The construction industry has a climate problem: cement.

Cement is an essential ingredient in the 30 billion tons of concrete used worldwide every year. But it’s also responsible for about 8% of the world’s carbon emissions. 

Demand for cement is high. It’s the second most consumed resource in the world after water—but in order to adhere to the Paris Climate Accord, annual emissions need to fall 16% by 2030.

A Somerville-based startup is aiming to help the industry reach that goal. Last month, Sublime Systems’ carbon-free cement was used commercially for the first time, right here in Boston. Sublime Systems CEO Leah Ellis joined GBH’s All Things Considered host Arun Rath to discuss the game-changing feat. What follows is a lightly edited transcript.

Arun Rath: To start off, would you mind giving us a quick breakdown of why exactly cement is so harmful to the environment? Where is the carbon working into the equation?

Leah Ellis: It’s all about how cement is made. Cement is made in massive fossil fuel-fired kilns. As you said, it’s the world’s biggest man-made industry. These kilns are often fired with coal. They operate at extremely high temperatures—around 1400 degrees Celsius—and that’s a temperature that you can only achieve by burning things like coal.

Also, the other half of cement emissions is mineral emissions from limestone. That’s calcium carbonate and it’s the key ingredient in cement. Calcium carbonate is 50% by weight CO2.

The way cement is made is by taking the calcium carbonate heated to these very high temperatures, at which point it decomposes. That combination of fossil fuel emissions from the heat and the mineral emissions from the limestone is what makes cement such a big CO2 problem, and also one that’s very difficult to solve.

Rath: It sounds complicated—talk to us about that. How do you take on the carbon byproduct?

Ellis: Sublime Systems and this technology spun out of my postdoctoral work at MIT. My co-founder and I are both electric chemists, so we have experience with battery technologies and electrochemical systems. Our idea was thinking about how we might use renewable energy—which we know has become more abundant, inexpensive and available—to eliminate the CO2 emissions from cement.

I like to call this technology the “electric vehicle of cement-making” because we’re replacing those high temperatures and combustion in the fossil fuels with an ambient temperature, an electrochemical way of breaking down cementitious materials into making cement.

Instead of thermally decomposing limestone—calcium carbonate—at those very high temperatures, we’re taking any rock that contains calcium and digesting it at ambient temperature with our system that’s powered by electricity. That’s how we break down those inert rocks into active cement powders that can be a drop-in replacement for the cement that’s made in a kiln, bypassing both the emissions for heat and the emissions from the minerals themselves.

Rath: That’s just remarkable. If you’re creating cement without this immense heat, is that a cheaper way to make it?

Ellis: Ultimately, yes — especially if you’re comparing it to other ways to make low-carbon cement. For any other way to make low-carbon cement, you’re going to have to add on carbon capture, either for the fuel emissions or from the limestone emissions. This way of doing it can not only be more energy efficient—which is important because we’re accelerating the deployment of renewables—but it’s not making a dent in the global appetite for energy. Any new technology has to be more efficient.

Also, we don’t have that extra cost of carbon capture. We’re simply replacing the kilns with a new system that costs about the same, so ultimately, it can be cost-competitive and have lower embodied energy.

Rath: Is the end product, the cement that you get out of this, different in any way from cement produced the conventional, carbon-emitting way? Is it stronger, weaker, or different at all?

Ellis: It’s similar in some ways, different in others. It’s designed to be a drop-in replacement for Portland cement, so in the key aspects of performance, such as strength, set time, flow and durability, we expect it to be the same and better.

Of course, cement adheres to very strict industry standards set forth by the ASTM International, the American Concrete Institute, and local building codes that specify how you’re supposed to measure these properties to make acceptable building materials, so our cement has very similar chemistry.

At the end of the day, it sets and hardens into the same concrete we use today, but in its fresh state, because it wasn’t made inside a kiln, it has a different crystal structure than the cement that’s made in a kiln. But that’s what we believe is the only significant difference, which really doesn’t have to do with performance at all.

Rath: Are you at the end of this process, or is it still something in the works? Are you still tweaking the formula and the process to work on this more?

Ellis: The innovation continues, but that’s not stopping us from deploying the materials. We started this journey about six years ago at MIT in 2018 with a proof of concept. In 2020, about four years ago, we spun out Sublime Systems to commercialize this technology.

Since then, we’ve gone from a gram of cement to 250 tons per year of cement-making capacity. We’re starting to deploy the cement locally, with partners in Boston and all over the country, but there’s still a long way to go until we can get to the scale at which we can compete on costs. The average cement plant makes about a million tons of cement per year. Obviously, at 250 tons of cement per year, we’re nowhere near being able to capture market share, but it’s a very important proof of concept.

We are gearing up to do a new level of scale. Now that we’ve validated our pilot plan and have about 7,000 hours of uptime over the past year, we are designing our next-sized plant. We do plan to build this year in Massachusetts, in the city of Holyoke, with a capacity of about 30,000 tons a year. That will be our first, industrial-scale and commercial plant.

From there, that will retire all the scale-up risks so that we can deploy this technology internationally, especially in the regions that we need it the most, which are the developing countries because they’re going to be using most of this cement in the coming decades, as their population grows and becomes more urban.

If we don’t act now to develop renewable technologies, they’ll be building cement plants with the legacy technology, which is very carbon intensive. These big cement plants are meant to last 75-100 years, so we have a window of opportunity now to work quickly and get a better solution in place that will avoid about 100 years of cement emissions.