The noncombustible part of coal is left over in the form of ash with unique characteristics. In addition to its ability to react with water like Portland cement, fly ash particles are smaller than grains of cement, and are spherical, like microscopic ball bearings. These two physical features make fly ash better than cement in several ways:
Fly ash has a lower unit weight than cement, so it contributes roughly 30 percent more volume of cementitious material per pound used; this helps the concrete flow better while it’s wet.
The smaller particles of fly ash fill in gaps within the wet concrete mixture better than cement, making the final product more consistent and therefore stronger.
This same characteristic means concrete can be made with slightly less water and sand, coating the larger aggregates more evenly and creating a more workable material.
While concrete made with fly ash does not equal the strength of cement-based concrete for about a month, it continues to gain strength after that, becoming more durable long after cement-based concrete has reached its maximum strength.
Concrete made with fly ash contains less lime, and converts some of the lime present into calcium silicate hydrate; this replaces one of the weakest ingredients in concrete with one of the strongest.
Lightweight concrete made with fly ash is easier to pump, and its smooth consistency reduces unwanted air pockets; the finished surface is more even where it is used with forms.
Fly ash makes concrete less permeable, which improves protection against corrosion; it reduces the intrusion of water, oxygen, and chemicals, protecting steel reinforcement from corrosion and expansion.
When cement reacts with water, it generates heat very quickly, which helps the concrete harden and gain strength; in some situations, rapid heat gain may lead to thermal cracking and loss of strength. Fly ash produces far less heat when it reacts with water, helping reduce the damaging effects of thermal cracking.
Considering all the practical benefits of using fly ash concrete, the environmental advantages may not seem as important - but they are substantial. According to industry estimates, for each ton of fly ash used instead of cement, we save as much landfill space as an average American uses in 15 months, reduce CO2 emissions by as much as an average car produces in 2 months, and save enough electricity to power an average household for 24 days.
For the past decade in the US, the use of fly ash in concrete has increased by more than 50 percent. At current rates, more than 12 million tons of coal fly ash are used in concrete products each year. Multiply that by the numbers above, and you can see why. One more thing: concrete itself is recyclable. Anywhere from 45 to 80 percent of crushed concrete can be used as aggregate in new construction.
Concrete made with fly ash qualifies for LEED credits from the U.S. Green Building Council. The U.S. Environmental Protection Agency requires that fly ash must be allowed on federally funded projects, and promotes the use of fly ash through its Coal Combustion Products Partnership (C2P2) program. Its use is endorsed by the U.S. Department of Energy; the U.S. Army Corps of Engineers requires fly ash concrete in most projects; and the U.S. Bureau of Reclamation has used fly ash extensively on dam projects. And all 50 states either allow or require the use of fly ash concrete in state-funded projects.
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