Fly Ash: A nuisance dust worth its weight in cement

Today's fly ash does not come from a volcano but from a power plant. In present day coal fired power plants, coal is first ground into a fine powder and then injected into a furnace, where it burns at a very high temperature. The burning process forms a glass-like, silica-rich residue known as fly ash. Once collected the fly ash is tested in accordance with ASTM C 618 and identified as either a class F fly ash, which is produced by burning anthracite or bituminous coal, or class C fly ash, which is produced by burning sub-bituminous coal.

Fly ash is one of our oldest building materials. The Romans first used it as a cement to build the Coliseum. They found that mixing volcanic ash with lime would produce crude but durable mortars and concretes. The ash that they used came from a volcano in the town of Pozzuoli. The term pozzolan is derived from the word "Pozzuoli", and is now used to classify this group of admixtures for concrete.

Fly ash can be safely used to economically replace as much as 25% to 30% of the portland cement in concrete.
Fly ash contains a high percentage of silica in the form of silicon dioxide, which appears as hollow glass spheres when viewed under a microscope. It is the silica in fly ash that reacts with calcium hydroxide found in the cement paste of hardened concrete. Calcium hydroxide, a by-product of the portland cement hydration process, is a very weak material that adds no structural strength to concrete. When the silica in fly ash reacts with calcium hydroxide, however, good things happen. Beneficial calcium silica hydrate (C-S-H) is formed. It is C-S-H that cements sand and large aggregates into a solid mass known to us as concrete. It has been found that fly ash can be safely used to economically replace as much as 25% to 30% of the portland cement in concrete.

The reaction between the fly ash and the calcium hydroxide takes slightly longer to produce calcium silicate hydrate than does the hydration process of portland cement.

Therefore, concrete mixes that have part of the portland cement replaced with fly ash require a longer time to gain design strength. A pure cement concrete mix will require approximately 28 days to reach 95 percent of its ultimate design strength, whereas a fly ash and portland cement concrete may require nearly twice as long. In cool weather a chemical accelerator admixture is commonly used to reduce the set time, in order to allow timely finishing for the concrete.

In addition to its strength building and cost saving properties, there are many reasons to consider using fly ash regularly in concrete mix designs.

The first use of fly ash concrete in the U.S. was at the Hoover Dam project in 1929. Due to the massive volume necessary for its construction, the exothermic heat generated by hydrating portland cement was anticipated to be a problem on this project. Too much internal heat in concrete was predicted to have a detrimental effect on concrete strengths. It was found that by reducing the amount of portland cement in a concrete mix and replacing it with fly ash, the same strength could be achieved while the internal temperature of the concrete could be greatly reduced. If this material, along with other techniques, had not been employed it is estimated that it would have taken approximately 150 years for the Hoover Dam face to cool to ambient temperature.

Fly ash has also been found to effectively and economically reduce the risk of ASR (Alkali-Silica Reaction). Under certain conditions, this problem can result in accelerated concrete deterioration. The inclusion of fly ash in suspect concrete mix designs containing unclean and deleterious aggregates has been found effective in mitigating the effects of ASR and is historically the most common method employed. For more information regarding ASR and the beneficial effects of fly ash in concrete mix designs, refer to the informative article appearing on these pages by Rachel Detwiller (Concrete News, Spring 2004).

Another benefit of fly ash is the finishing effect of the glass sphere shape of the fly ash. It acts as a finishing lubricant in the concrete mix. Easier placement can reduce the mixing water by 3 to 5 percent, while maintaining the same placement consistency (slump).

Finally, with the environment being the issue that it is today and recycling becoming a way of life, fly ash has been identified for its beneficial GREEN properties. Fly ash containing concrete mixes can receive LEED credit under the guidelines established by US Green Building Council. The LEED credit system has prompted more and more project managers to require fly ash to be used in their concrete.

In conclusion, the benefits of using fly ash in concrete mix designs are numerous. Its frequent use will help designers obtain better, stronger, and more cost effective concrete-something from which we can all benefit.

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© 2006 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2006.

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