Fly ash is one of the naturally-occurring products from the coal combustion process and is a material that is nearly the same as volcanic ash. Volcanic ash concrete was used thousands of years ago to produce Roman concrete structures that exist and function today; e.g., the Pantheon, Coliseum, and ancient aqueducts. When coal is burned in today’s modern electric generating plants, combustion temperatures reach approximately 2800°F. The non-combustible minerals that naturally occur from burning coal form bottom ash and fly ash. Bottom ash is a light-weight aggregate material that falls to the boiler bottom for collection. Fly ash is the material that is carried off with the flue gases, where it is collected and can be stored in silos for testing and beneficial use classification. The reuse of fly ash as an engineering material primarily due to its pozzolanic nature, spherical shape, and relative uniformity. Fly ash recycling, in descending frequency, includes usage in Portland cement & grout, as a structural fill, waste stabilization, and raw feed for cement clinkers, mine reclamation, stabilization of soft soils, road sub base, aggregate and flow able fill.
