Fly Ash is a powder that is a by-product of coal and can be used to help make concrete.  It does not have the same “glue-like” quantities that cement does, so you cannot use it by itself to make concrete, but you can substitute some of the cement to blend in with Fly Ash to make a durable product.

Fly Ash is a very common product, but it will slow down the set-time in comparison to a straight cement mix.