Experimental investigation on the mixture optimization and failure mechanism of cemented backfill with coal gangue and fly ash

Using coal gangue and fly ash for mining backfill materials can create a win-win situation for safe mining and waste disposal, with the critical challenge being the optimization of coal gangue grading and fly ash proportions. Uniaxial compression tests and scanning electron microscopy (SEM) observation were conducted on backfill specimens with different coal gangue grading and fly ash proportions, and in-situ monitoring of acoustic emission (AE) parameters and infrared radiation temperatures (IRT) was carried out. The elastic modulus, uniaxial compressive strength (UCS), failure pattern, infrared temperature, AE ringing count and energy evolution of the backfill were discussed. Further, the b-value and entropy-value of AE during the loading were analyzed. The results show that the UCS and elastic modulus decrease with the increase in the proportions of fly ash and large-size gangue. Tensile cracks generally dominate the failure of specimen under unaxial compressive load but shear cracks are more prone to occur for specimens with high proportions of fly ash and large-size gangue. The inner defects and weak interfaces between large-size gangue and cement pastes significantly affect the mechanical performance for specimens with high proportions of fly ash and large-size gangue. A sudden drop of the b-value occurs before the peak stress while the AE entropy-value reaches maximum at the peak stress. The methods and findings can guide the mixture optimization and mechanical failure analysis for backfill mining.