A new environmentally friendly cemented oil shale residue–steel slag–ground granulated blast furnace slag backfill (COSGB) was prepared using oil shale residue (OSR), steel slag (SS) and ground granulated blast furnace slag (GGBS) as constituent materials. Based on univariate analysis and the Box–Behnken design (BBD) response surface method, the three responses of the 28 days unconfined compressive strength (UCS), slump and cost were used to optimize the mix ratio. Using a combination of scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP), the reaction products, microscopic morphology and pore structure of the specimens with the optimal mix ratio at different curing ages were analyzed. The influence of heavy metal ions from the raw materials and the COSGB mixtures on the groundwater environment was studied by leaching tests. The research demonstrates that the optimal mix ratio is GGBS mixing amount 4.85%, mass ratio of SS to OSR 0.82, and solid mass concentration 67.69%. At shorter curing age, the hydration products are mainly calcium alumino silicate hydrate (C-A-S-H) and calcium silicate hydrate (C-S-H) gels. With the increase of curing age, ettringite (AFt) and C-S-H gels become the main source of the UCS. Meanwhile, the porosity of the filler decreases continuously. The leaching concentration of heavy metal ions from the COSGB mixtures is all lower than the leaching concentration of raw materials and meet the requirements of the Chinese groundwater quality standard (GB/T 14848-2017). Therefore, this new COSGB cannot pollute the groundwater environment and meets backfill requirements. The proposed technology is a reliable and environmentally friendly alternative for recycling OSR and SS while simultaneously supporting cemented paste backfill (CPB).

A new type of cemented paste backfill (CPB) was prepared using sodium hydroxide (NaOH) as the activator, slag and silica fume (SF) as the binder, and tailings as the aggregate. The effects of proportion of replacement of 0%, 5%, 10%, 15%, and 20% silica fume on the properties of CPB were studied. The strength formation mechanism of CPB was explored through a combination of scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and Fourier transform infrared (FTIR) spectroscopy. The SEM images were analyzed by IMAGE J software, and the porosity of CPB with different silica fume contents was obtained. The results show that as the amount of silica fume increases, the unconfined compressive strength (UCS) increases first and then decreases. When the amount of silica fume was approximately 5%, CPB with a larger UCS can be obtained. When the silica fume content increased from 0% to 5%, because silica fume has good activity and small particles, more calcium silicate hydrate (C–S–H) gels and Mg-Al type layered double hydrotalcites (LDHs) were generated in CPB, which made it denser and improved its strength compared with the non-silica fume group. C–S–H gels were the main source of CPB strength. With a further increase in the amount of silica fume, thaumasite produced inside of CPB, reducing the content of C–S–H gels. Moreover, due to the expansion of thaumasite, it is easy to generate a large number of micro cracks in CPB, which weakens the strength of CPB.

A new type of cemented paste backfill (CPB) was prepared by using the bottom ash (BA) from a thermal power plant as an aggregate, alkali-activated slag as a binder, and an air-entraining agent as an admixture. Based on the central composite design (CCD) response surface method, the mix ratio was optimized, and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) was performed on the optimal mix ratio. ImageJ software was utilized to determine the porosity of the experimental samples at various curing ages. The results indicate that the optimal mix ratio of the aggregate-binder ratio is 3.28, the alkali dosage is 3%, the solid content is 67.44%, and the air-entraining agent dosage is 0.1%. As the curing age increases, the porosity of CPB gradually decreases. A calcium aluminosilicate hydrate (C-A-S-H) gel is the main hydration product of alkali-activated slag. At the beginning of the hydration reaction, the slag gradually dissolves, and the C-A-S-H product binds the BA together. At 14 d, complete calcium hydroxide (CH) crystals appeared in the hydration product. Finally, the degree of C-A-S-H crystallization increased further to form a dense structure.