Particle size distribution of aggregates effects on mesoscopic structural evolution of cemented waste rock backfill

Bearing characteristics and damage rules of regenerated rock mass

This study investigates the bearing characteristics and damage evolution of regenerative rock masses formed under varying geological conditions through uniaxial loading tests, numerical simulations, and theoretical derivations. Regenerative rock mass samples with different water-cement ratios and cementing materials were prepared, and the mechanical behavior during the loading process was analyzed. The results indicate that the secondary damage process can be divided into three stages: pre-peak, weakening, and friction. As the mechanical properties of the cementing matrix improve, the bearing capacity increases, and the failure mode transitions from ductile to brittle. A damage constitutive model incorporating the Weibull distribution and a damage correction coefficient is proposed to predict the mechanical strength of regenerative rock masses. Numerical simulations using Particle Flow Code 3D (PFC3D) reveal that enhanced mechanical properties of the cementing material lead to a shift from tensile to shear failure. This study provides theoretical and practical guidance for the stability control of regenerative rock mass engineering, offering new insights into the design of support systems for mining operations. The findings have significant implications for the recovery of shallow residual coal resources and the stability control of mining roadways.

Study on mechanical properties of coal gangue and fly ash mixture as backfill material based on fractal characteristics

Backfill mining can effectively alleviate the problems of surface collapse and ecological water pollution, in which the mechanical properties of backfill materials, including coal gangue and coal fly ash, have a decisive role in the effect of filling mining. In this study, we analyze the permeability characteristics of coal gangue filler through a set of homemade percolation test systems and introduce fractal characteristics to investigate the key factors affecting percolation in complex pores of broken coal gangue. The results indicate that the fractal dimensions of crushed coal gangue particles show an increasing trend with increasing axial loading and that the variation range is from 2.15647 to 2.58933. The coal fly ash concentration has a positive relationship with the acceleration factor. The permeability of crushed coal gangue follows a hierarchical distribution law and the permeability changes in the magnitude range of 10-11 ~ 10-9 m2. The fractal dimension is inversely related to the permeability of crushed coal gangue. The experimental results show that the coal gangue will be further crushed and that adding a certain concentration of coal fly ash can achieve a better water barrier, which provides theoretical support and engineering significance for the stability analysis of geological engineering and backfill mining technology.

Microstructure and mechanical behavior of cemented gold/tungsten mine tailings-crushed rock backfill: Effects of rock gradation and content

Without appropriate and responsible waste management in place, the cursory storage of tailings and waste rocks on the surface can cause devastating damage to the planet's ecosystems. To proactively manage or abolish the damage, some techniques such as mine backfill have been already used repeatedly in mines. Microstructure and strength behavior of cementitious tailings-crushed rock backfill (CTCRB) with gold/tungsten tailings and rock contents (e.g., 10%, 20%, 30%, 40%, and 50%) were conducted in this study by using both UCS (unconfined compressive strength) tests (e.g., peak strengths, stress-strain curves, failure modes) and SEM micro-graphs. Key conclusions were shown that: when gradation and content of crushed rock was considered as 1-3 mm and 50% respectively, the UCS value of gold tailings based backfills was 1.02 MPa. In contrast, the UCS value of tungsten mine tailings based backfills was 1.36 MPa when the amount of crushed rock within the filling matrix became 10%. Tungsten tailings based backfills were more sensitive to crushed rock gradation than gold tailings based backfills. CTCRB's stress-strain curvatures were up-concave in the step of pore compaction. With the increase in the content and gradation of crushed rock, tungsten tailings based backfills showed swelling and crushing in complete destruction. Tailings' particle size, crushed rock content and gradation utterly affected the failure modes of CTCRB. Ettringite/CSH gel was found to be the leading hydration materials in the backfill matrix. The micro-cracks within CTCRB specimens were unfavorably correlated with its UCS data. To conclude, this study's main outcomes could give a significant guide for CTCRB's industrial uses.

Influence of the Graphene Oxide on the Pore-Throat Connection of Cement Waste Rock Backfill

The pore-throat characteristics significantly affect the consolidated properties, such as the mechanical and permeability-related performance of the cementitious composites. By virtue of the nucleation and pore-infilling effects, graphene oxide (GO) has been proven as a great additive in reinforcing cement-based materials. However, the quantitative characterization reports of GO on the pore-throat connection are limited. This study applied advanced metal intrusion and backscattered electron (BSE) microscopy scanning technology to investigate the pore-throat connection characteristics of the cement waste rock backfill (CWRB) specimens before and after GO modification. The results show that the microscopic pore structure of CWRB is significantly improved by the GO nanosheets, manifested by a decrease in the total porosity up to 31.2%. With the assistance of the GO, the transfer among internal pores is from large equivalent pore size distribution to small equivalent pore size distribution. The fitting relationship between strength enhancement and pore reinforcement efficiency under different pore-throat characteristics reveals that the 1.70 μm pore-throat owns the highest correlation in the CWRB specimens, implying apply GO nanosheets to optimizing the pore-throat under this interval is most efficient. Overall, this research broadens our understanding of the pore-throat connection characteristics of CWRB and stimulates the potential application of GO in enhancing the mechanical properties and microstructure of CWRB.

Rheological properties of cemented gangue backfill material based on fractal characteristics of waste coal gangue

The study herein was intended to evaluate the rheological properties of cemented gangue backfill material (CGBM). For this purpose, the rheological test of CGBM with fractal aggregate particle size distribution was carried out, and variations of static yield stress, dynamic yield stress, and plastic viscosity were investigated as a function of fractal dimension and time. The results reveal that aggregate particle size distribution exerts a significant influence upon the rheological properties of CGBM, and with the escalation of the fractal dimension of the aggregate, the yield stress and plastic viscosity initially decline and then increase. In addition, with elapsing time, the correlation between the static yield stress and the fractal dimension of CGBM specimens increases, while the correlations between the dynamic yield stress and the plastic viscosity and the fractal dimension decrease. The relationships between rheological parameters and fractal dimensions at different times are established based on the experimental results. The influence mechanism of aggregate particle size distribution on CGBM is analyzed from the perspective of the aggregate packing state. This study can provide a basis for the ratio design of CGBM in backfill mining.

Study on macro-meso mechanical properties of cemented tailings backfill with high fly ash content

The use of cement and fly ash (FA) to prepare cemented tailings backfill (CTB) lowers backfill mining costs while also reducing pollution caused by the accumulation of waste materials like tailings and FA, making it a green backfill mining process. While adding FA to CTB may reduce costs, too much FA might weaken CTB's strength property. Mechanical tests were used to explore the effects of FA content and curing time on the uniaxial compressive strength (UCS) and deformation modulus of CTB in this research. The effects of FA content on failure modes, strain energy, and crack evolution of CTB were studied using a numerical model that considered FA content and particle contact mode. The influence mechanism of different FA contents on CTB was also revealed at the microscopic level. The results demonstrate that the UCS of CTB has a quadratic polynomial and a linear relationship with FA content and curing time respectively, and that the elasticity modulus and secant modulus of CTB increase and then decrease with FA content under different curing times. The peak strain energy of CTB increases and subsequently declines with the FA content, and crack propagation inside CTB may be limited by regulating the FA content. A reasonable content of FA can optimize the size and distribution of CTB microscopic defects, enabling them to exhibit superior strength property. This study systematically explores the mechanism of different FA contents on the strength property of CTB from a macro-micro perspective, providing an essential reference value for improving the recycling of FA and waste residues such as tailings.

Deformation failure and acoustic emission characteristics of continuous graded waste rock cemented backfill under uniaxial compression

In order to study the effect of backfill aggregate particle size on the compressive strength and failure mode of cemented backfill, uniaxial compression tests were carried out on seven kinds of cemented backfills with different particle size gradations. By analyzing the AE characteristics during the failure process of the backfill, the damage evolution mechanism of the cemented backfill with different particle size gradations was discussed. The test results show that with the increase of the Talbot gradation index n, the compressive strength of the backfill specimens first increases and then decreases, and the failure mode gradually changes from shear failure to tensile failure. With the increase of particle size gradation, the particle size of aggregate increases, the interface between aggregate and cement matrix is more likely to be fractured, and the characteristic parameters of acoustic emission are more active. During the failure process of backfill, the AE energy rate increases rapidly in the plastic development stage, and reaches maximum value before and after the peak stress, which can be used as the precursor to judge the failure of waste rock cemented backfill. According to the test results, the damage model and constitutive equations of waste rock cemented backfill with different Talbot particle size gradations are established, which can provide engineering guidance for filling mined-out areas with waste rock to ensure safe production of mines.

Experimental Study on Strength Development and Engineering Performance of Coal-Based Solid Waste Paste Filling Material

To explore the strength development characteristics and engineering performance of different coal-based solid waste filling materials cemented into filling body, coal gangue was used as coarse material, fly ash, desulfurization gypsum, gasification slag, and furnace bottom slag as fine material, and cement as a gelling agent. The uniaxial compressive strength (UCS) and bleeding rate of coal-based solid waste cemented backfill (CBSWCB) were tested by an orthogonal experiment, and the influencing factors of mechanical properties and strength development were analyzed. The multiple generalized linear model of strength and bleeding rate was established, and the optimal filling material ratio was determined. The engineering performance index of CBSWCB with the optimal ratio was tested. The results show the following points: (1) the concentration and content of desulfurization gypsum had a great influence on the early compressive strength of CBSWCB, while fly ash, gasification slag, and furnace bottom slag had little influence on the early compressive strength. (2) High concentration, high content of fly ash and furnace bottom slag, low content of desulfurization gypsum, and gasification slag can significantly improve the early strength. High concentration and high content of fly ash, low content of gasification slag, furnace bottom slag, and desulfurization gypsum are beneficial to the later strength increase. (3) Under the optimal ratio scheme, the bleeding rate of CBSWCB was 1.6%, the slump was 16.6 cm, the cohesion was general, the segregation resistance was good, the initial setting time was 5.42 h, the final setting time was 7 h, and the early strength after curing for 8 h reached 0.24 MPa.

Failure properties and stability monitoring of strip and column cemented gangue backfill bodies under uniaxial compression in constructional backfill mining

The strip and column cemented gangue backfill bodies (CGBBs) are the main supporting components in the design of constructional backfill mining for coal mining, which determines the stability of goaf. Previous researches have mostly focused on the mechanical properties of column CGBB, but the mechanical properties of strip CGBB are still unclear. Herein, the uniaxial compression experiments for strip and column CGBBs were conducted to compare the failure properties. The acoustic emission (AE) and two types of resistivity monitoring were used to monitor the damage evolution. The effect of the length-height ratio on the mechanical characteristic of strip CGBB was analyzed by discrete element simulation. The results show that the strength and peak strain of strip CGBB under uniaxial compression is higher than those of column CGBB and the strip CGBB shows better ductility. The stress of column CGBB decreases significantly faster than that of strip CGBB at the post-peak stage. The strength and ductility of strip CGBB increase with the increase of length-height ratio. The strip CGBB is destroyed from both ends to the middle under uniaxial compression, and the core bearing area is reduced correspondingly. The AE signal evolution of CGBBs under uniaxial compression before the peak stress contains three stages, and the AE signals of strip CGBB at the peak stress will not rise sharply compared with column CGBB. The resistivity monitoring effect of the horizontally symmetrical conductive mesh is better than that of the axial. The horizontal resistivity increases gradually with the increase of stress under uniaxial compression, and increases sharply at the peak stress, and then drops after the peak stress. The damage constitutive models and the stability monitoring models of the CGBBs are established based on the experimental results. This work would be instructive for the design and stability monitoring of CGBB.

Effect of Particle Size Distribution on the Dynamic Mechanical Properties and Fractal Characteristics of Cemented Rock Strata

To investigate the dynamic mechanics and post-failure characteristics of fault-cemented rock strata, broken rock particles were reshaped to obtain cemented rock samples with various particle size distributions (PSDs). Split Hopkinson pressure bar (SHPB) dynamic impact tests were performed on the cemented rock samples under different strain rates. The test results show that plastic deformation occurs in the cemented rock sample as a result of its porous structure. Therefore, there is no linear phase in the dynamic stress–strain curves. With an increase in the Talbot index and mixture type, more large particles were contained inside the cemented rock sample, and the dynamic strength gradually increased. A power function can effectively describe the relationship between the strain rate and dynamic strength for various Talbot indices. After dynamic impact, the fragments of the cemented rock samples exhibit evident fractal laws, and the breakage of the samples includes breakage of the original rock particle itself and breakage between the rock particles and cementations. The breakage ratio and fractal dimension both decrease with the increase in the number of mixture type and Talbot index but increase with the increase in strain rate. It is worth noting that the breakage ratio and fractal dimension have a linear relationship regardless of the PSD or strain. The relationship between the dynamic strength and fractal dimension has different response laws for the PSD and strain rate effects. The dynamic strength is negatively linearly related to the fractal dimension under the PSD effect but positively linearly related to the fractal dimension under the strain rate effect. This research work can provide foundation support for investigating the instability mechanism of fault cemented rock strata under dynamic stress.

Random Forest Slurry Pressure Loss Model Based on Loop Experiment

A reasonable arrangement of filling pipelines can solve the problems of low line magnification, a high flow rate, large pipe pressure, etc., in deep well filling slurry transportation. The transportation pressure loss value of filling slurry is the main parameter for the layout design of filling pipelines. At present, pressure loss data are mainly obtained through the loop pipe experiment, which has problems such as a large amount of labor, high cost, low efficiency, and a limited amount of experimental data. In this paper, combined with a new generation of artificial intelligence technology, the random forest machine learning algorithm is used to analyze and model the experimental data of a loop pipe to predict the pressure loss of slurry transportation. The degree of precision reaches 0.9747, which meets the design accuracy requirements, and it can replace the loop pipe experiment to assist with the filling design.

Optimization of Parameters for Rheological Properties and Strength of Cemented Paste Backfill Blended with Coarse Aggregates

Cemented paste backfill (CPB) technology is widely used for environmental protection and underground goaf treatment. The influences of solid concentration, coarse aggregates dosage, and cement dosage on the rheological properties and compressive strength of CPB blended with coarse aggregates (CA-CPB) are investigated through three-factor and four-level orthogonal experiments. The dynamic shear stress and plastic viscosity are selected to characterize the rheological properties of CA-CPB. The uniaxial compressive strength (UCS) is used to describe the compressive strength. The effect of each factor on rheological properties is different from that on UCS. The most significant influences on rheological properties and UCS are solid concentration and cement dosage, respectively. The optimal levels of each factor for rheological properties and UCS are different, resulting in different optimal combinations obtained through range analysis. Therefore, the overall desirability function approach is employed to perform multiple response optimization. The optimal parameters for high fluidity and strength obtained provide valuable information for the CA-CPB process in the Chifeng Baiyinnuoer Lead and Zinc Mine.

Upcycling of wastes for sustainable controlled low-strength material: A review on strength and excavatability

In recent decades, the use of controlled low-strength material (CLSM) in densely populated cities has increased. CLSM is designed for future excavation with great fluidity, appropriate early strength, and low final strength. CLSM mixtures exhibit variable strength properties and performance due to the distinctive features of wastes (i.e., combustion residues, industry slags, and construction and other solid wastes) produced from various sources. CLSM should increase early strength quickly enough to allow traffic to resume within a few hours while maintaining a low strength for future re-excavation. It is suggested that the initial mixture design for each waste reported in the literature be changed until the combination meets the application standards defined in ACI 229R-13. The effects of adjusting other ingredients (i.e., cement, water, and admixtures) in the wastes incorporated into CLSM mixtures on the strength and re-excavatability properties are also detailed and discussed in this review. From practical and economic perspectives, the supply of materials in the waste streams, transport distance, and material properties and cost are important aspects to consider before their introduction to the construction industry.

Deformation and instability properties of cemented gangue backfill column under step-by-step load in constructional backfill mining

Constructional backfill mining with cemented gangue backfill column can solve the environmental issues caused by mining activities and the accumulation of waste gangue at a low cost. To study the deformation and instability properties of cemented gangue backfill columns during the advancement of coal mining face, five step-by-step loading paths were adapted to mimic the different loading processes of the roof. The lateral deformation at different heights and axial deformation of the sample were monitored. The results show that the deformation and instability of the backfill column have the properties of loading paths and are affected by the step-by-step loading path. When stress-strength ratio (SSR) is less than 0.6, the lateral of backfill column shrinks during the creeping process. In high-stress levels, lateral creep strain develops faster than axial creep strain. The backfill column has characteristics of axial creep hardening and lateral creep softening during the step-by-step loading process. The instantaneous deformation modulus and instantaneous Poisson's ratio show an upward trend. The strength of backfill column under the step-by-step load is related to loading paths and is no less than uniaxial compressive strength. The non-uniformity of the lateral deformation of backfill column leads to excessive localized deformation that mainly occurs in the middle, causing the overall instability. The development of cracks of backfill column under step-by-step load could be divided into 4 stages according to SSR. Under different step-by-step loading paths, the axial creep strain rate is nearly a constant before entering the accelerated creep stage. A nonlinear creep constitutive model with a creep strain rate trigger was proposed to depict the development of axial strain under step-by-step load. This research could provide a scientific reference for the design of the advancing distance and cycle for the hydraulic support, and reinforcement of the backfill column.