Fly ash is a byproduct from coal-fired power plants that is frequently used as an admixture in concrete to replace a portion of the Portland cement. Using fly ash in concrete is environmentally beneficial because it reduces the Portland cement (a major contributor of CO2) required in concrete. Fly ash in concrete improves performance in some ways, but it also has some disadvantages.
Slower Strength Gain
Concrete changes from a liquid to a solid a few hours after pouring, but the curing process may take much longer. It continues to gain strength for weeks after its initial setting period. The addition of fly ash can increase the length of time concrete takes to reach its full strength. This can cause problems when slow strength gain means delays in construction.
Longer Setting Times
Fly ash admixtures can lengthen the time it takes for concrete to set. Sometimes this is desirable, particularly in hot weather which speeds up concrete set times, but at other times it is an inconvenience and can cause delays in construction. Other admixtures may be necessary to adjust the set time of the concrete, depending on the percent fly ash in the mixture and the outside temperature.
Air Content Control
Concrete is susceptible to damage from freeze/thaw cycles if it does not contain air. Tiny air bubbles can be created in concrete by using air-entraining admixtures that cause the concrete to foam in the mixing and pouring stage. Fly ash reduces the amount of air entrainment, and concrete mixtures high in fly ash often require more air-entraining admixture.
The winter season is problematic for concrete pouring, and mixtures high in fly ash are even more susceptible to low temperatures. Low temperatures lengthen setting times and cause slow strength gain even in concrete mixtures without fly ash. When fly ash is added, low temperatures exaggerate these problems. Some regions have bans or restrictions on using fly ash in the winter months.
The structural effects of fly ash may be more critical, but cosmetic concerns also affect its use in concrete. It is more difficult to control the color of concrete containing fly ash than mixtures with Portland cement only. Fly ash also may cause visual inconsistencies in the finished surface, such as dark streaks from carbon particles.