Synthesis and characterization of fly ash geopolymer paste for goaf backfill: Reuse of soda residue Xianhui Zhao a , Chunyuan Liu a , Liming Zuo b , Li Wang a, * , Qin Zhu b , Youcai Liu b , Boyu Zhou a a School of Civil Engineering and Transportation, Hebei University of Technology, Tianjin, 300401, PR China b Hydrological Engineering Geology Survey Institute of Hebei Province, Shijiazhuang, 050021, PR China article info Article history: Received 2 November 2019 Received in revised form 28 February 2020 Accepted 9 March 2020 Available online 16 March 2020 Handling Editor: Prof. Bing-Jie Ni Keywords: Soda residue Geopolymer Paste backfill Compressive strength Microstructure 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 29 Si nuclear magnetic resonance ( 29 Si 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 con- centration is 2.0 mol/L and liquid-solid ratio is 1.2, in which gel product is the coexistence of calcium silicate hydrated (CeSeH) and Ca-containing geopolymer ((N,C)-A-S-H) according to the detected 2Ca/ Al ¼ 1.54 (more than 1.0) and Si/Al ¼ 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. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Ordinary Portland cement (OPC) production is an energy- intensive process consuming about 12e15% of industrial energy consumption in total world (Ali et al., 2011). Meanwhile, cement manufacturing generates huge amounts of CO 2 emission (where, each ton of cement produces nearly 950 kg CO 2 ) and results in the environmental pollution (Andrew, 2018; Hassan et al., 2019). The considerable energy demand and supply of raw materials are sig- nificant concerns for cement industries along with the CO 2 emis- sion (Zhang et al., 2018). Recently, the announced stricter environmental regulations limit the production and application of Portland cement in North China (Cai et al., 2016; Guo et al., 2019). Therefore, seeking the ecofriendly alternatives to cement has become the main subject of many studies (Oderji et al., 2019; Shekhovtsova et al., 2018; Wang, 2020). Geopolymer, widely regarded as sustainable alternatives to cement, has the potential to reduce CO 2 emissions and energy consumption by approximately 80% and 60% respectively (Assi et al., 2018; Hu et al., 2018; Nematollahi et al., 2015; Li et al., 2020; Ma et al., 2019). In Australia, it was estimated that solid waste-based geopolymer materials substitute OPC in construction engineering, which decreases the financial costs of 7% (regardless of the transport distance of wastes) and reduces 44e64% CO 2 emissions (McLellan et al., 2011; Zhou et al., 2019). Thus, geo- polymer production can consume huge amounts of local industrial wastes and eliminate the transport cost of raw materials (Hossain et al., 2018). Where, fly ash is the typical aluminosilicate wastes for manufacturing geopolymers, which is commonly available (about 500 million tons annually in China) (NDRCC, 2011; Shang * Corresponding author. Hebei University of Technology, Tianjin, 300401, PR China. E-mail address: wangl1@hebut.edu.cn (L. Wang). Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro https://doi.org/10.1016/j.jclepro.2020.121045 0959-6526/© 2020 Elsevier Ltd. All rights reserved. Journal of Cleaner Production 260 (2020) 121045