| Summary: | To investigate the issues related to significant environmental damage and poor resource utilization of soda residue (SR), the composition and microstructure of hydration products of the GGBS (ground granulated blast-furnace slag) synergistically activated by NaOH-SR are characterized by an X-ray diffraction (XRD), Fourier-transform infrared spectrometry (FTIR), thermogravimetric–differential thermogravimetric (TG-DTG) analysis, and scanning electron microscope X-ray energy dispersive spectrometry (SEM-EDS). The effect of SR proportion, activator dosage, and water-to-binder ratio on the hydration process is studied. Results indicate that the hydration products mainly include hydrated calcium chloroaluminate (3CaO·Al<sub>2</sub>O<sub>3</sub>·CaCl<sub>2</sub>·10H<sub>2</sub>O, FS), hydrated calcium aluminosilicate (Ca<sub>2</sub>Al<sub>3</sub>(SiO<sub>4</sub>)<sub>3</sub>OH, C-A-S-H), halite (NaCl), calcite (CaCO<sub>3</sub>), and C-S-H gel. With the increase in SR proportion (especially from 80% to 90%), the C-S-H gel yield decreases significantly, while the FS yield rapidly increases, and the T-O-Si (T = Al or Si) peak shifts to a higher wavenumber range (955 cm<sup>−1</sup> to 975 cm<sup>−1</sup>). With the decrease in activator dosage (40% to 15%), the hydration reaction gradually weakens, and the FTIR band of the T-O-Si (T = Al or Si) shifts to the lower wavenumber range (968 cm<sup>−1</sup> to 955 cm<sup>−1</sup>). Then, cemented paste backfills (CPBs) are prepared with iron tailings as the aggregate. At mass content of 75%, SR proportion of 80%, and activator dosage of 30%, the fluidity of the CPB reaches 267.5 mm with a 28-day unconfined compressive strength (UCS) of 2.4 MPa, confirming that SR- and NaOH-synergistically-activated GGBS has great application prospects in backfill mining.
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