Improving performance of Portland-limestone cements in sulfate exposures using supplementary cementing materials

Portland limestone cement (PLC) is a recent addition to North America, promoted for its environmental benefits while maintaining suitable properties in both the fresh and hardened state. However, due to concerns regarding the degradation of concrete in sulfate exposure, especially in cold environments, its use is currently limited. In Canada, this type of cement was first introduced in 2008, but its practice in sulfate exposure was prohibited according to CSA A23.1. In 2010, CSA A3001 introduced physical requirements and suggested minimum amounts of supplementary cementing materials (SCMs) to be used for a blended PLC to be considered in sulfate exposure. Alongside, CSA A3004-C8, the standard test method for evaluating performance of cement in sulfate exposure, was revised. In this study, different SCMs available in Canada were used in binary and ternary PLC blends in order to achieve sulfate resistance. For all PLC blends, ettringite sulfate attack (ESA) and thaumasite sulfate attack (TSA) were studied. It was found that the PLC blends containing the standard recommended amounts of SCMs were not resistant against TSA. However, when the SCM amount was increased in the blend, the desired resistance was achieved. Ternary blends of slag and fly ash were found to be the most resistant. In addition, mass changes in ESA and TSA as well as compressive strength and ultrasonic velocity in mortar samples were studied. Moreover, a combined differential scanning calorimetry and X-ray diffraction study were performed on samples in TSA and indicated formation of thaumasite in the deteriorated samples as well as on the surface of the intact ones. Interestingly, the mortar samples of the ternary PLC blend containing 40% slag and 20% fly ash were the only ones that did not show formation of thaumasite after a two-year TSA study. Overall, SCMs were found effective in improving the resistance of PLC blends in sulfate exposure. Additionally, it was found that, in general, the increase in the SCM content in a PLC blend promoted its resistance