Impact of Humidity and Freeze–Thaw Cycles on the Disintegration Rate of Coal Gangue in Cold and Arid Regions: A Case Study from Inner Mongolia, China

Abstract

Coal extraction in China is increasingly moving towards colder regions such as Xinjiang and Inner Mongolia. However, these mines face land restoration challenges due to a scarcity of fertile topsoil. This study explores the potential of coal gangue, a mining byproduct, as a viable substitute for topsoil. The study examines the effects of humidity fluctuations and freeze–thaw cycles, both individually and in combination, on the weathering disintegration of coal gangue. Coal gangue samples were subjected to controlled laboratory conditions simulating environmental factors. Fourteen interventions were analyzed, and the findings indicated that the combined application of humidity and freeze–thaw cycles significantly accelerated the disintegration process, outperforming the individual interventions. In addition, it was found that significant temperature variations caused the moisture and salts within the gangue to expand, which affected the rate of disintegration. The study showed that the rate of weathering disintegration was significantly higher in conditions of saturated humidity–freeze–thaw cycles compared to unsaturated humidity conditions. This highlights the essential role of ice crystals in accelerating the weathering process during temperature fluctuations. This study highlights the importance of humidity over temperature in the weathering and disintegration of coal gangue. It also suggests that freeze–thaw cycles can enhance this process. The study provides valuable insights for the management and utilization of coal gangue in cold and arid regions.

1. Introduction

In China, coal production results in the generation of substantial byproducts, with coal gangue being a significant one. Coal gangue consists of non-coal materials extracted alongside coal from mines. It primarily includes rocks and minerals from the coal seam and surrounding layers, such as mudstone, shale, sandstone, clay, and other minerals. Additionally, coal gangue may contain trace amounts of coal and organic matter [1]. Typically, for every hundred million tons of coal produced, approximately fourteen million tons of coal gangue are generated [2].

The accumulation of coal gangue poses environmental challenges due to its large volume and the potential release of harmful elements. As a byproduct of coal mining, it typically accumulates in large waste dumps near mining sites, posing environmental and disposal challenges [3]. Its composition varies based on the geographical location and geological conditions of the coal mine, influencing its physical and chemical properties. This variability is crucial in understanding its environmental impact and potential for reutilization [4].

Therefore, understanding its composition and the conditions under which it disintegrates is crucial for environmental management and land reclamation efforts. Recent studies have highlighted that weathering processes, particularly those influenced by humidity and freeze–thaw cycles, significantly impact the physical and chemical stability of coal gangue. These processes can lead to the gradual breakdown of the material, contributing to protosoil formation in mining areas. However, the rates of these processes and their impact on the surrounding environment depend heavily on the specific mineralogical and chemical characteristics of the gangue.

The northeastern region of China, known for its cold climate and significant coal production, generated approximately 336 million tons of coal gangue in 2020 [5]. National regulations require that tailing disposal sites within mining areas undergo land restoration and vegetation rehabilitation.

A key challenge in ecological restoration projects is the scarcity of topsoil. Limited topsoil reserves are a major constraint for land restoration and plant colonization in mining zones. In the open-cast coal pit in the Eastern Mongolian mining region of Inner Mongolia, the prevalent type of coal gangue is carbonaceous shale, which contains organic matter and has minimal pollutant content [6]. This shale, with its delicate layers and numerous fissures, readily weathers into protosoil. Studies show that rocks exposed to atmospheric conditions progressively weather and disintegrate due to physical, chemical, and biological factors [7,8,9].

The physical weathering of rocks, which marks the beginning of soil formation, is influenced by several factors, including the pH, temperature, humidity, salinity, and physical stress. Among these, the temperature and moisture are the primary determinants of the weathering rate [10]. In cold climates with significant day–night temperature variations, rocks are simultaneously affected by humidity and freeze–thaw cycles. The freezing and expansion of intrinsic pore water cause micro-cracks within the rock to deform and disperse [11,12]. This stress alters the rock’s layered structure, accelerating its weathering and transformation into protosoil [13].

Therefore, analyzing the weathering characteristics of coal gangue under humidity and freeze–thaw cycles is essential in understanding the process and mechanism of soil formation from coal gangue in cold mining areas. The results of these studies will be crucial in expanding the sources of topsoil for land restoration in mining zones [14,15]. Research has shown that the application of polymer curing agents can significantly enhance the stability of coal gangue slopes, as well as promoting vegetation growth, making it an effective method for ecological protection in cold mining regions [14]. Additionally, studies on the properties of recycled weathered rock materials have highlighted their potential applications in permafrost areas, underscoring the critical role of material selection in the success of land restoration initiatives [15]. These findings emphasize the importance of analyzing the weathering characteristics of coal gangue under various environmental conditions to optimize its use in protosoil formation and land reclamation in mining zones.

The current mainstream technology for the ecological restoration of waste dumps relies on soil covering methods that require natural soil resources [16]. However, natural soil is a vital resource and a key component of ecosystems, leading to its stringent protection. The large-scale acquisition of natural soil is not only costly but increasingly difficult and often impossible to implement. Therefore, developing restoration technologies for waste dumps that do not depend on natural soil is crucial.

Previous studies have primarily focused on the physical, chemical, and biological properties of coal gangue weathering byproducts and their environmental impacts [17]. However, there is a relative lack of quantitative research on the weathering decomposition of coal gangue and the associated mechanisms, especially regarding protosoil formation from coal gangue in cold regions and the factors influencing it [18,19,20,21].

The disintegration of coal gangue under environmental conditions is a significant concern due to its implications for environmental pollution and resource recovery. This study aims to explore the combined effects of humidity fluctuations and freeze–thaw cycles on coal gangue disintegration, which has not been extensively studied. Previous research has mainly focused on the individual effects of these factors, but their interaction remains underexplored. By filling this gap, the study contributes to the broader understanding of coal gangue weathering processes. This study examines the effects of humidity fluctuations and freeze–thaw cycles on the weathering disintegration rate of coal gangue in cold mining regions. By investigating the mechanisms of coal gangue weathering, this research provides insights to address the topsoil scarcity in restoration efforts within cold mining areas. It offers both a theoretical foundation and practical guidance for land restoration in mining regions.

2. Materials and Methods

2.1. Study Area

The study site (119°41′25″ E, 49°24′29″ N) is located in the northeastern part of Inner Mongolia, in China (Figure 1). It is one of 52 large-scale coal mines in China, with an annual production capacity exceeding ten million tons. The mine covers an area of approximately 25 km², with proven coal reserves of 4.166 billion tons and recoverable reserves of 1.37252 billion tons. The annual raw coal production capacity is 35 million tons. Alongside coal production, more than 60 million cubic meters of overburden and waste material are generated annually. The maximum accumulation height at the waste dump reaches 116 m, with a total volume of approximately 350 million cubic meters.

Inner Mongolia boasts a profusion of coal resources, with an estimated aggregate reserve of approximately 1.02 trillion tons, constituting over 20% of the Chinese forecasted reserves. Consequently, Inner Mongolia has emerged as a critical region for energy sustenance and a strategic energy reserve within China, identified by the state as one of the most crucial coal-producing regions.

The study region has an arid to semi-arid grassland climate, marked by significant annual and day–night temperature variations. Summers (June to August) are hot and oppressive, while winters (November to March) are intensely cold, with freezing conditions lasting about five and a half months and temperatures often dropping below −40 °C. The maximum frost depth reaches 3.1 m. The area receives average annual rainfall of 372 mm, with annual evaporation ranging from 900 to 1400 mm. Most precipitation occurs between June and August, with the highest recorded rainfall being 39.7 mm, while the winter precipitation (snow) is minimal. The region is consistently exposed to northwesterly winds, typically rated at 4–5 on the Beaufort scale, occasionally reaching 10. The open-cast mine lacks perennial surface water bodies, with temporary streams forming in valleys only after snowmelt and summer rains [22].

Based on the drilling data from the mining area (Figure 2), the overburden in the study area is categorized into three major rock formations: the Quaternary loose rock formation, the detrital rock formation, and the coal-bearing rock formation. The detailed engineering geological classification is provided in

Table 1. Engineering geological classification of rock and soil.

Engineering Geological Classification	Rock Formation	Space Distribution	Rock Mass Structure
Quaternary loose rock formation	Clay, powdery clay	Distributed on the surface	Discrete structure
	Gravel layer	Localized distribution
Cretaceous detrital rock formation	Weathered rock formation	The contact zone between the top of coal measures and the Quaternary system	Fragmented structure
	Sandstone, mudstone, and conglomerate	Distributed throughout the region
Coal rock formation	Coal seam	Distributed throughout the region	Layered structure

.

The Quaternary loose rock formations, listed from top to bottom, include surface humus soil, sub-sandy soil, sandy soil, fine sand, clay, gravelly clay, and small amounts of sub-clay, muddy gravel, and sandy gravel.

2.2. Experimental Methods

For the purpose of this study, unweathered coal gangue was collected from a surface mine for this investigation. Fresh coal gangue was harvested directly from the face to avoid any environmental contamination (Figure 3a). The specimens were sealed upon collection and promptly transported to the laboratory. To minimize the oxidation rate during transportation and storage, block samples were selected and securely wrapped in cling film. In the lab, the coal gangue, composed of carbonaceous shale, was carefully cut into 5 cm³ cubes, ensuring that each sample came from unweathered material by avoiding the outer surface.

This study aimed to examine the effects of moisture changes and freeze–thaw cycles on the weathering disintegration rate of coal gangue. The experiment included three categories with 14 distinct treatments, each comprising three parallel samples, resulting in a total of 42 samples. The three test protocols consisted of moisture variations, freeze–thaw cycles, and a combination of both (

Table 2. Categories of different treatments.

Experimental Processing Conditions/°C	Different Moisture Content Treatment Conditions
	15% of Saturated Water Absorption	30% of Saturated Water Absorption	50% of Saturated Water Absorption	100% of Saturated Water Absorption
20	M1	M2	M3	M4
−5/40	T1	\	\	\
−20/40	T2	MT1	MT3	MT5
−40/40	T3	\	\	\
−60/40	T4	MT2	MT4	MT6

). Figure 3b,c show the equipment used for the freeze–thaw cycle treatment.

The impact of moisture variations was tested at room temperature (20 °C), with the moisture levels set at 15%, 30%, and 50% of the saturated water absorption capacity, along with a fully saturated condition. Moisture treatments at 15%, 30%, and 50% were applied via surface immersion, while full saturation was achieved by soaking. The freeze–thaw cycle temperatures were set at −5 °C, −20 °C, −40 °C, and −60 °C, with a thawing temperature of 20 °C and an insulation temperature of 40 °C (Table 2). The freeze–thaw equipment was calibrated to maintain precise temperature settings, with thermocouples ensuring accuracy. The moisture content was controlled gravimetrically, with samples weighed before and after treatment to confirm the target levels. Calibration was performed before each run to ensure consistent conditions.

The determination of the saturated water absorption capacity for the coal gangue samples followed ASTM C 642-21, ensuring precise and reliable measurements [23]. The process began by determining the oven-dry mass of the samples. Each sample was dried in a bake oven at a temperature of 100 to 110 °C for a minimum of 24 h. After drying, the samples were cooled in dry air, ideally within a desiccator, to a temperature of 20 to 25 °C. The mass was then measured. If the initial mass measurement showed that the samples were dry and this mass was confirmed by a second, closely agreeing measurement, the samples were considered fully dry. If the samples were initially wet, they underwent additional 24 h drying cycles until consecutive mass measurements differed by less than 0.5% of the lesser value, ensuring complete drying. This final stable mass was recorded as the oven-dry mass.

After achieving the oven-dry condition, the samples were immersed in water at approximately 21 °C for no less than 48 h. The immersion process continued until the increase in mass, measured at 24 h intervals, was less than 0.5% of the larger value over two successive measurements, confirming full saturation. The surface moisture was then removed with a towel and the final surface-dry mass after immersion was recorded. The saturated water absorption capacity was calculated as a percentage of the weight increase relative to the dry weight of the samples. The formula used was

$$\mathrm{S}\mathrm{W}\mathrm{A}\mathrm{C}=\left({\displaystyle \frac{\mathrm{W}\mathrm{e}\mathrm{t} \mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}-\mathrm{D}\mathrm{r}\mathrm{y} \mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}}{\mathrm{D}\mathrm{r}\mathrm{y} \mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}}}\right)\times 100\mathrm{\%}$$

(1)

where SWAC is the saturated water absorption capacity (%). Wet mass is the weight of the coal gangue after soaking. Dry mass is the weight of the coal gangue before soaking.

2.3. Identification of Clay Minerals and Chemical Composition in Coal Gangue

The clay minerals in the coal gangue were identified using X-ray diffraction (XRD) analysis. The coal gangue samples were initially crushed and subsequently placed in an oven for drying at a constant temperature of 105 °C. After drying, the samples were finely ground with an agate mortar. The diffractograms were analyzed using standard reference patterns to identify the presence of key clay minerals such as kaolinite, illite, and montmorillonite. Quantitative analysis was performed to estimate the relative abundance of these minerals, providing insights into the mineralogical composition that could influence the gangue’s weathering behavior under varying environmental conditions. X-ray diffraction was employed to quantify the clay minerals in the coal gangue, focusing on particles less than 2 mm [24,25].

To determine the chemical composition, X-ray fluorescence (XRF) analysis was employed on the same powdered samples. The analysis focused on quantifying major elements like Si, Al, Fe, Ca, and trace elements relevant to the environmental stability of the gangue. The results were then correlated with the mineralogical data obtained from XRD to understand the relationship between the chemical and mineralogical composition. This comprehensive analysis highlights the role of specific minerals and elements in the disintegration process, particularly under the effects of humidity fluctuations and freeze–thaw cycles. X-ray fluorescence spectrometry was utilized to scrutinize the chemical composition of coal gangue particles smaller than 2 mm [26].

2.4. Experimental Design of Coal Gangue Weathering Disintegration Rate

At the start of the experiment, dried coal gangue samples were frozen at a set low temperature for 12 h and then thawed at 20 °C for 4 h and finally placed at 40 °C for 8 h. Each freeze–thaw cycle lasted 24 h and was repeated. For the combined moisture and freeze–thaw cycles, the conditions were the same, with the moisture content set at 30%, 50%, and full saturation. Temperatures of −20 °C and −60 °C were used, with water added at the beginning of each cycle.

Each treatment group consisted of three rock samples, with cycles conducted at intervals of 0, 5, 10, and 30 iterations. Samples were dried until their post-drying weight was no greater than their pre-soaking weight and then weighed. Generally, rock particles with a diameter of less than 2 mm are considered to have weathered into protosoil. After each treatment, the mass of particles ≤ 2 mm and particles between 2 mm and 20 mm was measured. The increase in the mass of particles ≤ 2 mm was used to calculate the weathering disintegration rate. Each treatment underwent 30 cycles.

The weathering disintegration rate of the coal gangue was calculated as follows:

$$\mathrm{W}\mathrm{D}\mathrm{R}=\left({\displaystyle \frac{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{s}\le 2 \mathrm{m}\mathrm{m} \mathrm{a}\mathrm{t} \mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}\mathrm{e} \mathrm{i}}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{b}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{e} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{f}\mathrm{i}\mathrm{r}\mathrm{s}\mathrm{t} \mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}\mathrm{e}}}\right)\times 100\mathrm{\%}$$

(2)

where WDR is the weathering disintegration rate; i = 1, 5, 10, …, 30.

3. Results

3.1. Main Physical and Chemical Parameters of Coal Gangue

The coal gangue samples used in the experiment were composed of high-carbon shale, with the mineral composition primarily consisting of clay minerals (approximately 40% by mass) and quartz (approximately 60% by mass). The clay minerals included illite, montmorillonite, kaolinite, and chlorite. The bonding force between the layers of the mineral structure was weak and the clay particles had strong hydrophilic properties, allowing them to absorb water easily (Figure 4). Water molecules can easily penetrate the layers of clay minerals, increasing the interlayer distance and causing the mineral particles to expand. This expansion accelerates the weathering process and protosoil formation (

Table 3. Physical properties of the coal gangue.

Density, g·cm−3	Natural Moisture Content, %	Coefficient of Water Saturation, %
2.21	2.54	13.31

).

The combustion experiments revealed that the coal gangue used in the study contained approximately 14.82% humus, indicating a significant amount of organic matter derived from plant fiber tissue. This suggests that, after weathering into protosoil, the coal gangue will be rich in nutrients. Chemically, the primary components of the coal gangue were SiO₂ and Al₂O₃. The composition analysis indicated that, after weathering into protosoil, the coal gangue can provide a substantial amount of organic matter and nutrients for reclamation plants (

Table 4. Chemical components of the coal gangue (mass percent).

Humus	SiO2	Al2O3	K2O	Fe2O3	TiO2	MgO	CaO	Na2O	SO3	P2O5	Miscellaneous
14.82	52.75	25.34	1.95	1.74	1.42	0.55	0.42	0.34	0.18	0.03	0.46

).

3.2. Influence of Moisture Change on Coal Gangue Weathering

Table 5. Effect of constant moisture on the rock decay rate of the sampled coal gangue (20 °C).

Sample Number	Percentage of Saturated Water Absorption	Number of Cyclic Experiments
		5		10		30
		1 Mean/%	2 SD	Mean/%	SD	Mean/%	SD
M1	15%	0.11	0.05	0.37	0.02	1.61	0.13
M2	30%	1.08	0.02	2.23	0.05	3.94	0.10
M3	50%	1.43	0.03	2.56	0.07	5.26	0.09
M4	100%	1.71	0.06	2.82	0.08	7.44	0.16

¹ Mean is the average of the weathering and disintegration rates of three parallel samples. ² SD is the standard deviation.

shows that the weathering disintegration rate of the coal gangue increased with higher moisture levels under constant-temperature (20 °C) conditions, with significant differences between the groups (p < 0.05 after 30 cycles). The test results for 5, 10, and 30 cycles indicate that the coal gangue with saturated moisture levels had a higher weathering disintegration rate than the other groups, suggesting that the moisture content can affect the weathering disintegration rate of coal gangue even at a constant temperature. Increased moisture content promotes the weathering disintegration rate of coal gangue. An analysis of the coal gangue layers shows that it is primarily composed of shale. When the moisture levels are low, the weathering process is incomplete, with mainly surface layers absorbing moisture and flaking off, resulting in a slower disintegration rate. As the moisture increases, especially when fully saturated, the clay minerals inside the gangue absorb water and expand, causing the more extensive development of shale fractures. This simultaneous surface flaking and internal cracking accelerates the weathering disintegration process of coal gangue.

3.3. Effect of Freeze–Thaw Cycle on Weathering of Coal Gangue

After 30 freeze–thaw cycles under constant moisture conditions, the highest weathering and disintegration rate of coal gangue was 0.67% (

Table 6. Effect of freeze–thaw cycles on the rock decay rate of the sampled coal gangue.

Sample Number	Temperature/°C	Number of Cyclic Experiments
		5		10		30
		1 Mean/%	2 SD	Mean/%	SD	Mean/%	SD
T1	−5/40	0.27	0.01	0.35	0.05	0.48	0.06
T2	−20/40	0.24	0.06	0.36	0.02	0.59	0.04
T3	−40/40	0.23	0.04	0.39	0.05	0.61	0.03
T4	−60/40	0.25	0.02	0.48	0.04	0.67	0.06

¹ Mean is the average of the weathering and disintegration rates of three parallel samples. ² SD is the standard deviation.

). A comparison between Table 5 and Table 6 reveals that the rate of weathering and disintegration of coal gangue due to 30 cycles of moisture change is significantly higher than that due to 30 freeze–thaw cycles, with no substantial variation between the different temperature treatment groups (p > 0.05). Under the condition of desiccated moisture in coal gangue, the number of freeze–thaw cycles does not significantly affect its weathering and disintegration rate. No significant difference is observed in the rate after 5, 10, and 30 cycles, nor is there any substantial difference between the groups. From Table 5, it is evident that when the moisture content is low, the freeze–thaw cycle marginally affects the weathering and disintegration rate of coal gangue. The rate is slow, which is consistent with other studies’ results [27,28,29].

3.4. Effect of Water and Freeze–Thaw Cycles on Weathering of Coal Gangue

Under the combined effects of moisture and freeze–thaw cycles, the rate of weathering and disintegration of the coal gangue saw a notable increase (

Table 7. Effect of constant moisture and freeze/thaw cycles on the rock decay rate of the sampled coal gangue.

Sample Number	Percentage of Saturated Water Absorption	Temperature/°C	Number of Cyclic Experiments
			5		10		30
			1 Mean/%	2 SD	Mean/%	SD	Mean/%	SD
MT1	30%	−20/40	4.33	0.66	7.52	0.91	15.55	2.01
MT2		−60/40	6.54	1.23	13.41	2.62	25.43	2.90
MT3	50%	−20/40	7.14	0.63	22.34	2.33	35.98	3.91
MT4		−60/40	12.45	1.78	30.55	2.92	61.31	2.96
MT5	100%	−20/40	15.88	2.90	41.14	2.75	67.59	2.39
MT6		−60/40	24.25	4.62	52.83	2.42	77.47	5.60

¹ Mean is the average of the weathering and disintegration rates of three parallel samples. ² SD is the standard deviation.

), accelerating the weathering of the coal gangue into protosoil. Additionally, Table 7 illustrates that, given the same moisture content, a lower temperature facilitates the weathering and disintegration of coal gangue. At the same temperature, an increase in water content is more beneficial to its weathering and disintegration. After 30 freeze–thaw cycles at −20 °C/40 °C, the rates of weathering and disintegration of the coal gangue samples with water content at 30%, 50%, and 100% of the saturated absorption amount were 15.55, 35.98, and 67.59, respectively. After 30 freeze–thaw cycles at −60 °C/40 °C, the rates were 25.43, 61.31, and 77.47, respectively. Considering the temperature variations in the study area and the challenges of reclamation engineering practice, the implementation conditions for the promotion of open-pit waste rock dump suggest the use of 50% of the saturated absorption amount under the treatment conditions of −20 °C/40 °C freeze–thaw cycles to expedite the weathering and disintegration of coal gangue, demonstrating strong practical operability.

The study area, located in an extremely cold region, has a topsoil layer thickness of only 20–30 cm [30]. The scarcity of soil resources impedes the progress of land reclamation in the mining area [31]. The analysis and experimental results demonstrate that the coal gangue in this mining area consists of layered structures bonded by clay minerals. The rock forms horizontal bedding, the lithology is relatively loose, and the shear resistance is poor. Bedding and weathering fractures are well developed. Influenced by moisture infiltration and the alternation of cold and heat and wet and dry conditions, the physical weathering effect is exceptionally strong and it is easily converted into protosoil.

The coal gangue’s chemical analysis shows high organic matter content, rich in nutrients such as C, Mg, K, Ca, Fe, etc. This composition promotes the rapid weathering of the coal gangue into protosoil, reduces the use of foreign soil in the reclamation process, and is beneficial in increasing the reclamation speed of the inner and outer waste rock dumps of the open pit [32]. From the results in Table 3, Table 4, Table 5 and Table 6 and Figure 5, the weathering rate of the coal gangue samples was the fastest under the combined effects of moisture and freeze–thaw cycles.

4. Discussion

4.1. Weathering Rate Influences Gangue Disintegration Processes

The slowest rate of weathering and disintegration of coal gangue was found under the single variable-temperature condition (freeze–thaw cycle), where layer-by-layer peeling occurs, resulting in a slow disintegration rate. Under constant-temperature conditions, as the degree of coal gangue immersion increases, the weathering of the coal gangue changes from mere surface layer peeling to simultaneous surface peeling and internal disintegration. This change is primarily due to the expansion of clay minerals after water absorption, which leads to the widening and deepening of the internal fractures in the coal gangue, resulting in gangue disintegration [32].

Furthermore, the mineral components in the coal gangue dissolve in the water, the dissolved salts enter the pores of the coal gangue, and the salts formed in the process exert stress on the internal pore walls of the coal gangue, accelerating its weathering and disintegration. The most pronounced weathering and disintegration effect occurred in the treatment group subjected to the combined action of moisture alteration and freeze–thaw cycles (variable temperature). In addition to the expansion of clay minerals due to water absorption, this effect can also be attributed to the expansion effect of ice crystals and the physical stress of salts [33].

4.2. Impact of Environmental Conditions and Statistical Analysis Insights

In the study area, the crystallization of the water and salt in the coal gangue at low temperatures causes irreversible damage to the internal fractures of the gangue. In particular, under the conditions of the freeze–thaw cycle, the crystallization and dissolution of a large amount of ice (salt) crystals alternate, causing continuous physical stress within the coal gangue, widening and deepening the internal fractures. The shale layer structure of the gangue causes it to continuously disintegrate into small fragments, which can quickly achieve the effect of protosoil formation (<2 mm) [31].

The experimental findings presented in Table 6 and Figure 5 support these observations. In particular, no rock particles larger than 20 mm were observed in the coal gangue from the treatment groups exposed to saturated water absorption and −60 °C after 10 cycles. Similarly, no rock particles larger than 20 mm were present after 30 cycles in the −20 °C + 50% saturated water absorption treatment conditions. The temperature in winter can achieve the necessary conditions by relying solely on the natural daily temperature fluctuations, thus negating the need for additional cooling measures.

In this study, hierarchical cluster analysis was employed to classify the sample data into distinct groups based on the similarity in their characteristics. The goal was to compare the effects of different environmental factors on the weathering and disintegration rate of coal gangue. Hierarchical cluster analysis was employed to classify the coal gangue samples based on their mineralogical and chemical characteristics. We applied Ward’s method to minimize the variance within each cluster during the agglomeration process. The input variables, including the concentrations of key minerals and elements identified in the samples, were standardized prior to analysis to ensure comparability. The resulting dendrogram provided a clear visual representation of the clusters, allowing us to identify distinct groups of samples with similar properties, which facilitated a deeper understanding of the relationships between the different coal gangue samples.

Hierarchical cluster analysis, a prevalent statistical method for the categorization of sample data (variables) based on their similarity and dissimilarity, was utilized in this study to examine the effects of different environmental factors on the weathering disintegration rate of coal gangue. A dendrogram (Figure 3) was generated. The hierarchical cluster analysis results showed that the outcomes could be divided into two primary categories: treatments involving individual moisture content or freeze–thaw cycles and treatments involving a combination of moisture content and freeze–thaw cycles. The cluster analysis revealed that the main factors influencing the classification were the simultaneous presence of enough moisture and freeze–thaw cycles, i.e., the factor that caused the acceleration of the weathering disintegration rate of coal gangue was whether there was enough moisture for crystallization.

In general, the rate of weathering and disintegration of coal gangue in a given coal mine is determined by the presence of adequate moisture and the ability to form a sufficient amount of ice crystals. When moisture and temperature treatments were applied simultaneously, the weathering and disintegration rate of the treatment group with higher saturated water absorption was greater than that of the group with lower water content (for the same temperature change). Under similar moisture conditions, the freeze–thaw group with a significant temperature difference had a higher rate than the group with a small temperature difference. The effects of individual freeze–thaw and moisture treatments were significantly smaller than the effects of combined moisture and freeze–thaw treatments.

4.3. Analysis of Causes of Coal Gangue Weathering

The wet–dry cycle is a primary factor in coal gangue weathering, especially in regions with significant seasonal variations in precipitation and humidity. During the wet phase, water can infiltrate the coal gangue’s pore spaces, promoting chemical weathering processes like hydrolysis, oxidation, and dissolution [34]. This also increases the pore pressure, potentially weakening the gangue’s structural integrity [35]. During the dry phase, the evaporation of water can cause salt crystallization, inducing mechanical stress and resulting in physical disintegration. The severity of weathering can vary with the frequency and intensity of the wet–dry cycles, with higher occurrences accelerating the weathering rate [36].

The freeze–thaw cycle is another crucial physical weathering mechanism that considerably affects the structural integrity of coal gangue. This process involves the penetration of water into the coal gangue’s pore spaces, which subsequently freezes under low-temperature conditions. The water’s expansion upon freezing exerts pressure on the surrounding material, instigating its fracture and disintegration. The rate of weathering is largely determined by the frequency and magnitude of the freeze–thaw cycles, with an increase in cycles leading to escalated weathering and disintegration [37].

The interaction of the wet–dry and freeze–thaw cycles can intensify coal gangue’s weathering. For example, the wet phase of the wet–dry cycle can encourage water infiltration, amplifying the impacts of the freeze–thaw cycle. Conversely, the freeze–thaw cycle can enhance the weathering effects of the wet–dry cycle by creating additional pore spaces for water penetration and salt crystallization.

5. Conclusions

• Weathering and Disintegration Rates under Different Conditions

Under the circumstances of exclusive freeze–thaw cyclic treatment, the disintegration rate of the coal gangue during weathering processes exhibits a slower pace, leading to suboptimal protosoil formation. As the water content within the coal gangue increases, it fosters an enhanced weathering and disintegration process. Therefore, the weathering and disintegration rate of coal gangue was more prominent in the treatment group with higher water content compared to the group with lower water content. Under the synergistic treatment of moisture variation and freeze–thaw cycles, the weathering and disintegration rate of the coal gangue was at its peak, leading to accelerated soil formation. Ranked from high to low, the weathering and disintegration rates under the three different treatments were as follows: the simultaneous application of moisture conditions and freeze–thaw cycles, treatment under varying moisture conditions (constant temperature), and the freeze–thaw cycle (at a constant humidity level).

2.

Cluster Analysis of Treatment Effects

The outcomes of the cluster analysis show that the influences on the weathering and disintegration rates of coal gangue under 14 different treatments are bifurcated into two significant categories, with pronounced disparities amongst the two. The cumulative impact of moisture and freeze–thaw cycles on the weathering process of coal gangue substantially surpasses the effects of the standalone factors.

3.

Recommendations for Onsite Conditions and Resource Utilization

Taking into account the practical conditions of the waste dump site, this study advocates for an onsite moisture level set at 50% of the saturated water absorption, and the most severe temperature condition should be below −20 °C. The coal mine is situated in a frigid, arid-to-semi-arid grassland area characterized by vast expanses of land, a small population, scarce topsoil resources, limited sources for mine land reclamation, and high reclamation difficulty. During land reclamation practices, it is beneficial to leverage the mine water resources discharged throughout the mining process and the local climatic conditions with vast day–night temperature fluctuations. Through human intervention, the coal gangue can be rapidly weathered into protosoil. This method can amplify the sources of topsoil in high-cold mining areas and expedite the efficiency of mine land reclamation.