Cemented Paste Backfill Research Articles

Mechanical properties and damage evolution characteristics based on the acoustic emission of gangue and high-water-content materials based cemented paste backfill

The long-term, continuous, high-intensity mining of coal resources has caused surface settlement, collapse of surface buildings, and stacking of gangue on ground. Backfill mining with high-water-content materials, as one of the core techniques in green coal mining, can control strata movement and surface settlement, while it fails to dispose of coal gangue. To dispose of coal gangue at large scale, single factor alternative tests were designed based on backfill mining with high-water-content materials and high-water-content materials were used as the binder and gangue as the aggregates. Furthermore, the mechanical properties and damage evolution of gangue and high-water-content materials based cemented paste backfill (GHW-CPB) under three main controlling factors (gangue particle size, gangue content, and water-cement ratio) were studied. The following conclusions were drawn: 1) GHW-CPB shows ductile failure under uniaxial compression and the material has high residual strength, exhibiting good stability after reaching the peak bearing capacity. 2) Relation equations of the three main controlling factors with the uniaxial compressive strength (UCS) and elastic modulus of GHW-CPB are obtained through regression, thus revealing the evolution of the mechanical strength of GHW-CPB. 3) Acoustic emission (AE) and failure characteristics of GHW-CPB in three periods in the uniaxial compression process are determined by using the monitoring systems for crack development and propagation. In addition, the damage equation of GHW-CPB is established based on the cumulative AE count, thus further revealing the damage evolution of GHW-CPB under uniaxial compression. These results provide a theoretical basis for the implementation of backfill mining with GHW-CPB.

Engineering Properties of Cemented Paste Backfill with Full-Range Water-Reducing Admixture

Engineering Properties of Cemented Paste Backfill with Full-Range Water-Reducing Admixture

Assessing the influence of geological stress induced damage and initial pH levels on the metal leaching behaviors of cemented paste backfill

Cemented paste backfill (CPB) is generally considered environmentally friendly when used in mining operations. However, the combination of geological stresses and groundwater seepage in an underground environment may cause the metallic elements contained in CPB to escape confined areas and potentially pollute the external environment. In this paper, geological stress-induced damage to CPB and the initial pH levels of the surrounding environment are assessed to determine their influences on the metal leaching behavior of CPB. Research methods used in the analysis include component characterization, static leaching tests, and kinetic theory analysis. The research results show that: as damage to CPB increased, both the pH and conductivity of the leachate increased, while the concentration of metallic elements in the leachate also increased except for aluminum (Al). Different metallic elements showed various leaching behaviors as the initial pH of the leaching solution increased. The release process of metallic elements from CPB can be segmented into three separate stages: the surface release stage, the internal release stage, and the adsorption and precipitation stage. Kinetic equations are obtained to describe the release rate of metallic elements. The leaching process is a dynamic equilibrium system formed by the release, absorption, and precipitation of metallic elements in the leachate. This research provides data for the evaluation of environmental effects on CPB.

Experimental study on evolutive fracture behavior and properties of sulfate-rich fiber-reinforced cemented paste backfill under pure mode-I, mode-II, and mode-III loadings

Tensile and shear fracture behaviors of fiber-reinforced cemented paste backfill (FRCPB) play critical roles in the safe and cost-effective engineering design of underground mine backfill structures. Sulfate pore solution is a key factor affecting the time-dependent evolution of fracture behavior and properties of FRCPB. However, no studies have been carried out to systematically investigate its effects on the fracture behavior and properties of mine backfill used in underground mines. The objective of this study is to discover the effect of sulfate concentration (0, 5000, 15,000, and 25,000 ppm) on FRCPB's fracture behavior and properties at different curing times (3, 7, 28, and 90 days) and under pure mode-I, mode-II, and mode-III loadings. The results show that, except for the 3-day FRCPB, the increase in sulfate concentration can continuously improve pre-peak stiffness and peak fracture resistance force and significantly enhance the post-peak residual resistance, especially under mode-III loading. Moreover, fracture energy shows a higher sensitivity to the sulfate concentration with respect to fracture toughness and stiffness. Furthermore, sulfate concentration has a very limited effect on tensile crack trajectory under mode I loading, while a transition of shear cracks from a curved crack pattern to a sharp and straight one is detected under mode II and III loadings. In addition, the fracture toughness possesses a stronger correlation with material stiffness and thus is recommended for the design of FRCPB to offer a more effective immediate support to surrounding rock. The obtained results provide an in-depth insight into the sulfate-induced evolution of fracture behavior and properties of FRCPB under various loading conditions and thereby promote the safe implementation of mine backfill technology.

A new acoustic emission characteristic parameter can be utilized to evaluate the failure of cemented paste backfill and rock combination

In backfill mining, there is a phenomenon of the joint bearing of cemented paste backfill and rock, so it is essential to study the destabilization damage mechanism of cemented paste backfill and rock combination (BRC). This study aimed to improve the evaluation system for the instability failure of BRC. A new evaluation index, Dr, based on acoustic emission (AE) characteristic parameters was proposed for predicting BRC instability failure caused by the load. This index comprehensively considers the time dependence and interaction of the AE rise time, amplitude, and average frequency during BRC failure. The study conducted uniaxial compression and simultaneous AE experiments for various ratio combinations of BRC to obtain the AE rise time/amplitude (RA) and average frequency (AF) values of BRC. Then, the RA/AF values were numerically calculated with fractal theory to obtain the Dr index, and the intrinsic relationship between Dr and BRC instability failure was further discussed. Finally, the index was validated through comparison with the statistical index b-value based on AE amplitude, and a discrimination criterion for BRC instability failure was proposed. The results show that: After reaching the yield stress, the number of AE events of the rock gradually decreases. In contrast, the number of AE events of the CPB fluctuates in the high-level range. The cement-tailings ratio significantly affects the BRC AE frequency and amplitude distribution. The decrease of the cement-tailings ratio enhanced the high-frequency amplitude signal of CPB, while weakening the medium and high-frequency amplitude signal of rock. The sudden drop of Dr in the near-failure stage can be used as a criterion for BRC failure determination. Compared with the b-value, using Dr as an evaluation index to predict BRC failure can obtain a slightly conservative result. The research provides a new and effective method for evaluating BRC instability failure, which can be used for real-time prediction of BRC instability failure in mining engineering practice.

Mechanical and Microstructural Properties of Cemented Paste Backfill with Chloride-Free Antifreeze Additives in Subzero Environments

Cement paste backfill (CPB) is an innovative technology of recycling mine waste (tailings) into construction materials to increase mine safety and productivity and promote sustainable mining. However, the application of CPB technology in mines located in cold or permafrost areas has been difficult due to freezing temperatures. These subzero temperatures can cause fresh CPB to freeze while being transported through the pipelines, which has significant financial implications for the mine. To overcome these problems, antifreeze additives are added to the CPB during its mixing or preparation to enhance the rheological properties of fresh CPB. The addition of antifreeze additives in CPB is an inexpensive and easy-to-operate method currently known to ensure its smooth delivery in subzero environments. But the mechanical and microstructural characteristics of CPB with chloride-free antifreeze additives in subzero conditions are not well understood. Therefore, this study experimentally examined the mechanical (strength, deformation behavior) and microstructural properties of CPB containing chloride-free antifreeze additives and cured under subzero conditions. Three kinds of chloride-free antifreeze (calcium nitrate, calcium nitrite, and urea) with different concentrations (0, 5, 15, and 35 g/L) were separately added to the CPB samples curing at different subzero environments (−1°C, −6°C, and −12°C) for different ages (7, 28, 60, and 90 days). The uniaxial compressive strength (UCS) tests, a series of microstructural (XRD, MIP, DT/DTG, SEM) analyses and monitoring (suction, volumetric water content) experiments of CPB were performed. The obtained results showed that the addition of antifreeze weakened the mechanical properties of CPB. Furthermore, the UCS of CPB was significantly affected by the antifreeze type, antifreeze concentration and subzero curing temperature. The findings in this research will support the successful application of antifreeze cemented backfill technology in cold regions.

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.

Usability of Chloride Mine Water in Preparing Cemented Paste Backfill

Usability of Chloride Mine Water in Preparing Cemented Paste Backfill

Strength Investigation and Prediction of Superfine Tailings Cemented Paste Backfill Based on Experiments and Intelligent Methods.

The utilization of solid waste for filling mining presents substantial economic and environmental advantages, making it the primary focus of current filling mining technology development. To enhance the mechanical properties of superfine tailings cemented paste backfill (SCPB), this study conducted response surface methodology experiments to investigate the impact of various factors on the strength of SCPB, including the composite cementitious material, consisting of cement and slag powder, and the tailings' grain size. Additionally, various microanalysis techniques were used to investigate the microstructure of SCPB and the development mechanisms of its hydration products. Furthermore, machine learning was utilized to predict the strength of SCPB under multi-factor effects. The findings reveal that the combined effect of slag powder dosage and slurry mass fraction has the most significant influence on strength, while the coupling effect of slurry mass fraction and underflow productivity has the lowest impact on strength. Moreover, SCPB with 20% slag powder has the highest amount of hydration products and the most complete structure. When compared to other commonly used prediction models, the long-short term memory neural network (LSTM) constructed in this study had the highest prediction accuracy for SCPB strength under multi-factor conditions, with root mean square error (RMSE), correlation coefficient (R), and variance account for (VAF) reaching 0.1396, 0.9131, and 81.8747, respectively. By optimizing the LSTM using the sparrow search algorithm (SSA), the RMSE, R, and VAF improved by 88.6%, 9.4%, and 21.9%, respectively. The research results can provide guidance for the efficient filling of superfine tailings.

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.

Preparing new supplementary cementitious materials with co-calcined waste glass-dolomite and environmental assessment

Recycling waste glass (WG) for supplementary cementitious materials is one of the most viable methods to address the environmental issues of waste generation. In this study, homogenised WG–dolomite with different mass ratios was first calcined at 800 °C and 1000 °C. Then non-cement clinker cementitious materials (NCCMs) were prepared by mixing calcined WG–dolomite, superfine slag (SS) and flue gas desulphurisation gypsum. Finally, coupled with silica tailings, NCCMs were used to prepare cemented paste backfill (CPB) samples. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), Isothermal calorimeter, Thermogravimetry (TG) and scanning electron microscopy (SEM) were employed to study the thermal transformation of calcined WG–dolomite and microstruction evolution of NCCMs hydration. The experimental results show that the main products of calcined WG–dolomite have high contents of MgO, calcite and CaO at 800 °C, whereas they have large proportions of C2S and CA at 1,000 °C. As the calcination temperature increases, the chemical shift of 29Si moves steadily towards higher frequencies, indicating the dominance of Q0/Q1 sites from silicate. In addition, the high contents of MgO and CaO in calcined WG–dolomite accelerate NCCM paste hydration, which is conducive to the formation of C–A–S–H and M−S−H. Moreover, due to SS hydration being more dependent on alkaline environments, co-calcined WG-dolomite at 800 °C enables the CPB sample to have better mechanical performance.

Formation mechanism and thermal decomposition properties of hydration products of superfine tailings cemented paste backfill

Filling mining with solid waste resources as aggregates is an important development direction for cleaner production in mines. The hydration products have an important effect on the mechanical properties of superfine tailings cemented paste backfill (SCPB). This paper investigates the mechanism of hydration product formation and thermal decomposition properties of SCPB by a series of experiments combined with thermal decomposition kinetics. Also, to optimize the mechanical properties of SCPB, the effect of Nano SiO2 (NS) on the related properties of SCPB is investigated. The results show that there are two main stages of the thermal decomposition of SCPB, and the kinetic models of these two stages are both reaction order models. After adding NS, the kinetic model of the second stage of thermal decomposition of SCPB is changed to random nucleation and growth model. The substances that undergo thermal decomposition reactions in the first stage are mainly hydration products such as ettringite and C-S-H, while the substances that undergo thermal decomposition reactions in the second stage are carbonates. The activation energy E and pre-exponential factor A of the first stage of thermal decomposition of SCPB with NS added are significantly improved, which indicates a more stable structure of its hydration products. The microscopic test results show that NS can promote the hydrolysis of C3S and gypsum and increase the production of hydration products, which in turn improves the strength of SCPB. In addition, the addition of NS to SCPB decreases the Ca/Si and increases the average silica chain length of C-S-H gels, while the H2O/Si decreases and the silica group increases, resulting in the enhancement of the structure of C-S-H gels.

Investigating the potential for porous ceramics as bacterial carrier in self-healing cemented paste backfill

Waste porous ceramic (WPC), which remains in the industrial manufacturing process, is difficult to be disposed of. This study proposes a new idea for resource utilization of WPC, which means that WPC as a bacterial carrier applies in self-healing cemented paste backfill. The authors investigated the effects of WPC pore size, initial crack size, and bacterial concentration on the crack self-healing ratio of the backfill, analyzed the crack self-healing evolution process by photographing, and finally used a series of instruments to characterize the physical phase at the sample fracture and explain the mechanism. The results showed that the crack self-healing ratio of WPC with small pore size (pore size distribution of 1–2 mm) as the bacterial carrier was 13–22% higher than that of WPC with large pore size (pore size distribution of 3–4 mm) at 3–7 days. Also, the strength recovery ratio of the cemented paste backfill can be improved to 10–23% when adding bacteria-WPC. The self-healing ratio can reach 100% in 28 days for initial crack widths less than 1 mm, while the self-healing ratio decreases to 98% for widths near 2 mm. Additionally, the biomineralization ratio of the 1.5 mol/L embedded bacterial solution was 2.77–4.68% higher than that of 1.0 mol/L. WPC can effectively release the embedded bacterial solution once the backfill cracks appear, which proves the feasibility of WPC as a bacterial carrier. This study also lays the experimental and theoretical foundation for porous ceramic management.

Effect of composite activators on mechanical properties, hydration activity and microstructure of red mud-based geopolymer

Portland cement is the most commonly used cementing material in cemented paste backfill. However, the production of it is limited by high energy consumption and environmental pollution. The development of cementless solid waste filling material has been attracting the attention of researchers. In this study, the potential application of CaO/CaSO4 composite activator in red mud based geopolymer was explored. The mechanical properties, hydration activity and microscopic structure of samples were analyzed by uniaxial compressive strength (UCS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric (TG), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results showed that the combination of CaO and CaSO4 can significantly increase the strength of red mud-based geopolymer at early ages. The ratio of CaO to CaSO4 played a crucial role in enhancing the mechanical properties of red mud-based geopolymer. The samples containing 5% CaO and 10% CaSO4 showed the highest UCS, which was 1.6 MPa after curing for 28 days. The simultaneous existence of CaO and CaSO4 can improve the polycondensation degree of red mud-based geopolymer. Under the action of composite activator, the chemical bonds of Si−O and Al−O in silicon oxygen tetrahedron [SiO4]4− and aluminum oxygen tetrahedron [AlO4]5− continue to depolymerize and recombine, so as to form Si−O−Al−O long-chain red mud-based geopolymer. This approach can help reduce the filling cost and promote the simultaneous utilization of red mud, fly ash, and gangue to protect the environment.

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.

The Failure Mechanical Properties of Cemented Paste Backfill with Recycled Rubber.

Understanding the mechanical properties and failure process of cemented paste backfill with recycled rubber (RCPB) is the foundation of backfill design in underground mining. In this study, physical and mechanical tests were conducted on RCPB to obtain its mechanical property parameters, such as its uniaxial compressive strength (UCS), toughness, and peak strain. The influence of the rubber dosage on the mechanical properties of RCPB was also analyzed. In addition, the deformation behavior, fracture development, and failure process of RCPB with different rubber contents were observed using the digital image correlation (DIC) technique. The experimental results suggested that, although the UCS of RCPB is reduced as more rubber is added, its toughness and ability to absorb energy is increased. Moreover, the impact resistance of RCPB is improved by this increased toughness. With the increase in the rubber content, the deformation corresponding to the plastic yield stage of RCPB increased, which resulted in better ductility and improved impact resistance. The failure of the RCPB specimens mainly showed an "X" shape. The results of this study help us to better understand the mechanical behavior of RCPB after backfilling underground.

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.

Effect of ice addition on the properties and microstructure of cemented paste backfill at early-age

Introducing ice slag into cemented paste backfill is one way to simultaneously meet the challenges in hot workplaces and promote tailings recycling. In this work, iced cemented paste backfill (ICPB) at early-age were investigated with a specific emphasis on the influence of ice content. A series of tests including hydration heat, real-time measurement of electrical conductivity, temperature and volume water, SEM-EDS, unconfined compressive strength (UCS), nuclear magnetic resonance, and free-free resonant column were carried out. The results show that the addition of ice slag influenced the cement hydration through the decrease of temperature and the supply of water. Hydration heat mainly focused on the supply of water and the hydration kinetics were increased with the increase of the ice-to-cement (ICR). The temperature was decreased while the electrical conductivity was increased with the increasing ICR. Compared with CPB without ice slag, the volume water content was evidently increased upon the addition of ice slag. The pore size and permeability were increased with the increase of ICR while the bound water volume, UCS and longitudinal wave velocity were decreased with the increasing ICR. SEM-EDS supported the above results from a microstructural point of view.

Preparation and Characterization of Low-Carbon Cementitious Materials Based on Soda-Residue-Activated Ground Granulated Blast-Furnace Slag: A Case Study on Cemented Paste Backfills

To investigate the issues related to significant environmental damage and poor resource utilization of soda residue (SR), the composition and microstructure of hydration products of the GGBS (ground granulated blast-furnace slag) synergistically activated by NaOH-SR are characterized by an X-ray diffraction (XRD), Fourier-transform infrared spectrometry (FTIR), thermogravimetric–differential thermogravimetric (TG-DTG) analysis, and scanning electron microscope X-ray energy dispersive spectrometry (SEM-EDS). The effect of SR proportion, activator dosage, and water-to-binder ratio on the hydration process is studied. Results indicate that the hydration products mainly include hydrated calcium chloroaluminate (3CaO·Al2O3·CaCl2·10H2O, FS), hydrated calcium aluminosilicate (Ca2Al3(SiO4)3OH, C-A-S-H), halite (NaCl), calcite (CaCO3), and C-S-H gel. With the increase in SR proportion (especially from 80% to 90%), the C-S-H gel yield decreases significantly, while the FS yield rapidly increases, and the T-O-Si (T = Al or Si) peak shifts to a higher wavenumber range (955 cm−1 to 975 cm−1). With the decrease in activator dosage (40% to 15%), the hydration reaction gradually weakens, and the FTIR band of the T-O-Si (T = Al or Si) shifts to the lower wavenumber range (968 cm−1 to 955 cm−1). Then, cemented paste backfills (CPBs) are prepared with iron tailings as the aggregate. At mass content of 75%, SR proportion of 80%, and activator dosage of 30%, the fluidity of the CPB reaches 267.5 mm with a 28-day unconfined compressive strength (UCS) of 2.4 MPa, confirming that SR- and NaOH-synergistically-activated GGBS has great application prospects in backfill mining.