Fly ash is a by-product of burning coal to create electricity, consisting of fine, glassy particles. It can be captured and recycled as an additive to cement, asphalt, and other building materials, but disposal presents environmental concerns due to its toxic heavy metal content.
Fly ash, also known as pulverized fuel ash (PFA), is industrial ash created when coal is burned to create electricity. As the gaseous emissions of burning coal cool, some of its chemical constituents solidify into spherical grains, forming fly ash. A fine, glassy powder, its chemical constituents vary but usually include oxides of silicon (SiO2), aluminum (Al2O3), iron (several types), and calcium (CaO). It is found in power plant chimneys and has several industrial uses, the most notable being as an additive to cement.
When coal is burned, the by-products of the combustion process separate into heavier components, which sink to the bottom of the burner and become bottom ash, and gaseous emissions, which escape from the top of the burner. Fly ash particles precipitate from the gas as it rises. In most plants, these are captured from the air using an electrical charge generated by a device called an electrostatic precipitator. Due to the conditions of their formation, the particles are extremely fine, mostly spherical and all of nearly the same size.
The main question surrounding fly ash is what to do with it. Once allowed to become airborne as an industrial pollutant, the law now requires it to be removed from coal-fired power plant emissions and disposed of as solid waste or recycled. Ash disposal presents problems, because a large part of it is produced. Most of the ashes are dumped in lagoons or landfills. The ash contains toxic heavy metals and conservationists fear these could seep into the soil or escape into the environment if lagoons break up.
Luckily, fly ash is a pozzolan, a material that can act as a cement when mixed with lime and water. For this reason, it is increasingly being recycled as an inexpensive extender for Portland cement, the common cement used to make concrete. There are a number of benefits: the resulting concrete is denser, smoother, easier to work with, more resistant to chemical erosion, and stronger in the long run. It also requires less carbon dioxide to produce and creates less pollution. The ash can also be used to make asphalt, brick, paint, tile and backfill.
Fly ash is one of several by-products of the coal burning process, called coal combustion products (CCPs). Others include bottom ash, boiler slag, and flue gas desulfurization (FGD) materials such as gypsum. Many of these can also be recycled as building materials to reduce their impact on the environment.
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