Characterization of Freeze-Thaw Cycle Damage to Mudstone in Open Pit in Cold Regions—Based on Nuclear Magnetic Resonance Method

Abstract

Damage deterioration of rocks in cold regions under seasonal changes and daily cycles of freezing and thawing generate a series of engineering geological problems. These problems will seriously affect the safe and efficient production of open-pit mines. In this paper, a freeze–thaw cycle test and uniaxial compression test considering the natural conditions of the slope were carried out. Mechanical properties and damage mechanisms of open-pit mine mudstone under freeze–thaw conditions were investigated based on nuclear magnetic resonance (NMR) technology. The test results show that the microscopic internal pore structure of mudstone was changed under the superimposed effect of freeze–thaw damage and hydration damage. The internal pore size of mudstone increased with the number of freeze–thaw cycles, while the average pore size of the natural mudstone test increased more. Macroscopically, the compressive strength and modulus of elasticity of mudstone varied linearly with the number of freezing cycles, and the compressive strength and modulus of elasticity showed a decreasing trend. The strain-softening characteristics of mudstone samples were significant for more freeze–thaw cycles. The study explains the microscopic causes of mudstone deterioration in open-pit mines in cold regions and offers guidance for solving engineering disasters caused by mudstone deterioration.

1. Introduction

About 50 percent of the world’s land is covered by perennial, seasonal, and transient permafrost zones, with China’s cold zones accounting for about two-thirds of its total land area [1,2]. The Cold Zone is the principal place of mineral resources in China; along with the continuous promotion of China’s Western development strategy and the proposal of the “Belt and Road” initiative, the development of mineral resources in the Cold Zone is facing a more severe test. The relevant data [3,4] show that Xinjiang’s prospective reserves of coal ranked first place in China. The Tarim Basin, Junggar Basin, Tulufan Hami Basin, Chai Basin, and the Xinjiang Cold Zone are the most essential areas in China. The exploration prospect of oil and gas in Tarim Basin, Junggar Basin, Tulufan Basin, Hami Basin, and Qaidam Basin is good, and the western region is the main storage area of gypsum, mica, magnesite, asbestos, jade, and other nonmetallic minerals in China. The research results of many scholars and a large number of engineering practices in cold regions show that the damage deterioration of rocks in the mean areas under the action of freezing and thawing is an important reason affecting the safety and stability of geotechnical engineering, such as freezing and swelling and cracking of peripheral rocks of the tunnel of Qinghai–Tibet Highway [5], instability of supporting structures, instability of slopes in open-pit mines and landslides [6], freezing and swelling and deformation of underground pipelines in Northwest China [7], freezing and thawing and weathering of rocky cultural relics in cold regions [8], and other problems. Freezing and thawing effects on open-pit mines in cold areas are more unique; this is because the slope-shaping process of open-pit mines has a significant time effect; with the continuous descending and expansion of the rock layer, the initial small slopes are gradually developed into tall slopes with multiple steps, and at the same time, the slopes in the production process of open-pit mines with the cyclical temperature changes show a two-state cycle of freezing in winter and thawing in spring. The process generally extends from several years to decades [9] due to the freeze–thawing deformation of underground pipelines in Northwest China, freeze–thawing weathering of rocky cultural relics due to the different exposure times of the slope at different step levels and locations, the number of freeze–thaw cycles experienced is different. The degree of freeze–thaw damage to the rock body at each mine site is also different. Therefore, it is of great significance to study the rule of change of physical and mechanical properties of rock bodies in open-pit mines under the freeze–thaw cycle and to solve the unfavorable impacts brought by the freeze–thaw cycle for the safe and efficient production of open-pit mines.

The change in rock’s physical and mechanical properties under the freeze–thaw cycle is affected by multiple factors. At present, scholars at home and abroad generally divide the factors affecting the freeze–thaw damage of rocks into internal and external factors. Internal factors refer to the properties of the stone itself, including lithology [10], porosity [11], water content [12], and degree of fissure development [13]. External factors refer to the environmental parameters in which the rock is located, including temperature [14,15], number of freezing and thawing cycles [16,17], and hydrochemical environment [18,19]. Many experimental studies have shown that the compressive strength, tensile strength, and modulus of elasticity of rocks are substantially reduced after undergoing multiple freeze–thaw cycles. The damage mode of rocks changes, and the reduction in rock physicomechanical properties is even more significant in internal fissures or particular environments [20].

Whether affected by internal or external factors, the significant reduction in rock’s physical and mechanical properties manifests its internal microstructural changes or damage. At present, scholars at home and abroad have studied the internal microstructural evolution law of freeze–thawed rocks mainly through nondestructive testing techniques, such as acoustic emission (AE) [21,22], scanning electron microscopy (SEM) [23,24], computed tomography (CT) [25,26], and nuclear magnetic resonance (NMR). As a commonly used nondestructive testing technique in medicine, NMR technology has the advantages of fast detection speed and high accuracy. It is widely used in the analysis and detection of petrophysical tests [27,28]. Porosity, free-fluid index, pore-size distribution, and permeability can be derived from NMR technology. However, NMR technology is primarily used in the study of freeze–thaw damage of concrete at present [29,30], revealing that there is less research showing the microscopic damage mechanism of natural rocks under the freeze–thaw cycle conditions of open-pit mines. Therefore, this paper designs the freeze–thaw cycle test of mudstone in open-pit mines under the consideration of natural conditions, analyzes the change rule of mechanical parameters of mudstone freeze–thaw damage, and examines the change rule of mudstone pore structure and the superimposed effect of freezing, thawing, and hydration damages of natural mudstone under different conditions of freeze–thawing based on the NMR technology. Meanwhile, it probes the hydration mechanism of mudstone freeze–thawing injury.

2. Materials and Methods

2.1. Open-Pit Mudstone

The rock samples used in this paper were taken from the South Open Pit Coal Mine of Tianchi Energy, Xinjiang, China. Concerning the test protocol [31], the rock samples were core drilled, cut, and prepared into standard cylindrical samples with a diameter of 50 × 100 mm (diameter height). After the sample preparation, the samples with obvious primary defects, fissures, and local defects were removed by observation, and the mudstone samples with similar longitudinal wave velocity were selected for testing. The processed samples are shown in Figure 1.

The surface of the mudstone sample was grey–white, with uniform texture and color, and its lithology was typical coal mudstone with a low degree of cementation. The mudstone under the initial state was subjected to three mercury pressure tests using the AutoPore IV 9520 (Micromeritics, Shanghai, China) fully automatic mercury pressure instrument (maximum pressure of 414 MPa, pore size measurement range of 3 nm–1000 μm). The average pore size of the mudstone used in the test was 97.75. The average pore size of the mudstone used in the trial was 97.75 nm, and the porosity was 3.34%. The density and water content were measured and listed in

Table 1. Mineral basic physical parameters of mudstone.

Sample	Capacity (g/cm3)	Dry Density (g/cm3)	Content (%)	Porosity (%)
Mudstone	2.04	1.98	2.89	3.34

.

The composition and content of the material were determined by X-ray diffraction (XRD) (Rigaku SmartLab SE, Japan) testing and are shown in Figure 2 and

Table 2. Analysis of XRD of mudstone.

Ingredient	Quartz	Illite	Chlorite	Potash Feldspar	Albite	Kaolin
Percentage (%)	23	47	8	5	7	10

. The mudstone is 65% clay minerals, with illite (47%) making up a large percentage. Pure illite is white, weathered from dolomite, etc. The sample has a high illite content, which gives it a greyish–white color. When immersed in distilled water for 15 min, the sample dissolves at the edges, and bubbles appear in the container. After two hours, the sample disintegrates completely into flaky debris and becomes muddy.

The molecular formulae and elemental compositions of the mudstone samples were determined by X-ray fluorescence spectrometry (XRF) (PANalytical Axios, The Netherlands) and are shown in

Table 3. Analysis of XRF of mudstone.

Formula	SiO2	Al2O3	Fe2O3	K2O	MgO	Na2O	TiO2	CaO	LOI
Percentage (%)	57.42	18.76	4.90	1.90	1.41	0.87	0.80	0.70	13.24

.

2.2. Test Equipment

In this paper, the rock samples were pretreated by the freeze–thaw cycle, the damage caused by the freeze–thaw cycle on the internal microstructure of mudstone samples was quantified by the NMR test, and, finally, the effect of the freeze–thaw cycle on the macroscopic mechanical characteristics of the samples was analyzed by uniaxial compression test. The test equipment and models used in the freeze–thaw cycle test, NMR test, and uniaxial compression test are shown in

Table 4. Test equipment.

No.	Title	Equipment Type	Manufacturer
1	Ultrasonic wave velocity measuring instrument	HS-YS4A	Beijing Jinyang Wanda Technology Co., Beijing, China
2	Automatic freeze–thaw cycle tester	JC-ZDR-5	Fushun Xinfu Xinyuan Electromechanical Instrument Factory, Fushun, China
3	Electronic universal testing machine	WDW-300	Jinan Chengyu Testing Equipment Co., Ltd., Jinan, China
4	Nuclear magnetic resonance core analyzer	MacroMR12-150H-I	NIUMAG, Shanghai, China

.

2.3. Test Methods

2.3.1. Freeze–Thaw Cycle Test

The freeze–thaw cycle test simulates the cycle of climate change in the natural environment through the freeze–thaw temperature and the number of processes to quickly simulate the cycle of water freezing and thawing. The source of water damaged by freezing and thawing of the rock body not only includes the inherent pore water inside the rock body but also includes the liquid water that continuously migrates and gathers from the unfrozen area to the frozen site in the process of freezing and thawing cycle, i.e., the water migration effect. To simulate the water migration of the slope rock mass in engineering practice, the mudstone samples in the freeze–thaw cycle were divided into two groups:

(1)

Plastic film samples were always used during the freezing and thawing process to prevent water from entering the samples during the thawing process and to cut off the source of water replenishment of the samples to simulate slopes with no water recharge from the rock formation;

(2)

Mudstone samples were left bare and natural during the freeze–thaw process to simulate mudstone slopes in a natural state with good conditions for moisture replenishment in the rock formation.

The changes in the mass of the sample before and after the freezing and thawing cycle of the natural sample were recorded in the freeze–thaw process, and the changes in water content were recorded. The test design freezing temperature was −20 °C, thawing temperature was 25 °C, and the number of cycles was set to 5 groups, respectively, 1, 5, 10, 20, and 30 [32]. To reduce individual error, each group of samples was set to 3; sample freezing and thawing conditions and numbering are shown in

Table 5. Grouping of freeze–thaw cycle test samples.

Environmental Conditions	Number of Freeze–Thaw Cycles (n)
	0	1	5	10	20	30
Sealed mudstone	I	FS-1	FS-2	FS-3	FS-4	FS-5
Natural mudstone		FR-1	FR-2	FR-3	FR-4	FR-5

.

According to the “Rock Physical and Mechanical Properties Test Procedure” in the rock antifreeze test, the freezing and thawing time of mudstone tested in this paper was four hours, and the freeze–thaw cycle test process of the sample is shown in Figure 3. A single complete freeze–thaw cycle process is represented by A.

Cooling process (I): Air freezing mode was used; the cooling rate was 0.5 °C/min, and it took 1.5 h for the room temperature (25 °C) to drop to −20 °C.

Freezing process (II): After reaching the set temperature, it was kept at a low temperature for four hours.

Warming process (III): Adopting the natural warming mode of water melting, it took 0.5 h to warm up from −20 °C to the average temperature (25 °C).

Melting process (IV): Kept at room temperature (25 °C) for 4 h.

For samples with more than one cycle, immediately after the completion of a single freeze–thaw cycle, the following freeze–thaw cycle process was entered after reaching the set number of freeze–thaw cycles to observe the shape of the sample and weigh the sample mass.

2.3.2. NMR Test

The theory of NMR technology measurement is based on the interaction between the magnetic properties of atomic nuclei and an applied magnetic field, which are used to characterize the chirality of fluid-bearing pores in rocks. After the sample is placed into the magnetic field, a radio frequency pulse of a specific frequency is emitted. The hydrogen proton (H proton) absorbs the energy to resonate, and when the radio frequency pulse ends, the H proton releases the energy absorbed at the time of resonance, and the change in power can be detected by the unique coil, which is the signal of nuclear magnetic resonance. The energy release rate is different for samples with other properties, and the difference in movements can be visualized to reflect the changes in the internal microscopic pore structure of the rock.

The changes in pore structure can be analyzed by T₂ mapping after freeze–thaw cycles. According to the NMR principle [33], the total NMR transverse relaxation rate 1/T₂ can be expressed as

$$\frac{1}{T_2}=\frac{1}{T_{2free}}+{\rho }_2{\left(\frac{S}{V}\right)}_{porosity}+\frac{D{\left(\gamma G{T}_E\right)}^2}{12}$$

(1)

where T_2free is the fluid free chirp time, ms; S is the pore surface area, cm²; V is the pore volume, cm³; ρ₂ is the transverse surface chirp intensity, μm/ms; D is the diffusion coefficient; γ is the spin-to-magnetism ratio, rad/(S*T); G is the magnetic field gradient, Gs/cm; T_E is the echo time, ms.

In the presence of only a single type of fluid in the pore space, the volume relaxation time is much larger than the area relaxation time, so T_2free can be neglected; when the magnetic field is uniform, and a short T_E is also used, the diffusion relaxation can also be ignored so that the above equation can be simplified as

$$\frac{1}{T_2}={\rho }_2{\left(\frac{S}{V}\right)}_{porosity}$$

(2)

From Equation (2), it can be seen that there is a correlation between the distribution of transverse relaxation time T₂ of the material and the pore size information, and there is a positive correlation growth between the two. The transverse relaxation time of the material is determined by the ratio of pore surface to volume (S/V) and the surface relaxation strength of the fabric together.

In the test process, it is necessary to calibrate the instrument with the standard oil samples with known NMR signals and then carry out NMR relaxation measurements on the normal mudstone samples in the initial state and under different freeze–thaw cycling conditions, according to the size of the porosity of the mudstone samples, the T₂ relaxation time, and the change of the recorded signal intensity, to analyze the evolution of the pore structure and porosity characteristics of the mudstone under the action of the freeze–thaw cycle. This is carried out to investigate the damage law of the mudstone under the conditions of freezing and thawing and to reveal the mudstone freezing and thawing microimpairment mechanism.

2.3.3. Uniaxial Compression Test

The samples after NMR were tested using a WDW-300 universal testing machine with displacement control, acceleration rate of 0.1 mm/min, and stopping condition of strain over 10%. At the end of the test, the damage characteristics of the sample were observed, the damage mode was analyzed, and the changes in the mechanical parameters of the mudstone after different numbers of freeze–thaw cycles were studied.

3. Results

3.1. NMR Spectrum

3.1.1. Sealed Mudstone

The internal water of mudstone includes interlayer water inside clay mineral particles and pore water between rock skeleton particles and clay particles. According to the principle of the NMR test, the T₂ pattern is determined according to the content of H protons in the sample, so the pore water referred to in the NMR test includes interlayer water between the mudstone skeleton particles, between the clay particles, between the skeleton particles and clay particles, and between the internal layers of the clay mineral particles.

As seen from Figure 4, the T₂ pattern of mudstone in the initial state is a single-peak type. The maximum value of the NMR signal corresponds to a chirp time of 0.34 ms; the chirp time is mainly distributed between 0.05 and 1.29 ms, and its area accounts for 99.49% of the spectral region. The other peak total area accounts for 0.51%, which can be ignored, and the characteristics of the T₂ pattern indicate that microporosity is dominant within the mudstone sample.

The size of the T₂ spectral area is positively correlated with the internal pore water content of mudstone sealed samples, and the change of mudstone pore volume can be reflected by the change of T₂ spectral area under different freeze–thaw cycles. The distribution of the T₂ spectral area can calculate the rock porosity and pore distribution, while the evolution of porosity can reflect the change of freeze–thaw damage of rocks. As can be seen from Figure 5, with the increase in the number of freeze–thaw cycles, the T₂ spectral area of mudstone samples increased, indicating that the original cracks within the mudstone under freeze–thaw conditions produce expansion or new cracks, and the cracks gradually develop under the repeated cyclic action and ultimately lead to damage.

The peaks of the T₂ spectra were counted for different number of cycles of the test to obtain Figure 6. From Figure 5 and Figure 6, it can be seen that for the confined mudstone, with the increase in freeze–thaw cycles, the distribution of T₂ spectral curves is the same shape; all of them are single-peak type. The NMR peak signal corresponds to a significant increase in the chilling time, from 0.280 ms (initial state) to 0.300 ms (1 freeze–thaw cycle), 0.321 ms (5 freeze–thaw cycles), 0.601 ms (10 freeze–thaw cycles), 0.560 ms (15 freeze–thaw cycles). Meanwhile, the NMR T₂ spectrum area increased significantly from 30,077 (initial state) to 30,394 (1 freeze–thaw cycle), 35,946 (5 freeze–thaw cycles), 52,117 (10 freeze–thaw cycles), and 53,548 (15 freeze–thaw cycles), with an increase of 1.05%, 19.51%, 73.27%, and 78.03%, respectively. The growth shows the law of rising first and then decreasing with the increase in freeze–thaw cycles when the number of freeze–thaw cycles is 6~10 times, and the average increase in pore volume of the confined mudstone samples is the largest with 7.327% per freeze–thaw cycle, which indicates that the pore volume expansion caused by water-ice phase change in the freezing process of the confined mudstone under the action of freeze–thaw cycles is different. With the increase in freeze–thaw cycles, the pore volume increases. The freeze–thaw damage accumulates, the pore volume grows with the increase in freeze–thaw cycles, and the freeze–thaw damage accumulates. With the rise in freeze–thaw cycles, the pore volume increases, and the freeze–thaw damage increases. Still, the pore volume increase rate shows the law of increasing first and then decreasing, and there is a maximum interval, which is between 6 and 10 times the freeze–thaw cycles in this test. With the continuous accumulation of freeze–thaw damage, the damaged pores gradually decrease under the condition that the total number of pores is certain.

3.1.2. Natural Mudstone

As seen from Figure 7, with the increase in the number of freeze–thaw cycles, the T₂ patterns of the natural mudstone samples and the confined mudstone samples were similar in shape. They remained single-peaked, with the peak signals corresponding to a significant increase in the chirp time, from 0.345 ms (initial state) to 0.455 ms (1 freeze–thaw cycle), 0.912 ms (5 freeze–thaw cycles), 1.203 ms (10 freeze–thaw cycles), and 1.29 ms (15 freeze–thaw cycles). The peak signals and corresponding relaxation times were quadratically related to the number of freeze–thaw cycles. Under the same number of cycles, the peak NMR signal intensity increased significantly from 1112 (initial state) to 1367 (1 freeze–thaw cycle), 2305 (5 freeze–thaw cycles), 3030 (10 freeze–thaw cycles), and 3191 (15 freeze–thaw cycles), with an increase of 22.93%, 107.28%, 172.48%, and 186.96%, respectively. After freeze–thaw cycles of the natural mudstone samples, the intensity of the NMR signals increased significantly, the new pores became more numerous, and the average pore size increased.

The peaks of the T₂ spectra were counted for different test cycles to obtain Figure 8. The T₂ spectral area of the natural mudstone samples increased with the number of freeze–thaw cycles. The average increase in each freeze–thaw cycle was different, and in general, the increase gradually decreased. According to the test program, the natural mudstone samples were divided into four stages with varying numbers of freeze–thaw cycles, namely, 0–1 freeze–thaw cycle (stage I), 2–5 freeze–thaw cycles (stage II), 6–10 freeze–thaw cycles (stage III), and 11–15 freeze–thaw cycles (stage IV), and the average increase in T₂ spectral area of the natural mudstone samples with one freeze–thaw cycle in the four stages was 6266, 4313, 5025, and 525, respectively. For 4313, 5025, and 572, the increased value of porosity in the natural mudstone samples was the largest in the first freeze–thaw cycle; the overall trend gradually decreased; after ten freeze–thaw cycles, the increase in porosity decreased significantly, which was different from the change rule of the porosity of the confined mudstone only under the action of freeze–thaw damage.

3.2. Mechanical Properties

3.2.1. Sealed Mudstone

To simulate the influence of seasonal temperature changes on the mechanical properties of mudstone in natural working conditions, the mudstone mechanical properties test was carried out under different numbers of freeze–thaw cycles (the freezing temperature was −20 °C, and the melting temperature was 25 °C), and the stress–strain curve of the confined mudstone sample is shown in Figure 8.

As can be seen from Figure 9, the uniaxial compression damage mode of the confined mudstone sample is still typical brittle damage; at the same time, the uniaxial compression strength of the confined mudstone sample in the freeze–thaw cycle number is lower, the change rule is not apparent, the freeze–thaw cycles for 1, 5, and 10 times correspond to the value of uniaxial compression strength of 7.46, 7.31, and 7.12 MPa, respectively, and the number of freeze–thaw processes is higher, the uniaxial compression strengths of the sealed samples were more prominently reduced, which were 7.17% (n = 20) and 12.06% (n = 30), respectively. Overall, the confined mudstone was damaged after freeze–thawing, and the micromechanical parameters were reduced to a certain extent. Still, the degree of freeze–thawing damage was low because the initial water content in the confined mudstone was small (2.89%).

3.2.2. Natural Mudstone

To investigate the specific effects of moisture replenishment conditions on mudstone slopes in natural working conditions, uniaxial compression tests were conducted on natural mudstone samples after different freeze–thaw cycles, and the stress–strain curves are shown in Figure 10.

The natural mudstone samples suffered more freeze–thaw damage than the sealed mudstone samples. This is why the natural mudstone samples were rehydrated during the cycling process. The full stress–strain curves of mudstone under different numbers of freeze–thaw cycles were categorized into two types: the first type of curves showed the stress–strain characteristics of mudstone under lower numbers of freeze–thaw cycles (n = 1, 5, 10), and the second type of curves showed the stress–strain characteristics of mudstone under higher numbers of freeze–thaw cycles (n = 20, 30). Compared with the former, the second type of curve has a significant strain-softening phase after the peak and exhibits significant plastic deformation characteristics. In addition, the second type of curve also shows substantial differences in the nonlinear extension stage of cracks. In contrast, the first type of curve is mainly characterized by a slight decrease in the slope of the curves and gradually reaches the peak. In contrast, the second type of curve shows significant ups and downs in this stage, with multiple small peaks before the peak.

The characteristics of the two types of curves differ mainly due to their different internal pore structures, natural state, and number of freeze–thaw cycles. In the case of mudstone with low internal porosity and a low number of freeze–thaw cycles, the prominent macromechanical properties exhibit brittle characteristics. However, in the case of mudstone with a higher number of freeze–thaw cycles, the internal cracks and pores are fully developed. Under the action of the load, the cracks close and expand, resulting in enhanced local plastic deformation and significant strain-softening characteristics.

According to the stress–strain relationship of mudstone under different freeze–thaw cycles in Figure 10, the change curves of mudstone compressive strength σd and elastic modulus Ed with the number of freeze–thaw cycles n can be obtained, as shown in Figure 11, where each datum is the average value of three times the datum under the same conditions. Overall, the three types of mechanical characteristic parameters with the number of freeze–thaw cycles n are linear distinct changes, the compressive strength and elastic modulus show decreasing change characteristics, and the peak strain shows increasing change characteristics. The specific characterization is as follows:

(1) Uniaxial compressive strength (UCS) σ_d characteristics of change

When increasing the number of freeze–thaw cycles n from 1 to 30, the uniaxial compressive strength of mudstone showed a linear decreasing trend. The average uniaxial compressive strength of mudstone in the number of freeze–thaw cycles of 1, 5, 10, 20, and 30 was 7.2 MPa, 6.82 MPa, 6.13 MPa, 4.5 MPa, and 3.58 MPa, with decreasing values of 7.33%, 12.22%, 21.10%, 42.08%, and 53.92%, which were smaller than those of mudstone compressive strength of 7.77 MPa at each temperature in the natural state, 42.08% and 53.92%, which are smaller than the uniaxial compression strength of mudstone in its natural state at each temperature mudstone compressive strength of 7.77 MPa.

$${\sigma }_d=-0.2622n+14.75$$

(3)

(2) Characterization of the change in modulus of elasticity E_d

The changes in the modulus of elasticity of the mudstone with the number of freeze–thaw cycles are consistent with the uniaxial compressive strength, which decreases linearly with a smaller magnitude than that of the uniaxial compressive strength. After freeze–thaw cycles, the modulus of elasticity of mudstone is smaller than the corresponding value at room temperature (9.15 GPa).

$$E_d=-0.102n+8.472$$

(4)

(3) Freeze–thaw coefficient

The freeze–thaw coefficient characterizes the ability of rocks to resist freeze–thaw damage. In this paper, concerning the protocol definition [34],

$$K_f=\frac{\overline{R_f}}{\overline{R_s}}$$

(5)

K_f is the coefficient of freezing and thawing; R_f is the uniaxial compression strength after the freezing and thawing test, MPa; R_s is the uniaxial compression strength before freezing and thawing test, MPa. The freeze–thaw coefficient of natural mudstone samples was calculated for the test program, as shown in

Table 6. Freezing and thawing coefficient of mudstone under different cycles.

Number of Freeze–Thaw Cycles	0	1	5	10	20	30
Freeze–thaw coefficient	1.000	0.926	0.877	0.789	0.579	0.460

. With the increase in the number of freeze–thaw cycles, the freeze–thaw coefficient of mudstone decreased obviously.

3.3. Macrofracture Characteristics

Sealed mudstone samples did not show significant damage characteristics after undergoing multiple freeze–thaw cycles, while natural mudstone samples exhibited specific patterns. The macroscopic damage characteristics of the samples under different numbers of freeze–thaw cycles are shown in Figure 12. There are longitudinally penetrating visible cracks on the samples. The number of macroscopic rupture surfaces of the samples increases gradually with the number of freeze–thaw cycles. Through the macrocrack characterization of the sample surface, the damage mode is summarized into two categories: shear damage (single-bevel shear injury and double-bevel shear damage) and tension damage (main crack parallel to the loading direction). When the number of freeze–thaw cycles is low, the sample is mainly dominated by shear damage; when the number of freeze–thaw cycles is high, the surface of the mudstone sample shows multiple parallel cracks expanding along the loading direction, with microcracks connecting between the shots, presenting a typical multisection tensile damage.

When the number of freeze–thaw cycles is small, the internal porosity is minor. Its internal structure is relatively dense, and the internal cracks of the sample will expand along the direction of minimum energy consumption, forming more significant shear damage; with the increase in the number of freeze–thaw cycles, the internal freeze–thaw cracks and other defects are gradually increased, and under the action of the load, the cracks will expand along the direction of the loading and expand rapidly. The multicross-section of tensile damage characterizes the macroperformance.

4. Discussions

4.1. Microporosity Changes in Mudstone

According to the results of mudstone sample component measurements, the proportion of clay minerals is 65%. The main components are illite (47%), kaolinite (10%), and chlorite (8%). Illite and kaolinite particles are small in diameter when water passes through the mudstone fissures. This is due to the particle adsorption of the water film becoming thicker, which leads to significant volume expansion, which is obviously nonhomogeneous, and, in the mudstone interior, triggers uneven tensile stresses which dilute and dissolve part of the intergranular cement, and eventually leads to the fragmentation and disintegration of the rock particles [35,36]. Rocks containing clay minerals (mudstone, muddy sandstone, sandy mudstone, shale, slate, etc.) will be subjected to microfracture expansion due to hydration, mainly in the rock skeleton.

4.1.1. Sealed Mudstone

The basic theory of nuclear magnetic resonance (NMR) shows a positive correlation between the relaxation time and the pore size of a rock. When the pore size is smaller than a certain threshold, the fluid present in the pore stops flowing under the influence of capillary force. Then, the pore size threshold corresponds to a limit value of the relaxation time on the map, which is called the cutoff value. The pore’s diameter grows positively correlated with the relaxation time, so when the relaxation time does not reach the cutoff value, the fluid state is bound at this time, and vice versa for the movable state. From the T₂ cutoff value, it can be seen that the relaxation time of the fluid in the mudstone sample is short, and the velocity is fast, and when there are tiny developed pores inside the mudstone, the fluid is in the bound state, and there are few movable fluids.

The porosity of confined mudstone increases with the number of freeze–thaw cycles, but the magnitude of the increase is not the same. During the beginning phase of the freeze–thaw cycle, the rock particles are less impacted by the water molecules in the microporous space and are arranged more compactly. As a result, the sample’s porosity is reduced, the development of pores is relatively slow, and the damage variable’s value and its rate of evolution are small. As shown in Figure 13, with the increase in the number of freeze–thaw cycles, due to the repeated water–ice phase transition and uneven shrinkage of mineral particles within the pore space, the pore space develops rapidly, gradually expanding and penetrating to form the pore space structure. At the same time, the water within the pore structure increases with the increase in the pore space and reacts with the hydrophilic clay minerals, further weakening the bonding between the clay mineral particles, affecting the internal microstructure of the mudstone and, ultimately, leading to the freeze–thaw damage variables and the damage evolution rate of mudstone increasing rapidly. Therefore, for the results of the NMR analysis of the sealed samples, there is a constant fixed difference between the absolute value and the total number of pores in the mudstone samples. Still, the pore volume increase law and the damage law of the mudstone samples are of reference significance.

To more intuitively compare the change rule of different sizes of pores in confined mudstone samples under other numbers of freeze–thaw cycles, the pore diameters were divided into (1, 10], (10, 30], (30, 50], (50, 100], (100, 1000] (unit: nm, the same hereafter), and there were five groups. From Figure 14, it can be seen that the mudstone pores in the natural state are mainly concentrated in less than 100 nm, accounting for more than 99.6%, of which the pore sizes in the intervals of (1, 10], (10, 30], (30, 50], and (50, 100] account for 18.1%, 42.4%, 23.99%, and 15.20%, respectively.

The internal pore structure and pore size distribution of the mudstone samples also changed after different freeze–thaw cycles. According to the change in the pore size percentage of the confined mudstone pores under other freeze–thaw cycle times (Figure 15), it can be seen that the freeze–thaw cycle caused damage to the pore structure of the mudstone. With the increase in freeze–thaw processes, the average pore size of the pores increased significantly. The pore size percentage within the range of the interval of (1, 10] decreased and then stabilized, and the pore size percentage of the pores within the scope of the (1, 10] break decreased from 18.1% to about 7.3% and remained stable after five freeze–thaw cycles. After the freeze–thaw cycles, it fell from 18.1% in the initial state to about 7.3% and remained stable, and the expansion of this part of the pores mainly occurred between one and five times at the beginning of the freeze–thaw cycles. The pores in the interval range of (10, 30] increased significantly between 6 and 10 freeze–thaw cycles, and the volume percentage remained stable after ten freeze–thaw cycles. The rate of pores in the range of (30, 50] showed an increasing and then decreasing trend with the increase in the number of freeze–thaw cycles, and it was the largest after five freeze–thaw cycles, reaching 29.44%. The pore size between 50~100 nm had a clear pattern, and the percentage of pore size in this range increased with the number of freeze–thaw cycles, with the most significant increase between 6~10 freeze–thaw cycles. The pore size in the field of 100 nm~1000 nm (1 μm) had a minor increase before five freeze–thaw cycles, from 0.4% to 0.6% (1 freeze–thaw cycle) and 0.7% (5 freeze–thaw cycles), and increased significantly between 6~10 freeze–thaw cycles, with the percentage directly rising to 13.3% (10 freeze–thaw cycles) and 17.5% (15 freeze–thaw cycles). With fewer freeze–thaw cycles (1~15 freeze–thaw cycles), the growth was evident between 6 and 10 times freeze–thaw cycles; the percentage directly grew to 13.3% (10 times freeze–thaw cycles) and 17.5% (15 times of freeze–thaw cycles), and the expansion of pores in the range of (1, 10] intervals and the increase in pores in the field of (30, 50] and (50, 100] apertures were the main reasons for fewer freeze–thaw cycles (one to five times). After six freeze–thaw cycles, the percentage of pores in the range of (1, 10] intervals was stable; mainly apparent were the decrease in pores in the field of (10, 30], (30, 50] apertures, and the increase in the percentage of pores in the area of (50, 100], (100, 1000] larger apertures. This indicates that pore expansion occurred in the mudstone after freeze–thaw cycles, with a decrease in tiny pore size pores and an increase in large pore size pores. With the rise in freeze–thaw cycles, the expansion of the smaller pore size occurred first, followed by the expansion of medium and large pores.

4.1.2. Natural Mudstone

Unlike the sealed sample, the natural mudstone sample will be recharged with water during the thawing phase. The water vapor inside the environmental chamber will attach to the surface of the piece, gradually penetrate the sample’s interior, and hydrate with the interlayer water between the clay mineral particles within the model and the interior of the clay mineral particles under the action of microscopic forces. Therefore, the freezing and thawing cycle process of the natural mudstone sample is a superimposed damage by the joint effort of the freezing and expansion force generated by the water–ice phase transition and the hydration force. Hydrating of clay minerals leads to the generation of interlayer and intergranular swelling, which will create swelling forces acting on the rock skeleton under the constraints of the rock skeleton. If the expansion force is large enough, it will lead to the expansion of existing cracks and the creation of new cracks. The size of the expansion force depends mainly on the type and content of the expansive clay minerals.

As depicted in Figure 16, an increase in freeze–thaw cycles leads to more natural mudstone samples undergoing the thawing process. During this process, water continuously penetrates the sample, causing the sample mass to increase gradually. Fitting curves for the sample mass changes during the three freeze–thaw cycles show that the group of natural mudstone samples grows linearly with the number of cycles. The water entering the piece moves to the voids between the mudstone skeleton, the pores between the clay mineral particles, and the interlayers within the clay mineral particles.

The increase in pore size during freeze–thaw cycling of natural mudstone samples consists of two aspects: the increase in pore area and number due to the volume change of the water–ice phase change under freeze–thaw, and the growth in intergranular and intraparticle interlaminar pore space in clay minerals due to hydration of the clay minerals of the mudstone.

By comparing the changes of T₂ spectral area between the confined mudstone sample and the natural mudstone sample under different freeze–thaw cycles, it can be seen that the increase in T₂ spectral area of the sealed mudstone sample occurs when the number of freeze–thaw cycles is 6–10. The increase in the size of the natural mudstone sample occurs after the first freeze–thaw process. The difference between the increase in the T₂ spectral area of the natural mudstone sample and the sealed sample can be regarded as the increase in the pore space caused by the hydration of the clay minerals of mudstone, i.e., hydration-induced mudstone damage. As seen from Figure 17, with the increasing number of freeze–thaw cycles, the pore area increase induced by hydration also increases gradually. In contrast, the rate of pore area increase decreases linearly.

4.2. Mudstone Hydration

According to the mudstone water immersion test, it is known that the clay mineral content of the sampled mudstone is significant, mainly composed of illite and kaolinite, with solid hydrophilicity and apparent expansion and disintegration in water, so the damage caused by hydration in the natural mudstone sample needs to start from the change of the microstructure of the mudstone after being exposed to water.

Clay minerals are a kind of aluminum–silicate crystal composed of two typical wafers superimposed on each other. One is a silica–oxygen wafer, with six silica–oxygen tetrahedra constituting a silicon sheet and adjacent tetrahedra sharing an oxygen atom; the other is an aluminum–hydroxide wafer, with four aluminum–hydroxide octahedra comprising an aluminum sheet. Silicon–oxygen wafers and aluminum–hydroxide wafers constitute two kinds of cell: the first is a 1:1 cell, the cell structure for a layer of silicon and a layer of aluminum together; the second is a 2:1 cell, the cell structure for the top and bottom of the two layers of silicon sandwiched between a layer of aluminum. The two types of cells can be arbitrarily combined, and different clay minerals are macroscopic manifestations of the various combinations of the two cell types. Montmorillonite, illite, and kaolinite are the three most common major clay minerals [37].

Figure 18 illustrates the individual crystal cell structure of montmorillonite, which comprises two layers of silica–oxygen tetrahedra held between a layer of aluminum–hydroxide octahedra. The O²⁻ to O²⁻ linkage of the two crystal layers is weak, making the crystal layer easily infiltrated by water. This results in a macroscopic manifestation of solid hydrophilicity. Ilmenite, on the other hand, is the weathered material of mica in an alkaline environment, and its single crystal cell junction consists of three layers. A large number of potassium ions between the two crystal layers strengthens the linkage, making it stronger than montmorillonite but weaker than kaolinite due to structural issues. Kaolinite is formed by the superposition of a layer of silica–oxygen tetrahedra and a layer of aluminum–hydroxide octahedra. Large particles and poor hydrophilicity mainly characterize it.

When natural mudstone samples come into contact with water, the microstructure of the mudstone changes. Initially, the water molecules penetrate the surface pores and cracks of the mud. This results in the formation of more pores on the surface. As the freezing and expansion force takes effect, the pore space starts expanding and connecting, resulting in an increase in volume. Gradually, water penetrates the interior of the mudstone, triggering the formation of secondary pores, which leads to a rise in the number of pores and damages the sample. There are two specific reasons for this: Firstly, under the action of water, some of the potassium feldspar in the mudstone transforms into kaolinite, creating secondary porosity due to the differences in structure, composition, and density between the two. The samples used in this paper contain potassium feldspar. Secondly, the clay mineral particles adsorb molecules that combine with water, resulting in local expansion and more significant tensile stress, which leads to the generation of secondary porosity.

5. Conclusions

(1)

The study in this paper utilized the NMR technique to determine the changes in the microporous structure of mudstone during the freeze–thaw process. The results indicated that the average pore size of mudstone increases as the number of freeze–thaw cycles increases. The process mainly affects the expansion of tiny pores, followed by medium and large pores. Additionally, the study found that the percentage of pores within the range of 50–100 nm increased with the number of freeze–thaw cycles, with the maximum gain observed between 6–10 cycles.

(2)

The uniaxial compression test confirmed the variations in the mudstone’s macroscopic mechanical properties. Both the uniaxial compressive strength and elastic modulus of the mudstone displayed a linear decrease as the number of freeze–thaw cycles increased.

(3)

The characteristics of mudstone damage changed with the increase in freeze–thaw cycles. The damaged sample gradually transformed from a shear mode with a single rupture surface to a double rupture surface with conjugate shear wear. Finally, it was transformed into tensile damage with multiple rupture surfaces.

(4)

The process of clay mineral hydration resulted in the expansion of interlayer and intergranular spaces, exerting pressure on the rock skeleton. This pressure caused the existing pores to widen and new pores to form. As a result, the average pore size of natural mudstone samples increased significantly more after freeze–thaw cycles than sealed samples.

(5)

The primary focus of this paper was to investigate the freeze–thaw damage of mudstone using NMR technology. The damage characteristics and micromechanisms of mudstone under different recharge conditions in open-pit mines were investigated. Currently, only the freeze–thaw damage mechanism of mudstone has been studied, but other lithologies in open-pit mines will be examined in the future. Additionally, the study aimed to explore the impact of freeze–thaw damage on slope stability.