profile="http://gmpg.org/xfn/11"> What is Fly ash composition?
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What is Fly ash composition?

In power plants that generate electricity a byproduct of burning pulverized coal is called fly ash. Mineral impurities in the coal, such as clay, feldspar, quartz and shale that form during combustion fuse in suspension and float out of the combustion chamber with the exhaust gases. It is converted into solid form on cooling. It turns into sphere  shaped glass particles called fly ash. The fly ash is collected from the exhaust gases by electrostatic precipitates or bag filters. This happens when the fused material rises. The fine powder has close resemblance to Portland cement.However, it’s chemical nature and configuration is completely different. It reacts chemically with the byproduct calcium hydroxide, which is released by the chemical reaction between cement and water. This forms into additional cementitious products that improve several desirable properties of concrete. This property of cementitious nature is exhibited in varying degrees depending on the chemical and physical properties of both the fly ash and cement. When compared to cement and water, the chemical reaction between fly ash and calcium hydroxide is typically slower. Thus resulting in delayed hardening of the concrete. Significantly, it creates challenges for the concrete producer, including the finisher when placing steel-troweled floors.

 

C class and F class are the two types of fly ash that are commonly used in concrete. C class contains high amount of calcium fly ash types. It contains less than two percent of carbon content. However, F class fly ash contains less than five percent carbon in it. At times it has ten per cent carbon content in it. In general, C class fly ash is produced from burning sub-bituminous or lignite coals. F class is formed from bituminous or anthracite coals. The quality of performance between C class and F class differs. It depends on the varying chemical and physical properties of the fly ash. Moreover, how it reacts with cement in the concrete. Several C class types on mixing with water become solid, which is very similar to the nature of cement. The F class ash will not. It will react only with the byproducts formed when cement reacts with water.

 

In US more than 50 per cent of the concrete structures contain fly ash in it. Only the dosage rates vary, which depends on the fly ash type (C or F) used and its reaction level. F class fly ash is used at dosages of 15 per cent to 25 per cent. C class fly ash at 15 per cent to 40 per cent with cement. However, fly ash is used in interior, steel-troweled slabs. This is because of the natural problems or challenges associated with fly ash, which is variable in nature in C class and F class. It is also due to the delay in the hardening of the concrete. The rate and uniformity of concrete hardening are critical parameters that establish the quality of the finished product. It can directly influence the quality of the final floor finish. A delay in the hardening of concrete can risk finishing of the floor. This will result in poor quality of steel-troweled finish. Till date, building constructions, concrete suppliers and masonry workers are reluctant to use fly ash in place of cement in steel-troweled floors. It is due to the increase in risk with it. They are stickiness in the surface, delay in concrete turning hard, including cracking due to early shrinkage in the volume, which is caused by delay in the setting.

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