Experimental Study of the Dynamic Mechanical Behavior and Degradation Mechanism of Red Sandstone in Acid Dry-Wet Cycles

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Introduction
Many rock engineering applications (such as mines, dams, and building foundations) experience varying degrees of dry-wet cycles in their service due to periodic changes in water conditions (including rainfall, tidal ebb and flow, and changes in groundwater levels) [1].The physical and mechanical qualities of the rock mass will deteriorate to variable degrees under long-term dry-wet cycles.If the degradation of rock materials in the field is not monitored or intervened timely, geological disasters can be caused [2][3][4][5].To this end, much research has been conducted on the physical and mechanical characteristics of rock mass under the impact of dry-wet cycles.
Hale [6] investigated the effect of dry-wet cycles on the unconfined compressive strength of six different types of sandstone.The results demonstrated that dry-wet cycles marginally diminish sandstone's compressive strength.Doostmohammadi et al. [7] studied the effect of dry-wet cycles on mudstone expanding characteristics and found that the time necessary for the mudstone to attain final expansion steadily reduces as the number of dry-wet cycles increases.Özbek [8] summarized that the unit weight, uniaxial compressive strength, and P-wave velocity of ignimbrite samples show a decreasing trend as the number of dry-wet cycles increases, while the polarity and water absorption by weight show an increasing trend.Lin et al. [9] discovered that calcite and chlorite in the rock matrix may be dissolved during dry-wet cycles, causing increased porosity and reduced rock strength.Zhao et al. [10] investigated the effect of the dry-wet cycle on the tensile strength of sandstone.The results demonstrated that the tensile strength of sandstone with a low clay mineral concentration reduces somewhat, owing to the fact that the expansion and dissolution of clay minerals play minimal part in the sandstone weakening process.SHPB tests were used by Du et al. [11] to examine the effect of dry-wet cycles on the dynamic fracture characteristics of red sandstone.The findings indicate that the dynamic fracture toughness of red sandstone does not increase when the critical loading rate is surpassed.
It can be seen from the above research that the existing dry-wet cycle test techniques are mostly based on neutral aqueous solutions.In fact, influenced by geological conditions, acid rain, water quality pollution, and other factors, the groundwater is rarely neutral (pH = 7), generally acidic or alkaline, and the acidic water solution has greater damage to the rock.Therefore, it is necessary to carry out research on the physical and mechanical properties of rocks under acid environment dry-wet cycle, which can provide reference for the safety assessment and maintenance of related geotechnical engineering.This research intends to determine how acidic dry-wet cycles affect the dynamic compression characteristics of red sandstone.Using the split Hopkinson pressure bar (SHPB) test equipment, dynamic compression tests were conducted on red sandstone following 0, 5, 10, 20, 30, and 40 dry-wet cycles in a neutral (pH = 7) and acid (pH = 4:5 and 2.5) environment, respectively.The deterioration mechanism induced by acid dry-wet cycles was summarized by the scanning electron microscope method.

Experimental Material and Methodology
2.1.Sample Preparation.The red sandstone rock was deep red and free of fissures.The major components of the red sandstone, according to X-ray diffraction study, were quartz and feldspar.Rock blocks were cored and sliced into disk samples of 50 mm in diameter and 30 mm in thickness [12], which is shown in Figure 1.The obtained rock samples were further screened to ensure good integrity and uniformity of rock samples.First, macro observation on rock samples was conducted, and the samples with obvious cracks or defects were removed.Second, the density comparison method was used to remove the samples with large density dispersion.Finally, the ultrasonic was employed to detect the longitudinal wave velocity of the samples, and the rocks with large wave velocity dispersion were removed.

Acid Dry-Wet Cycle
Design.In this study, NaCl and KHSO 4 were employed to create aqueous chemical solution due to the comparatively high quantity of Na + , K + , H + , Cl + , and SO4 + plasma in groundwater.We used the approach of increasing ion concentration and decreasing pH of the solution to reduce the test duration and allow the chemical solution to produce corrosion damage to the sample in a short period of time.In this test, three types of aqueous chemical solutions were designed.The solution's ionic concentration was 0.1 mol/L, and the pH values were 2.5, 4.5, and 7, respectively.Distilled water was used to create a neutral solution with a pH of 7. The pH and concentration of the solution must be rectified after each dry-wet cycle to guarantee that the pH and concentration of the solution before the next cycle were the same as the test design value.
A complete dry-wet cycle includes water saturation and oven drying.The selected samples were submerged in clean water for 48 hours at room temperature to saturation before being dried in an oven for 24 hours [13].The oven temperature was adjusted at 50 °C to limit the influence of temperature on the test findings.In this test, five levels of dry-wet cycles were set, namely, n = 0, 5, 10, 20, 30, and 40, and n = 0 means that the sample has not experienced a dry-wet cycle.
2.3.Dynamic Impact Test.The dynamic impact test on the SHPB system was completed (Figure 2) [11].During the test, the cylindrical punch impinged on the input rod along the axial direction at a constant speed, causing stress waves to form in the input rod.After multiple transmission and reflection cycles, the stress at both ends of the sample tended to be constant [14,15].The impact air pressure selected for this test was 0.35 MPa, and the impact test at each level of air pressure was performed more than three times.The threewave approach was used to analyze data based on the wave signals [16].The calculating formula is as follows [17]: where A 0 is the cross-sectional area of the compression bar, E 0 is the elastic modulus of the bar, C 0 is the wave velocity of the one-dimensional elastic bar, A s is the cross-sectional area of the sample, L s is the length of the sample, and ε I , ε R , and ε T represent the pulse signals generated by the incident wave, reflected wave, and transmitted wave in the pressure bar, respectively.

Test Results
Figure 3 depicts the dynamic stress-strain curves of red sandstone in various settings.The figure shows that acidic dry-wet cycles have a noticeable influence on the dynamic stress-strain curves of red sandstone.The more dry cycles performed in the given pH environment, the lower the peak point of the curve and the smaller the slope of the elastic section, demonstrating that the dry-wet cycles have a deteriorating impact on the mechanical characteristics of red sandstone.The peak strain of the sample tends to increase 2 Geofluids gradually as n increases, and the smaller the pH value, the more obvious this trend is, indicating that the stronger the acidity of the solution, the more significant the softening effect on red sandstone.The peak strain rose by 1.5% (pH = 7:0), 4.55% (pH = 4:5), and 10.95% (pH = 2:5) when n increased from 5 to 40.
The proportion of the stress strain curve before the linear elastic deformation segment occupies the peak gradually decreases as n increases, indicating that the more dry-wet cycles, the more severe the damage to the rock interior, and the internal microcracks expand and penetrate more rapidly, thereby decreasing the distance of the elastic deformation segment and entering the yield stage earlier.The elastic modulus was calculated according to the slope of the stress-strain curve between 40% peak strength and 60% peak strength.
According to the test results in Table 1 and Figure 4, the changing rule of dynamic compressive strength (UCS) subjected to dry-wet cycles in various acid solutions is depicted.The graph illustrates that, in a constant pH environment, the dynamic compressive strength of red sandstone gradually falls as the number of drying and wetting cycles increases, with the magnitude of the drop increasing as the pH value decreases.After 40 cycles of drying and wetting, the dynamic compressive strength of red sandstone fell by 30.56% (pH = 7:5), 45.52 (pH = 4:5), and 59.32% (pH = 2:5) relative   3 Geofluids to the reference sample (n = 0).It can be observed that the sample's compressive strength decreases with decreasing pH, showing that the acidic solution accelerates the sample's degradation.
Figure 5 depicts the change curve of the elastic modulus of red sandstone with n.The figure shows that, in the particular pH environment, the elastic modulus of red sandstone drops gradually as n increases, with the lowering speed being quick at first and later slow.The elastic modulus of red sandstone fell by 38.76% (pH = 7:5), 42.1 (pH = 4:5), and 50.14% (pH = 2:5) after 40 dry-wet cycles.It can be observed that the lower the pH of the solution, the higher the drop in elastic modulus, which is congruent with the dynamic compressive strength of the sample.According to the research results of dynamic compression performance of red sandstone after dry-wet cycles, the acidity of the solution should be considered.

Discussion
The dynamic mechanical qualities of red sandstone samples have degraded to varied degrees under the influence of an acidic environment dry-wet cycle, as demonstrated by the above research.In reality, the mechanical qualities and engineering properties of rock are directly connected to its interior micro and micro structural morphology [18].In order to determine the damage and degradation process of treated red sandstone under dry-wet cycles in  4 Geofluids various acidic environments, SEM scanning experiments were performed on treated red sandstone samples, as seen in Figure 6.First, produce a batch of cube samples with a side length of 10 mm, ensuring that the surface of each sample is flat and devoid of noticeable imperfections.Then, conduct the dry-wet cycle test in different acidic environments according to the dry-wet cycle design method described in Section 2.2.Dry the cube sample after drying and wetting cycles in a constant temperature (60 °C) drying oven for 12 hours, then adhere it to the base with latex for gold plating.Lastly, observe the micromorphological characteristics of the sample using scanning electron microscope equipment.

Micromorphological Characteristics of Red Sandstone
When pH = 7. Figure 7 displays SEM scanning images of red sandstone through several dry-wet cycles when pH = 7.
For the uncorroded sample (n = 0), the sample's overall structure is reasonably complete, with dense particles and few loose particles, and no newly developed microcracks have been discovered (Figure 7(a)).Due to the high degree of cementation, the tight arrangement of crystal particles, and the difficulty in connecting the micropores between particles, the sample that has not been corroded has excellent mechanical characteristics.The surface morphology of red sandstone samples altered dramatically as a result of dry-wet cycles.After 5 cycles (Figure 7(b)), some tiny particles are connected to the surface of the major mineral particles, and flake crystals cover the particle contact portion.This is due to the fact that the soluble cement dissolves and is gradually lost during the sample soaking procedure.The amount of fragments created owing to the dissolving of soluble cement steadily rises as n increases, as illustrated in Figures 7(c)-7(f).
Microcracks first formed near the particle-cement interface, as shown in Figure 7(d).This is because mineral particles shrink when the temperature drops during the soaking stage.The primary mode of particle contact is compressive stress.Mineral particles expand owing to temperature increase during the drying stage, and the contact between particles creates tensile tension.The tension and compression between mineral particles alternate frequently during the cyclic wetting drying cycle, making cracks at the particle cementation easy to form.Additionally, during the soaking process, the clay minerals in the cement expand after absorbing water, forming a part of additional tensile stress at the particle boundary.Because the mineral particles are mostly quartz and feldspar, they have low water sensitivity, whereas the cement is mostly clay minerals and calcium oxide, which have high water sensitivity.This disparity in water sensitivity causes deformation inconsistency, which is one of the causes of the initial break at the junction.
The fissures between mineral particles steadily enlarge as n increases, the depth and width of the cracks rise, and the cementation between particles weakens.The cementation   5 Geofluids region between particles is continually split due to the continuous initiation, extension, and penetration of fractures, which finally leads to the progressive loss of cementation between particles, as seen in Figures 7(e) and 7(f).Due to ion exchange and hydrolysis, feldspar in neutral aqueous solution can create new clay minerals such as kaolinite and silica colloid [19].Furthermore, calcium oxide in red sandstone will react with water.The dissolution degree of feldspar and the reaction degree of calcium oxide grow continually as cycle periods increase, resulting in a continuous increase in the secondary porosity of the rock.When internal fractures and micropores expand to a certain level, some visible holes appear on the sample's surface, as seen in Figures 7(e) and 7(f).

Micromorphological Characteristics of Red Sandstone
When pH = 4:5.The interaction between water and rock in the groundwater environment is primarily separated into two parts: physical and chemical.When the water solution is neutral, the dry-wet cycle causes mostly physical damage to the rock.The friction force of the particle interface or the crack surface decreases to varying degrees due to the lubrication and softening of water, resulting in a reduction of the bonding strength between particles, destroying the integrity of the internal structure of the rock, and the damage of this cementation will not recover with the drying of the rock.Chemical activity consists mostly of dissolution, hydrolysis, adsorption, and ion exchange [20].Water-rock chemical action causes more damage to rocks than physical action, and chemical damage is tightly connected to rock mineral composition, solution acidity and alkalinity, ion concentration, and other parameters.When an aqueous solution becomes acidic, hydrogen ions in the solution can react with iron/calcium 6 Geofluids cements, feldspars, carbonates, silicates, and other compounds [21].In neutral aqueous solutions, these are difficult to dissolve.Red sandstone clay minerals react with acid solution: In reality, the primary mineral component SiO 2 in red sandstone is insoluble in water and difficult to react with acid solution, but a little quantity of KAlSi 3 O 8 and NaAlSi 38 in feldspar will react with H + in acid solution [22]: When compared to neutral solution, the H + in acidic solution will react with most mineral components in red sandstone, producing dissolution and disintegration to variable degrees.The number of loose particles and lamellar crystals on the surface of the sample in the acidic environment tends to increase under the same n as in the neutral solution, indicating that the chemical reaction between the acid solution and mineral components leads to an increase in kaolinite, gypsum, and other precipitates.
The chemical action of water-rock causes quick loss of mineral particles in the sample, which causes rapid growth of internal fissures and pores and causes the internal structure of the rock sample to change from compact to loose [23].When n = 5, a number of cracks appear on the surface of the interparticle cement, and the cracks continue to expand and run through (Figure 8(a)).The cements between particles gradually decompose to form fragments of varying sizes as n increases, which are randomly distributed around the particles, as shown in Figures 8(b)-8(e).When n increased to 40 (Figure 8(e)), corrosion on the major mineral particles is observed as a result of hydrochemical action [24], which is not observed in the neutral solution environment.

Micromorphology Characteristics of Red Sandstone
When pH = 2:5.When pH = 2:5, the acidity of the solution increases more, and the degree of water-rock chemical reaction increases progressively.The cement between mineral particles exhibits visible dissolving and corrosion response after a few cycles, leading in the entire loss of cement between certain particles, as illustrated in Figure 9(a).As the concentration of H + in the solution increases, more mineral particles engage in the chemical reaction, and the flocs, alteration sites, and pits generated on the particle surface rise dramatically (Figures 9(b)-9(e)).Due to the exterior seepage of internal water molecules during the sample drying process, the outward migration of secondary minerals, rock cuts, and ions created by hydrochemical reactions is increased, resulting  7 Geofluids in more smooth edges of mineral particles (Figure 9(d)).Hydrochemical reactions have considerably enhanced degree of rock erosion and disintegration On the one hand, the cementation ability between particles has been seriously reduced, and several obvious cracks have formed between adjacent particles.The cracks have a tendency to gradually open and extend, which eventually leads to changes in the contact mode between particles, and the surface edge contact and edge contact modes between particles are gradually increasing.On the other hand, it is manifested by the dissolution of large mineral particles, a gradual decrease in particle size, the formation of secondary pores and fractures, and finally the softening of the mineral skeleton [26].

Conclusion
(1) The dynamic compression characteristics of red sandstone are significantly damaged by acid drywet cycles.The dynamic compressive strength and elastic modulus of red sandstone steadily decline as the number of dry-wet cycles increases, and the lower the pH of solution, the greater the reduction (2) The scanning electron microscope was used to investigate the microscopic degradation mechanism of red sandstone after acid dry-wet cycles.When the solution is neutral, the tensile and compressive stresses between mineral particles frequently alternate under the action of dry-wet cycle, and cracks are easily generated at the particle boundary and cementation, weakening particle adhesion.Moreover, partial cement dissolution increases the sample's internal porosity.These are the primary causes of red sandstone mechanical characteristics deterioration (3) In the acid dry-wet cycle environment, the mineral particles are partially dissolved, and the cement, feldspar, and other mineral components in the red sandstone react with the acid solution, weakening the cementation energy between the mineral particles, promoting the rapid development of secondary pores and fractures, and ultimately reducing mechanical properties

Figure 3 :
Figure 3: Dynamic stress-strain curve of red sandstone under different environments.

Figure 4 :
Figure 4: Variation of dynamic UCS of red sandstone versus n.

Figure 5 :
Figure 5: Variation of dynamic elastic modulus of red sandstone under different environments.

Table 1 :
SHPB test results of red sandstone in different environments.