1. Introduction
The softening and disintegration of rocks is the major cause of geologic hazards such as landslides, mudslides, and erosion [
1]. Economic development has increased the concentration of acid and alkali pollutants in industrial and domestic wastewater, as well as an increase in the frequency of acid rain [
2,
3]. This has led to a more complex environment for rock endowment. In addition, during rainfall, the occurrence of surface-breaking runoff with varying flow rates at different locations on the slope surface results in different degrees of disintegration of the acid- and alkaline-contaminated rocks [
4]. Therefore, investigating the effects and underlying mechanism of hydro-chemical damages on the disintegration characteristics of rocks subjected to the scouring effect can provide a scientific foundation for the prevention and prediction of geologic disasters.
The softening and disintegration of rocks occur due to the influence of various external factors such as freeze–thaw cycles [
5,
6], dry and wet cycles [
7,
8,
9,
10], and high temperatures [
11]. These factors interact in a complex manner. Additionally, the internal mineral composition of the rock and the macro-performance of microstructural changes play crucial roles in this process [
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23]. The effects of variations in the water chemistry on the deterioration and disintegration properties of rocks have attracted the attention of several researchers [
24]. Vergara et al. [
8] investigated the effect of aqueous solution chemistry on the physical and mechanical properties of rocks and demonstrated a strong influence of aqueous solution chemistry on the swelling and shrinkage of shales. Wakim et al. [
25] also suggested that the primary mechanisms responsible for the structural damage, structural decay, and weakening of rocks are the swelling and dissolution processes of clay minerals. Zhao et al. [
26] examined the relationship between rock disintegration and cation release in acidic environments through indoor experiments and observed a strong correlation between cation release and particles <20 mm. Li et al. [
27] documented that the primary cause behind the heightened disintegration of mudstones under the influence of acid rain hydrochemistry is the provision of an extra pathway for rock disintegration due to the damage inflicted by acid rain hydrochemistry. Moreover, Deng et al. [
28], and Huang and Zhan [
29] quantitatively characterized the disintegration properties of rocks treated with different pH solutions. Based on the fractal theory and energy dissipation principle, good results were achieved in their study. Zhang et al. [
9,
10,
30,
31,
32] provide an overview of indoor testing techniques for assessing rock disintegration. The study also investigates the impact of various testing methods on the properties of rock disintegration. However, the above-mentioned studies did not consider the effect of water scouring action on the disintegration characteristics of the rock and could not accurately simulate and reveal the effect of rainfall on the damage to the rock slope surfaces.
Zhao et al. [
33] demonstrated that the geotechnical particles under the action of water scouring were primarily subjected to the uplift force of water, the drag force of water, the underwater gravity force of particles, and the inter-particle bonding force. The equation for the estimation of the critical flow rate of geotechnical particles in the water flow was formulated in accordance with the principle of mechanical equilibrium. To investigate the initial phenomenon of slope geotechnical particles under different water flow conditions, Wang et al. [
34] designed a scour starting test device for experimental study and concluded that the initial flow rate of geotechnical particles is mainly affected by dry density, clay content, and slope. Wu et al. [
35] studied the scour damage process of slopes under different rainfall intensities through indoor slope scour experiments. In addition, few researchers [
33,
36] have evaluated the damage to geotechnical bodies on riverbeds and slopes when subjected to water scouring. The prior investigations concerning scour damage in geotechnical formations omitted the inclusion of the wetting caused by rainfall over various durations before the scouring process. Additionally, they did not address the consequences of the cementation weakening in geotechnical particles resulting from numerous wet and dry cycles. Consequently, an accurate simulation and examination of the influence of rainfall on the surface layer deterioration of geotechnical slopes could not be achieved.
The unique gypsum karst breccia at the Zhoukoudian site in Beijing contains a large number of mineral components, such as mud and clay minerals, which are easy to disintegrate after interacting with water, resulting in a decrease in rock integrity. This provides geological conditions for the occurrence of geological disasters under extreme climatic conditions. For example, in the “Beijing 7.21 heavy rain”, many landslides and collapses occurred at the Zhoukoudian site. This has led to several negative impacts on the protection of sites. Therefore, a comprehensive study of the disintegration characteristics of the gypsum karst breccia under different pH solutions and scouring conditions is crucial for predicting and preventing geologic hazards at the Zhoukoudian site.
In this research, we focus on the gypsum karst breccia as the subject of investigation. A bespoke disintegration test apparatus with a controlled flow rate was developed for this study. Subsequently, disintegration experiments were conducted on the gypsum karst breccia samples. These samples were soaked in solutions of varying pH levels, subjected to different flow rates, and exposed to varying cycle counts. The aim was to analyze how the pH of the soaking solution, flow rate, and number of cycles influence the disintegration characteristics of the rock samples. Through the utilization of NMR, SEM, XRD, and ICP-OES techniques, we delved into the investigation of several aspects. This included exploring the pore structure of the disintegrated rock samples, examining the microstructure of the immersed rock specimens, studying mineral compositions, and analyzing the release of cations into the solution. These analytical methods were employed to unveil the disintegration pattern of the gypsum karst breccia subjected to scouring and to elucidate the underlying mechanism of its formation. This comprehensive analysis reveals the disintegration patterns of gypsum karst breccias and illuminates their formation mechanisms.
4. Microscopic Test
Previous studies show that changes in the microstructure of porous media such as rocks are an important cause of changes in their macroscopic mechanical properties [
43,
44]. The pH values of the solutions and the flow rates have a significant influence on the pore structure as well as the mineral composition and content of the rock samples. To elucidate the formation mechanisms behind the disintegration characteristics and the variations in the stable flow rate of disintegration of the gypsum karst breccia, multiple tests, including XRD, SEM, and ICP analyses, were performed on rock samples exposed to solutions of varying pH.
4.1. Characterization of Particle Structure
SEM analysis helps to observe the distinct changes in the structural characteristics of gypsum karst breccia particles following treatment with various pH solutions. Due to space constraints, only SEM images of the gypsum karst breccia particles treated with solutions of pH 2, 7, and 11 were included in this study (
Figure 10).
This study analyzed the particle structural characteristics of the rock samples from four perspectives: particle morphology, cementing material, intergranular contact mode, and pore structure. Following treatment with a pH 7 solution, the particle morphology of the rock specimens appeared to be well-preserved, with complete and smooth surfaces, as shown in the figure. The grains contained a small amount of cementing material, primarily exhibiting point-to-face bonding, while the pore structure exhibited a combination of scaffolding and mosaic structures. The particles in the bracket structure exhibit low cementation strength, making them susceptible to peeling off under the influence of scouring, thereby intensifying the disintegration of the rock samples. After the rock samples were immersed in pH 2 solution, a large amount of cementing material dissolved, the original dense structure in the visual area became loose, and the proportion of point–plane contact between grains increased. At the same time, the corrosion effect of soil structure appears, which is intuitively manifested as the boundary between pores and particles in the visual field is no longer clear, the surface of skeleton particles becomes broken and disorderly, and the proportion of scaffold pores increases significantly, leading to the disintegration of rock samples. After treatment with pH 11 solution, the microstructure of the rock samples appeared relatively intact, with a higher density when compared with the rock samples treated with pH 2 and pH 7 solutions. There was a decrease in the proportion of the support structure alongside the loss of cementing material. In addition, a significant number of suspended particles were visible on the surface of the rock samples, while the pores within the support structure were filled with secondary cementing materials. This helps in minimizing the localized damage to the rock samples, thereby enhancing their integrity and bonding strength.
Furthermore, it can be inferred that the treatment of the rock samples with an acidic solution leads to an increase in the number of internal support pores and a decrease in the bonding strength between particles. These changes can be attributed to the corrosion and deterioration of the rock structure. As a result, at the same flow rates, there was an intensification of the rock sample disintegration, resulting in a reduced stable flow rate of disintegration. However, there was a decrease in the proportion of internal support pores and mosaic pores due to the enhancement of the secondary structure. This resulted in an increased cementation strength between the particles. As a result, the disintegration of rock samples caused by alkali contamination is reduced, and the stable flow rate of disintegration increases.
4.2. XRD and ICP Test
It is imperative to consider the impact of variations in mineral composition and content within the gypsum karst breccia on the disintegration characteristics, stable flow rate of disintegration, and microstructure. Therefore, the rock samples subjected to different pH treatments were analyzed using XRD analysis, and the corresponding results are presented in
Figure 11. Examination of data in
Figure 11 reveals that the treatment with an acidic solution leads to a significant decrease in the content of calcite and clay minerals (illite and kaolinite), while the quartz content shows a significant increase. When the pH value of the soaking solution is reduced from 7 to 2, the content of calcite decreases from 15.3% to 5.4%, the content of clay mineral decreases from 44.3% to 24.1%, and the content of quartz increases from 28.9% to 41.2%. Further, this trend became more pronounced with the decrease in the pH of the solution. Subsequent treatment of the rock samples with an alkaline solution resulted in a slight increase in the calcite content, while the content of quartz and clay minerals exhibited a slight decrease. When the pH value of soaking solution increased from 7 to 11, calcite content increased from 15.3% to 19.2%, the content of quartz decreased from 28.9% to 18%, and clay minerals decreased from 44.3% to 35.5%. Furthermore, this trend became more apparent at higher pH values of the solution.
To gain better insights into the variations in mineral composition in the gypsum karst breccia subjected to different conditions, ICP analysis of solutions obtained after immersing the rock samples was performed in this study. The results are presented in
Figure 12. The data in
Figure 12 reveal the following observations: the treatment of rock samples with an acidic solution leads to a decrease in the release of Na
+ ions in the solution, while the release of Al
3+ and Mg
2+ ions slightly increases. When the pH value of the soaking solution was reduced from 7 to 2, the release of the above ions ranged from 9.24 mg/L to 0.09 mg/L, 6.25 mg/L to 16.35 mg/L, and 0.813 mg/L to 3.79 mg/L, respectively. In addition, there was a significant increase in the release of Ca
2+; in a pH 2 solution, Ca
2+ is released about seven times as much as in a pH 7 solution. These trends became more pronounced with the decrease in the solution pH. On the other hand, following the treatment of the specimens in an alkaline solution, there was a slight decrease in the release of Na
+ and Mg
2+ ions in the solution, a significant decrease in the release of Ca
2+ ions, and a slight increase in the content of Al
3+ ions. When the pH value of the soaking solution increased from 7 to 11, Ca
2+ decreased by 14.602 mg/L, and Al
3+ ion increased by 1.71 mg/L. Furthermore, these trends became significant with the increase in the solution pH. The results of cation release from the immersion solutions at different pH levels were consistent with the changes observed in the mineral content in the rock samples.
The major reasons for the above-mentioned phenomenon can be explained as follows: in an acidic solution, a series of hydrolysis reactions occur within the gypsum karst breccia when it comes in contact with H+ ions, resulting in the dissolution of calcite, illite, and kaolinite present in the rock samples. This leads to the displacement of cations (Ca2+, Al3+, and Mg2+), thereby increasing the release of these ions into the solution. In alkaline solution, Ca2+ and Mg3+ inside the rock samples react chemically with OH– ions in the solution to form Ca(OH)2 and Mg(OH)2 precipitates, leading to a decrease in the release of Ca2+ and Mg2+ in the solution, and these newborn mucilaginous gels or flocs flow together with fine particles wrapped around them due to the Brown effect and fill the surrounding pores, and then form a stronger linkage with stronger contact points after embedded hardening. After embedding and hardening, a stronger linkage is formed, which directly strengthens the inter-particle cohesion and inter-particle friction. In addition, Ca2+ reacts chemically with OH– and CO2 to form CaCO3, which fills the internal pores of the rock samples, leading to an increase in calcite content and an improvement in the microstructure. At the same time, quartz is hydrolyzed under strong alkaline environment, which is the main reason for the decrease in quartz content and the increase in Al3+ release in the solution. Not only that, NaOH meeting CO2 produces Na2CO3, and with a slight change in temperature and humidity, it forms high water compounds and bring a certain expansion pressure, therefore, after soaking the NaOH solution, the sample, after coming into contact with the air for a few moments, develops a few white crystals on the surface. This shows that the potential reason for the disintegration inhibition and the increase in disintegration stabilization flow rate of the rock samples under alkaline environment should be that the improvement effect of the nascent structure inside the rock samples is stronger than the damage effect of the initial structure.
5. Mechanisms Underlying the Formation of Scouring and Disintegration Properties of Rock Samples
The pores within the rock samples facilitate the passage of the solution through the interior of the rock, where it undergoes physicochemical reactions that lead to changes in the mechanical parameters, including intergranular cementation.
Table 2 shows a significant presence of hydrophilic clay minerals and water-soluble mud cement within the interior of the gypsum karst breccia. The treatment of the rock samples with an acid solution resulted in the dissolution of calcite and clay minerals in the rock samples (
Figure 11 and
Figure 12), resulting in the deterioration of microstructural integrity (
Figure 10) as well as a decrease in particle cohesion. Macroscopically, it was evident that the cumulative relative disintegration of the rock samples increased with a decrease in the pH of the acidic solution while the flow rate remained constant.
Upon treating the gypsum karst breccia in an alkaline solution, two key reactions were observed. Firstly, the dissolution effect led to the dissolution of clay minerals and quartz particles in the rock samples. Secondly, the reaction of Ca
2+, Mg
2+, and other ions with OH
– led to the formation of precipitates that occupied the pores surrounding the rock, leading to a secondary structural improvement effect and the reduction in localized damages in the rock samples (
Figure 11 and
Figure 12). The SEM analysis of the rock samples after treatment with an alkaline solution reveals several findings: firstly, the improvement in the nascent structure outweighs the deterioration of the rock structure, resulting in enhanced microstructural integrity (
Figure 10). Secondly, the cohesion between particles is enhanced. Macroscopically, the cumulative relative disintegration of the rock decreases with an increase in the pH of the alkaline solution at the same flow rate. Additionally, the reaction between NaOH and CO
2 produces Na
2CO
3. Due to a slight variation in temperature and humidity, the highly hydrous compound Na
2CO
3·10H
2O is formed, which adheres to the surface layer of the rock sample, as shown in
Figure 10.
The process of rock disintegration due to scouring is significantly affected by the shear strength among constituent particles and the shear force exerted by water. The particle-induced shear force resulting from water scouring can be estimated using Equation (6):
In this study, the following parameters were considered as constants under constant conditions of rock samples and the scouring test environment: KS indicates particle surface roughness, χ is a function associated with the particle surface roughness, and C2 represents a parameter associated with resistance to the water flow in the opposite direction. The disintegrated particles collected and analyzed in this study have a diameter (yd) smaller than 2 mm. Therefore, for subsequent analysis, these parameters were considered as constants.
The shear strength between rock particles is estimated using the M-C criterion as follows:
where
and
indicate the normal stress and cementation force between particles, respectively, and
s the friction coefficient between the particles. In the surface geotechnical body of the slope, where there are no additional stresses, the normal stress
Qp between particles can be assumed as negligible and excluded from calculations. Therefore, the shear strength responsible for resistance to particle disintegration can be primarily attributed to the inter-particle cementation strength
Cp. When the cement type remains consistent, the cementation strength
Cp remains constant, and the inter-particle cementation force is determined by the cementation area.
Given the above, the disintegration characteristics of gypsum karst breccias under scouring, the effects of immersion treatment with solutions of different pH values, and the underlying mechanisms that influence the stable flow rate of disintegration are provided below.
(1) The particles surrounding the large pores of the rock samples exhibit minimal cementation area and the lowest resistance to shear disintegration. Therefore, the disintegration of the gypsum karst breccia predominantly occurs in these particle regions. As shown in
Figure 8, the cumulative relative disintegration of the rock samples decreases rapidly with a decrease in the volume of the large pores;
(2) As disintegration progresses, the macropores undergo a significant reduction, while the cementation area between residual particles and the surrounding particles on the rock sample surface significantly increases when compared to the particles surrounding the macropores. Hence, the cementation force of particles increases correspondingly. The process of disintegration ceases when this force becomes equivalent to the shear force exerted by water flow, indicating the attainment of the stable flow rate of disintegration;
(3) In acidic solutions, a decrease in solution pH increases the corrosion effect, leading to the pore space expansion and facilitating the penetration of the solution into the interior regions of the rock sample. This intensifies the water–rock interactions. Therefore, the decrease in pH of the soaking solution leads to an increase in the pores within the rock samples, a decrease in the cement strength between particles, and a weakening of the resistance to disintegration and water scouring. As evident in
Figure 5 and
Figure 6, a decrease in solution pH under acidic conditions is associated with an increase in cumulative relative disintegration as well as a decrease in the stable flow rate of disintegration;
(4) In alkaline solutions, with the increase in the solution pH, the positive impact of the nascent structure becomes more significant than the deteriorating effect. This results in an improved microstructure of the rock samples, a reduction in the pore volume, an increase in the cementation strength between particles, and enhanced resistance to water-flushing shear forces. As shown in
Figure 5 and
Figure 6, the cumulative relative disintegration decreases, and the stable flow rate of disintegration increases with an increase in solution pH under alkaline conditions.
In the design of practical engineering disaster prevention measures, to consider the influence of different influencing factors on the strength of the gypsum karst breccia layer, we should multiply the corresponding reduction coefficient based on obtaining the strength index of rock. However, at present, the specific value of the reduction coefficient cannot be determined because the relationship between the pH value and the flow rate of the solution and the disintegration degree of gypsum karst breccia is only obtained in this paper, and the strength value of the rock with different disintegration degrees is not studied. This paper is an exploratory experiment aimed at highlighting the effect of acid rain on rock disintegration behavior. Later, the author will establish the corresponding relationship between the disintegration degree of rock and its strength. Combined with the results of this paper, the strength reduction coefficient of rock under different pH acid rain is proposed to provide a theoretical reference for engineering design. At the same time, due to the limitation of test space, the test device in this paper can provide a maximum flow rate of 3 m/s, and it is necessary to change the test device when studying the influence of higher flow rates on rock disintegration.
6. Conclusions
In this paper, a self-designed disintegration test device was used to carry out disintegration tests at different flow rates for different numbers of cycles on the gypsum karst breccia soaked in different pH solutions, and the pore changes of the disintegrated samples and the changes in the microstructure and mineral composition of the soaked samples were investigated with the help of NMR, SEM, XRD, and ICP-OES, which led to the following main conclusions:
(1) The cumulative relative disintegration of the gypsum karst breccia increases with the decrease in solution pH. In addition, it increases with an increase in the number of cycles and the flow rate. When the flow rate reaches a certain threshold, the amount of cumulative relative disintegration does not exhibit a significant change (about 73%), indicating the attainment of a stable flow rate of disintegration. This stable flow rate of disintegration demonstrates a good correlation with the pH of the solution (or the number of cycles);
(2) Under acidic conditions (or alkaline conditions), the decrease (or increase) in the solution pH leads to a significant increase in the proportion of small pore volume within the gypsum karst breccia (maximum increase of 61.33%). The proportion of medium pore volume increases to a maximum value and then decreases, while the proportion of large pore volume decreases rapidly to zero. Upon reaching the flow rate associated with disintegration stabilization, the ratios of small and medium pore volumes reach a state of equilibrium. However, the proportion of large pore volume remains consistently at 0. Therefore, disintegration of gypsum karst breccia predominantly occurs in proximity to large pores;
(3) Under acidic conditions, a decrease in solution pH results in the dissolution of clay minerals and calcite within the rock samples. This leads to a decrease in the release of Na+, a slight increase in the release of Al3+ and Mg2+, and a significant increase in the release of Ca2+ in the solution (maximum increase of 90.89 mg/L). The rock samples exhibit noticeable deterioration due to corrosion, resulting in surface damage and increased pore space of the scaffolds. Therefore, the corrosion effect in an acidic environment is the main reason for the deterioration of rock disintegration and the decrease in the stable flow rate of disintegration;
(4) Under alkaline conditions, the increase in the solution pH decreases the content of clay minerals and quartz, while increasing the calcite content. Moreover, the release of Na+ and Ma2+ from the solution slightly decreases, the release of Ca2+ significantly decreases (reduced by 95.4%), and the release of Al3+ slightly increases. In addition, the surface of the rock samples remains intact, and there is a decrease in the pore space of the scaffold. Therefore, in alkaline environment, the improvement effect of the new structure is stronger than the corrosion effect, which is the main reason to restrain the disintegration of rock sample and increase the stable flow rate of disintegration;
(5) The influence of pH value of solution and flow velocity on rock disintegration characteristics is essentially caused by the dynamic change of relative proportion of large pore volume in rock samples.