Controlling aluminate phase hydration for sulfate resistance of Portland–limestone cements

Abstract

Sulfate optimisation of the aluminate phase (C3A) in Portland–limestone cements (PLCs) is critical for improving their hydration performance. This study investigates the changes in the sulfate attack performance of an undersulfated interground PLC (14.6% limestone) through the addition of gypsum. Isothermal calorimetry is used to measure the early-age hydration kinetics to optimise the sulfur trioxide content of the PLC. Sulfate expansion and strength loss of the undersulfated and optimised PLC in combination with fly ash is measured in both sodium sulfate and magnesium sulfate solutions. Sulfate optimisation of the PLC reduced its expansion to a greater extent in magnesium sulfate and improved strength at later ages to a greater extent in sodium sulfate. Less physical deterioration was also observed for the optimised PLC samples. Fly-ash incorporation resulted in improved expansion and strength performance for certain mixtures. Optimised samples underwent less sulfate attack as indicated by their higher calcium hydroxide contents measured by thermogravimetric analysis at later ages. The improved sulfate attack performance can be attributed in part to the formation of carboaluminate hydrates as determined by X-ray diffraction in the optimised PLC. This research study highlights the importance of sulfate optimisation in achieving PLCs with greater sulfate attack resistance.