What is flyash and why is it used in concrete products?

Flyash is a byproduct of the combustion of coal. It is a finely divided spherical residue that is captured from the exhaust of the boiler. Flyash is mainly harvested as a byproduct of coal fired electric generating plants.Approximately 30% of the flyash in the US is recycled into concrete. Flyash increases the strength, reduces permeability, and improves the flow and workability of concrete while reducing water demand. Concrete made with flyash is stronger than concrete made with cement alone, because the flyash reduces the amount of water required as well it reacts chemically with the lime given off by the cement forming a glue that holds the concrete together.  more