The development of strength in fly ash concretes is closely related to the mix proportioning. Three basic proportioning approaches have been developed to obtain either reduced heat hydration or to overcome difficulties in getting acceptable levels of strength in concrete at early ages. The three techniques are: partial replacement of cement; addition of fly ash as fine aggregate; and partial replacement of both cement and fine aggregate. In the first approach a portion of the Portland cement is replaced by an equal amount of fly ash. Replacement of Portland cement in concrete by fly ash on a one-for-one basis results in lower compressive and flexural strengths at ages up to about 3 months, though greater strengths develop at 6 months and beyond.
In the fly-ash-addition method, fly ash is added to the regular amount of cement. The cementitious content of the mix is thus increased, and usually the mix is adjusted by reducing the fine aggregate content. Essentially the fly ash has replaced fine aggregate rather than cement and this generally produces increased strength in concrete at all ages. The third method replaces a part of the cement with a greater weight of fly ash; then it compensates for the large amount of fines by a subsequent reduction in fine aggregate content. With this method, the low early strengths of the first approach are corrected.
Fly ash can be used successfully with water-reducing agents. Compared with non-fly-ash mixes, such concrete provides higher 90-day strengths, less bleeding in lean mixes, and higher 28-day tensile strengths. Fly ash can also be used with superplasticizers; slightly more sand is usually used in proportioning than in conventional concrete. Superplasticizers in fly ash mixtures do not seem to improve compressive strength as much, to extend the period of increased plasticity as long, or to reduce the water requirement as much, however, as in plain concrete; but they do increase early flowability equally for both fly ash and plain concrete.