Fly ash is a by-product of coal-fuelled thermal power plants.
Fly ash is an industrial waste or a by-product generated mainly by thermal power plants, which use coal as fuel for their boilers. It is formed by the rapid cooling of molten ash during the combustion and chemical recombination of mineral impurities found in coal. Due to its spherical shape, fly ash, which was once considered a threat to the environment, is now being used by the construction industry worldwide.
History
Fly ash is an artificial version of the volcanic ashes that were used for creating the first hydraulic cements known to man, in a small town named "Pozzuoli" around 300 B.C. in Italy. Fly ash has a large Silica and Alumina content optimal for the hydration chemistry in cements, and has become the best known and the most widely used pozzolan in the world today. A mammoth example of the durability and strength of fly ash is the Roman Colosseum, which was built in 100 A.D.
Hazards of Fly Ash
Before fly ash was rediscovered as a construction marvel, it was regarded as an industrial waste that causes air, water and topsoil pollution with the potential to disrupt ecological cycles and pose environmental hazards. Unless properly disposed, fly ash can become airborne and cause severe problems, like lowered crop yield, respiratory disorders in humans and livestock, and contamination of groundwater with toxic salts and metallic contents.
Fly Ash Disposal
The disposal of fly ash is extremely difficult. It becomes a heavy burden on land, water and energy resources, because the traditional methods of disposal call for either dry disposal by mass transportation, or the creation of ash ponds for dumping the fly ash in the form of a water-fed slurry.
Fly Ash Attributes
Fly ash has a perfectly spherical shape that allows it to blend seamlessly in mixtures. The particles create a ball-bearing lubrication effect while the cementitious compound is in a plastic state. It helps improve structural strength over time by continuing particle combination with free lime in the compound. It helps reduce bleed channels and permeability as a result of increased density. One of the most important fly ash attributes is that it stops sulphate attacks by engaging the free lime, which can otherwise react with sulphate and create unwanted expansions. It reduces efflorescence by chemically engaging efflorescence-prone salts and keeps such compounds protected inside. It also reduces drying shrinkage using its lubricating attributes that decrease water content. Its chemistry with lime creates reduced temperatures and thermal cracking when compared to ordinary Portland cement.
One of the most important fly ash attributes is that it reduces the chemical reactions between alkalis and silica by combining with the alkalis in the cement, thus ceasing unwanted expansions. It keeps tough compounds on the surface and enhances resistance to mild acids, soft water and sea water. Fly ash has a better slump life than Portland cement, hence increases reliability in warmer weather conditions. It is easier to pump than regular cement, hence consumes lesser efforts and travels longer distances. Fly ash has an easier workability due to better response to vibration. It provides better finishing and reduces segregation issues.
Uses of Fly Ash
The numerous fly ash attributes makes it suitable for making Portland cement concrete, in construction of roads, and as a filling material. Fly ash can be applied for the betterment of agricultural land. It can be used to modify soil for increased fertility and moisture, which in turn enhances water and nutrient intake, root development, soil binding, and carbohydrates storage in the crops. It also possesses cleansing properties and prevents soil borne diseases in plants.
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