Alkali-Aggregate Reaction in Concrete Containing Fly Ash


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Title: Alkali-Aggregate Reaction in Concrete Containing Fly Ash

Author(s): Josef Parbiarz and Ramon L. Carrasquillo

Publication: Special Publication

Volume: 100


Appears on pages(s): 1787-1808

Keywords: alkali-aggregate reactions; alkali content; cements; concrete durability; fly ash; portland pozzolan cements; silica; tests; Materials Research

Date: 4/1/1987

The effectiveness of the use of fly ash in concrete to reduce the damage to concrete due to alkali-aggregate reaction to acceptable levels was investigated. More than 1300 mortar bars were cast and tested according to the ASTM C 227 mortar bar test method, with 0, 17, 26, 34, 45, and 62 percent replacement of the volume of cement in the mixture with fly ash. The effect of silica fume in the mixture was compared to that of fly ash at 17, 34, and 45 percent cement replacement by volume. The variables studied included the type of aggregate, alkali content of the cement, type of pozzolan, percent of cement replacement, pH of mixing water, and blending of the cement with the fly ash. Both ASTM Class C and Class F fly ashes were investigated. The results indicate that the replacement of a portion of the volume of cement with an equivalent volume of fly ash tends to reduce the expansion caused by the reactivity between alkalies and reactive silica in the aggregate, provided the proper amount of cement is replaced. This amount appears to depend on the alkali content of the fly ash and on the chemical composition of the fly ash used, mainly calcium oxide content. For some high calcium fly ashes with more than 1.5 percent available alkali content, a "pessimum limit" was observed. Such a limit represents a percent replacement under which the addition of fly ash causes equal or greater expansions in the mortar bars than that of the mixture without fly ash, and above which the expansions are reduced. In some instances, more than one "pessimum limit" was detected for a given combination of materials.