Coal Gangue-fly Ash Research Articles

Mechanical properties under triaxial compression of coal gangue-fly ash cemented backfill after cured at different temperatures

The triaxial mechanical properties of cemented backfill are the key indicators that affect the stability of cemented backfill in underground mining areas, and the high temperature environment in mines has become one of the main factors affecting the mechanical properties of cemented backfill. Understanding the triaxial mechanical properties of cemented backfill in high temperature environments is crucial for the stability of cemented backfill in mining areas. To investigate the effect of temperature on the mechanical characteristics of cemented backfill under triaxial compression, triaxial compression experiments were done on coal gangue-fly ash cemented backfill that had been cured at three temperatures (20 °C, 35 °C, and 50 °C). The deviatoric stress-strain curve of cemented backfill under triaxial compression was produced, its deformation parameters, dilatancy, and failure characteristics were investigated, and a more appropriate strength criteria was proposed. The research results indicate that, peak strength and dilatancy stress of cemented backfill under the same confining pressure decrease first and then increase as the curing temperature increases. The failure patterns of cemented backfill under triaxial compression will exhibit layered splitting failure except tension and shear failure since the curing temperature has increased. By comparing it with several other criteria, the Mogi-Coulomb strength criteria was found to be a good reflection of the failure strength properties of cemented backfill cured at different temperatures under triaxial compression. With curing temperature rises, internal friction angle of cemented backfill steadily lowers, while the cohesion first decreases and subsequently increases. The alteration in cohesion is consistent with the peak strength change rule, illustrating that the primary factor influencing peak strength of cemented backfill is cohesion. This study provides a good theoretical reference value for guiding the design of deep mine filling and optimizing the mixture ratio of cemented backfill.

Investigation on deterioration mechanism of geopolymer cemented coal Gangue-Fly ash backfill under combined action of high temperature and salt corrosion environment

The geopolymer cemented coal gangue-fly ash backfill (GCGFB) is pumped to the goaf to maintain the stability of surface subsidence, which is inevitably affected by the high temperature caused by geothermal heat and ion erosion of mine water. To study the deterioration mechanism of GCGFB caused by salt corrosion and high temperature of mine water, in this study, a customized temperature-controlled corrosion chamber was used to carry out the corrosion experiment of GCGFB. A three-factor and three-level orthogonal experiment was designed. The uniaxial compressive strength (UCS), relative dynamic elastic modulus (RDEM) and weight deterioration of GCGFB under different combinations of temperatures, solution concentrations, and solution types were analysed. The deterioration mechanism of GCGFB was analysed through the testing of scanning electron microscopy (SEM)-energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). From the results, the most significant factors on the deterioration of GCGFB is solution type. Cl− and SO42− caused chemical-physical erosion on GCGFB, and the UCS, RDEM and weight of GCGFB increased first and then decreased with ages. The reason for the increase was due to the compacting effect of Friedel 's sat, ettringite, and gypsum filled in GCGFB. The reason for the decrease was due to the expansion pressure on the inner wall of the pores of the GCGFB caused it to crack or even peel off. The GCGFB was corroded in 20 % Na2SO4 solution at 40 °C for 90 days, and the UCS increased by 164.90 % compared to that of the uncorroded specimen. Mg2+ brought chemical erosion on GCGFB, and a decreasing law of the UCS, RDEM and weight with ages was shown. During the Mg2+ erosion, there are more and more M−S−H in matrix, which reduced the density of specimen apparently. The GCGFB was corroded in 5 % MgCl2 solution at 40 °C for 180 days, and the macropore area distribution of GCGFB is increased to 23.499% than that of uncorroded group. The expansion stress of corrosion products would squeeze the inner pore wall, resulting in microcracks inside GCGFB and high temperatures would accelerate the migration of ions and the formation of corrosion products. Overall, this study can provide a new insight or valuable data for the designing of ultra-high performance, low-caron, eco-friendly, and durable GCGFB structures in the harsh mining environments.

Properties of a backfill material prepared by cementing coal gangue and fly ash through microbial-induced calcite precipitation

Microbes can induce the production of calcium carbonate deposits, which cement solid wastes such as coal gangue to form backfill materials. Filling such backfill materials in underground goafs can facilitate the disposal of solid wastes and control surface subsidence. In view of this, the idea of preparing a backfill material by cementing coal gangue and fly ash through microbial-induced calcite precipitation (MICP) induced by B. pasteurii was proposed and the preparation procedure and test methods of properties of the backfill material were introduced. Changes in the slump flow and bleeding rate of the backfill material under conditions of different bacterial concentration, urea concentration, and mass concentration were studied. Influences of factors including the bacterial concentration, urea concentration, mass concentration, curing time, and curing temperature on the compressive strength of the backfill material were estimated. Results show that as the bacterial concentration increases, the slump flow increases, the bleeding rate decreases, while the compressive strength of the backfill material increases. An increase in the urea concentration reduces the slump flow and bleeding rate of the backfill material while first increasing, then decreasing the compressive strength; excessive concentrations of urea suppress bacterial activities. An increase in the mass concentration decreases the slump flow and bleeding rate of the backfill material, while increasing the compressive strength. As the curing temperature is increased, the compressive strength of the backfill material first increases, then decreases and the temperature range of 30 ∼ 40℃ is more conducive to bacterial hydrolysis of the urea and chemical reactions in the coal gangue-fly ash slurry. The prolonged curing time can significantly improve the compressive strength of the backfill material. The research results provide a theoretical basis for the application of microbial-induced calcite precipitation (MICP) in backfilling mining of coal mines.

Effect of the co-activation of sodium silicate and CO2 on setting and mechanical properties of coal gangue-fly ash backfill (CGFB)

Effect of the co-activation of sodium silicate and CO2 on setting and mechanical properties of coal gangue-fly ash backfill (CGFB)

Deformation and Failure Characteristics of Bimaterial Samples Consisting of Sandstone and Cemented Coal Gangue–Fly Ash Backfill under Uniaxial Loading

Based on the acoustic emission (AE) system and the digital scattered-spot deformation monitoring system, uniaxial compression tests were conducted on composite samples consisting of sandstone and cemented coal gangue–fly ash backfill (CGFB) to investigate their deformation and failure characteristics. The results showed that the average uniaxial compressive strength of the composite samples was 83.09% higher than that of the pure CGFB samples and 92.28% lower than that of the pure sandstone samples. In the composite samples, damage occurred in the CGFB part, and they showed obvious plastic damage characteristics. On both sides of the intersection, the sandstone and the CGFB deformed synergistically in the absence of a macroscopic failure. After a macroscopic failure, the interface effect promoted sandstone deformation and restrained CGFB deformation, transforming the sandstone and the CGFB on both sides of the intersection into a nonsynergistically deformed state. The interface effect had the most obvious influence on the horizontal deformation of the sandstone and CGFB monitoring points near the intersection. The failure of the CGFB samples induced sandstone springback deformation with a springback capacity of 0.0089 mm in the vertical direction and 0.0055 mm in the horizontal direction, which led to the further rupture and failure of the CGFB.

Study on Rheological Characteristics of Uncemented Coal Gangue-Fly Ash Backfill (UCGFB) Slurry Based on Fractal Theory

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.

Effect of velocity on flow properties and electrical resistivity of cemented coal gangue-fly ash backfill (CGFB) slurry in the pipeline

During the pipeline transportation of backfill slurry, flow velocity has direct effects on the transport properties and then influences pipe blockage and wear. In this paper, a novel flow loop-horizontal electrical resistivity (HER) test system and Eulerian model were used to investigate the flow behavior and HER characteristic of cemented coal gangue-fly ash backfill (CGFB) slurry under different flow velocity. The results showed that: 1) The pressure drop of CGFB slurry increased linearly with flow rate. 2) The increase in flow rate significantly reduced coarse aggregate contents near the bottom of the pipeline. 3) As the flow rate increases, the fluctuation of measured HER with time increased, the HER differences decreased, and the ranges of the high and low resistance area at the bottom and top of the pipeline, respectively, kept decreasing. The results can provide guidance for HER monitoring of flow state of CGFB slurry in pipeline.

Mechanical properties and damage evolution of cemented coal gangue-fly ash backfill under uniaxial compression: Effects of different curing temperatures

A high geothermal heat affects the physical properties of cemented coal gangue-fly ash backfill (CGFB). The mechanical properties, damage evolution, elastic modulus, and peak strain of CGFB under uniaxial compression were investigated at different curing temperatures (20℃, 35℃, and 50℃) and curing ages (3, 7, 14, and 28 days), and the stress-strain curves were obtained. The results show that the elastic modulus and peak strain of CGFB exhibit an increasing overall trend with a rise in the curing temperature; the 14th day of curing is the inflection point of the elastic modulus curve of the CGFB specimens. The theoretical results based on the piecewise damage constitutive model are in good agreement with the experimental results, indicating that the model is reasonable. Four damage stages of the CGFB are observed: no damage, initial damage formation, accelerated damage growth, and stable damage development. The CGFB specimens cured at 35℃ and 50℃ suffer complete damage, whereas the CGFB specimens cured at 20℃ are not completely damaged, with a damage value of only 0.5409. The damage trend under uniaxial compression indicates that the damage value increases slowly from 0 at the initial stress, rapidly increases after the peak stress, and increases slowly until the damage value reaches 1, and the CGFB suffers a complete failure. This paper lays a foundation for analyzing the stability of CGFB in a high geothermal environment.

Physico-mechanical and microstructure properties of cemented coal Gangue-Fly ash backfill: Effects of curing temperature

High environment temperatures in deep coal mines may affect the strength and stability of cemented Coal Gangue-Fly Ash Backfill (CGFB). This study aimed to analyze the consequences of temperature on the physical and mechanical properties and microstructure of CGFB. Uniaxial compression tests were carried out on CGFB specimens at different curing temperatures (i.e., 20℃, 35℃, and 50℃) and curing ages of 3, 7, 14, and 28 days. The moisture content of CGFB specimens was tested by the drying method. The microstructure and mineral composition of CGFB specimens were measured by X-Ray Diffraction (XRD), Thermogravimetric-Differential Scanning Calorimetry (TG-DSC), Mercury Intrusion Porosimetry (MIP), and Scanning Electron Microscopy (SEM). The results show that temperature has a significant effect on the strength and microstructure of CGFB. Higher curing temperatures can speed up the hydration reaction rate and generate more hydration products, making the pozzolanic effect of fly ash occur earlier. Early compressive strength of CGFB specimens increases, but later compressive strength decreases due to the decrease of the bonding of the internal structure due to the thermal damage. The change of curing temperature will lead to the change of failure mode of CGFB specimens. The failure mode of CGFB specimens changes from tensile failure at 20℃ to tensile-shear mixed failure at a higher temperature (35℃, 50℃). Under the influence of temperature, among the two physical property parameters (i.e., porosity and moisture content of CGFB), the porosity can better reflect the mechanical behavior of the backfill. This study provides a reference value for guiding filling in high ground temperature coal mine.

Effects of Kaolin Addition on Mechanical Properties for Cemented Coal Gangue-Fly Ash Backfill under Uniaxial Loading

In this investigation, six groups of cemented coal gangue-fly ash backfill (CGFB) samples with varying amounts of kaolin (0, 10, 20, 30, 40, and 50%) instead of cement are prepared, and their mechanical properties are analyzed using uniaxial compression, acoustic emission, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The uniaxial compressive strength, peak strain, and elastic modulus of CGFB samples decreased with the kaolin content. The average uniaxial compressive strength, elastic modulus, and peak strain of CGFB samples with 10% amount of kaolin are close to that of CGFB samples with no kaolin. The contribution of kaolin hydration to the strength of CGFB sample is lower than that of cement hydration, and the hydration products such as ettringite and calcium-silicate-hydrate gel decrease, thereby reducing strength, which mainly plays a role in filling pores. The contents of kaolin affect the failure characteristics of CGFB samples, which show tensile failure accompanied by local shear failure, and the failure degree increases with the kaolin content. The porosity of the fracture surface shows a decreasing trend as a whole. When the amount of kaolin instead of cement is 10%, the mechanical properties of CGFB samples are slightly different from those of CGFB samples without kaolin, and CGFB can meet the demand of filling strength. The research results provide a theoretical basis for the application of kaolin admixture in fill mining.

Effect of Concentration and Suspension Agent (HPMC) on Properties of Coal Gangue and Fly Ash Cemented Filling Material

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.

Experimental Study and Application of Rheological Properties of Coal Gangue-Fly Ash Backfill Slurry

From the perspective of resource recovery and environmental protection, coal gangue-fly ash cemented backfill coal mining has become an important direction for the green development of coal mines in recent years. Analysis of the rheological parameters of backfill slurry is the basic principle to design a backfill pipeline system. Coal gangue-fly ash backfill slurry has a mass concentration of 76% to 79% and a maximum particle size 20 mm. Therefore, it is difficult to use conventional rheometers for experimental analyses of the rheological parameters of such fluids. We developed a rheometer for high-concentration coal gangue backfill slurry (HCGS rheometer) based on the coal gangue-fly ash backfill practice of Gonggeying Mine, and analyzed the rheological properties of the backfill slurry. The experimental results showed that the Reynolds number of the coal gangue-fly ash backfill slurry was much smaller than the critical Reynolds number, indicating the flow state in the pipeline was laminar. Based on these results, it may be more appropriate to control the mass concentration to 77% to 78%, and the suggested fly ash content is 25%. This work provides a scientific basis to optimize the backfill parameters and pipeline system in coal mines.

Rheological Study on Cemented Coal Gangue Fly-Ash Backfill Slurry Associated with Fly Ash Impacts

The cemented coal gangue-fly ash backfill (CGFB) slurry has commonly been used to control subsidence damage caused by underground coal mining. This paper discusses the CGFB slurry rheological characteristic associated with the various percentage of the fly ash within the configuration of the mixture. A general description about CGFB slurry fluidity in the pipe transportation is provided. The physical, chemical and rheological characteristic of the CGFB have been determined in the test. The fly ash was mixed with 18%, 20% and 22% of solid concentration with respects to 12%, 10% and 8% for cement on various concentrated CGFB slurry ranges 76.05%, 78.05% and 80.05%. The tests verify the CGFB slurry exhibits non-Newtonian behavior in nature as Bingham plastic fluid. The experimental data show CGFB slurry rheological characteristic is noticeable influenced by the slurry solid concentration and the proportion of the fly ash, which affects the value of yield stress and viscosity. The lower value of the rheological parameters was observed on 22 % fly ash other than the 18 % case based on the same solid concentration of CGFB slurry.

A Coupled Thermo-mechanical Damage Modeling Application of Cemented Coal Gangue-Fly Ash Backfill Under Uniaxial Compression

Cemented coal gangue-fly ash backfill (CGFB), which is a mixture of coal gangue, fly ash, cement and water, is introduced and used for waste disposal and ground control. Once placed underground, the CGFB is required to possess satisfactory strength and stability for the support of overlying roof. Therefore, it is crucial to understand the mechanical behavior and damage evolution of CGFB, which is also affected by thermal factors. This paper develops a coupled thermo-mechanical damage model to describe the mechanical response and damage of CGFB under uniaxial compression. The developed model considers the thermal factors of heat conduction and binder hydration. The validity of the developed model is verified by the comparison of simulation results and data from an experimental study. The developed model is then used for some applications. The modeling results can contribute to a better design and preparation of stable and endurable CGFB structures.

Study of localized deformation in geopolymer cemented coal gangue-fly ash backfill based on the digital speckle correlation method

Abstract Uniaxial compression testing was combined with the digital speckle correlation method (DSCM) to explore the deformation field evolution characteristics and the Poisson’s ratio and energy release responses of a geopolymer cemented coal gangue-fly ash backfill (GCGFB) sample under uniaxial compression. The results showed that in the initial stage of loading, the deformation of the GCGFB sample was small and evenly distributed, and no localized deformation zone formed in the sample. As the loading progressed, a localized deformed zone gradually appeared, and the formation rate gradually accelerated. The maximum shear strain value at the start of localized deformation was 0.06. In the post-peak loading process, the localized deformed zone gradually expanded, and the degree of unevenness of the localized deformation was much greater than that before the peak; the maximum shear strain value at this time was 0.22. During the localization of the deformation to the peak intensity stage, the growth rate of the Poisson’s ratio of the GCGFB sample was markedly increased, and the growth rate after the peak was notably slowed and exhibited unstable fluctuations. When the stress reached the peak, the Poisson’s ratio was 0.24. The peak strain of the GCGFB sample was large, and the failure characteristics of the GCGFB sample showed multiple shear deformation zones, which is indicative of a typical cylindrical splitting failure. The deformation energy density of the GCGFB sample exhibited a slow decline after the peak, and the rate of decline gradually decreased. The deformation energy density drop time of the GCGFB sample was 96 s, which is much longer than that of high-strength rock.

Preparation and microstructure of fly ash geopolymer paste backfill material

As a new paste backfill material, geopolymer cemented coal gangue-fly ash backfill is prepared from fly ash geopolymer and spontaneous combustion gangue and can be beneficial to the cleaner production of coal mines and coal-fired power plants. A new mixing technique, in which a small amount of fly ash was mixed with cement, slag and water and allowed to react for a period of time before an aggregate and residual fly ash were added, was proposed. The influences of the solid mass concentration, the fine gangue ratio and the fly ash content on the workability and strength of the paste backfill material were analyzed, and ratio optimization was carried out using the response surface method, multi-objective multi-verse optimization and the desirability function approach. The results show that the optimal mixture is obtained when the mass concentration is 79.65%, the fine gangue rate is 57.19%, and the fly ash content is 15.67% of the total mass. The new mixing technique makes the fly ash more active to stimulate the early formation of calcium silicate gel and calcium silicate hydrate gel that improve the aggregate formation strength. In the later stage, amorphous hydrated calcium aluminate gel-coated ettringite and calcium carbonate formed a flocculation structure between the fly ash particles that gradually formed a condensation-crystallization structure. The research results can make extensive use of fly ash and gangue to promote cleaner production.

Effect of Curing Temperature on Mechanical Performance and Acoustic Emission Properties of Cemented Coal Gangue-Fly Ash Backfill

Cemented coal gangue-fly ash backfill (CGFB), which is a mixture of coal gangue, fly ash, binder and water, is introduced and employed to fill underground mined-out areas of coal mines. Once placed, the CGFB mixtures should have favorable stability, which is closely relevant to the mechanical performance of the CGFBs. The mechanical behavior of CGFB is affected by the environment temperature. In this study, with the help of acoustic emission (AE) and infrared thermal imaging technique, the influence of curing temperature on the mechanical behavior of CGFB specimens is investigated. A uniaxial compressive strength (UCS) testing apparatus coupled with AE monitoring and infrared thermography is equipped to investigate the mechanical (UCS, elastic modulus, stress–strain relation), acoustic (AE cumulative counts and energy), and thermal (average infrared radiation temperature) properties of CGFB samples cured for different ages (1, 3 and 7 days) at various curing temperatures (20, 50, 75 and 90 °C). This study provides an effective coupled monitoring method for evaluating the performance of CGFB structures, and this can contribute to a better understanding of the thermo-mechanical-acoustic behavior of CGFBs, and thus a better design of stable CGFBs.

Pressure Study on Pipe Transportation Associated with Cemented Coal Gangue Fly-Ash Backfill Slurry

Cemented coal gangue-fly ash backfill (CGFB) slurry has commonly been used to control subsidence damage caused by underground coal mining. This paper discusses the characteristics of CGFB slurry fluidity in its pipe transportation. A general description about the components of the CGFB is provided involving the percentage of composition, particle size distribution (PSD) and rheological performance. The CGFB flow characteristics of the slurry pipeline were simulated in a straight pipe and 90° elbow pipe, respectively, combined with the pressure loss and conveying velocity distribution. With the help of the commercial computational fluid dynamic (CFD) code FLUENT, the modeling was conducted with various slurry feeding velocities. These results showed the local resistance loss in a bending pipe is significantly higher than the resistance in a straight pipe under the same conditions associated with CGFB transportation. The velocity distribution of the slurry solid particles in the slurry’s movement forward is more decentralized as the hydraulic inlet velocity increases. Based on these simulation data, a correlation was developed to predict the resistance loss of the CGFB slurry as a function of the hydraulic inlet velocity, pipe diameter and CGFB slurry rheological characteristics.

Creep properties of geopolymer cemented coal gangue-fly ash backfill under dynamic disturbance

A testing machine was developed to test the creep disturbance effect and explore the creep disturbance law of geopolymer cemented coal gangue-fly ash backfill (GCGFB). A creep disturbance constitutive model of GCGFB was established by introducing a disturbance element that connects an improved fractional Burger’s model and nonlinear viscous dashpot with a strain trigger. Experimental results indicated that dynamic disturbances significantly influence the creep process of GCGFB. Increasing the disturbance impact energy increases the deformation and shortens the time to reach the failure strain. The disturbance deformation generated by GCGFB is negatively correlated with the static load stress at a low disturbance energy and positively correlated at a high disturbance energy. The calculated results of the disturbance constitutive model showed good agreement with the experimental data.

A coupled THMC modeling application of cemented coal gangue-fly ash backfill

Cemented coal gangue-fly ash backfill (CGFB) is prepared by mixing coal gangue, fly ash, cement and water. The CGFB mixtures are filled into mined-out areas of coal mines for ground control and waste disposal. Once placed underground, the performance of CGFB is subjected to the thermal (T), hydraulic (H), mechanical (M) and chemical (C) processes and THMC coupling effects. Understanding the coupled THMC influence on CGFB is crucial for predicting the behavior of CGFB. This paper develops a THMC coupling model to describe and analyze the coupled THMC processes that occur in CGFB. The developed model considers the courses of thermal conduction and convection, fluid flow, suction development, chemical shrinkage, thermal expansion, and binder hydration. The simulation and prediction results of the developed model are compared with the experimentally tested data from 3 case studies. Good agreement is found between the modeling results and experimental data, validating the capability of the developed model to well characterize the properties of CGFB under coupled THMC loadings. The modeling results can also contribute to a better design of stable, durable and environment-friendly CGFB structures.