Fly ash cement offers many advantages over regular cement, including decreased water permeability and increased strength. When improperly produced, however, poor quality fly ash will demonstrate disadvantages, such as heightened concrete permeability and slow curing times. Properly manufactured fly ash, as well as specific ratio calculations, normally produces a quality concrete product that outweighs any drawbacks.
The actual fly ash is a residue derived from burning coal; this precipitate can be mixed with other cement ingredients to make concrete. As the fly ash is mixed with the cement constituents, such as sand and aggregate, it fills small spaces within the mixture for an extremely cohesive substance. The resulting fly ash cement resists water infiltration, which can damage the final hardened concrete product; any water trapped inside a cement mixture can expand and contract with temperature variations to eventually crack the concrete.
The increased density of the fly ash cement mixture contributes to its high strength. Since the fly ash fills in small crevices within the cement mixture, the residue actually adheres to more constituents than an equivalent substitute, such as Portland cement. As a result, the concrete's final strength after curing is substantially better than other cement mixture types.
The way in which the fly ash is produced is very important to the properties of the cement mixture; fly ash that is not burned sufficiently will have larger particles. These larger pieces will not fit within the small constituent crevices of the concrete, and actually form new voids or spaces. The resulting fly ash concrete is subject to water permeability since the internal spaces are large enough to accommodate many liquid molecules.
Slow curing times can also be a drawback to fly ash cement. The dense concrete structure does not allow very much air to infiltrate the spacing between the molecules; as a result, the setting time is lengthened. For example, construction projects that require a rapid repair of a concrete wall may not want to use fly ash concrete since it will not harden fast enough to allow traffic to flow nearby in a safe manner.
In contrast, some concrete workers see the slow setting time as an advantage, rather than a drawback. Fast curing times tend to produce stress fractures across the concrete, either along the surface or localized internally. These cracks significantly reduce the strength of the concrete object. Slow curing times allow the fly ash concrete to steadily settle into position, even with fluctuating temperatures.