Fly ash is an inexpensive replacement for portland cement used in concrete, while it actually improves strength, segregation, and ease of pumping of the concrete. (gm-volt.com)
Its main characteristic is that it does not contain portland cement, which turns it into a more sustainable material. (sciencedaily.com)
Concrete strength - mostly a function of the sand to portland cement ratio - is supposed to be verified with cylinder compression tests of the material used on site. (dotearth.blogs.nytimes.com)