Synthesis and characterization of fly ash geopolymer paste for goaf backfill: Reuse of soda residue

Abstract

Soda residue (SR), an industrial Ca-containing solid waste, threatens and pollutes the coastal ecological environment in China. To promote the resource recycling of soda residue, this paper proposes the synthesis method and performance evaluation of fly ash-based geopolymer paste using soda residue for goaf backfill. The pre-treatment of SR and the solid-liquid mixing technique were determined, and the optimal mixing proportion of the paste backfill material was selected by orthogonal test through the fluidity, setting time and compressive strength. Meanwhile, the long-term compressive strengths of hardened paste and control were measured to clarify the role of soda residue in backfill paste. To evaluate the microstructure and products of optimal backfill paste, X-ray diffraction (XRD) tests was conducted to analyze mineralogical phases. Then, scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) were used to investigate the morphology and elemental composition of products. Thereafter, fourier transform infrared spectrometer (FTIR) and 29Si nuclear magnetic resonance (29Si NMR) were performed to characterize gel products by chemical bonds. The results show that the optimal backfill paste (SFN6) is obtained by one-step mixing technique when SR-FA ratio is 2:3, solution concentration is 2.0 mol/L and liquid-solid ratio is 1.2, in which gel product is the coexistence of calcium silicate hydrated (C–S–H) and Ca-containing geopolymer ((N,C)-A-S-H) according to the detected 2Ca/Al = 1.54 (more than 1.0) and Si/Al = 1.69 (more than 1.0). The results can make extensive utilization of soda residue and low-calcium fly ash in cemented paste backfill to promote cleaner production.