on its way to becoming the home of a big, industrial bet. If it pays off, what was once known as the Paper City could soon become the Clean Cement Capital — of the country, at least. Sublime Systems, a startup that has developed a process for producing the ubiquitous building material without releasing any carbon emissions, has chosen the site for its first commercial factory.

“It’s poetic justice,” Sublime’s CEO and co-founder, Leah Ellis, told me. “We’re excited about bringing clean technology to this community which has been damaged by a legacy of pollution from the old industry that used to happen there.”

The word cement is often used interchangeably with concrete, but it’s actually a key ingredient in the stuff that gets mixed and poured and hardened into sidewalks, roads, buildings. It is the glue that binds together sand, water, and gravel to form the fabric of our built environment. It’s also a major source of carbon emissions — 8% of the global total. And these aren’t like other kinds of emissions.

Scientists often split the climate problem into two categories. There’s the carbon that we know how to eliminate, like from power generation, home heating, and cars. And then there’s a group called the “hard to decarbonize” stuff — mostly emissions from industrial activities where clean solutions are still in early stages and not cost competitive. Cement is the poster child.

That’s because more than half of the emissions from cement come from a chemical reaction that’s intrinsic to its production. Cement consists of lime, silica, and water. It’s made by first heating up limestone in a kiln to more than 1,400 degrees Celsius (2,550 degrees Fahrenheit) — a level of heat that can typically be achieved only by burning coal or natural gas — to produce reactive lime. The bigger problem, though, is that limestone contains carbon, and as it heats up, that carbon is released as a gas. So even if you could heat the kiln with clean electricity instead of coal, there would still be carbon emitted by the process.

But Sublime has found another way. Ellis and her cofounder Yet-Ming Chiang — a serial entrepreneur who is also behind the buzzy battery startup Form Energy — developed a new way to make reactive lime that does not require limestone. Instead of heating up rocks in a kiln, they drive the chemical process with electric currents. This enables the company to avoid limestone and use a variety of other raw materials that do not contain carbon to produce lime.

When Ellis described her breakthrough to me, it sounded incredibly simple, like it might be obvious to anyone with a background in electrochemistry. “Why didn’t anyone else think of this?” I wanted to know.

“I believe this way of making cement is going to be obvious in retrospect, in a post-carbon world where you don’t use fossil fuels, where you’re penalized for CO2 emissions,” she told me. But she said there were three factors that led to this innovation in this moment.

First, we’ve reached a tipping point in figuring out how to decarbonize the electric grid. “That is the one key enabler. Once you’ve got a clean grid, that’s the tool to decarbonizing everything else.”

Second, Ellis happened to be in the right place at the right time. She started her career as a battery scientist and founded Sublime while completing a postdoc at the Massachusetts Institute of Technology. The interdisciplinary nature of the school, where she could collaborate with other departments, enabled her to expand the bounds of what she could do with her expertise.

And third, the technology Sublime uses to drive its chemical process — a device called an electrolyzer — has become much cheaper. Though electrolyzers have been around for a long time, they’ve recently benefited from increased economies of scale as interest in using them for applications like clean hydrogen production has grown.

For the past year, Sublime has been honing its process at a small pilot plant in Somerville, Massachusetts, a suburb of Boston. The new plant in Holyoke is designed to be many times larger — producing up to 30,000 tons of cement per year — though still smaller than the million tons per year that an average cement plant produces. The site is about half a mile down the river from a hydroelectric dam — a key consideration for the company, since it needs to power the plant with clean electricity.

The project is not yet fully financed, but Sublime has received $1 million in tax credits from the state of Massachusetts and is holding out hope for a federal grant from the Department of Energy’s Office of Clean Energy Demonstrations.

There are a number of other emerging methods to reduce emissions from cement, including alternative chemical combinations and installing carbon capture equipment on cement plants, but it’s far from being removed from the “hard to decarbonize” club. The question, as always, is whether Sublime and others will be able to produce a high-quality product — one that passes the strength and durability tests required for the construction industry — at scale, and at a competitive price.

Sublime’s product is certainly more expensive than conventional cement today. But its solution is cheaper than using carbon capture, Ellis said. Capturing the carbon from a cement plant generally requires a big increase in energy use. And while the technology has been under development for decades, it’s so far failed to be applied economically outside the natural gas processing industry. In a world where builders are required to use lower-carbon materials, or where there’s a price on carbon, Ellis thinks Sublime will have an advantage. Governments at various levels in the U.S. have already started to implement “buy clean” programs that require the use of lower-carbon cement for state and federal construction projects, so Sublime may have an edge in some markets once its Holyoke plant is up and running.

“Our process is true zero,” she said. “It just doesn’t emit.”

Emily Pontecorvo profile image

Emily Pontecorvo

Emily is a founding staff writer at Heatmap. Previously she was a staff writer at the nonprofit climate journalism outlet Grist, where she covered all aspects of decarbonization, from clean energy to electrified buildings to carbon dioxide removal.


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