Cemented Paste Backfill Material Research Articles

Study on the performance of alkali-activated phosphorus slag cemented paste backfill material: Effect of activator type and amount

Effective disposal of phosphorus slag (PS) is crucial for ecosystem protection. Herein, this study proposed that Na2SO4 (SS), Na2SiO3 (NS), Na2CO3 (SC) and NaOH (NH) were performed to prepare alkali-activated phosphorus slag cemented paste backfill material (AAPS-PB). The evolution mechanisms of setting time, rheological behavior and mechanical properties at different contents of various activators were systematically investigated. The mineral compositions, bonding structure, micromorphology and pore structure characteristics of AAPS-PB hydration products were further clarified by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) and mercury intrusion porosimeter (MIP). Results indicated that activators were proved to significantly promote the dissolution of PS at appropriate content. NH had stronger alkali activation ability in the early hydration period, whereas NS sufficiently improved the later strength, enhanced the denseness of hydration products and refined the pore structure characteristics. This study expands the avenues for high-value applications of PS.

Interaction of cemented paste backfill (CPB) and circumneutral mine water during column experiments

Cemented paste backfill (CPB) is used as a support and filler material in underground mines. In underground conditions, before mine closure and flooding, surface and groundwaters with variable quality are in contact with CPB material, which may result in the leaching of harmful substances from the CPB. In this study, we used actual mine water from a gold mine to flush CPB in a laboratory test to simulate underground conditions and the environmental influence of CPB on groundwaters more realistically than before.Different CPB materials were treated with mine water in columns for nine months. The columns were drained and filled with new mine water batches four times during the experiment to simulate intermittent flushing underground. The dissolved elemental content, major cations, pH and oxidation-reduction potential (ORP) of the waters were measured, and the chemical results were analysed by performing geochemical modelling with the widely used geochemical code PHREEQC. X-ray tomography was also used to examine the internal structure of the CPB specimens.The results demonstrated that calcium and sulphate present in CPB parent materials leached into the mine water, which might cause a deterioration of CPB. In contrast, the concentrations of dissolved As, Sb and Ni decreased in the mine water when it was in contact with CPB materials. However, arsenic started to be re-released from the materials at the end of the nine-month experimental period. The geochemical modelling suggests that the removal of metals/metalloids from the mine water was driven by the surface complexation reactions on originally present and/or freshly precipitated metal (hydr)oxide minerals. A longer and modified testing period is needed to thoroughly examine the conditions under which these adsorbed elements start to release over time.

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.

Study of fresh properties of cemented paste backfill material with ternary cement blends

This study is aimed at evaluating the effects of ternary blended Portland cement type I (PCI) with slag and limestone (LS) powder on key engineering fresh properties of CPB: (i) rheological properties and (ii) setting time. CPB samples with PCI/Slag ratios of 50/50 (Group A) and 80/20 (Group B) were considered in this study. The effect of LS in the ternary mix was investigated by replacing the slag with increasing doses of LS (5, 10, and 20 wt%). The specimens were cast and cured over periods of 0, 0.25, 1, 2 and 4 h and tested for rheological properties (yield stress, viscosity). In addition, tests were performed to identify the initial and final setting time of the samples. In addition, microstructural tests, zeta potential and pH measurements, and monitoring experiments (temperature, electrical conductivity) were carried out to better comprehend the underlying mechanisms behind the observed changes in rheological behavior and setting time. The findings have shown that the replacement of slag by a higher dosage of LS in the ternary binder significantly affects the rheological properties of the studied CPB. It reduces the yield stress and increases the viscosity of CPB. The effect of LS on the rheological properties of CPB is mainly attributed to the physical (e.g., filler effect, lubrication, particle size) and chemical (nucleation and changes in repulsive forces between CPB particles) effects of LS in the ternary system. The findings also revealed that increasing the proportion of limestone in the CPB ternary binder system prolonged the setting time (initial and final) of the CPB mixture due to the nucleation and dilution effects of LS. Overall, the synergy between slag and LS was observed; the optimal use of LS and slag in the ternary system can serve as a sustainable alternative to the predominantly used ordinary Portland cement (OPC) or OPC/Slag binary binder, thereby reducing the energy consumption and carbon footprint associated with cement and CPB technology. The results of this research will be valuable for a better comprehension of the impacts of ternary binder with an increasing percentage of LS on the rheology and setting time of the CPB mix, ultimately, the efficient design of the mixing plant, especially the backfill transportation system.

Utilization of mine waste heat in phase change rechargeable battery

Mine geothermal energy has been recognized as “heat hazard”, which is harmful to the health of miners and causes disasters. In order to realize the transformation of mine geothermal energy and waste heat from “hazard/waste” into “treasure”, this paper analyzes the distribution characteristics and genesis of mine geothermal energy with the field geothermal temperature monitoring data, evaluates geothermal reserves, and reveals the mutual influencing mechanism of stratified exploitation of two-layer geothermal reservoir and the law of thermal energy conduction and evolution in the stope of geothermal-coal synergetic mining (GE-COSM), using numerical models verified by experiments. In the end, the improvement effect of mine waste heat on the development potential of geothermal-coal mine (G-CM) was analyzed. The result shows that: The present geothermal field distribution characteristics of Anju Mine are high in the north and low in the south, high in the east and low in the west. There are two layers of zoned reservoir, with the temperatures of 43 °C and 54 °C respectively, the average heat flow value 70.1 mW/m2 and 80.9 mW/m2 respectively, and the geothermal water type SO42--Na+·Ca2+. The simultaneous exploitation of the two geothermal reservoirs will form a Coal-Austria geothermal reservoir joint influence area(C-Area) with lower temperature, which reduces the harm of “heat hazard” to miners. But the phase change heat storage function of functional cemented paste backfill material for phase change heat storage (F-CBM) is not fully utilized in the late stage of geothermal exploitation. The mine generates 4.6 × 1011 J of waste heat per day, and according to the type of heat source of mine waste heat can be continuously “heat charging” for a conventional backfill working face. Moreover, under the action of high temperature surrounding rock, the thermal energy recovery rate is greater than 1, and under the action of “heat charging”, the average maximum value of solid phase ratio is greater than 70 %. Therefore, the phase change rechargeable battery mode creatively proposed by this paper is significantly better than the traditional rock layer heat storage mode and is worth vigorously popularizing.

Study on energy dissipation and acoustic emission characteristics of fiber tailings cemented backfill with different ash-sand ratios

In this paper, acoustic emission (AE) monitoring technique was used to study the mechanical lifting mechanism of polyacrylonitrile (PAN) fibers on the CPB material under uniaxial compression and the specific energy characteristics and AE response characteristics and damage precursors during the damage of fiber tailings cemented backfill specimens with different ash-sand ratios. In this experiment, cemented paste backfill (CPB) and polyacrylonitrile fiber tailings cemented backfill (P-FTCB) specimens with 68% concentration and 1:6, 1:8, 1:10 and 1:12 ash-sand ratios were fabricated using PAN fibers. The study shows that the fracture arresting effect and bridging effect of PAN fibers inhibited the expansion of fracture development inside the CPB, thus enhancing the compressive strength and post-peak damage toughness of the CPB, while the fracture arresting effect and bridging effect of fibers depended on the fiber pull-out process, and the fiber pull-out process is divided into three stages: micro-slip stage, slip stage and detachment stage; the higher the ratio of ash-sand in P-FTCB specimens, the stronger the cementing force, the higher the energy required for material destruction, the greater the elastic strain energy that can be stored, and the greater the energy dissipated per unit volume; CPB specimens without fiber incorporation show a decreasing trend in AE parameters before damage; while P-FTCB specimens show a surge in AE parameters after the specimens reached their stress peaks under the effect of fiber fracture arrest; in addition, there is a significant relationship between the ratio of ash-sand of the filling material and its AE parameters: the AE energy parameters of CPB specimens and P-FTCB specimens increas as the ratio of ash-sand decreased. The results of the study can provide a reference basis for the use of P-FTCB materials in mines, and provide theoretical support for promoting the recycling of mine solid waste.

Properties and microstructure of total tailings cemented paste backfill material containing mining and metallurgical solid waste

Introduction: In order to improve the utilization efficiency of industrial waste discharged in the process of iron and steel metallurgy, a kind of material that can replace cement for mine filling is sought. In the test, steel slag (SS) and vanadium-titanium slag (VTS) were used as the primary raw materials to prepare cementing agents (CA). Then, combine it with vanadium-titanium iron ore tailings (VTIOTs) to make mine cemented paste backfill material (CPBM).Methods: The composition, properties and hydration mechanism of CPBM are studied through various tests, in-cluding mechanical property test, hydration heat test, X-ray diffraction (XRD), scanning electronic mi-croscopy (SEM), and fourier transform-infrared spectroscopy (FT-IR).Results: The results show that when the following conditions are met, the slump is 216 mm, and the 28-day flexural strength and compressive strength of CPBM reach 4.25 and 9.41 MPa, respectively, which meets the requirements of Chinese National Standard GB/T 39489-2020 Technical specification for the total tailings paste backfill: the mass percentage SS: VTS: phosphogypsum (PG): dicyandiamide waste slag (DWS) = 31:59:6:4; the content of compound phosphoric acid (CPA) accounts for 4% of the dry material; the cement sand ratio of CPBM is 1:4; the paste mass concentration (PMC) is 80%; and the content of water reducing agent (WRA) is 0.18%.Discussion: Mechanism studies show that the hydration product of the CA is mainly ettringite (AFt) and C-S-H gel. The addition of CPA promotes the hydration of active minerals in SS and VTS. The existence of PG pro-motes the formation of AFt, and the formation of AFt further promotes the fracture of [AlO4]5− and [SiO4]4− along the bridge oxygen in VTS and SS.

Preliminary Study of Preheated Decarburized Activated Coal Gangue-Based Cemented Paste Backfill Material

This study proposes a novel idea of the use of coal gangue (CG) activation and preheated decarburized activated coal CG-based cemented paste backfill material (PCCPB) to realize green mining. PCCPB was prepared with preheated decarburized coal CG (PCG), FA, activator, low-dose cement, and water. This idea realized scale disposal and resource utilization of coal CG solid waste. Decarbonization and activation of CG crushed the material to less than 8 mm by preheated combustion technology at a combustion temperature of 900 °C and a decarbonization activation time of 4 min. The mechanism of the effect of different Na2SO4 dosages on the performance of PCCPB was investigated using comprehensive tests (including mechanical property tests, microscopic tests, and leaching toxicity tests). The results show that the uniaxial compressive strength (UCS) of C-S2, C-S3, and C-S4 can meet the requirements of backfill mining, among which the UCS of C-S3 with a curing time of 3 d and 28 d were 0.545 MPa and 4.312 MPa, respectively. Na2SO4 excites PCCPB at different curing time, and the UCS of PCCPB increases and then decreases with the increase in Na2SO4 dosage, and 3% of Na2SO4 had the best excitation effect on the late strength (28 d) of PCCPB. All groups' (control and CS1-CS4 groups) leachate heavy metal ions met the requirements of groundwater class III standard, and PCCPB had a positive effect on the stabilization/coagulation of heavy metal ions (Mn, Zn, As, Cd, Hg, Pb, Cr, Ba, Se, Mo, and Co). Finally, the microstructure of PCCPB was analyzed using FTIR, TG/DTG, XRD, and SEM. The research is of great significance to promote the resource utilization of coal CG residual carbon and realize the sustainable consumption of coal CG activation on a large scale.

Mechanical Characterization of Cemented Paste Backfill

Mechanical characterization is important to the design and analysis of cemented paste backfill (CPB) structures. Unconfined compressive strength (UCS) tests have been widely used owing to their relative simplicity to characterize a material’s response to unconfined compressive loading. However, the UCS represents a single strength parameter and does not fully describe the material’s strength (or failure) envelope. In this study, we analyzed UCS tests with direct shear and uniaxial tensile strength tests conducted on the same CPB materials to provide mechanical characterization of CPB under a more complete range of loading conditions. The results demonstrate the Mohr–Coulomb failure envelope provides a consistent description of strengths arising from the three different test methods. Furthermore, a better estimate of the tensile strength is UCS/4, which is considerably higher than the conventional assumption that the tensile strength is equal to USC/10 or UCS/12. This has a significant impact on the assessed required strengths particularly for undercut designs using Mitchell’s sill mat analysis method and suggests that in future the conventional UCS tests should be complemented with direct tension and direct shear tests to improve underground designs using CPB.

Study of early-age performance of cementitious backfills with alkali activated slag under internal sulfate attack

The vulnerability to internal sulfate attack of calcium-rich ordinary Portland cement (OPC) and its relative high cost force mines to explore technically effective and low cost alternative binders to solidify sulfate-containing tailings. Alkali activated slag (AAS) has been established to be a promising substitute for OPC due to its superior strength, good resistance to chemical attack and low cost. This study shows results of a laboratory research dealing the characteristics of cemented paste backfill (CPB) with AAS under internal sulfate attack. A blend of two alkali activators (SS: sodium silicate, SH: sodium hydroxide) was considered. To assess the behavior of CPB materials subjected to diverse conditions, a total of 4 different SS/SH ratio (e.g., 0.5, 0.75, 1, and 1.25) were used within CPB. Results shows that initial/final setting times tend to drop with growing sulfate content up to 5000 ppm, followed by gradual increases. Increasing sulfate content leads to higher yield stresses and viscosities and the degree of this negative impact is much severer in samples having the high SS/SH ratio. The presence of sulfate can enhance or deteriorate the strength of AAS-CPB depending on activator type, cure age and initial sulfate content. CPB with high SS/SH ratio reveals weaker resistance to internal sulfate attack due to their lower pH values. The key outcome of this work will be beneficial to optimally design of sulfate bearing CPB-AAS structures.

Application of desulfurization gypsum as activator for modified magnesium slag-fly ash cemented paste backfill material

Recycling industrial solid waste for mine backfill is one of the best ways to achieve green production in multiple industries. In this paper, the desulfurization gypsum (DG) as an activator is combined with the modified magnesium slag-fly ash cementitious paste backfill (MFPB) technology for the co-disposal of solid waste and goaf treatment, and the influence of DG on the performance of MFPB was comprehensively analyzed through rheological properties, mechanical properties, durability, microscopic analysis and environmental characteristics experiments. The results show that the fresh MFPB mortar conforms to the Herschel-Bulkley model at different maximum shear rate (γ̇max) conditions. When the γ̇max is 100 s−1, the mortar exhibits shear-thickening properties. The apparent viscosity, yield stress and static yield stress of mortar decreased first and then increased with the increase of DG content, and all had the minimum value when DG was 2.5 %. The thixotropy of the mortar was significantly increased with the addition of DG, and the change in thixotropy was significantly correlated with the difference between the two yield stresses. Both the rheological and mini-slump results demonstrate that DG can improve the flowability of MFPB mortars. In addition, the UCS of D0 under steam curing and standard water curing conditions for 28 d were 4.342 MPa and 2.827 MPa, and the sample containing DG were 6.109–8.241 MPa and 6.669–9.492 MPa, respectively. The addition of DG not only improves the strength of MFPB, but also improves the durability of MFPB. Microscopic analysis (XRD, SEM, and TG-DTG) indicated that this was mainly because DG promoted the hydration reaction of the MMS-FA system and accelerated the generation of C-S(A)-H and AFt. Finally, in the results of in situ leaching based on durability and leaching based on standard HJ 557, all the indexes of MFPB meet the standard of class III groundwater in GB/T 14848-2017, and it has an effective stabilization/solidification effect on heavy metals (As, Cu, Ni, Ba, Zn and Mo, etc.). To sum up, the collaboration of DG and MFPB technology can not only efficiently clean and utilize a variety of solid wastes (MMS, FA and DG), but also greatly improve the performance of MFPB to promote its application.

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

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

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

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

Effective reuse of red mud as supplementary material in cemented paste backfill: Durability and environmental impact

Red mud (RM) is a potential supplementary backfill material. However, little is known about cemented paste backfill (CPB) preparation from reused RM. Here, the feasibility of Bayer RM in CPB technology is discussed to expand the utilization of solid waste. With Bayer RM, Fly ash (FA), and cement as raw materials, the strength performance of Bayer RM in cemented paste backfill (RM-CPB) was evaluated by the orthogonal experiment method, and the optimal ratio of RM-CPB was determined. The effects of slurry concentration, mixing ratio of RM to FA, and the cement dosage on uniaxial compressive strength (UCS) were studied. Its durability, microstructure, permeability, and environmental properties were investigated. The experimental results show that the strength of the optimum proportion reached 1.25 MPa and 2.37 MPa after curing for 3 d and 28 d respectively and meet the mine design standard when the ratio of RM-CPB is: 60% of the slurry concentration, the RM: FA is 3:2, and the dosage of cement is 5%. RM-CPB material has good durability, and the main hydration products are C-S-H gel, ettringite, and gmelinite, which is conducive to the development of UCS of the backfill material. Meanwhile, the permeability k20 of RM-CPB is only 6.005 × 10-6 cm/s, indicating that the material has few internal pores, which can effectively prevent external water intrusion. The leaching test results showed that both the heavy metal concentration and pH value of the leachate were within the range specified in the Chinese standard (DZ/T 0290–2015), and it was found that the effect of the three ion leaching tests is successive: toxicity leaching test > immersion test > eluviation test. The comprehensive analysis found that the reuse of Bayer RM as a supplementary cemented paste backfill material provides a new idea for the large-scale utilization of industrial solid waste.

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.

A New High Charge Density Superplasticizer Regulates the Rheological Properties of Tailings Slurry and Cemented Paste Backfill Materials

A New High Charge Density Superplasticizer Regulates the Rheological Properties of Tailings Slurry and Cemented Paste Backfill Materials

Research and Application Status and Development Trend of Alkali-Activated Binder Powder for Mine Backfill

To improve the economic and social benefits of mining and backfill, it is necessary to find the backfill materials suitable for mineral mining, improve the various properties of the filling materials, and develop low cost, high performance new filling materials. Portland cement is neither environmentally friendly nor economical. Currently, we have begun to study and apply some industrial waste, such as slag, fly ash, and other solid wastes, with certain activities as the primary component of cementing material that will not only meet the technical requirements of filling, but also comprehensively utilize industrial waste and possess a benign, sustainable lifecycle for environmental protection. This study expounds the composition and reaction mechanism of alkali-activated binder powder used for cemented paste backfill material, summarizes the research and application status of alkaliactivated binder powders, including mechanics, durability, economic and environmental, and discusses future development directions for mine binder powders. It needs to continue to expand the utilization range of solid waste and improve the utilization rate of solid waste in the future. The development and research of this binder powder plays an important role in the progress of filling technologies. Hence, the research about alkali-activated binder powder contains great development potential and broad prospects.

Further insight into the strength development of cemented paste backfill materials containing polycarboxylate ether-based superplasticizer

The utilization of water-reducing admixtures has become immensely popular in underground mine backfilling operations not only to enhance the workability of cemented paste backfill (CPB, a mine construction material), but also for their ability to enhance the mechanical strength and long-term durability of CPB. This study aims to evaluate the effect of a polycarboxylate ether-based superplasticizer (PES) on the mechanical strength of CPB made with different mix components (tailings, dosages of PES, cement and solids contents, tailings fineness, and water chemistry), and subjected to various curing conditions (stress-free, under stress, constant temperature, and non-isothermal temperatures). An extensive experimental program has been conducted to investigate the uniaxial compressive strength development, hydraulic (positive pore pressure, suction) and microstructural properties of CPB with PES. The results show that including PES in CPB mixtures significantly increases the unconfined compressive strength (UCS) as well as affects the microstructure and hydraulic properties of CPB. This UCS increase depends on the amount of PES added, type and fineness of the tailings, sulphate content, and curing time. After 7 days of curing, the UCS of the CPB increases as the dosage of the PES is increased. The CPB that contain PES and cured under stress exhibit a higher compressive strength after just 3 days of curing as compared to that without PES and cured under similar stress conditions. It is also found that curing CPB under non-isothermal conditions considerably increases the compressive strength and this increase is more significant with the presence of PES. These findings have significant impacts on the short- and long-term performances of CPB structures with PES and their associated economic implications for underground mine backfilling operations.

Experiment on hydration exothermic characteristics and hydration mechanism of sand-based cemented paste backfill materials

Cemented paste backfill technology can effectively control overburden movement, protect surface buildings and ecological environment. In this study, the early hydration exothermal characteristics, compressive strength evolution and hydration mechanism of sand-based cemented paste backfill materials (SCPB) were studied by hydration heat tests, uniaxial compressive strength tests, x-ray diffraction, scanning electron microscope and energy dispersive spectrometer tests; the hydration model of SCPB and fly ash were also constructed. The results show that: (1) the early hydration exothermic process of the SCPB can be divided into five stages: Dissolution stage, Induction stage, Acceleration stage, Deceleration stage and Slow deceleration stage. The heat release rates in the five stages are 5.64, 2.09, 3.63, 3.16 and 1.07 J·(g·h)-1, respectively; (2) the mechanical strength and elastic modulus of the SCPB increase with the increase of curing age; when the curing age of 7 d, the elastic modulus of the SCPB reaches 67.92% of the final value; (3) the primary hydration products of the SCPB are calcium silicate hydrate, ettringite, calcium hydroxide, Calcium carbonate and Calcium aluminate hydrate; (4) the hydration process of the SCPB can be divided into Dissolution stage, Hydration stage and Hardening stage, and the hydration process of fly ash experienced Adsorption stage and Hydration stage.

Mechanical Properties of Clay Based Cemented Paste Backfill for Coal Recovery from Deep Mines

Fly ash cement is used to solidify marine clay to prepare marine-clay-based cemented paste backfill (MCCPB) to fill the underground goaf of mines, which not only utilizes solid waste such as fly ash and marine clay, but also controls surface subsidence and protects the environment. To simulate the complex underground mine water environment of the filling body, a dry-wet cycle aquatic environment test under different material ratios and curing ages was designed. The water absorption and unconfined compression strength (UCS) of MCCPB with curing ages of 7 and 28 days under the action of 0, 1, 3, and 7 dry-wet cycles were investigated. The results indicate as the number of dry-wet cycles increases, the surface of MCCPB becomes significantly rougher, and the water content and the solid mass decrease accordingly. Different ratios and curing ages of MCCPB in dry-wet cycles of the UCS tend first to increase, then decrease. Meanwhile, the stress-strain curve of the specimen shows that the trend in the elastic modulus is consistent with that of UCS (first increasing, then decreasing), and that, the minimum UCS value of the specimen still meets the early strength requirements of cemented paste backfill in coal mine geothermal utilization. On the one hand, it proves the feasibility of fly ash cement-solidified marine clay for use as cemented paste backfill in coal mines; on the other hand, it also expands the available range of cemented paste backfill materials in coal mines.