Fracture Behavior and Crack Propagation Processes of Composite Structure of Goaf Backfill and Roof Rock Based on Acoustic Emission Monitoring

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The mechanical response of the composite load-bearing structure formed by the backfill and surrounding rock in underground backfill mining differs from that of either hardened backfill or the rock alone. To investigate how backfill type and curing age govern the strength development, fracture behavior, and crack propagation processes of this composite structure, three types of composite specimens were prepared by placing roof rock above hardened pure-cement grout, cement–fly-ash grout, or cement–water-glass grout: pure-cement grout–limestone (PC-L), cement–fly-ash grout–limestone (PC-FA-L), and cement–water-glass grout–limestone (PC-WG-L). Mechanical tests and synchronized acoustic emission (AE) monitoring were conducted at curing ages ranging from 1 to 35 d; detailed fracture analyses focused on 3, 14, 28, and 35 d. The peak load-bearing capacity increased in stages with curing age. PC-L passed through rapid, steady, and slow growth stages, with growth rates of 5.43, 1.89, and 0.27 MPa/d. After rapid early strength gain, PC-FA-L maintained a growth rate of 1.51 MPa/d and reached the highest 35 d strength. PC-WG-L showed the fastest early strength gain (6.44 MPa/d), but its growth rate decreased after 7 d and its 35 d strength was slightly lower than its 28 d strength. Analyses of the AE b value and the rise-time-to-amplitude ratio (RA)–average frequency (AF) parameters showed material-dependent crack evolution. PC-L changed gradually from shear-dominated to tensile-dominated macroscopic failure; PC-FA-L developed mixed tensile–shear failure with good post-failure integrity; and PC-WG-L exhibited pronounced axial tensile cracking. Tensile AE events accounted for 62–80% of classified events in all representative groups. These results provide a basis for selecting backfill materials according to the required curing age and for using AE parameters to assess damage evolution in grouted goafs.