The combustion of coal in coal-fired power plants produces several materials, including: fly ash, bottom ash, boiler slag, and flue gas desulfurization material.  Together, these materials represent what is generally referred to as coal ash, or sometimes as coal combustion byproducts (CCBs). 

Each material has unique characteristics that makes it  valuable for certain beneficial uses, including as a fill material for mine reclamation and subsidence control; for achieving mine reclamation requirements under the Surface Mining Control and Reclamation Act (SMCRA); for the elimination of dangerous highwalls; for producing concrete and Portland cement; as a low permeability capping material; to produce road base materials, structural fills and embankments; to make wallboard; as an additive to improve soil chemistry and reduce acidity; and to neutralize acid mine drainage.

[source:  http://www.nma.org]