Concrete is the most ubiquitous synthetic material on Earth. Only water is consumed by humans in greater amounts. It's office buildings and interstate highways, modern public sculptures and the ancient Roman aqueducts.
But producing it has the dubious side effect of generating roughly 5 percent of the total greenhouse gas emissions produced by humans. So earlier this month, a leading industry group announced that it intends to slash the sector's emissions beginning this year and cut by half the amount of greenhouse gas pollution it produces by 2030.
"We feel that it's important that producers of this basic building material should be falling in place with sustainable practices," said Robert Garbini, president of the National Ready Mixed Concrete Association (NRMCA), a trade organization representing owners of the nation's 5,500 concrete plants and 55,000 cement mixer trucks.
Concrete production is a $30 billion industry each year, and NRMCA's announcement speaks to the growing demand for cleaner building materials among architects, structural engineers and material scientists.
U.S. EPA regulations are increasingly bearing down on the industry, as well, targeting harmful emissions like mercury and acid gases.
But, Garbini said, the emissions reduction target is also a reflection of concrete producers' awareness of their economic bottom line and their willingness to get in the mix of 21st-century carbon and energy reduction efforts.
Reducing electricity and water consumption, trimming fuel costs, and recycling concrete rather than casting it off to landfills save producers money, Garbini added.
Rethinking a sustainable product
Concrete is composed of four basic materials: cement, water, small aggregate such as sand, and coarse aggregate, typically stones and rocks.
Producing cement requires baking limestone at 1,400 degrees Celsius in large, 200-yard-long rotating kilns, fueled by coal, natural gas or oil. The ovens spin at an angle. As the searing heat breaks down the stone into its constituent elements, balls of silicon carbonate, three-quarters of an inch to an inch in diameter, tumble down the incline.
This coarse material is then ground and mixed with gypsum, ending up in an industrial-scale silo or on your hardware store shelf as a ready-to-mix cement.
Add water to that dry powder and combine with small and large aggregates, and you have a versatile, long-lasting building material.
It's a sustainable material, Garbini said, pointing to its resiliency to the impacts of hurricanes, tornadoes, fires and floods. Other building materials couldn't withstand those natural stress, but concrete can. Reducing the carbon intensity of the material only enhances its already low-impact qualities.
In 2005, 458 million cubic yards of concrete was used in the United States, Garbini said. That required 122 million tons of cement. China, he said, consumes 250 billion tons of cement a year.
"Even on its worse day, China is still twice as large as the U.S.," he said.
The materials that go into concrete production -- cement, water, sand and stone -- are readily available anywhere in the world. Thus concrete is here to stay as humans' primary building material.
Recycling and producing lighter materials
Because there is no alternative to concrete on the immediate horizon, it's up to the sector to become more efficient and lower its emissions.
"For any industrial process, the first 90 percent of efficiency gains is achieved by grabbing the low-hanging fruit," Garbini said. "The next 5 percent becomes exponentially more difficult, more expensive."
The low-hanging fruit, he explained, is reducing electricity and water usage and improving fuel efficiency.
Recycling concrete, Garbini said, can yield huge gains, too. Instead of sourcing stone or rock quarried for the first time, producers can reuse broken-up pieces of concrete for small and large aggregate.
Using byproducts from burning coal and producing steel, such as fly ash and silica fume, instead of cement can further reduce costs and emissions, he added.
Regulators have been slow at accepting these efforts, Garbini explained, but they need not be worried about safety and compromised durability over the long term.
He said that when an Interstate 35 bridge collapsed in Minnesota, killing 15 people, the state's Department of Transportation selected a bridge replacement proposal that used half the amount of cement in a typical design. Instead of cement, the construction company used silica fume.
State officials were impressed and have become open to efforts to reduce cement use, Garbini said. But across the nation, he added, other states remain skeptical.
"The quality of concrete was not diminished by the company's sustainability commitment," he said of the I-35 project. "It's still a long, long road of education to get other agencies to understand this."
Green cement coming at 'blinding speed'
While producers attempt to pick that low-hanging fruit, scientists at the Massachusetts Institute of Technology are searching for methods to produce cement that emits far less greenhouse gases.
Hamlin Jennings, an adjunct professor of civil and environmental engineering at MIT and head of the university's Concrete Sustainability Hub, said half of the greenhouse gas emissions generated from concrete occur during the production of cement.
The efficiency of kilns is about as high as it can go, he said, so his research is focused on developing a method for churning out a less carbon-intensive cement from a process that's been around for millennia.
Jennings said deciphering the nano- and atomic-level reactions that occur during cement production might reveal chemical alternatives that require less energy, emit lower amounts of greenhouses gases and improve durability.
"We're exploring these possibilities using quantum mechanics, trying to determine the energetics of the bonding of the chemicals," Jennings said.
Five or seven years ago, he said, researchers warned him that understanding cement hydration would be a 20-year effort, perhaps one that would consume a researcher's lifetime.
Progress toward developing so-called green cement is "moving at blinding speed," he said, adding, "Things are emerging, and they may provide some breakthroughs."
Correction: A previous version of this story misstated the amount of cement China uses. The country, Garbini said, consumes 250 billion tons of cement a year.