Fly ash–cement composite backfill slurry, prepared by partially replacing cement with fly ash, has been demonstrated to effectively reduce the mine backfill costs and carbon emissions associated with cement production. However, the use of fly ash often results in insufficient early and medium-term strength of the backfill material. To address the demand for high medium-term strength in backfill materials under continuous mining and backfilling conditions, this study developed a silica fume–fly ash–cement composite backfill slurry. The effects of varying silica fume contents on the slurry’s flowability, uniaxial compressive strength, microstructure, and pore characteristics were systematically investigated. The results showed that increasing the silica fume content significantly reduced the slurry’s flowability. However, at a silica fume content of 5%, the slurry achieved optimal medium-term strength, with a 14-day uniaxial compressive strength of 3.98 MPa, representing a 25% improvement compared to the control group. A microstructural analysis revealed that a moderate silica fume content promoted the formation of calcium silicate hydrate gel, filled micropores, and optimized the pore structure, thereby enhancing the overall strength and durability of the material. Conversely, an excessive silica fume content above 5% led to a marked decrease in both flowability and strength. Based on a comprehensive evaluation of silica fume’s effects on the flowability, strength, and microstructure, the optimal silica fume content was determined to be 5%. This study provides a theoretical basis and practical guidance for improving the efficiency of continuous mining and backfilling operations, and for designing high-performance backfill materials suitable for continuous mining and filling conditions.

This study proposes a novel idea of the use of coal gangue (CG) activation and preheated decarburized activated coal CG-based cemented paste backfill material (PCCPB) to realize green mining. PCCPB was prepared with preheated decarburized coal CG (PCG), FA, activator, low-dose cement, and water. This idea realized scale disposal and resource utilization of coal CG solid waste. Decarbonization and activation of CG crushed the material to less than 8 mm by preheated combustion technology at a combustion temperature of 900 °C and a decarbonization activation time of 4 min. The mechanism of the effect of different Na₂SO₄ dosages on the performance of PCCPB was investigated using comprehensive tests (including mechanical property tests, microscopic tests, and leaching toxicity tests). The results show that the uniaxial compressive strength (UCS) of C-S2, C-S3, and C-S4 can meet the requirements of backfill mining, among which the UCS of C-S3 with a curing time of 3 d and 28 d were 0.545 MPa and 4.312 MPa, respectively. Na₂SO₄ excites PCCPB at different curing time, and the UCS of PCCPB increases and then decreases with the increase in Na₂SO₄ dosage, and 3% of Na₂SO₄ had the best excitation effect on the late strength (28 d) of PCCPB. All groups’ (control and CS1-CS4 groups) leachate heavy metal ions met the requirements of groundwater class III standard, and PCCPB had a positive effect on the stabilization/coagulation of heavy metal ions (Mn, Zn, As, Cd, Hg, Pb, Cr, Ba, Se, Mo, and Co). Finally, the microstructure of PCCPB was analyzed using FTIR, TG/DTG, XRD, and SEM. The research is of great significance to promote the resource utilization of coal CG residual carbon and realize the sustainable consumption of coal CG activation on a large scale.

With the wide application of the filling mining method, it is necessary to consider the influence of rock activity on the filling body, reflected in the laboratory, that is, the influence of loading rate. Therefore, to explore the response characteristics of loading rate on the mechanical and damage characteristics of aeolian sand paste filling body, DNS100 electronic universal testing machine and DS5-16B acoustic emission (AE) monitoring system were used to monitor the stress–strain changes and AE characteristic parameters changes of aeolian sand paste filling body during uniaxial compression, and the theoretical model of filling sample damage considering loading rate was established based on AE parameters. The experimental results show that: (1) With the increase in loading rate, the uniaxial compressive strength and elastic modulus of aeolian sand paste-like materials (ASPM) specimens are significantly improved. ASPM specimens have ductile failure characteristics, and the failure mode is unidirectional shear failure → tensile failure → bidirectional shear failure. (2) When the loading rate is low, the AE event points of ASPM specimens are more dispersed, and the large energy points are less. At high loading rates, the AE large energy events are more concentrated in the upper part, and the lower part is more distributed. (3) The proportion of the initial active stage is negatively correlated with the loading rate, and the proportion of the active stage is positively correlated with the loading rate. The total number of AE cumulative ringing decreases with the increase in loading rate. (4) Taking time as an intermediate variable, the coupling relationship between ASPM strain considering loading rate and the AE cumulative ringing count is constructed, and the damage and stress coupling model of ASPM specimen considering loading rate is further deduced. Comparing the theoretical model with the experimental results shows that the model can effectively reflect the damage evolution process of ASPM specimens during loading, especially at high loading rates. The research results have significant reference value for subsequent strength design of filling material, selection of laboratory loading rate and quality monitoring, and early warning of filling body in goaf.

Coal gangue slurry filling technology is an effective way of utilizing coal gangue solid waste resources rationally, and its fluidity and sedimentation behavior have an essential influence on filling performance. However, evaluation and optimization methods for the fluidity and sedimentation performance of coal gangue slurry filling materials (CSFMs) are still scarce. In order to solve this problem, based on the fractal grading theory, this paper carried out an experimental study on the influence of the fractal dimension on the flow characteristics of CSFMs, revealed the impact of the fractal dimension on the flow performance of slurry, and constructed a CSFM flow performance evaluation and optimization model based on the fractal dimension. At the same time, the influence of the fractal dimension on solid mass fraction and particle distribution in the CSFM sedimentation process was analyzed using a sedimentation experiment. Combined with fitting analysis and model construction, a CSFM sedimentation performance evaluation method based on fractal dimension D was proposed. The results show that (1) the slump, expansion, and yield stress of CSFMs increased first and then decreased with the increase in the fractal dimension, and the bleeding rate of CSFMs decreased with the rise in the fractal dimension. The analysis of the consistency coefficient of CSFMs shows that the increase in the proportion of fine particles will increase the consistency coefficient. (2) The fitting analysis indicates that the fractal dimension D of CSFMs is negatively correlated with the sedimentation performance P_S. The change in D is most significant in the range of 2.3 to 2.4, where the slurry’s stability is poor. When D exceeds 2.5, the slurry’s stability improves significantly. (3) Based on the evaluation of flow performance and settlement performance, the flow performance and settlement performance of CSFMs with fractal dimensions between 2.50 and 2.59 achieve the best balance, which ensures the reliability of long-distance transportation and construction quality. The research results can provide a reference for the pipeline transportation of whole gangue slurry and have important practical significance for realizing the large-scale disposal of gangue solid waste and green mining of coal mines.