Design, Property, and Interfacial Bonding Mechanism of a Novel Gangue-Cemented High-Porosity Backfill Material

In traditional gangue backfilling mining, the use of gangue as backfill material often leads to issues such as supply-demand imbalance, low crushing efficiency, and insufficient early strength. To address these challenges, a novel gangue-cemented high-porosity backfill material (GCBM) was developed, using coal gangue (CG, 15-20 mm) as aggregates bonded with an alkali-activated modified coal gangue powder and slag-based fast-setting cementitious material (FCM). The effects of alkali equivalent on the workability and mechanical properties of FCM were investigated to determine the optimal composition. The influence of FCM proportion on the porosity and strength of GCBM was analyzed, and a strength-porosity correlation model was established. The microstructure and bonding mechanism of the interfacial transition zone (ITZ) between FCM-CG aggregates were revealed through multiple microscopic techniques. Results indicate an optimal alkali equivalent of FCM is 6%. The FCM proportion is positively correlated with the compressive and flexural strengths of GCBM, with a critical proportion of 24% at which flexural strength increased by 172% compared with 22%, reflecting enhanced interfacial bonding. Microscopic observations further show that increasing the FCM proportion optimizes the ITZ structure and promotes secondary hydration products, thereby improving interfacial bonding density and the overall mechanical performance of the backfill material.