Luke M. Snell, P.E
These days, adding fly ash to concrete is standard practice. Research has shown that fly ash is a supplementary cementitious material (SCM) that can replace part of the cement in concrete. Thus, batch plants can provide a more environmentally friendly and durable concrete, often at a lower cost. As the costs of fly ash increase, let's review its history, benefits, and future as an additive in concrete.
Fly ash is created when coal is burned at a power plant. The resulting very fine ash initially “flew" out of the smokestacks, hence its name. However, since fly ash was an atmospheric pollutant, environmental regulations later required power plants to capture it, which they disposed of in ponds or dumps.
In the early 1900s, using fly ash in concrete was found to be effective as a partial cement replacement. However, it took decades for the U.S. Bureau of Reclamation to use it on large construction projects. A Hoover Dam spillway, and the Hungry Horse Dam in Montana in 1953, were the first significant occurrences of fly ash used by the agency. Afterward, fly ash became routinely added to concrete, but most was still landfilled. The abundance of fly ash kept its price low.
Although the use of fly ash is considered a 20th-century discovery, its use began with the Roman Empire. Roman engineers discovered that adding volcanic ash, the equivalent of modern fly ash, produced superior concrete. However, they didn't know that the ash was reacting with a byproduct of the chemical reaction between cement and water to form a secondary cement.
There are many advantages to using fly ash in concrete:
· Less cement is required, which reduces the cost of concrete,
· The resulting concrete is easier to place and pump since fly ash has a spherical shape and will increase the slump of the fresh concrete,
· The resulting concrete is more durable since fly ash fills the voids making it less permeable,
· It reduces the chemical reaction between cement and certain aggregates, which results in less cracking and deterioration, and
· It slows the setting time of the concrete, allowing more time to finish projects in hot weather.
Not all fly ash is suitable for concrete use, as it must meet the standards stated in ASTM C618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete.” A critical factor is the percent carbon in the fly ash, which must be less than 6 percent. Older power plants are sometimes inefficient in burning coal and cannot meet this standard.
A recent issue is the availability of fly ash, as many power plants are switching from coal to cheaper fuel sources and no longer produce fly ash. Fly ash is imported from China and India in the Middle East because their power plants use natural gas and oil. In response, Saudi Arabia is considering grinding volcanic material as a substitute for fly ash, a technique used by the Romans.
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This article originally appeared in the bimonthly Arizona Contractor & Community magazine, Jan/Feb 2022 issue, Vol. 11, No. 1.