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What is Fly Ash?

By D.M. Abrecht
Updated May 17, 2024
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Fly ash, also known as Pulverized Fuel Ash (PFA), is an industrial ash created when coal is burned to create electrical power. As the gaseous emissions of burning coal cool, some of its chemical constituents solidify into spherical granules, forming fly ash. A fine, glassy powder, its chemical components vary but usually include oxides of silicon (SiO2), aluminum (Al2O3), iron (several kinds), and calcium (CaO). It is found in power plant chimneys and has several industrial uses, the most noteworthy being as an additive to cement.

When coal is burned, the byproducts of the combustion process separate into heavier components, which sink to the bottom of the burner and become bottom ash, and gaseous emissions, which escape from the top of the burner. Particles of fly ash precipitate from the gas as it rises. In most plants, these are captured from the air using an electric charge generated by a device called an electrostatic precipitator. Because of the conditions of their formation, the particles are extremely fine, mostly spherical, and all nearly the same size.

The principal question surrounding fly ash is what to do with it. Once allowed to escape into the air as an industrial pollutant, the law now requires that it be removed from coal plant emissions and either disposed of as solid waste or recycled. Disposing of the ash presents problems, because so much of it is produced. Most of the ash is dumped in holding lagoons or in landfills. The ash contains toxic heavy metals, and environmentalists are concerned that these could leach into soil or escape into the environment if the lagoons rupture.

Fortunately, fly ash is a pozzolan, a material that can serve as a cement when mixed with lime and water. Because of this, it is increasingly recycled as an economical extender for portland cement, the common cement used to make concrete. There are a number of benefits: the resulting concrete is denser, smoother, easier to work with, more resistant to chemical erosion, and stronger over the long term. It also requires less carbon dioxide to produce, and creates less pollution. The ash may also be used to make asphalt, bricks, paints, tiles, and backfill.

Fly ash is one of several byproducts of the coal-burning process, called coal combustion products (CCPs). Others include bottom ash, boiler slag, and flue gas desulfurization (FGD) materials such as gypsum. Many of these can also be recycled as construction materials to reduce their impact on the environment.

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Discussion Comments
By anon987826 — On Feb 05, 2015

Does the fly ash in CMU concrete blocks make them undesirable to use as raised bed vegetable garden sides ?

By bangus — On Jan 23, 2013

There is nothing safe about recycling fly ash into concrete. I have first hand experience of this because I have my back yard done with it. The top never hardens solid and wear and tear loosens it up. Next thing you have is fly ash carried into the house and before you know it every floor in the house is white with it. The opening and closing of doors, traffic and draughts etc causes it to get airborne and then you are exposed to all the harmful metals and particulate matter that comes from it. You can seal it but this lasts no time at all and soon wears off. When this PFA gets wet it turns into an acidic solution.

I worked in a concrete plant for nine years. The first seven of them were OK. Then the company began to use a PFA blended cement product. In two years, it had destroyed my entire health to the point that I was medically retired at 44 and left with health so bad I am unable to work now to provide for my family.

This burned the paint and glass on a colleague's three month old Audi A3 that was parked 200 metres from our batching office. I developed a list of illnesses and diseases including pneumonia, interstitial lung disease, diverticulosis of the bowel which deteriorated into diverticulitis when my bowel became perforated. I have difficulties swallowing, stomach, intestines, bladder and prostate problems. It burned my skin, corneas, lips, throat, scalp, feet, tongue, eyelids, all of my mucous membranes and burned my gums away to the point that all of the roots of all of my teeth were left exposed and only the good Lord knows what else. Firstly, PFA when it gets wet, turns into sulphuric acid. It's also toxic because it's essentially a CDNP or combusted derived nano particulate. It's radioactive and because of the metals in it, it is also a carcinogen.

Between getting burned both internally and externally, it has my entire health in absolute ruin and all the time this was happening, the company did nothing to protect three of us employed at their plant. The other thing about it is that the symptoms start off mild and certainly in my case, they continued to deteriorate for a good while after I was medically retired and away from it.

By John57 — On Mar 29, 2012

When I was growing up, my dad was responsible for keeping the coal furnace at our church burning. That meant he would get there early enough before services to make sure the furnace was burning and the building was warm.

This also means he was responsible for somehow getting rid of the fly ash. I remember going with him many times to check on the furnace, but I have no idea what he did with the ashes.

Back then, they didn't have the environmental awareness like there is today. My parents recycled many things out of necessity, but I don't think he ever thought about a way to recycle fly ash.

By bagley79 — On Mar 28, 2012

@julies - I am wondering if there is too much cost incurred to use fly ash in making concrete. Otherwise, this would be much more environmentally friendly than just dumping it somewhere.

There are very few things that don't have some way they can be recycled. The problem is, many times it costs more to do it this way.

This reminds of playground material that is used from recycled tires. Someone found a great way to recycle old tires, but for some reason, you hardly ever see this being used.

There might be similar reasons that fly ash utilization is so low. I wouldn't be surprised if money was not the biggest reason.

By julies — On Mar 28, 2012

I would think as much concrete that is poured for construction projects every day, there would be no problem using up all of the fly ash. This sounds like the best way to take care of fly ash disposal.

When you know that concrete made with fly ash is smoother and easier to work with, why isn't it used in all concrete?

My husband works in construction and pours concrete several days a week. One of the things they always strive for is a smooth finish.

They also like to have cement that is easy to work with. I have no idea if the concrete they use has any fly ash in it or not. I don't even know if he is aware if the concrete they use has any fly ash in it.

By umbra21 — On Mar 27, 2012

@pastanaga - Coal fly ash was once just released into the atmosphere and is the reason the industrial revolution was such a polluted time. It caused all the smog and the acid rains they had back then.

Once people realized what was happening they made it so that it had to be disposed of properly. And yes, it's a perfect solution to make it into concrete as it is much more useful and safe in that state.

Unfortunately I don't believe they even manage to make half of it into concrete. It's just so much easier to dump it and even though it has a use so much is produced that it's not worth very much. When you take transport and so forth into consideration, it's cheaper to dump it, so people do so.

A very bad idea, as even when they put it somewhere they think is safe, there's no telling when it might eventually leach into the soil. I wish harsher laws were in place so that they had to use it all as concrete.

By pastanaga — On Mar 26, 2012

It's a shame it took them so long to come up with a way of using fly ash. It is a terrible pollutant and there are plenty of places in the world which have been ruined from the heavy metals leaching into the soils. Unfortunately, heavy metals are very difficult to remove from the environment once they have been put there so anywhere coal was burned and the ash wasn't disposed of properly, might be ruined for a very long time.

Coal was once the primary power source for all industry so you can imagine how much damage has been done over the years.

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