Use of Fly Ash in Concrete

Description

Fly ash, a by-product from the combustion of pulverizedcoal, is widely used as a cementitious and pozzolanic ingredientin hydraulic cement concrete. Because it improves manydesirable properties of concrete, it is introduced either as aseparately batched material (as in ASTM C 618, Class F or C) oras a component of blended cement (ASTM C 595 or C 1157). This report describes the use of fly ash in concrete and listsreferences concerning the characterization of fly ash, itsproperties, and its effects on concrete.

Guidance is providedconcerning specifications, quality assurance and qualitycontrol of fly ash, and concrete produced using fly ash.According to ACI 116R, fly ash is “the finely dividedresidue that results from the combustion of ground orpowdered coal and that is transported by flue gases from thecombustion zone to the particle removal system.” ACI 116Rdefines “pozzolan” as “a siliceous or siliceous and aluminousmaterial that in itself possesses little or no cementitious valuebut that will, in finely divided form and in the presence ofmoisture, chemically react with calcium hydroxide at ordinarytemperatures to form compounds having cementitiousproperties; there are both natural and artificial pozzolans.” Flyash possesses pozzolanic properties similar to the naturallyoccurring pozzolans of volcanic or sedimentary origin found inmany parts of the world. About 2000 years ago, the Romansmixed volcanic ash with lime, aggregate, and water to producemortar and concrete (Vitruvius 1960).

Similarly, fly ash is mixedwith portland cement (which releases lime during hydration),aggregate, and water to produce mortar and concrete.All fly ashes exhibit pozzolanic properties to someextent; however, some fly ashes display varying degreesof cementitious value without the addition of calciumhydroxide or hydraulic cement.