Changing up the recipe to make low-carbon cement

The chemical reaction at the heart of today’s cement-manufacturing process is a major reason why the production of this essential building material accounts for an estimated 7 percent of global carbon dioxide emissions from energy and industrial sources.

But a small group of tech entrepreneurs says they’ve found a better way. Cody Finke, co-founder and CEO of Brimstone Energy, is one of them.

“Our dream is to decarbonize cement, and we want to do it as fast as possible,” said Finke, who holds a doctorate in engineering and applied sciences from the California Institute of Technology.

Today, producers use limestone, a common rock, as the basis of cement. They mine it, grind it and then heat it up to temperatures roughly a quarter as hot as the surface of the sun. This drives carbon dioxide out of the rock and into the atmosphere, where it will hover for a century or more, soaking up and re-radiating the sun’s energy.

Limestone is a form of calcium carbonate, a chemical compound with the formula CaCO₃, made up of three main elements: calcium, carbon and oxygen. Extracting the calcium and oxygen needed for cement leaves behind the single carbon atom and two oxygen atoms — CO₂. Fifty to 65 percent of the CO₂ emissions from cement production result from the fact that the source rock is limestone, not from heating cement kilns to high temperatures.

Brimstone’s alternative recipe swaps out this limestone for calcium silicate rocks, which are also very common. The silicates contain the same calcium oxide, commonly known as lime. But ​“there is…no CO₂ in the rock,” Finke said.

Using an alternative source rock also allows Brimstone to lower the kiln temperatures — another way the company can reduce CO₂ emissions. Typical cement production requires temperatures of approximately 900 degrees Celsius for one key part of the process and 1,450 degrees Celsius for the other. Brimstone’s process, in contrast, requires temperatures above 500ºC for only 20 percent of its heat energy. Finke declined to specify further, citing a pending patent application.

Because of these lower temperature requirements, Brimstone can use electric kilns for most of its process instead of kilns fired by coal, petroleum coke or natural gas. For now, the company still expects to burn a fossil fuel for the hottest 20 percent of its process or use hydrogen if it’s available. It’s possible to electrify the entire process, Finke said; it’s just uneconomical to do so today. But using an electric kiln for even part of the production process lowers carbon emissions.

Brimstone Energy is an early-stage startup that’s currently raising money to build a pilot production facility. It raised an initial $2 million starting in 2019 from the U.S. Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-E), the National Science Foundation and a number of other sources. In the last year, it has rounded up another $5 million in seed money through Bill Gates’ Breakthrough Energy Ventures, DCVC, impact fund Accelr8 and the Collaborative Fund. The company is pursuing external verification of its novel cement-making process.

The fact that the startup received ARPA-E and NSF funding is meaningful, said Peter Saundry, a physicist who teaches environmental technology at Johns Hopkins University. The NSF proposal would have subjected the company’s chemistry to a peer-review process, he said, ​“a good sign that at least from a basic science point of view, they’ve got something that makes sense.”

Saundry also noted Finke’s doctorate from Caltech. ​“Not something to sniff at,” he said. The company’s other founder, Chief Technology Officer Hugo Leandri, worked as a research engineer at Caltech.

New recipe, new business model

Startups in this realm are chronically underfunded, and some fail due to insufficient funding, said Lionel Lemay of the National Ready Mixed Concrete Association. Others that do not fail outright never succeed in achieving widespread adoption of their product; Lemay cited the low-carbon cement LC3 as an example.

Brimstone has an answer for those who are skeptical about its prospects: The company will sell another product in addition to cement.

Cement companies tend to be vertically integrated. The largest cement companies operating in the United States, Cemex and Heidelberg Cement, for example, own cement plants plus quarries where they mine the rocks, or aggregate, that you see when you closely examine a slab of concrete.

To save money or to lower carbon emissions, cement makers may buy materials that allow them to use less cement in the final concrete product (supplementary cementitious materials, or SCMs in industry-speak). Those alternative materials often include fly ash, which is a powder-like byproduct of burning coal in a coal-fired power plant, or slag from a blast furnace.

Brimstone can provide both the cement and SCMs from the same calcium silicate rock, according to Finke. He said these will be ​“chemically and physically identical” to the fly ash and slag that are currently commonly used; they have been successfully tested against standards from the standards group ASTM International.

Finke envisions Brimstone’s process being adopted by existing cement manufacturers. ​“We work with cement or concrete company X, and that company owns an aggregate quarry, and we could turn the rocks that they already mine for aggregate into cement and supplementary cementitious material,” Finke said.

One challenge is that the concentration of lime in the calcium silicate rocks is lower than that of limestone. That requires processing more rock — anywhere from 30 to 250 percent more — to yield the same amount of lime. But because Brimstone’s process doesn’t include fly ash, the total tonnage of material required is comparable to that used by other cement makers, according to Finke.

Introducing new cements into the market is notoriously difficult, said Ian Riley, CEO of the World Cement Association. Manufacturers of structural concrete are limited by different construction standards and specifications as to what components and ingredients can be put into it.

“It’s not really a question of whether it works. It’s a question of whether everybody believes it is going to work,” he said. He gave the example of the company Calera, which had an alternative cement product ​“that worked perfectly, but they just could not make progress on the regulatory side.”

A recent report from the American Economic Liberties Institute reached the same conclusion, pointing to regulatory bodies as a key bottleneck holding back the development of innovative cement products. ​“Prescriptive standards, outdated or restrictive procurement criteria, and cozy industry relationships make it difficult for startups to receive significant procurement contracts or government grants,” the report states.

But there is a growing consensus that solutions to clean up cement are needed. It’s a topic of discussion everywhere from cement industry conferences to think tanks to the recently concluded COP26 climate summit.

“It sounds great,” Lemay said of Brimstone’s proposed business model. ​“I hope it’s a reality; I really do.”