By ALEC APPELBAUM

Concrete has existed since the public works of ancient Rome, when it was used to support aqueducts, and engineers love the material for its resilience in earthquakes and its utility in creating buildings of all shapes and sizes.
But as demand for concrete has soared, in part because of the rapid growth of cities in Asia, the environmental costs of the material are getting hard to ignore. “Concrete is a terrific material, which is why it’s been used for 2,000 years, said Tim Christ, an architect. “But it’s a very dirty material. The air-quality issues in China we observed during the Olympics derive from concrete construction.
That’s because every metric ton of new portland cement, the most common adhesive ingredient in concrete, releases roughly one metric ton of greenhouse gases from the kilns that bake it.
The most potent way to reduce that toll is to replace some portland cement with recycled material. Coal ash from power plants and blast-furnace slag are the easiest substitutes to find - and they tend to make concrete more valuable than portland cement alone.
Big concrete companies like France’s Lafarge and independent specialists also own and manage stocks of ash and slag. There are several ways that such companies agree to sell, store and share profits from the material.
Growing demand for the recycled material has caused it to become costlier. With America’s relatively few steel mills oversubscribed, some contractors are importing slag from Italy for their projects.
As often occurs with building techniques, the United States government gave this practice an early proving ground. It commissioned Mr. Christ’s firm, Morphosis, based in Santa Monica, California, and headed by Thom Mayne, the Pritzker Architecture Prize winner, to design the San Francisco Federal Building in 2001.
“We proposed a 50 percent slag replacement, where 15 percent was common, said Mr. Christ of the building, which was completed in 2007 .
The process was also gaining credibility on the East Coast around the same time. The green-minded Durst Organization opened Four Times Square, the Conde Nast Building, in 1999 as a showcase for environmentally responsible techniques, recalled the company’s co-president, Jody Durst.
To that end, the developer specified using coal ash in the building’s columns. The material held up so well there that Durst also used slag for the Helena, a 600-unit apartment tower in Manhattan.
By the end of that project, Mr. Durst said, contractors had come around. “I think we paid a premium, but by the end of the job the contractor said he liked the way it handled, Mr. Durst recalled. “The strength properties were superior. The developer’s standard procedure now includes ordering slag or coal ash for concrete.
More research into concrete’s environmental cost will probably change the industry further in the next 10 years. Even if ash or slag cement become universal, concrete will exact a big toll on the environment, said Christian Meyer, a civil engineering professor at Columbia University in New York, because up to three-quarters of it is aggregated sand or stone, which consumes lots of energy in production. Finding recycled material for aggregate will be the industry’s next challenge.