Cemented Paste Backfill Structures Research Articles

Sulphate influence on strength development of cemented paste backfill with superplasticizer under field-like curing conditions

This study investigates the impact of sulphate on the strength development of cemented paste backfill (CPB) with polycarboxylate ether-based superplasticizers (PES) under curing conditions mimicking real field environments. These conditions include thermal (T, non-isothermal field temperature), hydraulic (H, drainage), mechanical (M, overburden stress), and chemical (C, PES, sulphate ions) factors. CPB samples with varying sulphate and PES contents were examined through mechanical and microstructural tests under these THMC conditions. The findings reveal that traditional methods for assessing sulphate influence on CPB strength, typically under standard lab conditions, do not adequately capture the mechanical response of CPB structures to sulphate in field scenarios. Real field curing conditions such as temperature, drainage, and stress significantly affect CPB’s mechanical response to sulphate. CPB samples incorporating PES and sulphate show notable improvements in early-age strength, emphasizing the beneficial role of elevated temperatures and the complex relationship between drainage, sulphate, and strength development. Elevated curing temperatures counterbalance sulphate's negative effects, boosting initial strength, while drainage conditions play a critical role in strength development by facilitating the removal of sulphate ions and reducing capillary water. Additionally, stress application at the start of curing aids solid particle rearrangement, promoting the release of free water and enhancing mechanical strength. The competitive adsorption between sulphate ions and PES on the cementitious matrix influences the dispersion of particles and strength development. This research offers crucial technical insights for designing durable CPB mixes suited to various on-site curing conditions, particularly enhancing initial strength across diverse field curing scenarios.

Multiphysical testing of strength development of cemented paste backfill containing superplasticizer

Current laboratory procedures for curing and testing the mechanical strength of cemented paste backfill (CPB) do not take into account the complex Multiphysics (thermal, T; hydraulic, H; mechanical, M; chemical, C) processes that CPB structures are subjected to in the field. This oversight can lead to unreliable measurements and unsafe designs. In this study, a multiphysical curing and testing procedures for CPB with superplasticizer (CPB-PES) has been developed to evaluate its strength development under THMC curing conditions close to those encountered in the field. The obtained results demonstrated that the strength development of CPB-PES is greatly affected by the THMC factors and their interactions. The contributions of each one of the investigated THMC factors on the strength are not equally similar and greatly depend on the interaction between these factors and curing time. CPB samples with 0.125 % PES that underwent drainage during THMC curing showed a strength increase of up to 809 % after 28 days of curing, compared to the control samples. The strength of CPB-PES samples cured under THMC can be up to 57 % higher than that of samples cured under THC conditions. The results indicate a significant interaction between thermal (T; elevated field curing temperature) and chemical (C; superplasticizer and cement hydration) factors, between chemical (C) and mechanical (M; field curing stress) factors, as well as between thermal and mechanical factors. The influence of the mechanical factor on strength development was observed to be less pronounced compared to the impact of chemical and thermal factor, and is reduced at elevated curing temperatures. The findings underscore the critical importance of accounting for field-relevant THMC factors and their interactions in the determination of the CPB-PES strength development, which is essential for the design of safer and more economical CPB structures, ultimately enhancing mine productivity and safety.

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.

Effect of curing humidity on the deformation and mechanical properties of cemented paste backfill

ABSTRACT The influence of underground humidity on the mechanical performance of cemented paste backfill (CPB) in metal mines is a problem that cannot be ignored. To this end, this paper conducted the curing experiments of CPB samples under different curing humidity (60%, 80% and 95%). Physical properties such as moisture content and dry density as well as mechanical properties such as uniaxial compressive strength, cohesion and internal friction angle of the CPB samples were monitored during fixed curing time (3, 7, 14 and 28 days). TGA/DTG, XRD and SEM were used to analyse the microstructure of CPB samples under different curing humidity at 3 and 28 days, and a BP neural network prediction model of CPB strength was established based on the obtained data. The results show that the curing humidity has a great influence on the physical and mechanical properties of the paste, and the amount of hydration products and the change of pore structure are the main reasons that affect the properties. In order to ensure the safety and economy of the CPB structure, the influence of environmental humidity on the paste should be considered in the filling design.

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.

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.

Mathematical modelling and simulation for hydrating backfill body under cemented paste backfill/rock interface loading

ABSTRACT Cemented paste backfill (CPB)/rock interface shear resistance results in a spatiotemporal stress reduction in CPB and thus affects the mechanical stability of CPB structure in underground excavations. To study the stress distribution in CPB structure, an integrated model consisting of a cohesive zone model for CPB/rock interface behavior and a multiphysics model for hydrating CPB behavior is developed. The predictability of the proposed model is validated against measurements from laboratory tests and field studies. Thereafter, the validated model is used to investigate the influence of curing time, stope geometry, and field backfilling operation on the spatiotemporal stress development in CPB structure. It has been found that the CPB/rock interface loading is able to reduce the stress levels in CPB by up to 75%, and taking advantage of this phenomenon can significantly reduce the operation cost. Moreover, there exists a critical stope width that governs the extent of stress reduction in CPB. In addition, continuous mining activity in the secondary stope exerts limited impact on the stress redistribution in the primary backfill body. These findings provide practical guidance for the safe design of a CPB structure as well as a more strategic mining plan for the adjacent ore body.

Experimental investigation into the temperature effect on the shear behavior of the fiber-reinforced interface between rock and cemented paste backfill

Considering the importance of the interaction between cemented paste backfill (CPB) and rock in determining the stability of underground CPB structures, a comprehensive understanding of the shear performance of the fiber-reinforced CPB/rock (FR-CPB/rock) interface is crucial to the CPB design. Despite some tentative exploration performed on the shear performance of the FR-CPB/rock interface, the coupled effects of temperature, binder type and fiber content on the interface behavior remain unrevealed. To fill this research gap, direct shear tests are carried out on FR-CPB/rock interface samples prepared with diverse binders and fiber contents (cured at 2℃, 20℃, and 35℃). The obtained results indicate that the shear strength of the FR-CPB/rock interface generally increases with temperature and curing time. Besides, compared with samples made with Portland cement type I (PCI), the usage of PCI and fly ash (PCI/FA (50/50)) negatively affect the shear strength of the FR-CPB/rock interface (owing to the low sensitivity of FA to temperature) but the adoption of PCI/slag (50/50) in CPB preparation can improve the shear strength (due to the alkaline activation of slag). Moreover, the addition of fiber can positively or negatively influence the shear strength of the interface, up to the fiber content. However, the frictional angle of the FR-CPB/rock interface increases with the fiber content, but the cohesion shows a descending tendency with the fiber content. The findings in this research serves the purpose of a comprehensive understanding of the shear behavior of the FR-CPB/rock interface and are meaningful in CPB design.

Reactivity of cemented paste backfill containing polycarboxylate-based superplasticizer

Superplasticizers are used in the preparation of cemented paste backfill (CPB) to improve its rheological properties. CPB produced from sulphide-bearing tailings can generate acid mine drainage (AMD), which has negative environmental impact. The AMD potential can be assessed based on the reactivity of the CPB when exposed to the atmosphere. However, the effect of superplasticizers on the reactivity of CPB is unknown. This paper presents findings from experimental investigations on the effects of polycarboxylate superplasticizers (PCSP) on the reactivity of CPB containing pyrite. The reactivity of cured CPB specimens was determined by monitoring the rate of oxygen consumption in an enclosed chamber. Microstructural tests, namely mercury intrusion porosimetry, X-ray diffraction, and thermogravimetric analyses were also carried out to have a better understanding of the reactive mechanisms. The results obtained show that the addition of PCSP leads to a proportionate reduction of reactivity. The admixture causes the refinement of the pore structure of the CPB by mainly improving binder hydration and self-consolidation. It was also observed that less pyrite crystals were present in the CPB containing the superplasticizer. High curing temperature was also found to improve binder hydration, leading to a decrease in reactivity of CPB with PCSP. Due to the micro-filer effects on the CPB pore structure, partial replacement of cement with fly ash and blast furnace slag also decreases the reactivity of CPB. These research findings will be valuable for the design of cost-effective, more environmentally friendly and sustainable CPB structures.

Comparison of the mode I fracture toughness of different cemented paste backfill-related structures: Effects of mixing recipe

The mode I fracture toughness is an important property in evaluating the resistance of the laminar structure to the tensile failure. The various interfaces existing in underground cemented paste backfill (CPB) structures depict the potential failure planes. To better assess the stability of CPB structures, the semi-circular bending (SCB) test is conducted to compare the fracture toughness of the CPB matrix, the short-term CPB/long-term CPB interface, and the CPB/rock interface in this study. In addition, the effects of the mixing recipe of CPB (cement and fiber content, water-to-cement ratio) on the development of fracture toughness are investigated as well. The results obtained from the semi-circular bending test show that the mode I fracture toughness of the CPB body is generally higher than that of the interface structures owing to the denser capillary structure. Besides, the specimens prepared with a lower water-to-cement ratio and cement content experience a higher effective stress and thereby increasing the fracture toughness. Moreover, the addition of fiber increases the fracture toughness of the CPB body (due to the bridging effect) but decreases that of the interface structures by reducing the contacting area between diverse particles. These findings are significant to the stability analysis and the prediction of the stress distribution in CPB structures.

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.

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.

Insight into Saturated Hydraulic Conductivity of Cemented Paste Backfill Containing Polycarboxylate Ether-Based Superplasticizer

Recycling of tailings in the form of cemented paste backfill (CPB) is a widely adopted practice in the mining industry. Environmental performance is an important design criterion of CPB structures. This environmental performance of CPB is strongly influenced by its saturated hydraulic conductivity (permeability). Superplasticizers are usually added to improve flowability, but there is a limited understanding of their influence on the hydraulic properties of the CPB. This paper presents new experimental results on the variations of the hydraulic conductivity of CPB containing polycarboxylate-based superplasticizer with different compositions and curing conditions. It is found that the hydraulic conductivity of the CPB decreases with the addition of superplasticizer, which is beneficial to its environmental performance. The reduction is largely attributable to the influence of the ether-based superplasticizer on particles mobility and cement hydration. Moreover, both curing temperature and time have correlations with the hydraulic conductivity of CPB containing superplasticizer. In addition, the presence of sulfate and partial replacement of PCI with blast furnace slag reduces the hydraulic conductivity. The variations are mainly due to the changes in the pore structure of the CPB. The new results discussed in this manuscript will contribute to the design of more environmental-friendly CPBs, which is essential for sustainable mining.

Development of Mode I and Mode II fracture toughness of cemented paste backfill: Experimental results of the effect of mix proportion, temperature and chemistry of the pore water

Despite the comprehensive understanding of the development of the modes I & II fracture toughness of brittle materials, the evolution of the fracture toughness of cemented paste backfill (CPB) under different conditions remains unknown. Given the importance of modes I & II fracture toughness in analyzing the fracture initiation and propagation, many semi-circular CPB samples (with different water-cement (w/c = 6.5, 7.35, 8) ratios, cement contents (3%, 4.5%, 6%), curing temperatures (2 °C, 20 °C, 35 °C) and sulphate contents (0 ppm, 5000 ppm, 15000 ppm, 25000 ppm)) are prepared and subjected to semi-circular bending (SCB) test after being cured for 3 days, 7, 14, and 28 days. Irrespective of the mixing recipe or the curing conditions, all samples for mode I fracture toughness measurement generally experience a sudden failure, while the cracks in the samples for measuring the mode II fracture toughness propagate gradually. The obtained results show that a decrease of w/c ratio contributes to the modes I & II fracture toughness acquisition. Besides, owing to the acceleration effect of temperature on the cement hydration and the self-desiccation, the modes I & II fracture toughness of CPB increases with temperature as well. However, the effects of sulphate ions on the growth of the modes I & II fracture toughness of CPB depend on the initial sulphate content and the curing age. In other words, there is a competition effect between the factors contributing to the fracture toughness increase (refinement of the capillary structure by cement hydration products) and decrease (inhibition of the cement hydration by sulphate ions). The findings of this study provide critical information in the stability assessment of CPB structures, which is beneficial to the improvement of the mining safety.

Experimental study on the triaxial mechanical behaviors of the Cemented Paste Backfill: Effect of curing time, drainage conditions and curing temperature

The application of CPB (Cemented Paste Backfill) can realize the clean, efficient, and safe mining of underground metal mines. Clear understanding on the triaxial mechanical properties of CPB is important to the CPB design and the stability analysis of the backfilled CPB structure. The triaxial mechanical properties of CPB can be significantly affected by the different curing conditions. In this research, triaxial compression tests of the CPB samples were carried out using the GCTS (Geotechnical Consulting & Testing System), and the considered curing conditions include different curing time (1, 3, 7 and 28 days), drainage conditions (drained and undrained) and curing temperatures (20 °C, 35 °C and 45 °C). The measured mechanical parameters were compared and analyzed against the framework of the Mohr-Coulomb criterion. Then, the vertical stress distribution of the backfilled CPB structure was calculated and discussed using the measured mechanical parameters. The results show that with the increase of the lateral constraint ratio (σc/Sd0), the elastoplastic stage of the measured deviator stress versus axial strain curve of CPB sample is gradually obvious. The peak deviator stress (Sdp) and the ultimate axial strain (εu) show the linear and negative exponential increase with the σc/Sd0 respectively. The number of cracks on the fractured surface of the CPB samples gradually decreased with the increase of σc/Sd0. The failure types of CPB samples were changed from tensile failure (σc/Sd0 = 0%) to the mixed tensile-shear failure (σc/Sd0≈10%) and compression-shear failure (σc/Sd0≥20%). Moreover, with the increase of curing time and curing temperature or under the drained curing condition, the peak deviator stress and cohesion (cb) of CPB can be significantly increased, but the corresponding internal friction angle (ϕb) is decreased. The shear mechanical parameters of CPB can significantly affect the vertical stress distribution inside the CPB structure. Therefore, when estimating the vertical stress distribution inside the backfilled CPB structure in engineering practices, it is necessary to focus on the changes of CPB shear parameters (cb and ϕb) caused by different curing conditions.

Temperature induced changes in the behaviour of cementing fine-grained soils under dynamic loadings

ABSTRACT Cemented paste backfill (CPB) has been extensively used in the mine industry to backfill underground mine cavities for ground support and/or tailings management. Once placed in mine underground cavities, the geotechnical and liquefaction response of the CPB under dynamic (cyclic) loadings is a key design concern in mine backfilling. Moreover, fresh CPB might be exposed to several sources of heat that might affect its cyclic behaviour and liquefaction potential. This manuscript presents findings of assessing the effect of backfill temperature on its geotechnical and liquefaction response to cyclic loading by using shaking table. CPB mixtures were prepared under different temperatures, poured into a flexible laminar shear box, cured (under the same mixing temperature) for 2.5 hours, and then exposed to cyclic loading using 1-D Shaking table. Obtained results illustrate that CPB prepared and cured under the temperature of 20°C can be prone to liquefaction under the studied loading conditions. However, CPB prepared and cured below a temperature of 35°C is resistant to liquefaction. These results provide better comprehension of the effect of backfill temperature on the cyclic behaviour of CPB, and thus assist in designing more efficient CPB structures.

Non-isothermal evolution of mechanical properties, pore structure and self-desiccation of cemented paste backfill

In the field, cemented paste backfill (CPB) structures are subjected to non-isothermal curing conditions. In this study, the non-isothermal strength, pore structure development, and self-desiccation of CPB were investigated. CPB specimens with different types of binders and tailings were prepared and cured under isothermal (room temperature: 20 °C) and field non-isothermal conditions. Unconfined compressive strength (UCS) tests, microstructural tests and analyses as well as monitoring experiments, were conducted on CPB specimens cured over various ages. The obtained results show that the non-isothermal curing condition significantly increases the strength as well as the strength increase rate of the CPB regardless of the type of binder and tailings used. Moreover, the non-isothermal condition leads to more intense self-desiccation of CPB as well as the refinement of the pore structure of CPB due to the generation of more hydration products, such as calcium–silicate–hydrate (C–S–H), in the non-isothermal CPB. It is also found that a mine cavity (stope) backfilled with CPB material made with Portland cement type I (PCI) has a significant advantage over stope backfilled with CPB made with slag/PCI (50%/50%) in terms of strength formation and enhancement of self-desiccation due the difference in the non-isothermal curing conditions. The findings of this study provide insight into the development of the mechanical and microstructure properties as well as self-desiccation of CPB structures when exposed to field thermal curing condtions, thereby contributing to the improvement of mechanical stability and economic performance of future CPB designs. The impact of non-isothermal curing conditions on CPB properties need to be considered in mine backfill design and operations.

Effects of binder type and dosage on the mode I fracture toughness of cemented paste backfill-related structures

The mode I fracture toughness, which is an indication of the amount of stress required to propagate a preexisting flaw, is usually used to evaluate the resistance of interlaminar structure to tensile stress. Considering that the cemented paste backfill (CPB) includes various interfaces (e.g. early-age CPB/advanced-age CPB interface and CPB/Rock interface), a thorough understanding of the magnitude/order of the mode I fracture toughness of different CPB-related structures is critical in stability assessment. However, only limited researches have been dedicated to the fracture toughness of CPB so far, and none of them have addressed the effects of different binders on the development of the mode I fracture toughness. Hence, many semi-circular samples prepared with various binders (e.g., Portland cement type I (PCI), a mixture of PCI and Slag (PCI/Slag) or fly ash (PCI/FA), with a ratio of 25/75, 50/50, 75/25) are prepared and subjected to the semi-circular bending test after being cured for certain periods. The findings reveal that the binder has an observable influence on the mode I fracture toughness of different interface structures, and despite the binder types, CPB depicts the highest mode I fracture toughness in comparison with early-age CPB/advanced-age CPB interface and CPB/Rock interface. Furthermore, the samples prepared with PCI/Slag generally experience higher mode I fracture toughness as well. The results can contribute to the cost-effective design and the safety assessment of CPB structures.

Chemically induced changes in the geotechnical response of cementing paste backfill in shaking table test

Abstract Cemented paste backfill (CPB) is extensively used for underground mine support and/or tailings management. However, CPB behavior under cyclic loadings might be affected by the chemistry of its pore-water, which often contains sulphate ions. Till today, no studies have addressed the effect of sulphate on the response of CPB to cyclic loadings by using shaking table technique. This study presents new findings of assessing the effect of the sulphate in the pore water of CPB on its geotechnical response to cyclic loading by using shaking table. CPB mixtures were prepared (with and without sulphate), poured into a flexible laminar shear box, cured to 4 h, and then exposed to cyclic loading using one-dimensional (1D) shaking table. Several parameters (e.g. pore water pressure, settlement, lateral deformation, acceleration, electrical conductivity, effective stress, and liquefaction susceptibility) were monitored or determined before, during, and after shaking. Obtained results indicate that the sulphate-bearing CPB cured to 4 h can be prone to liquefaction under the studied conditions. However, sulphate-free CPB samples are resistant to liquefaction. These results are expected to contribute to a better understanding of the effect of water chemistry on the cyclic behavior of CPB, consequently enhancing the cost-effective design of CPB structures.

Sulfate-Dependent Shear Behavior of Cementing Fiber-Reinforced Tailings and Rock

A better understanding of the shear behavior of the interface between cemented paste backfill (CPB) and the surrounding rock is critical for constructing cost-effective, durable, and reliable CPB structures. In practice, CPBs suffer sulfate attack during their service life, and as a typical cementitious material, the CPB itself has disadvantages, such as high brittleness, easy cracking, and insufficient durability, which restrict the further popularization and application of CPB technology. Thus, in this study, direct shear tests, electrical conductivity (EC) and thermal gravity/differential thermal gravimetric (TG/DTG) analyses were conducted to research the effects of different amounts of monofilament polypropylene fibers (0%, 0.1%, 0.3%, and 0.5%; by mass of the sum of the dry tailings and cement) and initial sulfate concentrations (0 mg/L, 5000 mg/L and 25,000 mg/L) on the shear behavior of the fiber-reinforced CPBs and rock (FR-CPB/rock) interface, and the Mohr–Coulomb shear envelop was used to fit the shear strength of specimens with various periods (1 day, 3 days, 7 days, and 28 days) under various stresses (50 kPa, 100 kPa, 150 kPa). The experimental testing results indicated that the fibers generally enhance the performance of the shear behavior of the FR-CPB/rock interface and the optimal fiber content correlates to the initial sulfate concentration. For the same treatment time (7 days), a fiber content of 0.1% contributes to the best shear performance for the FR-CPB/rock interface with a sulfate concentration of 5000 mg/L. For the sulfate-free and 25,000 mg/L concentration specimens, 0.3% is the optimal fiber content. Furthermore, for the studied interface specimens, sulfate content can play a positive (the refinement of the pore structure) or negative (the sulfate retardation effect) role in the interface shear behavior between the FR-CPB and rock, depending on the treatment time, the initial sulfate concentration, and the fiber content. For the specimens treated for 7 days and 28 days, the specimens with initial sulfate concentrations of 5000 mg/L and 25,000 mg/L achieved the highest peak shear strengths, respectively. The outcomes of this paper present a substantial reference for the design and optimization of underground FR-CPB structures under sulfate attack.