Paste Backfill Research Articles

Development of Cemented Paste Backfill with Superfine Tailings: Fluidity, Mechanical Properties, and Microstructure Characteristics.

Previous studies have shown that the effectiveness of superfine tailings cemented paste backfill (SCPB) is influenced by multiple factors. To optimize the filling effect of superfine tailings, the effects of different factors on the fluidity, mechanical properties, and microstructure of SCPB were investigated. Before configuring the SCPB, the effect of cyclone operating parameters on the concentration and yield of superfine tailings was first investigated and the optimal cyclone operating parameters were obtained. The settling characteristics of superfine tailings under the optimum cyclone parameters were further analyzed, and the effect of the flocculant on its settling characteristics was shown in the block selection. Then the SCPB was prepared using cement and superfine tailings, and a series of experiments were carried out to investigate its working characteristics. The flow test results showed that the slump and slump flow of SCPB slurry decreased with increasing mass concentration, which was mainly because the higher the mass concentration, the higher the viscosity and yield stress of the slurry, and thus the worse its fluidity. The strength test results showed that the strength of SCPB was mainly affected by the curing temperature, curing time, mass concentration, and cement-sand ratio, among which the curing temperature had the most significant effect on the strength. The microscopic analysis of the block selection showed the mechanism of the effect of the curing temperature on the strength of SCPB, i.e., the curing temperature mainly affected the strength of SCPB by affecting the hydration reaction rate of SCPB. The slow hydration process of SCPB in a low temperature environment leads to fewer hydration products and a loose structure, which is the fundamental reason for the strength reduction of SCPB. The results of the study have some guiding significance for the efficient application of SCPB in alpine mines.

Mechanical Characterization of Cemented Paste Backfill

Mechanical characterization is important to the design and analysis of cemented paste backfill (CPB) structures. Unconfined compressive strength (UCS) tests have been widely used owing to their relative simplicity to characterize a material’s response to unconfined compressive loading. However, the UCS represents a single strength parameter and does not fully describe the material’s strength (or failure) envelope. In this study, we analyzed UCS tests with direct shear and uniaxial tensile strength tests conducted on the same CPB materials to provide mechanical characterization of CPB under a more complete range of loading conditions. The results demonstrate the Mohr–Coulomb failure envelope provides a consistent description of strengths arising from the three different test methods. Furthermore, a better estimate of the tensile strength is UCS/4, which is considerably higher than the conventional assumption that the tensile strength is equal to USC/10 or UCS/12. This has a significant impact on the assessed required strengths particularly for undercut designs using Mitchell’s sill mat analysis method and suggests that in future the conventional UCS tests should be complemented with direct tension and direct shear tests to improve underground designs using CPB.

Study of early-age performance of cementitious backfills with alkali activated slag under internal sulfate attack

The vulnerability to internal sulfate attack of calcium-rich ordinary Portland cement (OPC) and its relative high cost force mines to explore technically effective and low cost alternative binders to solidify sulfate-containing tailings. Alkali activated slag (AAS) has been established to be a promising substitute for OPC due to its superior strength, good resistance to chemical attack and low cost. This study shows results of a laboratory research dealing the characteristics of cemented paste backfill (CPB) with AAS under internal sulfate attack. A blend of two alkali activators (SS: sodium silicate, SH: sodium hydroxide) was considered. To assess the behavior of CPB materials subjected to diverse conditions, a total of 4 different SS/SH ratio (e.g., 0.5, 0.75, 1, and 1.25) were used within CPB. Results shows that initial/final setting times tend to drop with growing sulfate content up to 5000 ppm, followed by gradual increases. Increasing sulfate content leads to higher yield stresses and viscosities and the degree of this negative impact is much severer in samples having the high SS/SH ratio. The presence of sulfate can enhance or deteriorate the strength of AAS-CPB depending on activator type, cure age and initial sulfate content. CPB with high SS/SH ratio reveals weaker resistance to internal sulfate attack due to their lower pH values. The key outcome of this work will be beneficial to optimally design of sulfate bearing CPB-AAS structures.

Strata pressure behavior and overlying strata movement characteristics of slicing backfill mining in thick coal seams

To improve the coal recovery rate, paste backfill mining was used for 3# coal seam under the industrial square in Panji No.4 Coal Mine. Theoretical analysis, numerical simulation, and similar simulation were applied to study the underground pressure behavior and the overlying strata movement. Results demonstrate the following. (a) Caving mining exerts IV-level mining-induced damages to ground buildings, while slicing backfill mining only causes I-level mining-induced damages to ground buildings. (b) The maximum surface subsidence coefficient of backfill mining is 0.02, and the maximum surface subsidence value is only 4.12% of that under caving mining. (c) The front abutment pressure peak of the working face in caving mining is 28.31 MPa and only 17.82 MPa in backfill mining, which is only 62.95% of caving mining. The distance from the stress peak to the coal wall decreases from 11.14 to 3.04 m. Therefore, the paste backfilling mining of 3# coal seam below the industrial square of Panji No. 4 Coal Mine can effectively control the overlying strata movement and decrease the surface subsidence significantly. Compared with caving mining, backfill mining significantly relieves the strata pressure behaviors and narrows the distance of mining influence.

Eco-Sustainable Recycling of Cement Kiln Dust (CKD) and Copper Tailings (CT) in the Cemented Paste Backfill

Cement global demand shows continued growth and a significant increase in the production volume, which may negatively impact the non-renewable natural resources and the environment, which is incompatible with sustainability goals. Cement kiln dust (CKD) is a primary concern associated with clinker manufacturing as a waste byproduct. Similarly, the mining industry produces copper tailing as unwanted material while beneficiating the ore, creating environmental problems due to difficulty in managing worldwide generated quantities that reach billions of metric tons. This study investigated the beneficial utilization of cement kiln dust and copper tailing as undesirable wastes in industrial applications through underground mines’ cemented paste backfill (CPB). Sixty different mixtures were prepared with three types of CKD collected from various cement manufacturers and were accordingly used with a proportion of 5, 10, and 15% to partially replace ordinary Portland cement (OPC) and pozzolan Portland cement (PPC) binders, represented in hundreds of CPB samples. The hardened specimens were subjected to density, uniaxial compressive strength (UCS), and axial deformation measurements to evaluate the physical and mechanical properties at curing up to 90 days. Meanwhile, X-ray powder diffraction (XRD) was extensively applied to chemically investigate the hydration products of CPB-hardened mixtures. Moreover, we developed a UCS predictive model applying two techniques: multiple variables regression analysis and artificial neural network (ANN). The results showed that the tricalcium silicate (Alite) and dicalcium silicate (Belite) phases form C-S-H upon hydrations and provide high strength in the binary mixtures. Meanwhile, the CKD’s lime saturation factor (LSF) governed the strength value in the ternary mixtures that utilized copper tailings. That makes CKD practical in the CPB mixture when partially replacing the OPC and PPC binders, with a proportion of up to 15%. In addition, the ANN technique’s predictive model exhibited a significant positive correlation with excellent statistical parameters that achieved 0.995, 0.065, and 0.911 for R2, RMSE, and MAE, respectively.

Strength and microstructure of cemented paste backfill modified with nano-silica particles and cured under non-isothermal conditions

A series of mechanical and microstructural tests and monitoring experiments are performed on different cemented paste backfill (CPB) specimens made with different types of tailings (silica tailings, gold tailings) to investigate the effects of adding nano-silica (SiO2) particles on the strength and microstructure (pore structure, mineralogical composition) of CPB cured under isothermal and non-isothermal conditions that simulate the field thermal curing conditions of CPB structures. The results show that the CPB with nano-SiO2 (NS-CPB) has a higher strength and finer pore structure than the control samples for all curing times and in all test conditions. Nano-SiO2 particles are effective in increasing the UCS of CPB in isothermal room conditions and non-isothermal field curing conditions. The increase in mechanical performance is higher when CPB is cured under non-isothermal conditions. Most of the increase in strength of the NS-CPB occurs at the early ages (3 days) of curing. The reason for the improvement in the mechanical strength is due to increase in binder hydration and the filler effect of the nanomaterial, which are in agreement with the results from microstructural analyses and electrical conductivity (EC) measurements. It is also found that the increase in temperature due to field thermal curing conditions greatly affect the strengthening properties of nano-SiO2. The NS-induced pozzolanic reactions are much more sensitive to higher temperatures compared to Portland cement hydration. The strengthening properties of the nano SiO2 are affected by the sulphate content in the tailings due to sulphate attacks (inhibition of cement hydration by sulphate ions), as less CH is available for pozzolanic reactions and C-S-H production. The findings presented in this manuscript will help to provide a better understanding of the behaviour of CPB and design a more cost-effective CPB modified with nano SiO2 particles.

Using Artificial Intelligence Approach for Investigating and Predicting Yield Stress of Cemented Paste Backfill

The technology known as cemented paste backfill (CPB) has gained considerable popularity worldwide. Yield stress (YS) is a significant factor considered in the assessment of CPB’s flowability or transportability. The minimal shear stress necessary to start the flow is known as Yield stress (YS), and it serves as an excellent measure of the strength of the particle-particle interaction. The traditional evaluation and measurement of YS performed by experimental tests are time-consuming and costly, which induces delays in construction projects. Moreover, the YS of CPB depends on numerous factors such as cement/tailing ratio, solid content and oxide content of tailing. Therefore, in order to simplify YS estimation and evaluation, the Artificial Intelligence (AI) approaches including eight Machine Learning techniques such as the Extreme Gradient Boosting algorithm, Gradient Boosting algorithm, Random Forest algorithm, Decision Trees, K-Nearest Neighbor, Support Vector Machine, Multivariate Adaptive Regression Splines and Gaussian Process are used to build the soft-computing model in predicting the YS of CPB. The performance of these models is evaluated by three metrics coefficient of determination (R2), Root Mean Square Error (RMSE) and Mean Absolute Error (MAE). The 3 best models were found to predict the Yield Stress of CPB (Gradient Boosting (GB), Extreme Gradient Boosting (XGB) and Random Forest (RF), respectively) with the 3 metrics of the three models, respectively, GB {R2 = 0.9811, RMSE = 0.1327 MPa, MAE = 0.0896 MPa}, XGB {R2 = 0.9034, RMSE = 0.3004 MPa, MAE = 0.1696 MPa} and RF {R2 = 0.8534, RMSE = 0.3700 MPa, MAE = 0.1786 MPa}, for the testing dataset. Based on the best performance model including GB, XG and RF, the other AI techniques such as SHapley Additive exPlanations (SHAP), Permutation Importance, and Individual Conditional Expectation (ICE) are also used for evaluating the factor effect on the YS of CPB. The results of this investigation can help the engineers to accelerate the mixed design of CPB.

Mechanical and microstructural characteristics of cemented paste tailings modified with nano-calcium carbonate and cured under various thermal conditions

ABSTRACT This paper focuses on the evaluation of the strength development and microstructure of nano-calcium carbonate (CaCO3) cemented paste backfill experimentally cured under isothermal conditions at room temperature and non-isothermal conditions in the field. A series of mechanical (uniaxial compressive strength, UCS) and microstructural (thermogravimetric, mercury intrusion porosimetry, scanning electron microscope) tests as well as monitoring experiments are experimentally conducted on cemented paste backfill (CPB) specimens with and without nano-calcium carbonate and cured at different times and under isothermal or non-isothermal conditions. The results show that the addition of nano-CaCO3 particles to CPB significantly improves its mechanical strength, irrespective of the thermal curing conditions (isothermal, field non-isothermal conditions). However, the impact of nano-CaCO3 particles on the increase in strength of CPB is only effective or significant at the early ages (curing time≤7 days). It is also found that the higher temperatures improve the accelerating effect of nano-CaCO3 much more than they accelerate the PCI hydration reactions in the first 3 days. Moreover, it is also found the sulphate ions present in the natural gold tailings negatively affect the mechanical performance of nano-CPB and reduced the accelerating effect of nano-CaCO3 due to sulphate attacks.

Experimental study on the effect of fly ash with ammonium salt content on the properties of cemented paste backfill

The wide disposal of fly ash (FA) that contains ammonium is an essential method to realize green backfill mining and resource utilization of solid waste. In this paper, FA containing ammonium with different ammonia contents was simulated by adding NH4HSO4 to the FA. The law and mechanism of influence of ammonia content on the performance of FA-based cemented paste backfill (FA-based CPB) were explored using thorough experimentation, and a maximum ammonia content of FA containing ammonium used in backfill mining was proposed. The results showed that the fresh FA-based CPB slurry could be described by the Herschel–Bulkley model (R2 ≥ 0.991). With the increase in the ammonia content of FA, the yield stress and viscosity of the FA-based CPB slurry increased and the slump value decreased, due to the formation of specific firm bubble bridges between the cement particles, FA, and tiny bubbles and due to the fact that tiny bubbles need more water due to their large specific surface area. In addition, the uniaxial compressive strength (UCS) and elastic modulus of FA-based CPB decreased and the peak strain increased as the ammonia content of FA increased. Specifically, the 28-d UCS of FA-based CPB with ammonia contents of 239, 639, 1139, and 2139 ppm decreased by 12.4 %, 39.5 %, 42.4 %, and 47.0 %, respectively, compared to that of FA-based CPB with ammonia content of 139 ppm. Finally, MIP, SEM, and XRD were utilized to investigate the microstructure of FA-based CPB. Overall, the ammonium-containing FA adversely affected both the flowability and mechanical properties of FA-based CPB. The results provide a reference for the green and efficient use of FA containing ammonium salts.

Optimization of cemented paste backfill with carbon nanotubes as a sustainable treatment for lead-containing tailings

Optimization of cemented paste backfill with carbon nanotubes as a sustainable treatment for lead-containing tailings

The effect of coarse aggregate on the bleeding and mechanical properties of cemented paste backfill

The effect of coarse aggregate on the bleeding and mechanical properties of cemented paste backfill

Experimental investigation of evolutive mode-I and mode-II fracture behavior of fiber-reinforced cemented paste backfill: Effect of curing temperature and curing time

Experimental investigation of evolutive mode-I and mode-II fracture behavior of fiber-reinforced cemented paste backfill: Effect of curing temperature and curing time

Study on Paste Transformation and Parameter Optimization of Cemented Backfilling with Fine Tailings in Deep Gold Deposits

The key to cementation backfilling in underground stopes of metal mines is quality and efficiency of backfilling. Backfilling quality is inseparable from the cementitious material as well as the tailings properties. To explore the influence of different factors on the strength of the backfilling body, the ratio of backfilling cementitious materials in the preparation process of backfilling slurry was experimentally studied to determine the economical and reasonable proportion of backfilling cementitious materials. Under the multi-factor test, it is concluded that the proportion concentration of 1:6 and 66% in the cemented specimen of medium and fine tailings meets the strength requirements of the surface layer of the backfilling body. Using the numerical simulation software FLAC3D, the movement of rock mass under different backfillings is simulated, and the subsidence of overburden, the stress of the ore body, and the damage range of the plastic zone are analyzed. The results showed that, during the transition from full tailings cemented backfilling to medium-fine tailings paste backfilling, the vertical stress concentration area of the overlying strata shifts from the surrounding ore body to the backfilling body, the plastic zone decreases, and the complexity of failure forms gradually decreases. Finally, the transformation method from full tailings cementation backfilling to medium fine tailings paste backfilling is determined, and the medium fine tailings paste backfilling in a deep gold mine is realized.

Properties and application of thixotropic cement paste backfill with molybdenum tailings

Backfilling is considered worldwide as an effective treatment method for coal mining subsidence and mineral tailing disposal. A green thixotropic cement paste backfill with molybdenum tailing (MoT) sand was prepared which can rapidly accumulate in the coal mining cavity. A thixotropic coefficient method was also proposed to evaluate the thixotropy of the paste based on the correlation between shear stress and time. Through an orthogonal test regarding the optimal proportion design, the thixotropy, water separation rate and stone compressive strength of the thixotropic MoT-cement paste were tested. The range analysis showed that the optimal proportion of the paste was 6% mixed metal hydroxide (MMH), 0.8 water-cement ratio, and 6:4 MoT-cement ratio. The analysis in the rheological properties of the proposed paste indicated that the relationships of shear rate-shear stress of the optimal proportion conformed to the power rate rheological model. The influences of the addition amount of MMH, water-cement ratio and MoT-cement ratio on the mechanical property of the stone body of the proposed paste were tested and predicted theoretically. Through a simulated accumulation test, the relationships among the flowing distance, accumulation height and time of the thixotropic MoT-cement paste in the process of filling coal mining cavity were obtained. The cost of the thixotropic cement paste with MoT is 30–50% better economy than that with natural river sand and recycled fine aggregate whilst the thixotropic cement paste with MoT also shows good environmental performance by considering the depletion of stocked mineral tailing in coal mining cavity treatment.

Mechanical properties and microscopic characterization of cemented paste backfill with electrolytic manganese residue matrix binder

With the exploitation of mineral resources, more and more mined-out areas need to be backfilled. In order to meet the needs of backfill, a new cemented paste backfill (CPB) is developed. The new material uses electrolytic manganese residue (EMR), slag and red mud as binder, tailings as aggregates, and sodium hydroxide as alkali activator. Taking the EMR content, the mass concentration and bone glue ratio as the influencing factors, the unconfined compressive strength (UCS) of 3 d, 7 d and 28 d as the target, the single factor design was carried out, and the mixture proportion recommended was obtained. According to the 28 d UCS of CPB under mixture proportion recommended, a piecewise damage constitutive model with the end of initial compaction stage as the critical point was established. The three microscopic test methods of scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the CPB at different ages under the mixture proportion recommended. The internal morphology, composition, phase composition and chemical bond combination of the CPB were analyzed. The results show: when the EMR content is 20%, the mass concentration is 81%, and the bone glue ratio is 6, the mixture proportion recommended for the developed CPB was recommended; The established constitutive model can fit the UCS curve well; At the initial stage of reaction, C–S–H and C-A-S-H gels were formed inside the CPB; With the increase of age, the amount of gel formation increased, the diffraction peak of the reactants weakened, the diffraction peak of the product enhanced, the pore played a filling role, so that the internal structure was dense, porosity decreased; The corresponding characteristic bands originate from the asymmetric stretching and bending vibrations of H–O–H, Si–O, Si–O–Si, C–O and O–H chemical bonds. This new type of CPB can well meet the requirements of backfill, and can reasonably use solid waste.

Failure characteristics and the damage evolution of a composite bearing structure in pillar-side cemented paste backfilling

Failure characteristics and the damage evolution of a composite bearing structure in pillar-side cemented paste backfilling

Application of desulfurization gypsum as activator for modified magnesium slag-fly ash cemented paste backfill material

Recycling industrial solid waste for mine backfill is one of the best ways to achieve green production in multiple industries. In this paper, the desulfurization gypsum (DG) as an activator is combined with the modified magnesium slag-fly ash cementitious paste backfill (MFPB) technology for the co-disposal of solid waste and goaf treatment, and the influence of DG on the performance of MFPB was comprehensively analyzed through rheological properties, mechanical properties, durability, microscopic analysis and environmental characteristics experiments. The results show that the fresh MFPB mortar conforms to the Herschel-Bulkley model at different maximum shear rate (γ̇max) conditions. When the γ̇max is 100 s−1, the mortar exhibits shear-thickening properties. The apparent viscosity, yield stress and static yield stress of mortar decreased first and then increased with the increase of DG content, and all had the minimum value when DG was 2.5 %. The thixotropy of the mortar was significantly increased with the addition of DG, and the change in thixotropy was significantly correlated with the difference between the two yield stresses. Both the rheological and mini-slump results demonstrate that DG can improve the flowability of MFPB mortars. In addition, the UCS of D0 under steam curing and standard water curing conditions for 28 d were 4.342 MPa and 2.827 MPa, and the sample containing DG were 6.109–8.241 MPa and 6.669–9.492 MPa, respectively. The addition of DG not only improves the strength of MFPB, but also improves the durability of MFPB. Microscopic analysis (XRD, SEM, and TG-DTG) indicated that this was mainly because DG promoted the hydration reaction of the MMS-FA system and accelerated the generation of C-S(A)-H and AFt. Finally, in the results of in situ leaching based on durability and leaching based on standard HJ 557, all the indexes of MFPB meet the standard of class III groundwater in GB/T 14848-2017, and it has an effective stabilization/solidification effect on heavy metals (As, Cu, Ni, Ba, Zn and Mo, etc.). To sum up, the collaboration of DG and MFPB technology can not only efficiently clean and utilize a variety of solid wastes (MMS, FA and DG), but also greatly improve the performance of MFPB to promote its application.

Geomechanical behavior and properties of cemented paste backfill under passive interface loading and their influences on field-scale stability

Geomechanical behavior and properties of cemented paste backfill under passive interface loading and their influences on field-scale stability

Experimental and modeling study on the transient flow and time-dependent yield stress of superfine-tailings cemented paste backfill

The fresh superfine-tailings cemented paste backfill (SCPB) exhibits strong thixotropy, but quantification of the resulting transient flow (non-steady state) and time-dependent yield stress is lacking. In this study, a simple qualitative model was developed to describe the transient flow and time-dependent yield stress of SCPB. The effect of pre-shear time on the rheological behavior of SCPB was investigated. In addition, the adaptability of conventional non-Newtonian rheological models to SCPB was also evaluated. The results showed the Herschel-Bulkley model provides the most stable dynamic yield stress estimation of SCPB compared to the Bingham and modified Bingham models. A longer pre-shear time (within 500 s) led to smaller initial static yield stress and delayed recovery kinetics of static yield stress, but hardly affected the steady state of SCPB and the time required to reach it. The proposed model provides a good quantification of the transient flow at a given shear rate and time-dependent yield stress of SCPB.

Force Analysis and Strength Determination of the Cemented Paste Backfill Roof in Underhand Drift Cut-and-Fill Stopping

The stability of the cemented paste backfill roof (CPB roof) is critical to safe production in mines using the underhand drift cut-and-fill stopping. To investigate the scientific and reasonable design method of key parameters (size and strength) of the CPB roof and the stress state of the CPB roof during the mining process, field measurements were carried out with Jinchuan Group’s third mining area as the engineering background. Based on the measurement results, a mechanics model was constructed based on the thick plate theory. The field measurement results show that the overlying load on the CPB roof tends to increase first and then decrease with the gradual mining of the stope, and the maximum overlying load values of the two CPB roofs measured are 0.240 MPa and 0.244 MPa, respectively. With the gradual mining of the stope, the deformation of the CPB roof shows a trend of increasing first and then stabilizing. Based on the thick plate theory, the stress model of the CPB roof is constructed, and the error between the calculation results of the model and the field measurement results does not exceed 5%. Applying the research results to the three mines of Jinchuan Group, the span of the stope can be expanded from 5 m to 6 m under the existing strength standard of the filling body, which can increase its mining capacity by 20%. This study is the first to measure the overlying load and the tensile stress value on the CPB roof, which is an important guideline for related theoretical research.