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Fly Ash Concrete
Inexpensive replacement for Portland cement

Fly ash is a fine, glasslike powder recovered from gases created by coal-fired electric power generation. U.S. power plants produce millions of tons of fly ash annually, which is usually dumped in landfills. Fly ash is an inexpensive replacement for portland cement used in concrete, while it actually improves strength, segregation, and ease of pumping of the concrete. Fly ash is also used as an ingredient in brick, concrete block, paving, and structural fills.

Fly ash concrete was first used in the U.S. in 1929 for the Hoover Dam, where engineers found that it allowed for less total cement. It is now used across the country. Consisting mostly of silica, alumina and iron, fly ash is a pozzolan--a substance containing aluminous and silicious material that forms cement in the presence of water. When mixed with lime and water it forms a compound similar to portland cement. The spherical shape of the particles reduces internal friction thereby increasing the concrete's consistency and mobility, permitting longer pumping distances. Improved workability means less water is needed, resulting in less segregation of the mixture. Although fly ash cement itself is less dense than portland cement, the produced concrete is denser and results in a smoother surface with sharper detail.

Class F fly ash, with particles covered in a kind of melted glass, greatly reduces the risk of expansion due to sulfate attack, as may occur in fertilized soils or near coastal areas. It is produced from Eastern coal. Produced from Western coal, Class C fly ash is also resistant to expansion from chemical attack, has a higher percentage of calcium oxide, and is more commonly used for structural concrete.

Although the Federal government has been using the material for decades, smaller and residential contractors are less familiar with fly ash concrete. Competition from portland cement is one consideration. Because fly ash comes from various operations in different regions, its mineral makeup may not be consistent; this may cause its properties to vary, depending on the quality control of the manufacturer. There are some concerns about freeze/thaw performance and a tendency to effloresce, especially when used as a complete replacement for portland cement.

The Clean Air Act of 1990 requires power plants to cut nitric oxide emissions. To do so, plants restrict oxygen, resulting in high-carbon fly ash, which must be reprocessed for concrete production. Thus, fly ash could be less available or more costly in the future. Researchers at Brown University are studying why the high-carbon ash doesn't work for concrete, and other treatment options.

Article courtesy of:
NAHB Research Center

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