Concrete Has a Big Carbon Footprint. Can Green Tech Fix It?

ARLINGTON, Va.—At concrete plants across the world, diesel mixer trucks take in a polluting blend of sand, gravel, water and cement. A facility outside Washington, D.C., is adding a new ingredient to clean up the process: carbon dioxide.

The hybrid material—known as “green” concrete—reduces the carbon footprint of one of the dirtiest industrial sectors in the world and is emerging as an alternative to carbon storage options such as underground wells and pipelines that require regulatory approval and local support.

Green concrete can’t store the billions of tons of carbon needed to meet the world’s climate goals, but it offers an immediate, partial solution to the problem of concrete emissions until other options emerge. And thanks to government incentives and investments by the likes of Amazon and Microsoft, it is having a moment.

The 2022 Inflation Reduction Act introduced billions of dollars in tax credits for companies that capture carbon at factories or from the atmosphere and then sequester it underground or in products such as concrete.

Amazon used green concrete for its Arlington headquarters and invested directly in CarbonCure Technologies, a startup that manages carbon injection for concrete plants.

Microsoft also invested in CarbonCure and is among many companies and academic researchers experimenting with new concrete production methods.

“It feels like we’re on the cusp of a second industrial revolution,” said Scott Shell, an architect and industry director at the ClimateWorks Foundation, an environmental philanthropic organization.

Concrete manufacturers produce roughly 30 billion tons each year to construct the world’s buildings, roads and bridges, emitting several billion tons of carbon dioxide in the process, or roughly 7% of the global total. Most of the emissions come from heating limestone and clay to as high as 2,700 degrees Fahrenheit to make cement, the material that binds the other ingredients together.

To make green concrete, liquid CO2 is sprayed inside the mixing drums on concrete trucks. There, it reacts with calcium ions in the cement to form calcium carbonate, a mineral that is embedded in the construction material. The rocklike calcium carbonate reduces the amount of cement that is needed, cutting the concrete’s emissions by about 5% and locking away carbon that would have otherwise been emitted into the atmosphere.

“Anytime we can reduce the amount of cement, we can reduce the amount of CO2,” said Teck Chua, director of technical services for Vulcan Materials, a construction-materials company that operates the 83-year-old Arlington plant making green concrete.

Vulcan uses carbon injections at more than 20 of its roughly 140 concrete facilities in the U.S., producing more than 1 million cubic yards of green concrete last year, or nearly 10% of its total concrete shipments. The company has several hundred plants that produce crushed stone, sand and gravel and had more than $7 billion in total sales last year.

The CO2 used in Vulcan’s concrete is purchased from an ammonia plant about two hours away.

At the concrete plant, the liquid carbon dioxide snakes through a hose across a 50-foot-tall tower where the concrete materials are dumped into the mixing trucks. When the CO2 is inserted into the mix, it resembles the fog that comes off dry ice.

Two dozen trucks take turns driving through the plant and then to nearby construction sites, a symphony of activity that begins with ingredients sourced from all over the Mid-Atlantic region.

Because concrete typically has to be poured no more than 90 minutes after it is mixed to set properly, there are already plants all over the world, an existing network that could ease carbon storage bottlenecks—without requiring pipelines.

“The beautiful thing about concrete is there are concrete producers in every single community,” said Rob Niven, chief executive of CarbonCure, the Amazon- and Microsoft-backed startup that manages the carbon injection for Vulcan.

The modest carbon injections are just the beginning of a long process to reduce concrete’s environmental impact.

CarbonCure offers other solutions such as adding carbon dioxide at other points of the production process to bolster the emissions reductions.

Microsoft is testing concrete made with limestone derived from microalgae at one of its data centers in Quincy, Wash.

Other companies are working on new concrete mixes using everything from fly ash from coal plants or slag from steel mills to further reduce cement and its emissions, while a research team at the Massachusetts Institute of Technology found it could cut 15% of concrete’s carbon footprint by simply adding baking soda to cement before it is mixed.

“There is a lot that we can do to change this recipe,” Shell of the ClimateWorks Foundation said.

But the biggest reductions will come when cement plants switch to renewable energy, while capturing and storing industrial carbon emissions before they escape into the atmosphere.

In April, cement company Heidelberg Materials signed an agreement with the Canadian government to build a $1 billion unit to capture 1 million metric tons of carbon a year at the German company’s Edmonton plant.

That is too much carbon to easily store in concrete. The proposed Canadian plant will store its emissions underground, and a separate facility under construction in Norway will pipe 400,000 metric tons of carbon dioxide a year—about half the facility’s annual emissions—to a permanent underground storage site under the North Sea.

Such larger-scale solutions are needed to drastically cut the industry’s footprint.

“I think that’s great,” Steven J. Davis, professor of earth system science at the University of California, Irvine, said about small carbon injections in concrete. “But it’s not going to completely solve the problem of all of those emissions to begin with.”

Write to Eric Niiler at eric.niiler@wsj.com and Amrith Ramkumar at amrith.ramkumar@wsj.com