Strength Of Cemented Paste Backfill Research Articles

Mechanical response and damage constitutive model of early-age cemented paste backfill after cyclic loading

During mining operations, cemented paste backfill (CPB) is damaged in real time by different loading gradient, loading rate and loading time. And there is variability in the degree of damage and destruction of CPB by different loading conditions. This paper carries out uniaxial compression and ultrasonic velocity tests to investigate the uniaxial compressive properties and damage characteristics of CPB after cyclic loading. The results show that when the loading rate is 1.3 mm min−1, the loading gradient is σ0.5 and the loading time is 300 s, the uniaxial compressive strength (UCS) of CPB at 3, 5 and 7 days of curing age is the maximum, and the values are 1.7335, 4.0325 and 6.5301 MPa, respectively. The value of ultrasonic velocity variation is exponentially related to that of UCS. The CPB dissipation energy (DE), elastic strain energy (ESE), and unit volume strain energy (UVSE) improved with curing age raising. Under the age of 3 d, 5 d and 7 d, the failure mode of CPB is X shear- X-conjugate shear - Y shear. The correction coefficient α and coefficient of compaction η were introduced, and the two-stage damage constitutive model of CPB after cyclic loading was constructed based on the Weibull distribution, with the model validated. The magnitude of the correction coefficient α affects the peak strength and the residual stress duration strain length of the model, and has less effect on the damage development. When the value of α ranges from 0.90 to 0.92, the model curve is closer to the test curve.

Effects of Different Ratios of Fly Ash and River Sand on Mechanical Properties of Cemented Paste Backfill Materials

The mechanical properties of cemented paste backfill (CPB) are crucial for stope safety. Some mining areas in the Yellow River basin face a shortage of coal gangue, and these areas have abundant river sand (RS) resources. The use of RS as a fine aggregate can optimize the particle gradation of the material and improve the strength of the material. The effects of different fly ash (FA) and RS ratios on uniaxial compressive strength (UCS), elastic modulus, and microstructure of CPB were studied using uniaxial compression test and scanning electron microscopy. The results showed that with increasing FA content, UCS gradually increased at 7 and 14 days of CPB. However, at 28 days, the UCS of CPB first increased and then decreased. When the FA content was 40%, UCS at 28 days of backfill material reached its highest value of 14.76 MPa. The elastic modulus decreased as FA content increased and reached its maximum value of 1.54 GPa when FA content was 40%. FA optimized particle composition, promoted hydration reaction, and improved CPB strength; however, when FA content was more than 40%, it hindered cement hydration, thus deteriorating UCS. Based on Weibull distribution and prepeak correction factor, a damage constitutive model was established to describe the stress–strain process of CPB under load. The results of this research will provide a theoretical and experimental basis for CPB engineering design.

Deeper Understanding of the Strength Evolution and Deformation Characteristics of Sodium Silicate–Cemented Paste Tailing Material

This research aims to examine the impact of sodium silicate (SS) on the mechanical and microstructural properties of cemented paste tailings or cemented paste backfill (CPB), a cementitious construction material extensively used in underground mining operations. The study involved various compositions and curing conditions of SS-CPB samples, and their uniaxial compressive strength (UCS), stress–strain behavior, microstructure, and modulus of elasticity were evaluated by conducting mechanical (UCS) and microstructural (DT/DTG, MIP) tests, as well as monitoring experiments. Findings indicate that SS improves the mechanical strength of CPB and enhances its microstructure. The development of UCS is affected by SS dosage, curing time, water chemistry, curing temperature, and curing stress. Higher SS dosage, curing time, temperature, and stress lead to higher UCS, while sulfates decrease UCS. SS also increases CPB’s resistance to sulfate attack, and field curing temperature improves the binder hydration enhancement induced by SS. The presence of SS affects the stress–strain properties of CPB, including the shape of the stress–strain curve, maximum stress and strain, and modulus of elasticity. The modulus of elasticity of CPB with SS is higher than that without SS under field thermal curing conditions. Moreover, UCS and the modulus of elasticity have a linear relationship in SS-CPB, regardless of SS content. A relationship is proposed to estimate the modulus of elasticity of SS-CPB from its UCS. The study has significant practical implications for the cost-effective design of mine CPB structures and for improving underground mine work safety and productivity.

Data-driven approach in investigating and predicting unconfined compressive strength of cemented paste backfill

Eight machine learning (ML) models including 6 shallow ML including XGB, GB, RF, LGB, SVR, KNN and 2 hybrid ML models consisting of XGB_P (Particle Swarm Optimization) and XGB_R (Random-Restart Hill Climbing) were trained on the dataset collected from the literature for predicting unconfined compressive strength (UCS) of Cemented paste backfill (CPB). The dataset includes 359 samples and 8 input variables including of D50, Cu, SiO2, Al2O3, Fe2O3, Cement/tailing, Solid content, and Curing age. Three shallow ML models with the best performance are XGB, GB, and RF. The model with the best predictive performance is XGB with R2 = 0.9889, RMSE= 0.1344 MPa, MAE= 0.0925 MPa, and MAPE= 0.3761% observed on the testing dataset. The obtained XGB model has higher performance than the GB model with R2 = 0.9273 proposed in the previous investigation for predicting UCS of CPB. Shapley Additive Explanation (SHAP) and Partial Dependence Plot 2D are used for quantify the input variables effect on the UCS of CPB. The most important influenced input variables are cement/tailing, curing age, solid content, and Fe2O3. The cement/tailing feature has the greatest influence on the head variable and has a positive effect on the UCS of CPB. Fe2O3 content is the feature that is inversely related to the UCS of CPB. The modification of Fe2O3 content has a relatively large influence not only on the development of UCS of CPB, the smaller the Fe2O3 content, the higher the UCS of CPB. The results of this study are the basis for the establishment of Machine learning-based soft computing and the basis for selecting the mix design of CPB. That makes it easier for engineers to design CPB's component, as well as reduce testing costs and construction time.

Strength, Deformation Behavior, and Microstructure of Cement Paste Backfill with Calcium Chloride in Subzero Environments

This manuscript presents and discusses the results of an experimental study on the deformation behavior, strength, and microstructural development of cemented paste backfill (CPB) with chloride-bearing antifreeze (calcium chloride, CaCl2) in subzero environments. CPB samples with CaCl2 at various concentrations (0, 5, 15, and 35 g/L) were cured at different subzero temperatures (−1, −6, and −12°C) that represent temperatures to which CPBs may be exposed in mines located in permafrost or cold regions. Unconfined compressive strength (UCS) tests were conducted after specific curing time (7, 28, 60, and 90 days). Moreover, microstructural analyses and monitoring experiments (for volumetric water content and matric suction) were performed. The results showed that the addition of calcium chloride antifreeze reduced the strength of CPB and had a significant impact on its stress–strain behavior and microstructural characteristics. Furthermore, it was also found that the strength of CPB decreased with an increase of calcium chloride concentration and subzero curing temperature. These findings provide important guidance for mine backfilling work of CPB with chloride-bearing antifreeze additives and productivity improvement in subzero environments.

Application of deep neural network in the strength prediction of cemented paste backfill based on a global dataset

With the development of the mining industry, the increasing global content of tailings is causing environmental pollution and the use of tailings as a cemented paste backfill (CPB) for underground filling is an effective method of disposal. The application of the backfilling mining method is heavily dependent on the stability of the CPB. However, experimentally evaluating the unconfined compressive strength (UCS) of CPB is time-consuming and expensive. This study discussed the feasibility of using a deep neural network (DNN) model to predict the UCS of CPB. This study established the first global dataset of UCS values of CPB through a literature search and laboratory studies, for the first time. The dataset comprised 986 CPB samples, where each sample included 14 feature values and a target value. The dataset was divided into three parts, i.e., a training set, a validation set, and a testing set, which were used to build, validate, and test the model. The correlation coefficient (R), coefficient of determination, mean absolute error, and mean square error metrics were used for model evaluation. After the hyper-parameter tuning, the number of layers in the DNN model was 4, the learning rate was 0.001, and the dropout rate was 0.25. The optimal DNN model achieved excellent predictive performance on this dataset, with R values on the training, validation, and testing sets of 0.989, 0.964, and 0.967, respectively. The feature importance was analyzed using permutation importance and the Shapley Additive Explanation value, with the cement–tailings ratio found to be the most important feature value with the greatest impact on the model output. This study provides, for the first time, a UCS prediction based on the global CPB dataset, a method that can quickly and accurately obtain the UCS of CPB. The method can save a lot of experimental time and provide a theoretical reference for subsequent environmental management as well as tailings utilization, and promotion of paste filling.

Strength and suction development of nano-cemented paste tailings materials

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.

Utilization of rice straw as an inhibitor of strength deterioration of sulfide-rich cemented paste backfill

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.

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

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

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

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

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.

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

Mechanical strength and hydration exothermic behavior of cemented paste backfill with early-strength agent under low temperature

ABSTRACT Low ambient temperature is not conducive to hydration of cement material, resulting in low strength for cemented paste backfill (CPB). Therefore, the aim of this study is to investigate the effect of early-strength agent (ESA) on mechanical strength and hydration exothermic behaviour of CPB under low temperature. Influence factors including the ESA types, superposing patterns and dosages were evaluated on CPB by conducting unconfined compressive strength (UCS), hydration exothermic tests and thermogravimetric analysis. The obtained results show that with exception of sodium chloride (NaCl), the single ESAs of sodium sulphate (Na2SO4), calcium chloride (CaCl2), lithium bromide (LiBr) and triisopropanolamine (TIPA) exert positive effect on UCS of CPB. The optimal dosages of Na2SO4, CaCl2, LiBr and TIPA are 2.0-, 1.0-, 0.3- and 0.08- wt.%. By means of superimposing optimisation, the binary ESA that consists of 0.08 wt.% TIPA and 2.0 wt.% Na2SO4 is obtained. Adding different ESAs at optimum dosages can facilitate hydration reaction of cement. Both hydration exothermic rate, cumulative heat release and internal temperature increase, thereby more amount of hydration products form inside CPB. This contributes CPB to obtain higher initial temperature and UCS under low temperature.

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.

Optimization of Parameters for Rheological Properties and Strength of Cemented Paste Backfill Blended with Coarse Aggregates

Cemented paste backfill (CPB) technology is widely used for environmental protection and underground goaf treatment. The influences of solid concentration, coarse aggregates dosage, and cement dosage on the rheological properties and compressive strength of CPB blended with coarse aggregates (CA-CPB) are investigated through three-factor and four-level orthogonal experiments. The dynamic shear stress and plastic viscosity are selected to characterize the rheological properties of CA-CPB. The uniaxial compressive strength (UCS) is used to describe the compressive strength. The effect of each factor on rheological properties is different from that on UCS. The most significant influences on rheological properties and UCS are solid concentration and cement dosage, respectively. The optimal levels of each factor for rheological properties and UCS are different, resulting in different optimal combinations obtained through range analysis. Therefore, the overall desirability function approach is employed to perform multiple response optimization. The optimal parameters for high fluidity and strength obtained provide valuable information for the CA-CPB process in the Chifeng Baiyinnuoer Lead and Zinc Mine.

Experimental Study on Direct Tensile Properties of Cemented Paste Backfill

Cemented paste backfill (CPB) has been increasingly utilized in mines for efficient mineral obtaining and regional ground support. To guarantee the work performance, the mechanical properties of CPB have long been a topic of study among researchers. But the research progress on the tensile strength of CPB is limited, mainly because of the lack of an appropriate test method due to the low tensile strength of CPB. Therefore, instead of the conventional splitting indirect tensile strength test method, a new direct tension test method, which utilizes the specifically designed compression to tension load converter (CTLC) and dog-bone-shaped specimen, has been applied to study the direct tensile properties of CPB. In this study, the direct tensile strength (DTS) of 47 CPB mix designs were measured using CTLC, and the unconfined compressive strength (UCS) of the corresponding mix design was also tested. The experimental results showed that the increase in the binder content, solid mass content, and curing period led to higher CPB direct tensile strength, and the DTS of CPB was most sensitive to the binder content. Furthermore, the influence of the slurry mass solid content on the tensile strength of CPB was not linear. The influence of the binder content became increasingly notable with the increase in the solid content, especially if the binder content exceeded 75%. The effect of the curing period was found to be rather marginal due to the decreasing amount of un-hydrated cementitious materials left with the increase of the curing period. Overall, the DTS generated using dog-bone specimens and the CTLC apparatus are valid for better mine backfill designs. Finally, a linear correlative between UCS and DTS with a formula in the form of σDT (DTS) = 0.171 σc (UCS) was obtained, and the correlation was sufficient for further calculation of DTS using measured UCS.

Strength, Microstructure, and Deformation Behavior of Frozen Cemented Tailing Material

Cemented paste backfill (CPB) is a novel cementitious construction material that is used extensively for underground mine support and/or mine waste management worldwide. CPB consists of a mix of tailings (mine waste), water, and binder. In other words, tailings are transformed into construction materials. Key engineering properties required for the geotechnical design of CPB structures include strength and deformation behavior. However, there is a paucity of technical data and information about the effect of sub-zero temperatures on these properties. This paper presents an experimental study that focuses on the investigation of the influence of sub-zero curing temperatures on the strength, deformation behavior, and microstructure (e.g., pore structure, porosity, and binder hydration products) of CPB. Several CPB samples with various combinations of compositions (e.g., binder type, cement amount, and water content) cured at different temperatures (sub-zero and room temperatures) and times (7–90 days) are tested with regard to strength, stress–strain behavior, and microstructural characteristics. It is found that frozen CPB (FCPB) exhibits remarkable strength compared with unfrozen CPB and has a great resemblance to frozen soil. No obvious peak stress is observed from FCPB samples during compression, and strain hardening phenomena are commonly seen after the initial yield point. Factors that may affect the behavior of FCPB are thoroughly examined. Binder contents and types are found to be irrelevant; in contrast, water content plays a dominant role. This negligible effect of binder on the properties of FCPB is due to the inhibition of the binder hydration by sub-zero curing temperatures. FCPBs show coarser pore structure than unfrozen CPBs. The results obtained in this study have significant practical engineering applications for mines located in permafrost or cold regions.

Insight into the isothermal multiphysics processes in cemented paste backfill: Effect of curing time and cement-to-tailings ratio

Due to the complexity of multiphysics processes, cemented paste backfill (CPB) materials begin the “black box” curing period after placement into underground stopes. Moreover, due to the limited interference in the thermal process, CPB materials may experience locally isothermal curing conditions in underground excavations. Therefore, to yield an optimal design of CPB, it is necessary to investigate the isothermal multiphysics processes in CPB. In this study, the evolution and characteristics of isothermal multiphysics processes of CPB under the different cement-to-tailings ratios are studied. Through SEM, XRD, TGA, and MIP micro-analysis, the hydration products and pore structure of CPB at different curing times and cement-to-tailings ratios are analyzed. Moreover, an isothermal multiphysics monitoring program is implemented to quantitatively study the evolution of matric suction, volumetric water content, and electrical conductivity at various curing conditions. Based on the measured data, it has been found that the mechanical strength of CPB shows a strong dependency on matric suction, volumetric water content, and electrical conductivity. The correlation among multiphysics processes under isothermal conditions is established through regression analysis on the measured data. The findings have the potentials to improve the understanding of the complex field behavior of CPB and contribute to its optimal design.

Fresh and hardened properties of cemented paste backfill: Links to mixing time

Current technical knowledge is insufficient regarding the influence of mixing time on fresh and hardened properties of cemented paste backfill (CPB). This work focused on experimental studies to gain a deeper understanding of the influence of mixing time on the rheological properties (yield stress, viscosity), self-desiccation, permeability and mechanical properties or behavior (strength, deformation behavior) of CPB. A series of CPB specimens were prepared with various mixing times (1, 2, 4, 7, 10, and 15 min) and cured at different times (1, 2, 4, 7, 10 and 15 min) for rheological property testing. Unconfined compressive strength (UCS), hydraulic conductivity and microstructural tests were performed on CPBs that were made with various mixing times and cured for different times (up to 90 days). Monitoring experiments were also carried out on CPBs prepared with various mixing times. The results obtained indicate that the mixing time significantly affects the rheological, mechanical, hydraulic and microstructural properties of CPB. Moreover, the self-desiccation of CPB is a function of the mixing time. A longer mixing time increased the yield stress and decreased the viscosity. The longer mixing time promoted binder dissolution and hydration in the CPB, which increased the amount of hydration products and decreased the volumetric water. Longer mixing time intensified the CPB self-desiccation, which is beneficial for the early age mechanical strength of CPB. CPB specimens made with longer mixing time experienced a higher strength and elastic modulus due to the generation of more cement hydration products and the reduction of the volume of pores in CPB. A longer mixing time reduced the CPB hydraulic conductivity, due to the refinement of the CPB pore structure. This study provides relevant knowledge and technical information that will contribute to the design of more cost-effective and sustainable CPB structures.

Strength investigation of the cemented paste backfill in alpine regions using lab experiments and machine learning

Cemented paste backfill (CPB) is increasingly used in mines in alpine areas. The strength of CPB is unstable due to the influence of multiple factors such as low temperature. In order to obtain the uniaxial compressive strength (UCS) of CPB under the action of different factors, a large number of laboratory tests had to be conducted in the past, which is time-consuming and laborious. In this paper, a research on the UCS is conducted by means of a combination of indoor tests and machine learning. The sparrow search algorithm is used to optimize the extreme learning machine, thus solving the uncertainty of the extreme learning machine in determining the initial weights and thresholds. The strength prediction model of CPB based on SSA-ELM model is constructed with the curing temperature, sand-cement ratio, slurry concentration and curing time as input variables and UCS as output variables. Compared with the original ELM, the PSO-ELM and BP-ANN, the correlation coefficients improved from 0.8817, 0.9511, 0.9187 to 0.9979, which indicates that the SSA-ELM model has better prediction accuracy. In addition, correlation analysis shows that the low temperature(<20 °C) environment has the largest effect on the UCS, which affects the development of UCS by influencing the process of hydration reaction.