Coal combustion byproducts (CCBs) are considered to be four distinct and extremely different materials.
FLY ASH
Fly ash is the finest of coal ash particles. It is called "fly" ash because it is transported from the combustion chamber by exhaust gases. Fly ash is the fine powder formed from the mineral matter in coal, consisting of the noncombustible matter in coal plus a small amount of carbon that remains from incomplete combustion. Fly ash is generally light tan in color and consists mostly of silt-sized and clay-sized glassy spheres. This gives fly ash a consistency somewhat like talcum powder. Properties of fly ash vary significantly with coal composition and plant-operating conditions.
Fly ash can be referred to as either cementitious or pozzolanic. A cementitious material is one that hardens when mixed with water. A pozzolanic material will also harden with water but only after activation with an alkaline substance such as lime. These cementitious and pozzolanic properties are what make some fly ashes useful for cement replacement in concrete and many other building applications.
BOTTOM ASH
Coal bottom ash and fly ash are quite different physically, mineralogically, and chemically. Bottom ash is a coarse, granular, incombustible byproduct that is collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both. Bottom ash is coarser than fly ash, with grain sizes spanning from fine sand to fine gravel. The type of byproduct produced depends on the type of furnace used to burn the coal.
BOILER SLAG
Boiler slag is coarser than conventional fly ash and is formed in cyclone boilers, which produce a molten ash that is cooled with water. Boiler slag is generally a black granular material with numerous engineering uses.
FGD GYPSUM
Flue gas desulfurization (FGD) gypsum is also known as scrubber gypsum. FGD gypsum is the byproduct of an air pollution control system that removes sulfur from the flue gas in calcium-based scrubbing systems. It is produced by employing forced oxidation in the scrubber and is composed mostly of calcium sulfate. FGD gypsum is most commonly used for agricultural purposes and for wallboard production.
More details: What is coal ash? (source)
BOTTOM ASH
Coal bottom ash and fly ash are quite different physically, mineralogically, and chemically. Bottom ash is a coarse, granular, incombustible byproduct that is collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both. Bottom ash is coarser than fly ash, with grain sizes spanning from fine sand to fine gravel. The type of byproduct produced depends on the type of furnace used to burn the coal.
BOILER SLAG
Boiler slag is coarser than conventional fly ash and is formed in cyclone boilers, which produce a molten ash that is cooled with water. Boiler slag is generally a black granular material with numerous engineering uses.
FGD GYPSUM
Flue gas desulfurization (FGD) gypsum is also known as scrubber gypsum. FGD gypsum is the byproduct of an air pollution control system that removes sulfur from the flue gas in calcium-based scrubbing systems. It is produced by employing forced oxidation in the scrubber and is composed mostly of calcium sulfate. FGD gypsum is most commonly used for agricultural purposes and for wallboard production.
More details: What is coal ash? (source)