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Fly Ash Concrete

Inexpensive replacement for Portland cement

Fly ash is a fine, glass-like 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, 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 cement 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 cement, and other treatment options.


Affordability

Fly ash can be a cost effective partial substitute for Portland cement in markets that are experiencing supply shortages.

Environmental Performance

An industrial by-product that is otherwise waste, fly ash is environmentally friendly because it is recycled and has low embodied energy.


Easy

Packaged in bulk or bags, fly ash cement is generally available in two standard colors; coloring agents can be added at the job site. Manufacturers are developing specialty cements that should be widely available soon.


Fly Ash Concrete is currently cost competitive with Portland Cement Concrete.


Not Applicable


The first ASTM specification for fly ash cement was written in the 1950s, and amended in 1977 to include Class C fly ash from Western coal. Fly ash cement is ASTM listed and approved as a mineral admixture for use in mortar, patching, and structural concrete.


Not Applicable


Because fly ash cement requires less water than portland cement, it is easier to use in cold weather. It can be formulated to produce various set times, cold weather resistances, strengths and strength gains, depending on the job. Fly ash can be used sparingly as an admixture or in large amounts to replace portland cement. The material is somewhat lighter than portland cement.


Not Applicable


Fly ash is cost competitive with portland cement. Some manufacturer’s proprietary fly ash cement is considered a non-shrink material with advantages in workability, water retention, and strength. Because fly ash mixes with less water, it is less likely to crack. An industrial by-product that is otherwise waste, fly ash is environmentally friendly because it is recycled and has low embodied energy.

Fly ash comes from various operations in different regions, so its mineral makeup may vary between manufacturers. There are some concerns about freeze/thaw performance and a tendency of mixes made with fly ash to effloresce, especially when used as a complete replacement for portland cement.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.