Fly ash is a finely divided residue that is usually the result of the combustion of powdered coal in boilers at about 2,500 degrees collected by precipitations. There was a time when power plants, steel mills, and other large producers of this form of pozzolan paid to have the by-product hauled away. That was until fly ash was discovered to be a useful product.
The characteristics of fly ash that make it valuable for use in concrete are its (1) high fineness, (2) low carbon content, (3) uniformity, (4) high percentage of fused silica and (5) mostly spherical shape, which contributes toward great plasticity. Three reasons for which fly ash is most commonly employed in concrete mixtures are: to increase cementitious compounds developed, to correct deficiency in the extreme fines in some sands, and to counteract or reduce cement-aggregate reactions or damage due to sulphate attack.
Because it is much lower in cost than portland cement and can develop cementitious products, fly ash can be used as an economical cement replacement material to increase strengths. When fly ash is employed as an economy measure, it is best to test the fly ash thoroughly with the mix designs in which it will be used and compute the real cost of obtaining and storing the fly ash and batching the concrete. Although one of the main advantages of fly ash in concrete is its fineness, this characteristic can be bothersome when handling the material in much the manner as handling cement. The additional cost of handling this extra "cement" must be included if a true comparison of material costs is to be made.
Fly ash can be used effectively to correct a deficiency in fines in concrete sands. Proper use of fly ash under circumstances reduces the amount and rate of bleeding, increase workability, and improves finishability, especially of lean mixtures.
Fly ashes are usually not as effective in reducing alkali-aggregate reactions as are materials that are high in opal content. However, if fly ash is readily available locally, it is worth trying in concrete where a borderline reaction is occurring. Typically, from 20 to 35 percent fly ash is used by weight of the cement. Never use fly ash in concentrations of 10 percent or less in concrete that is susceptible to alkali-aggregate reaction because it many actually increase the rate and amount of the phenomenon.