## 1 Introduction  
The extraction of coal resources is often accompanied by the generation of solid waste products such as coal gangue, which brings many challenges to society and the environment. The large-scale discharge of gangue results in a range of issues, including land resource occupation, human health hazards, and surface desertification [1; 2; 3; 4; 5]. Furthermore, the movement and fracturing of overburden after coal mining leads to surface deformation and damage to buildings [6; 7; 8; 9; 10]. The disposal of gangue and control of surface deformation have become increasingly difficult problems in the protection of the mining ecological environment. In the gangue grouting backfilling technique [11], the backfilling slurry is prepared by mixing the gangue with different particle sizes and water at different ratios, and then transported to the underground backfilling area by filling equipment and pipelines. The distinguishing feature of this technique lies in the absence of additives in the backfilling slurry. The backfilling area includes the caving zone, fracture zone, and separation zone formed after coal mining. The backfilling effect is mainly affected by the bearing characteristics of the slurry. The on-site application research of the grouting backfilling technique shows [12] that the backfilling material flows in the form of slurry in the area during the grouting process; as the overburden slowly sinks, the water in the slurry continuously bleeds under pressure, ultimately forming a compacted body with certain strength. In this case, the compacted body remains in the backfilling area and plays a role in controlling surface deformation. The compression deformation and water bleeding of the slurry under pressure directly affect the effectiveness of the grouting backfilling technique in controlling the surface. Therefore, studying the bearing mechanism and mechanical properties of backfilling materials underground is of great significance. At present, research on the bearing deformation characteristics of backfilling materials (including solid and cementitious materials) has been extensively conducted from both theoretical and on-site perspectives. In terms of solid backfilling materials, [PERSON] et al. [13; 14; 15; 16; 17; 18] studied the bearing deformation characteristics and failure laws of solid waste under different particle size gradings, lateral confinement methods, and other conditions from both macro and micro perspectives through physical experiments, numerical simulations, and other methods. [PERSON] et al. [19; 20] investigated the crushing characteristics of gangue backfilling under different pressure conditions based on the on-site application of the solid backfilling technique. [PERSON] et al. [21; 22] analyzed the effect of additives on improving the strength characteristics of solid waste by adding materials such as silicon powder and fly ash to solid waste. For cementitious backfilling materials, [PERSON] et al. [23; 24; 25] explored the influence of the water content, fine particle size ratio, and shape of solid particles on the porosity, load-bearing performance, plasticity, and mechanical strength of cementitious backfilling materials after solidification. [PERSON] et al. [26; 27] obtained the compressive strength of cement coal gangue grouting materials activated by additives through physical experiments and analyzed the strengthening effect of cement on cementitious backfilling materials. [PERSON] et al. [28] tested the mechanical properties of cementitious backfilling materials by replacing cement with materials such as slag, fly ash, and silicon powder, and compared the strength of backfilling materials under different replacement materials and curing ages. The research results have shown that changing particle size grading, additives, and loading methods can affect the mechanical bearing characteristics of solid and cementitious backfilling materials. However, there is little research on the mechanical properties of slurry materials made from gangue and water.