It is a coal combustion byproduct from power plants and there are a few
types of coal ash according to its physical properties, mainly fly ash and
bottom ash.

- Fly ash is a very fine powder material that is carried with the stack
gases and is collected by electrostatic precipitators or a baghouse prior
to exiting the stack. It exhibits cementitious and/or pozzolanic properties,
which makes it useful in concrete and geotechnical construction
applications. When used in this way it actually makes the concrete
stronger than if fly ash were not added to the mix. And since it reduces
the amount of Portland cement that must be used, it saves energy that
would have been expended making Portland cement and reduces
associated greenhouse gas emissions.

- Bottom ash is much coarser than fly ash. It is an almost sand-like
material that is sluiced from the bottom of the boilers.