PATH - A Public Private Partnership for Advancing Housing Technology

Greening Concrete

Concrete has a major green attribute: it's strong and durable. But it also has some strikes against it from a green standpoint: cement production uses a lot of energy, which generates greenhouse gases. In addition, concrete requires aggregate, and that doesn't have to come from a quarry, it can be recycled from some other waste stream and save space in a landfill.

Enter cement substitutes and concrete aggregates. They make concrete a very green product. That's why they're on the 2007 PATH Top 10 list.

Recycled Cement Substitutes

[IMAGE: Fly ash, slag, and calcined clay or calcined shale are used in general purpose construction, such as walls for residential buildings. Courtesy PCA (www.cement.org).] Cement substitutes like coal fly ash, blast furnace slag, and other mineral admixtures can replace up to 75% of the cement in concrete, while improving the quality of concrete and reducing costs. Cement substitutes also use a byproduct that might otherwise end up in a landfill.

• Produced by coal-fired power plants, fly ash provides a durable alternative to regular cement. How durable is it? Think of the Pantheon. Built 2,000 years ago using fly ash, it still stands today. That kind of durability makes fly ash a very green alternative to cement.
• Slag, a byproduct of steel production, is another cement substitute that lowers concrete's CO2 footprint. Slag is more lightweight than cement, which increases its strength-to-weight ratio. It is often easier to find than fly ash.

Cement substitutes are widely available from most concrete suppliers. Just ask specifically for them.

You can also get one point on the LEED for Homes rating system, the U.S. Green Building Council's Leadership in Energy and Environmental Design program that rates "green" homes, for replacing at least 30% of your foundation's cement with a recycled cement substitute. (Credit MR 2.2)

[IMAGE: Recycled-concrete aggregate. Courtesy PCA (www.cement.org).] Concrete Aggregate Substitutes

Various solid wastes like fiberglass, discarded glass, and granulated plastics can substitute for sand, gravel and stones in concrete. This keeps the wastes out of the landfill, eliminates the need for producing virgin materials, and can produce an attractive and unique concrete.

The use of these recycled aggregates is becoming especially popular for interior uses, such as concrete floors and countertops.

Again, you can get a point on the LEED for Homes rating system for replacing at least 25% of your foundation's concrete aggregates with a post-consumer recycled material. (Credit MR 2.2)

Content updated on 3/20/2007

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