Fly ash typically replaces some of the Portland cement in a concrete mix. Portland cement, while inherently being a very environmentally beneficial product in that it is the primary binder in concrete, which is a highly environmentally beneficial building material, does have environmental impacts associated with its manufacturing. This of course can be said of any modern building material from wood to concrete to steel. Fly ash is a by-product of thermal power generation. It is typically disposed of as a waste if it is not used in some other way.