Microstructure and microhardness property of the interface between a metakaolin/GGBFS-based geopolymer paste and granite aggregate

The application of geopolymers as an alternative to Portland cement has received significant attention in concrete development due to the promotion of sustainability. Interfacial transition zone (ITZ) has been characterized as the weakest link in ordinary Portland cement concrete and usually deteriorates the mechanical properties and durability of concrete. To further understand the performance of geopolymeric concrete, the microstructure and properties of the interface zone in geopolymeric concrete are investigated in this study. Metakaolin and ground granulated blast-furnace slag (GGBFS) were used as the raw materials for geopolymer synthesis. Cubic splitting tests were conducted to investigate the interfacial bond strength between the geopolymer and the granite aggregate for different activator moduli and liquid/solid ratios. The morphology and mechanical properties of the ITZ between the geopolymer and granite aggregate were studied with the aids of scanning electron microscopy, energy-dispersive X-ray spectroscopy, and microhardness testing. The results indicate that the bond strength between the geopolymer and aggregate is higher than that between the cement paste and granite aggregate, and it decreases with the increase of the activator modulus and liquid/solid ratio. An ITZ was observed between the geopolymer paste and granite aggregate, and its microstructure and chemical components were different from those of the geopolymer. Obvious inhomogeneity can be observed for the ITZ along the aggregate body. The microhardness of the ITZ is lower than that of the geopolymer binder, and an increase in the activator modulus and liquid/solid ratio leads to an increase in the width of the crack in the ITZ and a decrease in the microhardness. (C) 2019 Elsevier Ltd. All rights reserved.