Sandstone in the Longshan Grottoes was the research object of this paper. The sandstone samples were soaked in distilled water, Na2SO4 solution and NaCl solution and subjected to 30 cycles of freeze-thaw testing. The quality, wave velocity, surface hardness, uniaxial compressive strength and other physical indices of rock samples were measured after different numbers of cycles. Combined with the results of mercury injection, scanning electron microscopy and X-ray diffraction, the damage characteristics and damage mechanism of rock samples under the combined action of chemical solution and freeze-thaw cycle were explored and studied. The results showed that the degree of damage of the sandstone samples increased logarithmically with an increase in the number of cycles. The damage degree of the sandstone samples soaked in the Na2SO4 solution was the highest, while the damage degree of the samples soaked in the NaCl solution and distilled water was similar. The difference in damage between the solutions is caused by the combination of frost heave, dissolution and salt crystallization, and the damage process generally occurs from the inside and the outside and is manifested as an increase in the number of macropores and the appearance of new pores. Freezing-thawing and salt erosion are the main reasons for the siltation of sandstone in the Longshan Grottoes.
Research Article
Study on the characteristics of damaged sandstone in the Longshan Grottoes using water chemistry and freeze-thaw cycling
https://doi.org/10.21203/rs.3.rs-2446332/v1
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Longshan Grottoes
sandstone
freeze-thaw cycle
chemical solution
damage characteristics
The Longshan Grottoes, which are the largest pure Taoist grottoes in China, were first built during the Tang Dynasty. The Longshan Grottoes have been exposed to air for a long time. With changes in the environment in the grotto area, the surface sandstone of the grottoes has become powdery, flaky and layered. This condition is generally observed in sandstone grottoes in Taiyuan and even Shanxi. Given the observed weathering of grotto sandstone, the need to study the weathering mechanism of grotto sandstone through indoor accelerated tests has become urgent.
Much research has been conducted on the influence of rock in the freeze-thaw process. In foreign countries, Nicholson [1], Khanlari [2], Kolay [3], Kodama [4] and others selected different kinds of rocks or different water contents to conduct freeze-thaw cycle tests to study the influence of the rock state on its strength. In China, Yang Hongrui et al. [5] conducted freeze-thaw cycle tests on the glutenite in Maijishan under three temperature intervals to explore the influence of temperature intervals on the freeze-thaw deterioration of the rock. Fang Yun [6] conducted freeze-thaw cycle tests on sandstones in Yungang Grottoes under different water-containing states, compared the changes in rock sample quality, wave velocity, compressive strength and microstructure, and summarized the main physical and mechanical properties of sandstones in Yungang Grottoes under cyclic freezing-thaw conditions. Hao Li [7] and Lin Y [8] studied the influence of chemical erosion on macro and micro indices of rock by setting solutions with different pH values and conducting nuclear magnetic resonance (NMR) and mechanical tests. Han Tielin [9] and Yu Jin et al. [10] discussed the water-chemical corrosion mechanism of sandstone samples by comparing the physical indices of sandstone soaked in different water-chemical solutions. Zhang Fengrui [11], Du Peng [12] and Xie Ni et al. [13] also conducted relevant studies on the damage effects of chemical solutions and freeze-thaw, but their research was more focused on objects of rock mechanics engineering such as tunnels and rock slopes in cold regions. In summary, many scholars have conducted research on the damage process of rocks under the action of single factors such as freeze-thaw and chemical erosion. However, the related research on the combined action of freeze-thaw and chemical erosion, especially the impact on cultural stone relics, has been limited.
This paper provides a method for establishing indoor accelerated test conditions based on environmental data such as temperature, humidity and rainfall in combination with conditions needed for the storage of cultural relics. In addition, to the best of our knowledge, this paper is the first to discuss the weathering characteristics and mechanism of the sandstone in the Longshan Grottoes. Based on the environmental monitoring data of the Longshan Grottoes, the author optimized the research methods of other scholars to make the test environment more similar to the field environment. The quality, wave velocity, surface hardness and compressive strength of the sandstone samples before and after cycling were measured by freeze-thaw tests with different immersion solutions and different cycles. The damage characteristics and damage mechanism were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and mercury injection (MIP) tests. The information collected here will provide some references for the subsequent protection of the Longshan Grottoes.
2.1 Research background
The Longshan Grottoes are located at the top of Longshan Mountain, Jinyuan District, Taiyuan City (Fig. 1a). The annual average temperature in the study area is 10.9°C, the extreme low temperature in winter is − 22°C, the extreme high temperature is 19.7°C, the number of days below 0°C is 205, the annual relative humidity is 56% and the average rainfall is 759 mm. After investigation, it appears that the Longshan Grottoes have suffered serious weathering, with the surface color of the statues altered and blurred. Flaky and layered spalling is well developed outside the caves. The thickness of the spalling layer is approximately 2\(–\)4 cm, and the rock mineralization of the cave body is very obvious (Fig. 1b–e). The weathering rate of the cave body in the south is up to 3 g/(m2·d). The weathering rate of the Longshan Grottoes is very fast and is greatly affected by the environment.
In winter, three snow water samples (1#, 2# and 3# are replicate samples) were collected in the Longshan area. The inductively coupled plasma emission spectrometer was used for the cation test, and the ion chromatograph was used for the anion test. The major anions are shown in Table 1. The cations were mainly Na+ and Ca2+, and the SO42- content was the highest among the anions, with a maximum concentration of 17.31 mg/L (0.18 mmol/L). Therefore, under the influence of a prolonged frost period and atmospheric precipitation, freeze-thaw and chemical damage are the important causes of weathering of sandstone in the Longshan Grottoes.
| Name | anion | cation | ||||||
|---|---|---|---|---|---|---|---|---|
| F− | Cl− | SO42− | \(\text{N}{\text{O}}_{3}^{-}\) | Na+ | Ca2+ | Mg2+ | K+ | |
| 1# | 0.1060 | 0.8267 | 2.7263 | 0.5882 | 1.319 | 1.646 | 0.2966 | 0.3122 |
| 2# | 0.0021 | 0.6705 | 17.3135 | 0.4248 | 9.465 | 0.597 | 0.2491 | 0.4785 |
| 3# | 0.0207 | 0.5757 | 9.6146 | 0.5123 | 4.593 | 1.538 | 0.3322 | 0.2511 |
2.2 Sample preparation and testing
Rock samples were taken from the rock mass around the Longshan Grottoes with the same lithology and weathering conditions. In accordance with the standard [14], the drill core was processed into cylindrical samples with a diameter of 50 mm and a height of 50 mm. Some samples are shown in Fig. 2. A non-metallic ultrasonic tester (model: NM-4B) was used to measure the wave velocity of rock samples. A Riehl hardness tester (model: BH200C, probe type D) was used to test the surface hardness of rock samples. The uniaxial compressive strength was measured using an electro-hydraulic universal testing machine (model: CSS-WAW 1000DL). The initial physical parameters of the sandstone are shown in Table 2.
| Dry density (g·cm-3) | Water absorption (%) | Porosity (%) | Surface hardness (HL) | Wave velocity (km·s-1) | Compression strength (MPa) | ||
|---|---|---|---|---|---|---|---|
| Drying regime | Native state | Saturation state | |||||
| 2.28 | 4.65 | 10.88 | 506 | 2.214 | 24.83 | 7.52 | 5.02 |
A powder crystal X-ray diffractometer (model: Dmax2600) was used for sample analysis, and the percent content of whole-rock minerals and clay minerals in the sandstone of the Longshan Grottoes was obtained, as shown in Tables 3 and 4. The results show that the non-clay minerals mainly consist of quartz with a small amount of albite and calcite, while the content of clay minerals is represented by illite.
| Rock lithology | Quartz | Albite | Calcite | Illite |
|---|---|---|---|---|
| Sandstone | 65.4 | 5.2 | 3.1 | 26.3 |
| Rock lithology | The relative content of clay minerals (%) | Mixed layer ratio (%) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| S | I/S | It | Kao | Chl | C/S | Cor | I/S | C/S | |
| Sandstone | / | / | 100.0 | / | / | / | / | / | / |
2.3 Freeze-thaw test scheme
In this freeze-thaw test, soluble salts (Na2SO4, NaCl) composed of main ions in atmospheric precipitation and distilled water were selected as the infiltrating solution, and the concentration was based on the concentration of SO42−, which was increased by a thousand times to 0.5 mol/L. Combined with temperature monitoring records, freezing and thawing cycles were performed at − 20°C to 25°C.
Freeze-thaw tests were conducted in a rapid temperature change humid heat test chamber (model: RHPS-408BT). Taiyuan climate belongs to the temperate continental monsoon climate. As the precipitation frequency is low, the rock mass does not undergo only a single freeze-thaw process following a precipitation event. Therefore, in this paper, the method of multiple freeze-thaw cycles with one soak process was adopted. All the grouped samples were soaked in the corresponding solution for 48 h and then put into the test chamber for five cycles. The freeze-thaw cycle process is shown in Fig. 3. The freeze-thaw cycles were set as 0, 5, 10, 15, 20, 25 and 30 times. The samples were tested for quality, p-wave velocity, Richter hardness, uniaxial compressive strength, MIP, SEM and XRD following freezing-thawing. Mercury injection was determined by an automatic mercury injection instrument (model: PoreMaster GT-60), and the micro-morphology was observed using Hitachi's new field emission scanning electron microscope (model: Regulus 8230).
3.1 Change in mass ratio
Changes in the mass ratio (the ratio of mass after circulation to mass before circulation) are shown in Fig. 4. The results show the following: 1) The sample mass of the Na2SO4 solution decreased the most, the decline rate gradually increased, and the mass ratio reached 78.03% after 30 cycles. The sample mass in water showed a trend of rapid change followed by slow change, and the sample mass loss was only 0.23% after 30 cycles. In the NaCl solution, the quality of the rock samples increased at first and then started to decrease after 15 cycles. 2) Within the same cycle, the mass loss of samples soaked in the Na2SO4 solution was much greater than that of samples soaked in the other solutions.
3.2 Changes in wave velocity
Changes in the longitudinal wave velocity of samples under different freeze-thaw conditions are shown in Fig. 5. The results show the following: 1) The P-wave velocity of the three groups of samples decreased with an increase in the number of cycles. 2) Throughout the whole cycle, the change in the longitudinal wave velocity of the sample after immersion in different solutions varied from large to small in the order of Na2SO4 solution, NaCl solution and distilled water. 3) The wave velocity of the rock samples in the Na2SO4 solution showed a decreasing trend of fast–slow–fast–slow. The variation in the wave velocity of the sample in the distilled water and NaCl solution was similar, and the trend was linear.
3.3 Changes in surface hardness
Changes in the surface hardness of each group of samples under different conditions are shown in Fig. 6. The results show the following: 1) The surface hardness values of the three groups of rock samples changed to different extents after the different cycles, and the trend was an exponential decline. 2) Throughout the whole cycle, the change in the surface hardness values of samples after wetting with different solutions varied from large to small in the order of Na2SO4 solution, NaCl solution and distilled water. 3) Under the same number of freeze-thaw cycles, the hardness values of the samples in different solutions varied from small to large in the order of distilled water, NaCl solution and Na2SO4 solution.
3.4 Changes in uniaxial compressive strength
The relationship between the number of cycles and uniaxial compressive strength under different freeze-thaw conditions is shown in Fig. 7. The changes are as follows: 1) The uniaxial compressive strength of the three groups of samples all decreased to different extents, the compressive strength of the samples in the Na2SO4 solution decreased exponentially, and the distilled water and NaCl solution showed a linear downward trend. 2) Within the same cycle period, the change in the uniaxial compressive strength of samples after freezing and thawing in different solutions was greatest for the Na2SO4 solution, followed by the NaCl solution and then the distilled water. 3) Under the same number of cycles, the degree of damage caused by freezing-thawing to samples after soaking in different solutions increased from low to high in the order of distilled water, NaCl solution and Na2SO4 solution.
3.5 Pore variation
3.5.1 Porosity change
The change in sandstone porosity was obtained using the MIP test (Fig. 8). The experimental results show the following: 1) With an increase in the number of cycles, there is an obvious increase in the porosity obtained by mercury injection, which shows a logarithmic trend. 2) During the cycle, the porosity of the Na2SO4 solution exhibits the greatest increase, followed by the NaCl solution and then distilled water. After 30 cycles, the sample porosity in the Na2SO4 solution increased from 11.07–16.86%, and the porosity in the distilled water and NaCl solution increased to 14.52% and 15.29%, respectively.
3.5.2 Pore radius distribution
The pore size distribution curve obtained from the mercury injection test is shown in Fig. 9. Using the mercury injection test results of Xiaobo [15] on tight sandstone, the pores were divided into micropores (0.1 µm), mesopores (0.1–1.0 µm), macropores (1–10 µm) and cracks (10 µm) according to their pore size.
The results show the following: 1) With an increase in the number of cycles, the number of pores and cracks with a pore size greater than 10 µm increased significantly in different solutions, and the proportion of mesopores and micropores was unchanged. This shows that under the action of freezing-thawing, pores in the rock expand to form larger pores, they connect to develop into cracks, and at the same time, new pores are generated. 2) In different solutions, the greatest number of cracks was observed for the Na2SO4 solution, followed by the NaCl solution and distilled water. 3) Damage from the freeze-thaw cycle was manifested in the rock samples as an increase in the number of macropores, the development of medium and small pores and the "birth" of new pores.
4.1 Qualitative injury analysis
4.1.1 Microstructural changes
To reveal the changes in the microstructure of the Longshan sandstone following five, 15 and 30 freeze-thaw cycles while immersed in three different solutions, 1000× SEM was performed on the surface of the sandstone samples, and the results are shown in Figure 10.
Figure 10 shows the following: 1) Under the freeze-thaw condition of the rock soaked in the distilled water, the initial change in the sample was small, and only some microcracks could be observed. Later, with an increase in the number of freeze-thaw cycles, microcracks gradually developed after 15 cycles, and the holes increased in size, forming linear cracks. Finally, after 30 cycles of freezing-thawing, the linear cracks expanded and the holes connected. 2) Under the freeze-thaw condition of the rock soaked in the Na2SO4 solution, a large number of pores developed on the sample surface. After 15 freeze-thaw cycles, the pores connected and formed a large number of linear fissures. Finally, after 30 freeze-thaw cycles, erosion on the mineral surface occurred, the linear fissures expanded, and pores connected and fused. 3) Under the NaCl solution, holes appeared on the sample surface after five cycles, linear cracks were formed after 15 cycles and cracks expanded after 30 cycles. However, the extent was smaller than that observed with the Na2SO4 solution.
4.1.2 Changes in mineral composition
Figure 11 shows the relative content analysis results of mineral components in the rock samples after freeze-thaw cycles in different solution environments. The results show the following: 1) Under the freeze-thaw effect of distilled water, the content of both feldspar and calcite in the samples decreased, and the relative content of feldspar decreased by 0.9%, while calcite disappeared. The quartz content increased and the clay mineral is also lost. 2) Under the condition of freezing-thawing while immersed in the Na2SO4 solution, the calcite in the sample also reacted and disappeared, and the relative content of feldspar decreased more significantly than in the distilled water, with only 0.6% remaining. The relative content of quartz increased more than in the distilled water, and the loss of clay minerals also occurred. 3) In the NaCl solution, the relative content changes of all minerals were the same as in the distilled water and Na2SO4 solution, and the relative content of feldspar finally decreased by 1.9%, which is between the values observed for the other two solutions.
4.2 Quantitative analysis of damage based on porosity
According to the analysis results of the microstructure and mineral composition, the damage of the sample under the action of freeze-thaw and chemical erosion is related to the increase in the number of pores and the connectivity of the pores. This is consistent with the research results of Wuxiu [16] and Kai [17-18]. The change in the porosity reflects the deterioration in the sample. The damage variable D[16] was established based on the change in the porosity, and the degree of damage to rocks under different chemical solutions and freeze-thaw action was quantitatively evaluated. The relationship between the damage variable and porosity is as follows:
In this formula, n0 is the porosity of the original rock, and nt is the porosity of the sample after n cycles. Damage variables of cycles under different freeze-thaw conditions were calculated using the above formula and porosity, as shown in Figure 12. The results show the following: 1) The damage variable D of the three groups of the sandstone samples soaked in different solutions exhibited a trend of logarithmic increase with an increase in the number of freeze-thaw cycles. 2) Over the whole cycle, the damage variable of samples after wetting with different solutions varied as follows: Na2SO4 > NaCl > D H2O.
To clarify the relationship between the microscopic damage variable and the macroscopic index, the damage variable D and the change in the wave velocity, surface hardness and uniaxial compressive strength were fitted by a function. As can be seen from the regression results (Figure 13), the fitting curves were all linear functions with correlation coefficients above 0.917 under the three freeze-thaw conditions. The results show that there is a significant quantitative relationship between the macroscopic damage index and the porosity of the microstructure under different freeze-thaw conditions.
4.3 Weathering mechanism
The above test results show that freeze-thaw cycles under different solution conditions, cause cumulative damage to the macroscopic properties, microscopic results and composition of the sample, which increases with an increase in the number of freeze-thaw cycles. Differences between the solutions are caused by the different combinations of frost heave, dissolution and salt crystallization, which result in the different number and connectivity of internal pores and pores.
The damage to sandstone in distilled water is mainly caused by frost heave and dissolution. After 30 cycles, there was no apparent change in the rock sample, and the mass loss rate and surface hardness decreased by 0.23% and 85 HL, respectively. The wave velocity and uniaxial compressive strength decreased greatly, with the wave velocity decreasing by 374 m/s, and the compressive strength by up to 50%. This means that the water's influence on the rock is more internal. When frozen, the rock mineral particles shrink, but different types of minerals shrink and expand differently, and the rock changes unevenly under the action of freezing and thawing. At the same time, the water-ice phase transition generates a 9% volume expansion [19], and the frozen rock contains various pores and tiny cracks of different sizes. The frost heave force generated further expands these pores and cracks (as can be seen from the SEM and MIP test results), and eventually causes rock damage. Feldspar minerals such as albite in sandstone are hydrolyzed in solution, according to the following reaction:
Calcite in sandstone is also chemically dissolved:
After several cycles, the small amount of calcite in the sandstone disappears, albite hydrolyzes and clay minerals are lost to a certain extent. As a result, holes, cracks and connectivity occur continuously on the surface and inside of the sample, resulting in surface dissolution. This is consistent with the results of the XRD analysis completed after the cycle. No calcite was detected after five freeze-thaw cycles, and the relative proportion of albite decreased with the increase in the number of cycles.
The damage to rocks caused by the NaCl solution is also small and only slightly greater than that caused by the distilled water, which is mainly due to frost heave and dissolution. After the completion of the test, there was no obvious change in the rock appearance, and the development degree of pores and cracks was greater than that of the rock samples in the distilled water. Furthermore, the relative content reduction of feldspar in the quantitative mineral analysis was also greater than that in the distilled water, indicating that the presence of NaCl increases the damage to the rock by freeze-thaw action.
The cumulative damage of the sandstone samples in the Na2SO4 solution is mainly caused by the joint action of frost heave, dissolution and salt crystallization. The frost heave and dissolution mechanisms are described above. When the temperature decreases during the freeze-thaw cycle, Na2SO4 in the sandstone pores absorbs water to form Na2SO4·7H2O and Na2SO4·10H2O crystals [20], and the crystal volume increases, which causes varying degree of damage to the rock samples in the initial Na2SO4 solution much greater than that of the rocks soaked in the other two solutions. The decreasing wave velocity and uniaxial compressive strength in the later period are due to the continuous expansion of pores, but Na2SO4 crystals cannot fill the entire pores, which leads to a decreased damage effect on rock samples. With the gradual expansion of the internal pores and fissures, the surface of the rock sample becomes rough on the inside and the outside, and the hardness is greatly reduced, and then the edge of the sample is damaged and pulverized. This is consistent with the condition observed in the grottoes, indicating that the weathering of the grottoes is mainly caused by sulfate and freeze-thaw action.
(1) The sandstone samples soaked in different solutions exhibit different degrees of damage under freeze-thaw action, and the quality, wave velocity, surface hardness and compressive strength of the sandstone tend to decrease with an increase in the number of cycles. The damage degree decreases in the order of Na2SO4 > NaCl > D H2O.
(2) Under different freeze-thaw conditions, the porosity of the rock samples increases with an increase in the number of cycles. The degree of damage imparted to the rock samples is reflected in the development and expansion of existing pores and the emergence of new pores. The damage variables established based on porosity in different solutions differ and decrease in the order of Na2SO4 > NaCl > D H2O. The fitting functions of the damage variable and wave velocity, surface hardness and compressive strength under different freezing-thaw conditions with different solutions are linear, and the R2 values are all greater than 0.917.
(3) The damage to the sandstone samples soaked in the distilled water and NaCl solution is mainly caused by frost heave and dissolution, while the rock samples in the Na2SO4 solution are affected by the combined action of frost heave, dissolution and salt crystallization.
(4) The destruction of the rock samples by sulfate is an internal and external process, which will eventually cause the pulverization and shedding of sandstone in the Longshan Grottoes. Freezing-thawing and salt erosion are the main causes of the pulverization of sandstone in the Longshan Grottoes.
SEM-- Scanning electron microscopy
XRD-- X-ray diffraction
MIP-- Mercury injection
UCS-- Uniaxial compressive strength
Availability of data and materials
All data generated or analysed during this study are included in this published article [and its supplementary information files].
Competing interests
The authors declare that they have no competing interests.
Funfing
This work was supported by the National Natural Science Foundation of China (No.52068050, No.41562015) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China(No.2017IRT17_51).
Authors' contributions
SUN Bo, CUI Kai and PENG Ningbo provided funding and revised the protocol, LI Xingyue performed the trial and wrote the main manuscript text, and HONG J.ie and CHEN Rui assisted with conducting the trial. All of the authors reviewed the manuscript.
Acknowledgements
Not applicable.
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No competing interests reported.