When coal and waste are burned in power plants, they create a non-combustible byproduct called coal ash. Two distinct types of ash particles make up coal ash: bottom ash, which collects on the bottom of coal furnaces, and fly ash, which is caught in the smokestacks. The vast majority of the non-combustible material is fly ash, which travels up in the flue gas and gets caught in the smokestacks and chimney. This residual needs to be cleaned out and disposed of on a regular basis. One way that this material can be reused is by combining it with other materials to create fly ash cement.

Concrete is traditionally made with Portland cement, a powdery substance made of ground clinker, calcium sulfate, and other minor additives. Clinker is a material usually made of limestone and minerals, which are crushed and ground together, then heated. Calcium sulfate is added, and the clinker is ground into cement powder. The process requires a large amount of energy; it has a huge carbon footprint, and accounts for approximately 7% to 8% of carbon dioxide emitted every year.

Fly ash, which is largely made up of silicon dioxide and calcium oxide, can be used as a substitute for Portland cement, or as a supplement to it. The materials which make up fly ash are pozzolanic, meaning that they can be used to bind — or cement — materials together. Pozzolanic materials, including fly ash cement, add durability and strength to concrete.

Fly ash cement is also known as green concrete. It binds the toxic chemicals that are present in the fly ash in a way that should prevent them from contaminating natural resources. Using fly ash cement in place of or in addition to Portland cement uses less energy, requires less invasive mining, and reduces both resource consumption and CO2 emissions.

While it is not considered a hazardous material by the US Environmental Protection Agency (EPA), fly ash may include considerable amounts of toxic materials. There have been incidents in which fly ash has leached into the groundwater causing cancer, birth defects, and other health problems. In many regions, there are no regulations or monitoring requirements for companies that create and dispose of fly ash. Ash that is not used to make products such as fly ash cement is often disposed of in landfills and abandoned mines.

It is currently unknown if the fly ash cement will leach chemicals into the air or ground; there is no data about what may happen in the future when concrete made with fly ash cement weathers and erodes. Several environmental agencies, such as the EPA, the Leadership in Energy and Environmental Design (LEED), and California’s Collaborative for High-Performance Schools (CHPS), have set limits on acceptable mercury levels in the fly ash used in fly ash cement. It is hoped that, by limiting some of the most dangerous chemicals found in fly ash, potential future problems can be minimized.