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

摘要

This paper reports results from ASTM C 1012 (sulfate resistance) tests on combinations of fly ash and silica fume with portland cements of varying C_3A 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-C_3A portland cement and generally meet the criteria for high-sulfate resistance provided a sufficient level of replacement is used (e.g. ≥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-C_3A 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-C_3A 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-C_3A 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 C_3A 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.