Interfacial Behavior of Slag, Fly Ash, and Red Mud-Based Geopolymer Mortar with Concrete Substrate: Mechanical Properties and Microstructure

Geopolymer, as a new type of solid waste-based inorganic cementitious material, exhibits outstanding behavior in terms of physical and chemical performance, macromechanical properties, long-lasting stability, and features potential application development tendency in the field of repair and reinforcement of existing concrete structures. This paper investigated the interfacial behavior of geopolymer mortar with OPC concrete substrate under different slag, fly ash and red mud mixing proportions, while cement mortar was used as a control group for the research. The interfacial bonding properties of the geopolymer mortar to the OPC concrete substrate were elaborated by carrying out split tensile test, double-sided shear test, and three-point bending test. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were employed to further analyze the microstructural characteristics and physical phase components of the interfacial transition zone between the geopolymer mortar and the OPC concrete substrate. The results indicated that the compressive strength of slag-fly ash-red mud-based geopolymer mortar under different mixing ratio conditions was consistently superior to that of cement mortar, where the optimum mixing ratio for the geopolymer mortar was S33F33R33. Overall, the interfacial bonding properties of the geopolymer mortar to the OPC concrete substrate gradually increased with the increment of the slag content, however, an evolutionary trend of minor enhancement followed by a gradual reduction was observed with the growth of the fly ash and red mud content.