Mine tailings-based geopolymer was proved to be successfully created with enhanced strength for potentially construction and building materials applications. Through geopolymerization, the mining wastes can be reused and the storage, economic, and environmental issues can be mitigated. However, the geopolymerization effect was relatively limited comparing to the ordinary Portland concrete. Therefore, the additions of amorphous supplementary fly ash were considered in the paper. In this study, class F fly ash was utilized as an amorphous additive and the additional aluminum source to adjust the Si: Al ratio for better geopolymerization effects. To better understand the mechanical and fracture behavior of the geopolymer after adding fly ash, the Brazilian indirect tensile tests were then conducted to evaluate the influence of different fly ash additions, 5%, 10%, 15%, and 20%, on the tensile strength of the geopolymer. Meanwhile, besides the load-displacement relationships, the digital image correlation was used to investigate the crack initiation and propagation in the geopolymer disks with respect to different fly ash additions. The crack evolution was evaluated from the full-field strain field from digital image correlation. Crack opening displacements, failure patterns, and strain evolutions for the geopolymer disks with different fly ash additions were examined. Results showed that, under the presented sample curing condition, the geopolymer with 15% fly ash addition had the largest tensile strength. Geopolymer disks without and with lower fly ash additions had plastic deformation prior to the peak load that can obviously be classified into three different stages. The geopolymer made without and with lower fly ash additions had both tensile and shear crack modes.
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