# Pore Changes in Purple Mudstone Based on the Analysis of Dry-Wet Cycles Using Nuclear Magnetic Resonance

Authors: Chen Dang; Zhili Sui; Xiuyuan Yang; Zhenlong Ge
Journal: Shock and Vibration
 (2022)

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## Abstract

The study on the change of rock pore structure during the weathering of purple mudstone is of guiding significance to the stability of the bank slope of the three gorges reservoir. In this paper, the pore changes in the wet and dry circulation of purple mudstone in the three gorges reservoir area are studied by means of nuclear magnetic resonance (NMR). The results show that the simulated weathering of wet and dry circulation has a great influence on the purple mudstone. With an increase in the number of dry-wet cycles, the purple mudstone pore volume ratio significantly changed. Originally, it consisted of a small pore structure with a single pore diameter of 0.01–0.1µm and changed to a variety of pore structures with various pore diameters of 0.001–100 µm. With the increase in the number of dry-wet cycles, the micropores (0.001–0.1 µm) were transformed into macropores (0.1–1 µm). The area of the second peak of the three samples (large pores 0.1–1 µm) increased from 0.9413, 0.9974, and 0.6779 to 0.9871, 1.1498, and 0.9901, respectively.


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## Body

## 1. Introduction

Owing to the requirements of engineering construction projects, including tunnels, mining, and rock excavations, current research on the effects of water-rock chemistry on the physical and mechanical properties of rock masses has become a hotspot for geotechnical engineering [1]. The dry-wet cycle has a great influence on rock properties and the geological environment. Water storage and drainage of the Three Gorges Dam on the Yangtze River, groundwater seepage, and seasonal precipitation cause changes in reservoir water levels, and the rock mass is periodically in a dry-wet state. Therefore, landslides, collapses, and deformation of the dam foundation are often triggered. In response to the problems associated with dry-wet cycles, a lot of research has been undertaken. Changes in physical properties have been used to evaluate rock deterioration and damage after dry-wet cycles. Using Yangtze River water and distilled water to study the effects of the dry-wet cycles, these authors compared the rock deterioration in these two cases and concluded that the degradation effect of distilled water on sandstone was lower than that of river water, thus providing a more accurate method for evaluating geological disasters and engineering faults [2]. River water is not composed of a single solution; thus, the combination of different anions and cations in the water causes the rocks to be under different water-rock interaction conditions. Testing the dry-wet cycle under acidic conditions, the corrosion of underground engineering by acid rain has been simulated, and the changes in the microscopic pore structure of sandstone under the combined effect of an acidic environment and dry-wet cycle were examined to provide a reference for the safety and durability of underground engineering [3]. The pore type and structure, as well as the porosity and permeability of coal have been studied previously by designing nuclear magnetic resonance (NMR) experiments to better estimate the permeability of coal [4]. The damage characteristics of the limestone mesostructure under different water chemistry conditions were studied, using the change in porosity to evaluate the degree of damage and thus creating a damage evolution model for rocks [5]. Li et al. [6] studied the changes in the cement composition of sandstone under different pH and acidic environments, and the chemical reaction rate was summarised as the damage factor of the rock corrosion process. A chemical damage model that can be used for the prediction of the mechanical properties of acid-corroded rocks during different periods was established. Previous studies have shown that an increase in the number of wet and dry cycles aggravates rock damage, creating an increase in the effective pores [7–15]. Many researchers have conducted laboratory tests on reservoir rocks. By controlling the wet and dry conditions, including the chemical composition, temperature, and timing of the aqueous solution, the physical and mechanical properties of bank rocks after wet-dry cycles have been tested, with the test results providing a reliable basis for engineering construction projects [16–18].However, few studies have applied NMR methods to purple mudstones after wet-dry cycles. Owing to the advantages of NMR technology, including being convenient, quick, and nondestructive, such technology has been widely utilised in many disciplines including medicine, civil engineering, geology, geophysics, biological sciences, and agricultural sciences [19–24]. In particular, NMR technology has been utilised in the field of geology to test the pore characteristics of rocks. In the traditional dry-wet cycle experiments involving rocks, the compressive and tensile strengths and the micropore structure of the rocks have been used to evaluate the physical and mechanical properties. The change in porosity can be used to evaluate the degree of damage and a damage evolution model of the rock established. However, the sample is discarded after one test using constant-pressure mercury injection, whereas the NMR method used to study the pore structure is fast and repeatable and does not damage the sample.Considering the new types of landslides caused by the storage and release of the Yangtze River Three Gorges Dam, the present study focused on the variation of the internal pore characteristics of purple mudstone on the bank slopes of the Three Gorges Reservoir under multiple dry-wet cycles. The research results provide a theoretical basis for the dry-wet multicycle physical-mechanical experimental evaluation project of a typical slope formation rock and soil and provide a basis for studying the formation of landslides in the Three Gorges area.


## 2. Materials and Methods

### 2.1. Rock Sample Preparation

The present study collected purple mudstones from the bank slopes of the Yangtze River in Wanzhou District, Chongqing, China. The geographical and sampling locations are shown in Figure1. The mineral properties of the purple mudstone samples are shown in Table 1. Table 2 shows the analysis results from the purple mudstone energy dispersive spectrometer (EDS) tests. The main mineral component was quartz with approximately 13%–15% and also contained a small amount of feldspar, rock debris, and mica. The diameter of the mineral particles was 0.25–0.50 mm, and they were irregularly round. A total of three purple mudstone samples were prepared for the analysis, namely, purple mudstone samples no. 1, 2, and 3.Figure 1
Sketch of the geographical position of the rock sample.Table 1
X-ray diffraction (XRD) mineral composition analysis results.
Rock samplesMineral compositionX-ray powder diffraction (mineral composition)Directional tablet (in terms of 100% of total clay mineral)Quartz (%)Feldspar (%)Calcite (%)Hematite (%)Total volume of clay minerals (%)Montmorillonite (%)Chlorite (%)Illite (%)Kaolinite (%)Purple mudstone sample no. 113332791015705Purple mudstone sample no. 212632771015705Purple mudstone sample no. 315562721520605Table 2
EDS major element ratio analysis results.
Rock samplesElement type and content (%)CONaMgAlSiSKCaFePurple mudstone sample no. 121.0443.001.830.826.4818.230.071.984.701.86Purple mudstone sample no. 214.6452.020.611.475.7118.180.071.414.061.82Purple mudstone sample no. 315.3653.720.761.205.0418.450.051.212.611.60


### 2.2. Test Equipment and Methods

In the present study, a Niu Man MacroMR low-field NMR instrument was used to test the porosity changes of the purple mudstones under multiple dry and wet cycle periods. The sample was placed in a blast drying box to dry (temperature 60°C, time 6 h), and then the sample was placed in a vacuum saturation tank to be saturated (vacuum for 2 h, soak for 24 h). This constituted the drying and wetting cycles. The saturated sample was removed from the tank, the surface water wiped off, and the sample was sealed with two layers of plastic wrap to prevent moisture from dissipating and any external interference. After each cycle, the prepared sample was placed into the NMR tester. The purple mudstone samples no. 1, 2, and 3 were destroyed after cycling for 8, 7, and 5 times, respectively. The detailed test process is shown in Figure2.Figure 2
Experimental flow chart.


## 2.1. Rock Sample Preparation

The present study collected purple mudstones from the bank slopes of the Yangtze River in Wanzhou District, Chongqing, China. The geographical and sampling locations are shown in Figure1. The mineral properties of the purple mudstone samples are shown in Table 1. Table 2 shows the analysis results from the purple mudstone energy dispersive spectrometer (EDS) tests. The main mineral component was quartz with approximately 13%–15% and also contained a small amount of feldspar, rock debris, and mica. The diameter of the mineral particles was 0.25–0.50 mm, and they were irregularly round. A total of three purple mudstone samples were prepared for the analysis, namely, purple mudstone samples no. 1, 2, and 3.Figure 1
Sketch of the geographical position of the rock sample.Table 1
X-ray diffraction (XRD) mineral composition analysis results.
Rock samplesMineral compositionX-ray powder diffraction (mineral composition)Directional tablet (in terms of 100% of total clay mineral)Quartz (%)Feldspar (%)Calcite (%)Hematite (%)Total volume of clay minerals (%)Montmorillonite (%)Chlorite (%)Illite (%)Kaolinite (%)Purple mudstone sample no. 113332791015705Purple mudstone sample no. 212632771015705Purple mudstone sample no. 315562721520605Table 2
EDS major element ratio analysis results.
Rock samplesElement type and content (%)CONaMgAlSiSKCaFePurple mudstone sample no. 121.0443.001.830.826.4818.230.071.984.701.86Purple mudstone sample no. 214.6452.020.611.475.7118.180.071.414.061.82Purple mudstone sample no. 315.3653.720.761.205.0418.450.051.212.611.60


## 2.2. Test Equipment and Methods

In the present study, a Niu Man MacroMR low-field NMR instrument was used to test the porosity changes of the purple mudstones under multiple dry and wet cycle periods. The sample was placed in a blast drying box to dry (temperature 60°C, time 6 h), and then the sample was placed in a vacuum saturation tank to be saturated (vacuum for 2 h, soak for 24 h). This constituted the drying and wetting cycles. The saturated sample was removed from the tank, the surface water wiped off, and the sample was sealed with two layers of plastic wrap to prevent moisture from dissipating and any external interference. After each cycle, the prepared sample was placed into the NMR tester. The purple mudstone samples no. 1, 2, and 3 were destroyed after cycling for 8, 7, and 5 times, respectively. The detailed test process is shown in Figure2.Figure 2
Experimental flow chart.


## 3. Experimental Results and Analysis

Nuclear magnetic resonance causes hydrogen protons to spin themselves in a magnetic field to align with the direction of the field. Then, the magnetic field changes direction and turns off again, causing the hydrogen proton to return to its original state. In this process, the induction signal received by the coil changes. This process is called relaxation [25, 26]. The transverse relaxation time T2 is the time constant for the exponential decay of the spin echo amplitude of a single pore body. For porous systems such as porous networks, exponential decay is the sum of individual decays. Using inversion processing technology, the original attenuation curve can be fitted to T2 spectrum. Most NMR interpretation and rock physical parameter estimation, such as porosity, pore size distribution, and permeability, are based on this T2 spectrum [27–29].Adding a corresponding magnetic field to the outside of the purple mudstone sample obtained the relaxation rate and intensity of the fluid hydrogen nuclei in the pores of the sample. From this analysis, the pore fracture state of the purple mudstone sample was obtained. Determining theT2 peak and distribution interval of the relaxation time is the main method used in NMR testing. In addition, the superposition of the three mechanisms of lateral volume relaxation, lateral surface relaxation, and diffusion relaxation constitutes the total lateral relaxation rate:(1)1T2=1T2B+1T2S+1T2D,where T2 is the total relaxation time; T2B is the volume relaxation time; T2S is the surface relaxation time; and T2D is the diffusion relaxation time.(2)1T2=ρ2SV,where S is the rock pore surface area; V is the rock pore volume; and ρ2 is the transverse surface relaxation strength of the rock (i.e. a constant). Equation (2) shows that the pore size of the rock is proportional to the surface relaxation time, i.e., as the pore size increases, the relaxation time increases accordingly. The T2 spectrum was obtained by performing NMR tests on the purple mudstone samples with different numbers of dry and wet cycles. However, the shape and volume of the purple mudstone samples changed greatly during the dry-wet cycles and the T2 spectrum had no reference significance. Therefore, the T2 spectrum data was converted (i.e., the signal amplitude value corresponding to each T2 time was divided by the total signal amplitude value) to obtain a pore volume ratio map and thus a different pore size development for each sample (Figure 3).(3)sv=Fsr,where FS is the shape factor; r is the size of the hole. According to equations (2) and (3), it can be concluded that(4)T2=rρsFs.Figure 3
Proportional distribution of the whole volume: (a) sample no. 1; (b) sample no. 2; (c) sample no. 3.
(a)(b)(c)So, the linear relationship between them is as follows:(5)r=CT2.Hole size can be measured by HPMI method,C = ρSFS, C can be calculated by linear regression [30–32].(6)σ=∑2nωriri−CT2i2∑2nωri.σ is the total error in the formula. In order to choose the most accurate C value, you must minimize sigma. ω(r(i)) is the increment frequency of pore size. So, T2 and PTSD can be switched back and forth (5). FS is pore shape factor (the rock pore structure can be regarded as a tubular model, where FS = 2). This is the result of averaging the coefficients after each cycle to minimize the total error σ of the linear conversion. If different spaces are filled with water, then T2 will vary depending on the size of the space. The more water you fill the space, the greater T2 is going to be. Therefore, equations (1)–(6) can obtain the distribution map of pore size of rock.The pore volume ratio in the purple mudstone samples varied with the sample pore size in the different samples and dry-wet cycles. When the dry and wet cycleNC was 1, the purple mudstone sample nos. 1, 2, and 3 all showed single peaks of 14.0287%, 14.7056%, and 13.52152%, respectively. With an increase in the number of dry-wet cycles, the pore sizes of sample nos. 1, 2, and 3 increased from 0.0341–0.0682 µm, 0.0318–0.0594 µm, and 0.0318–0.0636 µm to 0.0052–43.4224 µm, 0.0064–28.6292 µm, and 0.0040–32.8935 µm, respectively. As the number of dry-wet cycles increased, the pore volume ratio of the purple mudstone changed from a single peak to a camel-shaped double peak with a corresponding change in the sample pore diameter. In the camel-shaped doublet, the area of the first peak (corresponding to a pore size of 0.001–0.1 µm) of sample nos. 1, 2, and 3 decreased from 0.1277, 0.1181, and 0.1376 to 0.1209, 0.1047, and 0.1245, respectively. In the camel-shaped doublet, the area of the second peak of the three samples (with a corresponding pore diameter of 0.1–1 µm) increased from 0.9413, 0.9974, and 0.6779 to 0.9871, 1.1498, and 0.9901, respectively. In general, as the number of wet and dry cycles increased, the proportion of micropores with a pore diameter of 0.001–0.1 µm decreased compared to the total pore diameter and the large pores with a pore size of 0.1–1 µm increased compared to the proportion of total pores. The details of each cycle are shown in Table 3.Table 3
Peak area corresponding to the dry and wet cycle periods.
CyclesThe area of the first peakSecond peak areaRock samplesPurple mudstone sample no. 1Purple mudstone sample no. 2Purple mudstone sample no. 3Purple mudstone sample no. 1Purple mudstone sample no. 2Purple mudstone sample no. 3Nc = 20.13770.11810.13760.94130.99740.6779Nc = 30.03280.10620.12471.90671.29610.9315Nc = 40.11410.12940.15190.78330.65200.5445Nc = 50.13830.14200.12450.58550.64460.9901Nc = 60.12240.1403—0.75810.7608—Nc = 70.12090.1047—0.91661.1498—Nc = 80.1208——0.9871——The area of the first peak of the three purple mudstone samples decreased overall, whereas the peak areas of the second peak increased overall. However, there were fluctuations in the fourth and fifth cycles (Table3). First, the purple mudstone samples became weathered and disintegrated during the wet and dry cycles. After the sample was full, only the remaining intact blocks were removed for testing (Figure 4). Second, mudstone deposits were observed in the vacuum saturation tank before the fourth cycle. Mudstone adheres to the surface of the sample, making the water absorption incomplete; therefore, the measured signal amplitude was smaller than normal. Thus, the measured value of the proportion of large holes was smaller than the actual value, and the measured value of the proportion of small holes was larger than the actual value.Figure 4
Rock sample failure process.It can be seen from Figure5 that, with the increase in the number of wet and dry cycles, the change of porosity presents a three-stage trend of increase-decrease-increase. The number of cycles 1–3 is stage I, and the porosity increases sharply. The three samples increased to 24.59%, 25.59%, and 24.25%, respectively. In stage II, the porosity begins to decrease. The porosity of the three samples dropped to 15.49%, 17.99%, and 22.59% at the end of this stage. In stage III, porosity increases again. Each increase in porosity will cause the sample to peel off and collapse, which causes the porosity of the sample to drop to a value that will not be damaged. So, it repeated that the sample is completely destroyed. Table 4 shows the specific numerical changes.Figure 5
The change of porosity with the number of cycles.Table 4
Porosity values of different cycles.
Rock samplesPurple mudstone sample no. 1Purple mudstone sample no. 2Purple mudstone sample no. 3CyclesPorosity (%)Nc = 10.01220.092110.15882Nc = 218.7811722.0717522.19704Nc = 324.5857725.5927524.24514Nc = 420.1736118.8087122.58676Nc = 515.4854118.6361727.98095Nc = 619.725317.98795—Nc = 720.3657518.3578—Nc = 821.299——The cumulative pore volume ratio of the purple mudstone samples varied with the sample pore size for the different sample numbers and dry and wet cycle periods (Figure6). When the dry and wet cycle NC was 1, the pore distributions of the purple mudstone samples No. 1, No. 2 and No. 3 were 0.01–0.1 µm. The approximate slopes of the three sample curves were 2671.92, 3361.81 and 3334.40, respectively. As the number of dry-wet cycles increased, the slopes of the three samples decreased to 150.98, 149.84 and 162.00, respectively. In general, the higher the number of dry-wet cycles, the smoother the curve and the better the corresponding pore size grading curve.Figure 6
Cumulative pore volume ratio: (a) samples No. 1 Cumulative pore volume; (b) samples No. 2 Cumulative pore volume; (c) samples No. 3 Cumulative pore volume.
(a)(b)(c)


## 4. Discussion

Previous studies have shown that changes to the pore structure are related to the mineral composition of rocks and the external conditions [33, 34]. Many factors affect the pore structure during dry-wet cycles, including water, accumulation of salt crystals, and crystal expansion. These are discussed in detail.


### 4.1. Influence of Water

During the dry-wet cycles, the purple mudstone samples had a small pore structure with a single pore size of 0.01–0.1µm and changes to the micropore structure of less than 0.01 µm. This is because the pore water and calcite and quartz in the purple mudstone interacted to cause different degrees of dissolution and the contact of the minerals with water caused potassium feldspar and sodium feldspar in the purple mudstone to decompose and form kaolinite (see equations as follows). The EDS of the purple mudstone was tested by experiments (Figure 7). K+, Na+, Ca2+, Mg2+, and other cations in the purple mudstone minerals combined with the OH- ions in the solution and the original minerals were broken down to form new minerals. Under hydrolysis, the KOH solution (K+ combined with OH-) and NaOH solution (Na+ combined with OH-) were formed with the loss of water. The incomplete hydrolysis of the initial mineral caused the porosity in the purple mudstone to increase, resulting in new micropores smaller than 0.01 µm. With the progress of hydrolysis, the minerals were completely decomposed; therefore, the pores with a pore diameter of 0.01–0.1 µm in the rock were decreased, and the proportion of large pores of 0.1–1 µm was increased. The pH test found that, as the number of dry-wet cycles increased, the solution became more and more alkaline, which confirmed the occurrence of water and rock chemistry (Figure 8). Thus, various minerals in the rocks were damaged by the water-rock chemistry due to the chemical imbalance between the rocks and minerals. During this time, new micropores were generated, and the rock became loose and fragile, which was mainly manifested by the increase of pores in the purple mudstone samples that is consistent with the research findings of Feng et al. [35]. The result of the hydrochemical damage was a change of the microscopic composition of the rocks and the destruction of the original microstructure. The mechanism of geological disasters is closely related to this complex process.Figure 7
Proportion diagram of EDS experimental elements and SEM images of purple mudstone before and after dry-wet cycle.Figure 8
Relationship curves between the pH value and dry-wet cycle periods.


### 4.2. Changes in Bound Water and Cumulative Crystallisation of Salt

According to Figure9, as the number of wet-dry cycles increased, the bound water content in the sample clearly increased from 0.08876, 0.25918, and 0.3631 to 0.922, 1, and 0.9937. Table 5 shows the specific numerical changes.Figure 9
Diagram of the variation of irreducible water thickness with the number of dry and wet cycles.Table 5
The bound water thickness of different cycles.
Rock samplesPurple mudstone sample no. 1Purple mudstone sample no. 2Purple mudstone sample no. 3CyclesThickness of bound waterNc = 10.088760.259180.3631Nc = 20.386160.333310.40547Nc = 30.391930.411390.48394Nc = 40.415350.452590.53167Nc = 50.445480.480530.9937Nc = 60.476070.50031-Nc = 70.546150.98669-Nc = 80.92168--The existence of pore water in porous materials can be roughly divided into three types: bound water, pore water, and large amounts of free water [1, 36]. Mudstone contains a lot of clay minerals. When water penetrates into the pores and cracks of mudstones, the adsorbed water film (bound water content) of the fine clay minerals will thicken, which will cause volumetric expansion of rock particles [37]. Bound water mainly refers to water adsorbed on the surfaces of minerals or to interlayer water when clay minerals exist [38]. The T2 spectrum of pore water in a constant-temperature uniform magnetic field is mainly determined by ion surface relaxation [27, 39], which mainly depends on the nature of the liquid and its affinity for the mineral surface or inner surface [40], so the quantification of pore water can be derived from the T2 spectrum. The T2 of bound water in porous materials is generally less than 3 ms, while the range for capillary water is 3–33 ms, while values >33 ms indicate a large amount of free water [41, 42]. In this study, we used the above two critical values to divide the pore water component. The bound water content increases as the number of cycles increases. This is because the clay minerals first hydrate and swell upon encountering water. The cations between the layers combine with the water penetrating the surfaces of the minerals, causing the crystal lattice of the clay minerals to expand. In the dry state, cations are adsorbed on the surfaces of clay minerals and the interlayer spacing is almost zero. After adsorbing 1–4 layers of water molecules, the interlayer spacing expands to 12.6–21.6 Å, which is approximately 1 nm [43]. When the interlayer spacing is expanded to 3 nm, the clay minerals enter the electric double layer and expand. Gouy-Chapman proposed that the cations adsorbed on the crystal layer of clay minerals will diffuse around them due to the increase in the thickness of the combined water film. These cations are attracted by the negative charges of their own crystal layer and, at the same time, are far away from the crystal layer due to Brownian motion. The potential energy of the layer makes the negatively charged crystal layer and the cations in the water form a diffuse electric double layer. Clay minerals undergo hydration when in contact with water, and the forces generated by the expansions of the lattice and electric double layer drive the expansion and deformation of the clay bricks. The increase in bound water proves the influence of clay minerals on the pore structure. The mechanism of influence of salt crystallisation and clay mineral swelling on the pore structure is shown in Figure 10.Figure 10
Mechanism of clay mineral swelling and cumulative crystallisation of salt.Salt crystallisation is the main damage mechanism of rocks. Growing crystals can apply pressure in porous materials [44–46]. Experiments by Balboni [47]; Rivas [48]; Benavente [49]; and Bradley [50] have shown that growing crystals can exert pressure inside the rock.During saturation of the purple mudstone, the Cl-in the water combined with the Na+ in the rocks to form NaCl (i.e., salt). As shown in Figure 10, fine salts were loosely distributed among the pores during the initial stage. As the cyclic period increased, the salt crystals in the pores increased, which caused the NMR test results in the present study to change from a single peak with a distribution interval of 0.01–1 µm to a double peak with a distribution interval of 0.001–0.1 µm. When accumulated to a certain degree, the salt will squeeze against itself, causing stress on the inner wall of the rock hole. The accumulation of salt crystals causes this stress to increase and the pores to become enlarged. This explains the increase in the proportion of large pores with a total pore diameter of 0.1–1 µm in the NMR test results in the present study.


### 4.3. Crystal Expansion

The purple mudstone samples withstood a temperature of 60°C during the drying process, whereas rocks have stresses between 20°C and 90°C [51]. Rock is not a good thermal conductor; therefore, the surface temperature will be significantly different from the temperature inside the rock. Any expansion experienced during high-temperature heating will cause crushing of the outer layer of stones. This produced micropores smaller than 0.01 µm. Immediately after the rock was heated, it was placed into water, which caused thermal shock to increase the damage. The water quickly cooled the hot purple mudstone samples, causing further spalling and making the pores bigger, with pores larger than 0.1 µm being produced. Crystal expansion is the main mechanism for the deterioration of rock properties [52]. The calcite from the purple mudstone samples in the present study accounted for 3%–6%. Calcite is a mineral that expands in one direction and contracts in the other direction when heated (Figure 11). As it cools, it will shrink along the c-axis while expanding along the other axes. Therefore, calcite is very vulnerable to thermal cycling [53]. The results of mathematical modelling have shown that calcite is susceptible to thermal weathering [51]. For calcite, the stress caused by heating to 40–50°C causes cracks, resulting in the appearance of new micropores with a pore size of less than 0.01 µm. Heating causes cracking and increased porosity [54], increasing the proportion of large pores with a pore size of 0.1–1 µm.Figure 11
Calcite expansion and contraction process. (a) Single crystal (b) initial (c) expansion and contraction by heat (d) expansion and contraction by cold.
(a)(b)(c)(d)


## 4.1. Influence of Water

During the dry-wet cycles, the purple mudstone samples had a small pore structure with a single pore size of 0.01–0.1µm and changes to the micropore structure of less than 0.01 µm. This is because the pore water and calcite and quartz in the purple mudstone interacted to cause different degrees of dissolution and the contact of the minerals with water caused potassium feldspar and sodium feldspar in the purple mudstone to decompose and form kaolinite (see equations as follows). The EDS of the purple mudstone was tested by experiments (Figure 7). K+, Na+, Ca2+, Mg2+, and other cations in the purple mudstone minerals combined with the OH- ions in the solution and the original minerals were broken down to form new minerals. Under hydrolysis, the KOH solution (K+ combined with OH-) and NaOH solution (Na+ combined with OH-) were formed with the loss of water. The incomplete hydrolysis of the initial mineral caused the porosity in the purple mudstone to increase, resulting in new micropores smaller than 0.01 µm. With the progress of hydrolysis, the minerals were completely decomposed; therefore, the pores with a pore diameter of 0.01–0.1 µm in the rock were decreased, and the proportion of large pores of 0.1–1 µm was increased. The pH test found that, as the number of dry-wet cycles increased, the solution became more and more alkaline, which confirmed the occurrence of water and rock chemistry (Figure 8). Thus, various minerals in the rocks were damaged by the water-rock chemistry due to the chemical imbalance between the rocks and minerals. During this time, new micropores were generated, and the rock became loose and fragile, which was mainly manifested by the increase of pores in the purple mudstone samples that is consistent with the research findings of Feng et al. [35]. The result of the hydrochemical damage was a change of the microscopic composition of the rocks and the destruction of the original microstructure. The mechanism of geological disasters is closely related to this complex process.Figure 7
Proportion diagram of EDS experimental elements and SEM images of purple mudstone before and after dry-wet cycle.Figure 8
Relationship curves between the pH value and dry-wet cycle periods.


## 4.2. Changes in Bound Water and Cumulative Crystallisation of Salt

According to Figure9, as the number of wet-dry cycles increased, the bound water content in the sample clearly increased from 0.08876, 0.25918, and 0.3631 to 0.922, 1, and 0.9937. Table 5 shows the specific numerical changes.Figure 9
Diagram of the variation of irreducible water thickness with the number of dry and wet cycles.Table 5
The bound water thickness of different cycles.
Rock samplesPurple mudstone sample no. 1Purple mudstone sample no. 2Purple mudstone sample no. 3CyclesThickness of bound waterNc = 10.088760.259180.3631Nc = 20.386160.333310.40547Nc = 30.391930.411390.48394Nc = 40.415350.452590.53167Nc = 50.445480.480530.9937Nc = 60.476070.50031-Nc = 70.546150.98669-Nc = 80.92168--The existence of pore water in porous materials can be roughly divided into three types: bound water, pore water, and large amounts of free water [1, 36]. Mudstone contains a lot of clay minerals. When water penetrates into the pores and cracks of mudstones, the adsorbed water film (bound water content) of the fine clay minerals will thicken, which will cause volumetric expansion of rock particles [37]. Bound water mainly refers to water adsorbed on the surfaces of minerals or to interlayer water when clay minerals exist [38]. The T2 spectrum of pore water in a constant-temperature uniform magnetic field is mainly determined by ion surface relaxation [27, 39], which mainly depends on the nature of the liquid and its affinity for the mineral surface or inner surface [40], so the quantification of pore water can be derived from the T2 spectrum. The T2 of bound water in porous materials is generally less than 3 ms, while the range for capillary water is 3–33 ms, while values >33 ms indicate a large amount of free water [41, 42]. In this study, we used the above two critical values to divide the pore water component. The bound water content increases as the number of cycles increases. This is because the clay minerals first hydrate and swell upon encountering water. The cations between the layers combine with the water penetrating the surfaces of the minerals, causing the crystal lattice of the clay minerals to expand. In the dry state, cations are adsorbed on the surfaces of clay minerals and the interlayer spacing is almost zero. After adsorbing 1–4 layers of water molecules, the interlayer spacing expands to 12.6–21.6 Å, which is approximately 1 nm [43]. When the interlayer spacing is expanded to 3 nm, the clay minerals enter the electric double layer and expand. Gouy-Chapman proposed that the cations adsorbed on the crystal layer of clay minerals will diffuse around them due to the increase in the thickness of the combined water film. These cations are attracted by the negative charges of their own crystal layer and, at the same time, are far away from the crystal layer due to Brownian motion. The potential energy of the layer makes the negatively charged crystal layer and the cations in the water form a diffuse electric double layer. Clay minerals undergo hydration when in contact with water, and the forces generated by the expansions of the lattice and electric double layer drive the expansion and deformation of the clay bricks. The increase in bound water proves the influence of clay minerals on the pore structure. The mechanism of influence of salt crystallisation and clay mineral swelling on the pore structure is shown in Figure 10.Figure 10
Mechanism of clay mineral swelling and cumulative crystallisation of salt.Salt crystallisation is the main damage mechanism of rocks. Growing crystals can apply pressure in porous materials [44–46]. Experiments by Balboni [47]; Rivas [48]; Benavente [49]; and Bradley [50] have shown that growing crystals can exert pressure inside the rock.During saturation of the purple mudstone, the Cl-in the water combined with the Na+ in the rocks to form NaCl (i.e., salt). As shown in Figure 10, fine salts were loosely distributed among the pores during the initial stage. As the cyclic period increased, the salt crystals in the pores increased, which caused the NMR test results in the present study to change from a single peak with a distribution interval of 0.01–1 µm to a double peak with a distribution interval of 0.001–0.1 µm. When accumulated to a certain degree, the salt will squeeze against itself, causing stress on the inner wall of the rock hole. The accumulation of salt crystals causes this stress to increase and the pores to become enlarged. This explains the increase in the proportion of large pores with a total pore diameter of 0.1–1 µm in the NMR test results in the present study.


## 4.3. Crystal Expansion

The purple mudstone samples withstood a temperature of 60°C during the drying process, whereas rocks have stresses between 20°C and 90°C [51]. Rock is not a good thermal conductor; therefore, the surface temperature will be significantly different from the temperature inside the rock. Any expansion experienced during high-temperature heating will cause crushing of the outer layer of stones. This produced micropores smaller than 0.01 µm. Immediately after the rock was heated, it was placed into water, which caused thermal shock to increase the damage. The water quickly cooled the hot purple mudstone samples, causing further spalling and making the pores bigger, with pores larger than 0.1 µm being produced. Crystal expansion is the main mechanism for the deterioration of rock properties [52]. The calcite from the purple mudstone samples in the present study accounted for 3%–6%. Calcite is a mineral that expands in one direction and contracts in the other direction when heated (Figure 11). As it cools, it will shrink along the c-axis while expanding along the other axes. Therefore, calcite is very vulnerable to thermal cycling [53]. The results of mathematical modelling have shown that calcite is susceptible to thermal weathering [51]. For calcite, the stress caused by heating to 40–50°C causes cracks, resulting in the appearance of new micropores with a pore size of less than 0.01 µm. Heating causes cracking and increased porosity [54], increasing the proportion of large pores with a pore size of 0.1–1 µm.Figure 11
Calcite expansion and contraction process. (a) Single crystal (b) initial (c) expansion and contraction by heat (d) expansion and contraction by cold.
(a)(b)(c)(d)


## 5. Conclusion

The present study aimed to examine the changes in the porosity of purple mudstone samples under multiple dry-wet cycle conditions. The following results were obtained:(1)
With an increase in the number of dry-wet cycles, the purple mudstone pore volume ratio significantly changed. Originally, it consisted of a small pore structure with a single pore diameter of 0.01–0.1µm and changed to a variety of pore structures with various pore diameters of 0.001–100 µm.(2)
With the increase in the number of dry-wet cycles, the micropores (0.001–0.1µm) were transformed into macropores (0.1–1 µm). The proportion of small pores decreased from 17.50% to 56.32%, whereas the proportion of large pores increased from 8.56% to 15.00%. The area of the first peak (micropores 0.001–0.1 µm) of sample nos. 1, 2, and 3 decreased from 0.1277, 0.1181, and 0.1376 to 0.1209, 0.1047, and 0.1245, respectively. The area of the second peak of the three samples (large pores 0.1–1 µm) increased from 0.9413, 0.9974, and 0.6779 to 0.9871, 1.1498, and 0.9901, respectively.



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