Mixing fly ash, a coal-burning byproduct, into concrete is a smart use of waste material, plus it offsets some of the environmental damage caused by cement manufacturing’s CO2 emissions.

With applications ranging from floors, walls, and siding to kitchen countertops, shower stalls, and landscaping stones, concrete has become one of the most widely used materials in home building. Builders like it because it’s cheap and easy to work with. Homeowners are attracted to its durability (cracks and chips are easy to patch) and its good looks (it’s possible to conjure up a variety of finishes from polished to stained to acid-etched). Concrete is also one of the most inert materials available, which means no outgassing or leaching once it’s inside a house. All that said, it takes just one look into concrete’s traditional ingredients to bring its many benefits into question.

In its most basic form, hardened concrete is made up of sand, crushed stone or gravel, water, and a binding agent known as cement. During Roman times, cement was made by grinding limestone together with volcanic rock; the Pantheon in Rome was made entirely of concrete mixed with this volcanic cement. Today, by far the most commonly used type of cement is portland cement, which was patented in 1824 by English brickmason Joseph Aspdin and named for the grayish limestone found on the Isle of Portland. It’s made by heating limestone, chalk, aluminum, iron ore, and clay to 2,700 degrees Fahrenheit, a temperature at which chemical reactions occur within the mixture, fusing it into grayish-colored, marble-size pellets known as clinker. The clinker is then mixed with gypsum and ground into a superfine gray powder that’s mixed into concrete.

This sounds like a relatively harmless process until you consider that for every ton of clinker, approximately a ton of carbon dioxide (CO2)—the most damaging of the greenhouse gases—is released into the atmosphere. At last tally, the Environmental Protection Agency (EPA) estimated the cement-making industry accounts for 4 percent of all U.S. industrial CO2 emissions.

It’s no wonder, then, that green builders and green building programs have started looking into alternatives to portland cement. Blast furnace slag from steel-producing plants, crumb rubber from recycled tires, and rice hull ash from burned agricultural waste are several alternatives that are being effectively used to supplant some of the portland cement needed in concrete. The supplement that’s getting the most attention and the most use, however, is coal fly ash.