Frequently Asked Questions

Q. Where does fly ash come from?

A. Fly ash is produced through the combustion of coal used to generate electricity. After coal is pulverized, it enters a boiler where temperatures reach up to 1500 degrees Celsius. Upon cooling, the inorganic matter transforms from a vapor state to a liquid and solid state. During this process, individual and spherical particles are formed. This is fly ash. It is then collected by either using electrostatic precipitators, bag houses or a combination of both. Fly ash from these systems is collected in hoppers and then transferred to storage silos. Fly ash is tested for physical properties such as fineness, loss on ignition, and moisture, before it is shipped to its end user.


Q. What is the difference between Class C and Class F Fly Ash?

A. The primary difference between Class C and Class F fly ash is the chemical composition of the ash itself. ASTM C618 requires that Class F fly ash contain at least 70% pozzolanic compounds (silica oxide, alumina oxide, and iron oxide), while Class C fly ashes have between 50% and 70% of these compounds. Typically, Class C fly ash also contains significant amounts of calcium oxide (> 20%). Most Class F fly ash contains little calcium oxide; however, some Class F fly ash sources may contain intermediate levels (8% to 16%). 


Q. How Is Fly Ash Used?

A. The most common use of fly ash is as a partial replacement for portland cement used in producing concrete. Replacement rates normally run between 20% to 30%, but can be higher. Fly ash reacts as a pozzolan with the lime in cement as it hydrates, creating more of the durable binder that holds concrete together. As a result, concrete made with fly ash is stronger and more durable than traditional concrete made exclusively with portland cement.


Q. Is fly ash safe to use?

A. Fly ash is classified by ehe United States Environmental Protection Agency (EPA) as a non-hazardous product. As indicated by the material safety data sheet (MSDS), it is a relatively inert material. Fly ash is a product of coal combustion and thus possesses no significant risk of fire or explosion. Fly ash is similar to sand in composition and consistency. When transporting and handling fly ash, recommended precautions for safe handling as outlined on the product MSDS should be followed.


Q. What benefits accrue from the use of fly ash?

A. Re-use of fly ash into useful products can save utility customers millions of dollars a year. Unless they are recycled, the fly ash must be land filled, and the associated disposal expenses are passed along to the ratepayers. Recycling and re-use also saves energy because fly ash replaces products that require considerable energy to manufacture. Substituting fly ash for such products as limestone, clay, sand, and gravel also reduces depletion of these valuable resources.


Q. How does the use of fly ash help the environment?

A. Re-use of fly ash results in reduced dedication of otherwise productive land to landfill usage. Recycling fly ash also means reduced emissions of air pollutants, which result from making competing products. It also means fewer environmental problems, such as surface and groundwater problems, associated with mining virgin materials and reclaiming the land.


Inside power plant
Fly Ash Production and Utilization
Rate %
13.1 6.0 45.8%
6.8 2.3 33.8%
395.0 265 67.1%
Europe (EU15)
52.6 47.8 90.9%
105.0 14.5 13.8%
11.1 10.7 96.4%
United State of America
118.0 49.7 42.1%
Russian Federation
26.6 5.0 18.8%
Middle East & Africa
32.2 3.4 10.6%
Other Asia* 16.7 11.1 66.5%
Total/s 777.1 415.5 53.5%