How Fly Ash Concrete Works

What is fly ash?

In America, more than 50 percent of our electricity comes from coal-fired power plants, where coal is ground into a fine powder, placed in a boiler, and burned at very high temperatures. The heat from the boiler produces steam, which is used to spin a turbine and produce electricity. Burning coal is a very dirty process, but modern coal plants have pollution-control equipment that prevents all of the particulate matter from leaving smokestacks. When that equipment is cleaned, we're left with coal ash [source: Dewan].

Because coal power is so widely used, fly ash is one of the most abundant industrial byproducts on Earth. So instead of putting it in storage ponds that are just waiting to spill, it makes sense for companies to put it to good use as a construction material. Not only does using fly ash in concrete prevent those heavy metals from being released back into the environment, it also keeps cement makers from mining new raw materials.

As you might expect, coal ash is a cocktail of toxic chemicals -- it contains the same heavy metals that are found in coal, only in higher concentrations. Fly ash typically contains heavy metals like arsenic, lead and selenium that are known to cause cancer and other health problems. Because of the known risks, environmentalists have long fought to have fly ash categorized as a hazardous material, but they have faced stiff resistance from the energy industry, and now the construction industry.

Because it contains heavy metals, storing and safely disposing of fly ash presents a major challenge to power companies. It's typically mixed with water and stored in ash ponds that can be exposed to storms and cause spills. There are currently about 600 coal ash sites in 35 states across the country, and many of them have experienced spills in recent years [source: Merchant]. The biggest fly ash spill in U.S. history took place in 2008 at the Kingston Fossil Plant, which is about 40 miles west of Knoxville, Tenn. There, about a billion gallons of fly ash sludge and water were released into the surrounding land and waterways in an environmental catastrophe [source: Living on Earth].

One potential solution to the storage and disposal problem that could help prevent spills like the one in Tennessee is to use fly ash as a building material. When used in concrete, fly ash is generally believed to be fairly safe. Although it often does contain known carcinogens, the harmful effects are thought to be neutralized when fly ash is added to concrete mixes.