Experimental investigation into the influence of calcium aluminate cement on the micro- and macro-mechanical properties of the interfacial transition zone in geopolymer concrete

Abstract

This study aimed to analyze the properties of the interfacial transition zone (ITZ) in fly ash geopolymer concrete (FAGC) containing various concentrations of calcium aluminate cement (CAC) (5%, 10%, 15%, and 20%). The micro-mechanisms of the ITZ were investigated using nanoindentation, scanning electron microscopy–energy dispersive spectroscopy, and backscattered electron imaging. The macro-properties, such as the interfacial bonding strength and the interfacial fracture energy, were also evaluated. The results indicated a trend of decreasing ITZ width with increasing CAC content from the nanoindentation test. The minimum ITZ width was observed in the FAGCC20 sample at 10 μm, representing a 71.43% reduction compared with the 35 μm width of the FAGCC0 sample. The formation of greater amounts of polymerization products, i.e., N-A-S-H and C-A-S-H gels, filled the pores and weakened the cracks, which notably improved the porous state of the ITZ. The changes in the microstructure of the ITZ directly affected the macroscopic performance of the FAGC. The FAGC samples with CAC showed higher interfacial bonding strengths and fracture energies than the FAGCC0 sample. Consequently, the FAGCC20 sample exhibited the highest interfacial bonding strength and fracture energy of 2.37 MPa and 31.1 N/m, respectively.