Filling mining is an effective way to settle the dilemma of “Three Down and One Above” in coal mining. Fly ash and coal gangue can be used as filling materials with significant social, economic, and environmental benefits. Using coarse fly ash base as cementing material and coal gangue as aggregate, orthogonal experiment of filling paste was conducted in this study. The range analysis was performed for the strength and transportation requirements of filling paste, and the optimum proportion was determined by the comprehensive balance method. In order to verify the filling effect, a dynamic filling simulation device was designed, and a comparative simulation test of caving mining and dynamic filling mining was carried out. Results show that the filling paste with fly ash and coal gangue as the main component can meet the requirements of filling design and application. This research provides a reference for the material selection and proportion design of paste filling.

In order to study the effects of different concentrations of sulfate on the strength of fly ash‐based coal mine filling paste, using variable control, mechanical analysis, and other means, the changes in the uniaxial compressive strengths of filling paste blocks soaked in different concentrations of sodium sulfate solution for different durations are studied, and their stress‐strain curves are discussed. The hydrated products of each block are analyzed at different stages by XRD, and the results indicate that different concentrations of sodium sulfate solution have different effects on the strength of the filling paste after soaking for different durations. A sodium sulfate solution with a concentration of 5% had an activator effect on the fly ash‐based filling paste and enhanced the strength of the filling paste. A sodium sulfate solution with a concentration of 10% and 15% increased the early strength of the paste test block faster, but after 60 d, the strength decreased. The stress‐strain curves for these blocks show that the elastic moduli of the filling paste test blocks change irregularly, and it was found that with the increase in soaking time, the blocks soaked in the 10% and 15% sodium sulfate solutions developed fissures in the later stage that adversely affected the strength of the filling paste. The XRD results show that the filling paste test block hydration products are hydrated calcium silicate (C‐S‐H) based and that ettringite (AFt), beneficial to strength of the filling paste in proper quantities, appeared in the main product of the filling paste test blocks that were soaked in the sodium sulfate solution. With the increase in the concentration of the sodium sulfate solution, the AFt is generated in larger quantities, and gypsum crystals begin to appear, which is not conducive to the filling paste block strength.

This study was conducted on the impact of carbonization reaction on the performance of gangue–cemented paste backfill(GCPB). Tests were conducted using a carbonization box with CO₂ concentrations of 0%,5%,and 10%. Each test was conducted for 3,7, and 28 days. We then conducted uniaxial compressive strength(UCS)testing, as well as X-ray diffraction(XRD)and scanning electron microscopy(SEM)analyses to explore the effects of the carbonization reaction on the mechanical properties of the GCPBs under the varying carbonization concentrations. The test results showed that before 28 days, the UCS of the GCPB increased with time. The higher the CO₂ concentration, the larger the contact area with the gel material;and the longer time, the slower the growth. Hydration products, such as needle-shaped ettringite(AFt)and white fibrous calcium-silicate-hydrate(C–S–H)gel, carbonize with CO₂ in the air, mainly generating CaCO₃,and combine with incompletely carbonized hydration products to act on the GCPB, increasing the UCS in its growth state. Moreover,we established a CO₂ diffusion and mass transfer equation, indicating a negative correlation between the UCS of the GCPB and the CO₂ concentration,which verified the macroscopic test results. Controlling the CO₂ concentration provides the theoretical and experimental basis for the impact of the carbonization reaction on the performance of GCPBs.

The spatial variability of surface roughness can affect the overland flow processes. To accurately simulate the overland flow processes of watersheds, many theories have been developed which consider the spatial variability in surface roughness. Several theories claim that the surface roughness has anisotropic attributes, but there is little experimental evidence to support this claim. The authors developed a hydraulic laboratory model to simulate land surfaces, and replicated water flows in different directions over the exact same overland surface. Two typical strip overland surfaces were simulated, and the Manning’s roughness coefficient is calibrated. The results show that in a completely turbulent regime when the angle $\theta$ of the flow direction and the; overland strips direction are 90° and 45°, respectively; the average Manning’s roughness coefficient values $n$ are 0.0438 and 0.0492 over a $60\times 60\mathrm{mm}$ simulation overland surface, and 0.0276 and 0.0382 over a $120\times 60\mathrm{mm}$ simulation overland surface, respectively. Over the exact same strips of surface, there are different Manning’s roughness coefficient values in different flow directions. This proves that the surface roughness of watersheds has anisotropic attributes, which will have effects on overland runoff path and distribution.