For the last two centuries, nearly all the concrete used in buildings, bridges, dams and roads has been held together with a key ingredient: Portland cement. The limestone and clay fusion is ubiquitous, inexpensive — and extremely carbon-intensive to produce.
Recently, startups and established construction firms alike have begun devising lower-carbon ways of making concrete, the world’s most widely used building material. Pilot-stage and commercial-scale facilities are attempting to prove out technologies that could help reduce emissions from one of the economy’s hardest-to-decarbonize industries.
Now, a new, high-tech initiative is getting underway on a dusty industrial lot in Flagstaff, Arizona.
Earlier this month, Block-Lite, a family-owned masonry business, announced plans to produce concrete by combining CarbonBuilt’s alternative-cement process with Aircapture’s technology that removes carbon dioxide from the sky. The three partners received a $150,000 grant from the 4 Corners Carbon Coalition, a network of local governments in Colorado, Utah, Arizona and New Mexico, to design the first-of-its-kind manufacturing facility.
The companies plan to retrofit one of Block-Lite’s facilities and begin production in 2024. Initially, the plant will churn out 30,000 metric tons per year of concrete, while also removing some 500 tons of atmospheric CO2 annually. The novel processes are expected to reduce overall emissions from concrete-making by over 70 percent, according to the developers.
Experts say that initiatives like the one in northern Arizona offer a promising solution for immediately slashing at least some of the industry’s enormous emissions footprint. Today, the world makes more than 4 billion metric tons of cement per year and, as a result, cement production accounts for between 7 and 8 percent of global CO2 emissions. That’s more than the annual emissions from air travel and maritime shipping combined.
However, neither direct air capture nor CO2-cured concrete will be able to fully address the technical and financial challenges involved with transforming the globe-spanning, energy-intensive industry. Even if both technologies become mainstream, concrete producers will still need to develop other methods and materials to make the literal building blocks of our modern economy.
“A lot more needs to happen [to achieve] the complete decarbonization of cement and concrete,” said Anu Khan, deputy director of science and innovation for Carbon180, a nonprofit focused on carbon dioxide removal.
Still, she added, “If there’s anything we can do today to decarbonize the industry, we need to take those steps.”
Cutting the carbon out of concrete
To clean up the construction industry, companies are working to tackle the two main sources of cement’s planet-warming emissions.
About 40 percent come primarily from burning coal, gas and oil to heat kilns to scorching temperatures — up to 1,450 degrees Celsius, hotter than molten lava. (A smaller fraction stems from using electricity to power machinery and transporting products by truck.) Companies are experimenting with using hydrogen, electric plasma heating and biomass in order to replace fossil fuels.
The remaining 60 percent of cement’s emissions come from the chemical process of calcination. When limestone is heated, it breaks down into its constituent parts of calcium oxide and CO2.
One way to deal with those emissions without altering today’s cement-making methods is to capture carbon from the cement plant’s flue gases. Major manufacturers, including Mitsubishi Heavy Industries, are developing test units that can be installed directly on existing equipment. But some startups — such as CarbonBuilt, Brimstone Energy and Sublime Systems — are trying to avoid creating such emissions in the first place by ditching limestone or drastically reducing the use of Portland cement.
“If we want to decarbonize concrete in the near term, we have to use much less cement,” said Rahul Shendure, CEO of CarbonBuilt. “Other ways we can tackle that problem are going to take too long and be too capital-intensive.”
CarbonBuilt’s own approach involves creating a new kind of cement using calcium-rich industrial waste materials, which are then combined with water and aggregates. The mixture is pressed into molds and placed inside a temperature-controlled chamber. CarbonBuilt then flows CO2 into the chamber, driving a chemical reaction that forms solid concrete — and permanently traps carbon in the blocks. Gaurav Sant, CarbonBuilt’s founder, has previously likened the process to baking cookie dough in a convection oven.
After forming in UCLA's engineering school in 2014 and then spinning out in 2019, CarbonBuilt has demonstrated its technology at coal- and gas-fired power plants in northern Wyoming and central Alabama, using the plants’ CO2-rich flue gases to cure concrete. In 2021, it won $7.5 million from the NRG Cosia Carbon Xprize for its technology.
For the new project in Arizona, CarbonBuilt will source its CO2 directly from the sky. Aircapture, based in Berkeley, California, plans to build an array of direct air capture machines on Block-Lite’s property, with each unit standing about 15 feet tall and stretching 6 feet wide. CarbonBuilt will then use that CO2 as an input for making concrete.
The project comes as efforts across the United States are ramping up to tackle the climate impacts of heavy industrial sectors.
In early March, the Biden administration announced a $6 billion initiative to accelerate technologies that decarbonize the production of cement, chemicals, steel and other raw materials. California, New Jersey and New York — plus the cities of Austin, Honolulu, New York City and Portland, Oregon — have all adopted some form of “buy-clean” policies to drive demand for lower-carbon products used in government-funded construction projects.
Such initiatives are chipping away at the construction industry’s historical reluctance to add new equipment or source different ingredients for fear of raising costs or invoking customers’ concerns about the material’s safety, said Ankit Kalanki, a manager with the Carbon-Free Buildings program at RMI, a clean-energy think tank. (Canary Media is an independent affiliate of RMI.)
“A lot of startups are beginning to emerge and trying to collaborate with industry, and industry is equally receptive to these new ideas now,” he said.
Along with designing the Arizona project, CarbonBuilt is now commissioning its first commercial plant in Childersburg, Alabama. The facility is similar in size to the one in Flagstaff, though it doesn’t involve direct air capture. Instead, CarbonBuilt is sourcing its CO2 from forestry waste that’s processed in an onsite furnace. The startup is partnering with Blair Block, a local masonry manufacturer, to retrofit an existing facility to make the low-carbon blocks.
Shendure said the Alabama project has cost a couple of million dollars to build, though he expects future project costs to decline with experience.
Using cement for CO2 storage
The Block-Lite initiative isn’t the only effort to pair CO2-harnessing technologies with concrete production.
Heirloom uses limestone slabs to absorb CO2 from the ambient air, then crushes it in a kiln and separates the gas from waste rock. CarbonCure, meanwhile, injects CO2 into wastewater that has been used to clean out concrete trucks, producing a mineral that strengthens concrete. In February, the two companies said they successfully combined their technologies in a demonstration at a Central Concrete Supply facility in San Jose, California.
These “CO2-to-concrete” projects are gaining steam not just because they supply a source of CO2 for making building materials — they also provide a place to put the carbon emissions once they’re captured. Most direct air capture projects today are small-scale and sprinkled across the country. They aren’t yet harnessing large enough volumes of CO2 to be able to access underground wells for permanent storage.
“Concrete [production] is a really smart way to get around what is potentially a bottleneck around carbon storage,” Khan of Carbon180 said. “You can put CO2 from direct air capture into [concrete], and you can do so in a way that provides really durable storage for hundreds of years.”
Direct air capture faces other barriers to achieving widespread use, not least of which is its enormous energy consumption. Removing a ton of carbon from the sky requires around 1,200 kilowatt-hours of electricity, according to an analysis by MIT Energy Initiative’s Howard Herzog. That’s more than the average American home uses in a month. If that electricity comes from fossil fuels, not renewable energy, it could boost the project’s overall emissions, despite removing CO2.
In Arizona, Aircapture’s machines will consume a mix of electricity from Arizona Public Service’s grid — which runs on coal, gas, nuclear and, to a lesser extent, renewables — as well as from Block-Lite’s on-site 157-kilowatt solar array.
Aircapture is “aiming to increase the proportion of solar (or other renewables) as much as possible, and plans to work with CarbonBuilt and Block-Lite on that,” a representative said by email. The startup declined to disclose its energy-consumption data but noted that it uses an “energy-efficient DAC design.”
Beyond energy use, another potential limit to direct air capture’s role in decarbonizing concrete is that the facilities may eventually suck up more CO2 than the industry can reasonably accommodate. Using too much of the gas to make concrete can potentially weaken the strength of building materials or extend the time it takes for the concrete to settle, according to RMI’s Kalanki.
While he said it’s “very encouraging” to see more projects combining captured CO2 and concrete, more research is needed to better understand how the resulting materials will perform over time, and more innovations are required to drive down the costs of operating such facilities.
“We need a lot of these early demonstration projects to really prove that these products, these technologies, are durable and can meet the same requirements as traditional cement and concrete,” Kalanki added.