Fly ash is becoming more popular as a substitute for cement in concrete. The Federal government has been allowing and encouraging its use for quite some time. It was actually used extensively in the construction of the Hoover dam. Another less known fact is that the Romans used volcanic ash from the solfatara volcano near Pozzuoli, Italy. The ash from this volcano was essentially fly ash and was mixed with lime to create their concrete. You may have heard of pozzolan admixtures for concrete.
The use of fly ash in concrete has a couple environmental considerations. The first is lowered carbon dioxide emissions. Another is less water usage. This is because the shape of the material is more spherical and which make it easier to work thus reducing water needs by up to 5%. Fly ash cement also has a greater ultimate strength and is more resistant to sulfate attacks and ASR, Alkali-Silica Reactions which can cause accelerated deterioration and failure of the concrete.
Class F fly ash is generally produced from coal mined and burned in the eastern states. Class C fly ash is generally produced from coal in western states. The difference comes from the typical makeup of the coal deposits in these two areas. This variation in the properties of fly ash is one of the aspects that have slowed its acceptance on a wider scale. These variations make it harder to control the quality of the material.
Fly ash can help garner LEED points as a post-industrial product and potentially count as a locally sourced material. Its use also diverts it from being placed in landfills and also reduced the demand for typical cement and the associated energy costs (lower CO2 emissions) in its production.
It is actually quite interesting to see how so many things can be interlaced and tied to our energy sources. Many construction materials are by-products of gasoline production. The future is going to be an interesting place in many ways as our energy sources evolve.