Microstructure of Biomass-Coal Mixed Combustion Ash Geopolymer Backfill

Abstract

Biomass-coal mixed combustion ash (BCC) is a solid waste rich in silica and aluminum, and its application to mine backfill is of great scientific importance for green mine backfill and solid waste disposal. In this article, using a new type of BCC and slag as cementitious material, gangue as aggregate, and sodium hydroxide and water glass as composite alkali exciters, a new type of biomass-coal mixed combustion ash geopolymer backfill (BCCGB) was developed. The mixing ratio was optimized by Box–Behnken design (BBD) response surface design and the satisfaction function method, and the three response values were 28-day uniaxial compressive strength (UCS), slump, and cost. The internal morphology, mineral composition, chemical bond and pore structure of BCCGB with different curing ages were analyzed by Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and mercury intrusion porosimetry (MIP). When the mass concentration was 82%, the aggregate–binder ratio was 3.585, and the proportion of fine aggregate was 40%, which were optimal for backfill. Initially, C-S-H, C-A-S-H gel, and CaCO₃ were generated inside the backfill. With increasing age, gel generation increased, diffraction peaks increased, pores were backfilled, and the internal structure was densified. As the porosity decreased, asymmetric stretching and bending vibrations appeared for H-O-H, Si-O, Si-O-Al, and C-O bonds. The backfill is highly capable of fulfilling the filling requirements.