Paste Backfill Research Articles

Cemented paste backfill compressive strength enhancement via systematic water chemistry optimisation

Systematic study of the evolution of the uniaxial compressive strength (UCS) of cemented paste backfill (CPB) is presented using various mine site hypersaline water and synthetic brines. 5 and 9 wt% binder CPB were hydrated up to 56 days. CPB prepared with 1/3 hypersaline water reached the benchmark of 1 MPa at 14 and 7 days respectively for 5 and 9 wt% binder, while 2/3 and 85 % hypersaline water CPB showed improvement compared with both tap and hypersaline water. Synthetic brines replicating mine site water composition were prepared and showed similar enhanced mechanical performance (<7 days of hydration to reach benchmark). SEM-EDX probe of micro-morphology showed that cationic (Na+ and Mg2+) and anionic (Cl- and SO42-) concentrations at optimal performance produced distinct calcium silicate hydrate and ettringite formations, which combine to facilitate observed mechanical performance enhancement. Findings demonstrate systematic mixing water utilisation for CPB preparation can contribute to improved underground mine scheduling.

Monitoring the failure process of cemented paste backfill at different curing times by using a digital image correlation technique

• Volumetric water content (VWC) was used to quantify hardening process of cemented paste backfill (CPB). • Fracture development in CPB having different curing times was visualized by using the DIC technique. • Fracture process was divided into four stages, i.e., the gestation, initiation, propagation and coalescence of fractures. Understanding the mechanical properties and failure process of cemented paste backfill (CPB) is fundamental for the backfill design in underground mining. In this research, physical and mechanical tests were conducted on CPB having different hardening times. The volumetric water content (VWC) was used to reflect the CPB hardening process because it is a confirmed controlling variable that strongly correlated with the peak strength, brittleness index and maximum strain at fracture initiation. Moreover, the deformation behavior, fracture development and failure pattern of CPB having different curing times were observed by using a digital image correlation (DIC) technique. The experimental results suggest that the fracture development can be divided into four stages, the gestation, initiation, propagation and coalescence of fractures. The failure pattern changed from shear to tensile-shear composite and finally to tensile failure as the water content decreased. The results contribute to better understanding the time-dependent mechanical behavior of CPB after it is back-filled into underground stopes.

Enhancing the sustainable immobilization of phosphogypsum by cemented paste backfill with the activation of γ-Al2O3

The in-situ solidification/stabilization of phosphogypsum (PG) has been deemed to be an efficient technology for large-scale stockpiled waste remediation. But there is still great uncertainty and controversy concerning fluorine and phosphorus transformation. Further, with a great concern for the high carbon footprint of the typical Portland cement binder, developing a low-carbon binder is of increasing importance for sustainable development. Herein, γ-Al2O3, an alternative with lower CO2 emissions and energy consumption, was preliminarily explored in PG remediation. Results showed that γ-Al2O3 promoted ettringite and amorphous precipitation. The SEM/EDS evidenced the role of aluminum-containing assemblages in pollutants retention. The pH-dependent leaching tests illustrated that the addition of γ-Al2O3 increases the fluorine retention capacity, but it is less dominant in phosphorus and sulfate immobilization. The modeling approach verified Al-F, Ca-F, and Ca-P phases as the primary phases for pollutants retention. The deviation in experimental and modeling leaching results indicates that the physical encapsulation cannot be ignored, which is evidenced by the higher mechanical strength that γ-Al2O3 modified mixtures achieved, presenting mechanically stable assemblages for engineering application and pollutants retention. This study proved that the utilization of γ-Al2O3 can expand the scalable application of PG, both in view of low-carbon strategy and high-performance solidification/stabilization.

Experimental Investigation on Flow Behavior of Paste Slurry Transported by Gravity in Vertical Pipes

In order to dispose of large quantities of mineral tailings, paste backfill is proposed and widely adopted. Paste backfill, which has high concentration and yield stress and behaves as a non-Newtonian fluid, is prepared at the surface plant and then transported underground. Vertical pipelines are more likely to suffer various failures, such as pipe breaks, wear and plugging, during the transport process. Few present studies focus on the flow behavior of paste slurry flowing in vertical pipes. In this work, two L-shaped pipeline systems with internal diameters (ID) of 40 mm and 65 mm with electrical resistance tomography (ERT) were manufactured to visualize flow regimes and measure the axial velocity and pipe pressure of slurry flowing in the upper and lower zones of vertical pipes. Flow regimes were extracted from ERT-reconstructed images stacked by time-series. Based on this, four typical flow regimes were summarized, and the characteristics of velocity and pressure change with vertical pipe depth were discussed.

Strain-Rate Effects on the Crack Evolution Pattern and Damage Characteristics of Cemented Paste Backfill

Strain-Rate Effects on the Crack Evolution Pattern and Damage Characteristics of Cemented Paste Backfill

Experimental investigation of fiber content and length on curing time‐dependent mode‐I fracture behavior and properties of cemented paste backfill and implication to engineering design

AbstractThe inclusion of fiber in cemented paste backfill (CPB) can significantly alter the mechanical response of the CPB body. The intrinsic defects in CPB and the potential dynamic loading condition make it necessary to investigate the fracture properties of fiber‐reinforced CPB (FR‐CPB). The mode‐I fracture behavior and properties are crucial to the successful engineering application of FR‐CPB technology used in underground mines. The contribution of fiber length and content to the evolutive mode‐I fracture behavior and properties of FR‐CPB was examined in this study. The results show that the addition of fiber reduces the prepeak stiffness but improves the mode I fracture toughness (KIc), and the improvement in KIc increases with fiber length. In contrast, the initial increase in fiber content (from 0% to 0.5%) benefits the KIc acquisition, while a further increase in fiber content from 0.5% to 0.75% poses a negative influence on the KIc development. Moreover, with the adoption of the cement hydration model, four predictive functions are proposed to describe the contribution of fiber length and content to the development of fracture properties. In addition, KIc is identified as a more reliable fracture property for assessing the immediate ground support role played by the FR‐CPB structure. The findings are helpful when it comes to the determination of the fiber length and content in FR‐CPB design.

A Green Sintering-Free Binder Material with High-Volumetric Steel Slag Dosage for Mine Backfill

Cemented paste backfill (CPB) is a sustainable mining method that has been increasingly utilized. Demand for high-performance and low-cost binder material is one of the limitations in CPB utilization. This work aims to examine a new, green, and economical steel-slag-based binder for CPB and to explore valorization techniques of steel slag (SS). Proportioning experiments were performed to obtain the best binder recipes for various steel slag dosages. The hydration heat, hydration products, and pore structure of a high-volumetric steel slag binder (H-SSB) were further inspected. Results show that the H-SSB, which contains 50 wt.% of SS, has a competitive strength performance superior to ordinary Portland cement (OPC) regardless of its 30–50% lower cost than OPC. The 7-day and 28-day strengths of H-SSB CPB are 1.24 and 0.74 MPa, respectively, which meets the meets of most free-standing backfill applications. The H-SSB generates less hydration heat and a larger amount of gel and ettringite hydrates than OPC in its early hydration, which can reduce the thermal expansion risks and strengthen the mechanical properties of CPB. Though the H-SSB CPB has a larger porosity than OPC-CPB at 28-day curing (45% vs. 37%), most pores are small and uniform in diameter (500–2000 nm), which is less harmful to CPB strength development. The H-SSB has secondary hydration effects in the long-term age, which can fill and refine the pore structure. The proposed H-SSB has benefits in reducing backfill costs, minimizing green gas emissions, and extending steel slag valorization techniques that can promote sustainable development of the mining and steel industries.

Pipeline Transportation Characteristics of Different Cementing Materials in CPB Slurry

Research on the underground filling mining of Cemented Paste Backfilling (CPB) slurry shows that it alleviates the underground subsidence and environmental problems caused by mining, making a strong contribution to the realization of green mining. To optimize performance, cost reductions, and the recycling of agricultural solid waste, some researchers have studied using volcanic fly ash (FA) to replace part of cement. Corn straw fly ash (CSFA) may be used to replace the same amount of part of cement or FA, as corn straw can improve the mechanical properties of FA to make the CPB slurry for filling goaf. On this basis, in order to ensure a good flow of the CPB slurry in the pipeline, we need to conduct pipeline tests on the slurry of these different cement materials to verify their feasibility. In this article, Fluent has been used to study the CPB slurry transport properties in the two cases mentioned above through numerical simulation. Comparing the velocity distribution and pressure loss changes of the CPB slurry with different proportions in the horizontal pipeline provides a reference for future CPB slurry research. The results show that: (1) FA forms a lubricating layer in the pipe wall due to its "ball effect", which effectively reduces the yield stress and pressure loss of the CPB slurry in the pipe, and plays a positive role in the transportation of CPB in the pipe; and (2) CSFA increases the CPB slurry transport pressure loss due to its irregular particle size, increasing the risk of pipe wear. The results are helpful for better understanding and designing paste filling material for mine.

Experimental and Numerical Analysis of Pressure Gradient and Flow Characteristics of Cemented Paste Backfill Based on Carbonate-Rich Tailings

Experimental and Numerical Analysis of Pressure Gradient and Flow Characteristics of Cemented Paste Backfill Based on Carbonate-Rich Tailings

Strength prediction and application of cemented paste backfill based on machine learning and strength correction

Cemented paste backfill (CPB) is wildly used in mines production practices around the world. The strength of CPB is the core of research which is affected by factors such as slurry concentration and cement content. In this paper, a research on the UCS is conducted by means of a combination of laboratory experiments and machine learning. BPNN, RBFNN, GRNN and LSTM are trained and used for UCS prediction based on 180 sets of experimental UCS data. The simulation results show that LSTM is the neural network with the optimal prediction performance (The total rank is 11). The trial-and-error, PSO, GWO and SSA are used to optimize the learning rate and the hidden layer nodes for LSTM. The comparison results show that GWO-LSTM is the optimal model which can effectively express the non-linear relationship between underflow productivity, slurry concentration, cement content and UCS in experiments (R=0.9915, RMSE = 0.0204, VAF = 98.2847 and T = 16.37 s). The correction coefficient (k) is defined to adjust the error between predicted UCS in laboratory (UCSM) and predicted UCS in actual engineering (UCSA) based on extensive engineering and experimental experience. Using GWO-LSTM combined with k, the strength of the filling body is successfully predicted for 153 different filled stopes with different stowing gradient at different curing times. This study provides both effective guidance and a new intelligent method for the support of safety mining.

Recycling of arsenic-containing biohydrometallurgy waste to produce a binder for cemented paste backfill: Influence of additives

Biohydrometallurgy waste (BW) was used as sulfate sources for supersulfated cement (SSC) production with the aim of providing an alternative binder for cemented paste backfill (CPB). Additives, including carbide slag (CC), Al2(SO4)3 (AlS) and Na2SO4 (NaS), were used to modify the binder properties, when considering the poor mechanical strength at the early curing ages. The reaction kinetics of binder modified by additives were studied via isothermal calorimeter test; mechanical strength and hydration products modifications due to the addition of additives were also explored. Additionally, arsenic (As) leaching from the binders was tested and the immobilization mechanism was explored using density functional theory (DFT) calculation. The results showed that the addition of CC increased the early strength strongly, but higher amount of CC restricted the strength development after curing for a long time; NaS presented marginal influence on the binder properties. The coupling addition of 25 wt% CC and 1 wt% AlS showed the best performance; the compressive strength after curing for 3 and 28 days reached around 13.67 MPa and 36.97 MPa, which was increased by about 47.69% and 18.34%, respectively, compared to the control sample. The superior performance was attributed to the generation of more reaction gels, especially ettringite (AFt). Rising in the AFt content contributed to better immobilization effects on As, because that it led to lower pore volume and permeability; besides, AFt showed higher immobilization effects according to the DFT calculation.

Numerical Study on Transportation of Cemented Paste Backfill Slurry in Bend Pipe

With the development of coal mining, the use of elbows has diversified the forms of underground backfill pipelines, which has inevitably complicated the transportation characteristics of filling slurry in the pipeline, thus affecting the entire backfilling system. The objective of this study is to numerically investigate the running state of cemented paste backfilling (CPB) slurry and coarse particles at different velocities by transporting in bend pipes. To better understand the transportation state of CPB slurry in pipeline, a computational fluid dynamics (CFD) model—mixture model was developed to study the transportation of CPB slurry. The volume distribution of coarse particles in slurry under different pipe types and different flow rates, as well as the velocity profiles of slurry at different positions, were simulated and analyzed, and the pressure losses under different pipe types were compared. The results show that the distribution of coarse particles varies with the tube type, and the effect of coarse particles on the position of tube wall changes with the increase in velocity. The high-speed zone of CPB slurry will move toward the outer wall of the elbow with the increase in velocity. The pressure loss of CPB slurry in the vertical–horizontal pipeline is larger than that in the horizontal–vertical pipeline, and the difference is larger in the bend section. This study provides a theoretical and meaningful reference for CPB slurry backfilling operations in different bends.

Mesoscopic damage evolution and acoustic emission characteristics of cemented paste backfill under different loading rates.

The cemented paste backfill (CPB) has a significant loading rate (LR) effect. The damage evolution process of CPB is closely related to the characteristics of acoustic emission (AE). This paper analyzes the damage evolution law of the filling body under different loading rates using indoor test and numerical simulations. We introduce the moment tensor theory to simulate the AE characteristics of the whole process of filling loading and explore the LR effect of the backfill with the help of the energy conservation. The results indicate the following: (1) when LR increases from 0.1 to 2mm/min, the UCS of the backfill first increases and then decreases, contributing to the occurance of the critical LR. (2) There are no microcracks occurred in the backfill at the initial stage of loading, and the microcracks increase slowly, which is not obviously affected by LR. After the peak value, the microcracks in backfill expand and propagat rapidly to form mesoscopic cracks. (3) The mesoscopic AE events based on the moment tensor theory are in good agreement with the laboratory tests results, which can be divided into the initial period, quiet period, slow raising period, rapid raising period, and rapid falling period. (4) The temporal and spatial distribution characteristics of AE are consistent with the evolution law of microcracks. There are fewer AE events before the peak value, and AE events increase significantly and frequently with large magnitude events after the peak value. AE events dense zone and AE events with larger magnitude increase under higher LR. (5) Besides, the boundary energy and dissipation energy also gradually increased; at the same time, the strain energy first increased and then decreased. The results can provide a reference for understanding the damage evolution characteristics of backfill by different LR and AE mesoscopic simulation.

Investigation of the Effect of Using Different Fly Ash on the Mechanical Properties in Cemented Paste Backfill

Investigation of the Effect of Using Different Fly Ash on the Mechanical Properties in Cemented Paste Backfill

A new method for exploiting mine geothermal energy by using functional cemented paste backfill material for phase change heat storage: Design and experimental study

The research on mine geothermal energy exploitation has attracted global interest for many years. This paper proposes an innovative new method for geothermal-coal synergetic mining (GE-COSM) to expand the valorization of coal-based solid waste (C-BSW), reduce the environmental damage caused by coal utilization, and achieve a low-carbon transformation of the energy structure. A functional cemented paste backfill material (F-CBM) utilizing C-BSW for phase change heat storage was invented through orthogonal testing. This study obtained the main factors influencing raw material proportions on F-CBM working properties. Moreover, the optimal ratio of F-CBM raw materials was determined. F-CBM prepared with the optimal ratio showed improved working properties: the thermal conductivity (TC) and specific heat capacity (SHC) were increased by 75.70 % and 111.90 %, respectively, and the highest latent heat value was 17.37 J/g. The F-CBM's strength loss mechanism was discussed in terms of its mesostructure. In addition, it is shown that the backfilling layer formed by F-CBM in the goaf act as a thermal insulation and a phase change heat storage layer. Finally, the mine design principles of GE-COSM at different stages are summarized. These findings provide a reference for the application of F-CBM in engineering practice, and lay the groundwork for exploiting the mine geothermal energy.

Retention of phosphorus and fluorine in phosphogypsum for cemented paste backfill: Experimental and numerical simulation studies

The solidification/stabilization of phosphogypsum using cemented paste backfill (OCPB) provides a low-cost and alternative in-situ technique for recycling phosphogypsum stockpiles. But the OCPB is far from obtaining steady states in which the pollutants would redistribute as a response to dynamic environmental conditions. Further, the associated chemical interactions and the mineralogy information of the solubility-controlling phases of contaminants (fluorine and phosphorus) have not been thoroughly studied or fully understood. In this study, a framework coupling the chemical, mineralogical, and morphological analyses is used to determine the fluoride and phosphate retention mechanisms of immobilized OCPB. Then the pH-dependent leaching tests and numerical simulation is applied as a useful tool to identify the minerals controlling stabilized OCPB leaching behavior. The overall findings proved that aluminate-rich calcium silicate hydrates play an essential role in fluoride and phosphate retention. Both experimental and simulational acid neutralization and leaching curves indicate that the cementitious matrix works as a strong buffering material ensuring high pH conditions that are necessary for fluorine and phosphorus retention. Although discrepancies were observed in absolute fluorine and phosphorus leaching values at highly acidic conditions, the simulations are able to describe highly amphoteric leaching behavior. The simulation suggests that the aluminum species and calcium phosphates governed the solubility of fluorine and phosphorus, respectively. The results of this work would have implications for predicting the leaching behavior of OCPB in detrimental and multiple environments.

Cemented Paste Backfill Hydraulic Conductivity Evolution from 30 Minutes to 1 Week

AbstractThe first one to four days constitutes a critical period in filling mining voids with cemented paste backfill (CPB, a mixture of mine tailings, binder, and water) and determining the saturated hydraulic conductivity (ksat) during this period is important to rational engineering design. However, most published studies started testing 24 h after specimen preparation, and those that started earlier did so by preconsolidating the specimens, which results in void ratios lower than those occurring in the field. This study uses a new test method that retains the backfill’s representative bulk properties and starts testing about one-half hour after specimen preparation. During backfilling, CPB undergoes three distinct ksat stages: a relatively constant high-value ksat stage associated with the portland cement’s (PC’s) dissolution phase; a rapidly declining ksat stage associated with PC’s acceleration phase; and then a slowly declining ksat stage associated with PC’s steady-stage and subsequent deceleration phase. The last two stages can be represented by modified Kozeny-Carman equations, in which the hydration effects are constant but different in each stage, and the ksat changes can be related to void ratio changes in each stage. In contrast, the distinct stages are not adequately represented by traditional PC “maturity” models, and, more importantly, the ksat values determined here are at least an order of magnitude higher than previously published results on similar materials.

An integrated management strategy for acid mine drainage control of sulfidic tailings

The oxidation of sulfide minerals releases acidic leachate enriched in sulfate, iron, and toxic metals and metalloids (e.g., As, Sb, Cu, Pb, Cd, Zn, Hg, etc.) known as acid mine drainage (AMD). This eco-hydrological hazard is imminent when mineralogical and geochemical analysis indicates that acid-generating (sulfides) overcome the acid-neutralizing (carbonate) minerals in the ore and mining waste. AMD represents a multifaceted problem faced by the mining industry. The most widespread management technique is to store the tailings into engineered impoundments and actively neutralize the acidity produced by the oxidation of the sulfide minerals by adding alkaline chemical agents. However, this method involves a constant and long-term commitment, elevated operational costs, and several geotechnical and environmental drawbacks. In a pre-mining stage, geological, geochemical, mineralogical, and textural characterization of the mine waste could guide proper and site-specific mining waste management.In this paper, we present an integrated strategy to reduce the acid mine drainage (AMD) potential of the Dundee Precious Metals Chelopech (DPM-Ch) tailings through a combination of environmental desulfurization and cemented paste backfill (CPB) – the latter one being already practiced on-site. In the proposed scenario, a desulfurization plant would be installed downstream to re-float the non-valuable reactive pyrite. The reactive pyrite concentrate would be combined with the CPB material, whereas the remaining tailings would be discharged at the tailings management facility (TMF). The lack of neutralizing gangue (NP of untreated tailings < 8 kg CaCO3/t) precludes converting the tailings into non-acid generating. However, reducing nearly six times the amount of lime required to neutralize the acidity produced by pyrite oxidation was feasible without substantially increasing operational costs. Supporting the environmental benefits of combining desulfurization and backfill, an X-ray mapping of the CPB material revealed that this approach effectively encapsulates the reactive gangue, ultimately avoiding pyrite oxidation and leaching of heavy metals.

Effect of Waste Materials on the Strength-Stability Performance and Production Cost of Cemented Paste Backfill

Bu çalışma ile çimentolu macun dolgu (ÇMD) karışımında sülfürlü-maden atığı (S-MA) yerine %15 oranında ikame edilen atık malzemelerin (inşaat-yıkıntı atığı: İYA, kireçtaşı: KT, uçucu kül: UK ve yüksek fırın cürufu: YFC) ÇMD’nin dayanım/duraylılık performansına ve üretim maliyetine etkisi incelenmiştir. Bu amaçla, üretilen ÇMD numuneleri (kontrol: ve %15 atık malzeme ikameli) 7-360 gün boyunca dayanım testine tabi tutulmuştur. ÇMD üretim maliyetinin araştırılması için karışımlarda kullanılan çimento miktarları ve maliyetleri belirlenirken, atık malzemelerin öğütme sırasındaki enerji tüketimleri ve maliyetleri hesaplanmıştır. Tüm numuneler kısa ve uzun dönemde istenen 1,0 MPa dayanımı sağlamış ve %15 UK ve YFC ikameli ÇMD numuneleri kontrol numunelerine kıyasla 4 kata kadar daha yüksek dayanım üretmiştir. 1,0 m3 ÇMD üretimi için kontrol karışımında 130,0 kg çimento (%8,5) kullanılırken, atık malzeme ikameli karışımlarda bağlayıcı oranının %1,0 düşürülmesi yaklaşık %15 çimento tasarrufu sağlamıştır. Atık malzemelerin öğütme işlemlerinde 6,55-18,89 kW-saat/ton elektrik enerjisi tüketilmiş ve böylece 2,99 ₺ (UK)-8,85 ₺ (İYA) arasında para harcanmıştır. ÇMD üretimi için toplam maliyet değerlendirildiğinde, kontrol karışımı için 153,4 ₺ harcanırken, %15 atık malzeme ikameli karışımların maliyetinin 133,5 ₺’ye kadar (%12,98’e kadar tasarruf) düşürülebilmesi öngörülmüştür. Sonuçlar, ÇMD’nin dayanım/duraylılık performansı ve üretim maliyeti konusunda en uygun karışım tipinin %15 UK ikameli ÇMD karışımı olduğunu göstermiştir.

Application of Cement Paste in Mining Works, Environmental Protection, and the Sustainable Development Goals in the Mining Industry

Cement paste is an already well-known material used in ore mining. It is mainly used to fill excavation areas and the tailings from the surface return to underground mines. In this way, the amount of deposited material and degradation of the surface of the terrain are reduced. The paste itself can be used as an artificial barrier between mining works and underground watercourses. Significant economic and environmental benefits can be expected from using cement paste, which would contribute to sustainable development. The basic materials that make up cemented paste backfill (CPB) are flotation tailings, cement, and water. For CPB to be adequately and safely applied to the filling of excavation spaces and indirectly to the protection of the groundwater, environment, and sustainable development of the mining industry, it must meet certain physical–mechanical, physicochemical, and deformation properties. This paper presents the results of synthesized and analyzed samples of different compositions based on flotation tailings (from the production of ZiJin Copper in Bor, Serbia), cement, and water. The methods used for chemical and mineralogical tests include inductively coupled plasma atomic emission spectroscopy (ICP-AES), atomic absorption spectroscopy (AAS), X-ray diffraction analysis (XRD), and nephelometric turbidity units (NTUs; turbidimetry). The results prepared with CPB consisting of 5% cement, 24% water, and 71% flotation tailings were the most acceptable.