Six Startups Take CO2 Out of Cement and Concrete – GWC Mag

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.