An experimental investigation on the damage mechanisms of red glutenite in the Mount Wuyi cultural and natural heritage site subject to acid rain and wet-dry cycles: a macro-to-micro approach

The safety of rock landscapes in Mount Wuyi is significantly impacted by acid rain and wet-dry cycles. In this paper, the decay characteristics of the physical–mechanical properties of red glutenite were investigated under acidic wet-dry cycles. A systematic approach, including cold field emission scanning electron microscopy (CFE-SEM), image processing techniques, and X-ray diffraction (XRD), was proposed to investigate the damage mechanism of red glutenite under acidic wet-dry cycles. The results indicate that with increasing solution acidity and wet-dry cycles, dry density (DD), longitudinal wave velocity (LWV


Introduction
Against the backdrop of continuous global environmental evolution, acid rain poses a significant threat to many natural and cultural heritages based on rocks (e.g., grottoes, Danxia landscapes, karst landscapes, fossil sites, etc.).Renowned for their unique geological composition, cultural inscriptions, and historical significance, these heritages stand not only as precious landscapes but also as testimonies to the development of civilization.Unfortunately, over time, they have become exceedingly fragile due to prolonged natural weathering and anthropogenic disturbances.The erosion caused by acid rain further exacerbates this process [1][2][3].
Mount Wuyi is one of the foremost national scenic destinations in China.This remarkable region possesses unparalleled ecological resources, and the cultural heritage of this hallowed mountain reflects the storied historical civilization of Fujian Province.In December 1999, Mount Wuyi was inscribed on the UNESCO World Heritage List (No. 911) as a site of both cultural and natural significance, following the World Heritage selection criteria (III, VI, VII, and X).Since then, it has become one of China's four world cultural and natural heritage sites.Its cultural and natural heritage exemplifies the harmonious coexistence of humankind and nature over countless generations.The designation of Mount Wuyi as one of the nation's inaugural national parks in 2022 underscores the imperative to safeguard its natural and cultural resources.[4,5] The rock mass landscape at Mount Wuyi is both unique and captivating, comprising a quintessential Danxia landform that constitutes a vital component of the region's scenic natural splendor.Late Cretaceous red glutenite formations have given rise to the scenic Danxia landforms that characterize the predominant rock types.Mount Wuyi is situated within China's acid rain distribution zone.It lies within the subtropical monsoon humid climate zone, blessed with abundant rainfall and synchronous precipitation and warmth, undergirded by a well-developed surface water system.These environmental factors catalyze the repeated acidic wet-dry cycles that drive the deterioration of the rock landscape due to rainfall recharge and evaporation, as well as fluctuations in river levels, thereby directly impacting its stability and safety (Fig. 1).Indeed, many Danxia landform cultural heritage sites worldwide face similar challenges of natural weathering and anthropogenic degradation, urgently necessitating in-depth research and the formulation of practical conservation measures in relevant fields.Therefore, this study focuses on the red glutenite of Mount Wuyi as the research subject, aiming to provide references and foundations for the protection of stone cultural and natural heritage worldwide by investigating its damage mechanism through acid rain wet-dry cycles damage tests.
Acid rain, containing corrosive chemicals such as sulfuric acid and nitric acid, poses a significant threat to stone cultural and natural heritage.It chemically reacts with minerals in rocks, leading to mineral transformation and the formation of new minerals [6,7].The resulting soluble salts migrate with water to the rock surface, forming salt crystals, which is a primary cause of rock surface weathering [8,9].At the same time, acid rain weakens the cementation between rock particles, leading to a reduction in the load-bearing capacity and durability of rocks [7,10,11].This phenomenon directly endangers the preservation of cultural heritage, such as stone carvings, petroglyphs, and monuments, causing surface discoloration, damage, and even the loss of historical inscriptions, thereby severely impacting their aesthetic and historical value [12][13][14].For instance, Huang et al. [15] found that acidic solutions caused the three-dimensional surface morphology of limestone carvings to become rougher, and this phenomenon was significantly correlated with the pH value of the solution.In their study of limestone relics in Fars Province, Iran, Taghipour et al. [16] found that acid rain primarily reacts with calcite in the limestone, leading to increased porosity and reduced compressive strength.Geng et al. [11] reported that in Fig. 1 A rock landscape instability area in Mount Wuyi the Yungang Grottoes, the cementitious material in the sandstone dissolved first under the impact of acid rain, leading to decreased longitudinal wave velocity and compressive strength.Zhang et al. [17] reported that acid rain induced the formation of subflorescence on the surface of the sandstone in the Nankan Grottoes, causing severe damage to the stone carvings and petroglyphs.
The mineral composition and structural characteristics vary among different rocks, leading to different degradation mechanisms of various types of rocks under acidic environments [18][19][20][21].Numerous scholars emphasize the importance of revealing the mechanisms of the chemical deterioration of rocks in acidic environments by analyzing changes in the microstructure and chemical composition of rocks [21][22][23].Ciantia et al. [21] found that acidic solutions accelerate the dissolution of calcite in carbonate rocks, thereby facilitating the debonding of carbonate particles.Li et al. [22] revealed the primary reaction between hydrogen fluoride and sandstone minerals in acidified sandstone through theoretical analysis and laboratory experiments.They also demonstrated the presence of hydrochloric acid catalysis during the dissolution of sandstone minerals by a mixed acid of HCl/ HF and analyzed the mode of this catalysis.Miao et al. [23] used scanning electron microscopy (SEM) and XRD techniques to investigate the chemical reactions between granite and acidic solutions, as well as the mechanisms of damage.It was found that the microstructure and mineral content of granite changed after acidic solution treatment.In this study, variations in the microstructure of the red glutenite are analyzed based on CFE-SEM and image analysis techniques, and the variation in the mineral composition of red glutenite is examined using XRD.A comprehensive method for analyzing the damage mechanism is proposed.
In addition, the effect of wet-dry cycles has garnered the attention of numerous scholars due to its frequent occurrence in practical situations and significant deterioration effect on the rock mass [6,[24][25][26][27][28][29][30][31][32][33].Research has shown that wet-dry cycles cause the entry and exit of water within rocks, leading to significant volume expansion and contraction [26,27,30,31].This volume change generates significant internal stress within the rock.Additionally, repeated infiltration and evaporation of water can cause micro-cracks in the rock to gradually extend, ultimately forming larger fractures or cracks, which have a significant impact on the pore structure and permeability of the rock [28,32,33].
Research in this domain holds significant importance for the conservation of stone cultural and natural heritage sites.However, most existing studies on the degradation characteristics of microstructures have remained at a qualitative level, lacking the necessary quantitative data to bolster their conclusions.Additionally, the research objects are limited to a few rock specimens, and the damage characteristics of different types of rocks necessitate further investigation.This study selects red glutenite as the research object, conducting a series of tests including DD tests, LWV tests, UCS tests, CFE-SEM tests, and XRD tests under various acidic wet-dry cycles.The study analyzes the decay characteristics of the physical and mechanical properties of red glutenite under acidic wet-dry cycles from a macro perspective and explores the microstructural damage process and mechanism of red glutenite under acidic wet-dry cycles from a micro perspective.This research offers a theoretical foundation and practical guidance for the preservation of stone cultural and natural heritage, while also holding practical engineering value.

Specimens preparation
The red glutenite samples were obtained from the same location in Mount Wuyi to ensure consistent lithology and representativeness.These samples were then processed into 50 mm × 50 mm × 50 mm square specimens using a rock cutter and a sample grinder.To meet the accuracy requirements of specimen preparation, the adjacent surfaces of the specimens were perpendicular to each other, with a permissible deviation of ± 0.25°.The opposite surfaces were required to be parallel, with a permissible deviation of ± 0.05 mm for non-parallelism.
The ZT802-90958 non-metallic ultrasonic testing analyzer was used to conduct LWV measurements in three orthogonal directions on each specimen.Specimens with LWV within the range of 3000 ± 100 m/s were selected for the subsequent tests.Notably, each test group included three parallel specimens for physical and mechanical experiments, as well as microstructure and mineral composition experiments.

Petrographic characteristics
Figure 2 shows the microscopic observation of the red glutenite specimen slice.Figure 2 illustrates that the rock has a medium-coarse-grained sandy structure and a blocky structure with gravel.It is generally composed of calcareous cement and argillaceous cement.The gravel particles are mostly angular with poor roundness.The clastic grain size ranges from 0.5 to 5.0 mm, and the sorting is relatively poor.The specimen shows vigorous effervescence when treated with dilute hydrochloric acid.The clastic components mainly consist of quartz, feldspar, and fragments, occasionally including muscovite.The quartz is colorless with a low protrusion, an interference color of first-order grey to white, and a clean surface due to its non-development of alteration.The feldspar exhibits a crystalline character development with alkaline feldspar and plagioclase present.The surface of feldspar appears relatively cloudy due to alteration.The clasts are mainly volcaniclastic and metamorphic rocks.The cementing material is distributed among the clastic particles, mainly calcareous and siliceous mud.The calcareous cement occurs in granular form with grain sizes mostly below 0.8 mm, displaying high-grade white interference color.Siliceous mud is cryptocrystalline, mostly light yellowish brown.

Experimental methods Solutions
Chemical composition analysis and pH testing of the five groups of rainwater specimens from the Wuyi Mountain scenic area indicated that the rainwater contained high levels of Na + , K + , H + , Cl − , and SO 4 2− . The pH values of the five groups of rainwater specimens were measured to be 5.0, 5.2, 5.5, 5.8, and 5.9, with an average pH of 5.48.NaCl and KHSO 4 were used to prepare a solution with a pH of 5.5 to simulate rainwater.In contrast, solutions with pH values of 4.0 and 7.0 were designed for comparative experiments by decreasing and increasing the pH value.The ion concentration of the solutions was maintained at 0.1 mol/L.

Wet-dry cycles
The wet-dry cycles of the specimens were implemented by drying and free water absorption at room temperature.The processed red glutenite specimens were placed in a drying box for 24 h at a temperature of 105 °C to ensure complete evaporation of water.After switching off the power supply, the specimens were allowed to cool naturally to room temperature within the drying box.Subsequently, the specimens were placed in the configured solution to absorb water freely for 24 h, then removed, wiped clean, and placed back in the drying box to dry for an additional 24 h.This constituted a completely acidic dry-wet cycle process.To minimize the impact of temperature on the mechanical properties of the rock, the drying temperature was set at 60 °C [34,35].A plastic grid was placed at the interface between the container and the specimen to ensure complete water absorption by the specimens.The experiment consisted of five levels of wet-dry cycles, namely 0, 5, 10, 20, and 25 cycles.The initial state of the dried specimens was labeled as the 0th cycle specimens, indicating no prior exposure to wet-dry cycles.After each wet-dry cycle, the pH and concentration of solutions were adjusted to maintain their stability until the start of the next cycle.It is important to note that all specimens reached a fully dried state after the drying treatment.Physical-mechanical and microscopic tests were performed on the treated specimens.

Physical and mechanical tests
The DD, LWV, and UCS tests were performed on specimens that completed 0, 5, 10, 20, and 25 acidic wet-dry cycles.The LWV of the specimens was measured after varying numbers of cycles using a ZT802-90958 nonmetallic ultrasonic testing analyzer.To improve the accuracy of measurement results, a uniform layer of petroleum jelly was uniformly applied to the contact surface between the transducer and the specimen, improving their contact coupling properties.The MTSe45.305 electronic universal testing machine was used as the UCS testing apparatus.The maximum load capacity of the machine is 300 kN, with an accuracy level of ± 0.3%.Its displacement loading speed range is from 0.001 to 254 mm/min.The strain produced during the loading process was measured using resistance strain gauges attached to the surface of the specimen.The UCS tests were conducted with displacement control at a 0.06 mm/ min loading rate.The acquisition frequency of the strain gauges was set to 10 Hz.

Microscopic tests
After the uniaxial compression test was carried out on the specimens with different numbers of wet-dry cycles, some intact rock samples were taken from the center of the tested specimens.The microstructural morphology of these damaged specimens was obtained using the Hitachi S-4800 Cold Field Emission Scanning Electron Microscope (CFE-SEM).The microstructure of the specimens was objectively analyzed by employing PCAS software for image processing and quantitative identification.The CFE-SEM tests were conducted at 3.0 kV voltage and 10 k magnification.The mineralogical composition Fig. 2 The red glutenite slice micrograph was analyzed using a Bruker D8 ADVANCE X-ray diffractometer (XRD) on specimens after 0, 5, 10, 20, and 25 cycles of wet-dry cycles at pH = 4, as well as on specimens after 25 cycles of wet-dry cycles with solution pH = 4.0, 5.5, and 7.0, respectively.The 2θ scanning range of XRD was 5-60°, the data point interval was 0.02°, and the scanning speed was 4°/min.

Deformation characteristic under acidic wet-dry cycles
The stress-strain curves (Fig. 4) of the specimens exhibit four typical stages: the compaction stage (OA), the elastic stage (AB), the yielding stage (BC), and the post-peak damage stage (CD).As illustrated in Fig. 4, under the same pH conditions, the stress-strain curve gradually flattens out as the number of wet-dry cycles increases.The strain values during the OA stage increase with an increase in the number of wet-dry cycles and a decrease in the pH of the solution.The wet-dry cycles cause the generation and expansion of micro-cracks within the specimen, reducing the density, while the increase in the acidity of the solution aggravates the damage to the internal structure of the specimen, resulting in a gradual increase in strain values during the OA stage.The strain values during the AB stage decrease with the increase in the number of wet-dry cycles.During the BC stage, the strain values are relatively small but exhibit complex variation.Due to the brittleness of the specimen, the destruction is accompanied by a crisp sound, and the stress of the specimen rapidly decreases after reaching the peak value.The CD stage almost appears as a straight line.
The general form of these curves aligns with previous research.More precisely, in some previous rock mechanics studies, uniaxial compression stress-strain curves exhibit a noticeable compaction feature at the initial stage of loading [18,36].This feature is a macroscopic result of micro-cracks and pore closure.In the red glutenite specimens tested in this study, the compaction feature becomes increasingly significant with an increase in the number of wet-dry cycles.Previous studies have indicated that the effect of wet-dry cycles often leads to an increase in the peak strain of rocks [37,38].However, in the red glutenite specimens tested in this study, the evolution of peak strain is unpredictable, probably due to its greater heterogeneity.

Decay characteristic of physical-mechanical properties
An exponential decay function model (Eq. 1) was used to fit DD, LWV, and UCS.
where I denotes the value of the test data; I n is the value of the test data for n times of wet-dry cycles; γ I is the decay coefficient, which characterizes the rate of decay of the test data; n represents the number of wet-dry cycles.Constants a and b were determined through repeated data fitting to ensure a goodness-of-fit R 2 ≥ 0.95, as shown in Table 1. (1)

DD and LWV
The data processing results for DD and LWV are shown in Figs. 5 and 6.Both DD and LWV exhibit similar evolution characteristics.Under the same pH conditions, both  DD and LWV exhibit a gradual decreasing trend as the number of wet-dry cycles increases.This indicates that wet-dry cycles significantly affect the internal structure of the specimens, causing particle gaps to expand and the number of cracks and pores to increase, thereby reducing the DD and LWV values of the specimens.
With the same number of wet-dry cycles, an increase in solution acidity causes a significant decrease in both DD and LWV.This indicates that the acidic environment accelerates the dissolution and corrosion processes of the specimens, resulting in the dissolution of mineral components, the fragmentation of particles, and the expansion of pores, consequently reducing both DD and LWV values.
The exponential decay function model accurately reflects the evolution characteristics of DD and LWV under acidic wet-dry cycle conditions, fitting well with the experimental data.This model is significant for predicting DD and LWV values.As the acidity of the solution increases, the attenuation coefficients γ DD and γ LWV increase.This indicates that the increase in solution acidity accelerates the decay rate of DD and LWV of the specimens.
Under acidic wet-dry cycles, the internal structure of the specimens is continuously eroded.Simultaneously, the chemical solution reacts with mineral particles, leading to the release of dissolved soluble mineral ions along fissures and pores.This results in a continuous decrease in the compactness of the specimens.In contrast, wetdry cycles have a lesser impact on the specimen dimensions.Therefore, the DD and LWV of the specimens show a gradual decreasing trend.

UCS and prediction model
To investigate the long-term decay pattern of the UCS of the red glutenite under acidic wet-dry cycles, it is necessary to establish a reasonable prediction model based on experimental data to effectively predict the UCS of the red glutenite over an extended period.
Figure 7 shows the variation of UCS.The UCS of the specimens decreases as the frequency of wet-dry cycles increases in solutions with varying pH.This observation suggests that the wet-dry cycles have a substantial detrimental effect on the bearing capacity of the specimen.Under the same number of wet-dry cycles, as the pH of Fig. 5 The variation of DD Fig. 6 The variation of LWV Fig. 7 The variation of UCS the solution decreases, the UCS gradually decreases.This indicates that the stronger the acidity of the solution, the stronger the corrosive effect of the solution on the specimen, leading to a greater reduction in the specimen's bearing capacity.The exponential decay function model is found to fit well with the UCS data, effectively reflecting the evolution pattern of UCS under acidic wet-dry cycles.The UCS decay coefficient γ UCS increases as the pH of the solution decreases.This indicates that acidic environments accelerate the rate of UCS decay.
Based on previous research, it is known that neutral environments have the least damaging effect on rocks compared to acidic and alkaline environments [18,39].Therefore, the decay coefficient γ UCS reaches its mini- mum value at pH = 7.The expression for γ UCS is: where α is a constant, obtained by fitting the data.
Figure 8 shows the processing results of the decay coefficient γ UCS , indicating a good fitting between γ UCS and Eq. 2. The three-dimensional prediction model for UCS: Using Eq. 3, a three-dimensional fitting was performed for UCS, wet-dry cycles, and solution pH.The fitting result is shown in Fig. 9, with a correlation coefficient of 0.98, indicating that the obtained relationship between the UCS of the specimen and the wet-dry cycles and solution pH is reasonable.The derived UCS decay prediction model provides valuable insights for predicting the strength of red glutenite in engineering.Currently, research on the relationship between (2) γ UCS = 0.0379 + α(pH − 7) 2   (3) rock strength and wet-dry cycles, as well as solution pH, is limited.Therefore, this strength decay prediction model also has an important reference value for predicting the strength of various types of rocks.

EM
The average EM represents the value of the slope of the approximately straight-line portion of the stress-strain curve, and the test environment has a minor impact on this value.To minimize this impact and improve the reliability of test results, the average EM is adopted as the EM of the specimen.
The EM was obtained from the stress-strain curve, and the EM was fitted using a linear decay function model (Eq.4).
where EM n is the EM for n wet-dry cycles; EM 0 is the EM for 0 wet-dry cycles; β is the decay coefficient of EM, which characterizes the rate of decay of EM.
The data processing results of EM are shown in Fig. 10.With the increase in wet-dry cycles, the elastic modulus of specimens in different pH solutions gradually decreases, indicating that wet-dry cycles significantly reduce the ability of specimens to resist deformation.This observation aligns with the decreasing slope of the AB stage of the stress-strain curve discussed in Sect.3.3.1.When the number of wet-dry cycles is the same, as the pH of the solution decreases, the EM gradually decreases.This suggests that as the acidity of the solution increases, the faster the production of new cracks and pores, leading to a more pronounced decrease in the specimen's resistance to deformation.The decay coefficient β increases as the

Deterioration sensitivity analysis
To gain insights into the deterioration sensitivity of various parameters of the specimen under acidic wet-dry cycles, we introduce the concept of deterioration degree [26,28,30] and quantitatively calculate it for each parameter using Eq. 5.
where D is the deterioration degree.
Table 2 displays the evolution of deterioration degrees for various specimen parameters.According to Table 2, under different pH conditions, the deterioration degrees of DD, LWV, UCS, and EM increase with the increase of (5) D = |(I n − I 0 )/I 0 × 100%| wet-dry cycles.This indicates that the damage inflicted on the specimen by wet-dry cycles accumulates gradually.Furthermore, when the number of wet-dry cycles is constant, the deterioration degrees of DD, LWV, UCS, and EM all increase with an increase in solution acidity.Notably, there are certain differences in the deterioration degree patterns of each parameter under different wetdry cycles and pH conditions.However, a general trend emerges: UCS > LWV > EM > DD.This suggests that there are distinct differences in the sensitivity of various specimen parameters to both wet-dry cycles and pH values, with UCS being the most sensitive, followed by LWV, EM, and DD.

Evolution of microstructural characteristics of the red glutenite
To investigate the evolution of microstructural characteristics of specimens under acidic wet-dry cycles, CFE-SEM was utilized to observe the microstructures of specimens under various acidic wet-dry cycles.As Fig. 11 Illustrates, the acidic wet-dry cycles altered the microstructural morphology of the red glutenite.In the initial drying state (n = 0), the structural surface was relatively smooth, with mineral particles bonded tightly and intact, interrupted only by a few scattered natural micro-cracks and pores.The presence of initial micro-cracks and pores provided a prerequisite for solution entry into the specimen.In vertical comparison, as the number of wet-dry cycles increases, the cement between mineral particles is continuously lost, and the mineral particles are also fractured due to repeated drying and wetting.Micro-cracks and pores continuously developed and connected, ultimately leading to a gradual deterioration of structural  integrity.In horizontal comparison, the microstructural damage of specimens under low pH conditions was often more severe than that under high pH conditions.The enhancement of solution acidity accelerated the destruction rate of the microstructure, which corresponds with the conclusion that the decay coefficients of DD, LWV, UCS, and EM increase with increasing solution acidity in Sect."Evolution of the physical-mechanical properties under acidic wet-dry cycles".Under conditions of low pH values and a high number of wet-dry cycles, the micro-morphology of the red glutenite exhibits more pronounced dissolution, erosion, and destruction characteristics.For example, at pH = 4, n = 25, the structure is severely fragmented and a large number of mineral grains have no cement attached and appear as isolated grains under the CFE-SEM.The microstructure of the red glutenite experienced continuous destruction under acidic wet-dry cycles.These changes could severely impact the physical-mechanical properties and the stability of the red glutenite.This is similar to the deterioration characteristics of sandstone and limestone as reported by Yuan et al. [40] and Li et al. [41], respectively.Above, the evolution of microstructural features of the red glutenite was analyzed by observing the CFE-SEM images.This approach follows the qualitative analysis of the deterioration characteristics of rock microstructures performed by many scholars [30][31][32]40], However, there is a notable lack of quantitative research in this area.Below, the evolution of microstructural parameters of the red glutenite will be analyzed using image processing techniques.The CFE-SEM images were binarized and vectorized (Fig. 12) to obtain various parameters including microporosity, average shape factor, and fractal dimension.
(1) Microporosity Porosity is defined as the proportion of pore volume within a specimen relative to the overall volume of the specimen, which directly reflects the degree of compactness of the specimen [42].Porosity in this paper refers to the ratio of pore area to total area in the twodimensional plane.Changes in this two-dimensional porosity indirectly reflect changes in the three-dimensional porosity of the specimen.Figure 13 displays the variation pattern of porosity with the number of wet-dry cycles under different pH conditions.According to Fig. 13, as the pH of the acid solution remains the same, porosity increases as the number of wet-dry cycles increases.With a constant number of wet-dry cycles, microporosity shows an increasing trend with the decrease in the pH value of the acid solution.This conclusion corresponds to the experimental conclusions on DD and LWV, further confirming that wet-dry cycles result in a decrease in specimen compactness.Moreover, higher solution acidity intensifies the impact on the decrease in specimen compactness.Increased microporosity is an important cause of increased strain in the compaction stage and decreased DD, LWV, and UCS.
(2) Average shape factor The shape factor is usually used to describe the morphological characteristics of two-dimensional shapes [43].In this study, it reflects the complexity of pore geometry.It is defined as: where T i is the shape factor of the pore, B b is the equal area circular circumference of a pore, S a is the actual (6) 12 Image processing (n = 0 as an example) Fig. 13 The variation of microscopic porosity with the number of wet-dry cycles circumference of a pore, x is the number of pores, and T is the average shape factor.
The average shape factor is calculated by averaging the shape factors of all pores in an image.According to Eqs. ( 6) and ( 7), the shape factor is equal to 1 when the pore is perfectly circular.The shape factor ranges from 0 to 1, with more complex pore shapes corresponding to smaller values.
Figure 14 illustrates the variation of the average shape factor with the number of acidic wet-dry cycles.As seen in Fig. 14, the average shape factor of pores decreases as the number of wet-dry cycles increases under the same pH condition.This indicates that wetdry cycles increase the complexity of pore geometry. (

3) Fractal dimension
The fractal dimension is commonly employed to characterize the variation in the complexity of a pore system with its area and can reflect the self-similarity of the pore system [44].If the pore system exhibits fractal characteristics, the pore area, perimeter, and fractal dimension conform to the following relationship: where R 1 is a constant, S is the pore area, R is the perimeter, D f is the fractal dimension.
The variation in the fractal dimension with the number of wet-dry cycles is shown in Fig. 15.The fractal dimension gradually increases as the number of wetdry cycles increases, indicating that the pore structure (8) lg R = D f /2 • lg S + R 1 becomes increasingly complex and irregular, deviating from its original self-similarity.
The above analysis explores the evolution of the microstructure of the red glutenite from three perspectives: microporosity, average shape factor, and fractal dimension.This provides a fresh perspective on studying the damage characteristics of rocks under acidic wet-dry cycles.
To analyze the effect of the number of wet-dry cycles on the mineral composition of the red glutenite, the variation of the relative content of each mineral component is plotted based on the XRD test results, as shown in Fig. 17.
As seen in Fig. 17, an increase in the number of wetdry cycles leads to an increase in the relative content of quartz, while the relative content of albite, calcite, and illite experiences a decrease.On the other hand, the relative content of zeolite remains relatively stable without significant changes.Compared to its initial state at 0 wet-dry cycles, the relative content of quartz exhibited a notable increase of 28.8% after 25 wet-dry cycles.Conversely, the relative content of albite, calcite, and illite experiences significant decreases of 46.1%, 45.9%, and 68.6%, respectively.The most notable alteration is observed in the illite content, followed by albite and calcite, while the change in quartz content is quite minor.This finding demonstrates that the impact of acidic wet-dry cycles on various minerals varies.The primary cause for the significant reduction in the Fig. 14 The variation of the average shape factor with the number of wet-dry cycles Fig. 15 The variation of fractal dimension with the number of wet-dry cycles proportion of illite may be attributed to the acceleration of illite disintegration and dissolution resulting from acidic wet-dry cycles.This process induces alterations in the crystal structure of illite, leading to a decline in its overall content.The decline in the relative proportions of albite and calcite can be attributed to their vulnerability to oxidation and disintegration during acidic wet-dry cycles.Meanwhile, the quartz content shows an increasing trend.This is because the In order to investigate the effect of solution pH on the mineral composition of the red glutenite, XRD analysis was performed on the red glutenite specimens subjected to 25 cycles of wet-dry cycles at different pH conditions (pH = 4.0, 5.5, 7.0).The results are shown in Fig. 18.
The relationship between the pH value of the solution and the relative content of each mineral component is depicted in Fig. 19, utilizing data from XRD tests.
Figure 19 reveals that as the acidity of the solution increases, the relative content of quartz increases, the relative content of albite, calcite, and illite decreases, and the relative content of zeolite remains relatively unchanged.When the pH of the solution decreases from 7.0 to 4.0, the relative content of quartz increases by 12.0%.Additionally, the relative contents of albite, calcite, and illite were decreased by 21.7%, 50.1%, and 44.6%, respectively.The content of calcite and illite exhibits the most notable changes, followed by albite, whereas the relative change Fig. 17 The variation of the relative content of mineral components with the wet-dry cycles Fig. 18 XRD patterns under different pH conditions in quartz content is comparatively minor.This suggests that albite, calcite, and illite are more susceptible to reacting with acidic ions and undergoing mineral dissolution in an acidic solution, while quartz remains relatively stable and less prone to dissolution.Consequently, as the acidity of the solution increases, the relative content of albite, calcite, and illite decreases, while the relative content of quartz increases.

Damage mechanisms
The physical-mechanical properties of rock are mainly determined by its mineral composition and structure.Based on the CFE-SEM test results, it is evident that under the action of repeated acid wet-dry cycles, the microstructure of the red glutenite undergoes continuous changes, and the interstitial materials between some skeleton particles are continuously lost.The micro-cracks and pores are developed and connected, and the integrity is gradually deteriorated.The porosity increases, and the cement transitions from being uniform and dense.Furthermore, XRD test results indicate that the mineral composition of the red glutenite is also constantly evolving.The primary factors contributing to alterations in the mineral composition and microstructure of the red glutenite are solution-rock physical interactions, solution-rock chemical interactions, and desiccation.

Solution-rock physical interactions
There are three main types of solution-rock physical interactions between the solution and the red glutenite specimen: (a) Lubrication and softening effect: Water molecules from the acidic solution can penetrate into the red glutenite specimen altering the connections between mineral particles by lubricating the contact surface between them inside the red glutenite specimen.This causes the structure inside the red glutenite specimen to become loose and the density to decrease, making the red glutenite specimen more prone to damage from external forces.(b) Scouring and transport effects: The presence of water molecules inside an acidic solution has the potential to dissolve the cement and debris present in the red glutenite specimen.This dissolution process leads to a decrease in the connection between mineral particles.(c) The splitting effect of pore water pressure: Water molecules can penetrate the pores of the red glutenite specimen in acidic solutions.As the number of water molecules gradually increases, pore water pressure forms, exerting tremendous pressure on the interior of the red glutenite specimen.This pressure causes the splitting and destruction of the red glutenite specimen.

Solution-rock chemical interactions
Solution-rock chemical interactions alter both the mineral composition and the microstructural characteristics of the specimen's particles, pores, and fissures, as evidenced by the CFE-SEM and XRD test results.According to the XRD analysis, the primary minerals present in the red glutenite specimen are quartz, albite, calcite, and illite.Quartz (SiO 2 ) exhibits the highest stability in acidic environments.However, albite (NaAlSi 3 O 8 ), calcite (CaCO 3 ), and illite (K(Al 4 Si 2 O 9 (OH) 3 )) are more prone to reacting with acidic solutions.The reaction equations are as follows: In addition, certain cations (e.g., K + , Na + , Ca 2+ , etc.) present in the red glutenite specimen readily react with OH − in water, leading to the decomposition of the original minerals and the formation of new minerals.These newly formed minerals are highly soluble in water, ultimately leading to a loss of mineral components within the red glutenite specimen.Concurrently, the internal (9) Fig. 19 The variation of the relative content of mineral components with pH pores of the specimen continue to develop, increasing the contact surface area for hydrolysis reactions, thereby exacerbating the hydrolysis reaction [7,10].This iterative effect gradually weakens the structural integrity of the red glutenite specimen.X-ray diffraction analysis of the red glutenite specimen and the chemical reaction equations involving mineral composition and acid solution indicate that the primary influence of an acidic environment on the specimen is manifested in the alteration of albite, calcite, and illite.The stronger the acidity of the solution, the stronger the chemical reaction.At the macroscopic level, the UCS and EM of the red glutenite specimens exhibit a more significant decrease in a highly acidic environment.

Desiccation
The specimen underwent repeated cycles of wetting and drying.During the drying process of the specimen, water molecules carried mineral particles and secondary minerals generated through solution-rock interactions, seeping out along micro-cracks and pores.This process resulted in the formation of new secondary cracks, which in turn provided additional reaction surfaces for further solution-rock interactions.The repetitive wet-dry cycles cause frequent expansion and contraction of the red conglomerate specimen, leading to decreased inter-particle bonding strength and reduced friction between mineral particles.This facilitates the development of cracks and pores in the specimen, thereby easing the penetration of water solutions into the interior [25,27,28].
During the repeated process of solution-rock physical interactions and desiccation, the rock specimens undergo absorption and evaporation of moisture, leading to impact and friction between the particles.This process gradually expands the gaps between the particles, thereby forming or enlarging cracks and pores.As the increase in the number of wet-dry cycles, DD, LWV, UCS, and EM gradually decrease.This decrease is attributed to the increase in pores and cracks, which weakens the physical contact and mechanical interactions among the rock particles.Solution-rock chemical interactions result in an increased proportion of soluble fillers dissolving, which promotes the expansion and aggregation of cracks and pores, and increases the contact area between the solution and the specimen, as well as the intensity of the corrosion reaction.As the acidity of the solution increases, the solution-rock chemical interactions become more intense, leading to an increase in the rate of decay of DD, LWV, UCS, and EM.

Conclusions
(1) During acidic wet-dry cycles, the variations in DD, LWV, UCS, and EM observed in the red glutenite specimens are remarkably consistent.At a constant pH value, repeated wet-dry cycles damage the internal structure of the red glutenite specimens by lubricating, softening, scouring, migrating, and pore water pressure.This leads to a decrease in specimen DD, LWV, UCS, and EM with an increase in the number of wet-dry cycles.Under the same wet-dry cycles, acidic solutions demonstrated a corrosive impact on the red glutenite specimens, accelerating the formation and development of internal cracks and pores.As acidity increases, this corrosive impact becomes more pronounced, leading to a reduction in specimen DD, LWV, UCS, and EM as the pH values decrease.(2) The UCS prediction model is established based on the exponential decay function, reflecting conditions during acidic wet-dry cycles.This model reasonably explains the relationship between UCS, wet-dry cycles, and solution pH.This model holds significant reference value for predicting the strength of red glutenite in engineering.(3) Based on CFE-SEM and image processing technology, it has been observed that acidic wet-dry cycles alter the microstructure of the red glutenite.Acidic wet-dry cycles cause a continuous reduction in cementation between mineral particles, an increase in microporosity, and an increasingly complex pore structure.In addition, heightened solution acidity accelerates the deterioration rate of the microstructure.(4) The relative content of quartz increases while albite, calcite, and illite decrease as the number of wet-dry cycles increases under acidic conditions (pH = 4.0).After 25 cycles, the quartz content increases by 28.8%, while albite, calcite, and illite decrease by 46.1%, 45.9%, and 68.6%, respectively.As the solution pH decreases from 7.0 to 4.0, the quartz content increases by 12.0%, while albite, calcite, and illite decrease by 21.7%, 50.1%, and 44.6%, respectively.Acidic environments and wet-dry cycles significantly affect the mineral components of the red glutenite.(5) The primary causes of the red glutenite damage are solution-rock interaction and desiccation, which interactively promote constant microstructural and mineral compositional changes.These actions ultimately alter the macroscopic physical-mechanical properties of the red glutenite.
Experiments involving wet-dry cycles under various acidic conditions were conducted on the red glutenite of Mount Wuyi to study its physical-mechanical properties.Although the destructive effects of different acidic wet-dry cycles on the red glutenite varied, all accelerated its degradation.Therefore, targeted measures should be implemented to protect Danxia landform rocks based on specific environmental characteristics.Emphasizing environmental protection, reducing harmful gas emissions, and controlling acid rain formation are effective strategies for preventing acid rain damage to Danxia landforms.This study provides new insights into the prevention of acid rain damage to these unique cultural and natural heritage sites.
Figure 3 provides a detailed schematic diagram of the test procedure.

Fig. 3
Fig. 3 Schematic diagram of the test procedure

Fig. 10
Fig. 10 The variation of EM

Table 1
Fitting parameters a and b

Table 2
Deterioration degree