Boston startup raises $40 million to develop new low-carbon cement technology

(Bloomberg) — Boston-based startup Sublime Systems has raised $40 million to develop breakthrough technology for producing low-carbon cement.

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The cement industry accounts for 8% of global greenhouse gas emissions. Achieving global climate goals under the Paris Agreement will require dropping them to zero within decades.

However, cement has proven to be one of the more difficult industries to clean up, as it is much more expensive to manufacture low-carbon cement using existing processes than to produce traditional equivalents.

Another major difficulty is that emissions from cement are not just from the fossil fuels burned during production. Most cement today is made by throwing limestone and sand into coal-fired kilns, which produce gases that warm the planet. Even if coal is replaced by a cleaner energy source, the chemistry of limestone means some carbon dioxide will still be released.

The most favored solution among large cement companies seeking to reduce emissions is to build carbon capture plants that capture the carbon dioxide released by factories before it enters the atmosphere. However, the process is energy-intensive and can more than double the cost of the final product.

Sublime says it has come up with a process that reduces both energy use and carbon emissions. The breakthrough was made in the lab of Yeming Jiang, a professor at the Massachusetts Institute of Technology. The latest round of funding comes from Lowercarbon Capital, along with previous investors Energy Impact Partners and MIT’s The Engine. Sublime has also signed up Southeast Asian company Siam Cement Group as a strategic investor.

Sublime’s solution consists of splitting the cement manufacturing process into two steps. The first step is to make calcium — a key element in limestone — in a form that can chemically react with silicon — a key element in sand. Sublime reduces energy use and carbon emissions in this step by avoiding limestone and using electricity instead of coal-fired heat.

The second step, which Sublime has not yet mastered, involves processing the silicon into a reactive form. Silicon in sand is very stable, so making it reactive isn’t easy, but Chiang says some early research suggests it can be done. For the first batch of cement, Sublime used active silica available in nature. Chiang would not confirm exactly what the startup is using, but examples include calcined clay, coal ash, steel slag and minerals such as olivine.

The advanced cement will then be sold as a mixture of activated calcium and activated silica. When you add water and gravel, a chemical reaction begins and the mixture begins to harden into concrete. Sublime CEO Leah Ellis says the hardened cement meets or exceeds the standards set by the industry.

“It’s a creative approach,” said Venkat Viswanathan, a materials science professor at Carnegie Mellon University and the man behind another cement startup that uses electricity. “It’s unclear exactly how much energy consumption is reduced.”

Currently, Sublime has a small facility that can produce 100 tons of this low-carbon cement. It will use some of the $40 million it has raised to scale it up to 40,000 tons a year by 2025, depending on its success in making active silicon from nature. Meanwhile, Chiang said Sublime is working to scale up the process for making synthetic active silicon. There is still a long way to go, and Sublime aims to have a commercial-scale factory by 2028.

“Roman cement is proof,” Jiang says, that Sublime’s technology works. “You can make a very durable, long-lasting hydraulic cement.”

— With assistance from Mark Bergen and Christine Driscoll.

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