## 3 Results and Discussion
### Stress-Strain Curve
Consequently, BF proves to be more effective in enhancing the compressive strength of coal-gangue concrete than PF. The impact of the hybrid BF and PF reinforcement on compressive strength falls between the two previously mentioned reinforcements and does not exhibit a noticeable hybrid synergistic effect [44]. BF, which is a hydrophilic fibre, exhibits strong bonding properties with the coal-gangue concrete matrix. In contrast, PF, which is a hydrophobic fibre, displays relatively weak bond performance with the matrix [45,46]. Studies by [PERSON] et al. [47] have presented bonding properties between BF and PF and cement mortar with similar compositions, concluding that the bond strength of BF to cement mortar is higher than that of PF to cement mortar, further substantiating the superior bonding characteristics of BF with cement-based materials. Additionally, when the fibre volume content is equivalent, BF exhibits a significantly smaller monofilament diameter than PF, resulting in the presence of more BF monofilaments than PF monofilaments. Therefore, the effect of BF on the compressive strength of coal-gangue concrete is enhanced relative to PF. However, when mixed BF and PF have an excessive volume content, the number of internal defects within coal-gangue concrete is increased, consequently diminishing compressive strength. When the volume content of fibre is small, after mixing, the fibre can be evenly distributed into the concrete. However, when the volume content of fibre added increases, because of the high consistency of the concrete itself, it is difficult for the fibre to be evenly and ideally distributed into the concrete. Basal fibre (BF) has high strength and good bonding properties with cement-based materials. The strength of basalt fibre of the same volume is higher than that of concrete of the same volume, but polypropylene fibre (PF) is a kind of filamentous flexible fibre. When the amount of polypropylene fibre is large, the filamentous polypropylene fibre is easily wound, thus forming a mass in the concrete. These agglomerate structures are unevenly distributed in coal-gangue concrete, which not only leads to the low strength of concrete in the agglomeration but also leads to the concrete not filling the pores in the mass well, resulting in holes and congenital defects of concrete. In addition, compared with basalt fibre (BF), polypropylene has relatively weak bonding properties with cement-based materials. More polypropylene in concrete means more interfaces between the polypropylene and the concrete. The weak bonding on the interfaces will deteriorate the concrete; thus, the interfaces are also regarded as internal defects in concrete. The internal defect in concrete is one of the important reasons why the strengthening effect of coal-gangue concrete with less hybrid fibre is more obvious than that with more fibre. Relative to the benchmark coal-gangue concrete, HBPRGC-20-0.2 and HBPRGC-30-0.2 experience respective reductions of 13.75% and 13.07% in compressive strength. Figure 7b presents the variations in the elastic modulus of HBPRGC concerning the fibre volume content, type, and matrix strength. The elastic modulus reflects the material resistance to deformation in its initial undamaged state, where internal defects within coal-gangue concrete have not yet developed, and the bridging effect of the fibre remains unapparent. Hence, the effects of BF and PF on coal-gangue concrete's elastic modulus do not align with their effects on compressive strength. Specifically, at a 0.1% fibre volume content, the elastic modulus of BC-20-0.1 and BC-30-0.1 increase by 15.22% and 4.77%, respectively, relative to the base coal-gangue concrete group. Conversely, PC  
20-0.1 and PC-30-0.1 experience reductions of 5.23% and 5.07% in their elastic modulus, respectively. For HBPRGC-20-0.1 and HBPRGC-30-0.1, the elastic modulus increase by 8.01% and 5.10%, respectively. Both BF and PF exhibit the capacity to mitigate shrinkage cracking in coal-gangue concrete during the hardening process.