Cemented coal gangue-fly ash backfill (CGFB) mixtures are utilized as the filling materials for backfilling the underground openings in coal mines. The freshly prepared CGFB slurries are commonly transported into the gobs through a pipeline. The mixture ratio of slurry concentration and suspending agent (HPMC) plays an essential role in transporting the slurry to goaf smoothly and efficiently. In this paper, the influence of slurry concentration and HPMC on the performance of coal gangue-fly ash cemented filling material was studied based on the response surface method. The prediction model of CGFB slurry slump flow, segregation rate, and bleeding rate was constructed. It is concluded that the segregation rate and slump flow of slurry are more sensitive to the variation of concentration. On the other hand, the bleeding rate of slurry is more sensitive to the change of HPMC content. Based on the established model, the reasonable mix proportion range of slurry concentration and suspending agent (HPMC) was obtained. In addition, three new CGFB mixtures have been tested, and the experimental results are in good agreement with the predicted values.

In order to explore the influence of gradation and concentration on the rheological parameters of uncemented coal gangue-fly ash backfill (UCGFB) slurry, based on the fractal theory, the particle size distribution of the mixture of coal gangue and fly ash under different mixing ratios are analyzed in this paper. On this basis, the influence of gradation and concentration on rheological parameters of UCGFB slurry are studied and a numerical simulation of slurry transportation is also carried out. The results show that (1) the fractal dimension can well characterize the grading characteristics of UCGFB mixtures, the larger the fractal dimension, the more fine particles in the material. (2) The fractal dimension of 2.628 is a critical point, when the fractal dimension of the mixture is greater than or equal to 2.628, the content of fine particles in the slurry can meet the requirements. On this basis, by adjusting the concentration of the slurry, the slurry can reach a good state. (3) When the slurry concentration reaches 79%, no matter how the gradation of the mixture changes, the rheological parameters of the slurry are at a high level. (4) In this paper, the average pressure loss per unit length pipeline is between 3000–8000 Pa for slurry with different mixing ratios, with a minimum value of 3070 Pa and the maximum value of 7697 Pa. Moreover, the pressure loss of bend is greater than that of straight pipe.

Understanding the mechanical properties and failure process of cemented paste backfill with recycled rubber (RCPB) is the foundation of backfill design in underground mining. In this study, physical and mechanical tests were conducted on RCPB to obtain its mechanical property parameters, such as its uniaxial compressive strength (UCS), toughness, and peak strain. The influence of the rubber dosage on the mechanical properties of RCPB was also analyzed. In addition, the deformation behavior, fracture development, and failure process of RCPB with different rubber contents were observed using the digital image correlation (DIC) technique. The experimental results suggested that, although the UCS of RCPB is reduced as more rubber is added, its toughness and ability to absorb energy is increased. Moreover, the impact resistance of RCPB is improved by this increased toughness. With the increase in the rubber content, the deformation corresponding to the plastic yield stage of RCPB increased, which resulted in better ductility and improved impact resistance. The failure of the RCPB specimens mainly showed an “X” shape. The results of this study help us to better understand the mechanical behavior of RCPB after backfilling underground.

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