Study on the Development Rule of Mudstone Cracks in Open-Pit Mine Dumps Improved with Xanthan Gum

Abstract

The stability of open-pit mine slopes is crucial for safety, especially for spoil dump slopes, which are prone to cracks leading to landslides. This study investigates the use of xanthan gum (XG) to enhance the stability of mudstone in spoil dumps. Various concentrations of xanthan gum were mixed with mudstone and subjected to dry–wet cycle tests to assess the impact on crack development. Pore and crack analysis system (PCAS) was utilized for image recognition and crack analysis, comparing the efficiency of crack rate and length modification. The study found that xanthan gum addition significantly improved mudstone’s resistance to crack development post-drying shrinkage. A 2% xanthan gum content reduced the mudstone crack rate by 45% on average, while 1.5% xanthan gum reduced crack length by 46.2% and crack width by 26.3%. Xanthan gum also influenced the fractal dimension and water retention of mudstone cracks. The optimal xanthan gum content for mudstone modification was identified as between 1.5% and 2%. Scanning electron microscopy imaging and X-ray diffraction tests supported the findings, indicating that xanthan gum modifies mudstone by encapsulation and penetration in wet conditions and matrix concentration and connection in dry conditions. These results are expected to aid in the development of crack prevention methods and engineering applications for open-pit mine spoil dump slopes.

1. Introduction

In geotechnical engineering, the stability analysis of slopes is a crucial research topic, especially in areas prone to landslides. Given the frequent landslide occurrences in China, the stability issue of open-pit mine slopes is particularly prominent [1,2]. Compared to conventional rock slopes, the waste dump slopes of open-pit mines are loose body slopes formed by the accumulation of topsoil and rocks stripped during the mining process. These include different grain sizes and types of soil and rock masses, making their mechanical properties more complex. At the same time, the stability of waste dump slopes is influenced by geological conditions and natural factors such as rainfall [3], earthquakes, typhoons, and other extreme weather conditions that can lead to landslide disasters. Surface shrinkage and cracking on waste dump slopes are extremely common. Under rainfall and wet–dry cycles, small cracks can develop into large fissures or gullies, significantly increasing the risk of landslides on waste dump slopes [4,5]. Therefore, studying the laws of shrinkage and cracking of open-pit mine waste dumps and taking corresponding measures is significant for the safety of open-pit mine production.

The human-induced slow construction of open-pit mine waste dumps provides the feasibility of improving their properties by adding specific materials. In the field of geotechnical engineering, modified materials are being widely applied to enhance engineering performance and efficiency. These materials include microbially modified materials [6], nanomaterials [7], and reinforced materials [8], among others. Currently, some materials and technologies at home and abroad can suppress the cracking of soil and rock masses. By incorporating natural fibers [9] (e.g., lignocellulose, bagasse, and plant roots), adding natural gels [10,11] (guar and xanthan gums, etc.), spraying soil consolidating solutions [12,13] (MICP, EICP, etc.), and biochar chemical treatment [14], the solidification of soil and rock masses can be effectively achieved, realizing the effect of inhibiting the shrinkage and cracking of the surface of soil and rock masses. Haipeng Wang et al. [15] enhanced the erosion resistance and water retention of waste dumps by spreading PAM powder particles and spraying PAM consolidating solutions. The results of indoor experiments showed that this treatment method can significantly improve the erosion resistance of the slope surface under rainfall conditions and reduce porosity, effectively reducing soil and water loss. Liu Bo et al. [16] evaluated the impact of MICP technology on inhibiting the drying cracks of clay through a series of field experiments. The CaCO₃ crystals produced by the MICP process effectively inhibited the generation of soil and rock cracks by filling the crack space, consolidating soil particles, and reducing soil moisture evaporation. Dai Chengjiang et al. [17] explored the impact of different amounts of xanthan gum and sand content on the crack resistance of surface soil to suppress the development of cracks caused by surface drying. The results showed that combining xanthan gum biopolymers and a certain amount of sand can effectively increase the cohesion within the soil and enhance the crack resistance. Cai Yangyang et al. [18] found that adding xanthan gum provided good anti-seepage properties to the clay layer and significantly reduced the macroscopic crack rate when studying the impermeability function of biopolymers in clay layers. Although there are many studies on the modification of soil and rock masses, most of them focus on improving the macroscopic mechanical properties of soil and rock masses, and there are fewer studies on the modification of complex soil and rock mixtures such as open-pit mine waste dumps. The effects and mechanisms of modification are still unclear.

This study selects the environmentally friendly xanthan gum biopolymer as a modifying material widely used in geotechnical modification. A series of drying and wetting cycle experiments on the cracking performance of improved mudstone are carried out to comprehensively evaluate the impact of adding xanthan gum on the water retention and crack resistance of open-pit mine mudstone from multiple perspectives. This research aims to explore the optimal content of xanthan gum to suppress the cracking of mudstone, reveal its microscopic mode of action, and summarize the modification mechanism. It is hoped that through the study of xanthan gum-modified mudstone, contributions will be made to the development of solidification techniques for waste dump slope technology and reduce the accidents of open-pit mine slopes.

2. Materials and Methods

2.1. Specimen Preparation

2.1.1. Open-Pit Mine Mudstone

Mudstone is a widespread type of rock in open-pit mines. It is one of the principal rock types in many open-pit mines. The mudstone used in this study is from the waste dump of an open-pit mine, collected from a large open-pit coal mine in Changji City, Xinjiang Uygur Autonomous Region, China. Figure 1a displays the sampling location. The height of the open-pit mine waste dump is 15 m, and sampling was conducted at a safe location at the base of the slope. Figure 1b shows the appearance of the experimental materials. The mudstone is grayish-white, with various shapes and distinct edges, a rough and uneven surface, and some brown spots. After simple indoor drying and measurement, the mudstone’s density was 2.391 t/m³, the dry density was 2.304 t/m³, and the natural water content was 3.23%. The saturated water content was obtained through a saturation test to be 27.40%. Due to the maximum particle size limit of indoor experiments, it is necessary to scale down the original grading curve of the mudstone. According to the procedures [19,20], considering the limitations of sieve mesh size and test container, sieve mesh sizes of 10 mm, 5 mm, 2.5 mm, 1.25 mm, 0.63 mm, 0.315 mm, 0.16 mm, and 0.08 mm were selected. Based on the on-site rock block size and the similar grading method, a laboratory-usable particle grading was formulated, and the materials were screened and mixed; the particle mesh morphology and grading are shown in Figure 1c,d.

2.1.2. Xanthan Gum

Xanthan gum, or Hanxin gum, is a pale yellow fine sand-like substance. It is an extracellular acidic polysaccharide produced by the fermentation of Xanthomonas bacteria, and it functions as a thickener, suspending agent, and emulsifier [21]. It is easily soluble in water but insoluble in most organic solvents, with the chemical formula C₈H₁₄C_l2N₂O₂. Due to its stability under complex conditions such as low pH values, high temperatures, and freeze-thaw cycles, XG can be used alone to improve geotechnical materials’ properties and as an essential ingredient in composite curing agents [22,23,24]. The XG used in this study was purchased from Hefei Qiansheng Biotechnology Co., Ltd. (Hefei City, China), and the reagent is intended solely for experimental and research purposes. The morphology of XG is shown in Figure 1b.

2.1.3. Preparation Standards

Prepare the on-site retrieved material by crushing and screening to obtain mudstone materials of various particle sizes. Dry the material at a temperature of 105 °C for 12 h to achieve dry and uniform mudstone specimens of different particle sizes. According to the particle size distribution ratio determined earlier, mix and prepare several individual mudstone specimens weighing 200 g. Referring to previous research on modifying geotechnical materials with biopolymers [25,26], the optimal mass fraction of XG for improving the crack development properties and strength characteristics of geotechnical materials is 0~3%. Therefore, the mass fraction of XG added to the mudstone specimens and the corresponding specimen numbers are shown in

Table 1. Xanthan gum content in specimens.

Specimen	U1	M1	M2	M3	M4	M5	M6
Mass fraction	0%	0.5%	1%	1.5%	2%	2.5%	3%

below.

The experimental containers are uncovered cylindrical stainless-steel molds with an inner diameter of Φ10 cm × 5 cm height (To prevent the tolerance of the container manufacturing from affecting the experimental results, the base weight of the containers is recorded, respectively). A layer of petroleum jelly is evenly applied to the inner wall, and sandpaper is laid at the bottom of the mold to prevent the surface properties of the container from affecting the experimental results during the experiment. Considering the natural water content of this specimen is 3.23%, water is added to the uniformly mixed specimens using a spray bottle to simulate the natural water content state. The specimens are placed in the containers, wrapped with plastic to seal, and left to cure for 24 h.

2.2. Experimental Conditions

2.2.1. Specimen Air-Drying Conditions

Due to the sampling site being inland and having a temperate continental climate, there is a significant temperature difference between day and night, with precipitation typically concentrated in the summer. The sampling area experiences hot summers with the highest temperatures reaching up to 42 °C, and high temperatures are quite common, while surface temperatures usually exceed air temperatures. To simulate the characteristics of dry and hot conditions during the day in the northern Xinjiang region in summer, it is necessary to conduct drying experiments in a drying oven at a temperature of 50 °C. During the experiment, the weight of each specimen container is measured and recorded every 2 h until the difference in weight between two measurements is less than 0.1 g, at which point the specimen is considered completely dry.

2.2.2. Wet–Dry Cycle Test

The principle of the wet–dry cycle test is to simulate the cyclical drying and wetting conditions that materials undergo in their natural environment. During the drying phase, materials may experience moisture loss, leading to volume reduction, crack formation, or decreased mechanical properties. Conversely, during the wetting phase, materials absorb water, which can cause volume expansion, damage caused by freeze–thaw cycles, or changes in chemical composition. By implementing such cycles, the durability of materials and the evolution of their performance can be assessed.

In this study, one wet–dry cycle is defined as the process from the natural dehydration of a moist and saturated specimen to a dehydrated state. Although the saturation water content obtained from the vacuum saturation test is 27.4%, the average value achieved under atmospheric pressure is 24%, so the specimen is considered saturated at this water content.

After preparing the specimens, water is added to the container using a fine mist sprayer until the water content of the specimen reaches 24%. Record the weight of the specimen at this time and calculate the water content. The saturated specimens are then placed in a drying oven for drying, and they are weighed and photographed every 2 h until they are scorched, ending this cycle. Pure water is added again to start the next wet–dry cycle, and the test is concluded after six wet–dry cycles. The first wet–dry cycle in the experiment is named N = 1, and so on. The wet–dry cycling process is shown in Figure 1e.

2.2.3. Surface Hardness Measurement

The working principle of a Shore hardness tester is to use a spring-driven indenter to press vertically into the surface of the material being tested. The force applied by the indenter is proportional to the depth of penetration. The reading of the hardness tester reflects the depth to which the indenter penetrates the surface of the material: the smaller the penetration depth, the higher the material hardness; the more significant the penetration depth, the lower the material hardness [27]. The advantage of the Shore hardness tester is its simple operation, quick measurement, portability, and suitability for on-site testing, but it can only provide relative hardness values.

In the preliminary experiment, the surface hardness of all specimens was below 100 HA. Therefore, in this study, a flat surface of the specimen was selected after each wet–dry cycle, and the surface hardness was measured using a Shore hardness tester. Multiple measurements were recorded, and the average value was taken as the surface hardness value of the specimen.

2.2.4. Microscopic Tests

A scanning electron microscope (SEM) is an instrument that scans the specimen surface with a focused electron beam, obtaining high-resolution images of the specimen surface by detecting signals generated from the interaction between the electron beam and the specimen [28]. The working principle of SEM involves using an electron gun to emit a narrow beam of electrons, which are accelerated by an accelerating voltage and directed toward the specimen surface. When the electron beam interacts with the atoms in the specimen, signals such as secondary electrons, backscattered electrons, and characteristic X-rays are produced. These signals are collected by corresponding detectors and converted into images, revealing the microstructure and composition of the specimen surface.

X-ray diffraction (XRD) testing is essential for analyzing materials’ crystal structure, chemical composition, and physical properties. It is based on the diffraction phenomenon that occurs when X-rays interact with materials, and it obtains detailed information about the material by measuring the diffraction pattern. XRD testing is based on Bragg’s Law, which is nλ = 2dsinθ, where n is the order of diffraction, λ is the wavelength of the X-ray, d is the distance between crystal planes in the material, and θ is the angle between the incident X-ray and the crystal plane. When X-rays are directed at a crystal specimen, due to the regular arrangement of atoms within the crystal, X-rays are scattered at crystal planes, resulting in a diffraction phenomenon. By measuring the diffraction angle and intensity, the atomic arrangement and lattice parameters within the crystal can be deduced [29].

As mentioned above, some test specimens from U1 and M6 were selected to undergo SEM and XRD testing after the experiment to study the micro and compositional changes in the mudstone.

2.3. Experimental Equipment

The experimental methods in this study mainly include four parts: specimen preparation, wet–dry cycle test, surface hardness measurement, and microscopic test. The experimental equipment and models involved are shown in

Table 2. Model of experimental equipment.

NO.	Equipment Name	Model	Manufacturer
1	Drying oven	101-1B	Shanghai Jingmai Instrument Equipment Co., Ltd., Shanghai, China
2	Electronic scale	LQC-50002	Shanghai Yaoxin Electronic Technology Co., Ltd., Shanghai, China
3	Shore durometer	0-100HA-A	Xingweiqiang Hardware Tools Co., Ltd., Dongguan, China
4	Scanning electron microscope	Zeiss Gemini 300	Carl Zeiss AG, Oberkochen, Germany
5	X-ray diffraction instrument	Rigaku SmartLab SE	Rigaku Holdings Corporation, Tokyo, Japan

.

2.4. Image Capture and Analysis

Before each wet–dry cycle, the specimen is in a saturated state with a relatively flat mudstone surface, and after the cycle, the surface will develop numerous cracks and pores. The surface of the cracked mudstone is photographed with a fixed camera position. Adobe Illustrator is used to crop the resulting photos to the same pixel area to ensure the accuracy of subsequent image analysis. The pore and crack analysis system (PCAS) is employed to process and analyze the images of the dried and cracked surfaces. Taking the U1 specimen in the first cycle as an example, the image processing procedure is shown in Figure 2. Through image capture and analysis, parameters of crack development after each wet–dry cycle can be obtained, such as crack density, crack length, etc. Additionally, the calculations for probability entropy and fractal dimension were conducted using the built-in algorithms of the PCAS, which has been instrumental in further studying the patterns of cracks.

3. Results

3.1. Image Analysis

3.1.1. Crack Ratio

In this study, the crack ratio is calculated by processing the images to binary, where the sum of the white pixel areas represents the total area of cracks or pores, and the ratio of crack pixel area to the total image area is defined as the crack ratio (J). The calculation method for crack ratio can be represented by the following formula.

$$J(\%)={\displaystyle \frac{S_c}{S_t}}\times 100$$

(1)

where S_c is the total area of the cracks; S_t is the total area of the image.

By compiling the crack ratios of the specimens from each cycle, a three-dimensional graph showing the development of crack ratios under different numbers of cycles can be plotted, as seen in Figure 3a. Among them, the U1 specimen without adding XG has a significantly higher crack ratio than the modified specimens, reaching 11.26% during the first wet–dry cycle. According to the trend shown in the three-dimensional graph, the final crack ratios of the modified specimens in different cycles are similar. This leads to the conclusion that the crack ratio does not fluctuate significantly with the increase in wet–dry cycles. The lower data observed in the third cycle for the U1 and M5 specimens may be due to variations in the experimental environment or image processing conditions.

The concept of modification efficiency is proposed to demonstrate the difference between the mudstone before and after modification. Modification efficiency is defined as the reduction in the crack ratio of the specimens to the crack ratio of the U1 specimen at each cycle, which serves as the reference quantity. A graph of the average modification efficiency at different mixing ratios is plotted, as shown in Figure 3b. The graph includes error bars for the data and fits the data curve along with its 95% confidence interval, which helps to understand the overall trend of the data and indicates the precision of each data point. Although there is an error of about 5% in the average modification efficiency, a quadratic function relationship between the mixing ratio and efficiency can still be observed. The average modification efficiency is positively correlated with the mixing ratio when the XG mixing ratio is between 0.5% and 2% and negatively correlated when it exceeds 2%. In this experiment, a mixing ratio of 2% is the optimal mixing ratio, with an average modification efficiency of 45%, and it is estimated that the optimal mixing ratio is between 1.5% and 2.5%.

3.1.2. Crack Length and Width

In this study, the unit of length is pixels. The shape of the cracks includes the average length and average width of the cracks, which are used to describe the geometric characteristics of the cracks [30]. Length and width refer to the maximum and minimum Feret diameters, respectively. The Feret diameter (dF) is the orthogonal distance between a pair of parallel tangents to the feature at a specified angle to the unit. The average length and width of the cracks in each cycle of this study are summarized in Figure 4a,b. To clarify the differences before and after modification, the definition of modification efficiency from the previous text summarizes the specimen modification efficiency in Figure 4c. First, a specific positive correlation exists between crack length and width related to the statistical method of crack shape characteristics. Second, incorporating XG can reduce the length and width of the cracks, which is one of the ways to affect the overall crack ratio. Finally, the crack parameters of the modified specimens maintain a relatively balanced value in each cycle, with less influence from the number of cycles.

The experimental results show that different amounts of XG can reduce mudstone specimens’ crack width and length under wet–dry cycles. After adding XG, the crack length of the mudstone is decreased significantly, with a modification efficiency of more than 35%. Among them, the M6 specimen with 3.0% XG has an average modification efficiency of up to 47.8%. In comparison, the improvement effect of XG on crack width is limited, with an average modification efficiency between 15 and 30%, among which the M3 specimen with 1.5% XG has a modification efficiency of 26.3%. The mudstone with 1.5% XG shows the best modification effect on both crack length and width.

3.1.3. Fractal Dimension and Probability Entropy

The fractal dimension is a concept that describes the shape, contour, or forms in nature without considering their complexity. Generally, the fractal dimension gives the autocorrelation of the profile or the rate of change in length (perimeter) in response to a change in the measurement scale (area). It essentially reflects the change in the morphological factor (complexity of crack boundaries) when the crack area increases, which can be calculated by the Form factor (ff) [31]. The form factor is often used to describe the shape of features and is defined by the perimeter (C) and area (S) as follows:

$$ff=4\cdot \pi \cdot S/C^2$$

(2)

The formula gives the relationship between fractal dimension and form factor:

$$Df=1-{log}_{s}ff\cdot a^{-1}$$

(3)

where a is a constant, and Df is the fractal dimension.

The expression for probability entropy, commonly used to describe the directionality of pores, can be used in this paper to describe the directionality of crack development. The expression is as follows:

$$H=-{\displaystyle \sum _{i=1}^n{P}_i}{log}_n{P}_i$$

(4)

H represents the probability entropy; P_i indicates the percentage of cracks within a specific range, where it is the identifier for the direction range division, and n is the number of direction ranges divided. The fractal dimension and probability entropy of the specimen obtained by image analysis are shown in Figure 5.

The value of fractal dimension generally lies between 1 and 2, and the complexity of the boundary increases with the increase in fractal dimension. The larger the fractal dimension, the rougher and more complex the crack boundary. The experimental results show that adding a small amount of XG (0.5–1%) does not change the complexity of the mudstone specimen’s crack boundary, which remains above 1.3. After adding an appropriate amount of XG, the fractal dimension of the specimen decreases significantly, indicating that the cracks produced by the specimen’s drying shrinkage become more straightforward and regular. The change in fractal dimension can affect the permeability of the rock and soil body, and many studies have shown that the fractal dimension of cracks is negatively correlated with the permeability [32,33]. The addition of XG can improve the permeability of mudstone to a certain extent, and the importance of the impact needs to be proven by subsequent experiments.

According to the formula, probability entropy is between 0 and 1. When the crack direction is the same, H = 0; when the crack direction is entirely random, H = 1. The more chaotic and random the pore direction, the greater the H value. The experimental results show that with the increase in the mixing ratio, the probability entropy of the specimen shows a trend of first decreasing and then increasing. The unmodified U1 specimen has a probability entropy that remains above 9.6 in multiple wet–dry cycles, essentially in a state of random direction. Among the modified specimens, the M3 specimen has a probability entropy that decreased to 0.879 in the 5th cycle, which is the lowest value in the entire experiment; the M4 specimen has the lowest average probability entropy of 0.943, and the crack development direction is still highly random. The decrease in probability entropy can affect the specimen’s crack development disorder. Through image comparison, the cracks of the modified specimens are relatively fixed in direction, but no obvious directionality is observed. This study’s slight decrease in probability entropy is due to the reduced crack ratio and the decline in effective cracks.

3.2. Physicochemical Characteristics

3.2.1. Water Content

Under conditions where the initial water content of each specimen is similar, they are dried simultaneously. The moisture on the specimen surfaces continuously evaporates, and the humidity within the soil body slowly dissipates due to capillary action until dry cracks appear. The change in water content of the specimens reflects their water retention performance. By comparing the water content changes during the drying process of each specimen, the effect of XG content on the water retention performance of the specimens can be assessed. The method for calculating the water content(C) in this paper is as follows:

$$C(\%)={\displaystyle \frac{w_w-w_d}{w_d}}\times 100$$

(5)

where w_w is wet mudstone weight; w_d is dry mudstone weight.

As shown in Figure 6, the initial water content of each specimen is 24%, and the water content is recorded every 2 h. After 16 h of drying, all specimens reach a dry state, with the final water content between 1% and 3%. According to the trend displayed in the figure, it is observed that there is no significant correlation between the water content of the mudstone itself and the modified specimens with the number of wet–dry cycles. By calculating the average water content of the specimens during each drying period, the water content reduction rate of the M2 and M3 specimens is significantly higher than that of the U1 specimens, and their final water content is also lower. The M4 specimen exhibits better water retention performance. It has a higher final water content, while the water content reduction curves of the M5 and M6 specimens are similar to those of the U1 specimen. Therefore, this study concludes that the number of wet–dry cycles does not influence the improvement of the water retention performance of mudstone by XG. Additionally, a small amount of XG content is not conducive to retaining moisture in the specimens. In contrast, an appropriate amount of XG can enhance the water retention performance of the specimens.

3.2.2. Surface Hardness

Studies have shown that incorporating XG can enhance the strength of geotechnical materials [34,35]. Surface strength refers to the ability of a material’s surface to resist external forces such as scratching, wear, and impact, which is significant for the mudstone in this study. Increasing surface strength can improve the slope’s wear, erosion, and impact resistance, contributing to slope safety. The experimental results were analyzed and presented in Figure 7.

This study’s surface strength measurement results show high variability and large error values. This is due to the uneven surface of the specimens after wet–dry cycles, which brings difficulties to the use and measurement of the instrument. The surface strength of the mudstone decreases after incorporating a small amount (0.5%) of XG. As the amount of incorporation increases, the surface strength also increases, but the average increase in surface hardness is not significant, with the M5 specimen showing the largest increase of only 5%. It can be seen that the addition of XG in this study does not significantly improve the surface strength of the mudstone specimens after wet–dry cycles.

4. Discussion

4.1. Microscopic Changes

4.1.1. X-Ray Diffraction

To investigate the microstructural changes during the modification of open-pit mine mudstone with XG, this study selected specimens from the U1 specimen, M6 specimen, and XG for X-ray diffraction (XRD) experiments. The XRD diffraction curves obtained are shown in Figure 8. The XRD pattern of the mudstone specimen exhibits sharp and intense diffraction peaks, indicating good crystallinity. By comparing with the standard diffraction database, it is found that the mineral is mainly composed of SiO₂, Al₂Si₂O₅(OH)₄, KAlSi₃O₈, NaAlSi₃O₈, FeCO₃, KAl₂(Si₃Al)O₁₀(OH)₂. As a polysaccharide organic substance, XG does not have distinct diffraction peak characteristics. There are two broad peaks at 10° and 20°, which are amorphous substances, and the presence of NaCl is detected at positions of 30°, 45°, and 75°. By comparison, adding 3% XG in the open-pit mine mudstone does not change the position of the specimen’s diffraction peaks, indicating that its crystal structure has not changed. Compared with the U1 specimen, the M3 specimen lacks the KAlSi₃O₈ phase, and the rest of the phases remain the same, indicating that the introduction of XG has changed the types of phases in the specimen to some extent.

Using the Rietveld method for full-spectrum fitting, the X-ray diffraction (XRD) experimental data were refined and quantitatively analyzed to obtain the crystal structure and phase content information of the U1 and M6 specimens. Figure 8 shows that the main mineral components of the unmodified mudstone specimen are quartz, albite, kaolinite, microcline, siderite, and illite. The addition of XG leads to a decrease in the microcline content and an increase in the illite content, confirming the XRD pattern analysis results. After consulting the literature, it is found that the formation of illite involves the regional metamorphism of clay minerals in rocks, and there is currently no research proving that microcline can be transformed into illite under average temperature and pressure conditions. Therefore, the decrease in microcline content and the increase in illite content may be due to experimental and specimen errors.

In summary, the introduction of XG keeps the shape and position of the specimen’s diffraction peaks unchanged. The results of the quantitative analysis also show that no new compounds were produced after modification. Therefore, it is concluded that the crystal structure and phase composition of the specimens in this study were not affected.

4.1.2. Scanning Electron Microscopy

To explore the effect of XG on the micromorphology of mudstone specimens, scanning electron microscopy (SEM) was utilized to capture the microstructures of the U1 specimen (0% XG) and the M6 specimen (3% XG). Comparative analysis was performed by selecting images at different magnifications of 500×, 1000×, 2000×, and 5000× from various regions, and the results are depicted in the figure.

The analysis of the electron microscopy results reveals that at a magnification of 500×, the U1 specimen exhibits dispersed fine particles. At the same time, the M6 specimen shows loose, small particles and some consolidated particles. At a magnification of 1000×, the U1 specimen still consists of loosely distributed particles on the conductive adhesive. In contrast, the M6 specimen has more consolidated particles that are more uniformly sized. Finely dispersed soil particles characterize the mudstone specimen without XG after drying. In contrast, the modified mudstone specimen shows some soil aggregates.

When the electron microscope magnification is increased to 2000×, the mudstone particles in the field of view have become relatively concentrated. Although gathered together, the soil particles in the U1 specimen still belong to a dispersed distribution, with no apparent aggregation or binding between particles. In contrast, the M6 specimen shows apparent, compact aggregation, as shown in Figure 9g, where soil particles of varying sizes aggregate into a single entity and exhibit a specific stability. Upon close observation, some striped flocs are observed to connect and interweave between particles, forming a network. The characteristics of the modified specimen at this magnification are further pronounced, with the addition of XG causing the mudstone particles to aggregate and combine.

When the electron microscope magnification is increased to 5000×, the particles in the U1 specimen are relatively concentrated and enlarged. It can be seen from Figure 9d that there are apparent macropores between the microscopic particles, indicating that the particles are not closely connected. The M6 specimen has become an agglomerate with a smoother and flatter surface, and small particles can be seen adsorbed on the surface. Combining all the electron microscopy results, the conclusion is drawn that after drying, the natural mudstone specimen has loose particles that do not have aggregative solid properties; the specimen with XG added after drying still has some aggregates composed of large and small mudstone particles; the XG matrix becomes striped flocs after drying, which are distributed between the particles, and the aggregates have a particular ability to adsorb and fix small particles on the surface.

4.2. Modification Mechanism

Based on the XRD analysis results, it is known that the open-pit mine mudstone contains a portion of clay particles (kaolinite, illite, and montmorillonite), accounting for 31.4%. Negatively charged clay particles form an electric field around them, and surrounding water molecule dipoles, cations, etc., are adsorbed on the surface due to electrostatic attraction [36]. XG is an anionic polysaccharide composed of D-glucuronic acid, D-mannose, pyruvylated mannose, 6-O-acetyl D-mannose, and 1,4-linked glucan. The primary chemical structure of xanthan gum is a linearly linked β-D glucose backbone with a trisaccharide side chain on every other glucose [37]. As shown in Figure 10a, due to the presence of a large number of carboxyl (-COOH) groups and hydroxyl (-OH) groups in the chemical structure of XG, under conditions of sufficient moisture, these anionic groups can form bonds with cations, oxygen atoms, and water molecules on the mineral surface. This bonding can enhance the inter-particle linkage force and resist the tension of drying shrinkage in the form of bond energy during drying and water loss [38].

In addition, under static conditions, a small amount of XG can cause a significant increase in the viscosity of liquids. While adding water for wetting, XG dissolves in water to form a gelatinous liquid with a specific viscosity, filling the gaps between the soil particles. It shrinks during drying and water loss and becomes a hard plastic-like substance. XG gradually shrinks and accumulates during the drying and curing process, merging into bridge links to form a dense network that tightly connects the framework of adjacent soil bodies [39]. Therefore, in the electron microscope image of the unmodified U1 specimen, the soil particles are dispersedly arranged with many pores. In contrast, in the M6 specimen with added XG, they appear dense and reticulated.

Based on the analysis above, we can summarize the mechanism by which xanthan gum (XG) modifies the properties of mudstone in open-pit mines. When the specimen is in a water-saturated state, the XG solution can effectively carry fine particulate matter, which then extends and fills into the pores of larger mudstone particles. Electrostatic bonding and hydrogen bonding allow the fine particles in the XG solution to firmly adhere to the surface of the soil. As the drying process progresses, the water in the XG gradually evaporates, causing some soil particles to detach from the adsorption on the surface of the larger mudstone. At the same time, other soil particles interact with the XG matrix during the drying shrinkage process, forming a filamentous network structure. These filamentous networks act to connect and fill between the mudstone bodies, enhancing the overall structural stability of the soil. The entire process is depicted in Figure 10b.

During the wet–dry cycling process, XG continuously transitions between a gel-like solution and a filamentous matrix. This transition is crucial for maintaining the stability and strength of the soil. Due to XG’s inherent stability and durability, its performance does not show a significant decline even after six wet–dry cycles. This indicates that mudstone modified with XG can maintain good performance under repeated changes in moisture conditions, which is of great significance for improving the engineering performance of mudstone in open-pit mines. Through this modification mechanism, XG not only improves the pore structure of mudstone but also enhances its adaptability and durability under various environmental conditions, thereby playing a greater role in engineering practice.

5. Conclusions

The slopes of open-pit mine waste dumps are prone to many small cracks under the influence of wet–dry cycles and self-weight. If timely preventive measures are not taken, the slope’s stability will be threatened. This paper uses xanthan gum to modify the crack development characteristics of mudstone in open-pit mine waste dumps, and the following conclusions are drawn.

(1)

The analysis of the specimen images shows that adding XG to mudstone can significantly reduce the area of cracks after drying shrinkage, manifested explicitly in the changes in crack ratio, crack length, and crack width. The modification efficiency index is introduced to evaluate the effect of modification. A quadratic function relationship exists between the specimen’s crack ratio and XG’s content. When the content of XG is between 0.5% and 2%, the average modification efficiency is positively correlated with the content. After 2%, it shows a negative correlation, with the maximum average modification efficiency reaching 45%. Similarly, although there is no apparent functional relationship between the length and width of the cracks and the content, adding XG can reduce the height and width of the crack development to a certain extent. Among them, the average modification efficiency of the crack length and width with 1.5% content of XG is 46.2% and 26.3%, respectively.

(2)

Taking the fractal dimension and probability entropy as indicators, the shape of the cracks and the direction of crack development are comprehensively analyzed. A small amount of XG does not affect the complexity of the mudstone crack boundary. As the content of XG increases, the fractal dimension gradually decreases, indicating that a specific content helps improve mudstone permeability. Since the calculation of probability entropy is greatly affected by the image quality and the number of cracks, combined with the actual performance of the specimen cracks, it is found that the addition of XG does not reduce the disorder of crack development.

(3)

Based on the results of micro-experiments and the properties of XG itself, the modification mechanism of XG affecting the crack development pattern of mudstone is revealed. During the wet–dry cycle, no new substances were produced in the specimen. In the wet state, XG powder dissolves in water to form a viscous XG colloidal solution, which carries fine soil particles to fill the pores between the mudstone particles and adheres to the surface of the soil body through electrostatic bonding and hydrogen bonding. During the drying process, due to the loss of water, the XG colloidal solution gradually transforms into an XG matrix, forming connections and fillings between the mudstone bodies to resist the tension of mudstone water loss and cracking. The transformation between the XG colloidal solution and the filamentous matrix in the wet–dry cycle is stable.

In summary, this paper explores the modification efficiency and optimal mixing ratio of XG in improving the crack development pattern of mudstone. It reveals the modification mechanism of XG, which is significant for developing crack prevention technology and the safety of open-pit mine slopes. Future research will consider the use of environmentally friendly waste materials to achieve more sustainable and eco-friendly mudstone modification techniques.