Coal Fly Ash in Making Cement, Grout and Concrete

Qualities

• Concrete made with fly ash is lighter and less porous than concrete made with only Portland cement. Fly ash concrete stays workable longer and is easier to pump or pour. Fly ash in concrete helps protect metal rebar and other metals submerged in the concrete from corrosion. Adding fly ash increases the durability of cement and concrete. Fly ash also reduces shrinkage when used in cement, concrete and grout.

Environmental Benefits

• Using fly ash in concrete, cement and grout recycles a substance that would otherwise sit in landfills. The United States uses about 30 percent of its fly ash in concrete production. Fly ash replaces part of the Portland cement used in making concrete and grout, conserving the cement. Production of Portland cement uses large quantities of sand and limestone. Concrete made with fly ash also requires less water for mixing, conserving water.

Fly Ash Proportions

• The proportion of fly ash used in concrete ranges from 15 percent up to 50 percent. According to the U.S. Department of Transportation, the most common ratio is 1 to 1 ½ pounds of fly ash for each pound of cement. The higher the volume of fly ash in the mixture, the longer the mixture takes to set. The concrete continues to get stronger for up to 90 days after it sets. For grout, the proportions are typically 1 part cement to 3 parts fly ash.

Types of Fly Ash

• Fly ash is designated as either Class C or Class F. Burning anthracite or bituminous coal produces Class F fly ash, and burning sub-bituminous and lignite coal produces Class C fly ash. Class C contains some lime, which gives it many of the properties of cement when mixed with water. Class F contains no lime and will not cement together when mixed with water. Both Class C and Class F fly ash are used in concrete, cement and grout.