Cemented Paste Backfill Research Articles

Innovative AI-based multi-objective mixture design optimisation of CPB considering properties of tailings and cement

ABSTRACT Cemented paste backfill (CPB) is a profitable tailings management strategy for underground mines. CPB is designed to meet specific objectives and constraints such as strength and workability at reasonable costs. Traditional design of CPB is costly due to large number of variables such as properties of tailings/cement. This paper proposes an innovative AI-based approach for multi-objective mixture design optimisation of CPB considering properties of tailings and cement. This approach contributes to meliorating mine economy according to the United Nations Sustainable Development Goals by reducing the amount of cement considering pozzolanic potential of tailings, thereby reducing carbon footprint and operational costs.

Research on fracture behavior and reinforcement mechanism of fiber-reinforced locally layered backfill: Experiments and models

Locally fiber-reinforced layered cemented paste backfill (FR-LCPB) were prepared for three-point bending experiments to examine the influence of fiber content and reinforcement position on its fracture resistance properties, in an effort to reduce the significant demand for cement during mine filling and improve the characteristics of the CPB brittle damage. Further, scanning electron microscopy (SEM) and PFC 3D numerical simulation were used to investigate the failure mode of FR-LCPB and the reinforcement mechanism of fibers. The results demonstrated that the addition of fibers significantly affects the FR-LCPB's fracture resistance, with 0.6% being the ideal fiber content. Flexural strength, peak deflection, and bending modulus of FR-LCPB increase with the decrease of the distance between the fiber reinforcement position and the lower layer. Additionally, the addition of fibers enhances the CPB's brittle damage characteristic, ensuring that it retains some strength even after failing. It is primarily due to the fibers' ability to deform and rub against the surrounding matrix while resisting the external load that FR-LCPB retains its strength even after damage. PFC 3D simulation results demonstrated that FR-LCPB damage exhibits hysteresis. FR-LCPB breaks down in the fiber-containing layer after first cracking through the non-fiber layer during the loading process, which differs from the damage pattern of regular layered CPB and further suggests that the fiber has an inhibitory effect on the FR-LCPB's fracture. The outcomes will serve as a guide for low-carbon mining development and safe mining operations.

Performance investigation of blast furnace slag based cemented paste backfill under low temperature and low atmospheric pressure

To promote application of cemented backfill mining in underground mines located at high-altitude regions, there is a necessary to investigate mechanical performance of cemented paste backfill (CPB) under low temperature and low atmospheric pressure. In this study, the orthogonal experiment was first scheduled to synthesize the new cementitious material by combining industrial byproducts of blast furnace slag (BFS), carbide slag (CS), desulfurization gypsum (DG) with cement clinker (CC). Then, CPB samples were prepared with the new cementitious material and cured under different temperatures of 5, 10, 20 ℃ and atmospheric pressures of 50, 75, 101 kPa for 3, 7 14, and 28 days. Unconfined compressive strength (UCS) and microstructural analyses were conducted to evaluate strength and microstructure evolution of CPB. Experimental results indicate that UCS values of CPB increase with BFS, but decrease with CS and DG. The optimal proportion of the new cementitious material is determined as: 70 wt% BFS, 12 wt% CS, 1 wt% DG, and 17 wt% CC. Under curing of low temperature and low atmospheric pressure, CPB samples exhibit lower UCS values than those under 20 ℃ with 101 kPa. Especially, the coupling of low temperature and low atmospheric pressure exerts significantly adverse influence on the later strength of CPB after curing of 14 and 28 days. The declines of 57.99 % and 41.75 % are observed from UCS values of 14-day and 28-day CPB under 5 ℃ with 50 kPa, respectively. Regardless of curing temperature and atmospheric pressure, UCS values increase with the ratio of cementitious material to tailings and curing time. Low curing temperature and low atmospheric pressure reduce the hydration degree of the new cementitious material. The amount of hydration products decreases, the number of micropores (0.1 ∼ 1 μm) and the total porosity increases, as well as more loose microstructure forms among solid particles. This is the underlying reason behind performance degeneration of CPB under low temperature and low atmospheric pressure.

Novel SCMs produced by the calcination of secondary aluminium dross with dolomite and their potential usage in cemented paste backfill

In this study, novel supplementary cementitious materials (SCMs) were produced by the mixed calcination of secondary aluminium dross and dolomite. The thermal activation effect on calcined mixes, hydration properties of pastes with the novel SCMs and workability of cemented paste backfill (CPB) were experimentally investigated. The results demonstrated that high-temperature calcination evidently increased the activity of SCM, and the improvement by thermal activation was positively correlated with calcination temperature. Pastes with calcined SCMs hydrated to form more CsbndSsbndH, ettringite and Ht but less portlandite because of the accelerated pozzolanic reaction of SCMs. Moreover, CPB samples with novel SCM had an imporved transportation performance and mechanical strength with denser pore structure. Furthermore, heavy metals (Cu, Pb, Zn and Cr) were well-immobilised/stabilised by CPB with novel SCMs calcined at 900 °C and 1,000 °C, which can make secondary aluminium dross be used without environmental risks.

Determination of the Required Strength of Artificial Roof for the Underhand Cut-and-Fill Mine Using Field Measurements and Theoretical Analysis

For the underhand cut-and-fill mining method, to ensure safe and economic mining, a key issue is to correctly determine the required strength of the artificial roof made of cemented paste backfill (CPB). However, the determination of the required strength is typically based on historical experience and analytical beam formulas, resulting in the obtained required strength being unsuitable for the actual situation. Therefore, in order to determine the required strength of the CPB roof reasonably and accurately, field measurements based on sensors were proposed and carried out in the Jinchuan mine, and then formulas based on thick plate theory were derived to verify the measured results. The results show that the required strength obtained by field measurement is 0.325 MPa and that obtained by thick plate theory is 0.304 MPa, with an error of 6.78% between them, verifying the accuracy of the measurements. However, the strength standard currently used by Jinchuan is 0.59 MPa, which far exceeds the optimal strength and results in many additional, unnecessary expenses. To ensure economical mining, the span of the drift was enlarged from 5.0 m to 6.0 m based on the results of the actual measurements and the current production status of the mine. The measurements show that the maximum cumulative subsidence of the drift roof is 11.69 mm and the maximum convergence deformation of the sidewalls is 8.34 mm, which indicates that the stability of the span-enlarged drift is satisfactory. Meanwhile, enlarging the drift span allows for a 20% increase in production capacity per mining cycle. This field measurement method and theoretical analysis model can be used as an efficient guide to facilitate the design of underhand cut-and-fill mining.

Experimental research and numerical simulation of the multi-field performance of cemented paste backfill: Review and future perspectives

Experimental research and numerical simulation of the multi-field performance of cemented paste backfill: Review and future perspectives

Experimental study on thermal and mechanical properties of tailings-based cemented paste backfill with CaCl2·6H2O/expanded vermiculite shape stabilized phase change materials

Experimental study on thermal and mechanical properties of tailings-based cemented paste backfill with CaCl2·6H2O/expanded vermiculite shape stabilized phase change materials

Rheological and physicomechanical properties of rod milling sand-based cemented paste backfill modified by sulfonated naphthalene formaldehyde condensate

Rheological and physicomechanical properties of rod milling sand-based cemented paste backfill modified by sulfonated naphthalene formaldehyde condensate

Research on the Homogenization Evaluation of Cemented Paste Backfill in the Preparation Process Based on Image Texture Features

In China, cemented paste backfill (CPB) is a common treatment method after the exploitation of basic energy. The homogeneity of slurry influences the performance of CPB. However, the online monitoring and characterization of homogeneity lack relevant technologies and unified standards. This article discusses an online image analysis technique applied to the online monitoring of cemented paste backfill mixing, which is based on the evolution of the texture of images taken at the surface of the mixing bed. First, the grayscale distribution of the image obtained by the high-speed camera in the CPB preparation process was analyzed by Matlab and its variance (s2) was solved, and the texture features of the image were analyzed by the variance of grayscale distribution. Then, a homogeneity discriminant model (cst) was established. The results show that the variance value of the grayscale distribution of the slurry image increases rapidly at first, then gradually decreases, and becomes stable in the final stage since it turns a constant value. When the s2 value tends to be stable, the slurry gradually reaches homogenization, and the discriminant coefficient of paste homogenization based on the homogenization discriminant model reaches 0.05. The homogenization prediction of CPB proves to be consistent with the backfill performance comparison results. The evolution of the texture allows obtaining important information on the evolution of different formulations during mixing, which can be used for intelligent monitoring of CPB preparation process.

Mechanical grinding activation of modified magnesium slag and its use as backfilling cementitious material

The utilization of industrial solid waste with potential cementitious activity to prepare low carbon, economic and environmental backfilling cementitious materials is an urgent issue in cemented paste backfill (CPB). In this work, the mechanical grinding activation of modified magnesium slag (MMS), including grinding kinetics, crystallinity, and the influences on backfilling paste characteristics, have been investigated systematically. The indexes of particle size distribution (PSD), fineness, and sieve residue indicate that the grinding efficiency of MMS is high when grinding time ranges [20,80] min. The particle coarsening appears when the grinding time increases to 120 min from 100 min. A fitted grinding kinetics function of MMS has been established based on the measured PSDs with six grinding time. The XRD shows that the diffraction peak intensities of CaSO4 and SiO2 phases decreases with the increasing of grinding time, indicating the increasing of lattice defects and amorphization degree. Both of grinding dispersion refinement and amorphization increase of MMS contribute to the activation of cementitious reactivity. Grinding time has a great influence on the properties of modified magnesium slag-based (MMS-based) backfilling pastes. The fresh backfilling pastes conformed to both of the H-B model and revised Bingham model well. The critical shear rate, critical apparent viscosity, and extended degree increase with the increasing of grinding time, indicating that the fluidity decreases. The TG-DTG, SEM tests show that hydration rate of the MMS-based backfilling pastes is slow and there are many interconnected micropores when the grinding time is below 60 min. Besides, the uniaxial compressive strengths (UCS) of MMS-based backfilling mortar samples also show a similar pattern. All tests showed that grinding for 60 min could stimulate most of the potential cementitious reactivity of MMS. Meanwhile, the corresponding backfilling paste behaves good working condition. Taken together, 60 min is suggested as the optimal grinding time for MMS in consideration of energy consumed and technical benefits, and the corresponding specific surface area (SSA) and 45 µm sieve residue (SR) are 503 m2/kg and 3.83 %, respectively.

Confined yield stress lubrication flows for cement paste backfill in underground stopes

Along with the increasing excavation of underground spaces, the mining industry has developed various techniques for refilling these voids. One of them is cemented paste backfill (CBP), coming from the mixing of tailings with water and a binder. These pastes are then transported to underground cavities through pipes and into voids, which they fill in a layering pattern. In this paper we explore the capabilities of a simplified thin-film model to describe the evolution of the paste shape within the stope. The mathematical difficulties are to include the yield stress behaviour of the paste and to deal with the walls in a consistent manner. We show the possible behaviours of the associated solutions to these problems, governed by 2 dimensionless parameters. Two regimes of filling are exhibited, when studying physical parameters typically used in CBP. Finally, we compare the simulations with physical experiments performed in a scaled lab setting using realistic tailings pastes from lead–zinc and nickel mines in North-West China. A good agreement is found between simulations and experiments.

Effect of curing pressure on the stability of bottom cemented paste backfill under different types of barricade

In this paper, a multi-physical field coupling curing experimental instrument was developed to study the effect of curing pressure on the stability of bottom cemented paste backfill (CPB) under two different barricades. The effect mechanism was analyzed by using 5TE, MPS-6 sensor, scanning electron microscope (SEM), and binary processing of SEM images. And then, the strength prediction model of CPB under different curing conditions was established. The findings show that the strength of CPB increases with curing pressure regardless of permeable or impermeable barricade, and the relationship between CPB strength and curing pressure conforms to the law of linear function, and the change law of strength grow rate can also be expressed by a linear function. The curing pressure will accelerate the hydration reaction rate and compress the internal pores of the CPB, making the structure of the CPB more denser, thus promoting the formation of strength. It is worth noting that after the curing pressure was increased to 600 kPa, the strength growth rate of CPB slowed down in the later period, and obvious cracks appeared in the bottom CPB during sampling. The results will be helpful to better consider setting different types of barricade at different stope heights

Synthesis and characterization of cemented paste backfill: Reuse of multiple solid wastes

The alkali-activated composite cementitious materials composed of fly ash (FA) and ground granulated blast-furnace slag (GGBS) can effectively overcome the performance deficiency of alkali activated single precursors. In order to make full use of the synergistic cementing effect among multi-source solid wastes, cemented paste backfills (CPB) were prepared with iron tailings as the aggregate, FA and GGBS composed as the binder, soda residue (SR) and calcium carbide slag (CS) composed as the activator. The effects of varying FA/GGBS and SR/CS proportions on the workability, mechanical, and microstructural properties of SR–CS–activated FA-GGBS CPB were analyzed. The results indicate that the fluidity of the freshly-mixed backfilling slurry increases first and then decreases with the increase in FA content; whereas it increases with the decrease in the SR content. The unconfined compressive strength (UCS) of CPB decreases with the increase in FA content; it first increases and then decreases as SR content decreases. When the mass ratio of GGBS, FA, SR, and CS was 5:3:1:1, the 28d UCS of CPB was 2.66 MPa, while the fluidity of the freshly-mixed slurry was 158 mm. The microscopic analysis shows that the hydration products are mainly C–S–H gel, C-A-S-H, Friedel's salt (FS), Hemicarbonate (Hc), and Hydrotalcite (Ht). The amount of C–S–H gel produced increases gradually with the curing time and decreases continuously with the increased FA content. More dissolved free aluminum ions promote the conversion from C–S–H to C-A-S-H, while the formation of C-A-S-H inhibit the formation of FS.

Mechanical Properties and Microstructure of Iron Tailings Cemented Paste Backfills Using Carbide Slag-Activated Ground Granulated Blast-Furnace Slag as Alternative Binder

For rational disposal of solid wastes, low-cost cemented paste backfills (CPB) were prepared with iron tailings, ground granulated blast-furnace slag (GGBS), and calcium carbide slag (CS). To ensure that CPB can be effectively utilized in mine backfill projects, the effects of solid contents, GGBS content, CS/GGBS ratio, and mixing water on the workability and mechanical properties of CPB were investigated. The results indicated that when the solid content was 70%, the GGBS content was 14%, the CS/GGBS ratio was 0.4, the mixing water was tap water, the fluidity of fresh CPB slurry was 167 mm, and 28d unconfined compressive strength (UCS) of CPB reached 2.89 MPa, at which time the effect of the activation of the GGBS with CS was optimal. The analysis of X-ray diffraction (XRD), scanning electron microscopic imaging (SEM), thermogravimetric analysis (TG-DTG), and Fourier transform infrared spectroscopy (FTIR) demonstrated that the hydration products are mainly C-S-H gels, C-A-S-H, hemicarbonate, and hydrotalcite in CS-GGBS cementitious material. The C-S-H gel content gradually increased with increasing curing time and CS content (15%–35%). When the CS content was increased to 25%, the C-S-H gel content significantly increased, which improved the overall structural compactness and increased the UCS of CPB.

Effect of superplasticizer on the setting behaviors and mechanical properties of tailings-waste rock cemented paste backfills

Jinchuan Mine is facing the problem of a large number of tailings and waste rock, which forced the mine to establish tailings-waste rock cemented paste backfilling (TW-CPB) system. However, the transport ability and setting process of TW-CPB slurry have aroused the mine's attention, and adding appropriate superplasticizer is expected to be one of the options to solve this problem. This paper investigates four types of superplasticizers: Polycarboxylate- (PC-P), Naphthalene- (FDN), Aliphatic- (AK), Melamine- (SM), and five different contents: 0.5 %, 1.0 %,1.5 %, 2.0 %, 2.5 % (by mass of cements),tests the bleeding rate, setting time, internal water evolution, hydration products, and microstructure during the setting period, and assesses the unconfined compressive strength (UCS) of the TW-CPB samples at different curing times. Test results showed that the appropriate superplasticizers can effectively improve the water distribution in the slurry, which is conducive to the to slurry transportation in the initial process, promote the hydration reaction and change the morphology and microstructure of the main hydration products, which is important to the UCS improvement of TW-CPB sample. Finally, considering the effect on the fluidity and UCS, it can be concluded that the reasonable addition type and content of the superplasticizer in TW-CPB is 0.5 % of PC-P in the Jinchuan Mine. Furthermore, the successful application of superplasticizer of the TW-CPB would be significance to the utilization of tailings and waste rock.

Study on Shear Characteristics and Failure Mechanism of Inclined Layered Backfill in Mining Solid Waste Utilization

To eliminate the massive accumulation of tailings and promote the recycling of mine solid waste to develop green mining, a two-layered cemented paste backfill (CPB) with layering angles of 5°, 10°, 15°, 20° and 25° and complete CPB were prepared to conduct direct shear experiments; thus, the effect of the layering angle on their shear mechanical properties can be investigated. Meanwhile, the particle flow analysis program PFC 2D was used to simulate the micro-crack propagation law and reveal the damage mechanism. The results showed that the layered structure weakens the integrity of CPB and significantly reduces its shear strength, cohesion and internal friction angle. When the layered angle increased from 20° to 25°, the shear strength under normal stress of 100 kPa was decreased by 35.13% and cohesion by 43.43%. As the layered angle increased from 5° to 25°, the internal friction angle decreased first and then increased and reached the minimum when the layered angle was 15°. With the increase in the layered angle, the layered CPB gradually generated tension cracks along the layered surface, and the number of cracks gradually increased. The failure mode of each specimen is mainly a shear crack through the shear surface failure, crack expansion in the middle of shear surface on the upper and lower sides of the spindle-shaped failure and an inverted Z-shaped conjugate shear failure. This research can provide a substantial reference in the design and application of layered CPB in underground mines.

In-situ remediation of phosphogypsum in a cement-free pathway: Utilization of ground granulated blast furnace slag and NaOH pretreatment

In-situ remediating phosphogypsum (PG) for cemented paste backfill (CPB) in the contaminated site is economic management for promoting sustainable developments in the phosphate industry. This study concerns the combined use of NaOH pretreatment and ground-granulated blast furnace slag (GGBFS) additives to promote the solidification/stabilization of PG with a lower carbon footprint pathway. According to physico-chemical analyses, the NaOH pretreatment effectively removed approximately 95% of F within the PG, which may originally be present as sparingly soluble fluorides or coexisting with silicates. The micro mineralogical characterization illustrates that the pretreatment can accelerate the early age hydration, with more hydration products observed, including calcium silicate hydrates and ettringite, effective F and P retention candidates. Whereas the incorporation of GGBFS plays an essential role in promoting the generation of additional cement hydrates at the following stages. The macro mechanical performance analysis indicates that the mixtures of pretreated-PG-OPC-GGBFS exhibit an excellent mechanical performance satisfying the design criteria. Subsequent elemental mapping and toxicity characteristic leaching procedures demonstrate that this combined approach has a competitive F and P immobilization ability compared to the typical OPC binder and individual GGBFS addition. The newly formed phases effectively controlled the concentration of F and P through adsorption, incorporation, or encapsulation. Objectively, the proposed methodology can be a promising candidate pathway for extrapolating the in-situ immobilization of PG. This study opens up new perspectives for synergetically recycling PG and GGBFS in a profitable and low carbon footprint way.

Yield Characteristics of Cemented Paste Backfill

Cemented paste backfill (CPB) plays an increasingly important role in the mining industry due to its operational and environmental benefits. CPB is placed in the mined-out stope to form a self-supporting structure. The strength and stability of the CPB is of great concern in its engineering applications. Indeed, CPB must remain stable during the extraction of adjacent stopes to ensure the safety of the mine operations. Although significant research has been conducted on the shear properties of CPB, there are limited studies on its post-failure behavior, in particular the yield characteristics of CPB. This paper presents the finding on the post-peak and yield property of CPB. The study is conducted on three cemented contents and six stress intervals based on the mining practice and field study. The results show that CPB exhibits dilative behavior under strain softening and contractive property under strain hardening conditions. Our study demonstrates that pure frictional resistance could exceed the cohesion strength at high stress levels.

A Novel Design Concept of Cemented Paste Backfill (CPB) Materials: Biobjective Optimization Approach by Applying an Evolved Random Forest Model.

For cemented paste backfill (CPB), uniaxial compressive strength (UCS) is the key to ensuring the safety of stope construction, and its cost is an important part of the mining cost. However, there are a lack of design methods based on UCS and cost optimization. To address such issues, this study proposes a biobjective optimization approach by applying a novel evolved random forest (RF) model. First, the evolved RF model, based on the beetle search algorithm (BAS), was constructed to predict the UCS of CPB. The consistency between the predicted value and the actual value is high, which proves that the hybrid machine learning model has a good effect on the prediction of the UCS of CPB. Then, considering the linear relationship between the costs and the components of CPB, a mathematical model of the cost is constructed. Finally, based on the weighted sum method, the biobjective optimization process of the UCS and cost of CPB is conducted; the Pareto front optimal solutions of UCS and the cost of CPB can be obtained by the sort of solution set. When the UCS or the cost of CPB is constant, the Pareto front optimal solutions can always have a lower cost or a higher UCS compared with the actual dataset, which proves that the biobjective optimization approach has a good effect.

The Rheological Properties and Strength Characteristics of Cemented Paste Backfill with Air-Entraining Agent

Clogging pipelines is one of the most common and urgent problems in paste backfill mining. The aim of the present study was to solve the problem of pipe blockage in paste backfill mining. In this paper, paste mixed with coal gangue, fly ash, cement, and additives is used to investigate the influence of three air-entraining agents (AEAs) (including sodium dodecyl sulfate (SDS), triterpene saponin (SJ), and sodium abietate (SA)) on the flow characteristics and strength characteristics of the paste. A series of relevant tests was conducted on the paste, such as air content experiments, slump and expansion experiments, viscosity and yield stress tests, and the uniaxial compressive strength (UCS) test. The results show that the air content of the paste increases with increasing AEA content, but the increase is limited and reaches a maximum at 0.9 AEA. The slump of the paste increased by up to 10–13 mm, and expansion increased by up to 66–130 mm compared to the paste without AEA. The viscosity of the paste decreased by up to 0.13–0.20 Pa·s, and the yield stress decreased by 81.47%–93.7% of the original. The strength of the paste was also reduced, and after 28 days of curing, the strength was reduced by up to 1–1.2 MPa. Taking into account the strength requirement of 3 MPa for the paste from the Linxi mine, it was considered that the dosage of 0.9 B was a good choice, as it could better change the flowability of the paste and reduce the pipeline transportation resistance and transportation energy consumption. At the same time, the strength was also acceptable. The study in this paper can provide a reference for performance studies of pastes mixed with coal gangue, fly ash, cement, and additives as materials.