Of course, fly ash isn't without controversy. Because fly ash is a byproduct of coal, which itself is full of heavy metals and toxins that can be dangerous, concern has been raised that buildings made from fly ash concrete could be harmful to people.
The biggest challenge to fly ash came in 2008, following the massive fly ash spill at the Kingston Fossil Plant in Tennessee (see sidebar). Then, a few weeks later, there was another smaller fly ash spill in Alabama that kept fly ash in the news. Those spills and the resulting cleanup and news coverage put fly ash on the Environmental Protection Agency's (EPA) radar and prompted environmental activists to call for tougher regulations. Since then, the EPA has been weighing whether or not to classify fly ash as hazardous waste, and although the agency has stated that such a classification wouldn't affect fly ash that's used to make concrete, observers from the construction industry have been skeptical [source: Purdy].
But is it really safe to live in a building that's made of coal? The safety of fly ash in concrete has been hotly debated. While geopolymer concrete has been proven to leach dangerous elements like arsenic, chromium and selenium, at least one study has shown that adding calcium can help reduce the leachability of heavy metals [source: Sanusi]. However, many questions still remain about what will happen to the heavy metals and other substances in fly ash once the concrete breaks down and is disposed of [source: Post].
The concrete industry is worried that even a hybrid law that classifies only stored fly ash as a hazardous material would still stigmatize fly ash concrete, placing it on a similar level as asbestos or lead paint. Even if it's safe in concrete applications, hazardous classification for fly ash could potentially make construction companies more vulnerable to lawsuits.