Fly ash is produced from the burning of a coal-fired boiler which is usually extracted by means of electrostatic precipitators, bag houses, or mechanical collection devices[i]. During the process of extraction or combustion, all the ash that leaves the furnace becomes fly ash.
ACAA or the American Coal Ash Association, an organization that represents the coal combustion byproducts industry, has reported various benefits from coal fly ash. Among them are: Cement production and/or concrete products; structural fills or embankments; stabilization of waste materials; road base or sub base materials; flowable fill and grouting mixes; and mineral filler in asphalt paving. Although not all fly ashes are converted into a renewable product, larger amounts (tons) are deposited in dumpsites and landfills.
Based on a survey conducted in 1993 by ACAA, about 80.3 million metric tons of CCP (coal combustion by-products) were produced in the United States alone. CCPs were mostly coming from fly ash, bottom ash, boiler slag, and FGD (Flue Gas Desulfurization) material[ii]. There has been a notable volume of CCPs produced from fly ash. Sadly, only low volumes of CCPs are reported to be used every year.
Distribution, utilization, importations were just some of the common underlying reasons why CCPs were not very popular. Industry sectors were doubtful about the quality of these engineered materials from coal ash. Processing costs might also be among the contributing factors, which have highly affected the underutilization of these materials. The support provided by the government did not make any sizable contribution. But as the years go by, more and more efforts for further development and desire for sustainability have become more prevalent.
Today, fly ash is gradually gaining popularity from reliable sectors that are highly engaged in coal ash production. Fly ash is now being used to create cement and concrete products, with various kinds of concrete products made of fly ash. Quality is absolutely not compromised and in many cases can surpass that of traditional materials due to characteristics intrinsic in fly ash. With the application of state-of-the-art technologies, fly ash could possibly be the primary substance for concrete materials in the near future.
Flowable mixture or CSLM (Controlled Low Strength Materials) is another innovative fly ash application which also works for backfills and road base/ sub base construction. The controlled density and cohesiveness of fly ash makes it a desirable component for CSLM. It works the same way as traditional soil back fillings. It should be noted though, that there are certain guidelines or specifications in the application of CSLM to better ensure the quality of the end-product.
Fly ash is also used for structural fills. When it is compacted, it forms a structural fill. Structural fills can be used as foundations for building construction and other structures. Fly ash has a considerable amount of high shear strength making it the best substitute over soil and rock.
While the current market for fly ash is still in its infancy, there is no telling the long term potential and positive impact that it can have on the world.