Critical Appraisal of Coal Gangue and Activated Coal Gangue for Sustainable Engineering Applications

Abstract

Coal gangue, a primary solid waste by-product of coal mining and processing, constitutes approximately 10–15% of total coal output. Its accumulation poses substantial environmental challenges, including land occupation, spontaneous combustion, acid mine drainage, and heavy metal leaching. Despite its high silica and alumina content (typically exceeding 70% combined), the highly stable and crystalline structure of raw coal gangue limits its pozzolanic activity and adsorption efficiency. To address this limitation, this review emphasizes recent advances in activation strategies such as thermal (500–900 °C), mechanical (dry/wet grinding to less than 200 µm), chemical (acid/alkali treatments), microwave, and hybrid methods. The activated coal gangue resulted in an enhanced surface area (up to 55 m²/g), amorphization of kaolinite to metakaolinite, and the generation of mesoporosity under optimal conditions. This review critically examined the geotechnical applications, such as soil stabilization and mine backfill, highlighting the replacement of 50–75% of cementitious binder in backfilling and meeting the subgrade/base material strength criteria (UCS > 2 MPa). In geoenvironmental applications (adsorption of phosphate, dyes, heavy metals, and CO₂ mineralization), more than 90% of pollutant removal is attained. In construction applications, supplementary cementitious materials and sintered bricks are examined. Several critical knowledge gaps, including limited understanding of long-term durability, inconsistent activation optimization across different coal gangue sources, and insufficient assessment of environmental impacts during large-scale implementation, are clearly addressed. This review provides a roadmap for advancing sustainable coal gangue utilization and highlights emerging opportunities for cost-effective applications in the mining and construction sectors.

1. Introduction

Coal is a combustible organic rock composed of carbon, hydrogen, and oxygen. Coal is the dominating global energy source, as it offers a cost-effective and reliable source of electricity, ensuring power availability on demand to meet rising energy needs. In 2023, coal-based power generation rose by 1.9%, and global demand for coal reached a record high of 8.70 billion tons, representing a 2.6% increase compared to the previous year [1]. Growth in countries like China and India, primarily relying on coal, was the main driver of this surge. Coal consumption increased in China (+3%), India (+5%), and Indonesia (+11%), with China and India together accounting for 71% of global coal consumption. The leading countries producing more than 200 million tons of coal are China, India, the United States, Indonesia, and Russia [2]. Nonetheless, coal usage has drastically decreased in 2023 in nations like the US and the EU, with decreases of 17.7% and 23%, respectively. Energy transitions, a rise in the use of renewable energy sources, and the shutdown of coal-fired power facilities were the main causes of this decline [1]. A significant issue for coal-producing nations is the waste produced during coal mining and processing, which makes up as much as 10–20% of the raw coal [3]. Coal mine waste includes coal gangue, coal mine water, coal sludge, and coal bed methane [4].

Coal gangue, a solid waste produced during coal mining and washing, accounts for about 10–15% of total coal production [5] and 40% of solid waste generated from mining [6]. It is collected during coal washing and mining from the coal seam roofs, which are generally composed of siltstone and sandstone with charcoal bands [7]. Generally, coal gangue is grey in color, fissile, and laminated, containing organic substances such as coal [8]. The mineralogy of coal gangue includes kaolinite, quartz, muscovite, and illite [9]. Raw coal gangue has a stable crystalline structure, so its activity is poor [10]. In 2024, China produced 717 million tons of coal gangue, representing the highest production [11]. Based on the coal production reported by IEA (2024) [1], India conservatively produced 94 million tons of coal gangue. The utilization rate of coal gangue in developed countries like the US and EU is more than 90%, while the largest coal-producing country, China, had a rate of 60–70% in 2013 [12].

Coal gangue has a stable nature and a low utilization value, so it is discarded at mining locations, which causes environmental hazards. The coal gangue occupies 70 km² of land, causing landscape degradation and preventing its use for agriculture and other purposes [13]. The accumulated coal gangue contains heavy metals such as Zn, Cu, Cd, Pb, and Ni [14]. Heavy metal concentrations and leaching are significantly affected by weathering, with cadmium posing a substantial ecological risk and chromium being highly prevalent [15]. The overall ecological risks associated with heavy metals in coal gangue range from 351.51 to 412.27, indicating a high level of risk [16]. Additionally, coal gangue hills are prone to spontaneous combustion due to residual coal and pyrite, which oxidize and release heat [17], and the high ash content promotes greater heat transfer [18]. Spontaneously combusted coal gangue releases As (as aerosol) and Pb, heavy metals that have a greater impact on the air than on soil [19].

By considering environmental concerns associated with the coal gangue, instead of managing it, it can be used in different applications. Early stages of utilization of coal gangue occur in backfilling the mines to avoid subsidence failures [20], roadbed filler [21], and coarse aggregate in the concrete production. The utilization of coal gangue as filler material avoids the land accumulation problem. The high coal-producing countries like India and China have limited coal gangue utilization to filling material, power production, and construction material [22]. The amount of coal gangue utilized as filling material in China is limited to 30% of the coal gangue generated. Coal gangue is also a viable and sustainable alternative to natural soil for embankment construction. Optimizing transport, ground improvement methods, and design can significantly reduce carbon emissions [23]. Raw coal gangue finds applications mainly in low-value construction and geotechnical use.

Coal gangue in its natural state has the binding energies of 74.41 eV for Al_2P and 102.90 eV for Si_2P, and these binding energies make the materials chemically inert [24]. Coal gangue’s low reactivity nature, crystalline form, and harmful minerals are the primary factors hindering its high use rate. Researchers have investigated several activation techniques to improve coal gangue’s qualities and use it as a high-value product in various industries to boost its utilization rate and lessen its environmental problems. The activation methods that are employed on coal gangue are mechanical activation, thermal activation, chemical activation, microwave activation, and microbiological activation [11,25]. These activation methods significantly improve the reactivity and enable its transformation into value-added cementitious binders, soil stabilizers, and environmental materials.

In geotechnical applications, Qiu et al. [24] first studied the utilization of activated coal gangue powder for filling bodies. When used in mine filling, it enhances pumpability, compressive strength, and workability due to its water-retaining properties. It improves the microstructure by forming dense C-S-H and C-A-S-H gels, reducing harmful pores, and enhancing strength and durability. A replacement rate of 50–75% is optimal. Its use increases coal gangue utilization to 72%, supporting sustainable and cost-effective resource management. The mechanical characteristics of coal gangue treated with neutral, acidic, and alkaline aqueous solutions were examined, which increases cohesion (C) by 3.72-fold [26]. Scanning electron microscopic results reveal that thermal activation leads to a looser, porous structure with many broken bonds and increased internal energy, supporting improved reactivity [27]. Thermally activated coal gangues exhibited higher pozzolanic reactivity than raw coal gangue and increased the compressive strength and flexural strength of cement mortar by 46.4% and 16.1%, respectively [28]. Guo et al. [29] proposed a wet grinding mill as an environmentally acceptable method of producing coal gangue-based cementitious material. The resulting cement composites have a high compressive strength, minimal porosity, and a quicker initial set-in time. In geoenvironmental applications, coal gangue is used as an adsorbent, zeolite, and porous material [30]. Natural coal gangue has a mesoporous structure with limited micro- and macropores. Thermal treatment increases pore volume but reduces surface area, especially at 600 °C. Chemical activation with H₂NO₃ and H₂O₂ boosts surface area and microporosity, making coal gangue a potential low-cost adsorbent for heavy metals and large molecules [8]. From the results, it is observed that activated coal gangue gives better results than raw coal gangue. Although activation increases coal gangue’s utilization rate, the literature lacks optimized activation methods for lower energy use, environmental impact, secondary pollution, and circular economy integration. In addition, a cost analysis is imperative to choose the optimal and economical activation technique for different targeted applications.

This review emphasizes research published between 2006 and 2025 to ensure the inclusion of recent advancements in coal gangue activation and applications. This extensive literature comprises peer-reviewed technical articles, review articles, and authoritative technical reports from various databases, including Web of Science and Scopus. The selection criteria were based on the relevance of studies to activation methods (thermal, mechanical, chemical, microwave, and hybrid); the resulting changes in physical, chemical, and mineralogical properties; and their performance in geotechnical, geoenvironmental, and construction applications.

This review article aims to comprehensively analyze the role of activation in enhancing the properties of coal gangue in desired applications. The article addresses the physical, chemical, mineralogical, geotechnical, and geoenvironmental properties of coal gangue and activated coal gangue. It also explores the various methods of transforming and activating coal gangue to enhance its value and suitability. Additionally, the review will examine applications of coal gangue and activated coal gangue, highlighting their potential in the civil engineering field. Furthermore, the review will assess the broader benefits of coal gangue, such as reducing waste and promoting sustainability. Challenges and limitations associated with its use are addressed, particularly in terms of technical, economic, and regulatory factors. Ultimately, this review aims to highlight the potential of coal gangue as a resource for sustainable development while identifying key areas for further exploration and innovation.

2. Characterization of Coal Gangue

Coal gangue and activated coal gangue are widely used as construction material, filling material, adsorbent, zeolite, porous materials, soil stabilizer, and soil conditioner. Studying the characteristics of raw and activated coal gangue helps determine the best use in geotechnical and geo-environmental applications. This section examines the properties of coal gangue and how those properties vary under different activation methods.

2.1. Chemical and Mineralogical Properties

The original rock properties and the geological circumstances of the mining location significantly impact the chemistry and mineralogy of coal gangue. The composition is further changed by weathering processes [31]. Understanding the chemical properties of coal gangue is crucial for its various applications. Figure 1 presents the chemical characteristics of coal gangue that various researchers have investigated using X-ray fluorescence (XRF). The predominant oxides in coal gangue are SiO₂ and Al₂O₃, which comprise approximately 40–65% and 15–30% of the total composition, respectively, and account for 65–80% of the total composition [27]. These two elements play a significant role in the utilization of coal gangue. Coal gangue’s siliceous and aluminous qualities allow it to be used instead of clay to make bricks, clinker, cementitious materials, and landfill liner [32,33,34,35,36]. It also aids in the extraction of alumina to produce aluminum hydroxide, aluminum chloride, and poly-aluminum chloride [37,38,39] as well as in geo-environmental applications as an adsorbent and zeolite [40,41].

Based on its silica and alumina content, coal gangue can be categorized into four types: clay stone gangue (SiO₂: 40–70% and Al₂O₃: 15–30%), sandstone gangue (SiO₂ > 70%), aluminous stone gangue (Al₂O₃ > 40%), and calcareous stone gangue (CaO > 30%) [12]. Figure 1 reveals that most studies have focused on understanding the chemical composition of claystone gangue. On the other hand, Fe₂O₃, CaO, and others, including Mg, Mn, Na, S, Ti, K, etc., are available in small quantities. The loss on ignition indicates the presence of organic matter and volatile matter, which varies between 0.63 and 28.3%.

The chemical composition of coal gangue changes during the thermal, mechanical, and chemical activation methods. Figure 2 depicts the changes in the chemical composition when coal gangue transforms to activated coal gangue. The silica and alumina content relatively increased due to physical and chemical changes during the activation, while the remaining components show only slight modifications. This is attributed to the decomposition and dissolution of adsorbed water, organic matter, and volatile matter. This causes the mass loss and shows a higher proportion of stable silica and alumina oxides in the remaining mass in thermal and chemical activation [8,50]. In mechanical activation, the reason might be that the breakage of particles causes an increase in exposure of surface area and detection of Si and Al content [49]. The maximum increase in thermal activation resulted in 5–21% silica and 1–10% alumina. Thermal activation produces the largest proportion of silica and alumina, followed by mechanical and chemical activation. The combined silica and alumina content of activated coal gangue exceeded 80%, indicating a high level of pozzolanic activity and enhancing the adsorption capacity of coal gangue [50]. However, in chemical modifications, reduced silica and alumina content is observed.

The literature shows that major minerals available in the coal gangue are quartz and kaolinite, followed by illite, hematite, and muscovite [48,51]. Coal gangue can be used in different applications based on the type of mineral present. The benefit of having kaolinite is its surface hydroxyl groups and regular morphology, which act as a potential adsorbent for wastewater treatment. In addition, kaolinite-rich coal gangue can be utilized as paint additives, alumina extraction, mullite formation [61], and zeolite [6]. Quartz, a predominant mineral in coal gangue, can be used as a cementitious material [61]. However, these minerals exhibit a crystalline nature during their origin. E et al. [65] estimated the crystallinity of kaolinite, quartz, and hematite to be 97.98%, 73.24%, and 96.69% respectively.

Figure 2. Chemical composition of coal gangue and activated coal gangue (Zhang et al. [61], Xiong et al. [63], Guo et al. [66], Wang et al. [54], Ye et al. [67], and Zhang et al. [50]).

Figure 2. Chemical composition of coal gangue and activated coal gangue (Zhang et al. [61], Xiong et al. [63], Guo et al. [66], Wang et al. [54], Ye et al. [67], and Zhang et al. [50]).

Though it has a crystalline nature, it exhibits pozzolanic property when added with additives like gypsum and lime to the coal gangue [68]. These minerals are altered under various activation techniques, making coal gangue more suitable for various applications. In the thermal treatment of coal gangue, the calcination temperature changes the chemical composition of coal gangue due to the dehydroxylation. The change in the mineral phase of coal gangue starts at 500 °C. Frias et al. [69] reported that at 650 °C, the kaolinite mineral transformed into metakaolinite. This transformation occurs early in microwave activation and is more active when combined with acid activation [70]. Kaolinite mineral transforms into metakaolinite due to the removal of hydroxyl groups and the breakdown of its crystal structure [13]. The kaolin mineral is stable up to 180–300 mesh. Similarly, mechanical activation can substantially alter the mineralogical features of coal gangue. A study revealed that milling coal gangue to 400 mesh can destroy the kaolin’s crystal structure and transform it into partially ordered semi-crystalline metakaolin [71]. Moreover, mechanical activation combined with other activation methods can be adopted to maximize the activity of the coal gangue. The combined effect of thermal and chemical (Na₂CO₃) activations resulted in a complete transformation of stable/inert minerals into active minerals [37].

2.2. Physical Properties

2.2.1. Particle Size Distribution and Specific Gravity

The size of the coal gangue at the mining location varies from boulders to clay fractions. Authors have reported that for 0 h, 0.5 h, 1 h, and 1.5 h of mechanical grinding, the particles fall in the range of 20–200 µm, 0.8–60 µm, 0.7–20 µm, and 0.8–20 µm, respectively. As grinding time increases, grinding force causes the particle size reduction up to 1 h of grinding; after that, binding of finer particles will occur, which again increases the particle size. The specific gravity of raw coal gangue varies between 1.98 and 2.69 [36,48,68,72,73]. Specific gravity values of 0.425 mm and 4.75 mm passing fractions are 1.98 and 2.52, respectively, and the lower specific gravity of coal gangue exerts lower pressures on typical retaining structures and steeper slope embankments [48].

2.2.2. Specific Surface Area and Porosity

According to the literature (Figure 3), the specific surface area of raw coal gangue ranges from 1.665 to 9.57 m²/g, and activated coal gangue varies from 1.105 to 55.361 m²/g. After the activation, not all the coal gangue’s specific surface areas are increased. In every activation method, there is a certain limit where an increase in specific surface area occurs; after that, it decreases. While combustion of organic and volatile matter is the reason for reduced surface area in thermal and chemical activation, particle size reduction plays a crucial role in mechanical activation [8]. The conversion of macropores into mesopores during thermal activation increases specific surface area up to a certain temperature, after which it decreases due to melting and sintering [13]. During mechanical activation, the specific surface area increases due to a reduction in particle size. The specific surface area of mechanical activation increases as grinding time increases, but as particle size decreases, agglomeration occurs, which again reduces the specific surface area [74]. Wet grinding is an alternative mechanical method to bypass the agglomeration of reduced fine particles [29].

2.3. Geotechnical Properties

2.3.1. Atterberg Limits

The raw coal gangue showed non-plastic and shrinkage behavior due to the lack of fines and surface charges [48,68,73]. However, in some regions, coal gangue exhibits a plastic nature. Wu et al. [36] reported that the 1 mm size of coal gangue showed 11.1% plasticity and a 17.2% plasticity index. The liquid limit of coal gangue passed through a 425 micron sieve was observed to be 28%, which is due to the presence of carbon and residual clay [48,68]. When cement stabilizes coal gangue, the liquid and plastic limit increase, while plasticity and linear shrinkage decrease. This is because of high pH conditions, cementitious compounds are formed, which cause coal gangue to bind with nearby particles, becoming coarser and reducing finer particles [79].

When coal gangue is blended with the black cotton soil, the liquid limit and plasticity index decrease as coal gangue content increases, while the plastic limit is increased by replacing finer, water-absorbing clay particles with the coal gangue particles, leading to a reduction in diffuse double-layer thickness and water absorption [73]. A similar trend of reduction in the plasticity index was observed when coal gangue is blended with expansive soil; however, this reduction is more for lime- and fly ash-blended expansive soil [80]. Overall, these results demonstrate that using coal gangue can effectively lessen the plasticity of fine-grained soils, making it appropriate for use in subgrade, embankment, and geotechnical applications.

2.3.2. Compaction Properties

The literature shows that the optimum moisture content (OMC) and maximum dry density (MDD) of raw coal gangue vary between 9.8 and 16% and 1.65 and 2.31 g/cc, respectively [48,68,73]. The increase in the density of raw coal gangue due to the non-plastic nature of coal gangue offers shearing resistance at the particle level. When coal gangue is stabilized with lime, coal gangue density is slightly reduced because lime converts the non-plastic nature of coal gangue to a plastic nature [81]. The increase in lime content increases the MDD due to the immediate formation of primary hydration products. Stabilizing the coal gangue with the cement increases dry density and decreases OMC [79]. This is attributed to the fact that changes in gradation characteristics of coal gangue and the cement paste cause the agglomeration of cement around the soil particle and cause an increase in the fine particle size. The OMC and MDD of cement stabilized coal solid waste, which include coal gangue, coal bottom ash, and fly ash, increase and decrease, respectively, as the coal bottom ash content increases. This is attributed to its rough surface, porosity, and low density [82]. A similar reason applies if coal bottom ash is replaced with the furnace slag [83]. In cement-stabilized coal gangue, as the cement content increases, OMC and MDD values also increase. Guan et al. [84] studied the OMC and MDD characteristics of cement-stabilized coal gangue. The results showed that the maximum OMC and MDD values were obtained at 7% of cement, with values of 5.5 OMC and 2.33 MDD. The cement-stabilized macadam shows higher OMC and MDD, even under the same cement content in cement-stabilized coal gangue. Figure 4 represents the compaction characteristics of black cotton soils and expansive soil stabilized with coal gangue. Gaddam et al. [73] examined the variation in OMC and MDD of black cotton soil stabilized with coal gangue. They found that coal gangue enhances the compaction characteristics of black cotton soil up to an ideal dose of 20%. The addition of coal gangue causes the soil to reorganize, and pore size increases the MDD of black cotton soil. At a 20% dose, flocculated structures form, lowering the OMC of black cotton soil. Because coal gangue has a low specific gravity, black cotton soil begins to reduce the MDD at 30% dosing.

Ma et al. [80] stabilized the expansive soil with coal gangue, fly ash, and lime. By adding these materials, it is observed that MDD values are decreasing due to a lower relative density than expansive soil. In modification, coal gangue powder influences the OMC, though it has a lesser impact than fly ash. This is because coal gangue causes the structural modification, causing the flocculation along with the lime. From this, it is observed that coal gangue acts as a secondary stabilizer to expansive soil.

2.3.3. Unconfined Compressive Strength

According to Guan et al. [84], unconfined compressive strength is influenced by cement content and raw coal gangue grading, and it increases as cement content increases in cement-stabilized coal gangue over 7 days. This is explained by the fact that when the cement content rises, the pores surrounding the coal gangue become denser, enhancing the cement-stabilized coal gangue’s unconfined compressive strength. Figure 5 illustrates the changes in the unconfined compressive strength of the coal gangue with the incorporation of various additives. The addition of lime increased the UCS of coal gangue at early curing periods, with a high strength observed at a 2% lime dosage. While further increases in lime dosage showed marginal gains, the curing period played a major role, lasting up to 7 days, after which the strength increment was marginal. The addition of gypsum along with the lime produces the synergistic effect, markedly enhancing the UCS values by about 10 times compared to the untreated case by accelerating the formation of secondary hydration products such as calcium alumina silicate hydrates (CASHs) [68]. A similar result was observed when lime and gypsum were added to fly ash [85].

The UCS of cement- and fly ash-stabilized slag–coal gangue mixture (curing periods of 7,14,28,56, and 90 days) for a pavement base mixture application was analyzed by Li et al. [83] by keeping the fly ash and cement content constant and replacing the coal gangue with 0, 25%, 75%, and 100% of furnace slag. From the results, it is observed that the early curing period strength of the mixture decreased as slag content increased, due to its low density. In addition, the cement–fly ash slurry and slag exhibited lower cohesion and interlocking properties compared to the coal gangue. Early strength was reported due to the formation of gel substances, calcium silicate hydrate, and calcium aluminum hydrate from hydration due to the presence of cement. Later, the 28-day mixture showed significant improvement in the compressive strength growth rate in all the specimens. Moreover, it is observed to be greater than 60%, which is due to the presence of active substances (SiO₂ and Al₂O₃) in fly ash and furnace slag undergoing secondary hydration under the excitation of cement hydration product Ca(OH)₂, generating crystal substances such as CSH and CAH. These crystals reduce the spacing among coal gangue, slag, and cement mortar.

Based on the findings, 50% and 75% should be used in real-world applications. Similar results were found when coal bottom ash is replaced in the furnace slag, but the average growth rate observed is more than 65% with a maximum of 72% [82]. Similar results were observed in the unconfined compressive strength of red mud- and coal gangue-based underground backfilling material (RGUBM) using a composite binder, which includes fly ash, desulfurized gypsum, and cement. At 40% red mud, the highest unconfined compressive strength of RGUBM is 6.9 MPa after 28 days of curing [86]. Cao et al. [47] stabilized coal gangue with inorganic binders, cement, lime, and fly ash for different ratios and calculated the unconfined compressive strength for various curing periods, including 7, 28, and 90 days. Cement-stabilized coal gangue showed early strength due to hydration reactions between cement and coal gangue, and hydrated products strengthened the interlock between cement and coal gangue particles, improving compressive strength. The lime and fly ash binders improved the strength at later stages of the curing period because the reaction between fly ash and coal gangue is slow at first. Then, the highly pozzolanic active property of fly ash increases the strength. A coal gangue mixture of steel slag and mineral powder showed 7-day unconfined compressive strength ranging from 2 to 6 MPa, which meets the requirement of pavement base of grade II and below grade II roads in the Chinese standard [87]. The road performance of coal gangue mixture is influenced by the proportion of spontaneous combustion and unspontaneous coal gangue continues, particularly when only the fine aggregated portion is varied. The unspontaneous combustion coal gangue continues to be used as the coarse aggregate, indicating that the substitution strategy primarily affects the fine fraction and plays a key role in determining the material performance. The 7-day UCS of cement-stabilized coal gangue follows a cubic polynomial curve relationship with cement dosage [88].

By nature, coal gangue is inert and less pozzolanic. This is why, in most studies, it is combined with other binding materials to enhance the strength of problematic soils. As depicted in Figure 6, Zhao et al. [55] and Li et al. [89] studied the mechanical properties of dispersive soil stabilized with coal gangue and calcinated coal gangue, respectively. Figure 6 shows that, in comparison to the curing period, the content of coal gangue and calcinated coal gangue has a significant impact on enhancing the strength of dispersive soil.

According to the findings, coal gangue significantly increases the strength of the soil when compared to calcinated coal gangue. At the lowest addition of calcinated coal gangue, the UCS of the dispersive soil stabilized with the coal gangue showed relatively similar values. Gaddam et al. [73] investigated the impact of coal gangue in the presence of lime to determine the durability and strength properties of clayey soil. Figure 7 illustrates the influence of coal gangue and calcinated coal gangue on dispersive and non-dispersive soil. The addition of lime enhanced the pozzolanic activity of coal gangue in black cotton soil. Lime aids in increasing the solubility of silica and breaking the Si-O bond within the silica-rich glassy phases of coal gangue, and it becomes hydrated. The results revealed that the optimum dosage of coal gangue, 20% and 6% lime, is the dosage at which the soil obtained a maximum strength of 3256 kPa after 28 days of curing, which satisfies the minimum strength requirement of sub-base course, as per IRC: 37-2012. Ma et al. [80] stabilized expansive soils using a mixture of admixtures, lime, coal gangue, and fly ash. The UCS of the expansive soil doubled. The UCS of the soil increased linearly with the addition of lime and fly ash, reaching up to 11% and 6%, respectively. The optimal mixture identified in this study is coal gangue, fly ash, and lime in the ratio of 8:11:6. SEM results showed that soils treated with different mixtures developed a denser, more stable microstructure characterized by fewer flaky and sheet-like clay particles and the formation of spherical fly ash particles and lime-induced micro-agglomerates. These agglomerates connect soil particles and aggregates, filling structural pores and reducing overall porosity. Coal gangue introduces rhombohedral-shaped silicon minerals and calcium-bearing particles into the soil matrix, promoting larger aggregates and granular pile structures. This leads to a coherent and compact skeletal framework within the soil, enhancing its strength and reducing swelling behavior.

3. Activation of Coal Gangue

Raw coal gangue cannot be directly utilized because of its organic matter, impurities, and carbon. Though the coal gangue is rich in SiO₂ and Al₂O₃, they are inactive, and activation is required to use it as a pozzolanic material [90]. Coal gangue, in its raw form, has a high degree of crystallization, which results in less alkali hydration [71]. Activation increases the specific surface area and porous nature of coal gangue. Researchers have improved the properties of coal gangue by using different activation methods in various applications to utilize the maximum amount of coal gangue and reduce dependency on natural resources. The existing activation methods of coal gangue are mechanical activation, thermal activation, chemical activation, and microwave activation (Figure 8) [61,67,91,92]. As depicted in Figure 9, each activation method involves a different mechanism to enhance the surface and structural properties of coal gangue. Many researchers have implemented two or three activation methods to improve the activity of the coal gangue compared to the single activation method. However, implementing the composite activation is complex. Activation is applied to coal gangue to decrease its crystallinity and increase its specific surface area and porosity. This results in improved pozzolanic properties for civil engineering materials and enhanced adsorption capacity for geoenvironmental applications.

3.1. Mechanical Activation

Mechanical activation alters the physical and chemical properties of coal gangue through grinding. Mechanical activation is easy to operate and environmentally friendly, making it particularly suitable for small-scale applications [11]. This process involves milling, which incorporates friction, collision, and extrusion, gradually wearing down the edges of the particles. As a result, there are blurred boundaries, and particles become finer [11]. As shown in Figure 9, there are two methods for mechanical activation: wet grinding and dry grinding. In dry grinding, due to charges on the surface of the coal gangue, particle agglomeration occurs, which inhibits the reduction of particle size. In wet grinding, agglomeration is prevented due to the presence of water, which disperses the coal gangue particles [29].

Mechanical action causes the breakdown of the kaolinite Al-O octahedral structure, resulting in an increase in the amorphous Si-O structure and Al^IV-O tetrahedral structure, thereby enhancing the activity of coal gangue [91]. The kaolin mineral is stable up to 180–300 mesh. The crystal structure of kaolin was destroyed and transformed into partially ordered semi-crystalline metakaolin when coal gangue was milled for 300 to 400 mesh and more than 400 mesh [71]. Due to this, mechanical activation is combined with other activation methods to maximize the activity of the coal gangue. The prolonged milling of coal gangue leads to significant structural changes in its mineral components. Mechanical activation of coal gangue increases the reaction rate by lowering the activation energy required for reactions. This is achieved through the deformation of the crystal lattice and the creation of structural defects, which enhance the energy storage of the material. Kaolinite’s crystal structure is destroyed, becoming disordered and amorphous. The diffraction peaks of muscovite, anorthite, and calcite weaken, indicating the breakdown of their crystal structures into smaller crystals [71]. While the surface of the mechanically activated coal gangue powder is more reactive due to the increased surface energy, the internal active substances within the particles are not fully exposed [71]. After 0.5 h of milling, the gangue particle size is reduced by at least 90%. As milling time increases, the particle size decreases, but the reduction rate slows down.

3.2. Thermal Activation

Thermal activation is the process through which thermal energy/heat is supplied to a material, which causes a change in the material. There are two procedures for calcination: fluidized calcination and static calcination. The former is generally chosen [93]. Thermal activation is the most commonly used method to increase the activity of the coal gangue. Thermal activation is widely used in construction materials, adsorbents, soil conditioning, and soil stabilization. In this method, coal gangue grinding is first performed, followed by thermal activation [72]. The change in the mineral phase of coal gangue starts at 500 °C. As illustrated in Figure 9, kaolinite mineral transforms into metakaolinite due to the removal of hydroxyl groups and the breakdown of its crystal structure [13]. Hao et al. [28] reported that complete dehydroxylation occurs at 800 °C. The temperature at which the transformation of kaolinite occurs is known as the optimum calcination temperature. Which varies predominantly between 600 °C and 800 °C [72]. The optimum calcination temperature proposed by different researchers is depicted in Figure 10. The optimum calcination temperature is not uniform; it is greatly influenced by the mineralogy of coal gangue and its content. For example, coal gangue with a high kaolinite content begins to show early structural disorder at around 700 °C. As the kaolinite content decreases and proportions of illite and quartz increase, structural disorder is observed at higher temperatures, specifically at 900 °C and 1200 °C [61].

Thermal treatment changes the aluminum from six-coordinated aluminum (Si–O–Al^VI) in kaolinite to four-coordinated aluminum (Si–O–Al^IV) in metakaolinite. During the thermal activation of kaolinite, external hydroxyls at the edge of the double-layer structure were removed, reducing the coordination number of Al³⁺ from six to five, as shown in Figure 9. As the temperature increased, the remaining external and internal hydroxyls were eliminated, lowering the coordination number of Al³⁺ to four. This led to the destruction of the Al^VI-O octahedron and the transformation of kaolinite into disordered metakaolinite. These changes make the coal gangue active by forming metakaolinite [94]. Kaolinite decreases as temperature rises, owing to Al-OH bond breakage and lamellar structure degradation [72]. Kaolinite mineral plays a significant role in activating the coal gangue by thermal activation, followed by illite mineral [61]. Mullite forms when the calcination temperature is raised to 950 °C, and it will form when recrystallization of kaolinite occurs [95]. As temperatures rise from 500 to 800 °C, macropores in coal gangue transform into mesopores, improving the surface area. At temperatures higher than 800 °C (i.e., 900 °C), the surface area decreases, likely due to further structural changes, material degradation, or pore collapse [13]. Thermally activated coal gangue yields a slack and porous nature. After thermal activation, the coal gangue will change color to brown. This is due to the presence of iron and titanium [28]. Calcinated coal gangue undergoes a color change after being treated at 600 °C due to the appearance of hematite [13]. Here, 700 °C was proposed as the optimum calcination temperature to eliminate carbon and attain strong chemical reactivity [61]. Calcination of small particles releases more heat than the larger coal gangue. The reason is that difficult-to-crush minerals become coarse grained during the selective crushing of coal gangue, while fragile minerals, such as clay and carbonaceous particles, form fine particles [94]. These fine particles have higher carbon content and release more heat than the coarse particles. Li et al. [96] reported that 800 °C is the optimal activation temperature. After this temperature, there are no significant weight changes, signifying the end of the activation reactions. The TG and DTG patterns revealed that primary decomposition occurs with kaolinite at about 13% at 400 °C–600 °C, followed by carbon combustion and calcite decomposition at about 83.4%.

Figure 10. Variation in the optimum calcination temperature of coal gangue (Wang et al. [97], Yang et al. [98], E et al. [65], Zhang et al. [61], Li et al. [96], Li et al. [35], Jiu et al. [99], Fan et al. [13], and Zhou et al. [100]).

Figure 10. Variation in the optimum calcination temperature of coal gangue (Wang et al. [97], Yang et al. [98], E et al. [65], Zhang et al. [61], Li et al. [96], Li et al. [35], Jiu et al. [99], Fan et al. [13], and Zhou et al. [100]).

3.3. Microwave Activation

Microwave activation is similar to thermal activation. In thermal activation, the heat is applied externally; however, microwave activation employs electromagnetic waves to heat a material’s interior directly. Among various enhancement strategies, microwave-assisted activation has demonstrated significant potential in improving both the efficiency of synthesis and functional groups due to its volumetric heating of samples [71]. The reason is that microwave activation operates via dipole rotation of water and hydroxyl groups and thermal motion of ions, rapidly raising the internal temperature and accelerating dehydration and dehydroxylation, which transforms kaolinite into metakaolinite more rapidly in less time and with reduced energy input. The optimal calcination temperature for microwave activation is reported to be 600 °C. Similar to thermal activation, microwave activation also undergoes calcination, and dehydroxylation occurs, as shown in Figure 9. The activity index of coal gangue crossed the value of 0.65, which means kaolinite is dehydroxylated to the greatest extent. Microwave activation depends on ion conduction, during which Al³ and Si⁴ ions accelerate in the microwave field, generating heat. Their vibration and migration create internal friction, enhancing heating efficiency. This process disrupts and reorganizes the –Si-O- and –Al-O- bonds in CG, breaking mineral lattices and triggering phase transitions [101]. Using microwave-activated coal gangue and limestone powder as partial cement replacements cuts carbon emissions by about 30%, enhancing sustainability in cement production [102].

3.4. Chemical Activation

Chemical activation is achieved by adding an acidic solution (H₂O₂, H₂SO₄, and H₂NO₃) or an alkaline solution (Na₂SiO₃, NaOH, and KOH) to the coal gangue. Chemical activation causes the dissolution of unstable substances, like organic and mineral substances, and increases the activity of the coal gangue by increasing the specific surface area [8]. The chemical activation method is preferred mostly in the adsorption of pollutants and the extraction of valuable elements [37,67]. In chemical activation, an increase in surface area occurs due to the removal of organic and mineral substances, as shown in Figure 11. Chemical activation effectively improves the micropore content and specific surface area of coal gangue by 10%, while thermal activation at 250 °C and 600 °C reduces its content by 17% and 15% [8]. Coal gangue activated with H₂SO₄ reacts with the metal ions present in coal gangue and generates defects and holes.

Chemical modification of coal gangue with HCl and KOH made the surfaces of HCG and KCG looser with reduced particle size and irregular layered structures, increasing their surface area. Chemical modification of coal gangue with KOH solution is depicted in Figure 11. In contrast, fresh CCG retained a dense surface with minimal pores or fractures [76]. Complete decomposition occurs in chemical activation along with thermal activation. Kaolinite is transformed to metakaolinite and the quartz mineral decomposes into nepheline (CaAl₂SiO₆) and sodium aluminum silicate, CaTiO (SiO₄), when Na₂CO₃ is added, and coal gangue is calcined at 800 °C [36]. Na₂CO₃ is a charge-compensating ion and promotes the depolymerization or decomposition of aluminosilicates. When coal gangue is modified with calcium under different pH conditions, it shows a reduction in silica and alumina content as well as increases in CaO and SO₃. Alkali modification led to the formation of C-S-H gel and AFt, enhancing the structural stability and chemical resistance of coal gangue, thereby extending its service life. This modification is employed to utilize coal gangue as the filling material and a soil conditioner and to promote adsorption and desorption toward the phosphate [67]. Calcium modified the coal gangue with the CaCl₂, increasing the CaO content in coal gangue from 1.87 to 3.29 [103]. Alkali activation methods provide the fundamental chemical pathway to transform coal gangue into reactive aluminosilicate gels. While the alkali activation consumes more energy, the hybrid techniques (a combination of two or more activation methods) are proven effective and economical. At a microwave output power of 400 W, porous materials treated with alkali-activated coal gangue resulted in an increase in compressive strength (from 3.7 MPa to 11.5 MPa) [104].

4. Applications of Coal Gangue and Activated Coal Gangue

4.1. Construction Applications

4.1.1. Cement-Based Materials

Cement is a binding material that holds building elements, such as stones and bricks, together. As it is the most widely used building material worldwide, it depletes natural resources, consumes energy, and leads to increased carbon dioxide emissions. Coal gangue is used to make cement because it contains a lot of clay minerals, SiO₂, and Al₂O₃. According to ASTM C618-17 [105], coal gangue is considered a class N pozzolan material. In addition to coal gangue composition, coal gangue requires less energy to produce clinker, due to its calorific value of 6688 kJ/kg, which replaces the coal in the calcination process. To produce one ton of activated coal gangue, the coal consumption is equivalent to 27.8 kg of coal equivalent, which is 76% less than the coal equivalent required for producing one ton of clinker. Replacing clinker production with activated coal gangue production saves approximately 90.7 kg of coal equivalent energy per ton, combining both coal and electricity savings [106]. The combination of coal gangue and copper tailing as a substitute for clay in cement clinker production offers significant benefits in terms of the utilization of resources and minimum energy requirement. The synergetic effect of sulfide minerals in copper tailings and oxygen-rich minerals in coal gangue reacts thermally during calcination, reducing the temperature requirement by more than 100 °C [107]. The composite cement was prepared by replacing 30% of clinker with thermally activated coal gangue. The incorporation of thermally activated coal gangue significantly enhanced the early hydration of cement and itself. The highest hydration rate and mechanical strength were observed when coal gangue was activated at 700 °C [82].

Table 1. Different activations facilitated the production of cement-based materials.

Materials	Activation Method	Optimum Temperature	Application	Compressive Strength (MPa)			Reference
				3 Days	7 Days	28 Days
Coal gangue, copper tailings, low calcium cement, and high calcium cement	Thermal	1450 °C	Clinker production	55		80	Qiu et al. [107]
Coal gangue	Thermal	800 °C	Cementitious material		28.4	38.5	Guo et al. [66]
Coal gangue blended with cement	Thermal	700 °C	Cementitious material	30	42		Li et al. [108]
Coal gangue	Thermal	800 °C	Cementitious material		15.51	22.43	Su et al. [109]
Coal gangue	Thermal	-	Cement mortar	26		52	Wang et al. [110]
Clinker, gypsum, and coal gangue	Thermal	700 °C	Clinker production	25.2		52.8	Guo et al. [111]
Red mud and coal gangue	Thermal	600 °C	Cementitious material	24	28		Zhang et al. [112]
Cement and coal gangue	Thermal	-	Cementitious material	3	22		Zhao et al. [60]
Coal gangue	Thermal	750 °C	Cementitious material	21.2		47.2	Lu et al. [113]
Coal gangue	Thermal	700 °C	Admixture		35	49	Wang et al. [58]

shows different activations employed to produce cement-based materials. Li et al. [106] successfully built an industrial prototype to produce eco-cement containing 56% of activated coal gangue, 25% of slag, 15% of clinker, and 4% gypsum on a large scale for road pavement application. The 28-day compressive strength of C20 grade concrete was determined using this cement; the findings indicated that it was 35.9 MPa, exceeding the standard value. Activated coal gangue, when used as a mineral admixture in cement, reduces compressive strength, though less than the proportion added (8–30% reduction). This suggests some pozzolanic activity, albeit weak due to low kaolin and clay content. Therefore, the recommended mixing ratio should be kept below 20% [27].

Zhang et al. [50] developed an auxiliary cementitious admixture using thermally activated high-alumina coal gangue. According to reports, the ideal temperature for producing auxiliary cement or concrete is 700 °C, and the dose should be less than 30%. This results from a lack of calcium hydroxide from cement hydration to react with metakaolin and calcinated coal gangue. At 7 and 28 days, the auxiliary cement mortar’s flexural and compressive strengths are 16.5% and 12.6% as well as 8.27% and 11.85% higher than those of the cement mortar, respectively. Cement pastes mixed with calcined coal gangue showed higher compressive strength compared with the cement mixed with river sand, which is due to the transformation of kaolinite into metakaolinite in the cementitious system, undergoing a pozzolanic effect, which generates hydrated calcium silicate (C-S-H), hydrated calcium aluminate, and ettringite (AFt) [94]. Liu et al. [101] suggested the practical application of microwave-activated coal gangue as a sustainable and high-performance supplementary cementitious material in construction.

4.1.2. Bricks

In the literature, authors have used various industrial waste materials to produce bricks and reduce dependence on natural clay. Sintering temperature plays a major role in the production of bricks. When making bricks with raw coal gangue, the optimum sintering temperature is 1200 °C. At this temperature, primary mullite and cordierite are formed, and quartz is transformed into tridymite, and a glassy phase will evolve [5]. This variation is due to changes in the constituents of fluxes in coal gangue. Luo et al. [114] prepared a sintered brick using waste materials like coal gangue and iron ore tailings as the raw materials and shale and sewage sludge as a binder. These bricks were produced using an optimum content of 54:30:10:6 for iron ore, tailings, coal gangue powder, shale, and sewage sludge, respectively, with a molding pressure of 20 MPa, a sintering temperature of 1100 °C, and a holding time of 3 h. Under sintering, crystal-like minerals in brick are wrapped and cemented by the molten glass phase. Xu et al. [32] utilized coal gangue as the sole raw material for making sintered bricks. Coal gangue is mixed with water and molded into a rectangular, cylindrical shape under uniaxial cold pressure at 80 MPa for 3 min and then dried. These bricks were sintered at 900 °C, 1000 °C, 1100 °C, 1200 °C, and 1250 °C in an electric furnace at a heat rate of 1 °C/min and then cooled. As the sintering temperature increases from 900 °C to 1100 °C and reaches a maximum at 1200 °C, the bulk density, compressive strength, flexural strength, and linear shrinkage increase. In contrast, the water absorption value shows an inverse trend. Zhu et al. [4] prepared permeable bricks using coal gangue as aggregate and tailings as the binder using the partial sintering method. Compressive strength and permeability showed the reverse trend.

4.2. Geotechnical Applications

4.2.1. Backfill

Surface subsidence is a major problem in the mining area. Surface subsidence in mining areas causes water accumulation and loss of useful land. Based on the severity, it also damages the nearby structures. This approach not only mitigates waste disposal issues but also enhances the stability of mine structures. Bo et al. [20] emphasized the role of coal mine solid waste backfill in protecting geological formations and water resources, supporting sustainable mining in China. It reviews advancements in backfill technology and addresses technical challenges. Zhang et al. [62] studied the mechanical properties of in situ cemented paste backfill containing coal gangue and fly ash and explored the mechanical properties of a novel backfill material used in coal mining. The study concludes that CGFACPB is a viable option for backfilling in coal mines, providing significant ground support and reducing surface subsidence. However, due to the three-dimensional stress in the field, there may be significant differences in mechanical properties between the unconfined compression test and triaxial compression test results. Howladar et al. [115] emphasized that backfilling should be recognized as a significant research area with considerable potential for development in mining practices. The study concludes that a hydraulic backfill is preferred, recommending a mixture of sand fill with 5–7% cement and 8–10% ash, considering the required volume, material characteristics, and cost implications. The authors have encountered a solution to bad fluidity and pipe wearing of coal gangue flow by adding fly ash to cemented coal gangue, and the mass ratio of cement to fly ash to gangue is 1:4:15 [62]. Li et al. [116] mentioned that the optimal ratio of coal gangue to fly ash is 1:0.35, which showed the lesser deformation due to the large size of the gangue formed as a skeleton structure, and fly ash particles fill gaps and become stiffer. The authors concluded that gangue and fly ash can be used as underground backfill materials to address environmental issues and promote sustainable mine development through the comprehensive utilization of solid waste resources. A composite backfill material using coal gangue and construction solid waste was developed to recycle these materials and address waste management issues. The study showed that the material’s flow performance is affected by the reducing agent, mass concentration, fly ash, and construction solid waste substitution rate. Calcium hydroxide (CH) content initially increases and then decreases, aiding secondary hydration reactions that strengthen the material’s structure. The optimal ratios were 21% fly ash, 50% construction solid waste, 80% mass concentration, and 0.6% reducing agent [117]. Li et al. [118] conducted a thorough analysis of the permeability coefficient and acceleration factor of crushed coal gangue specimens utilizing a customized percolation test apparatus and fractal theory. They established that adding a specific amount of fly ash to coal gangue enhances the adhesion of the broken coal gangue samples, effectively controlling subsidence in the coal mine area. An increase in the fractal dimension leads to the filling of pores by smaller particles, subsequently reducing permeability. In addition, Li et al. [119] investigated the compaction properties of crushed gangue backfill material across various particle sizes. They proposed a double yield model for simulating the behavior of coal gangue backfill material (CGBM), delivering essential insights for optimizing backfill materials and ensuring stability. Compaction tests have conclusively shown that CGBM with particle sizes between 2.5–50 mm mirrors subsidence patterns seen in those sized 2.5–16 mm while yielding lower results compared to larger sizes (20–31.5 mm and 31.5–50 mm).

4.2.2. Pavements

Guan et al. [84] studied the feasibility of using the cement-stabilized coal gangue with optimum gradation as a pavement base material. The results reported that at 4% cement content, it exhibited satisfactory results of 7-day UCS of 2.3 MPa, a 90-day splitting tensile strength of 0.87 MPa, a frost resistance index BDR of 75.16%, a moss loss rate of 0.07%, and a 31-day dry shrinkage strain of 627.5 × 10⁻⁶. It can be used in medium and light traffic bases and lower categories, and the heavy traffic sub-base of class II highways. For the road pavement test, concrete was prepared using a mix ratio of coal gangue-based cement:sand:stone of 2:3:4, designed for C20 grade concrete. The compressive strengths recorded at 3, 7, and 28 days were 18.1 MPa, 27.5 MPa, and 35.9 MPa, respectively, significantly exceeding the standard strength requirement for C20 concrete [105]. Gaddam et al. [73] stabilized the black cotton soil using coal gangue and lime to reduce the dependency on chemical additives. Based on UCS and CBR results, authors have concluded that 20% CG and 6% lime are beneficial and suitable for subgrade and sub-base as per IRC:37 (2012) and IRC:SP:72 (2007). Based on the mechanical properties of inorganic binder stabilized coal gangue, it is recommended to use lime and fly ash/cement stabilized coal gangue in base or sub-base material in low-volume rural roads, and these materials satisfy the requirements of class III roads in China [47]. The field trial in Yinchuan City, China, confirms that a cement-stabilized coal solid waste, consisting of 4% cement and 50% CBARR, provides the 7th day UCS of 3.88 MPa in the field test, which met the Chinese standards for heavy traffic of class II highways. This demonstrates a sustainable and high-performance alternative for road base construction, promoting the large-scale reuse of coal industry solid wastes [82]. The utilization of binary blending for different waste materials meets the requirements for the low-volume roads [120]. Coal gangue mixture of steel slag and mineral powder showed 7-day unconfined compressive strength ranging from 2 to 6 MPa, which meets the requirement of pavement base of grade II and below grade II roads in the Chinese standard [87].

4.3. Geoenvironmental Applications

4.3.1. Phosphate Adsorption Performance of Coal Gangue

The adsorption efficiency of raw coal gangue toward phosphate is low, even lower than that of agricultural soil [67]. When this raw coal gangue is used in reclaiming the soil and purification of wastewater, it causes eutrophication and unbalanced nutrient loss. Ye et al. [67] suggested the use of calcium-modified coal gangue as a soil conditioner, filling material in mining areas, due to its higher adsorption capacity toward phosphate (3.599 mg/g). Under alkaline conditions, the best results were obtained due to the formation. The development of C-S-H and ettringite phases, which raised the specific surface area and supplied additional active sites like Ca²⁺, Al²⁺, and hydroxyl groups for phosphate adsorption, is responsible for the improved adsorption performance of Al-CG. The adsorption efficiency for the removal of phosphate using calcinated coal gangue and zirconium oxide-added calcinated coal gangue increased from 35% to 93%. This increase in adsorption efficiency is attributed to temperatures above 400 °C, where orthorhombic ZrO₂ transforms into tetrahedral or mixed-phase ZrO₂, with the tetrahedral phase likely having a higher affinity for phosphate adsorption. The maximum phosphate adsorption capacity with CCG-Zr was reported as 8.55 mg/g [63]. When the adsorption efficiencies of thermally activated coal gangue, lanthanum-modified coal gangue, and polymerized ferric sulfate are compared, the lanthanum-modified coal gangue showed the best results. This is due to its hexagonal structure crystals, with the crystal rods growing in various directions, resulting in making the material porous and increasing the specific surface area up to 122.231 m²/g. This modification can be employed in real-time applications like lakes, reservoirs, and other slow-flow water bodies [100].

Although coal gangue is porous, it lacks interconnectivity and is filled with mineral debris, preventing gases and water from infiltrating. Removing heavy metals with low-cost adsorbents holds significant promise due to the availability and affordability of materials like natural resources, agricultural waste, and industrial by-products. The existing literature emphasizes the use of coal gangue as an adsorbent for removing heavy metals and dyes from wastewater and for purifying mine water. In the adsorption process, coal gangue is employed in various forms, including calcined coal gangue, chemically modified coal gangue, and geopolymer. Treating mine water on-site and utilizing mining waste is a cost-effective strategy to prevent surface and groundwater pollution from the discharge of mine water during mining activities. Cheng et al. [121] conducted a study on mine water treatment, analyzing the sorption of calcium ions and total dissolved solids (TDS) in the roof rock layers of coal mines under varying pH conditions. The results indicated that both acidic and alkaline conditions enhanced gangue solubility, increased void size, and improved adsorption capacity. Medium-grained coarse sandstone demonstrated a higher TDS adsorption capacity compared to muddy siltstone. Mohammadi et al. [122] successfully synthesized and characterized the alginate-combusted coal gangue (ACCG) composite, testing its effectiveness in removing heavy metals, specifically zinc (Zn (II)) and manganese (Mn (II)), from aqueous solutions. The incorporation of alginate significantly enhanced the adsorption capacity of coal gangue, an inorganic waste material. In addition to removing pollutants from water, researchers also focused on agriculture and soil reclamation in mining areas.

4.3.2. Removal of Dyes

Synthetic dyes in industrial wastewater pose serious risks to aquatic life, soil health, and human well-being due to their toxicity, persistence, and carcinogenic potential. Coal gangue and its modified forms are inexpensive, efficient, and environmentally friendly solutions that show promise in lowering dye pollution, safeguarding ecosystems, and encouraging environmentally sound behavior. The methods that are followed used coal gangue are the response surface test and the hydrothermal method. Response surface analysis showed that the preparation parameters of calcium-based coal gangue, especially the mass ratio of calcium chloride to coal gangue, followed by ultrasonic water bath time and stirring time, significantly influence dye removal. At optimal conditions, removal efficiencies for methylene blue, malachite green, crystal violet, and methyl violet reached 100%, 99%,95%, and 100%, respectively [123]. Wang et al. [54] reported that the percentage of calcium chloride and coal gangue is the major factor that affects the adsorption of dyes. The removal efficiency of calcium-modified coal gangue exhibits the following order: malachite green > methylene blue > crystal violet > methyl violet and other dyes in water. In line with economical and environmentally friendly water treatment methods, recent developments have investigated its conversion into a variety of functional adsorbents, such as zeolites and ceramic materials. The shortest equilibrium rate is achieved with the ceramic adsorbent, achieving 90% adsorption within 1 min [124].

The significant presence of SiO₂ and Al₂O₃ in coal gangue makes it suitable for use as a zeolite [125]. Many researchers have used coal gangue as an adsorbent to mitigate the accumulation of coal gangue and eliminate dyes from wastewater, as it is cost-effective compared to other adsorbents [126]. Raw coal gangue has a low adsorption capacity due to its relatively low surface area and lack of functional active sites. To enhance the adsorption capacity of coal gangue, it is modified in various ways. Yang et al. [124] prepared an adsorbent by altering coal gangue with calcium carbide to evaluate the removal efficiency of malachite green, malachite blue, crystal violet (CV), and methyl violet (MV) dyes in wastewater. The optimal preparation parameters for calcium-based coal gangue are a molar ratio of calcium chloride to coal gangue of 2.5 g:5 g, an ultrasonic water bath time of 25 min, and a stirring time of 686 min. MB, MG, and CV removal rates are 100%, 99%, and 95%, respectively. Wang et al. [57] reported that when coal gangue is modified with CaCl₂, calcium ions penetrate and gradually remove interlayers of coal gangue, thereby increasing both the specific surface area and negative charge. The adsorption efficiency exhibits the following the order, namely, malachite green > crystal violet > methylene blue > methyl violet, indicating rapid adsorption. The dye-adsorbed Ca-CG can be regenerated via desorption pathways and reused in water pollution control. A ceramic microsphere adsorbent made from coal gangue effectively removes cationic red and blue dyes from water. It is low-cost and eco-friendly and offers rapid adsorption. At pH 8, it removed 99.7% of cationic blue, while at pH 12, over 100% of cationic red was achieved. The substitution of Al for Si increases electronegativity, thereby enhancing cationic dye adsorption [124].

A high-performance silicate adsorbent synthesized from coal gangue efficiently removes pollutants, supporting the “waste-to-resource” approach. It exhibits high capacities for Cd (II) (168.92 mg/g) and MB (234.19 mg/g), with removal rates exceeding 99% [125]. It is effective in real wastewater and reusable over five cycles, holding strong potential for continuous purification through column adsorption. Sodalite zeolite synthesized from coal gangue using a hydrothermal method adsorbs Cd²⁺ and methylene blue. Sodalite demonstrated high Cd²⁺ removal due to its low Si/Al ratio, high Na⁺ content, and negative framework, and it removed methylene blue through electrostatic forces, hydrogen bonding, and porosity. In contrast, raw coal gangue had low Cd²⁺ removal but adsorbed methylene blue well due to the presence of hydroxyl-rich aluminosilicates and residual organics [126]. Coal gangue-based zeolite granules were synthesized using an alkali fusion hydrothermal method. The zeolite granules made from coal gangue exhibited a microporous structure (0.44–1.89 nm), effective for methylene blue (MB) adsorption. The process adhered to the Langmuir isotherm, indicating monolayer adsorption, and pseudo-second-order kinetics, suggesting chemical adsorption via ion exchange [127].

4.3.3. Removal of Heavy Metals

The study of the adsorption of heavy metals by gangue in a goaf environment is influenced by particle size distribution, inflow velocity, and ion concentration. Smaller particle sizes and higher inflow conditions enhance adsorption due to increased surface area and contact time [128]. Physical and chemical interactions drive adsorption, and optimizing these factors can significantly improve the effectiveness of gangue in purifying mine water [129]. Shang et al. [77] prepared mercapto-modified coal gangue through surface grafting to eliminate heavy metals in an aqueous solution. Through oxidation with oxidants (K₂MnO₄ and H₂SO₄) and intercalation with N₂H₄ during the modification process, the surface of mercapto-modified coal gangue became rough and increased its specific surface area. The maximum theoretical adsorption capacities were found to be 332.8 mg g⁻¹ for Pb (II), 110.4 mg g⁻¹ for Cd (II), and 179.2 mg g⁻¹ for Hg (II). Mei et al. [130] successfully prepared and characterized geopolymer (GP), demonstrating a surface area of 5.93 m²/g and a mesoporous structure with a pore size of 5.60 nm. The adsorption efficiencies of different coal gangue-based materials are depicted in Figure 12.

Due to electrostatic effects, negative charges favor the adsorption of Cu²⁺ [131]. The highest adsorption capacity for Cu (II), 32.8 mg/g, was achieved with a SiO₂/Na₂O ratio of 1.2 under optimal conditions (1.67 g/L adsorbent, 40 °C, pH 5.5, and 20 mg/L Cu (II) concentration). GP exhibited excellent stability, maintaining 80.88% removal of Cu (II) after five reuses, making it a promising and cost-effective adsorbent for wastewater treatment.

4.3.4. Carbon Sequestration

Carbon sequestration is a crucial strategy for mitigating global CO₂ emissions. Many countries are looking for the neutralization of carbon content. Coal gangue serves as a low-cost, abundant, and reactive material (calcium and magnesium) that can be used in both direct CO₂ mineralization and in developing CO₂ adsorbents, contributing to sustainable carbon management. Utilizing coal gangue for CO₂ sequestration not only reduces greenhouse gas emissions but also offers a sustainable solution for waste management. Amine-modified MCM-41 synthesized from coal gangue shows strong potential for low-temperature CO₂ capture, including from vehicle exhaust or ambient air. Optimal preparation conditions (pH 9, 550 °C) yielded a maximum surface area of 642 m²/g. Loading polyethyleneimine (PEI) onto MCM-41 significantly increased CO₂ adsorption from 0.125 to 1.742 mmol/g as PEI content reached 60%. After five cycles, 70% of the initial capacity was retained, demonstrating robust stability and reusability [132]. Zhu et al. [133] studied the role of carbonization pressure on CO₂ sequestration efficiency and rheological behavior of coal gangue-based backfilling slurry. Coal gangue-based backfilling slurry was prepared by coal gangue, fly ash, and water. The study reported that carbonization pressure enhances CO₂ uptake by generating carbonization products that refine pore structure and densify the matrix. At a constant slurry mixing ratio of 70:30 (coal gangue: fly ash), CO₂ sequestration capacities were determined, varying different carbonization pressures (0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 MPa). As a result, the maximum CO₂ adsorption of 0.38% was achieved at 0.7 MPa.

Chen et al. [6] synthesized the Na-X type molecular sieve by using the alkali fusion hydrothermal method to remove Cu²⁺ from wastewater and CO₂ capture. Adsorption experiments demonstrated strong performance, with maximum adsorption capacities of 185.35 mg/g for Cu²⁺ ions and 5.51 mmol/g for CO₂ gas. The approach not only addresses the environmental hazards of gangue accumulation but also offers a cost-effective route for producing commercial-grade adsorbents suitable for heavy metal and greenhouse gas removal. Yi et al. [134] successfully demonstrated the synthesis of 13-X molecular sieve porous materials using coal gangue. Utilizing an “alkali fusion hydrothermal” reaction system, the molecular sieves displayed well-defined crystalline structures and uniform pore distribution. The synthesized materials exhibited a notable CO₂ adsorption capacity of 1.82 mmol/g at 0 °C. In this study, four novel silicate-based nanomaterials (SBNMs)—MgSiO₃, MnSiO₃, CuSiO₃, and ZrSiO₄—were successfully synthesized from coal gangue (CG) via thermal treatment for the first time. The process involved extracting SiO₃²⁻ from CG using a NaOH solution, followed by a reaction with the corresponding metal salts to form SBNMs. The resulting materials exhibited excellent thermal stability, high surface area, and strong CO₂ selectivity over N₂ [135].

4.4. Recovering Valuable Metals from the Coal Gangue

Coal gangue is traditionally regarded as waste; however, it is also a valuable resource for critical metals such as aluminum, lithium, gallium, rare earth elements, and metal oxides like iron oxides, as well as nonmetallic compounds, including unburnt carbon. China estimates the alumina and iron resource content in coal gangue to be 262 million tons and 196 million tons, respectively, based on oxide content [136]. Aluminum is primarily sourced from depleting bauxite reserves, prompting the search for alternatives. The aluminum content in raw coal gangue ranges from 15 to 30%, as illustrated in Figure 1, largely due to the presence of kaolinite clay, which remains inert. Only 17% of aluminum is recoverable from raw coal gangue [74]. Silica and alumina in coal gangue separate easily when kaolinite is distorted [137]. The activation of coal gangue promotes the dissolution of valuable minerals. Zhang et al. [61] reported that 700 °C is the optimal temperature to extract alumina. The grinding extraction rate of alumina is lower than that of other activation methods and is time-consuming. As the grinding time increases, the extraction of alumina also increases from 7.24 to 62.33% for a grinding duration of 0–30 min [91]. Research indicates that one or more activation methods can enhance alumina extraction from coal gangue within a shorter timeframe. For instance, coal gangue ground for 20 h alone achieved a 70% aluminum extraction rate, whereas calcinated coal gangue with just 2 h of grinding produced similar results [74].

Kong et al. [39] found that hydrochloric acid performed better when extracting iron and aluminum from coal gangue than sulfuric acid. A leaching time of four hours was found to be the most effective and energy efficient. The addition of Na₂CO₃ significantly enhances alumina extraction from calcinated coal gangue from 72% to 90% by promoting mineral phase transformations during calcination. The charge-compensating effect of Na₂CO₃ aids in breaking the aluminosilicate network, enhancing overall extraction efficiency [37]. The choice of the calcination process also affected the pace at which aluminum leached. For instance, the highest leaching rate under the fluidized calcination method is 74.42% at 500 °C, and the static calcination condition is 66.33% at 600 °C [93]. The form and leachability of rare earth elements (REEs) in coal waste are greatly impacted by calcination. Leaching efficiencies of up to 98% are made possible by the conversion of REEs into more extractable forms, which is facilitated by ideal temperatures between 600 °C and 900 °C. These results establish coal waste as a viable supplementary source for these essential elements by showing that regulated calcination can significantly improve REE recovery [138]. The raw coal gangue contains 437 ppm of lithium [59]. By employing a sequential chemical extraction method as well as a roasting activation and sulfuric acid leaching process, the research demonstrates high leaching rates for these metals, featuring a standard extraction mechanism for Al, Li, and Ga. Zhang et al. [61] examined how chemistry and minerals affect coal gangue structural evolution and chemical reactivity during calcination for practical use. It has been reported that various minerals that develop at different temperatures can be employed in industries such as alumina extraction, cement, and paint additives.

To provide a concise comparison,

Table 2. Summary of activation methods, their performance, and suitability for different applications of coal gangue.

Activation	Applications							Limitations
	Cementitious Materials		Geotechnical		Geoenvironmental
	Cement Mortar/Concrete	Bricks Production	Backfill	Pavements	Removal of Phosphorus	Removal of Dyes	Removal of Heavy Metals
Mechanical	Faster initial setting time, improves compressive strength, reduces porosity.	Improves the strength and uniformity of bricks	*	Enhances compaction and stability	Limited effectiveness in phosphorus removal	Moderate adsorption	Moderate adsorption efficiency	High energy consumption for fine grinding
Thermal	High setting time	Enhance water absorption and compressive strength	High compressive strength	Improves durability	Energy intensive	Good adsorption for cationic dyes	Highly effective for Pb2+, Cd2+, and Zn2+	Requires high temperatures (500–900 °C), leading to significant energy demand
Chemical	Improves workability	Lower density, water absorption, and thermal conductivity	*	Enhances pozzolanic reaction when combined with lime	Efficient removal of phosphorus, risk of secondary pollution if the reagents are not recovered	High effective	Efficient removal of heavy metals, potential secondary pollution	Requires high temperatures (500–900 °C), leading to significant energy demand
Microwave	Increases the alkalinity of the cement slurry and secondary hydration of the coal gangue enhances the strength	*	*	*	*	*	Similar benefits as thermal activation; high adsorption potential, but scaling up remains challenging	Limited penetration depth for large particle sizes or bulk materials

* It indicates limited studies/no studies, with insufficient data to summarize.

summarizes the suitability of different activation methods for various applications, highlighting their performance characteristics, limitations, and observed trends.

5. Secondary Pollution

Secondary pollutants of coal gangue refer to the new contaminants generated due to physicochemical and environmental interactions during its utilization. The leaching behavior of coal gangue dumps is strongly influenced by both the degree of weathering and the mineralogical forms in which metal occurs. Elements such as Cu, Pb, and Mo are typically bound to sulfides and carbonates, making them more susceptible to release during long-term weathering and oxidation. This indicates that coal gangue can act as a long-term secondary source of heavy metal pollution to the environment through continuous leaching [16]. The dynamic leaching experiment results revealed that coal gangue is a source of multiple inorganic pollutants, which include Fe, Na, SO₄²⁻, Mg, Ca, Mn, and Zn, leading to acidic leachate. Despite dilution, Fe, Na, and SO₄²⁻ remain high under rainfall conditions (903.66, 687.13, and 1307.94 mg/L, respectively). Pyrite oxidation drives this process, sustaining acidity and pollutant release. Thus, coal gangue dumps pose long-term risks to soil and groundwater [46]. The leaching toxicity of coal gangue is considered safe when it is combined with other solid waste. Zhang et al. [139] revealed that the addition of expansive soil to coal gangue reduces Cr leachate and improves the UCS of coal gangue. Therefore, authors have suggested using the combination of coal gangue and expansive soil in embankment filling, backfilling, and road construction. Li et al. [140] reported that when coal gangue is utilized in the solid mine backfilling, it promotes the release of heavy metals due to the combined action of effective stress and mine water. The pH influences the leaching ratios with Pb, Zn, and Mn, and Cu and Cr are closely related to the stress level. The leaching ratios of heavy metals are Zn > Pb > Mn > Cu > Cr. Coal gangue in a mixture of magnesium coal-based backfill materials acts as a stable, low-risk aggregate, ensuring bulk utilization and environmental safety, while working synergistically with CGCS to balance mechanical performance and heavy metal immobilization [141].

Thermal activation reduces the leaching of heavy metals by reconstructing mineral phases and removing volatile or semi-volatile elements. Fan et al. [13] reported that leaching of Ni, Cd, and other metals decreased sharply at higher calcination temperatures (500–800 °C). The maximum decrease in leaching rate is observed at 800 °C, and the leaching of Ni, Cd, Mn, Cu, Zn, and Pb are 99%, 67%, 86%, 40%, 99%, and 93%, respectively. In a similar pattern, mechanical grinding causes the leaching of heavy metals. However, when it is mixed with cement OPC (Ordinary Portland cement), it reduces the leaching of heavy metals due to an increase in binding property [60].

6. Cost Analysis

Coal gangue is a waste byproduct of coal mining and processing, and it is traditionally associated with disposal costs. Raw material cost is negligible as it is a waste. Coal gangue as a secondary resource involves processing and activation costs, but generates savings in raw materials. From an engineering and economic standpoint, large-scale utilization of coal gangue in composite production is both feasible and profitable. With a fixed asset investment of 550 million RMB, the project yields an annual revenue of 2600 million RMB and a profit of 375 million RMB, achieving a 68.2% return on investment. Compared with similar products, the production cost is reduced by 44–55%, demonstrating that coal gangue-based composites not only enable bulk and high-value utilization of waste but also deliver significant economic and environmental benefits [142]. Substituting lime and sand with heat-treated coal gangue (HCCG) in autoclaved aerated concrete (AAC) production lowers costs (from 51.86 USD to 33.15 USD/m³), boosts profits, and provides an environmentally sustainable way to utilize solid waste. It is proven that the unit cost of using treated coal gangue is one-third the unit cost of lime [143]. The energy and cost analysis of coal gangue roasting for silicon fertilizer production shows that both the basic heating energy (Q1) and heat conduction loss (Q2) increase with higher roasting temperatures, leading to higher overall energy demand. Among the additives studied, Na₂CO₃ demonstrated the most efficient and economical performance, with lower heat conduction losses, reduced energy consumption, and lower material costs, while still meeting the effective silicon content requirements [144].

7. Conclusions

This review comprehensively assessed the transformation of raw coal gangue into activated coal gangue and its implications for geotechnical, geoenvironmental, and construction applications. The study critically examined changes in physical, chemical, mineralogical, geotechnical, and environmental characteristics resulting from various activation techniques. By mapping the relationships among activation conditions, modified material properties, and application performance, the following key conclusions are drawn:

• Activation induces significant physical and chemical modifications in coal gangue, including increased amorphous content, specific surface area, and availability of reactive components, which enhance its suitability for engineering applications.
• Hybrid activation methods, combining thermal, mechanical, chemical, or microwave techniques, are more effective than single-mode approaches in achieving desired material properties.
• Activated coal gangue demonstrates strong potential in construction materials and geoenvironmental applications, followed by geotechnical uses, due to the enhanced formation of reactive silica and alumina phases.
• Mechanical activation is environmentally safe but less effective in converting kaolinite fully; thermal activation ensures complete transformation into metakaolinite.
• Activation significantly improves surface area and porosity, making coal gangue suitable for geoenvironmental applications such as adsorbents, zeolites, and porous materials.
• When blended with problematic soils, activated coal gangue improves geotechnical properties to meet the requirements of IRC:37 (2012) and IRC:SP:72 (2007) for use in subgrade and subbase layers.
• Replacing clinker with activated coal gangue in cement production significantly reduces energy consumption. Producing one ton of activated coal gangue requires only 27.8 kg of coal equivalent, approximately 76% less than that needed for one ton of clinker, resulting in energy savings of up to 90.7 kg of coal equivalent per ton.
• Coal gangue, in its natural state, acts as a source of secondary pollutants. The leaching of heavy metals in coal gangue follows the order of Zn > Pb > Mn > Cu > Cr. Nevertheless, the effect of secondary pollutants can be mitigated by the activation techniques.
• The maximum decrease in leaching rate is observed when coal gauge is thermally activated at 800 °C; the leaching of Ni, Cd, Mn, Cu, Zn, and Pb is 99%, 67%, 86%, 40%, 99%, and 93%, respectively.
• CG-based composites allow for the high-value and bulk use of waste, lowering production costs by 44–55% while also providing notable environmental and economic advantages.

This study provides a valuable foundation for both researchers and practitioners aiming to transform coal gangue from an environmental burden into a functional engineering material. It presents a holistic framework for developing activation strategies adapted to enhance specific material properties for targeted applications. A fundamental understanding of the physicochemical transformation mechanisms involved, particularly in hybrid activation approaches, is essential to advance the sustainable and efficient reuse of coal gangue in civil engineering practice.

The future of coal gangue and activated coal gangue is trending strongly toward industrial innovation, environmental sustainability, and economic opportunity. With activation technologies maturing and research advancing, we can expect growing adoption in sectors like construction, soil improvement, removal of pollutants, and resource extraction. It is important to explore the different activation methods in geotechnical applications where bulk utilization of coal gangue occurs, and the combination of activation methods that gives good efficiency should be identified.