Fly ash is a fine, powdery material that is produced by burning coal to produce electricity, primarily in pulverized coal combustion (PCC) boilers. It is composed mainly of non-combustible inorganic material, but also contains some carbon that is leftover from partially combusted coal. Although fly ash particles are generally largely spherical in shape, there are usually irregularly-shaped particles present also, such as angular particles of quartz. The spherical shape of fly ash results from the formation of tiny molten droplets as the ash travels through the boiler. The droplets form spheres because this shape minimizes the surface area relative to the volume. Since it is so fine, removal and collection of fly ash from combustion gases (flue gas) requires specialized equipment such as electrostatic precipitators or fabric filters.
Fly ash (class C) sample as taken from an ash impoundment - Enlarge Photo
Fly ash (class F) sample as taken from an ash impoundment - Enlarge Photo
Fly ash is classified by ASTM, based primarily on its chemical composition. Class C fly ash has a relatively high calcium, sodium and magnesium content and is produced from burning low rank coals such as lignite and subbituminous coal. The calcium-bearing phases give cementitious characteristics to Class C fly ash when it is mixed with water, which allows it to be used in place of portland cement for certain applications such as soil stabilization and flowable fill. Class F fly ash has little calcium content and (usually) a higher silica and iron content and is typically produced by burning bituminous coal. It is comprised mainly of alumino-silicate glassy particles, but also contains crystalline material.
The glassy material in fly ash is what causes it to be useful as a concrete additive. The glass reacts with calcium hydroxide, which is produced within the concrete during hydration of the portland cement component, to form additional cementitious material that increases strength and durability. The carbon (unburned coal) in fly ash is usually detrimental because it adsorbs air entraining admixtures from fresh concrete. These admixtures are used to improve the concrete's resistance to freeze-thaw damage. Another important property of a fly ash is the overall particle size because smaller particles give the fly ash a greater reactivity in concrete compared to larger particles. Thus, a high-quality Class C or Class F fly ash for use in concrete should have a low carbon content and a high proportion of small, glassy particles.
Fly ash is frequently used and often required in:
concrete structures such as roads, bridges and dams ...
"Coal combustion products are the solid residues generated by coal-burning electric utilities in the production of electricity. In 1998, electricity accounted for about 35% of the primary energy use in the United States and was produced by electric power generators designed to convert different fuel types into electricity. More than one-half of the electricity in the United States was generated by burning coal. (Courtesy of USGS)".