Sulfate Resistance of Mortars Containing High-Calcium Fly Ashes and Combinations of Highly Reactive Pozzolans and Fly Ash

Abstract

This paper reports results from ASTM C 1012 (sulfate resistance) tests on combinations of fly ash and silica fume with portland cements of varying C3A content. Tests on different types of fly ashes confirm previous findings regarding the effects of fly ash composition. Low-calcium ashes invariably improve the sulfate resistance of mortars made with high-C3A portland cement and generally meet the criteria for high-sulfate resistance, provided a sufficient level of replacement is used (e.g.> or = to 20 to 25% fly ash by mass of cementitious material). Moderate-calcium ashes were generally less effective, but could still be used to produce blended cements of moderate to high sulfate resistance when combined with a high-C3A cement at replacement levels of 20%. High-calcium (i.e. CaO > 20%) fly ashes showed highly variable performance and in many cases replacement levels of 20% to 40% actually lead to increased expansion when compared with high-C3A cement used on its own. Results for a classified ultra fine fly ash indicate improved performance with relatively low levels of ash (e.g. 8 to 16%) producing a high level of resistance when combined with high-C3A cement. It was found that mortars containing moderate to high levels of high-calcium fly ash could be produced to meet the criteria of high-sulfate resistance by using either portland cements of lower C3A content or ternary cement blends containing relatively low levels of silica fume (e.g. 3 to 6% by mass of total cementitious material). However, in view of the highly variable performance observed for mortars containing high-calcium fly ash, the need to test individual combinations of materials is stressed.