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Fly Ash in Concrete

What is Fly Ash?

Fly ash is a coal combustion byproduct. It is collected via bag house filtration equipment or electrostatic precipitators. These power generation furnaces typically burn at high temperatures causing the inorganic materials to melt and quickly cool in exhaust fumes. The quick air cooling process creates amorphous, particle spherical shapes. These spheres can often improve rheological properties in fresh cement, as there is an almost ball-bearing effect. Cementing grades of fly ash will have higher concentrations of SiO2, Al2O3, and CaO. These oxides can have good pozzolanic or hydraulic cementing properties when classified to at least 66 percent being -325 mesh.


Where does Fly Ash Come from?

Fly ash comes from coal burned in electric utilities or industrial boilers. Fly ash is the leftover material that was unable to be burned. The mineral composition is directly related to the coal burned and the temperature coal is burned. Depending on the particle size and chemical composition, the residual ash might be “good” enough to qualify as a type C or type F fly ash.


What’s the difference between Type C and Type F?

Type C Fly Ash comes from lignite or sub-bituminous coal. Typically, class F tends to be hydraulic, meaning, in the presence of water, it contains all the minerals to be cementitious.


Type C can provide higher concrete strength in the early stages of construction, though class C may have higher water demands.


Typically, an optimal dosage for type C fly ash is recommended to be between 20-35 percent cement replacement by weight.


Type F Fly Ash comes from anthracite or bituminous coal and typically tends to be pozzolanic as it lacks the calcium required to produce calcium silicate hydrate.


Type F can provide better strength and provide lower permeability over time.


Typically, an optimal dosage rate for type F fly ash is between 15-25 percent cement replacement by weight.

How can fly ash improve concrete quality?

Both types of fly ash can improve the quality of concrete. The silicon dioxide and alumina in the fly ash react with the calcium hydroxide and water to create the additional cementing binders. These additional cement binders increase strength and reduce porosity, allowing the cement to protect the rebar more effectively from additional chemical attacks. The spherical shape can reduce the water-to-cement ratio, directly impacting the overall strength and cement matrix.

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