by Michael Cockram

With the growing awareness of the environmental harm of greenhouse gases, one major culprit in the construction industry is beginning to attract attention. The production of Portland cement, a key ingredient of concrete, releases substantial amounts of carbon dioxide (C0₂) — 8 percent of greenhouse gases worldwide. The United States consumes 110 million tons (100 million metric tons) of Portland cement annually and China now produces and places five times that amount.

Alternatives to Portland cement are available and are becoming easier to implement. One compelling innovation is the substitution of coal fly ash. Fly ash is a by-product of coal-burning power plants. Up to 22 million tons (20 million metric tons) of fly ash is removed annually from U.S. coal plants.

Historically, this abundant material was considered a nuisance waste product and left in huge piles near coal plants or in landfills. Left exposed to the elements, and in particular to percolating water, fly ash can leach trace amounts of heavy metals into the ground, posing an environmental danger. However, once locked in concrete, these small amounts of heavy metals are rendered relatively harmless.

Fly ash consists primarily of tiny beads of silica, which act as a binder in the hydrating process. These beads can make concrete more workable and fill voids better than Portland cement-based concrete. Fly ash also allows for reduced amounts of water in the mixture. The result is a material that is stronger and less permeable to moisture and to agents like sulfur that can degrade the integrity of the concrete.   >>>

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The McIrwin House by Common Practice Design uses an alternative to conventional masonry, Rastra block, which will be covered with standard exterior finish materials. Photo: Rudy Berg A mixture of cement and polystyrene foam beads make stay-in-place concrete form products like Rasta block lightweight and a good insulator. They are also easy to cut; note the mitered corners. Photo: Rudy Berg   Click on thumbnail images to view full-size pictures.