Fly ash is a fine ash that is collected from the flue gas after coal combustion.

Fly ash is similar in appearance to cement and varies in color from milky white to grayish black. The color of fly ash is an important quality index, which can reflect the amount and difference of carbon content. To a certain extent, it can also reflect the fineness of fly ash. The darker the color, the finer the particle size of fly ash and the higher the carbon content. Fly ash is divided into low calcium fly ash and high calcium fly ash. Usually the color of high calcium fly ash is yellow, and the color of low calcium fly ash is gray. The fly ash particles have a porous honeycomb structure, large specific surface area and high adsorption activity, and the particle size ranges from 0.5 to 300μm. And the bead wall has a porous structure, the porosity is as high as 50%-80%, and has a strong water absorption.

The main oxides of fly ash in China's thermal power plants are SiO2, Al2O3, FeO, Fe2O3, CaO, TiO2, MgO, K2O, Na2O, SO3, MnO2, etc., in addition to P2O5 and so on. Among them, silicon oxide and titanium oxide come from clay and rock sheet; Iron oxide mainly comes from pyrite; Magnesium oxide and calcium oxide are derived from their corresponding carbonates and sulfates.
The element composition (mass fraction) of fly ash is: O47.83%, SI11.48%-31.14%, Al6.40%-22.91%, Fe1.90%-18.51%, CA0.30%-25.10%, K0.22%-3.10%, Mg0.05%-1.92%, Ti0.40%-1.80%, S0.03% ~ 4.75%, Na0.05% ~ 1.40%, P0.00% ~ 0.90%, Cl0.00% ~ 0.12%, other 0.50% ~ 29.12%.

Adding fly ash to concrete saves a lot of cement and fine aggregate. Reduced water use; Improved workability of concrete mix; Enhancing the pumpability of concrete; The creep of concrete is reduced; Reduce hydration heat and thermal expansion; Improve concrete impermeability; Increase the modifiability of concrete.
National standard first class concrete: The modern concrete new technology using high-quality fly ash and high efficiency water reducing agent composite technology to produce high grade concrete is developing rapidly throughout the country.
National standard Grade 2 concrete: high quality fly ash is especially suitable for the preparation of pumped concrete, mass concrete, impermeable structural concrete, sulfate resistant concrete and soft water erosion resistant concrete and underground, underwater engineering concrete, grouting concrete and roller compacted concrete.
National standard Grade 3 concrete: fly ash concrete has the advantages of good workability, strong pumpability, improved finishing, improved impact resistance, and enhanced freezing resistance.