Tom Elko, writing for the Minnesota Independent, reports that the superstructure of the new I-35W bridge in downtown Minneapolis is composed of 25 percent coal fly ash, a byproduct of coal-fired electrical generation. Fly ash has a reputation as a green building material because it can be used in place of Portland cement, which is notorious for using huge amounts of energy and releasing large amounts of CO2 during its production.
"If you use a 25 percent fly ash content instead of the standard nine percent, you get an 11 percent reduction in CO2 emissions," Dan Handeen (Center for Sustainable Building Research) tells Elko. "A fly ask content of 35 percent reduces CO2 potential 21 percent."
But coal fly ash contains toxins -- including arsenic, dioxins, lead, and mercury -- which may be of concern.
Handeen tells Elko that, "There are tradeoffs to anything: there's no perfect material. Fly ash, in a way, is still solving a symptom. In one sense, yes, you're using up this waste material. In another way it's justifying the burning of coal as a fuel source. Until we find better ways to produce energy, it is a good use of the byproducts."