Concrete in the midst of a green makeover
On any given day, Central Concrete does what concrete companies have been doing for centuries: combining sand, gravel, water and cement to create the slurry that is used in construction.
But Central — located in San Jose, California and one of a handful of companies at the forefront of a movement to make a greener concrete — is increasingly experimenting with some decidedly new mixtures.
In one part of the plant, carbon dioxide from a chemical gas company is injected into the concrete, locking in that greenhouse gas and keeping it out of the atmosphere, where it would contribute to global warming. Elsewhere, engineers tinker with the recipe for concrete, trying out substitutes for some of the cement, which makes up about 15% of the mix and functions as the glue that holds it all together. Cement, however, is responsible for most of concrete’s carbon emissions — emissions so high that some have abandoned concrete for alternative building materials like mass timber and bamboo.
Concrete, it turns out, has a serious pollution problem.
The most widely used construction material on the planet, concrete has given the world sculptural buildings, sturdy bridges and dams, parking garages and countless other structures that surround us. But it is also responsible for about 8% of global carbon emissions. If concrete were a country, it would rank third in emissions behind China and the United States.
In the U.S. alone, 370 million cubic yards of concrete were produced last year, with nearly 40% of it going into commercial real estate, according to the National Ready Mixed Concrete Association, a trade group.
In recent decades, architects, developers and policymakers seeking to lower the carbon footprint of buildings have focused on reducing energy use by improving the efficiency of lighting, heating and other systems. Now, they are looking beyond such operational matters to the carbon emitted in the production and transportation of the materials that make up the structures, or so-called embodied carbon.
“People are getting smarter about where global-warming impacts are coming from,” said Amanda Kaminsky, principal of Building Product Ecosystems, a consulting firm in New York. “Concrete is responsible for a disproportionate chunk.”
Central, part of U.S. Concrete, a manufacturer based in Texas, is making progress tackling the problem: Lowemission concrete makes up 70% of the material the company produces annually, up from 20% in the early 2000s.
Guiding Central’s effort is U.S. Concrete’s national research laboratory at the plant in San Jose. Headed by engineer Alana Guzzetta, the lab scrutinizes technology and products developed by other companies, deciding whether to put them to the test and, ultimately, incorporate them into its operations.
Several new ways to make concrete greener employ waste carbon dioxide.
Carboncure Technologies, a company based in Halifax, Nova Scotia, invented a process that involves shooting liquid carbon dioxide into concrete during mixing. Doing so not only keeps the greenhouse gas out of the air but also strengthens the concrete and reduces the amount of cement needed.
So far, Carboncure concrete has a net carbon reduction of only 5% to 7%, but the technology has already been installed at 225 plants in the United States.
Blue Planet, based in Los Gatos, California, uses carbon dioxide collected from the exhaust stack of a power plant to produce a synthetic limestone that functions as a substitute for the sand and gravel in concrete. Although Blue Planet is still piloting its technology, Central has already used its aggregate in concrete poured at San Francisco International Airport.