Insights into factors influencing coal gangue-filled backfill cemented by self-consolidating alkali-activated slag grouts

Abstract

The cemented backfilling is an effective measure to mitigate surface subsidence caused by mine goaf and to consume solid waste generated from mining activities. In this study, sodium hydroxide (SH) and sodium silicate (SS) were used as activators to respectively prepare self-consolidating alkali-activated slag grout (AASG) and alkali-activated fly ash-slag grout (AAFSG) for disposal of iron tailings (TS) and coal gangue (CG). These grouts were casted into a pre-placed CG skeleton to obtain coal gangue-filled backfill (CGFB). The factors influencing the mechanical properties of CGFB were investigated through fresh property tests (rheological properties, bleeding and segregation, setting time, and filling ability) and mechanical property tests of AASGs and AAFSGs. The micro-mechanical properties of the interfacial transition zone (ITZ) between grouts and CG aggregates were investigated by microhardness test. The microstructure of AASGs and AAFSGs was characterized using scanning electron microscope-energy dispersive spectrometer (SEM-EDS) and thermogravimetric analysis (TGA). The results showed that the filling ability of fresh AASGs and AAFSGs was dependent on the flowability and increased with the modulus (SiO2/Na2O molar ratio). The compressive strength of CGFBs was shown to be influenced by the filling index and mechanical strength of AASGs and AAFSGs, which were also related to the modulus of activators. The microhardness gradually increased with distance from ITZ and then tended to be stable, higher modulus led to enhanced microhardness around ITZ, which benefited the mechanical properties of CGFB by strengthening the weak ITZ.