Fly Ash

Fly ash is a product recovered from coal-fired electric power generation plants. It is a fine powder, and U.S. power plants make millions of tons of it every year, most of which is dumped in landfills, rather than recycled. It can be used as an admixture to replace Portland cement in concrete. It improves concrete strength and ease of pumping. It can also be used in the creation of brick, paving, blocks, and structural fillings.

First used in 1929 in the construction of Hoover Dam, it is now used across the U.S.

It can be purchased in bags or in bulk and is competitive with Portland cement on price.

The first ASTM specifications for fly ash cement was written in the 1950s and amended in 1977 to include ash from western coal.

It is ASTM approved for use in mortar, structural applications, and for patching concrete. Fly ash concrete has a reduced risk of expansion from sulfate attack. This is a risk in concrete structures in fertilized soils, or near coastal regions.

Class C fly ash also resists expansion from chemical attack, but has more calcium oxide and is used more in structural concrete projects.

The U.S. government has used it for several decades, but small contractors are less familiar with it.

Since it comes from different operations in different regions of the U.S., the mineral makeup of the ash will vary.

When used as a total replacement for Portland cement, there have been reported concerns with freeze and thaw performance.

If you define "fly ash" generously, you can say that it was used over 2000 years ago in the construction of Roman structures like the Colosseum - which is still around!

The ash used in building the ancient Roman structures was actually volcanic ash, but structurally and chemically, it was not unlike today's admixture.

Ash has high levels of silica and alumina in their reactive forms. They conveniently complement the hydration chemistry of cement. When cement and water react, the process is called hydration. Hydration of cement produces C-S-H gel, the strongest part of the "paste" in concrete, and it also produces calcium hydroxide, or free lime.

Excessive free lime can cause concrete to be too porous, but coal ash binds free lime over a long period of time, increasing compressive strength.

Adding ash generally creates a longer lasting concrete structure.

Benchmarks evaluating concrete performance with respect to strength and permeability show that adding ash makes concrete denser by reducing water demand in concrete and improving concrete's microstructure.

It affects the concrete's plastic properties, improving workability, lowering the heat of hydration, and reducing alkali-aggregate reaction.

Concrete with coal ash reaches its maximum strength more slowly than traditional concrete, but the compressive strength is generally greater due to reduced permeability. Also, it substantially contributes to chemical resistance of concrete.

Fly ash is one of three general coal combustion by-products. Using it in concrete benefits the environment by diverting fly ash from the waste stream. Therefore, it reduces the energy investment in processing virgin materials, conserving them, and reducing pollution. The state of Texas alone makes 13 million tons of coal ash per year. Replacement of Portland cement with coal by-product reduces the "carbon footprint" of concrete, because production of one ton of Portland cement results in production of approximately one ton of carbon dioxide.

No further carbon dioxide is used with extant coal ash (although new ash production - burning of coal - produces 20 to 30 tons of carbon dioxide per ton of ash).

Coal ash is not known to be risk-free. It often contains contaminants like arsenic and lead, and in situations where ash is used to sculpt land, such as the creation of a golf course, there is not enough evidence yet to conclude whether contaminants do or do not leach into groundwater.

There is still much to learn about any potential hazardous effects of coal ash in concrete, but evidence so far suggests that reusing the by-product in concrete is a safer way of dealing with it than putting it in a landfill.

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