Fly ash is a fine, powdery material created as a byproduct of industrial combustion processes. It consists of non-combustible mineral residue carried away with exhaust gases, which is how it earns its name. Billions of tons are generated annually, making it one of the largest industrial solid wastes produced globally.
The Primary Industrial Source
The vast majority of fly ash originates from coal-fired electric and steam generating plants. These facilities continuously combust coal to heat water, creating the high-pressure steam necessary to drive turbines and generate electricity. Fly ash represents the largest fraction of the total solid waste generated by these thermal power plants, often comprising about 60% of all coal combustion residuals (CCRs). For example, in 2015 alone, the global volume of fly ash production was estimated to be around 1.143 billion tons.
The Mechanism of Formation
The creation of fly ash begins with preparing the raw coal fuel source. To ensure rapid and efficient burning, the coal is first pulverized into an extremely fine powder, increasing its surface area. This powdered coal is then injected with air into the combustion chamber, or boiler, where it ignites almost instantly. The intense heat, which can reach 1500°C, causes the organic carbon structure of the coal to burn away.
Coal contains non-combustible inorganic mineral matter, such as quartz, feldspar, and clay minerals. These minerals do not burn but melt under the high temperatures of the furnace, forming tiny molten droplets. As these liquid droplets are carried upward by the rapidly moving exhaust gases, known as flue gas, they are swept out of the hottest part of the combustion zone.
The quick cooling that occurs while the droplets are suspended in the gas stream causes them to solidify before they can crystallize. This rapid cooling gives fly ash particles their characteristic amorphous, glassy, and predominantly spherical shape. These spheres are very small, typically ranging in size from 0.5 micrometers to 300 micrometers.
Once formed, the fine ash particles remain suspended in the flue gas, distinguishing them from bottom ash, which falls to the bottom of the boiler. Before the flue gas is released, environmental regulations require the fly ash be removed. Specialized equipment, such as electrostatic precipitators or fabric filter baghouses, captures these fine particles, separating them from the gas stream.
Classifying Fly Ash Based on Source Coal
The final chemical makeup of fly ash is linked to the specific type of coal that was combusted. The American Society for Testing and Materials (ASTM) recognizes two primary classifications for ash used in construction, designated as Class F and Class C under standard C618.
Class F fly ash is produced from burning older, harder coals, specifically bituminous and anthracite coal. These coal types yield an ash characterized by a lower calcium oxide (CaO) content, often less than 10%, and higher amounts of silicon dioxide and aluminum oxide. This composition means Class F ash possesses pozzolanic properties, requiring an activator to react and form cementitious compounds.
Conversely, Class C fly ash results from the combustion of younger, softer coals, such as sub-bituminous coal and lignite. Ash derived from these sources contains a higher concentration of calcium oxide, frequently exceeding 20%. The presence of this elevated calcium content gives Class C ash self-cementing properties, allowing it to harden and gain strength without the addition of other cementing agents.