Reducing concrete’s carbon footprint: The Silo Eco-Home and some other high-tech solutions

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The Silo Home used recycled concrete from debris left after a devastating tornado in 2007.

One way to reduce the carbon footprint of concrete is to change the typical mixture in its production. Another is to use recycled aggregate, and the Silo Eco-Home in Greensburg, Kansas did both.

Greensburg, in southwest Kansas, is home to about 1,400 people. It was devastated by an EF5 tornado in 2007. After the tornado, the city embraced the idea of rebuilding as a green community. The $300,000 silo house is the first of 12 demonstration homes, called the chain of eco-homes, each of which will showcase a particular kind of green building technique. The 2,000-square-foot silo home is all pre-cast concrete, walls and roof, and can withstand winds of 200 mph.

The 2007 tornado created a lot of debris, an estimated 87,000 truckloads, and much of it was concrete from foundations, basements, driveways, and sidewalks.

The image here (borrowed from the Portland Cement Assocation) shows the typical mixture for concrete—6% air, 11% Portland cement, 41% gravel or crushed stone (coarse aggregate), 26% sand, and 16% water. The Silo Eco-Home replaced the 41% gravel component with crushed concrete salvaged after the storm. The result was a less energy intensive concrete building. The aggregate was sourced no more than a few miles away from where it was crushed; no new material was extracted to produce the concrete; and thousands of pounds of concrete was diverted from the landfill. The Silo Home also used 30% fly ash (a by-product of coal combustion) in the cement mixture.

The Silo Home.

The outside of the silo home was treated with Dryvit System’s Mojave E, a premixed, lightweight finish that provides better performance characteristics than standard elastomeric finishes over stucco and concrete, is 100% acrylic-based and offers improved flexibility, ease of application and superior crack resistance. The company claims it is an energy efficient choice for a cast concrete building’s exterior because it contains 20% post-consumer recycled glass, is 40% lighter than traditional finishes, and will provide superior durability for years to come.

The big culprit and some high-tech solutions

Cement, which binds the concrete like a glue, is one of the most carbon-intensive materials to make. It produces one ton of CO2 for every ton of cement made. Portland cement is the most common type of cement. Its production involves crushing raw materials and heating them up to 3,000 degrees Fahrenheit.

Ways to reduce the amount of Portland cement necessary in concrete include replacing a portion with fly ash, using a paste made from fly ash called geopolymer, and using slag, a byproduct from smelting ore.

Ceratech, Inc., a Baltimore company, offers cement products that are completely free of Portland cement. Their products contain industrial waste like fly ash and wood ash, and they add a chemical mixture to activate the fly ash to create concrete. Ceratech’s products have been approved in 41 states, and they have been used on bridges, highways, and other road work.

The Geopolymer Laboratory at Louisiana Tech University makes a paste of fly ash with waste products from chemical and petrochemical industries and liquid sodium silicate. The paste is added to to the aggregate, and Portland cement isn’t needed in the concrete mix. Concrete made with geopolymer sets faster, lasts hundreds more years, resists corrosion and fire better, has higher tensile and compressive strength and reduces greenhous gas emissions from its lifecycle by 90%.

Other companies, like U.S. Concrete, are using slag to replace up to half the Portland cement. Slag is ground up and combined with water to make a cement-like material that strengthens over time and is less permeable and more durable than Portland cement. Slag requires about 90% less energy to make than Portland cement.

A Cradle to Cradle approach

The New Jersey company Hycrete is making a water-based concrete admixture (a material added to the concrete at the time it’s being mixed) that makes concrete waterproof. It makes concrete recyclable, reduces water absorption, reduces corrosion, extends the life of concrete and eliminates all of the materials, time and work related with adding waterproof barriers to concrete. Hycrete has earned a silver Cradle to Cradle certification, and the company says the waterproof admixture provides a 20% to 60% savings over using waterproof membranes.

For more information

Check out the Cement Sustainability Initiative for more information. You can even take a virtual tour of a cement plant.

Posted by on Monday, October 25th, 2010. Filed under Technology. You can follow any responses to this entry through the RSS 2.0. Both comments and pings are currently closed.

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