Gray to Green: How to Make Cleaner Concrete

Concrete gets its stickiness from cement, which is typically made by heating limestone and clay up to 2600 degrees Fahrenheit. (The resulting hard substance gets ground into a powder that becomes sticky when mixed with water.) But the process of making cement is responsible for 85 percent of concrete's carbon dioxide emissions. The CO₂ comes from two places: the fact that limestone releases CO₂ when heated, and that cement makers burn plenty of fossil fuels to reach those high temperatures. For every 100 pounds of cement produced, 90 pounds of carbon dioxide is released into the atmosphere.

To cut those emissions, about 50 percent of ready-mixed concrete in the U.S. now replaces some of the cement with fly ash, or dust collected from the smoke stacks of coal-burning power plants. Fly-ash particles are finer than cement particles, so they pack together more densely, making the concrete stronger and less permeable to water. Replacing cement with fly ash also decreases costs, prevents the ash from being sent to the landfill, and reduces concrete CO₂ emissions by up to 15 percent. The downside is that concrete made with fly ash sets more slowly. And some health experts have raised concerns about the trace levels of heavy metals, such as mercury and arsenic, which can be found in fly ash; however, others say the harmful substances are locked into the concrete and pose little to no harm.

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