What is fly ash? Fly ash is the bi-product of the coal combustion process used to make electricity at many power plants across the country. It is the light-weight material that is separated from bottom ash, which is then collected and stored in silos for testing or recycling. Why does using fly ash benefit the environment? There has been a lot of discussion in recent years about the use of fly ash and how it can benefit the environment. When fly ash is used as a replacement for Portland cement in the process of producing cement, it helps to: Reduce greenhouse …