Fly ash bricks are constructed at least partially from the residue of burnt coal. Depending on the type of coal that is burned, the resulting fly ash can take a number of different forms. Some kinds of fly ash require a cementing agent, such as quicklime or Portland cement. Other types contain a large enough percentage of lime that they are self setting, requiring only the addition of water to be turned into fly ash bricks. These bricks have been utilized in construction since the 1950s in some parts of the world, and in certain configurations are able to meet or exceed specifications for clay or cement bricks.
When coal is burned, one of the results is a fine powder known as fly ash. While other ashes might settle at the base of the furnace where the coal is being burned, fly ash is light and fine enough that flue gases often are able to carry it up through the chimney. In the past, this ash was typically allowed to escape into the atmosphere, though in many locations environmental regulations the fly ash must be captured for safe disposal or reuse. Since toxic materials like mercury and lead may be highly concentrated in fly ash, this can lead to environmental or health concerns.
Since the 1950s in places like the United Kingdom (UK) and the 1970s in parts of Africa, houses and other structures have been build with fly ash bricks. The construction and quality of these bricks has varied over the years, with certain mixes being stable and others swelling or deforming when in contact with water or other moisture. Developments in the early 21st century led to fly ash bricks that contained a type of fly ash that is naturally high in lime and which can meet or exceed specifications for traditional types of bricks. This technology may also be used to create fly ash pavers and other applications.
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In addition to using fly ash to make bricks, it is also often used in concrete manufacturing. When used in concrete, fly ash typically replaces 30% or less of the cement that would otherwise be used in the mixture. In certain other cases, fly ash may replace as much as 70% of the Portland cement. Depending on the specific mixture and use of the concrete, the presence of fly ash instead of Portland cement can actually result in stronger concrete.