Study of interfacial transition zone in concrete with SCMs

Abstract

The interfacial transition zone is one of the weakest areas in concrete. Microcracks and new phases often form in this zone, affecting the resulting concrete’s mechanical properties. In this research, concrete in which Portland cement was partially replaced by powdered limestone and/or metakaolin was cured for 1 and 28 days and then mechanical properties were tested. The interfacial transition zone was studied by scanning electron microscopy and selected phases were analyzed by electron microprobe. The addition of a small amount of limestone or metakaolin increases the strength of the concrete. The strength significantly decreases with a content of 10% or more supplementary cementitious materials (SCMs) in concrete. The microcrack’s thickness at the binder-aggregate interface reaches up to 0.8 μm. In concretes with limestone, rhombohedral calcite crystals up to 0.5 μm in size accumulate in the interfacial zone. This accumulation can significantly reduce the strength of the concrete when the amount of limestone is high. Zeolites with a composition corresponding to Mg-mazzite are formed at the interface of cement paste, quartz and albite, partially replaced by sericite.