Performance Optimization and Characterization of Soda Residue-Fly Ash Geopolymer Paste for Goaf Backfill: Beta-Hemihydrate Gypsum Alternative to Sodium Silicate

Solid waste soda residue (SR), as an industrial pollutant of water, air and soil environment, can be utilized to prepare the low-calcium fly ash (FFA)-based geopolymer paste activated by sodium silicate (NS) solution for goaf backfill. However, the high addition of NS produces the high cost and high strength of synthesized backfill material in the previous study. The objective of this research is to investigate the cost optimization method and performance evaluation of SR-FFA-based geopolymer backfill paste. The alkaline beta-hemihydrate gypsum (BHG) alternative to partial NS was proposed. Scanning electron microscopy (SEM), X-ray diffraction (XRD) as well as Fourier transform infrared spectrometer (FTIR) tests were performed to clarify the role of BHG and evaluate the microstructures and products of backfill pastes. The results show that 10% BHG alternative ratios effectively improve fluidity, setting time and compressive strength to satisfy the performance requirement of goaf backfill material. The gel products in the optimal backfill paste C4 with 10% BHG alternative ratios are determined as the coexistence of C-S-H gel, (N,C)-A-S-H gel and CaSO(4)·2H(2)O at 28 d. The research results can make extensive utilization of SR and FFA in cemented paste backfill to synthesize cleaner material at a larger scale.