What Is Fly Ash Cement?

Fly ash cement is a primary ingredient in certain concretes that is often used as an alternative to Portland cement, which is a more traditional concrete base. The ash itself is a byproduct of coal combustion, but once formed into a cement the ash usually binds in such a way as to neutralize many of the chemicals and toxins that have made coal burning so controversial in recent years. As such, the cement is often thought of as a way to not only clean up the coal industry but also leave less of a footprint and reduce the energy required to form all sorts of concrete projects and structures. It’s often advanced as a "green" concreting solution, though it's not without its critics. Fly ash is normally considered suitable for all sorts of concrete projects, but many places have set limits on how much of it can be used at once pending more environmental impact reports. Though many of the harmful chemicals associated with coal burning do get bound in the cement, it’s unclear if or under what conditions they may begin to leach out once more.

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 as it "flies" up and away. 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 a cement.

One of the most common uses for fly ash cement is as an additive to form concrete. 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 generally add durability and strength to concrete.

Concrete made with fly ash is often thought to be environmentally green. It binds the toxic chemicals that are present in the fly ash in a way that should prevent them from contaminating natural resources. Using green concrete 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 most of the world’s regulatory agencies including 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 cement made from coal burning byproducts will leach chemicals into the air or ground, and there is very little data about what may happen in the future when concrete made with it 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.