Mechanical properties and compression damage characteristics of coal gangue-filled backfill cemented by fly ash modified self-consolidating grouts

Cemented backfill is an effective method for controlling subsidence in mined-out areas. This research investigated the effect of different fly ash (FA) substitution rates (0 %, 10 %, 20 %, and 30 %) on the flowability and filling ability of fresh self-consolidating cement-fly ash tailings grouts (CFTGs). Additionally, the damage process of coal gangue-filled backfill (CGFB) specimens under uniaxial loading was monitored using acoustic emission (AE) technology. Results reveal the enhancement mechanism of FA on the flowability, filling ability and mechanical strength of CFTGs, which matters the mechanical strength and compression damage characterization of CGFBs. It was found that the filling ability of CFTGs was enhanced with increasing FA contents, but the effect of enhancing the filling ability of FA was weakened at high water to binder ratio (W/B), and thus the CGFBs specimens exhibited higher strength and less AE activity (growth period and transition period) at a W/B of 0.6 and FA substitution rate of 20 %. The CGFB specimens achieved a compressive strength of 16.6 MPa at 28 days of curing, representing a 9.1 % increase compared to the group with a lower FA substitution rate (10 %). As the W/B increased, the positive effect of FA on flowability, filling ability, and mechanical strength of CFTGs at 28 days diminished. These findings highlight the critical balance between FA dosage and W/B ratio for optimizing both flowability and damage resistance in sustainable backfill systems.