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Cemented paste backfill (CPB) is easy to withstand the coupling effect of the mining operation equipment's crushing, water immersion, forming all kinds of intrinsic or extrinsic defects affecting its load-bearing capacity. In this paper, the initial immersion age and immersion time were used as variables, the damage and uniaxial compression characteristics of CPB under the coupling effect of water-static load were explored. Results show that the damage of water-immersed CPB under static load are mainly affected by water lubrication and pore water pressure and it improves the plastic deformation of CPB and weakens the energy storage capacity. When the initial immersion age was 3d, the effect is more significant. Water immersion increases the rate of damage with strain before peak strain and decreases the rate of damage with strain after peak strain. The strength of CPB varies from 0.3 MPa to 0.8 MPa at the same initial immersion age. The damage constitutive model of CPB under water-static load coupling is established, and the damage mechanism is revealed. Compared with the immersion time, reducing the initial immersion age is the key factor to improve the structure stability of CPB.

During the process of constructional backfill mining, the cemented paste backfill (CPB) typically exhibits a high degree of brittleness and limited resistance to failure. In this study, the mechanical and damage evolution characteristics of waste tire steel fiber (WTSF)-modified CPB were studied through uniaxial compression tests, acoustic emission (AE) tests, and scanning electron microscopy (SEM). The results showed that the uniaxial compressive strength (UCS) decreased when the WTSF content was 0.5%, 1%, and 1.5%. When the WTSF content reached 1%, the UCS of the modified CPB exhibited a minimal decrease (0.37 MPa) compared to that without WTSF. When the WTSF content was 0.5%, 1%, and 1.5%, peak strain of the WTSF-modified CPB increased by 18%, 31.33%, and 81.33%, while the elastic modulus decreased by 21.31%, 26.21%, and 45.42%, respectively. The addition of WTSF enhances the activity of AE events in the modified CPB, resulting in a slower progression of the entire failure process. After the failure, the modified CPB retained a certain level of load-bearing capacity. Generally, the failure of the CPB was dominated by tensile cracks. After the addition of WTSF, a gradual increase in the proportion of tensile cracks was observed upon loading the modified CPB sample to the pore compaction stage. The three-dimensional localization of AE events showed that the WTSF-modified CPB underwent progressive damage during the loading, and the samples still showed good integrity after failure. Additionally, the response relationship between energy evolution and damage development of WTSF-modified CPB during uniaxial compression was analyzed, and the damage constitutive model of CPB samples with different WTSF contents was constructed. This study provides a theoretical basis for the enhancement of CPB modified by adding WTSF, serving as a valuable reference for the design of CPB constructional backfill.

This paper examines the influence of various factors, including the addition of nano-calcium carbonate particles (chemical processes, C) to CPBs, non-isothermal field curing temperatures (thermal factor, T), and field vertical curing stress (mechanical factor, M), on the self-desiccation capacity of CPB plugs. The assessment of self-desiccation involves the determination of pore water pressure (PWP) and volumetric water content (VWC) in the examined CPBs. Thermo-mechanical-chemical (TMC) column experiments were conducted to investigate the interaction among these factors and their collective impact on the evolution of PWP and VWC within the CPB plug. The findings underscore that each factor, both individually and in combination, significantly affects the magnitude and progression of self-desiccation in CPB plugs. Notably, the factors of elevated field temperature and the introduction of nano-calcium carbonate (NC) to the CPB exert the most substantial influence on the extent and enhancement of self-desiccation, whereas the impact of field curing stress in isolation is comparatively much weaker. The integration of nano-calcium carbonate, especially under non-isothermal conditions and concurrent field stress, emerges as a key contributor to enhanced PWP and VWC dissipation rates. The role of non-isothermal field curing temperature is underscored as a significant contributing factor in the self-desiccation of CPB plugs. The results presented in this paper will contribute to a more cost-effective design of CPB plugs and the improved optimization of CPB mixtures with nano-calcium carbonate particles.

The backfill is the safety barrier of the backfilling stopes, and the uniaxial compressive strength (UCS) of the backfill is the key indicator to keep the stability of the backfill. To investigate the actual distribution of the UCS of the backfill in the stopes, a large-scale nickel mine in Northwest China is taken as the research object, and field experiments are carried out to investigate the actual distribution characteristics of the UCS of the backfill at different locations in the underhand drift stopes with different curing times. Further, the machine learning algorithm is used to construct a spatial-temporal distribution prediction model of the backfill UCS with different locations and different curing times as variables, and with the UCS of the backfill as the prediction target, to identify the weak parts and corresponding strengths of the backfill. The results show that the UCS of backfill has the distribution characteristics of increasing and then decreasing in the horizontal direction, and gradually decreasing in the vertical direction. When the curing time of the backfill in the stopes exceeds 80 days, its UCS growth tendency slows down. Compared with the back propagation neural network (BPNN), radial basis function neural network (RBFNN), and general regression neural network (GRNN), the prediction accuracy of extreme learning machine (ELM) is improved by 4.4 %, 3.4 %, and 13.9 %, respectively. Compared with the grey wolf algorithm (GWO) and particle swarm algorithm (PSO), the prediction accuracy of ELM optimized by the sparrow search algorithm (SSA) is improved by 7.2 % and 8.4 %, respectively. The SSA-ELM spatial-temporal distribution prediction model constructed in this study achieves fast and high-precision prediction of backfill UCS, which can provide guidance for stability analysis and quality control of underhand drift stopes.

In the field of coal mine backfilling, the method of grading the coal gangue and the effect it has on the rheological properties of the cemented paste backfill (CPB) are still to be fully understood. In this study, the CPB is composed of coal gangue as the coarse aggregate, fly ash and cement as the fine aggregate. The properties of the coal gangue were characterised in terms of porosity and friction angle, and the comprehensive influence of different coal gangue grading, volume fraction and CPB mixture ratios on the rheological properties of the CPB was investigated. The experimental results indicate that both large and small coal gangue particle sizes can increase the porosity and friction angle. Compared with single grade and gap grade, coal gangue with continuous grade (G32) has a lower porosity of 42.1 %, a friction angle of 43.8 °, a yield stress of 7.6Nm and a viscosity of 6.5Nm, a higher slump of 221 mm, and a water film thickness of 1.68 μm. The yield stress has been identified as the primary factor influencing the slump of CPB. The yield stress and viscosity of the CPB are negatively correlated with the particle size, mortar ratio, and slump, and positively correlated with the volume fraction of coal gangue. In addition, the significant impact of various factors on the rheological parameters of the CPB is ranked as porosity> mortar ratio >volume fraction>friction angle. The relationship between yield stress, viscosity and water film thickness (WFT) is a power function, and the established WFT model can conveniently and quickly predict the rheological parameters of the CPB.

Solid waste filling is an important development direction for filling mining. This research proposes the preparation of solid waste-based cemented paste backfill (SWCPB) to fill the underground mined area by using waste rock and copper slag as the mixed aggregate and granulated blast furnace slag, steel slag, and desulfurization gypsum as raw materials of solid waste binder to reduce the environmental hazards caused by the industrial solid wastes. Based on the response surface method, experiments are designed to analyze the effect patterns of different factors on the mechanical properties of SWCPB. The formation mechanism of SWCPB mechanical properties is explored based on various microscopic analysis techniques. The mix proportions of SWCPB are designed based on machine learning. The results show that the strength of SWCPB increases and then decreases with the increase of the proportion of copper slag, and gradually increases with the increase of the steel slag dosage and binder-aggregate ratio. The alkaline environment provided by the steel slag causes the glass body in granulated blast furnace slag to depolymerize and react with desulfurization gypsum to produce hydration products, which are the main source of mechanical properties formation in SWCPB. The machine learning model constructed in this research paper has high prediction accuracy for the strength of backfill, which can achieve high-precision optimization of backfill mix proportion. The research results could guide the recycling of solid waste and the efficient use of filling mining technology.

In order to improve the mechanical properties of backfill, steel slag and straw fiber were used as materials to prepare backfill. The law of sand-cement ratio, mass concentration, steel slag content and straw fiber on strength and rheological parameters of backfill was analyzed by orthogonal test method, and the influence mechanism of steel slag content and straw fiber on physical properties of backfill was revealed. The results show that mass concentration exerts the most substantial impact on rheological parameters, emerging as a significant determinant. Moreover, the yield stress diminishes with escalating tailings-cement ratio, while it escalates with higher mass concentration, steel slag content and straw fiber content. Augmenting the tailings-cement ratio and steel slag content serves to impede the amplification effect of mass concentration on yield stress. Notably, the tailings-cement ratio emerges as the most influential factor affecting the uniaxial compressive strength (UCS) of the backfill, albeit the UCS diminishes with rising tailings-cement ratio. Elevating the mass concentration and steel slag content can counteract the debilitation caused by an increase in the tailings-cement ratio on the UCS of the backfill. Furthermore, heightened mass concentration can bolster the reinforcing impact of steel slag content on early UCS. The increase of tailings-cement ratio increases the size and range of the void inside the backfill, subsequently leading to the degradation of its macroscopic compressive strength. The adhesion between straw fiber and mortar matrix constitutes a pivotal factor in improving the integration of backfill, whereas steel slag contributes to improving the mechanical properties by enhancing the distribution of void structures within the backfill. This research not only offer data support and scientific guidance for the design of backfill ratio parameters but also facilitate the broader application of straw fiber and steel slag in mine backfilling operations.

If the cemented paste backfill (CPB) does not contact the roof, it will cause mining safety problems such as roof collapse, and the passive roof contact technology cannot effectively solve the problem. Therefore, the active roof-contact technology based on expansion agent was developed. The orthogonal experiments were carried out to study the influence of mass concentration, tailings-to-cement ratio and expansion agent content on the expansion ratio and mechanical properties of CPB. Through the range analysis of the expansion ratio and unconfined compressive strength (UCS), the significance of influencing factors on the expansion ratio and UCS was obtained. The experimental results show that the tailings-to-cement ratio has the greatest influence on the UCS, and the expansion agent content influence the expansion ratio more. The multivariate nonlinear regression models can accurately predict the expansion ratio and UCS of CPB. It is found that the sensitivity of expansion agent content to the expansion ratio decreases with the increase of tailings-to-cement ratio. According to the requirement of the mine for the expansion ratio and UCS, the expansion ratio and UCS were optimized by multi-objective optimization, and the optimal proportioning schemes suitable for the CPB in mines were obtained. Finally, the expansion mechanism was revealed by scanning electron microscope (SEM), and the evolution of the expansion agent with hydration process was analyzed. This study is of great significance to realize the roof-connect of metal mines, and has important practical value for the safe mining.

The present study conducted an extensive investigation into the durability of Na2SO4-activated binary slag cementitious materials for use in cemented paste backfill (CPB) under sulfate attack conditions. The primary objectives were to assess the feasibility of employing a cost-effective and environmentally friendly binder in CPB applications. Samples were subjected to exposure to Na2SO4 and MgSO4 solutions for a duration of 30 weeks, and multiple aspects including dimension stability, micro and macro mechanical strength evolution, changes in phase assemblage, microstructure following sulfate attack, and heavy metals leaching were examined. The study revealed that Na2SO4-activated binary slag (OPC-slag/S) exhibited superior resistance to sulfate attack, demonstrating enhanced dimension stability. Notably, sample OPC-slag/S displayed an increase in compressive strength, even after prolonged exposure to sulfate solutions for 30 weeks. This increase amounted to approximately 10.76% and 4.81% following exposure to Na2SO4 and MgSO4 solutions, respectively. The heightened resistance of binder OPC-slag/S to sulfate attack can be attributed to its low portlandite content and the early consumption of aluminates, primarily due to the formation of ettringite. This phenomenon effectively prevents the formation of detrimental and expansive products within the cement matrix during sulfate attack. On the other hand, CPB samples with binder OPC-slag/S showed the best performance in heavy metals immobilizaton capacity due to the high resistance to sulfate attack. In conclusion, this study underscores the potential of utilizing OPC-slag/S as a cost-effective and environmentally friendly binder for CPB applications in sulfate-rich environments.

The unconfined compressive strength (UCS) of alkali-activated slag (AAS)-based cemented paste backfill (CPB) is influenced by multiple design parameters. However, the experimental methods are limited to understanding the relationships between a single design parameter and the UCS, independently of each other. Although machine learning (ML) methods have proven efficient in understanding relationships between multiple parameters and the UCS of ordinary Portland cement (OPC)-based CPB, there is a lack of ML research on AAS-based CPB. In this study, two ensemble ML methods, comprising gradient boosting regression (GBR) and random forest (RF), were built on a dataset collected from literature alongside two other single ML methods, support vector regression (SVR) and artificial neural network (ANN). The results revealed that the ensemble learning methods outperformed the single learning methods in predicting the UCS of AAS-based CPB. Relative importance analysis based on the best-performing model (GBR) indicated that curing time and water-to-binder ratio were the most critical input parameters in the model. Finally, the GBR model with the highest accuracy was proposed for the UCS predictions of AAS-based CPB.

The rapid concentration of tailings is a crucial initial step in the cemented paste backfill (CPB) process, as it ensures the stability of the backfilling slurry concentration and the continuity of the backfilling process. The structure of flocs plays a pivotal role in tailings concentration, directly contributing to such critical technical parameters as settling speed and bottom flow concentration. However, most of the current studies focus on flocculant addition rather than the tailings slurry itself. Also, there is a lack of uniform characterization of the flocculant structure in the flocculation and settling system. This study establishes a semi-quantitative assessment system for evaluating the fine structure of tailings flocs based on indicators such as the exposure state of coarse particles, floc size, and floc compactness (porosity). The findings reveal that, during the settling process, the higher true density of tailings (3.156 g/cm3) and intermediate particles (40 wt% of 38 μm–74 μm particles) attenuates the clamping effect of turbulence on settling, leading to a faster settling rate to 0.65 cm/s and 0.29 cm/s respectively. Moreover, in the thickening process, tailings located at the bottom of the mud layer expedite the precipitation of interstitial water within the floc structure through the combined influence of gravity and rake frame shear. Consequently, a denser floc structure forms, as evidenced by a higher concentration of bottom flow at a macroscopic level. This conclusion provides a theoretical basis for faster preparation of highly concentrated tailings slurry in CPB.

Cemented paste backfill (CPB) has been a practical approach for mining waste management, underground goaf treatment, and green mining. Strength deterioration because of high sulfide content (SC) is a severe challenge for sulfide-rich CPB. To investigate the influence of rice straw (RS) on the strength of sulfide-rich CPB, RS was utilized as an inhibitor of strength deterioration. RS dosage (RSD), RS length (RSL), and sulfide content (SC) were chosen as the factors, and two stages of the experiment were carried out. It was found that high SC produced pores, cracks, and loose microstructure of CPB, resulting in the strength deterioration of sulfide-rich CPB. The 90-day uniaxial compressive strength (90 d-UCS) was the maximum of the UCS of sulfide-rich CPB. The hydration products and microstructural characteristics observed by XRD and scanning electron microscopy (SEM) illustrated that RS has the effect of bridging and water retention. Accordingly, adding RS improved the long-term strength of sulfide-rich CPB. The optimal RSD, RSL, and SC for 90 d-UCS of sulfide-rich CPB were obtained through the response surface methodology approach. The optimal conditions within the experimental range of this study for maximum 90 d-UCS of CPB with high SC (17 wt%) were RSD of 0.032 wt% and RSL of 0.28 cm, producing a 90 d-UCS of 5.14 MPa.

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.

Cemented paste backfill (CPB) technology, a densified mixture of mine tailings, binding material, and water, thickens to form a non-settling paste that is transported to fill mined cavities. CPB could provide environmental advantages by preventing the release of contaminants through hydraulic binder stabilization/solidification. Up to now, only few studies have been carried out to examine the feasibility of arsenic (As) fixation within cemented paste backfills. The main objective of the current study is to assess the impacts of incorporating pure arsenic trioxide on the geomechanical and geochemical properties of cemented paste backfill. CPB samples were prepared using pure ground silica (Sil-Co-Sil®) mimicking mine tailings, general use (GU) cement, deionized water, and reagent-grade pure As2O3 at different arsenic contents (0, 5, 10, and 15%). The samples were cured for up to 28 days and unconfined compressive strength (UCS) tests were conducted after 7 and 28 days of curing. Moreover, to evaluate the geochemical behavior of the pastes, mixtures of GU cement, pure As2O3 (0–15%), and deionized water with the same proportions as the CPB samples were prepared and mixed for up to 28 days. Then, pH, EC and chemical composition of the mixtures were evaluated. Results showed that the addition of pure arsenic trioxide (as a partial substitution for pure silica) leads to a substantial decrease in the mechanical strength of the CPB samples. However, this adverse effect was more substantial for the arsenic content of 5 wt%. The strength of the As2O3-containing samples did not improve significantly after 7 days of curing and arsenic addition lowered the pH of the solutions Microstructural analysis using SEM proved the formation of some C–S–H gels that contained arsenic in their structures as well as some complex mixture of oxides for the case of the As2O3-cement mixtures.

Cemented paste backfill (CPB) and rock interface interaction causes the formation of an interfacial loading and affects the thermal, hydraulic, mechanical, and chemical processes in bulk CPB and thus its in-situ behavior. In this study, a new meter-sized column model is developed to systematically investigate the multiphysics processes in CPB under interfacial loading. The obtained results discover that for the mechanical process, the interfacial loading leads to a reduced settlement and a weakened stress level in CPB. For the hydraulic process, lower matric suction and smaller moisture content coexist in CPB under interfacial loading. For the thermal process, the interfacial loading weakens the porosity-dependent thermal conduction and causes retardation in temperature variation relative to the ambient temperature. For the chemical process, weakened cement hydration with smaller electrical conductivity was observed in CPB under interfacial loading. Therefore, the obtained results reveal the linkage between the interfacial loading and multiphysics processes in CPB and thus contribute to an in-depth understanding of the multiphysics behavior of CPB in underground mines.

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

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.

This paper presents an experimental study of the strength and suction development of cemented paste backfill (CPB), which is an innovative cementitious construction material for mining (made by recycling mine waste into a construction material), and modified with nanoparticle (NP) additives. The effects of different amounts of four types of NP additives, including nano-silica (SiO2), nano-calcium carbonate (CaCO3), nano-iron oxide (Fe2O3), and nano-aluminum oxide (Al2O3), on the key engineering properties of CPB are investigated. An ether-based polycarboxylate superplasticizer (SP) is added to the backfill to help the NPs to better disperse in the mixture. Ordinary Portland cement is used as the binder in the CPB mixture. Uniaxial compressive tests (UCS) are conducted to determine the strength of the CPB, while suction monitoring experiments are performed to evaluate changes in suction with time. To understand the effects of the NP additives, different microstructural analyses and tests, including thermal analyses (thermogravimetry (TG), differential thermogravimetry (DTG)), mercury intrusion porosimetry (MIP), and X-ray diffraction (XRD) are conducted on the nano-CPB and the cement paste of nano-CPB. The results indicate that the addition of NP additives in the absence of SP results in lower strength due to high likelihood of agglomeration. In contrast, samples with SP and NP additives show a higher UCS and more suction than the control sample at the early ages of curing. It has been observed that the addition of NP additives results in the generation of more hydration products which enhance the interparticle friction and packing density of the CPB structure. Higher strength is obtained by increasing the SP content (0.25%) with the same NP content (1%). Enhancement of the strength of CPB and increase in suction, particularly at early ages, can have great importance in speeding up the mining cycle and thus increasing mining productivity, which is obviously associated with financial benefits to the mine.

Reducing solid waste emission is the development direction of cleaner production in the future. This paper proposes to use the agricultural solid waste rice husk ash (RHA) as an alternative cementitious material for the underground filling of mines. The pore structure and mechanical properties of cemented paste backfill with different RHA dosages (RCPB) are analyzed. The results reveal that: (1) The pore structure of RCPB is divided into three types: micropores (100 μm). The total number of micropores accounts for the largest proportion, followed by secondary pores and main pores. The total volume of secondary pores accounts for the largest proportion, followed by micropores and main pores. (2) The porosity decreases and the UCS increases with the increase of RHA dosage and curing time. The UCS decreases linearly with the increase of RHA dosage. The UCS has a linear negative correlation with porosity. (3) The cracks of RCPB initiated at the yield point, gradually expanded at the peak point, and finally penetrated at the post-peak point. The failure mode of RCPB is mainly a tensile failure, accompanied by a small number of secondary tensile cracks. (4) The hydration products of RCPB are mainly C–S–H and AFt gels. The greater the RHA dosage and CT, the more hydration products, the fewer pores, the greater the strength, and the greater the average gray value. The outcome of this work will afford a new ideas and methods for using solid waste (Rice husk) as backfilling.

In this paper, the internal pore structure and mechanical properties of cemented paste backfill (CPB) with different mix proportions are studied by low field nuclear magnetic resonance (NMR) and unconfined compressive strength (UCS) test, and the relationship between UCS, pore structure characteristics and curing time is quantitatively characterized. The following conclusions are obtained: (1) The internal pores of CPB are divided into micropores, secondary pores, and macropores, among which secondary pores account for the largest proportion, accounting for about 70% of the total pores. The proportion of macropores is inversely proportional to the UCS, while the proportion of micropores is proportional to the UCS. (2) The porosity of CPB decreases with the increase of curing time, and UCS increases with the increase of curing time. The curing time has an exponential function relationship with porosity. UCS decreases exponentially with the increase of porosity. (3) A damage model of CPB is established considering the initial pore damage, which is highly consistent with the experimental results. The total damage evolution curve presents an “S'' type distribution. The total damage change rate evolution curve showed a normal distribution.

This work systematically investigates the effects of coarse aggregate (CA) type, dosage, and slurry concentration on the backfill slurry's setting, bleeding, and hardening strength properties. Rod milling sand (RMS) and waste rock (WR) were selected as CA, the backfill slurry's concentration ranged from 76% to 79%, its replacement rate of tailings with RMS and WR was between 40% and 60%, and its cure period was 3, 14 and 28 days. Then, the fresh backfill slurry was subject to setting and bleeding experiments, and the hardened backfill slurry was submitted to UCS tests. Finally, a CPB curing time of 28 days was chosen for microstructure experiments to reveal the mechanism of the effect of slurry concentration and CA dosage on CPB hardened strength. The results showed that the initial and final setting times reduce as concentration increases and increase as tailings/CA (T/CA) decreases. The effect of concentration and CA dosage on the bleeding properties of backfill slurry is not apparent. Increasing the slurry concentration increases the amount of C-S-H gel inside the CPB, which improves its mechanical behavior. The CPB has optimal physical properties when the slurry concentration is 77%− 78%, and the amount of WR is 50%. The study results can provide a theoretical basis for the preferential selection of backfill materials in mines.

Mine water is rich in various chlorides, which will inevitably affect the performance of the cemented paste backfill (CPB). In this study, the TAM Air isothermal calorimeter was used, the effects of typical chlorides (NaCl, CaCl2 and MgCl2) in mine water on the hydration heat and hydration heat flow in the solidification stage of fresh CPB were studied, and the effects of chlorides on the setting time of fresh CPB were analyzed. The hydration products of CPB samples containing different chlorides were tested by the X-ray diffractometer and thermal gravimetric analyzer. The results show that NaCl and CaCl2 can promote the early hydration reaction of CPB; and the promoting effect of CaCl2 is more significant; while MgCl2 consumes OH- in the solution and reduce the pH of the solution, thus inhibiting the early hydration reaction of CPB. With the increase of chloride concentration, the promoting effect of NaCl on the early hydration of the CPB first increases and then decreases, the promoting effect of CaCl2 increases constantly, and the inhibitory effect of MgCl2 also increases continuously. Besides, CaCl2 can significantly shorten the hydration time of CPB in the induction stage and acceleration stage, and the higher the CaCl2 concentration, the higher the cumulative hydration heat and the hydration heat release rate of CPB in the same hydration stage. This study comprehensively evaluates the influence of typical chlorides in mine water on the early hydration process and setting time of the fresh CPB, providing a reference for the clean utilization of mine water, fly ash, gangue and other solid wastes.

In this study, a series of experiments was conducted to examine the mechanical and damage properties of cemented paste backfill (CPB) with different water saturations (ws = 0%, 22.1%, 47.0%, 65.3%, 82.8%, and 100%). The results reveal that the UCS, peak strain, and elastic modulus first increased and then decreased, and the porosity and fractal dimension of cracks first decrease and then increase with the increase of ws. In addition, the UCS, peak strain, and elastic modulus is maximum, and the porosity and fractal dimension are minimum when the ws is 65.3%. The UCS increases with the increase of peak strain and elastic modulus, and decreases with the increase of porosity and fractal dimension, and the UCS present good quadratic polynomial functions relationship with the porosity and fractal dimension. The SEM images show that with the increase of ws from 0% to 65.3%, the hydration products increases and the pore structure decreases. When the ws exceeds 65.3%, and the pore structure begins to increase. The failure mode of the CPB is mainly tensile failure. A new damage constitutive model of CPB with different ws was constructed. The total damage D of the CPB is composed of initial damage Dp and load damage DL. The total damage evolution curve shows an S-shaped distribution, and the total damage rate evolution curve shows a normal distribution. With the increase of ws, the damage rate and the peak value of damage rate of CPB first increases and then decreases.

Slag-based cementitious material was synthesized from blast furnace slag, clinker, gypsum, and activator to replace cement in cemented paste backfill (CPB). We researched the influence of slag-based cementitious material dosages and curing times on the properties of CPB, including unconfined compressive strength tests, leachate toxicity and chemical speciation of heavy metal as well as microstructural tests and analyses. The results indicated that the addition of slag-based cementitious material improved the compressive strength of the CPB, which attained the compressive strength requirements (≥1.0 MPa) at 28 days. The leachate concentrations of Pb, Cr, Cu, and Cd in CPB decreased as the slag-based cementitious material dosage and curing period increased, which met the standard (GB 5085.3-2007). The dosage of 10% slag-based cementitious material could effectively immobilize the heavy metals in the tailings, and the immobilization performance was similar to that of 20% cement, which indicated the amount of slag-based cementitious material was only half the quantity of cement in CPB. Microstructural analysis showed the hydration products included calcium silicate hydrate, ettringite, and portlandite, which could enhance the bonding force between the tailing grains.

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.

The Cemented Paste Backfill (CPB) yield stress is a key rheological parameter for paste filling technology, which has significant practical value for pipeline optimization and equipment selection of pipeline conveying systems. However, the slurry yield stress is affected by many factors. In order to accurately analyze and predict the CPB yield stress, this study uses the sparrow search algorithm to optimize the relevance vector machine (SSA-RVM) and proposes the prediction model of SSA-RVM CPB yield stress regression. Based on 136 sets of rheological tests for copper mine, different waste rock/tailing sand ratios, mass concentrations, and water-cement ratios select to predict the yield stress at different training set ratios (78%, 85%, 92%). Compared with the traditional relevance vector machine (RVM) regression model, the SSA-RVM regression prediction model has higher prediction accuracy. In addition, the coefficient of determination R2 of the predicted and true values obtained from the SSA-RVM model increased by 0.0407, 0.0438, and 0.0500 for the training set ratios of 78%, 85%, and 92%, respectively. The results suggested that SSA-RVM can efficiently predict the CPB yield stress, which can be a reference for the design of paste-filled pipe conveying systems.

The stability of cemented paste backfill (CPB) is threatened by dynamic disturbance, but the conventional low strain rate laboratory pressure test has difficulty achieving this research purpose. Therefore, a split Hopkinson pressure bar (SHPB) was utilized to investigate the high strain rate compressive behavior of CPB with dynamic loads of 0.4, 0.8, and 1.2 MPa. And the failure modes were determined by macro and micro analysis. CPB with different cement-to-tailings ratios, solid mass concentrations, and curing ages was prepared to conduct the SHPB test. The results showed that increasing the cement content, tailings content, and curing age can improve the dynamic compressive strength and elastic modulus. Under an impact load, a higher strain rate can lead to larger increasing times of the dynamic compressive strength when compared with static loading. And the dynamic compressive strength of CPB has an exponential correlation with the strain rate. The macroscopic failure modes indicated that CPB is more seriously damaged under dynamic loading. The local damage was enhanced, and fine cracks were formed in the interior of the CPB. This is because the CPB cannot dissipate the energy of the high strain rate stress wave in a short loading period.

The pumping ability and placement performance of fresh cemented paste backfill (CPB) in underground mined cavities depend on its rheological properties. Hence, it is crucial to understand the rheology of fresh CPB slurry, which is related to CPB mixture design and the temperature underground. This paper presented an experimental study investigating the effects of binder type, content, water chemical properties and content, and temperature, on the rheological properties of CPB material prepared using the tailings of a copper mine in South Australia. Portland cement (PC), a newly released commercially manufactured cement called Minecem (MC) and fly ash (FA) were used as the binders added to the mine tailing materials. Various amounts of two different water types were added to the mixtures in the preparation of backfill material slurry. Six different temperatures ranging from 5 to 60 °C were to investigate the effect of temperature on CPB rheology. Overall, the increasing water content and decreasing temperature lead to lower yield stress. Based on the results obtained from the rheological properties of CPB slurry, it was found that at room temperature (25 °C), with regards to the unconfined compressive strength (UCS) performance, the replacement of 4% PC mixed CPB (28 days UCS 425 kPa) to 3% MC mixed CPB (28 days UCS 519 kPa), reduced the slurry yield stress from 210.7 to 178.5 Pa. The results also showed that the chemical composition of water affects the yield stress of CPB slurry and that MC mitigates the negative effect of mine-processed water (MW) and thus lead to improve the rheological properties of the slurry. However, the results suggested that the rheological properties of a mixture using MC is very sensitive to the water volume and temperature change. Therefore, using MC in backfill requires better quality control in slump mixing.

Urbanization and economic development have increased the demand for fertilizers to sustain food crop yields. Huge amounts of by-products, especially phosphogypsum (PG), are generated during the wet processing of rock phosphate to produce fertilizers. Chronic exposure to fluoride in phosphogypsum in groundwater as a result of the weathering of fluoride-containing waste poses a significant health risk to millions of people. We propose a method for using calcium aluminate cement (CAC) to remediate high fluoride contents in solid waste. Column leaching tests under harsh rainfall conditions confirmed the efficient fluoride immobilization capacity of a CAC binder. Although the fluoride concentrations in leachates during the first 1-2 days (1.25 mg/L) slightly exceeded the threshold of 1.00 mg/L, the concentrations over 3-28 days (ranging from 0.98 to 0.83 mg/L) consistently remained well within the acceptable range. Furthermore, our characterization and geochemical modeling revealed the fluoride retention mechanisms of CAC-stabilized PG under laboratory-simulated conditions of torrential rainfall. During leaching, physical encapsulation prevents fluoride from contacting leachate. However, an unfavorable pH value can cause the release of fluoride from the cement matrix, which is subsequently captured by aluminate hydrate through adsorption or co-precipitation. We quantified the carbon footprint of CAC for immobilizing 1 mg of fluoride in PG, obtaining a remarkably low value of 4.4 kg of CO 2 , in contrast to the emissions associated with the use of ordinary Portland cement (OPC). The findings suggest a unique opportunity for extensive PG remediation. This opportunity extends the horizons of achieving zero-waste emissions in the phosphorus industry and has practical significance in the context of reducing carbon emissions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Caving mining method could lead to massive waste rocks hauled to surface while leaving a large void in underground. This would eventually result in the surface subsidence and damage to the environment and surface infrastructures. In this study, we proposed three different backfilling methodologies to minimise the surface subsidence being 1) 100 % mining and 100 % backfilling (method 1); 2) leaving one slice of coal between two backfilled slices (method 2) and 3) leaving one slice of coal between one backfilled slice (method 3). The backfilling materials are made of waste rock, fly ash and cement and the optimal ratio has been found through the test program designed based on the orthogonal experiment design method. The strength of the backfilling paste is 3.22 MPa at the axial strain 0.033. The mine scale numerical simulation has also been conducted and it was concluded that the method 1 would lead to 0.098 m roof deformation in underground roadway whereas the method 2 and method 3 only induced a roof deformation around 32.7 % and 17.3 % of that induced by the method 1, respectively. All three methodologies have been approved to minimise the roof deformation and disturbance to the rock by mining operations. At last, the surface subsidence has been scientifically evaluated based on the probability integration method of surface movement. It indicated that the surface subsidence, horizontal movement, inclined movement and curvature of rock surrounding the panel void were all below the minimum value required by regulation. This confirmed that the selected backfilling mining is able to ensure the integrity of the surface infrastructures. This technology provides a new way to control the surface subsidence caused by coal mining., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Large volumes of carbon dioxide are released during mining and ore resource development, and cemented paste backfill (CPB) materials are placed in the mined-out stopes where can be discharged from polluted air containing CO 2 . The construction of green mines and the goal of achieving carbon neutrality have become an inevitable choice for the mining industry to achieve the harmonious development of rational exploitation of resources and environmental protection. Against this background, to minimize the carbon emissions from the mining industry and promote the efficient utilization of CPB, this study investigated the carbon-uptake characteristics and mechanical property of CPB in underground mined-out stopes with 1.5 % concentration CO 2 . The results show that the carbonation curing (CC) increased the carbonation rate by nearly 4 times compared to natural curing, while the samples exhibited total carbonation within 28 days. This indicates that CO 2 uptake could occur within the CPB. The CO 2 was absorbed as calcium carbonate minerals, and each ton of CPB can ideally absorb about 78.4 kg CO 2 and treat 2600 m 3 of dirty air in the mined-out stopes. The increase in early uniaxial compressive strength (UCS) during CC required a higher cement concentrate, and the CC would retard the development of later compressive strength. Microstructure analysis indicated that the CC refined the pore structure and reduced the porosity of the CPB. It also affected the crystal growth and distribution of hydration and carbonation products, further influencing the difference in strength. In summary, CPB technology can potentially be useful during carbon uptake and may assist in mitigating carbon emissions from the mining industry and promoting environment friendly development., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022. Published by Elsevier B.V.)

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., (Copyright © 2022. Published by Elsevier Inc.)