EPA's “Cement and Concrete Containing Fly Ash; Guideline for Federal Procurement” as published in the Federal Register

While it is true that fly ash contains trace amounts of certain elements, which can be toxic in larger concentrations, it is unlikely that fly ash as used in concrete would exhibit leaching characteristics. First, the permeability of concrete containing fly ash is negligible compared to the permeability of fly ash as typically disposed. This reduced permeability prevents water or other liquids from penetrating concrete and providing a leaching medium through which contaminants could travel.