This article is part of our special series "The Tough Stuff: Decarbonizing steel, cement and chemicals." Read more.
One of the first batches of modern cement was cooked up in a kitchen. In 1824, the British bricklayer Joseph Aspdin began experimenting with clay and limestone, mixing the ingredients with water and heating them in a furnace. After grinding the fusion into a fine powder and adding more water, he found he’d created a particularly strong material.
He called it “Portland cement,” after a famous type of stone from England’s Isle of Portland.
Aspdin’s concoction and later iterations have since become an essential ingredient for making concrete — one of the world’s most widely produced materials. Every year, some 4.2 billion metric tons of Portland cement are made, serving to bind together sand, gravel, water and other components that form our concrete infrastructure.
Yet cement has a critical flaw: It’s extremely carbon-intensive to make, contributing around 8 percent of human-caused carbon dioxide emissions every year. About 40 percent of cement’s emissions come from fossil-fueled kilns that can get hotter than molten lava. The other 60 percent comes from the chemical process of calcination. When limestone is heated, it breaks down into its constituent parts of calcium oxide and CO2, sending planet-warming gases into the atmosphere.
Startups and manufacturers are striving to develop new methods and formulas for making carbon-free cement. Many of these efforts are still early-stage and face major financial and technical hurdles to scaling up. Still, investors have recently poured hundreds of millions of dollars into these companies — reflecting the rising demand from construction firms and government agencies for lower-carbon concrete.
Here are six of the leading startups that are working to transform this foundational building block of modern society. (As a bonus, watch Canary Media’s video on companies capturing carbon dioxide and injecting it into concrete blocks in New York City.)
1. Curbing the Portland cement in concrete
One way to reduce concrete’s massive carbon footprint is to reduce its reliance on Portland cement. Terra CO2 Technologies is doing just that by developing a “supplementary cementing material” (SCM) made from the world’s most abundant and commonly used minerals. The company plans to start construction next year in Texas on its first full-scale facility.
Major cement and concrete companies already use millions of tons of SCMs every year, both to reduce their concrete’s carbon footprint and to cost-effectively strengthen the material. But most SCMs today are made from fly ash from coal-fired power plants or slag from steel blast furnaces. As coal plants close and steel furnaces shutter, those materials are expected to become less readily available and more expensive to get.
Golden, Colorado–based Terra CO2 is developing a drop-in replacement for traditional SCMs using a variety of silicate rocks, including granite, basalt, alluvial sand and gravel, glacial flood gravel and clay-sand mixtures. The seven-year-old company estimates that every metric ton of cement replaced by these materials will result in 70 percent lower CO2 emissions compared to pure Portland cement.
Silicate rock “for the most part doesn’t have any embodied CO2,” Bill Yearsley, CEO of Terra CO2, recently told Canary Media. Limestone, on the other hand, is “by weight about 50 percent embodied CO2.” Virtually all of that carbon is released into the atmosphere when heated and processed into clinker, the precursor to Portland cement.
2. Standard cement, innovative new process
Brimstone is aiming to kick limestone out of cement altogether.
The Oakland, California–based startup has developed a new process to make industry-standard cement using basalt and other calcium-bearing silicate rocks, in lieu of limestone. These replacement rocks are rich in calcium oxide — a key ingredient in cement-making — but, crucially, they don’t contain any carbon atoms.
Silicate minerals make up over 80 percent of the earth’s crust, said Brimstone CEO Cody Finke, who noted that the company’s innovations lie in processing common rocks to produce both cement and SCMs.
“We think it could lead to a lower-cost solution, which is essential for scaling globally on a reasonable timescale,” he said.
Along with avoiding CO2 from limestone, Brimstone’s approach has another potential climate benefit. Its cement-making process produces a magnesium compound as a byproduct, one that binds with carbon dioxide to permanently remove it from the atmosphere. Brimstone claims this step can make its process carbon-negative.
Brimstone is working to build its first pilot production plant in Nevada. Still, the four-year-old startup is likely years away from getting its cement to market. Beyond the difficulty of developing new technologies, emerging cement-makers also face the challenge of convincing regulators, standards-setters and construction workers that their products are safe to use.
To that point, Brimstone recently achieved an important industry milestone. In July, the company received third-party certification that says its Portland cement is structurally and chemically identical to conventional supplies — a key designation for a fledgling company looking to break into the risk-averse construction industry.
Any new cement formula that requires construction companies to alter tried-and-true methods could well be a deal-breaker, according to Finke. That’s why “you need to make [Portland cement] in order to decarbonize quickly,” he said.
3. Decarbonizing literal building blocks
Another cement-focused startup, CarbonBuilt, announced a key achievement of its own last month. The Los Angeles–based company said it had eliminated the use of Portland cement in its concrete blocks, drastically reducing the amount of embodied carbon associated with its products.
CarbonBuilt’s approach involves creating an alternative cement “binder” with calcium-rich waste materials from undisclosed industrial sources. To start, the company combines the ingredients with water and aggregates, which are then pressed into molds and placed inside a temperature-controlled chamber.
CarbonBuilt next flows CO2 into the chamber, driving a chemical reaction that forms solid concrete. The waste materials are “calibrated to react specifically with CO2 curing to enable carbon removal,” JJ Steeley, CarbonBuilt’s head of marketing and strategic partnerships, said in a recent interview.
The startup claims its novel technology can reduce overall CO2 emissions from concrete-making by 70 to 100 percent.
CarbonBuilt spun out of UCLA’s engineering school in 2019. In May, the company began commercial production of its sustainable concrete at a partner facility in Alabama, where an on-site biomass furnace captures and supplies CO2. The company says it now plans to produce its Portland-cement-free blocks at “commercial volume” in Alabama starting in 2024.
CarbonBuilt is also partnering with companies in Flagstaff, Arizona to combine its process with technology that removes CO2 directly from the sky. In both locations, CarbonBuilt is mainly focusing on making masonry blocks. Because the blocks are cured with CO2 in a chamber — not mixed and poured on-site — they’re “ideally suited to serve as a carbon-removal mechanism,” Steeley said.
4. Tossing out the thermal heat
Sublime Systems isn’t just searching for lower-carbon ingredients. The Boston-based startup is developing an entirely different way of driving chemical reactions that turn minerals into cement — one that doesn’t involve any fossil-fuel-burning, high-temperature kilns.
Sublime, which spun out of MIT in 2020, uses an electrochemical process instead of thermal heat. At the heart of its approach is an electrolyzer, which splits water to produce an acid and a base. A range of carbon-free rocks, minerals and industrial waste materials can be dissolved in the acid to extract calcium. The calcium is then reacted with the base, a step that creates calcium hydroxide, or lime.
Finally, the company blends the lime with silica — just as the ancient Romans once did — to produce what it calls “Sublime cement.” Once mixed with water, the cement hardens by forming “calcium silicate hydrate,” the same product that Portland cement makes. The entire process takes place at ambient temperatures; by replacing limestone inputs with alternative materials, Sublime says it can further reduce the emissions that typically occur when limestone decomposes in fiery-hot kilns.
“That’s how we avoid the kiln entirely: by using electricity,” Leah Ellis, Sublime’s CEO, told Canary Media. Ellis and her co-founder Yet-Ming Chiang, a renowned material sciences professor at MIT, both focused on batteries before pivoting to low-carbon cement.
The company completed its first pilot plant in late 2022, which can produce as much as 100 metric tons of cement per year. It’s also pursuing plans for a first commercial facility that can produce tens of thousands of tons of cement per year. In September, Sublime obtained a key industry designation that says its product meets certain performance-based standards for hydraulic cement.
“It’s a first-of-a-kind technology that we have to scale up, and that’s going to be hard,” Ellis said. “You have to get to a certain size before you’re even real to the cement or the concrete industry.”
5. Fortera: Channeling coral reefs
As Sublime draws inspiration from ancient Rome, the startup Fortera is taking its cues from coral reefs.
The San Jose, California–based company uses a novel mineralization process to capture carbon dioxide from existing Portland cement facilities. To start, Fortera combines the lime and the CO2 that emerge from the two ends of a limestone calcination kiln and dissolves them in a proprietary solvent. The end result is a form of calcium carbonate called vaterite.
Ryan Gilliam, CEO of Fortera, noted that vaterite exists in nature “only a fraction of a second” before transitioning into minerals that form shells and corals. Fortera can manufacture vaterite “at an industrial scale” but still make it “stable enough to sit on a shelf,” he said.
“We’re going from an inert rock to a reactive carbonate” that has valuable cementing properties, Gilliam added.
The company’s fundamental technology comes from Calera, a Silicon Valley startup founded in 2007 that claimed it could mix seawater with carbon dioxide from coal plants to make carbon-negative cement. It was also quite controversial, with prominent scientists casting doubt on its promises. But Gilliam said Calera’s patents have informed Fortera’s method of making vaterite.
This process also improves cement-making economics by recapturing the nearly half of the weight of the limestone that’s lost as carbon dioxide. “We effectively make double the product” compared to traditional methods, he said.
Fortera’s first demonstration project, at a CalPortland cement plant in Redding, California, will make its first commercial product in early 2024. The supplementary cementing material is designed to be blended into Portland cement at up to a 15 percent ratio, potentially lowering the product’s carbon footprint by up to 70 percent compared to standard cement on a ton-for-ton basis, he said.
Fortera can also make a “zero-emissions cement” product to completely replace Portland cement, he said — but that will require approval from cement standards bodies like ASTM before it can be used for structural cement.
Fortera told Axios in September that it is seeking project financing of more than $1 billion to install its technology in six to seven cement plants over the next five years. That’s a big jump from the $30 million Series B it raised in 2021. “We’re putting the right people around the table from a strategic-partnership perspective,” Gilliam said.
6. Turning living organisms into cement
Prometheus Materials is likewise looking to the seafloor to make its “bio-concrete.”
The Colorado-based startup is making novel cement using algae and biomineralization — a process that occurs when living organisms produce minerals to form sea shells, corals, starfish and more. The idea is that the company can biologically “grow” calcium carbonate instead of relying on quarried limestone that’s heated in ultra-hot kilns.
Prometheus Materials was formed in 2021 out of a research project at the University of Colorado. Today, the spinout’s pilot plant in Longmont produces microalgae in 1,350-liter tanks filled with simulated seawater and nutrients. LED lights surround the tanks to mimic sunlight, while Prometheus bubbles in air to feed carbon dioxide to the algae.
Next, the team harvests the algae, places it in separate tanks and, using its proprietary process, stimulates biomineralization. The resulting calcium carbonate is then dried into a powder and combined with other natural binders and aggregates to create “bio-concrete” products, including blocks, paving stones and panels.
The process can potentially eliminate 100 percent of the CO2 emissions associated with making conventional concrete blocks, Loren Burnett, the company’s CEO and co-founder, said in an interview.
Prometheus Materials plans to make its line of precast products commercially available in the first half of 2024. The company is also seeking to raise up to $35 million in venture funding to build a 35,000-square-foot factory. Eventually, the startup aims to license its bio-based technologies to established producers of cement and concrete.
“That’s ultimately how we’ll be able to impact CO2 reduction on a global scale and in a large way,” Burnett said.
Maria Gallucci is a senior reporter at Canary Media. She covers emerging clean energy technologies and efforts to electrify transportation and decarbonize heavy industry.