As mining depth increases, the backfill mining method is more and more widely used in underground mines. The dynamic load generated by the blasting can affect the stability of the cemented tailings backfill (CTB). The CTB samples were prepared to conduct a test of the split Hopkinson pressure bar (SHPB) to investigate the dynamic disturbance of CTB. The present paper discusses dynamical mechanics, energy dissipation, and microstructure analysis of CTB. Micro-computer tomography (micro-CT) scanning of CTB samples after the SHPB test was performed to analyze the evolution of internal cracks. The experimental results showed that when the average strain rate (ASR) increased from 30 to 98 s⁻¹, the dynamic uniaxial compression strength (DUCS) of the CTB showed a trend of first increasing and decreasing with the increase in ASR. The dynamic stress–strain pre-peak curve of CTB directly enters the linear elastic stage. As ASR increases, the absorbed energy of the CTB shows a trend of first increasing and then decreasing. Moreover, according to the micro-CT scanning results, the crack area of CTB accounts for about 16% of the sample near the incident bar and about 1% near the transmitted bar. The crack area ratio is exponentially related to the specimen height. These findings can provide reasonable dynamical CTB strength data selection for underground pillar mining.

It is of great significance to study the effect of solid contents (SC), binder-to-tailings (b/t) ratio, types and dosage of fly ash (FA) on the viscosity (V) and uniaxial compressive strength (UCS) of backfill. It can improve filling efficiency and reduce filling costs to understand the relationship between SC, b/t ratio, FA dosage and viscosity, and UCS of backfill. Consequently, this paper carried out uniaxial compression tests and rheological tests on five different types of backfill specimens. Experimental results indicate that, with the increase of SC, the viscosity and UCS of all backfill samples increases as a power function. With the decrease of b/t ratio, the viscosity and UCS of all backfill samples decreases as an exponential function. The coupling effect of SC and b/t ratio has a great influence on the viscosity and UCS of backfill samples. The relationship between SC, b/t ratio and viscosity, and UCS is a quadratic polynomial function. The order of the viscosity of the backfill slurry is: pure tailings < backfill slurry mixed with Ordinary Portland Cement (OPC) < backfill slurry mixed with FA1 < backfill slurry mixed with FA2. The higher the FA dosage, the greater the viscosity. The order of the UCS of backfill is: backfill with OPC > backfill with FA1 > backfill with FA2. The higher the FA dosage, the smaller the UCS. The UCS of all backfill samples increased with the increase of curing time (CT). The relations between the viscosity and UCS of backfill present the positively linear functions. It is feasible to use viscosity to predict the UCS of backfill, and the error between the UCS predicted value and the test value is mostly controlled within 10%. Ultimately, the findings of the experimental work will provide a scientific reference for the mine to design the strength of the backfill.

In underground metal mines that use sublevel or stage open-stope and backfilling mining methods (SSOBMMs), there is a special structure around which both sides of the rock pillar are wrapped by backfill. As a permanent part of an underground mine, how much can backfill improve the rock pillar’s compressive strength? What is the difference in the mechanical properties between the special structure and the signal rock? To explore these questions, a composite structure made of a cement-tailing backfill (CTB) and rock core (RC) was designed. Uniaxial and triaxial compressive strength tests and scanning electron microscope (SEM) were used to research the mechanical properties, failure process, failure characteristics, and microstructure characteristics of the cement-tailing backfill and rock core (CTB-RC) specimens. It was found that the full stress–strain curve of the CTB-RC specimen under triaxial compressive strength (TCS) test had two times the stress increases reaching a lower peak deviator stress two times after the RC was destroyed. The CTB can reduce the destruction and slow down the deformation speed of the inner rock cor (IRC). It can also prevent rigid slip of the IRC after it is damaged and maintain the stability and integrity of the overall structure. The findings of this study can provide some basic knowledge on the mechanical properties of the CTB-RB and provide theoretical guidance for the optimization direction of the width of the rock pillar and the room in mines using SSOBMMs.

Aiming to solve the problems related to the slow settling speed and the long-term consumption of ultra-fine tailings in mine filling, the effect of flocculant type on the flocculation and settling performance of ultra-fine tailings was studied through static sedimentation experiments on tailings. The microstructure of the flocculation was observed and analyzed using an electron microscope. On this basis, the selection of the optimum flocculant type and dosage parameters was carried out. The results show that the best addition amount of the AZ9020 anionic flocculant was 30 g/t, a solution concentration of 0.3%, and a stirring time of more than 45 min. The floc structure of the full-tailings flocculation solution was formed by the AZ9020 anionic flocculant. Moreover, the size of less than 0.1 μm was still relatively large; thus, the overall size of the structure was small and uniformly dispersed. The floc solution had the smallest porosity, the fractal dimension was the largest, the molecular weight of the floc was the largest, and the floc was the most compact, making it appropriate for the rapid removal of floc structures from water. Sedimentation is also the best flocculant for flocculation and sedimentation. The size of the flocs decreased as the height of the flocculation sediment bed increased during flocculation and sedimentation. The research results provide a microscopic view for the selection of the best flocculant type.

The backfill in the stope usually forms a composite structure with the surrounding rock in order to bear pressure together to support the goaf and ensure the safe mining of subsequent ores. Based on laboratory tests and theoretical analysis, the energy and damage evolution of the rock–backfill composite materials (RBCM) are studied deeply. The results show that: (1) The σ_p (peak stress), ε_p (peak strain), and E (elasticity modulus) decreased with the increase of the internal backfill diameter. When the diameter of the backfill increases from 10 mm to 40 mm, σ_p decreases from 50.15 MPa to 18.14 MPa, ε_p decreases from 1.246% to 1.017%, and E decreases from 7.51 GPa to 2.33 GPa. The U^T shows an S-shaped distribution, the U^E shows an inverted U-shaped distribution, and the U^D first increases slowly and then increases rapidly. The U^T_p, U^E_p, U^D_p, U^E_p/U^D_p, and U^E_p/U^T_p decrease by 67.38%, 97.20%, 58.56%, 32.64% and 13.64% respectively, and the U^D_p/U^T_P increases by 20.93% with the increases of the backfill diameter. (2) A damage constitutive model of the RBCM is established based on the energy consumption characteristics. The damage evolution curve shows an S-shaped distribution, and the damage rate evolution curve shows an inverted U-shaped distribution. (3) The AE correlation fractal dimension decreases with the increase of the strain gradient and damage value, and the AE correlation fractal dimension presents linear and exponential functions with them, respectively. With the increase of stress, microcracks first appear and gather in the internal backfill of the RBCM, and then microcracks appear and gather in the peripheral rock, which together lead to the macro penetration failure of the RBCM.