Some producers of concrete use fly ash in concrete. What is fly ash? And what does using fly ash in concrete accomplish?

Fly ash (aka flue ash) is often used as a replacement for Portland cement in the production of concrete.

As the Federal Highway Administration describes it, fly ash “is the finely divided residue that results from the combustion of pulverized coal.”

Origins of Fly Ash

Where does it come from? The FHA explains that, too, in tremendous detail.

“Fly ash is produced by coal-fired electric and steam generating plants,” the FHA writes. “Typically, coal is pulverized and blown with air into the boiler’s combustion chamber where it immediately ignites, generating heat and producing a molten mineral residue. Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash. Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in the flue gas. Prior to exhausting the flue gas, fly ash is removed by particulate emission control devices, such as electrostatic precipitators or filter fabric baghouses.”

And then what? This is where producers of precast concrete enter the picture. Fly ash is added to Portland cement concrete, in addition to a number of other applications. These additional uses of fly ash include asphalt filler and soil stabilization.

“The use of fly ash in Portland cement concrete (PCC) has many benefits and improves concrete performance in both the fresh and hardened state,” the FHA writes. “Fly ash use in concrete improves the workability of plastic concrete and the strength and durability of hardened concrete. Fly ash use is also cost-effective. When fly ash is added to concrete, the amount of Portland cement may be reduced.”

Using this ash in concrete means the finished product is stronger, more durable, and less permeable to water.

Environmental Benefits

Best of all, using fly ash in concrete means there will always be a demand for it, eliminating the need to send the ash to landfills. In fact, before the enactment of strict environmental measures, fly ash used to be released directly into the sky during coal firing.

The FHA lists four “significant environmental benefits” of using fly ash in concrete.

  1. Increases the life of concrete products
  2. Reduces energy use, greenhouse gases, and other adverse air emissions
  3. Reduces the amount of coal combustion products dumped in landfills
  4. Conserves natural resources and materials.

Fly ash can also be used as a replacement for clinker, which is a main component of Portland cement.

As we explained in a blog post on the different types of sustainable concrete, “The making of cement — a big component of concrete — itself can be a laborious process. Clinker … is created by heating waste materials at very high levels, creating an ashy substance. In some specific applications, fly ash can be used as a replacement.”

So there you have it! Using this material in concrete helps manufacturers continue to provide concrete products to contractors and developers. And it helps maintain ecologically friendly production standards.

Let us know if you have any questions.

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