Pore structure evolution and sulfate attack of high-volume slag blended mortars under standard curing and steam curing

Partial replacement of cement with high-volume slag reduces the amount of cement in concrete and thereby lessens the environmental pressure caused by cement production. This paper investigates the pore structure evolution and sulfate attack of high-volume slag blended mortars under standard curing and steam curing, discusses the effects of slag content and pore structure on sulfate attack, and evaluates the methods for determining the sulfate resistance performance. The pore structure results show that the hydration of cement and/or slag reduces the porosity and most probable pore diameter (MPPD) and refines the pore structure of mortars. Steam curing primarily affects the pore size distribution but not the porosity for the mortars within 3 d. Increasing slag replacement increases the porosity for standard-cured and steam-cured mortars at the age of 28 d; however, it significantly reduces the air voids that are detrimental to sulfate resistance. Sulfate attack results show that the combination of the flexural strength test with the sulfate-ion content can evaluate the sulfate resistance well. The sulfate resistance improves with increasing slag content, and 70% of slag improves the sulfate resistance significantly for both standard-cured and steam-cured mortars. In addition, the MPPD, gel pores, and mesopores do not seem to determine the sulfate resistance. Reducing the volume fractions of large capillary pores, especially air voids, can improve sulfate resistance.