Mechanical performance and ultrasonic properties of cemented gangue backfill with admixture of fly ash

Rheological and mechanical performance analysis and proportion optimization of cemented gangue backfill materials based on response surface methodology

Cemented backfill mining is a green mining method that enhances the coal mining rate and the safety of mined-out regions. To transport the cemented gangue backfill material (CGBM) into the mined-out regions, it is essential to ensure high flowability and adequate compressive strength after hardening. Based on the response surface methodology (RSM), 29 experiments were conducted in this paper to test the yield stress and plastic viscosity of CGBM slurry. Cubic specimens with dimensions of 100 mm were prepared and underwent uniaxial compression tests to obtain the compressive strength at a curing age of 28 days. Quadratic polynomial regression models were established for yield stress, plastic viscosity, and compressive strength to explore the effects of fly ash content, water-cement ratio, mass concentration, and superplasticizer dosage on the properties of CGBM. Multi-objective optimization was conducted to determine the optimal material proportion of CGBM. The research results indicate that (1) the mass concentration most profoundly affected the yield stress and plastic viscosity of CGBM, and it increased with an increase in mass concentration. Fly ash content had an inverse relationship with compressive strength. Superplasticizer was found to improve the flowability and strength of CGBM. (2) The established response surface model could reflect the relationship between CGBM's material proportion and rheological and mechanical properties, and predict relevant parameters. (3) Multi-objective optimization determined the optimal proportion of CGBM to be 80% fly ash content, 54% water-cement ratio, 79% mass concentration, and 3% superplasticizer dosage. The research findings offer valuable guidance to mining backfill engineering.

Coupled effects of fly ash and calcium formate on strength development of cemented tailings backfill

Cemented tailings backfill (CTB) is widely adopted to ensure the safety of underground goafs and mitigate environmental risks. Fly ash (FA) and calcium formate (CF) are common industrial by-products that improve the mechanical performance of CTB. How the coupling of the two components affects the strength development is not yet well-understood. Neural network modelling was conducted to predict the strength development, including the static indicator of uniaxial compressive strength (UCS) and the dynamic indicator of ultrasonic pulse velocity (UPV). Sobol' sensitivity analysis was carried out to reveal the contributions of FA, CF and curing time to CTB strength. SEM microstructure investigation on CTB samples was implemented to reveal the mechanism of strength development and justify the predictions by neural network modelling and sensitivity analysis. Results show that the combination of FA content, CF content and curing time can be used to predict both UCS and UPV while providing adequate accuracy. The maximum of UCS of 6.1215 MPa is achieved at (FA content, CF content, curing time) = (13.78 w%, 3.76 w%, 28 days), and the maximum of UPV of 2.9887 km/s is arrived at (FA content, CF content, curing time) = (11.67 w%, 3.08 w%, 10 days). It is also implicated that prediction of UCS using UPV alone, although common in field application is not recommended. However, UPV measurement, in combination with the information of FA dosage, CF dosage and curing time, could be used to improve UCS prediction. The rank of variable significance for UCS is curing time > FA content > CF content, and for UPV is FA content > curing time > CF content; variable interaction is strongest for FA with CF for UCS development, and for FA with curing time for UPV evolution. Influence of FA on CTB strength development is due to improved polymerisation and consumption of Ca(OH)₂. Influence of CF on strength development is a result of accelerated hydration and increased combined-water content in calcium silicate hydrate (CSH). Effect of curing time is attributed to the evolution of CSH product and pore-water content during cement hydration.

Expansion Properties of Cemented Foam Backfill Utilizing Coal Gangue and Fly Ash

The cemented backfill (CB) utilizing coal gangue (CG) and fly ash (FA) is widely applied in coal mines. However, the bleeding and shrinkage of CB leads to insufficient contact with surrounding rock, which is not beneficial for controlling roof subsidence and even stope stability. Herein, a cemented foam backfill (CFB) formulation is demonstrated, employing hydrogen dioxide (H2O2) as a chemical foaming agent. The cement and FA show noticeable inhibiting effects on volume expansion due to the network formed by their hydrates. Moderately lower cement, FA, and solid concentration are beneficial to improve volume increment and prolong expanding duration. A foaming coefficient (k) is proposed in theory to evaluate the foaming efficiency. The kem values, determined by volume evolution experiments of CFB slurries, provide a calculation basis for the needed dosage of H2O2 solution targeting specific volume increment. CFB specimens with expanding ratios of 21%~103% and densities of 994~592 kg/cm3 were prepared, with an actual foaming coefficient of 52.40 cm3/g and uniaxial compressive strength (UCS) of 0.32~0.55 MPa. The mass of H2O2 solution was 1.9%~11.3% of cement and 0.29%~1.67% of total solid materials by weight. The UCS decline compared to CB was attributed to rich pores observed by CT and carbonation indicated by X-ray diffraction (XRD).

A Joint Experiment and Discussion for Strength Characteristics of Cemented Paste Backfill Considering Curing Conditions

As lots of underground mines have been exploited in the past decades, many stope instability and surface subsidence problems are appeared in the underground mines, while the cemented paste backfill (CPB) technology has been applied for more than 40 years, and it can solve these problems. As it is shown that the effect of backfilling is mainly affected by the mechanical properties of the CPB, and there are lots of factors which can influence the strength of the CPB, but the coupled effects of curing conditions has not been reported. In this research, the coupled effects of curing conditions are importantly considered, and the uniaxial compressive strength (UCS) is adopted as the important evaluation index of CPB, then the evolution law of the UCS for CPB are analyzed, also the mathematical strength model of CPB is established. The findings suggest that the relationship between the UCS of CPB and curing stress develops the function of quadratic polynomial with one variable, while the UCS of the CPB shows the power function as the curing temperature increases. Moreover, the established mathematical strength model is verified on the basis of laboratory experiments, the error between the measured UCS and the prediction UCS is less than 15%. It shows that the established strength model of the CPB by considering the curing conditions can predict the UCS very well, it has great significance for the safety design of CPB.

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

The most economical, environmental, and friendly method for recycling gangue is filling mining with cemented waste rock backfill (CWRB), which solves the environmental problems caused by gangue discharge and reduces the mining damages. Evaluating the mesoscopic structure of CWRB is of great significance for maximizing the utilization of gangue recycling and improving the economic benefits of filling mining. This paper constructed the particle flow model of cemented waste rock backfill (CWRB) considering particle size distribution (PSD) of aggregates and hydration of cementing material to investigate the effect of the PSD of aggregates on its mesoscopic structural evolution. The strain energy, crack, force chain, and particle fragment of CWRB during the whole loading were discussed. The binary processing and calculation on the crack image were performed to analyze the fractal dimension of crack distribution by compiling program. The influencing mechanism of the PSD of aggregates on the strength of CWRB is revealed from the mesoscopic levels of crack evolution, force chain structure, and particle fragment. The results show that the strain energy increases firstly and then decreases with the PSD fractal dimension, while the crack number decreases firstly and then increases with that. The cracks with less number and more uniform distribution present the smaller fractal dimension, CWRB with a low fractal dimension of crack distribution has higher strength, the fractal dimension of crack distribution exhibits a correlation with the PSD of aggregates. CWRBs with the PSD fractal dimensions of 2.4-2.6 have the largest strain energy and the smallest crack number, performing the superior structural evolution during loading. This study presents the huge potential of optimizing PSD in CWRB application from a new perspective, it is of great significance for strengthening the internal structure of CWRB and reducing engineering cost.

Ultrasonic evaluation of strength properties of cemented paste backfill: Effects of mineral admixture and curing temperature

This paper presents the findings of a research study designed and conducted to investigate the effects of mineral admixture and curing temperature on uniaxial compressive strength (UCS) and ultrasonic pulse velocity (UPV) behavior of laboratory-prepared cemented paste backfill (CPB) samples. A total of 290 CPB samples were prepared at different replacement rates (10-80%), cured at various temperatures (10-50 °C), and respectively subjected to both UPV and UCS testing after curing times of 3, 7, 14, 28, 56 and 90 days. The obtained experimental results show that the addition of fly ash (FA) can lead to an increase or decrease trend in UCS and UPV behavior of CPB samples, depending on the replacement level of admixtures. There is a competition between the strength-increasing factor (micro-filler effect of FA) and strength-decreasing factor (lower amount of cement hydration products induced by replacement ratio). Both UPV and UCS are found to decrease with increasing blast furnace slag (Slag) replacement level mainly attributable to its low pozzolanic reactivity. Besides, the curing temperature has a significant influence on UCS and UPV behavior, depending on the curing time. Results also suggest that UPV is less sensitive to the variation in the admixture dosage and curing temperature than UCS. As a result, there exists a clear linear relationship between UPV and UCS behavior of both CPB samples prepared with FA and/or Slag admixtures, and CPB samples tested at each curing temperature. The main findings of this research study suggest that the UPV test can be reliably used for predicting CPB's strength properties, saving money and time to mine operators.

Recycling Lead–Zinc Tailings for Cemented Paste Backfill and Stabilisation of Excessive Metal

This study demonstrates the feasibility of recycling lead–zinc tailing (LZT) as a cemented paste backfill (CPB) by considering the mechanical properties and environmental effects, thus providing an approach for safe and environmentally friendly treatment of LZT. First, the mechanical properties of CPB samples were tested. When the cement/tailing ratio was 1:6 and the slurry concentration was 70%, the maximum unconfined compressive strength (UCS) of the CPB cured for 28 days reaching 2.05 MPa, which could ensure safe mining. Then, the metals with pollution potential in the backfill slurry were investigated through static leaching. Finally, after adding immobilisation materials to stabilise excessive metals, the environmental stability of the CPB was demonstrated through dynamic leaching and a toxicity characteristic leaching procedure. The results show that the lead leached from the backfill slurry still exceeds the Chinese standard for groundwater quality (GB/T14848-2017 Class III). The addition of 2 mg/L polyaluminium sulfate (PAS) can further improve the strength of the CPB and maintain the environmental friendliness of the CPB. Therefore, the technology of recovering LZT as a CPB proposed in this study is an effective alternative to deal with LZT, which can help lead–zinc mines meet the requirements of cleaner production.

Evaluation for the Leaching of Cr from Coal Gangue Using Expansive Soils

Coal gangue can cause significant heavy metal pollution in mining areas, which would have a negative impact on the environment and human health. The objective of this research is to investigate the relationship between expansive soil amount and the leaching behavior of Chromium from coal gangue and the engineering properties of coal gangue used as building materials. The leaching behavior of Chromium from coal gangue was observed using atomic absorption spectrometry. A column leaching experiment was conducted to examine the impact of leaching time and heavy metal concentration. Furthermore, the unconfined compressive strength test was employed to evaluate the engineering properties of coal gangue with expansive soil. The results of the study demonstrate that pH of leachate solutions, leaching time, and expansive soil amounts in mixtures have important influence on Chromium concentration. The leachate solutions, which behave like alkaline, provide a positive environment for adsorbing Cr. Adding expansive soil can reduce leached concentrations of Chromium from coal gangue when compared to leachate of original coal gangue. It was found that 30% expansive soil was an improved solution because it delayed the cumulative concentration to reach the limitation line. Moreover, the unconfined compressive strength of coal gangue was boosted through adding expansive soil.

The role of gangue on the mitigation of mining-induced hazards and environmental pollution: An experimental investigation

Gangue, produced from coal mining and washing process, is a serious threat to the ground environment. Gangue backfilling mining method can solve this problem and reduce mining-induced hazards, e.g., controlling surface subsidence and preventing water inrush from seeping into goaf by cracks in overlying strata. In this paper, effects of the original particle size distribution (PSD) and water content on the particle crushing behavior and seepage properties of granular gangues were investigated. Experimental results show that the crushing behavior can promote the compaction of gangue particles; the variation of PSD after crushing reveals distinct fractal characteristics. With the increasing compression stress, the particle crushing ratio and fractal dimension increase, while the permeability decreases. Due to the rearrangement of particles and newly generated fine particles filled the gap among larger particles, it is difficult to reduce the permeability by increasing the compressive stress. In addition, the variation of fractal dimensions is similar to the crushing ratio, so the particle crushing can be illustrated by fractal dimensions. The relationship between porosity and permeability established by the Kozeny-Carman equation can model the effect of particle crushing in this research. The reliability of the equation is verified by the comparison of model result and experimental data. To increase the mitigation rate of mining-induced hazards and environmental pollution by GBM method, granular gangues can be crushed into smaller particles and dehydrated before backfilling.

An experimental study of the influence of lithology on compaction behaviour of broken waste rock in coal mine backfill

The research aims to explore the influences of lithology on the compaction behaviours of broken waste rocks. For this purpose, a WAW-1000D servo test machine and a self-made bidirectional loading test system for granular materials were used to conduct axial and lateral compaction tests on four typical types of broken waste rocks: sandstone, mudstone, limestone and shale. On this basis, we analysed the relationships between lateral and axial stress with the strain in, and porosity of, the four types of broken waste rocks. In addition, the relationship of axial stress with lateral stress and lateral pressure coefficient, and the changes in the particle size distribution of broken waste rocks before and after compaction were discussed. The test results demonstrated that the samples of higher strength were found to have low lateral and axial strains as well as a lower porosity in axial and lateral loading tests, while samples of lower strength showed low lateral stress and lateral pressure coefficient under axial load. After being compacted, the samples of the four types of broken waste rocks were found to have a higher proportion of small particles, indicating some particle crushing. Moreover, the samples of lower strength were broken to a greater extent.

Aggregate gradation effects on dilatancy behavior and acoustic characteristic of cemented rockfill

Investigating the effect of the aggregate gradation on the material properties of cemented rockfill is significant for the green mining, economic benefit and engineering safety. Consequently, the ultrasonic test, uniaxial compression experiment and acoustic emission (AE) monitor on cemented rockfill were carried out, for which the aggregate satisfied Talbot gradation. The dilatancy behavior and AE characteristic of cemented rockfill under load were investigated. The damage in the internal structure under compression was revealed by the deformation and AE signals of cemented rockfill. The effect of the Talbot index on the ultrasonic pulse velocity (UPV) and the strength parameters such as stress of dilatancy onset and uniaxial compressive strength (UCS) of cemented rockfill was analyzed. The mechanical properties of cemented rockfill materials were evaluated by the establishment of the relation between the UPV and the strength parameter. The results show that The difference between the stress of dilatancy onset and the UCS, the deformation performance and the activity of AE signals during dilatancy are positive correlated with the Talbot index of aggregate in cemented rockfill. The relation between the UPV and the strength parameters (stress of dilatancy onset and UCS) of cemented rockfill can be characterized by the positive linearity, and the UPV is also suitable for characterizing the stress of dilatancy onset of cemented rockfill material. The cubic polynomial is more suitable for describing the relations between the parameters of strength and UPV and the Talbot index of aggregate than the quadratic polynomial, and the Talbot index with optimal aggregate gradation reflected the maximum strength of cemented rockfill material should be around 0.45-0.47.

Properties and Application of Backfill Materials in Coal Mines in China

Coal is the basic resource underpinning energy generation in China, however, constant, large-scale mining of coal results in many problems such as ecological destruction of mining areas. As a result, backfilling of solid waste underground is proposed to control strata and surface subsidence and to protect the environment. At present, these materials, such as granular material, cemented material and high-water-content materials are mainly used for backfilling. This study summarised the types of backfill materials that are used in coal mines in China along with the backfilling process. Moreover, distribution and characteristics of mines backfilled with these backfill materials were obtained and analysed. Considering the socio-environmental aspects that affect backfilling, this research proposed a guideline for the selection of backfill materials and then analysed specific engineering cases of three backfill materials. In addition, the future development of backfill materials was discussed. With extensive extraction of shallow coal resources in China and, therefore, rapid depletion of coal resources in eastern regions of China, coal mining depth is increasing significantly. As a result, it is required to investigate new backfill materials suited for the deep high-stress environment.

Slurry Preparation Effects on the Cemented Phosphogypsum Backfill through an Orthogonal Experiment

The cemented phosphogypsum (PG) backfill technique provides a new method for massive consumption of PG, and therefore alleviating the environmental pollution of PG. This study considered the effects of slurry preparation on the performance of cemented PG backfill. A L16(44) orthogonal experiment was designed to analyze four factors, namely the solid content, phosphogypsum-to-binder ratio (PG/B ratio), stirring time and stirring speed, with each factor having four levels. According to the range analysis, the solid content played the dominant role in controlling the bleeding rate, while the setting times strongly depended on the PG/B ratio. In terms of strength development of the backfill, the PG/B ratio was shown to be the most significant factor determining the unconfined compressive strength (UCS), followed by the solid content, stirring time and stirring speed. Furthermore, the results showed that the slurry preparation affected the environmental behavior of impurities that originated in PG. By analyzing the concentrations of impurities in the bleeding water of the slurry as well as the leachates of the tank leaching test, the results showed that the release of F− and SO42− was aggravated clearly with the increase in the PG/B ratio, while the release of PO43− always remained at relatively low levels.

Failure analyses of unconfined CCWBM body in uniaxial compression based on central pressure variation

Cemented coal waste backfill material (CCWBM) is made of coal gangue, fly ash and cementitious materials. It has been widely used in the field of backfill mining to control surface subsidence and protect the environment. A large number of unconfined backfill bodies without lateral support are formed in partial backfill mining. To study the failure characteristics of unconfined CCWBM body in partial backfill, the stress-strain curves of the CCWBM were obtained by uniaxial compression tests at different ages (1-28 d). The central pressure was measured by the embedded pressure sensors. The failure characteristics of the specimen were monitored by acoustic emission (AE) positioning technique. Three observations can be made. 1. The central pressure variation curves lag behind the mean stress change curves. The central pressure curve can be divided into three stages: slow increase stage, rapid growth stage and decline stage. It has two pressure manifestations: early appearance and peak appearance. They can be as the failure precursor and instability critical, respectively. 2. The specimen forms a central elastic bearing area in the process of compression. The plastic area develops to the inner side with the increase of pressure, and an upper and lower compound cone-shaped residual area is finally formed. 3. The embedded pressure sensor can be used to monitor the instability of the unconfined backfill body. The research results can provide guidance for the in-situ stability monitoring and reinforcement of unconfined CCWBM body in partial backfill.

Core size effect on the dry and saturated ultrasonic pulse velocity of limestone samples

This study presents the effect of core length on the saturated (UPVsat) and dry (UPVdry) P-wave velocities of four different biomicritic limestone samples, namely light grey (BL-LG), dark grey (BL-DG), reddish (BL-R) and yellow (BL-Y), using core samples having different lengths (25-125mm) at a constant diameter (54.7mm). The saturated P-wave velocity (UPVsat) of all core samples generally decreased with increasing the sample length. However, the dry P-wave velocity (UPVdry) of samples obtained from BL-LG and BL-Y limestones increased with increasing the sample length. In contrast to the literature, the dry P-wave velocity (UPVdry) values of core samples having a length of 75, 100 and 125mm were consistently higher (2.8-46.2%) than those of saturated (UPVsat). Chemical and mineralogical analyses have shown that the P wave velocity is very sensitive to the calcite and clay minerals potentially leading to the weakening/disintegration of rock samples in the presence of water. Severe fluctuations in UPV values were observed to occur between 25 and 75mm sample lengths, thereafter, a trend of stabilization was observed. The maximum variation of UPV values between the sample length of 75mm and 125mm was only 7.3%. Therefore, the threshold core sample length was interpreted as 75mm for UPV measurement in biomicritic limestone samples used in this study.