Crack evolution characteristics of cemented-gangue-fly-ash backfill with different proportions of fly ash and cement

To explore the strength boundaries and corresponding failure characteristics of the backfill body, the basic bearing capacity and failure performance of cemented-gangue-fly-ash backfill (CGFB) with different ratios of fly ash to cement in the binder (F/C) were tested using a uniaxial compressive stress experiment, acoustic emission, and digital image correlation. The results indicate that CGFB has an upper-limit strength of 15.35 MPa, and the strength varies significantly from F/C 0.5 to 2. With the increase in F/C, the brittleness of CGFB decreases. The change in failure mode from tensile accounts for 69.7%, and the change in failure mode to shear accounts for 70.66%. When F/C is low, the crack initiation stress is approximately 0.4-0.5 the peak strength, the crack development is concentrated in the middle of the entire bearing stage, and the scale of the crack is large and develops along the main crack. With the increase in F/C, the initiation stress of the crack decreases to 0.25-0.4 the peak strength, and the development of cracks begins to concentrate in the early and late stages during the loading stage. The scale of the crack is smaller than when F/C is low, and the failure is scattered throughout the specimen.