What is fly ash? What are the varieties?

A: Fly ash is the powder collected from the flue gas of the pulverized coal-fired power plant boiler and belongs to the mixed materials of pozzolanic activity. Its main components are silicon, aluminum and iron oxides, with potential hydration activity. Fly ash is gray brown and usually acidic, with a density of 1.77-2.43/cm3 and a  specific surface area of 250-700m2/kg. Fly ash particles are mostly spherical and smooth on the surface, with a particle size mostly less than 45μm, and can be directly used in ready-mixed mortar without grinding.

According to different ways of discharge, fly ash can be divided into dry-discharged and wet-discharged fly ash. The wet-discharged fly ash is high in the water content, great in the decrease of activity and not as good as dry-discharged fly ash. According to the difference in collection methods, the dry-discharged fly ash can be further divided into the dust of electrostatic collection and dust of machinery collection. The dust of electrostatic collection is fine in particles and good in quality, while the dust of machinery collection is coarser in particles and poorer in quality. In order to improve the quality of fly ash, fly ash can be processed. The grinding-processed fly ash is called milled fly ash, but the unprocessed fly ash is called raw fly ash.

According to the content of calcium oxide, fly ash can be divided into high and low calcium fly ash. The fly ash formed by the combustion of lignite is higher in the content of calcium oxide (generally CaO>10%), brownish yellow in color and called high calcium fly ash, with a certain hydraulicity. The fly ash formed by the combustion of bitumite and anthracite is lower in the content of calcium oxide (generally CaO<1 0%), gray or dark gray in color and called low calcium fly ash, generally with pozzolanic activity.

The low calcium fly ash has a wide variety of sources, is the concrete admixture with the maximum amount and widest range of use currently at home and abroad. As for the high calcium fly ash, due to higher content of calcium oxide, if used improperly, it could easily lead to expansion cracks of hardened cement and should test its safety before use.

The quality of fly ash is quite different due to the differences in the coal varieties and combustion conditions. The composition and performance of fly ash discharged from different power plants at different time are also quite different. Therefore, attention should be paid to the fluctuation of its quality during use.

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