Factors Affecting Sulfate Resistance of Mortars

摘要

The ability of a mortar made using portland cement, fine aggregate, and water to resist attack by sulfates is affected by the proportions of the mortar (water-cement ratio and cement content), by the maturity, by the amount of tricalcium aluminate in the portland cement, by the presence of tricalcium aluminate-sodium oxide solid solutions with different structures and reactivities, and by the composition, reactivity, and amount of pozzolan used together with the portland cement. Cements investigated included: portland cements of Types (I-III), and V meeting ASTM C 150; blended cements including Type IP's meeting ASTM C 595 that were made from the same clinkers as the Type I's; and Type I's blended with pozzolans including fly ashes produced by burning bituminous, subbitumous, and lignitic coals, calcined natural volcanic glass high in silica, and silica fume. Silica fume forms glassy microspheres that may contain over 90% SiO2; it is a by-product of the production of silicon metal. Some of the fly ashes produced from subbituminous and lignitic coals replacing 30% by volume of cements increased the expansion of mortars containing the blends when stored in sulfate solutions. This behavior reflects SiO2 below 50%, Al2O3 16 to 26%, and CaO 5 to 30%. Al2O3 and CaO in the fly ash glass are readily available to combine with sulfate to form ettringite. With cement of lower C3A content, some of the subbituminous and lignitic fly ash blends improved the sulfate resistance of mortars, except when SiO2 in the fly ashes was 38% or less.