Innovative Application and Research of Industrial Solid Waste in Mining Filling Materials in China

Abstract

The swift advancement of China’s mining sector has led to the generation of substantial amounts of industrial solid waste, which poses significant risks to the ecological environment. This study aims to investigate effective methods for utilizing industrial solid waste in the production of mine filling materials, thereby facilitating green mine construction and the efficient use of resources. The study employs the PRISMA methodology to conduct a systematic review of the pertinent literature, analyzing the current status, challenges, and developmental trends associated with the use of coal-based solid waste, smelting waste, industrial by-product gypsum, and tailings in filling materials. The findings indicate that, while the use of individual coal-based solid waste in filling materials shows promise, there is a need to optimize the ratios and activation technologies. Furthermore, the synergistic application of multi-source coal-based solid waste can enhance the overall utilization rate; however, further investigation into the reaction mechanisms and ratio optimization is required. Smelting slag can serve as a cementing agent or aggregate post-treatment, yet further research is necessary to improve its strength and durability. Industrial by-product gypsum can function as an auxiliary cementing material or activator, although its large-scale application faces significant challenges. Tailings present advantages as aggregates, but concerns regarding their long-term stability and environmental impacts must be addressed. Future research should prioritize the synergistic utilization of multi-source solid waste, performance customization, low-carbon activation technologies, and enhancements in environmental safety. Additionally, the establishment of a comprehensive lifecycle evaluation and standardization system is essential to transition the application of industrial solid-waste-based filling materials from empirical ratios to mechanism-driven approaches, ultimately achieving the dual objectives of green mining and the resource utilization of solid waste in mining operations.

1. Introduction

The mining sector in China serves as a crucial foundational industry, contributing significantly to the provision of energy and raw material security, thereby facilitating economic and social development [1]. Since the onset of the new era, China has experienced rapid industrialization, during which the mining industry has consistently exhibited high-speed growth. Notably, fixed asset investments have shown positive growth for three consecutive years, the consumption of major minerals has steadily increased, and energy production has reached unprecedented levels [1,2]. However, the processes involved in mining, metallurgy, and other industrial production and processing activities have resulted in the generation of substantial amounts of derived residues [3,4]. These residues manifest in various forms and originate from a diverse array of sources, including dust, slag, sludge, and other by-products produced during manufacturing, collectively categorized as general industrial solid waste [5,6].

Solid waste can be classified based on its source and characteristics into categories such as coal-based solid waste, industrial by-product gypsum, tailings sand, and smelting slag. Numerous experts and scholars have undertaken extensive research on the potential for resource utilization of industrial solid waste across various sectors, including mining, environmental protection, construction and building materials, agriculture, and the chemical industry [7,8,9,10,11,12], as illustrated in Figure 1. Some studies have focused on the utilization of silica-aluminate-rich solid wastes, such as fly ash and slag, as raw materials for the synthesis of geopolymers, leveraging the silica-aluminate and alkali reaction to produce gelling agents [13,14]. In this context, mining operations may employ geopolymer-based gelling materials as alternatives to traditional cement in the formulation of new cemented filling materials [15,16,17]. Furthermore, due to their developed pore structure and high specific surface area, geopolymers are being explored for applications in carbon sequestration, as adsorbents, and in chemical engineering within the environmental protection domain [18,19,20]. Additionally, metallurgical slag (including steel slag and red mud) contains components such as SiO₂ and Al₂O₃, which can be utilized in the chemical industry to produce microcrystalline glass through nucleation and crystallization reactions [21,22].

Data on solid waste production from various industrial sectors in 2023 indicates that the majority of the top five industries contributing to waste output are situated within the mining sector [23], which collectively represents over fifty percent of the total waste generated, as illustrated in Figure 2. The inherent harmful physical and chemical properties of general industrial solid waste can lead to significant adverse effects on the ecological environment when managed improperly. Such effects include soil and water contamination, encroachment upon land resources, and the potential initiation of geological disasters [24,25]. In alignment with sustainable development objectives, China has increasingly prioritized ecological protection and has actively promoted the establishment of green mining practices. The mining sector is currently experiencing pressures to transform and upgrade towards sustainability and green development, resulting in a heightened investment in the management of industrial solid waste. Research focused on the resourceful and high-value utilization of solid waste has made substantial progress, leading to a continuous increase in the comprehensive utilization rate [23], as illustrated in Figure 3. This shift represents a transition from rudimentary management practices to more refined utilization methods, thereby facilitating advancements in green development and the circular economy.

Considering the large volume and wide range of solid wastes, parties involved in the construction of green mines need to consider them as usable resources, expand the multiple resourceization channels for solid waste, and improve the comprehensive utilization rate [26]. Filling mining technology has attracted a lot of attention as an ecological protective production technology in recent years [27,28,29], which has developed rapidly. This technology is based on the preparation of filling materials to be placed into the mining tunnel to replace the mineral resources, which has significant advantages in terms of green production and safe mining. The filling material is a mixture of different components in appropriate proportions, including water, cement as a cementitious material, sand and gravel as aggregate, and various additives that are used to optimize different performance indexes.

The development of filling mining technology both domestically and abroad has gone through four general stages, from dry waste rock filling, water–sand fillings to cemented filling and paste filling, high water fillings, and so on. At present, compared with other filling processes, cemented filling has become the main filling technology used in mining due to its advantages of fast filling speeds and large filling volumes. However, cemented filling also has some disadvantages. According to statistics, the cost of cemented filling accounts for about one third of the mining cost, and cement accounts for about 30% to 60% of the cost of cemented filling [28]. Cement, as the main cementitious material, means that filling materials have high preparation costs, and mine applications are limited by economic benefits. On the other hand, due to the geological conditions, mine fillings vary from place to place; the preparation of filling materials needs to account for the adaptability of the complex environment, but also stimulate the demand for special engineering properties of the material, such as seepage resistance, the expansion of the top of the joints, and other properties.

These shortcomings constrain the mining application of mine filling technology materials, so many domestic experts and scholars have sought to improve filling material research [30,31,32,33,34] in order to meet the basic requirements of mine filling strength according to local conditions. There has been an effort to add components related to external industrial solid waste replacement materials, reduce the cost of preparation, and improve the engineering characteristics of the material. At the same time, the existing research has explored the strength formation mechanism and reaction mechanism of the newly developed filling materials. Promoting research on related areas can help to solve the contradiction and problem of the high intensity and underutilization of solid waste generation in China today. In this study, we first conduct a systematic literature review of the research on solid-waste-based filling materials according to the PRISMA method. In addition, we focus on the current status of the application of industrial solid waste in filling materials; we then objectively analyze and summarize the research progress of various solid-waste-based filling materials and the constraints related to the study of the preparation of solid waste-based filling materials, in order to provide a reference for the optimization of the proportioning and the engineering characteristics of the improved solid-waste-based filling materials in the future.

2. Literature Search Methodology

A systematic review process is employed for the literature survey, providing a comprehensive overview of key research related to the specific topic of investigation. This approach systematically identifies, selects, evaluates, and synthesizes the highest-quality evidence relevant to the subject matter [35].

2.1. Criterion of Selection and Sources of Data

A systematic literature review was conducted following the appropriate PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) framework [36,37]. The PRISMA checklist contains a total of 27 checklist items that describe, in order of the organization of the body of a research study (from the title to the abstract, introduction, methods, results, discussion, and other additional information), each step that should be followed in a scientifically rigorous and reproducible review study. Each step of a scientifically rigorous and reproducible review study is described.

The process consists of four steps: a literature search, literature de-duplication, initial screening, and full-text review with final inclusions. Given the disparities in database coverage between Chinese and English journals, the China National Knowledge Infrastructure (CNKI) was chosen as the Chinese database, while the Web of Science served as the English database. Utilizing selected keywords, domestic literature was screened in accordance with the aforementioned classification of solid wastes, resulting in a total of 2178 records. The search keywords, along with the applied limiters, are detailed in

Table 1. Keywords, and preliminary search results for each database explored.

Data Source	Keywords	No. of Papers
Documents identified using the CNKI database	TOPIC (((coal-based solid waste) OR (fly ash) OR (coal gangue) OR (coal gasification slag)) AND (filling material))	612
	TOPIC ((tailings) AND (filling material))	142
	TOPIC ((gypsum) OR (by-product gypsum)) AND (filling material))	155
	TOPIC (((smelting slag) OR (blast furnace slag) OR (red mud) OR (non-ferrous smelting slag) OR (steel slag)) AND (filling material))	66
Documents identified using the Web of Science database	TOPIC (((coal-based solid waste) OR (fly ash) OR (coal gangue) OR (coal gasification slag)) AND (filling material))	511
	TOPIC ((tailings) AND (filling AND material))	182
	TOPIC ((gypsum) OR (by-product gypsum)) AND (filling material))	146
	TOPIC (((smelting slag) OR (blast furnace slag) OR (red mud) OR (non-ferrous smelting slag) OR (steel slag)) AND (filling material))	364
	Total=	2178

. Following the removal of 690 duplicate entries, 1488 unique documents were identified. A thorough review of the titles and abstracts of all records led to the selection of 86 and 121 documents, respectively. Subsequently, after further evaluation of the abstracts and titles, 207 records were chosen for the full-text assessment, ultimately yielding 86 potentially eligible records. The selection process for the various studies is illustrated in Figure 4.

2.2. Publication Information

The temporal distribution of reviewed articles is presented in Figure 5. All 86 studies that were considered for the systematic review process were published between the years 2013 and 2024. All 86 studies that were considered for the systematic review process were published between the years 2013 and 2024. In the past five years, a total of n = 61 studies were reported. However, the highest number of studies (n = 50) was reported between the years 2022 and 2024. The maximum number of studies, i.e., n = 24, was published in the year 2022. This establishes that, recently, there has been a steady rise in interest in the topic of consideration.

3. Research Progress on the Preparation of New Mine-Filling Materials from Solid Mining Waste

The utilization of industrial solid waste in the development of mine-filling materials has emerged as a significant focus for researchers both domestically and internationally in the realm of green mining technology. Scholars are investigating the relationship between the source of solid waste and its associated physical and chemical properties, tailoring their approaches to meet the specific requirements of various mining operations based on local conditions. This research encompasses the categorization of innovative filling materials, the optimization of their composition, and the examination of the mechanisms influencing the strength and engineering properties of these materials, as illustrated in Figure 6.

3.1. Preparation of Filling Materials from Coal-Based Solid Wastes

3.1.1. Single Coal-Based Solid Waste

Coal-based solid waste encompasses various solid residues produced throughout the entire life cycle of coal resource utilization. This waste is generated during the complete industrial chain associated with coal development and utilization, which includes critical stages such as mining, washing, processing, combustion, and transformation. Based on the stage of formation and variations in physical and chemical properties, coal-based solid waste can be primarily categorized into several types, including gangue, fly ash, gasification slag, and desulfurization gypsum, among others. This category of solid waste exhibits notable “homologous and heterogeneous” characteristics; despite originating from the same coal resource development system, the mineral composition, physical and chemical properties, and environmental behavior of this waste differ significantly due to varying generation mechanisms and processing conditions. It is particularly important to highlight that, in China, coal-based solid waste is characterized by the phenomenon referred to as “two highs and one wide”: the annual production reaches billions of tons, the total stockpiles exceed 70 billion tons, and the geographical distribution spans major coal-producing and energy-consuming regions. However, the comprehensive utilization rate remains below 70%, and large-scale resource utilization continues to encounter substantial technical challenges.

Table 2. Main types of coal-based solid waste and their characteristics.

Source Link	Solid Waste Products	Characteristics
Coal Mining Segment	Coal gangue	Rock or low-calorific-value coal with low carbon content and high ash content separated during coal mining and washing. Accounting for 10% to 20% of coal production, the accumulation is prone to spontaneous combustion, dust, and heavy metal pollution.
Coal Washing Segment	Washing gangue	Impure and low-quality coal sorted out by coal washing plants. It is similar to gangue but with higher water content, and it is easy to slate.
	Coal slurry	Fine particles of suspended matter produced in the coal washing process and the mud formed after dewatering. Characteristics: high moisture, high viscosity, difficult to handle.
Coal power generation/heating link	Fly ash	Fine ash collected from boiler flue gas in coal-fired power plants. Rich in silicon, aluminum and iron oxides, it can be used as a raw material for building materials.
	Desulfurization gypsum	By-product from flue gas desulfurization (e.g., the limestone–gypsum method).
	Furnace slag	Molten residue discharged from the bottom of coal-fired boilers. It has coarse particles, a porous structure, and can be used for road building or brick making.
Coal gasification/chemical link	Gasification slag	Residue after the high-temperature gasification of coal in coal gasifiers, divided into coarse residue and fine residue (fly ash). It has a low carbon content and contains silicon, aluminum and other inorganic components; the difficulty of resource utilization is high.
	Coal chemical waste products	Waste catalyst and tar slag produced in the coal-to-oil, coal-to-gas and coal-to-olefin processes.

below delineates the primary types of coal-based solid waste along with their sources and characteristics. Coal-based solid waste is an inert material that requires the activation of its gelling properties through mechanical excitation, chemical excitation, and complex excitation. Physical excitation can destroy the original glassy dense structure of the solid waste; at the same time, physical excitation can also increase the specific surface area of the solid waste, to promote the contact of reactive substances and the reaction rate. The chemical activation of coal-based solid waste for the production of cementitious materials involves the incorporation of suitable chemical exciters, which initiate a chemical reaction. This process alters the structure and characteristics of the vitreous components within the solid waste, leading to the transformation, polymerization, or cross-linking of its constituents, ultimately resulting in the formation of products with cementitious properties. Commonly utilized chemical exciters encompass bases, salts, and acids.

Since the onset of the 21st century, significant advancements have been observed in China’s coal mining technology, particularly in the development of environmentally sustainable and efficient filling techniques, such as paste/paste-like fillings and overburden off-layer slurry filling. Overburden slurry filling involves the injection of filling materials into voids created by the subsidence and fracturing of overburden rock following coal extraction, utilizing high-pressure slurry pumps to facilitate the process. Currently, the predominant material used for single off-seam grouting is fly ash slurry, which limits the advancement of grouting and filling technologies. Consequently, numerous studies [38,39] have been conducted to explore the incorporation of gangue to reduce the reliance on fly ash. For instance, Han et al. [40] have investigated the combination of coal slurry with finely ground gangue powder to formulate overburden off-seam grouting and filling materials. They conducted orthogonal experiments to ascertain the optimal slurry composition while also integrating various types of coal-based solid waste to expand the range of raw materials available for grouting applications. This approach effectively addresses the issue of surface subsidence and ensures the maintenance of the injection ratio. In the context of paste filling mining technology, coal-based solid waste is combined with cement to serve as both aggregate and cementitious materials, resulting in a paste filling material characterized by fluidity and adequate strength. This material is subsequently transported or pumped to the underground mining site for filling purposes, as illustrated in Figure 7. Wang et al. [41] conducted a study examining the mechanical properties and damage characteristics of coal-based solid waste paste filling materials with varying water content, as illustrated in Figure 8b,c. Figure 8a reveals the preparation process of coal-based solid waste paste filling materials and the related engineering performance tests. The infiltration of water into microcracks within a material results in an acceleration of the microcracks under compressive stress, causing them to expand and evolve into larger fractures. This process ultimately contributes to a decrease in the structural integrity of the object (Figure 8d). The experimental findings indicate that the compressive strength of the filling material exhibited a gradual decline as the water content increased, with a more pronounced effect observed at higher water levels. Utilizing the experimental data, the authors developed a damage ontology model to investigate the mechanisms by which water content affects the filling strength from multiple perspectives. Wang et al. [42] conducted a study on the formulation and physicochemical characteristics of a novel soil paste filling material, utilizing coal zircon, laterite, and cement as the primary raw materials. Through strength testing, the optimal ratio of coal zircon, laterite, and cement was established as being 6:2:1, with a paste concentration of 80%. The findings indicated that the incorporation of laterite enhanced particle gradation and improved the overall structural integrity of the material. However, the preparation and application of paste filling materials encounter challenges related to inadequate bonding rates, which are attributed to water secretion and gravitational settling effects.

The current methodologies for the preparation and application of paste filling materials encounter challenges related to water secretion and the effects of gravitational settling, which result in suboptimal performance. Consequently, expansion foam materials have emerged as a pivotal innovation in the realm of traditional paste filling materials. Extensive research has been conducted on the development of expansion foam materials derived from coal-based solid waste. In the context of coal mining, the utilization of bentonite, often regarded as a companion mineral, remains underexploited. Bentonite, which is primarily composed of montmorillonite, possesses the ability to absorb water and expand, characteristics that can be advantageous when incorporated into the formulation of filling slurries for expanded filling materials. Zhang et al. [43] have explored the combination of cement and fly ash to create composite cementitious materials, utilizing coal gangue as an aggregate to develop self-expanding paste filling materials. Their findings indicate that the fine particles of bentonite, upon contact with water, expand and fill the pores within the cemented paste, thereby reducing the porosity and permeability of the filling body. Additionally, fly ash, recognized for its pozzolanic properties [44], can significantly enhance the compatibility of the paste material. Wang et al. [45] incorporated calcium-based bentonite into gangue filling slurry and subsequently evaluated the slump, water secretion rate, and compressive strength of the resulting material. The findings indicated that the addition of bentonite enhanced the flow characteristics, mechanical properties, and permeability of the gangue cemented backfill, thereby improving the performance of the novel filling material. Furthermore, Lin et al. [46] integrated ultrasonic testing technology with uniaxial compression tests to assess the sensitivity of various acoustic parameters on compressive strength, ultimately establishing a predictive model for the strength of the filling body based on different dosages of sodium bentonite. Collectively, these studies have advanced the adoption and application of expansion fill mining, while also broadening the avenues for the resource utilization of bentonite.

The process of coal gasification generates solid waste known as coal gasification slag, which possesses complex chemical properties and a significant amount of residual carbon. This residual carbon adversely affects the gelling activity of the slag, leading to a low rate of resource utilization. However, coal gasification slag shows considerable potential for application in mine filling. Research conducted by Zhang et al. [47] investigated the impact of coal gasification slag on the performance of gangue paste filling materials. Their findings indicated that substituting gangue fine aggregate with slag enhances the fluidity of the paste material. The spherical particle characteristics of the slag facilitate lubrication among the gangue aggregates, thereby reducing the yield stress and plastic viscosity of the filling paste. Furthermore, it is essential to consider the activation of coal gasification slag as an inert material during the cementation of the paste. In a separate study, Qu et al. [48] examined the feasibility of utilizing coal gasification slag (CS) as a paste cementation filling. They prepared a modified coal gasification slag filling material using sodium sulfate as an activator. The results demonstrated that sodium sulfate effectively activated the slag, leading to the generation of additional hydration products, such as calcium alumina. This activation improved the ductility and reduced the shrinkage of the filling body, resulting in a significant increase in compressive strength. In addressing the significant issue of the substantial accumulation of gasification slag and insufficient resource utilization in the Ningdong mining area, Chen et al. [49] conducted an experimental study utilizing the response surface methodology. This investigation focused on three key variables: the mass fraction of gasification slag, the mass ratio of gasification slag to cement, and the slurry content. The findings indicated that optimal performance of the filling material was achieved with a gasification slag dosage of 48%, a mass ratio of gasification slag to cement of 3, and a slurry content of 80%. The above study not only reveals the strength formation mechanism of the coal gasification slag filling body but also selects the corresponding proportioning scheme and parameters for different functional requirements (e.g., surface settlement control, fast filling to reduce pipe plugging, and strength and cost balance).

3.1.2. Multi-Source Coal-Based Solid Waste

In alignment with the strategy aimed at enhancing the large-scale utilization of industrial solid waste, the comprehensive utilization of multi-source coal-based solid waste has emerged as a prominent area of research. Investigating the use of individual solid waste types for the production of filling materials is anticipated to significantly impact the overall utilization rates of various solid wastes, which is crucial for achieving efficient disposal objectives in subsequent stages. The optimization of ratio design can initially be conducted using a response surface methodology and orthogonal experiments. Subsequently, the optimization of the cementation system involves the careful selection of complementary materials, such as the combination of fly ash, slag, and cement, to leverage the beneficial properties of volcanic ash and enhance water resistance. Furthermore, the utilization of multi-source solid waste necessitates the incorporation of composite excitation, which may include the addition of alkaline activators (such as sodium hydroxide or water glass) or sulfate activators (such as gypsum) to facilitate the formation of silicoaluminate networks. Yu et al. [50] conducted a study to enhance the flow and mechanical properties of multi-source coal-based solid waste (MCSW) filling materials. Using a response surface experimental design, they optimized the formulation to achieve a 28-day compressive strength of 5.18 MPa and a minimum initial setting time of 140 min, utilizing desulfurization gypsum (DG), gasification slag (GS), and furnace bottom slag (FBS) as variables. This optimization provides a theoretical foundation for the scientific application of these materials. The generation of silica gel was found to mitigate the segregation of gangue, strengthen inter-particle bonding, and enhance the overall strength of the filling body.

Zhang et al. [51] concentrated on the mechanical properties and damage characteristics of cemented filling materials derived from multi-source coal-based solid waste (MCSW),as illustrated in Figure 9a. Their research identified five key factors (the mass concentration, fly ash/gangue ratio, gasification slag/gangue ratio, furnace bottom slag/gangue ratio, and desulfurization gypsum/gangue ratio) and employed uniaxial compression tests, acoustic emission monitoring, and microstructural analysis(Figure 9c). The findings indicated that the uniaxial compressive strength of CBSWCB exhibited regional variations over time, categorizing the development into three distinct phases: a low-enhanced zone, a medium-stable zone, and a high-strength zone. Notably, fly ash was found to significantly influence early strength, while concentration had a more pronounced effect on late strength (Figure 9b,d). The macro-deformation transitioned from plastic deformation and minor splitting damage to brittle deformation and shear damage. The evolution of the acoustic emission parameters was classified into four stages, illustrating a gradual progression of internal damage within the material, which was further delineated into three stages of damage development. Remarkably, the residual structure maintained a certain level of load-bearing capacity even after reaching peak stress [52], as illustrated in Figure 10 and Figure 11.

Gong et al. [53] addressed the challenges associated with the long-distance transportation of multi-source coal-based solid waste cementitious filling materials during underground filling processes. Through a combination of physical experiments and numerical simulations, they investigated the flow characteristics and pipeline transport performance of these materials. Their findings elucidated the primary and secondary relationships between various factors, such as the content of solid waste and the mass concentration of the mixture, as well as the flowability of the filling materials, specifically in terms of slump and water bleeding rates. Similarly, Hua et al. [54] examined the acoustic emission characteristics and microstructural features of multi-source coal-based solid waste filling bodies, revealing the significance and evolutionary patterns of different factors affecting the strength of the filling bodies. These research outcomes have substantial theoretical and practical implications for optimizing the performance of multi-source coal-based solid waste cementitious filling materials. By employing appropriate mixing ratios and activators, it is possible to transform these materials into filling substances with favorable engineering properties. This not only facilitates the resource utilization of solid waste but also mitigates environmental pollution and reduces carbon emissions, thereby contributing positively to the sustainable development of the coal industry.

3.2. Preparation of Filling Materials Based on Smelting Slag

3.2.1. Preparation of Filling Materials Based on Metal Smelting Slag

Blast furnace slag is a solid byproduct generated during the blast furnace ironmaking process, resulting from the interaction between veinstone and solvent, which produces molten slag that is subsequently discharged, cooled, and processed. This slag predominantly consists of amorphous vitreous material, with amorphous content typically exceeding 80%. The primary constituents of slag include CaO, SiO₂ and Al₂O₃. Due to the unique chemical composition and spatial structure of slag, which gives it a high potential gelling activity, a number of research attempts have been made to develop a variety of low-carbon-footprint activation techniques to stimulate it, as shown in

Table 3. Low-carbon-footprint activation technology to stimulate slag.

Low-Carbon-Footprint Activation Technology	Type of Technology	Principle
Low-temperature thermal activation technology	Low-temperature calcination	Reconstruction of slag’s vitreous structure via low-temperature heat treatment to release reactive SiO2 and Al2O3.
Low-carbon chemical stimulation technologies	Industrial by-product exciters	Activate the potential activity of slag/steel slag by utilizing the alkaline or sulfate content of other industrial wastes.
	Carbon dioxide mineralization activation	Carbon dioxide is employed to react with calcium and magnesium oxides present in the slag, resulting in the formation of carbonates. This process not only sequesters CO2 but also contributes to the densification of the material.
Bio-activation technology	Microbially Induced Carbonate Precipitation	Urease-producing microorganisms facilitate the hydrolysis of urea, resulting in the generation of carbonate ions. These carbonate ions subsequently interact with calcium ions to form a precipitate of calcium carbonate.
Mechanical–physical activation technology	Powder grind	Increasing the specific surface area of slag through mechanical energy increases the reactivity.

.

For instance, Liu et al. [55] successfully formulated a cost-effective mine-filling cementitious material (FGC cement) utilizing slag powder, calcium carbide slag, and composite activators. The optimal ratio for the FGC cement was established through response surface optimization, revealing that the curing efficiency of FGC cement for lead ions (Pb²⁺) was 1.04 to 1.24 times greater than that of conventional silicate cement. Additionally, ferrochrome slag, which contains substantial amounts of toxic ions such as chromium (Cr⁶⁺) and chromium (Cr³⁺), poses a risk of leaching and groundwater contamination over extended periods of natural leaching. In response, Zhou et al. [56] developed a novel filling material by synergistically activating ferrochrome slag with a combination of slag, lime, desulfurization gypsum, and cement clinker. Microstructural analyses indicated that the hydration products, including calcite and calcium silicate hydrate (C-S-H) gel, formed a dense lattice structure that effectively immobilized heavy metal ions.

Nonferrous metal metallurgical slag constitutes a solid waste byproduct generated during the process of smelting nonferrous metals. This slag primarily encompasses the byproducts produced during pyrometallurgical smelting as well as the residues released during hydrometallurgical smelting. To harness the latent gelling properties of these smelting slags, it is necessary to subject them to mechanical grinding and chemical activation. Upon activation, these materials can serve as supplementary cementing agents for filling applications, thereby enhancing their overall utilization efficiency. Wang et al. [57] conducted a study in which they finely ground nickel slag, desulfurization gypsum, and cement clinker to create a salt–alkali composite activator. They employed a controlled-variable methodology to ascertain the optimal proportions of this composite activator and examined how variations in the composition of raw materials and the dosage of the activator influenced the mechanical properties of cemented paste backfill (CPB) materials. Similarly, Na et al. [58] explored the use of copper–nickel smelting slag in conjunction with hydrogen peroxide (H₂O₂), cetyltrimethylammonium bromide (CTAB), and sodium silicate to formulate a novel swelling material. Their findings indicated that the utilization rate of the copper–nickel smelting slag could reach 70% following appropriate ratio selection, thereby mitigating the environmental impact associated with the copper–nickel smelting slag. Sodium silicate was identified as a crucial component that provides an alkaline environment, facilitating the activation of the SiO₂ content within the copper–nickel smelting slag. This activation process not only simplifies the foaming mechanism but also enhances the early compressive strength of the resultant material. The expanded material produced from this process holds promise for applications in roof contact filling within mining airspaces, potentially improving filling efficiency and reducing associated costs. Ruan et al. [59] developed a modified magnesium-slag-based cementitious material (M⋅C) intended for mine-filling applications, utilizing materials such as magnesium slag (MMS), fly ash (FA), and diatomaceous earth (DG). The optimal preparation ratio for the filling material was established through comparative testing of various performance indicators. Additionally, the study examined the mechanical properties, hydration exothermic process, and synergistic mechanisms associated with M⋅C, as illustrated in Figure 12. The synergistic interaction between FA and DG was found to enhance the hydration reaction of M⋅C, resulting in the formation of significant quantities of calcium silicate hydrate (C-S(A)-H) and ettringite (AFt), thereby increasing the strength of the material. The gelling agent developed in this research demonstrated commendable performance in industrial applications, confirming its viability for mine filling and offering innovative approaches for the resource utilization of MMS, which holds substantial engineering significance.

Deng et al. [60] utilized lead smelting slag as a partial substitute for cement in the formulation of paste cementitious materials. The experimental findings indicated that the incorporation of lead smelting slag had a pronounced impact on the strength of the filling body, with the strength exceeding that of conventional silicate cement when the slag content reached 16%. Similarly, Lan et al. [61] explored the development of a novel controlled low-strength filler material (CLSFM) using mechanical and chemical activation methods focused on the sustainable utilization of copper slag. Their research demonstrated that the reactivity of copper slag was significantly enhanced by high-energy ball milling, along with the addition of lime, sodium hydroxide, and triethanolamine as activators, resulting in uniaxial compressive strengths of 1.2 MPa and 2.5 MPa at 7 and 28 days, respectively. Furthermore, Li et al. [62] employed a combination of steel slag, vanadium–titanium slag, dicyandiamide waste slag, phosphogypsum, a limited quantity of cement clinker, and composite phosphoric acid, which were synergistically optimized through chemical stimulation and mechanical milling, to create a cementing agent intended to replace traditional cement. Additionally, vanadium and titanium iron tailings were incorporated to formulate a complete tailing sand filling material. The study investigated the influence of varying mass fractions of raw materials and activators on the performance of the filler, ultimately determining the optimal ratio of each binder component.

3.2.2. Red-Mud-Based Mineral Filling Materials

Red mud is a highly alkaline industrial solid waste product with iron oxide as the main component, which is produced after the extraction of alumina from bauxite [63]. Because red mud contains a large number of highly alkaline chemicals that are susceptible to rainfall solubility and the contamination of surface and groundwater bodies, many scholars have carried out resource utilization studies of red mud in order to reduce the large amount of red mud stockpiles, which pollute the environment. Chen et al. [64] used highly alkaline red mud as an alkali activator to add to cemented gangue gypsum filling materials, to study the effect of red mud on the material’s mechanical properties and microstructure. Shi et al. [65] used Bayer method red mud, calcium carbide slag, mineral powder, and coal gangue to prepare red-mud-based all-solid-waste mine-filling materials, and they optimized the mechanical properties and workability of the materials by adjusting the dosage of red mud and calcium carbide slag. The experiment shows that, when red mud accounts for 50% of the cementitious material and the mass concentration is 75%, the flow degree and compressive strength of the material meet the requirements of mine filling. The appropriate amount of red mud can effectively stimulate the activity of gangue, significantly improve its early strength, meet the requirements of mine filling, and have obvious advantages in terms of mechanical properties and economic benefits. Zhu et al. [66] mixed red mud into filling slurry containing an expansion agent, and they analyzed the effect of different dosages of red mud, cement, and mineral powder on the performance of expansive filling material by conducting indoor tests and numerical simulations. Liang et al. [67] used red mud as the main cementitious material and waste concrete blocks as coarse aggregate to prepare paste filling material; they determined that its compressive strength can be characterized by the resistivity index, and the compressive strength is the highest when the ratio of red mud and recycled concrete aggregate is 1:2. Zhang et al. [68] analyzed the effects of the red mud/gangue mass ratio, desulfurization gypsum dosing, and the water/cement ratio on the flowability of red-mud-based composite filling material using an orthogonal experimental design, as illustrated in Figure 13a. They explored the red-mud-based composite filling material’s fluidity and uniaxial compressive strength, as well as analyzing its micro-mechanism and synergistic effect (Figure 13b,c); they successfully developed a composite filling material with red mud content up to 70%.

It is feasible to use red mud to partially replace silicate cement for the preparation of cementitious paste filling materials, which is not only conducive to reducing the filling cost of coal mine airspace but also provides a new method for the resource utilization of red mud. However, the red-mud-based cemented paste filling material will inevitably soak and leach in the groundwater environment in practical applications, which is potentially polluting and may cause secondary pollution to the environment of the air-mining area. Many scholars in China [69,70,71] have used XRD, leach tests, and other means to study the evolution of the microstructure of the red-mud-based cemented paste filling material, as well as conducting leach tests to investigate the harmful ion leaching concentration and curing mechanism of the filling body. Regarding the leaching concentration of ions and the curing mechanism of the filler, the hydrochemical reaction of the filler material generates calcium alumina and C-S-H gel and other gel-like substances to adsorb and wrap the harmful ions on their own surfaces, reducing the leaching of heavy metal ions and decreasing the potential pollution risk that red mud causes to the environment.

3.3. Preparation of Filling Materials Based on Industrial By-Product Gypsum

Industrial by-product gypsum refers to the by-products or waste residues with calcium sulfate as the main component, generated through chemical reactions in industrial production activities [72]. These mainly include desulfurization gypsum (DG), phosphogypsum, fluorogypsum, etc., of which DG and phosphogypsum account for about 85% of the total amount of all industrial by-product gypsum generated. Desulfurization gypsum (DG) is a by-product of the wet desulfurization process of coal-fired flue gas, and its main component is calcium sulfate dihydrate, which is similar to natural gypsum [73]. It has cementitious properties, and, after reacting with water or other materials in alkaline environments (e.g., cement, fly ash), hydration products such as calcium alumina can be formed, which enhances the early strength of the filling body. As an auxiliary cementitious material, FGD gypsum can partially replace cement, reduce the cost of filling, and at the same time reduce carbon emissions from cement production. At present, most of the means of its resource utilization are concentrated in the field of building materials [74], e.g., as a cement retarder, gypsum board, etc. The research on its application for the preparation of mining filling materials mostly focuses on the synergies of multi-source solid waste. For instance, Chen et al. [75] used FGD gypsum, fly ash, and cement as raw materials and designed an experimental scheme using the response surface method to investigate the effect of the FGD gypsum mass fraction and other factors on the strength and mobility of the filling materials; they optimized the strength of less-hydrated coal ash. The effect of the FGD gypsum mass fraction and other factors on the strength and fluidity of the filling material was investigated, and the ratio of the less hydrated coal-based solid waste paste filling material was optimized. The microstructure analysis showed that the sulfate ions in desulfurization gypsum can react with the aluminate in cement to form calcium alumina, which can regulate the setting time of the filling material and significantly improve the late strength of the filling material. Desulfurized gypsum can also be used as an exciter [76] and compounded with cement clinker, etc., in an alkaline environment to stimulate the activity of multi-source solid waste and accelerate the hydration reaction process of materials. Ruan et al. [77] used modified magnesium slag and fly ash as cementitious materials and desulfurized gypsum as an active agent for material optimization proportioning experiments. It was found that the increase in desulfurization gypsum (DG) content significantly improved the rheological properties and compressive strength of modified magnesium slag fly ash cemented filler. Gao et al. [78] used calcium carbide slag, desulfurization gypsum, and slag as the cementing components to prepare the filling material, and desulfurization gypsum provided sulfate ions to stimulate the activity of slag in the alkaline environment during the hydration reaction of the filling body. The C-A-S-H gel and calcium alumina crystals generated were tightly bonded with the tailing sand to improve the microstructure of the filling body. However, the above studies neglected the effect of desulfurization gypsum itself on the modified filling materials. Zhou [79] prepared fiber-desulfurization gypsum-based composite cementitious materials by modifying desulfurization gypsum with fibers, and the bridging effect of the fibers significantly filled the body of the engineering properties; the damage pattern of the specimen was changed from brittle to ductile damage. The advantage of adding waste gypsum as an admixture to prepare new mining filling materials is that it can be used to stimulate the activation of inert or potentially active components; however, as an admixture, its content in the material is very rare, and there is still a long way to go before we achieve the large-scale resource utilization of waste gypsum.

Based on the strategy of promoting the large-scale resource utilization of industrial solid waste, domestic scholars have begun to study the use of waste gypsum as the main material for preparing waste-gypsum-based cementitious filling materials. The successful implementation of large-scale applications hinges on the processes of pretreatment and impurity management, which are essential for the elimination of detrimental constituents and the regulation of phase composition. The incorporation of washing, calcining, and grinding modules is proposed to establish cost-effective pretreatment systems. Phosphogypsum, as the main industrial byproduct of phosphoric acid production, contains many harmful impurities. To safely and efficiently apply phosphogypsum in the preparation of filling materials, various pretreatment methods are required, such as lime neutralization, water washing, and citric acid leaching [80,81,82]. Lan et al. [83,84] used hemihydrate phosphogypsum (HPG) as the main material and added gas-phase-introducing agent GPA and surface hydrophobic agent HA to prepare a new type of multi-phase water-expanding material. They found that GPA can react with water in the filling material to generate acetylene gas, forming gas-phase cavities that promote the volume expansion of the filling body. An increase in the amount of GPA significantly improves the expansion rate of the material while reducing its compressive strength. The incorporation of HA can reduce the loss of mechanical strength after the filling reaction. Wang et al. [85] used fluorogypsum (FG), an industrial byproduct generated during the production of hydrofluoric acid, as the main material, combined with coal gangue, fly ash, and lime to prepare a new type of slurry filling material. When the ratio of coal gangue, fly ash, and fluorogypsum (FG) was 10:3:3, with a mass concentration of 78%, the uniaxial compressive strength of the filling body could reach 4–5 MPa after 28 days. Wang et al. [86] studied the effect of fluorogypsum content on the setting time and compressive strength of high-water filling materials. An analysis of the hydration products indicated that fluorogypsum has a fast hydration rate, but a high content can lead to increased porosity, affecting the later strength. The above studies demonstrate that it is feasible to use fluorogypsum and multi-source solid waste to collaboratively prepare mining filling materials, which also offers environmental and economic benefits for filling mining.

3.4. Preparation of Filling Materials Based on Tailings

In mineral processing, “tailings” refers to the byproduct resulting from a sorting operation that contains a minimal concentration of valuable target components and is unsuitable for production [87]. Tailings are typically categorized into four primary classifications based on industry standards: the first category encompasses ferrous metal tailings, which include minerals such as iron, manganese, and chromium; the second category pertains to nonferrous metal tailings, which consist of metals such as copper, aluminum, lead, and zinc; the third category includes tailings from rare and precious metals, containing valuable resources such as gold, silver, molybdenum, and platinum group metals; and the final category consists of nonmetallic tailings, which include materials such as graphite, phosphorus, sulfur, fluorite, and barite. The composition of these tailings is notably diverse and complex. In China, tailings production constitutes over fifty percent of the total industrial solid waste generated, making it the largest category of industrial waste in the country. The utilization of tailings sand as an aggregate for the preparation of geopolymers aimed at filling mining areas has emerged as a significant research focus within the industry, as it seeks to promote resource reuse and mitigate environmental impacts, as illustrated in the technological process depicted in Figure 14. Certain tailings are composed of sulfide minerals, including pyrite, which undergo oxidation in the presence of atmospheric oxygen, resulting in the production of sulfuric acid. This process contributes to the development of acid mine drainage. The application of cementitious materials has been shown to effectively encapsulate sulfides, such as FeS₂, within the tailings, thereby preventing their exposure to oxygen and water. Additionally, the incorporation of lime or steel slag, with high calcium oxide (CaO) content, can elevate the pH of the system to levels above neutrality. This increase in pH serves to passivate the sulfides and suppress the activity of acidophilic bacteria responsible for acid generation.

The tailings sand cement filling method is widely used in metal mines, and a reasonable filling ratio is the key to ensure the filling quality, slurry flow characteristics, and filling cost. Scholars at home and abroad have optimized the filling ratios using orthogonal experimental design, uniform design, etc. Gao et al. [88] used molybdenum tailing sands to prepare thixotropic cement slurry filling material, which has good thixotropy, water retention, and compressive strength; it is suitable for the management of coal mine airspace, and the cost is also reduced by 30–50% compared with the traditional material, which helps to reduce the accumulation of tailings. Wu et al. [89] proposed a mine filling material ratio optimization method based on response surface method–satisfaction function, which was successfully applied to optimize the ratio of tailing sand cemented filling in a gold mine. The results of the proportioning test show that the sand–ash mass ratio has an extremely significant effect on the response volume. Zhang [90] used ultrafine copper tailings as an aggregate and substituted some cementitious materials with red clay to prepare cementitious fillings; mechanical strength tests were carried out on the cementitious tailing filling under different mass concentrations, ash-to-sand ratios, and red clay substitution rates. It was found that, the larger the mass concentration and ash-to-sand ratio, the larger the strength of the cementitious tailings filling is. Moreover, an increase in the substitution rate of red clay will lead to a decrease in the strength of the tailings-based cementitious filling. Wang et al. [91] developed a tailings sand-based active roof contact technology based on expanding agent, and they investigated the effects of the tailing ash ratio, slurry mass concentration, and expanding agent content on the expansion ratio and mechanical properties of the collodion paste filling body (CPB) through orthogonal tests (Figure 15). The results of the tests showed that the tailing ash ratio had the greatest effect on the mechanical properties, and the expanding agent content had a greater effect on the expansion ratio; as the tailing ash ratio increased, the sensitivity of expansion agent dosage to the expansion ratio decreased.

An et al. [92] proposed a clinker-free consolidant based on ultrafine iron tailings powder for the preparation of whole-tailings filling materials. They experimentally investigated its rheological properties, mechanical properties, and hydration and hardening mechanisms, which provides a new research direction for low-carbon mining technology. The ultrafine iron-tailings-based consolidant has a high-strength consolidation effect on the whole-tailings-filled specimens. At the same slurry consistency, the specimens using this consolidant were comparable in compressive strength to those using cement. When there are specific requirements for the strength and stability of the filling material in mining projects, the performance of the filling material can be optimized by adjusting the proportion of the consolidant and the slurry concentration.

4. Current Problems and Development Trends

As a key technology for realizing green mine construction and efficient resource utilization, the core of the mine-filling method lies in the research, development and application of industrial solid-waste-based filling materials. Although significant progress has been made in this field, the following key issues and challenges remain, which need to be overcome through technological innovations and systematic research:

(1)

The economy and scale application of solid waste resource utilization are limited

The comprehensive utilization efficiency of industrial solid waste is limited by the fluctuation in market demand in the construction and building materials industry, resulting in mining enterprises facing the dilemma of escalating solid waste disposal costs, insufficient space for stockpiling, and intensifying pressure on environmental protection. Although many studies have shown that multi-source coal-based solid waste (such as coal gasification slag, desulfurization gypsum) and smelting waste (such as red mud, steel slag) can be optimized to achieve a high mixing (>70%) ratio to replace cement, the actual application depends on the mine characteristics of the “local conditions”. For example, coal gasification slag needs to be activated by exciters to enhance the cementitious activity, and red-mud-based filling materials need to be cured through the hydration products of heavy metals. Future research should focus on the development of low-energy, low-cost solid-waste pretreatment process and promote the mine filling system and solid-waste disposal chain of the depth of the coupling, to achieve local consumption and large-scale application.

(2)

The disconnect between laboratory research and engineering practice is prominent

Current research focuses on the uniaxial compressive strength, mobility, and microstructure of filling materials under laboratory conditions (e.g., calcium alumina and C-S-H gel generation), but the durability and stability of the actual filling body, which is subjected to complex three-way stress, groundwater seepage, and dynamic loading for a long period of time, still need to be verified in depth. For example, Zhang Haibo et al. found that coal gasification slag can improve the fluidity of the paste, but that study did not address the risk of abrasion and pipe blockage during pipeline transportation. Zhu Jingkai et al. analyzed the effect of expansion material on the roof through numerical simulations, but their study lacked long-term monitoring data on site. Therefore, it is necessary to build a “laboratory–pilot–engineering application” whole-chain research system, combined with in situ monitoring technology (such as acoustic emission, resistivity characterization) and numerical simulations, to reveal the performance evolution of the filling body under the action of multi-field coupling.

(3)

There is weak research on the mechanism of the synergistic activation of composite solid waste

Existing studies mostly focus on the activation mechanism of single solid waste (e.g., mechanical grinding, chemical excitation), but there is a lack of systematic knowledge on the reaction path and interface effect under the synergistic effect of multi-source solid waste. For example, desulfurization gypsum can be used as a sulfate exciter to promote slag hydration, but its coexistence with red mud, gangue and other multi-component competitive reaction mechanisms is still unclear; multi-source coal-based solid waste (such as gangue, gasification slag, furnace slag) has a synergistic gelling effect through the response surface method to optimize the ratio, but the microscopic level of the ionic mobility and product distribution still need to be analyzed in depth. In the future, it will be necessary to combine multi-scale characterization techniques (such as XRD, SEM-EDS, and molecular dynamics simulation) to reveal the synergistic mechanism of the activation–gelation-consolidation of composite solid-waste systems, and to develop efficient composite exciters to promote the solid-waste-based filling materials from “empirical ratios” to “mechanism-driven”. The development of efficient composite exciters will promote the transformation of solid waste-based filling materials from “empirical ratio” to “mechanism-driven”.

Future research should focus on the following directions:

(1)

Multi-source solid waste synergistic utilization and performance customization: based on the geological conditions of the mine and functional requirements (such as seepage resistance, expansion roofing, fast solidification), we build an intelligent model of “solid waste characteristics—ratio design—performance prediction” to achieve the precise control of material performance.

(2)

Low-carbon activation technology and environmental safety enhancement: scholars should develop green excitation agents (e.g., bio-based activators), optimize the curing path of harmful components (heavy metals, alkaline substances), and reduce the risk of leaching.

(3)

Evaluation of the whole lifecycle and construction of a standardized system: the inventory data encompass the entire lifecycle of materials, spanning from the acquisition of raw materials through pretreatment, material production, transportation, construction, service, dismantling, and ultimately recycling or disposal. The primary indicators within this inventory consist of environmental metrics (such as carbon emissions and heavy metal leaching) and technical metrics (including compressive strength and permeability). In the assessment process, the inventory data, which include emissions of CO₂ and SO₂, are initially classified into relevant environmental impact categories, such as climate change and acidification. Subsequently, the contributions of various substances to a specific environmental issue can be quantified through scientific modeling and translated into a standardized equivalent. The development of the standardization system is organized into three main components: the formulation of technical performance standards, environmental safety standards, and process management standards.

5. Conclusions

With the rapid development of the mining industry, the large amount of industrial solid waste generated poses a serious threat to the ecological environment. The application of industrial solid wastes in the preparation of mining filling materials has become an important way to realize the construction of green mines and the efficient use of resources. By reintroducing industrial by-products back into the production chain through backfill materials, the waste-as-resource (WAR) cycle is realized, which promotes the resourcefulness of solid waste and reduces resource depletion in a linear economy. Solid waste-based cementitious materials can also replace highly carbon-emitting materials to achieve direct carbon emission reductions, revealing the potential for carbon sequestration. Terrestrial ecological protection is reflected in the reduction in land occupation and the promotion of surface reclamation in the mining area. This study systematically reviews the research progress on the preparation of filling materials based on coal-based solid waste, smelting slag, industrial by-product gypsum and tailings, as well as discussing the current status, problems, and development trends of the application of different solid waste types in filling materials.

It is found that single coal-based solid wastes such as fly ash and gangue have broad application prospects in filling materials, but the further optimization of proportioning and activation technology is needed to improve their performance. The synergistic use of multi-source coal-based solid waste can effectively improve the comprehensive utilization rate of solid waste, but the reaction mechanism and proportioning optimization need to be studied in depth. Smelting wastes such as slag, steel slag, etc. can be used as binder or aggregate for filling materials after appropriate treatment. However, the strength and durability need to be strengthened; industrial by-product gypsum can be used as an auxiliary cementing material or exciter in filling materials, but its large-scale application still faces challenges; tailings have natural advantages as the aggregate of filling materials, but attention should be paid to their long-term stability and environmental impact.

Future research should focus on the synergistic utilization and performance customization of multi-source solid wastes, the development of low-carbon activation technologies, the enhancement of environmental safety, and the construction of a full lifecycle evaluation and standardization system. Through these efforts, we will promote the transformation of industrial solid-waste-based filling materials from empirical ratios to mechanism-driven processes and achieve the dual goals of green mining and solid waste resource utilization.