## 3 Results and Discussion
### SEM Results  
The microstructure of RSCPB was observed. Figure 11 shows the SEM of specimens A2 and A8. The solid mass concentration of both is 62%; the difference is that the cement content of A2 is 25% and A8 is 17%. It can be seen from Figure 11 that the pore size of the A2 specimen is significantly smaller than that of A8 at the same observation scale, indicating that the hydration reaction products are more abundant at a higher cement content, which will wrap the RS fibers more tightly and contribute to enhancing the denseness and tensile properties of RSCPB. Moreover, a bent RS fiber that has a rough surface can be clearly seen in the SEM photograph of specimen A8 (Figure 11b). RS fibers uncoated by hydration products are present in RSCPB as weakly structured surfaces. The lower part of the fiber is fully wrapped by the hydration products, which indicates that the surface of the RS fibers can adsorb the hydration products and form a large-scale aggregate, which improves the compactness inside the CPB and, thus, the strength of the CPB.  
Figure 10: The influences of RS fiber on peak strain and \(Re_{\varepsilon}\). (**a**) Average \(\varepsilon\) at different RS fiber contents; (**b**) average \(\varepsilon\) at different RS fiber lengths; (**c**) average \(Re_{\varepsilon}\) at different RS fiber contents; (**d**) average \(Re_{\varepsilon}\) at different RS fiber lengths.  
The image in Figure 12 is an enlargement of the segment in the red box in Figure 11. It can be seen that C-S-H (spherical, flocculent), AFt (needle-like crystals), and C-H (hexagonal plates) [41] are interspersed and interlaced with each other. A large amount of ettringite, which is an expansive product generated by S\({}^{2-}\) in the tailings [42], causes the formation of fractures inside the CPB, forming the microcrack structure shown in Figure 12. The cracks connect and develop when subjected to tensile forces, which is one of the main reasons for its brittle damage and reduced tensile strain.  
In previous studies, RS fibers have been shown to improve the compressive and tensile properties of the backfill, and the low price and wide availability of RS make it feasible to use RS fibers as a backfill additive material. The use of RS fibers to backfill can reduce the amount of cement and thus the cost, while maintaining the strength in the early curing time. The composition of RS is dominated by organic matter, and the organic component can be further removed by pyrolysis or alkali treatment, thus increasing the inertness of the RS fiber may further improve the strength of the backfill [26,31]. However, the removal of the fiber structure will reduce the toughness of the RS, which is a double-edged sword. In addition, further resizing of the RS fibers by reducing the length and narrowing the diameter may further improve the linkage effect between the RS fibers and the aggregate, thus improving the strength of the backfill and reducing its effect on the flowability of the backfill slurry, which will be further worked on in subsequent studies.  
Figure 11: SEM images of A2 and A8 cured for 28 days. (a) A2; (b) A8.  
Figure 12: SEM images of A2 and A8 cured for 28 days.