Study on the Protective Effect of Organosilicon Materials on Weathered Red Sandstone of the Leshan Giant Buddha

Study on the Protective Effect of Organosilicon Materials on Weathered Red Sandstone of the Leshan Giant Buddha

Meng Yuan

Faculty of Architecture and Civil Engineering Huaiyin Institute of Technology, Huaian, China

Ningbo Peng

Faculty of Architecture and Civil Engineering Huaiyin Institute of Technology, Huaian, China

Bo Sun

Northwest Research Institute Co. Ltd. of China Railway Engineering Corporation, Lanzhou, China

Fengrui Wang

Key Laboratory of Mechanics on Disaster and Environment in Western China, the Ministry of Education of China School of Civil Engineering and Mechanics, Lanzhou University, Lanzhou, China

Abstract – Aiming at the weathered red sandstone of the Leshan Giant Buddha, four organosilicon protective materials including Wacker BSOH-100, Remmers KSE-300, fluorosilane KY-60 and modified siloxane were selected for reinforcement and protection in this study. The protective effects of the above four organosilicon materials on the highly weathered red sandstone of Leshan were comprehensively evaluated through indoor reinforcement performance tests and accelerated aging tests of sunlight and rainfall. The results showed that the red sandstone of the Leshan Giant Buddha suffered from a high degree of weathering and severe weathering damage. For its weathering protection, anti-weathering reinforcement materials with strong permeability, remarkable cementation effect and good water resistance should be selected, among which KSE-300 exhibited the best protective effect. This study provides a feasible scheme for the protection of the weathered red sandstone of the Leshan Giant Buddha, and has important reference significance and popularization value for the scientific protection of the same type of large open-air sandstone cultural relics at home and abroad

Keywords – Leshan Giant Buddha; weathered red sandstone; organosilicon materials; cultural relic protection; differentiated protection

1. INTRODUCTION

   China boasts abundant grotto temple heritage, the main body of which is mostly carved out of mountain rocks. Under the natural environment, these stone cultural relics are continuously subjected to the coupling effect of multiple weathering factors such as physical, chemical and biological ones [1-3], leading to the increasingly severe weathering diseases including surface pulverization, particle shedding, crack development and biological erosion [4-5], which seriously endangers the safety and historical value of cultural relics and thus an effective protection is urgently needed. As the main host rock mass of numerous grotto temples, sandstone is particularly sensitive to water erosion and temperature

   changes due to its porosity and the easy dissolution of cementing materials, making it a hard-hit area of weathering damage [6-7]. Faced with this challenge, the application of protective materials for weathering reinforcement has become the mainstream technical approach for the protection of stone cultural relics at present.

   Figure 1 Weathering diseases of red sandstone of the Leshan Giant Buddha a Fracture development; b Biological diseases; c Rainwater erosion; d Spalling accumulation; e Crusting and warping;f Surface efflorescence

   The system of protective materials has been continuously developed, evolving from early inorganic materials such as lime water and silicate, to organic materials such as acrylic resin and epoxy resin, and now to organic-inorganic composite materials and high-performance organosilicon materials [8]. The core goal of the application of protective materials is to improve the mechanical strength and reduce the water absorption of sandstone, thereby enhancing its anti-weathering capacity. With their unique molecular structure and properties, organosilicon materials have the advantages of good compatibility with sandstone cultural relics, strong permeability, high stability, excellent hydrophobicity and weather resistance, showing remarkable advantages in the field

   of weathering protection of stone cultural relics and attracting extensive attention [9-12]. Internationally, Vicini et al. [12] evaluated the reinforcement effect and durability of siloxane products on Italian sandstone. Dunková L et al. [13] explored the reinforcement effects of three organosilicon materials on different types of sandstone. In China, Li Zuixiong et al. [14] successfully applied potassium silicate to reinforce the weathered sandstone of the Northern Grotto Temple in Qingyang, Gansu in the 1980s; the Sichuan Provincial Cultural Relics Bureau [15] adopted the KSE series products of Remmers Company of Germany to treat the Yuanguang Cave of Anyue Grottoes, which significantly improved the mechanical strength and surface hydrophobicity of the weathered sandstone; He et al. [16] discussed the protective effect of fluoropolymers on sandstone and confirmed that they could significantly improve the strength and water resistance of sandstone; the high molar ratio potassium silicate solution developed by Chen et al. [17] as a new type of inorganic consolidant significantly improved the mechanical strength and anti-salt crystallization capacity of weathered sandstone by virtue of its good compatibility and long-term effectiveness.

   Large grotto temple cultural relics are usually subject to a high degree of weathering due to their open-air location, the host rock mass mostly being soft sandstone suitable for carving, and long-term natural weathering and human damage [18-19]. Therefore, the demand for weathering protection is extremely urgent. In terms of the selection of protective materials, current studies mostly focus on the comparison between protective materials with different properties, lacking the horizontal comparison of protective materials with the same property applied in the same environment. When evaluating the protective effect, scholars mostly focus on the immediate performance improvement of sandstone after protection, while neglecting the long-term durability of protective materials. However, verifying the durability of protective materials in the natural environment has limitations such as a long cycle, difficult monitoring and potential damage to cultural relics. Therefore, researchers mostly adopt laboratory accelerated aging experiments to simulate the weathering effect of the natural environment [20-23] to evaluate the long-term durability of protective materials.

2. Experimental Materials and Methods
   1. Research Object

      Figure 2 Characteristics of the Leshan Giant Buddha and sandstone samples a Leshan Giant Buddha; b Microscopic image of weathered sandstone; c XRD

      diffraction pattern of weathered sandstone

      The Leshan Giant Buddha is built on the red sandstone mountain at the west cliff of Lingyun Mountain in Leshan City, Sichuan Province, at the confluence of the Minjiang River, Qingyi River and Dadu River. The area belongs to a subtropical monsoon climate with an average annual temperature of 17.2 , an extreme maximum temperature of

      38.6 , an average annual rainfall of 1384.8 mm and a relative humidity of 81%. The high temperature and humid climatic environment have caused severe weathering damage to the Leshan Giant Buddha [24,25]. Microscopic observation (Figure 2b) shows that the weathered red sandstone of the Leshan Giant Buddha has large surface particle size with irregular inlay, a large distribution of rock debris, serious loss of cementing materials between particles, and features a porous structure and weak cementation characteristics. Its mineral composition s mainly quartz and feldspar (Figure 2c). The basic physical properties and mechanical parameters of the weathered red sandstone of the Leshan Giant Buddha were determined under laboratory conditions. To ensure accuracy, the average value of 5 samples was taken as the measurement result for each group of parameters (Table 1)

      Table 1 Basic physical and mechanical properties of sandstone

      Taking the highly weathered red sandstone of the Leshan Giant Buddha as the research object, this study selected four organosilicon protective materials including Wacker BSOH- 100, Remmers KSE-300, fluorosilane (Sikang KY-60) and modified siloxane for reinforcement treatment. By systematically comparing the reinforcement effect and long- term durability, the weathering mechanism of sandstone was analyzed, and the action mechanism and failure mode of different organosilicon materials were revealed. Finally, the

      Gro up

      Stron gly weat

      Leeb hard ness (HL)

      Longit udinal wave velocity (km·s¹)

      Natur al water absorp tion (%)

      Satura ted water absorp tion (%)

      Compr essive strengt h (MPa)

      Cohesi on (MPa)

      Inte rnal frict ion angl e (°)

      47.1

      entropy weight-TOPSIS method was used for the comprehensive evaluation of protective effects to screen out

      here

      d sand

      312 1.430 8.65 12.89 2.93 0.34 0

      the optimal protective materials and protection schemes for the weathered red sandstone of the Leshan Giant Buddha.

      stone

   2. Protective Materials

      In view of the particularity of stone cultural relics, the selected protective materials must be colorless and transparent, without changing the appearance and morphology of cultural relics after curing, and have a chemical composition compatible with that of sandstone [26]. The names, sources, appearances, treatment methods and serial numbers of the four organosilicon materials used in this study are shown in Table 2.

      Serial number

      Table 2 Information of protective materials

      Name Source Product appearance

      Treatment method

      W Wacker

      BSOH-100

      K Remmers KSE-300

      Fluorosilane

      1. (Sikang KY- 60)
      2. Modified siloxane

      Wacker Chemie AG

      Remmers Company, Germany Sikang New Material Development Co., Ltd.

      Shanghai University

      Colorless to pale yellow liquid Colorless and transparent

      Colorless and transparent

      Colorless and transparent

      Brushing Brushing

      Brushing

      Brushing

   3. Sample Preparation

      The preparation, reinforcement and curing process of samples are shown in Figure 3. To avoid damaging the cultural relic body and ensure the rigor of the experiment, sampling was carried out in the area matching the strata of the two types of sandstone of the Leshan Giant Buddha. Firstly, cylindrical sandstone with a size of 70 mm × 500 mm was drilled on site, then cut into cylindrical samples with a

      Figure 4 Schematic diagram of experimental methods a Reinforcement performance test; b Air permeability test; c Contact angle test; d Sunlight and rainfall cycle test

      1) Reinforcement Performance Test

      The reinforcement effect was tested from the aspects of appearance characteristics, physical indicators, mechanical properties, microstructure and composition by the methods shown in Table 3.

      Table 3 Reinforcement performance test methods

      specification of 50 mm × 50 mm. After cleaning, a non- metallic acoustic wave detector was used to screen and group

      samples with similar longitudinal wave velocities. To ensure the full penetration of materials, the brushing method was

      Group Appear

      Test Test method

      index

      The unreinforced sandstone was taken as

      Test equipme nt

      LS175

      adopted with 3 times of brushing and an interval of 15 min each time. After completion, the samples were placed in a constant temperature and humidity chamber (temperature 24 , humidity 70%) for curing for 14 d according to the on- site curing conditions of materials simulated by the climate environment of Leshan. The cured samples were marked as SW, SK, SF, SG and SB, among which SB was the unreinforced sandstone as the control group of the experiment.

      Figure 3 Sample preparation process a Sampling; b Cutting; c Cleaning; d Screening; e Reinforcement; f Curing

   4. Experimental Methods

   To evaluate the protective effects of the four organosilicon materials on the two types of weathered red sandstone of the Leshan Giant Buddha, the experimental methods used are shown in Figure 4. Three parallel samples were set for each index, and the average value was taken as the result to ensure accuracy.

   ance

   charact eristics

   Physic al propert ies

   Mecha nical propert ies

   Micros tructur e and compo

   sition

   Color

   differenc

   e (E)

   Water absorptio n rate (%)

   Air permeabi lity reduction rate (%)

   Contact angle (°)

   Longitud inal wave velocity improve ment rate (%)

   Compres sive strength improve ment rate (%)

   SEM

   the standard color, and the surface color

   difference of the reinforced sandstone was measured by comparing with the standard color.

   Calculate the percentage of the difference between the dry mass of the reinforced sandstone and the mass before reinforcement in the mass before reinforcement.

   Adopt the wet cup method [27], seal the sandstone disc sample with a diameter of 5 cm and a height of 1 cm at the mouth of a glass bottle filled with 150 g of distilled water. Weigh it every 24 h until the change rate is less than 5%, and calculate the air permeability reduction rate of the reinforced sandstone relative to the original sandstone.

   Measure the static contact angle of water droplets on the surface of reinforced sandstone.

   Measure by the opposite measurement method under dry condition, set 3 parallel samples for each group, test each rock sample 3 times and take the average value, then calculate the longitudinal wave velocity improvement rate.

   Set the descending speed to 0.3 mm/min, set 3 parallel samples for each group and take the average value, then calculate the compressive strength improvement rate.

   Spray gold on the sandstone before testing, select a typical area and observe it at a magnification of 500 times.

   color

   differenc e meter

   JA5003

   electroni c balance

   JA5003

   electroni c balance

   HYB- 302

   manual contact angle tester

   NM-4B

   non- metallic ultrasoni c detector

   TAWY- 200

   universal testing machine

   S-3000N

   scanning electron microsco pe

   XRD Analyze after grinding the sandstone D8

   until there is no obvious sand feeling, set

   Discover

   w 1 ej

   the test voltage to 40 KV, the scanning speed to 5 °/min and the scanning angle to 5 °~60 °.

   X-ray powder diffracto

   meter

   1. Sunlight and Rainfall Test

      A xenon aging test chamber produced by Huai’an Zhongya Test Equipment Co., Ltd. was used for the sunlight and rainfall test. According to the meteorological data of Leshan City, the average annual temperature in the Leshan Giant Buddha area is 17.2 , the extreme maximum temperature is 38.6 , the multi-year average precipitation is 1291.6 mm, and the annual cumulative radiation is about 800-

      950 KWh/. Therefore, the experiment was set with 6 cycles,

      The entropy weight wj reflects the variation degree of the index data itself; the greater the variation, the richer the information and the higher the weight.

      Step 3: Constructon of the weighted decision matrix and determination of ideal solutions

      Combine the normalized matrix R with the weight vector to construct the entropy weight decision matrix (i=1,2,,m; j=1,2,,n), then define the positive ideal solution and negative ideal solution:

      each cycle with 30 periods, and one period included 60 min of

      illumination (irradiation intensity 800 W/m², temperature range 15 -35 ) and 5 min of spraying, simulating the weathering effect of rainfall after sunlight on sandstone in nature. This test is designed to simulate the natural weathering process of the Leshan Giant Buddha under the combined action of solar radiation and rainfall, so as to evaluate the long-term durability of protective materials in a short time.

      Step 4: Calculate the Euclidean distance from each evaluation object i to the positive ideal solution V+ and the Euclidean distance from V- to the negative ideal solution

   2. Evaluation of Protective Effect

   To scientifically evaluate the long-term durability of different organosilicon protective materials in accelerated

   aging, the entropy weight-TOPSIS method was used for comprehensive evaluation in this study. The core principle of the entropy weight-TOPSIS method [28-29] is to optimize the index weights by the entropy weight method, determine the

   Step 5: Calculate the TOPSIS score: Calculate the relative closeness between each evaluation object i and the ideal solution:

   S

   positive and negative ideal solutions of each index by the

   TOPSIS method, and sort the fitting degree by calculating the

   distance between the evaluation object and the positive and

   negative ideal solutions. The specific steps are as follows:

   Step 1: Construction and normalization of the original decision matrix

   Construct the original decision matrix with the performance index data (contact angle, mass loss rate, longitudinal wave velocity loss rate, compressive strength loss rate) measured after each cycle, where m is the number of evaluation objects and n is the number of evaluation indexes. Then normalize the original matrix to obtain the normalized decision matrix by the following formula:

   The value ranges from 0 to 1. A larger value indicates better comprehensive durability of the material and closer to the ideal state.

3. RESULTS
   1. Reinforcement Performance
      1. Physical and Mechanical Properties

         The changes of various indexes of the two types of sandstone after reinforcement are shown in Table 4.

         Appearance change: After protection with the four organosilicon protective materials, the surface color difference

         x max x min

         rij x max xij (i 1,2,…, m; j 1,2,…, n) x max x min

         (1)

         of sandstone was all less than 5, which met the requirements for appearance change in the protection of stone cultural relics

         [30]. The color difference in descending order was: SG SF

         Step 2: Calculation of index weights by the entropy weight method

         Calculate the characteristic proportion of the i-th evaluation object under the j-th index:

         SK SW.

         Physical properties: Water absorption rate reflects the infiltration amount of protective materials on sandstone. The

         water absorption rate in descending order was: SW SK

         Calculate the entropy value of the j-th index:

         SG SF.

         The initial contact angle of sandstone was 0 °, showing

         1 m hydrophilicity. After reinforcement with all materials, the

         ln m

         When the value is 0, the entropy value is 0. Calculate the weight of the j-th index:

         (3)

         contact angle was greater than 90 ° (Figure 5), transforming into hydrophobicity. The contact angle in descending order was: SF SK SG SW.

         The four organosilicon materials all had a certain impact on the air permeability of the two types of sandstone. The air permeability reduction value of sandstone after protection ranged from 10.55% (SW) to 17.34% (SF), rated as moderate.

         The air permeability reduction rate in descending order was: SF SG SK SW.

         Mechanical properties: The mechanical properties of sandstone were all improved after reinforcement. The order of longitudinal wave velocity and compressive strength improvement rate was consistent with that of water absorption rate: the improvement rates of longitudinal wave velocity and compressive strength of weathered sandstone after protection in descending order were: SW SK SG SF.

         Table 4 Changes of various indexes of sandstone after reinforcement

         high porosity and complex pore structure. A certain “glossiness” could be observed on the sandstone surface reinforced by SF and SG; a uniform, transparent and dense protective layer was formed on the sandstone surface and pore walls, which adhered to the particle surface, had a certain adsorption and fixation effect on the clastic particles on the sandstone surface, and did not completely block the original intergranular pores of sandstone, still maintaining a certain air permeability. Due to its small molecular weight and good permeability, SK had relatively poor film-forming property,

         Serial

         physical properties

         number

         mechanical property

         with only a small amount remaining on the sandstone surface,

         and had little change to the surface morphology and pore structure of sandstone. After protection with SW, the

         Group

         Stro S

         Color differen ce (E)

         Water absorpt ion rate (%)

         Cont act angl e (°)

         00. 10.55 51.90 209.40 34
         20. 12.06 37.30 193.33
         55
         22. 17.34 27.30 56.58
         64
         09. 16.58 29.10 64.10 88

          

         W
         SK	2.36	2.93
         SF	2.89	0.72
         SG	4.23	1.89

          

         1

         Air permeab ility reductio n rate (%)

         Longitu

         dinal wave velocity improve ment rate (%)

         Compre ssive strength improv ement rate (%)

         protective material penetrated into the surface pores of sandstone with no obvious surface coverage, but colloidal precipitation could be observed in the intergranular pores of sandstone, playing a role in filling and reinforcement, while almost no change to the original micro-particle morphology of sandstone.

         ngly weat here d sand ston e

         Figure 7 Microscopic images of sandstone after reinforcement

         SEM:

         Observing the SEM images of sandstone before and after reinforcement (Figure 8), the unprotected rock sample SB showed loose particle distribution, large intergranular gaps,

         Figure 5 Contact angle images of sandstone after reinforcement

      2. X-ray Diffraction Analysis

         By comparing the X-ray diffraction patterns of sandstone before and after reinforcement (Figure 6), no new crystal phases were found, indicating that the four organosilicon protective materials did not hve chemical reactions with sandstone minerals, and their action modes were mainly physical filling, surface coating and physical adsorption.

         Figure 6 XRD diffraction pattern analysis of sandstone before and after reinforcement

      3. Microstructure

         Microscopy:

         Comparing the micro-surface properties of the two types of sandstone before and after reinforcement, the untreated rock sample SB showed an uneven dark red hue, with quartz, rock debris and other particles on the surface in irregular inlay, large pores between particles which were mostly open pores,

         broken edges and a large number of suspended rock debris particles on the surface, with obvious weathering characteristics. After protection, the suspended rock debris and rock particles on the surface of SW sample were adsorbed and bonded into a whole by the (SiO·aq) colloid generated by the polymerization reaction of the protective material; the colloid was deposited between the pores and cementing materials, having a good filling effect on the pores of sandstone. The action mechanism of SK was similar to that of SW; the main component tetraethyl orthosilicate formed SiO colloid through hydrolysis and condensation reactions, and then solidified into a cementitious state. These colloids were deposited in the pores and cracks of sandstone, and bonded with rock debris particles to form new cementing materials, which closely connected the sandstone particles together and improved the strength and integrity of sandstone. SF took organic solvent as the carrier, and after penetrating into the sandstone interior, formed a monomolecular hydrophobic film layer on the surface of sandstone particles. A film-like structure could be observed on the surface of SG particles, some pores were filled, and it had a wrapping and adsorption effect on the surface suspended clastic particles, making the structure denser and more integral.

         Figure 8 SEM images of sandstone after reinforcement

   2. Durability Cycle Test
      1. Physical and Mechanical Properties

         The sunlight and rainfall test was used to simulate the weathering effect of abundant precipitation and ultraviolet radiation on sandstone in the Leshan Giant Buddha area. Combined with the change laws of contact angle, mass loss rate, longitudinal wave velocity loss rate and compressive strength loss rate and the subsequent SEM images, the weathering mechanism of sandstone and the durability performance of protective materials were explored.

         The change of sandstone contact angle in the durability cycle is shown in Figure 9a, among which SB was always 0 °, the contact angle of SK and SF was still greater than 90 ° after 6 cycles with good water resistance, the contact angle of SW began to be less than 90 ° after the 4th cycle with a steep decline trend, and the contact angle of SG began to be less than 90 ° after the 5th cycle. The mass loss rate, longitudinal wave velocity loss rate and compressive strength loss rate in descending order were as follows: mass loss rate (Figure 9b): SB SK SW SG SF; longitudinal wave velocity

         loss rate (Figure 9c): SW SB SG SF SK;

         Figure 9 Changes of durability cycle performance indexes of strongly weathered sandstone a Contact angle change; b Mass loss rate; c Longitudinal wave velocity loss rate; d Compressive strength loss rate

         In view of the one-sidedness of single index evaluation, the entropy weight-TOPSIS method was used to comprehensively evaluate the long-term durability of materials. According to the directionality of evaluation indexes, the indexes were divided into two categories: positive indexes were mass loss rate, longitudinal wave velocity loss rate and compressive strength loss rate, with larger values indicating worse durability; the negative index was contact angle, with a larger value indicating better durability. The index weights calculated are shown in Table 5, and the comprehensive scores of the four organosilicon materials on the two types of sandstone after 6 cycles are shown in Figure

         10.

         Table 5 Index weights calculated by the entropy weight-TOPSIS method

         compressive strength loss rate (Figure 9d): SB SG SF

         Group

         Contact angle

         Mass loss rate

         Longitudinal wave velocity loss rate

         Compressive strength loss

         SW SK.

         During the cycle period, the weathering of SB was mainly manifested as mass loss caused by surface particle shedding, and internal structural damage caused by water erosion and temperature change, leading to the reduction of mechanical properties. Although the mass and compressive strength loss of SW was less than that of SB, the internal compactness changed greatly, and the longitudinal wave velocity loss was greater than that of SB. SK maintained hydrophobicity all the time; although the mass loss was large, the internal structural damage was small, and the longitudinal wave velocity and compressive strength loss rates were low with a uniform decline. Although the mass loss of SF and SG was not obvious, their mechanical properties were poor, and the internal structural damage of sandstone was significant after the cycle.

         rate

         Strongly

         weathered 0.2542 0.1866 0.2793 0.2798

         sandstone

         Figure 10 Comprehensive scores of entropy weight-TOPSIS

      2. Scanning Electron Microscopy Detection

   Observing the SEM images of sandstone after the durability cycle (Figure 11): the surface particle shedding of SB was aggravated, the pores were enlarged, and the physical disintegration caused by weathering was very obvious. The protective material of SW cracked, with pore development and loose structure, and a large number of open pores generated by particle shedding were distributed on the sandstone surface,

   with obvious damage caused by water erosion. Cracks appeared in the protective material between SK particles, and the protective material on the particle surface had a peeling trend, but the arrangement was still tight, and the adsorption effect of the protective material on clastic particles was still effective. The protective material on the surface of SF sandstone particles peeled off obviously, and the hydrophobic layer was damaged. The protective material of SG peeled off seriously, gaps caused by water erosion appeared between the material and sandstone particles, and cracks developed on the sandstone surface with water dissolution holes.

   Figure 11 SEM images of sandstone after durability cycle

   Fig.4 Numerical model and groups

4. DISCUSSION

   The differences in reinforcement effect and durability of the four organosilicon protective materials on the weathered red sandstone of the Leshan Giant Buddha stem from the different reinforcement mechanisms of the materials themselves and the different original properties and weathering mechanisms of sandstone.

   1. Reinforcement Mechanism

      The different chemical compositions, molecular weights and reaction mechanisms of protective materials lead to significant differences in their reinforcement effects. Wacker BSOH-100 has a small molecular weight and strong permeability. After protection, under the action of a neutral catalyst, it undergoes a hydrolysis reaction with water in the atmosphere or sandstone pores to form a glass-like silica gel binder (SiO·aq), which deeply fills the pores and cracks and significantly improves the mechanical properties of sandstone. Remmers KSE-300 forms SiO colloid through hydrolysis and condensation reactions, and solidifies into a cementitious state, which bonds with rock debris to form new cementing materials and deposits in the pores and cracks of sandstone, enhancing the bonding strength between particles and greatly improving the mechanical properties.Fluorosilane (Sikang KY-60) undergoes a condensation reaction with hydroxyl groups in sandstone to form a three-dimensional network protective layer. In addition, the fluorine element in the fluorosilicon polymer has an extremely low surface free energy, which reduces the surface tension of sandstone and forms a hydrophobic layer on the sandstone surface. The modified siloxane forms a flexible siloxane network (Si-O-Si) after curing, which can fill most of the pores and form a hydrophobic layer on the sandstone surface. However, its permeability is relatively poor, the improvement of

      mechanical properties is small, and it has a certain impact on the color of the sandstone surface.

      Wacker BSOH-100 and Remmers KSE-300 have small molecular weights and strong permeability, and the colloid generated after curing has a good filling effect on sandstone pores, significantly improving the mechanical properties and integrity of sandstone. Fluorosilane (Sikang KY-60) and modified siloxane have large molecular weights and low water absorption rates, with poor filling property for sandstone and relatively weak improvement of mechanical properties. However, they have good film-forming properties, and the formed hydrophobic layer can significantly improve the hydrophobic capacity of sandstone.

   2. Differences in Sandstone Reinforcement Effect

   The weathering state of sandstone, including pore structure, cementing material loss degree and weathering mechanism, determines the differences in the protective effects of protective materials.

   The weathered red sandstone of the Leshan Giant Buddha has high porosity and loose structure, with a large water absorption rate of protective materials, and the mechanical properties are significantly improved after protection. In the durability cycle, its weathering damage is mainly manifested as mass loss and mechanical property decline caused by structural disintegration. The protection of such sandstone needs to rely on the deep penetration and cementation filling of materials to strengthen the sandstone structure. Remmers KSE-300 has a small molecular weight and good permeability, and the generated SiO colloid can effectively improve the bonding force between particles with a significant improvement of mechanical properties. In the durability cycle, the cementitious network formed by it has good water resistance and maintains hydrophobicity all the time. Although there is a certain peeling of surface materials and sandstone particles causing mass loss, the sufficient penetration depth effectively resists water erosion and temperature stress, maintaining the structural stability of sandstone with the best comprehensive performance. Although tetraethyl orthosilicate also has strong permeability and deep filling performance, it has insufficient water resistance, with large losses of various indexes after the cycle, and the longitudinal wave velocity loss even exceeds that of unreinforced sandstone, showing poor durability performance.

   The weight distribution of the entropy weight-TOPSIS method further corroborates the above conclusions. The longitudinal wave velocity loss rate and compressive strength loss rate of weathered sandstone have the highest weights, reflecting that the loose sandstone structure is prone to disintegration, and the attenuation of mechanical properties is the key factor leading to its long-term durability performance. The performance of Remmers KSE-300 is highly matched with this, with the highest score among the four organosilicon materials.

5. CONCLUSION

In this study, four organosilicon materials were used for the weathering protection of the weathered red sandstone of the Leshan Giant Buddha, and the weathering diagnosis, demand matching and effect verification processes were integrated to reveal the protection demand of the weathered

red sandstone of the Leshan Giant Buddha and the differences in the protective effects of organosilicon materials. The main conclusions are as follows:

For the weathering protection of large open-air stone cultural relics, the weathering mechanism determines the protection direction. A profound understanding of the damage mechanism of weathered sandstone is the premise for formulating an effective protection strategy. A protection scheme should be established based on the accurate diagnosis of the specific weathering state and core protection demand of sandstone.

The selection of protective materials must be highly compatible with the physical state and chemical properties of sandstone. Even for organosilicon materials of the same type, the differences in chemical composition and reinforcement mechanism lead to distinct protective effects on weathered sandstone. Fine screening must be carried out during protection to make the advantageous characteristics of protective materials highly match the core demand of weathered sandstone.

The short-term improvement of sandstone reinforcement performance by protective materials does not represent the quality of protective effect, which needs to be comprehensively evaluated in combination with long-term durability. Some materials have excellent immediate reinforcement effects but insufficient durability or excessive intervention on the sandstone structure, which is counterproductive, accelerates weathering damage and endangers the safety of cultural relics themselves. Therefore, long-term durability is a necessary link for screening effective protective materials.

When evaluating the long-term durability of protective materials, a single index is one-sided. The entropy weight- TOPSIS method can integrate multi-dimensional performance indexes, which is a scientific and objective evaluation tool and suitable for the comprehensive evaluation of complex protective effects of weathered sandstone.

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