Compressive Strength Characteristics of Cemented Tailings Backfill with Alkali-Activated Slag

A Review on Development of Industrial Solid Waste in Tunnel Grouting Materials: Feasibility, Performance, and Prospects

With rapid infrastructure development worldwide, the generation of industrial solid waste (ISW) has substantially increased, causing resource wastage and environmental pollution. Meanwhile, tunnel engineering requires large quantities of grouting material for ground treatment and consolidation. Using ISW as a component in tunnel grouts provides a sustainable solution to both issues. This paper presented a comprehensive review of the recent advancements in tunnel grouting materials using ISW, focusing on their feasibility, mechanical characteristics, and future development directions. Initially, the concept and classification of ISW were introduced, examining its feasibility and advantages as grouting materials in tunnels. Subsequently, various performances of ISW in tunnel grouting materials were summarized to explore the factors influencing mechanical strength, fluidity, durability, and microstructure characteristics. Simultaneously, this review analyzed current research trends and outlines future development directions. Major challenges, including quality assurance, environmental risks, and lack of standardized specifications, are discussed. Future research directions, including multifunctional grouts, integrated waste utilization, and advanced characterization techniques, are suggested to further advance this field. These findings provided useful insights for the continued development of high-performance and environmentally friendly ISW-based grouting materials.

Experimental Study on Mechanical Properties of Coal-Based Solid Waste Nanocomposite Fiber Cementitious Backfill Material

Previous studies have shown that coal-based solid waste can be utilized in combination with cement, silica fume, and other modified materials to create a cemented backfill material. However, traditional cemented backfill materials have poor mechanical properties, which may induce the emergence of mining pressure and trigger dynamic disaster under complex mining conditions. In this study, the nanocomposite fiber was used to modify the traditional cemented backfill materials and a new cemented backfill material was developed using coal-based solid waste, nanocomposite fiber and other materials. Specifically, coal gangue, fly ash, cement, and glass fibers were used as the basic materials, different mass fractions of nano-SiO2 were used to prepare cemented backfill materials, and the mechanical enhancement effect of the compressive strength, tensile strength, and shear strength of the modified materials was analyzed. The results show that when the nano-SiO2 dosage is 1%, the optimal compressive strength of the specimens at the curing age of 7 d can be obtained compared with cemented materials without nano-SiO2, and the compressive strength of the modified specimens raises by 84%; when the nano-SiO2 dosage is 1%, the optimal tensile strength and shear strengths of the modified specimens can be obtained at the curing age of 28 d, increasing by 82% and 142%. The results reveal that nanocomposite fibers can be used as additives to change the mechanical properties of cemented backfill materials made using coal-based solid waste. This study provides a reference for the disposal of coal-based solid waste and the enhancement of the mechanical properties of cemented backfill materials.

Influencing Factors of Sulfuric Acid Resistance of Ca-Rich Alkali-Activated Materials

In this paper, we distinguished the degradation of alkali-activated material (AAM) exposed to sulfuric acid as physical (scaling, spalling, cracking, breaking, etc.) and chemical degradation (neutralization), because the mechanisms of these two types of degradation are different. Then, the effects of curing method, raw materials, and their mixing proportions on the two kinds of degradation of AAMs containing GGBFS were investigated in detail, including liquid-filler ratio, component of alkali activator, chemical admixture, inactive filler alternative to fly ash (FA), addition of municipal waste incineration bottom ash (BA), etc. The experimental results show that (a) small liquid-filler ratio, heat-curing, and the use of blended alkali activator solution of sodium silicate and NaOH can reduce both physical and chemical degradation of AAMs; (b) large GGBFS content or AE agent addition decreases the physical degradation, but increases the chemical degradation; (c) using crushed stone powder to replace FA and adding BA or a retarder would increase the physical and chemical degradation; but (d) the use of drying shrinkage reducer composed of polyether derivatives does not affect acid resistance. We also discussed the applicability and limitation of XRD and SEM-EDS in analyzing the chemical compositions of Ca-rich AAMs exposed to sulfuric acid, and found that (e) XRD analysis can identify the gypsum formation, and the gypsum peak intensity is related to the physical degradation of the Ca-rich AAMs; (f) by SEM-EDS analysis, the decalcification and dealkalization of C-A-S-H gels can be judged from the decrease in the average Ca/Si atomic ratio and the average Na atomic percentage in the acid corrosion area, but dealumination can be only determined from the dissimilarity of Al and Si elemental maps; and (g) if the CaO/SO₃ molar ratio ranges from 0.8 to 1.0, gypsum formation can be estimated.

Investigation on Dynamical Mechanics, Energy Dissipation, and Microstructural Characteristics of Cemented Tailings Backfill under SHPB Tests

As mining depth increases, the backfill mining method is more and more widely used in underground mines. The dynamic load generated by the blasting can affect the stability of the cemented tailings backfill (CTB). The CTB samples were prepared to conduct a test of the split Hopkinson pressure bar (SHPB) to investigate the dynamic disturbance of CTB. The present paper discusses dynamical mechanics, energy dissipation, and microstructure analysis of CTB. Micro-computer tomography (micro-CT) scanning of CTB samples after the SHPB test was performed to analyze the evolution of internal cracks. The experimental results showed that when the average strain rate (ASR) increased from 30 to 98 s−1, the dynamic uniaxial compression strength (DUCS) of the CTB showed a trend of first increasing and decreasing with the increase in ASR. The dynamic stress–strain pre-peak curve of CTB directly enters the linear elastic stage. As ASR increases, the absorbed energy of the CTB shows a trend of first increasing and then decreasing. Moreover, according to the micro-CT scanning results, the crack area of CTB accounts for about 16% of the sample near the incident bar and about 1% near the transmitted bar. The crack area ratio is exponentially related to the specimen height. These findings can provide reasonable dynamical CTB strength data selection for underground pillar mining.

Establishment of the Controlled Low-Strength Desulfurization Slag Prediction Model for Compressive Strength and Surface Resistivity

In this study, the desulfurization slag used the volume method to replace fine natural aggregates in controllable low-strength materials (CLSM); the desulfurization slag content (DS) and curing time (t) were used as variables to test the compressive strength and surface resistivity of CLSM and simulated a prediction model on the results. The test results showed during that the compressive strength on the 28th day, the average desulfurization slag replacement amount increased by 10%, and the compressive strength decreased by 0.9 MPa. The surface resistivity increases with age, and each ratio increases from seven days to 28 days, and the surface resistivity value increases from 9.3% to 20.6%. After that, a hyperbolic function and exponential function with multiple variables were used to establish a simulation model of the effects of the DS content and curing time on the compressive strength and surface resistivity of CLSM. Compared with the test results, the statistical analysis shows that the average absolute percentage error (MAPE) of the compressive strength is 9.17%, and the surface resistivity is 10.67%. From the results, the predictive analysis model developed in this paper provides good predictive results in terms of compressive strength and surface resistivity.

Rheological Properties of Cemented Paste Backfill with Alkali-Activated Slag

This study investigates the time-dependent rheological behavior of cemented paste backfill (CPB) that contains alkali-activated slag (AAS) as a binder. Rheological measurements with the controlled shear strain method have been conducted on various AAS-CPB samples with different binder contents, silicate modulus (Ms: SiO2/Na2O molar ratio), fineness of slag and curing temperatures. The Bingham model afforded a good fit to all of the CPB mixtures. The results show that AAS-CPB samples with high binder content demonstrate a more rapid rate of gain in yield stress and plastic viscosity. AAS-CPB also shows better rheological behavior than CPB samples made up of ordinary Portland cement (OPC) at identical binder contents. It is found that increasing Ms yields lower yield stress and plastic viscosity and the rate of gain in these parameters. Increases in the fineness of slag has an adverse effect on rheological behavior of AAS-CPB. The rheological behavior of both OPC- and AAS-CPB samples is also strongly enhanced at higher temperatures. AAS-CPB samples are found to be more sensitive to the variation in curing temperatures than OPC-CPB samples with respect to the rate of gain in yield stress and plastic viscosity. As a result, the findings of this study will contribute to well understand the flow and transport features of fresh CPB mixtures under various conditions and their changes with time.

The Mechanical and Microstructural Properties of Composite Structures Made of a Cement-Tailing Backfill and Rock Core

In underground metal mines that use sublevel or stage open-stope and backfilling mining methods (SSOBMMs), there is a special structure around which both sides of the rock pillar are wrapped by backfill. As a permanent part of an underground mine, how much can backfill improve the rock pillar’s compressive strength? What is the difference in the mechanical properties between the special structure and the signal rock? To explore these questions, a composite structure made of a cement-tailing backfill (CTB) and rock core (RC) was designed. Uniaxial and triaxial compressive strength tests and scanning electron microscope (SEM) were used to research the mechanical properties, failure process, failure characteristics, and microstructure characteristics of the cement-tailing backfill and rock core (CTB-RC) specimens. It was found that the full stress–strain curve of the CTB-RC specimen under triaxial compressive strength (TCS) test had two times the stress increases reaching a lower peak deviator stress two times after the RC was destroyed. The CTB can reduce the destruction and slow down the deformation speed of the inner rock cor (IRC). It can also prevent rigid slip of the IRC after it is damaged and maintain the stability and integrity of the overall structure. The findings of this study can provide some basic knowledge on the mechanical properties of the CTB-RB and provide theoretical guidance for the optimization direction of the width of the rock pillar and the room in mines using SSOBMMs.

Ultrasonic evaluation of strength properties of cemented paste backfill: Effects of mineral admixture and curing temperature

This paper presents the findings of a research study designed and conducted to investigate the effects of mineral admixture and curing temperature on uniaxial compressive strength (UCS) and ultrasonic pulse velocity (UPV) behavior of laboratory-prepared cemented paste backfill (CPB) samples. A total of 290 CPB samples were prepared at different replacement rates (10-80%), cured at various temperatures (10-50 °C), and respectively subjected to both UPV and UCS testing after curing times of 3, 7, 14, 28, 56 and 90 days. The obtained experimental results show that the addition of fly ash (FA) can lead to an increase or decrease trend in UCS and UPV behavior of CPB samples, depending on the replacement level of admixtures. There is a competition between the strength-increasing factor (micro-filler effect of FA) and strength-decreasing factor (lower amount of cement hydration products induced by replacement ratio). Both UPV and UCS are found to decrease with increasing blast furnace slag (Slag) replacement level mainly attributable to its low pozzolanic reactivity. Besides, the curing temperature has a significant influence on UCS and UPV behavior, depending on the curing time. Results also suggest that UPV is less sensitive to the variation in the admixture dosage and curing temperature than UCS. As a result, there exists a clear linear relationship between UPV and UCS behavior of both CPB samples prepared with FA and/or Slag admixtures, and CPB samples tested at each curing temperature. The main findings of this research study suggest that the UPV test can be reliably used for predicting CPB's strength properties, saving money and time to mine operators.

Properties and Application of Backfill Materials in Coal Mines in China

Coal is the basic resource underpinning energy generation in China, however, constant, large-scale mining of coal results in many problems such as ecological destruction of mining areas. As a result, backfilling of solid waste underground is proposed to control strata and surface subsidence and to protect the environment. At present, these materials, such as granular material, cemented material and high-water-content materials are mainly used for backfilling. This study summarised the types of backfill materials that are used in coal mines in China along with the backfilling process. Moreover, distribution and characteristics of mines backfilled with these backfill materials were obtained and analysed. Considering the socio-environmental aspects that affect backfilling, this research proposed a guideline for the selection of backfill materials and then analysed specific engineering cases of three backfill materials. In addition, the future development of backfill materials was discussed. With extensive extraction of shallow coal resources in China and, therefore, rapid depletion of coal resources in eastern regions of China, coal mining depth is increasing significantly. As a result, it is required to investigate new backfill materials suited for the deep high-stress environment.

Laboratory and Field Experiments on the Effect of Vinyl Acetate Polymer-Reinforced Soil

Soil stabilizer can enhance the soil properties, which can improve ecological environmental problems such as soil erosion and slope instability. This study investigates the water-related and mechanical properties of soil stabilization using a polymer soil stabilizer solution synthesized from vinyl acetate polymer. The water properties test, mechanical properties test, durability test and seed growth test were carried out in the laboratory. Also, the effect of vinyl acetate polymer was verified by field tests. The results revealed that vinyl acetate polymer can enhance the water-retaining property, anti-erosion resistance, strength property and durability of soil, all of which increase with the increase of polymer concentration. At the same time, the polymer also has a certain promoting effect on vegetation growth. In addition, field tests proved that vinyl acetate polymer has good effects on soil engineering. These results could be applied as a reference for vinyl acetate polymer-improved soil engineering.