The protection of stone cultural relics mainly includes three links: cleaning, strengthening and protection.
1. Clean
The cleaning objects of stone cultural relics are all harmful substances on the stone surface, including microorganisms, weeds, soluble salts, insoluble hard shells, dust and smoke.
According to the characteristics of cleaning agent and treatment technology, cleaning methods can be divided into two categories: water cleaning method; Chemical cleaning method;
1. 1 water cleaning method
This cleaning method is very effective for removing soluble salts from stone cultural relics. The therapeutic effect depends on the operation method.
(1) The water immersion method is suitable for small stone cultural relics. The method is to soak the stone cultural relics in deionized water. However, if the stone products are not in good condition, it is more dangerous to completely soak them to remove salt, because rapid hydration can quickly dissolve salt, which will lead to the stone products falling off in pieces.
(2) Steam cleaning method This method can be used for damaged porous stone surfaces, cultural relics carving and ancient buildings. The industrial realization pressure is (5- 10) × 105 Pa, and the effect is good.
(3) Atomized water leaching method Water is sprayed through a very narrow nozzle to form atomized water. Atomized water slowly falls on the surface of stone cultural relics in the air. This leaching is very mild and won't have any influence. Moreover, cleaning is relatively blocked, because atomized water occupies a large area.
(4) Adsorption desalination can prolong the contact time between water and stone surface and reduce the penetration depth of water. Its method is to use some fiber materials as adsorbents, such as paper pulp, paper towels, absorbent cotton, wood pulp, sepiolite, activated clay and other clay minerals. First, the adsorption material is wetted with deionized water, and paper pulp is applied on the part to be cleaned. In order to prevent the rapid evaporation of water, the adsorption material can be covered with plastic film. After a certain period of time, the film was uncovered, and with the evaporation of water in the adsorption material, the adsorbed crystalline salt was precipitated. Repeat the operation for many times, and when the conductivity of the adsorption material is constant, it means that the cleaning is in place. Wetting adsorption materials with other chemical solvents can help us to remove water-insoluble substances on the surface of stone cultural relics. Such as organic solvents, surfactants, etc. Adsorption desalination method is very practical and effective.
1.2 chemical cleaning method
(1) strong acid and alkali cleaning method is commonly used in industry, but it is not suitable for stone cultural relics.
(2) ion exchange resin cleaning method can get deionized water with ion exchange resin, and remove pollutants on the surface of stone cultural relics with paste made of ion exchange resin. According to their chemical properties, the active groups of ion exchange resins can be acidic or basic. Soluble carbonate, silicate, silicon dioxide, etc. In the actual treatment process, its dissolution is quite slow, only in the part where the wet ion exchange resin contacts with the stone surface. As long as the paste is scraped off, the dissolution reaction stops immediately without any danger of infiltration. So it is easier to control.
The ion exchange resin used for cultural relics protection must be pure (analytically pure) and the particles should be fine (100-200 mesh). This method is very expensive and suitable for cleaning stone cultural relics with high value.
(3) Cleaning method of adhesive paste This paste can be prepared by adding adhesive modifier into diluted solution. Can be used on vertical surfaces and ceilings without falling off. In addition, this paste can be treated for a long time, which can inhibit its solution from penetrating into the stone. Plastic film can also be used to moisturize.
Mora and his wife developed a common paste "AB57" in the Roman Restoration Center, and its formula is as follows:
1000ml water, 30 g ammonium bicarbonate, 50g sodium bicarbonate, 25g sodium EDTA, 10ml Desogen (quaternary ammonium base) and 50 g hydroxymethylcellulose.
The PH value of its solution is about 7.5. Two kinds of bicarbonate have scavenging effect and can dissolve gypsum and other salts. Deoxy is a surfactant and scavenger. EDTA can dissolve calcium-containing pollutants such as gypsum, calcite and dolomite. Hydroxymethyl cellulose is a thixotropic agent and also plays an adhesive role.
The cleaning effect of paste AB57 is slow, but it is very effective for removing black hard shell containing more gypsum.
(4) Chemical cleaning agents used in special occasions can be used to remove copper pollutants, including 10% sulfamic acid solution and 2%- 10% ammonium carbonate solution, and can also be made into paste.
Iron dirt can be removed by making a paste with the following solutions: mixing potassium oxalate with water to make a paste; Saturated diammonium hydrogen phosphate. 10% EDTA sodium salt solution.
The removal of plant organisms on the surface of stone cultural relics can be achieved by combining mechanical methods with enzyme inhibitors and fungicides.
strengthen
The purpose of reinforcement and protection is to improve the strength of weathered cultural relics. Its basic principle is to supplement the natural cement lost due to weathering by infiltrating the reinforcing agent into the stone cultural relics. Reinforcement is mainly aimed at porous cultural relics that have been weathered, are in danger of disintegration and are eroded by wind and sand.
Requirements for reinforcing materials: (1) can form new weathering-resistant mineral rock cement; (2) Salt-containing by-products that destroy rocks are not formed; (3) Some main characteristics of rocks, such as water vapor permeability, are adversely affected. (4) It has good permeability in rock, at least it should be able to penetrate into the unweathered part, and the reinforced mechanical section should be smooth, resulting in excessive mechanical strength near the surface. (5) It will not change the color of the rock surface.
Strengthening method: in practical operation, it is often used: melting-solidification; The transferred solvent volatilizes; Chemical reaction between reinforcing agents-chemical reaction between reinforcing agents and minerals.
Reinforcing materials are divided into organic materials and inorganic materials. The difference between them is that the reinforcement of inorganic materials is realized by the reaction or hydration of some components in stone with CO 2 to form new substances. The bond between new substances and minerals is fragile, and the width of the bond crack should not be greater than 10-50um. It is impossible to bond these two broken parts with inorganic reinforcing agent. Compared with organic materials, organic materials are anti-aging, but fragile and have poor elasticity. It is difficult for chemical reaction to achieve good penetration effect, because once the chemical reaction starts, the reactants will block the cracks on the rock surface, thus inhibiting the progressive penetration of reinforcing agent.
Compared with inorganic reinforcing agents, organic reinforcing agents are more susceptible to environmental aging, mainly due to the generation or physical change of organic substances by oxygen, ozone, water, ultraviolet and infrared radiation. However, if the reinforcing material is in the gap of stone products, the influence of the above factors will be limited. Another disadvantage of organic materials is that their thermal expansion coefficient is higher than that of rocks, but they have good adhesion and flexibility, so they have good compressive properties. On the other hand, it is difficult for organic reinforcement to penetrate as easily as inorganic reinforcement, which is mainly due to the long molecular chain and excellent viscosity of organic reinforcement. To solve this problem, some people are using or experimenting with prepolymers and monomers, especially silicone materials and acrylic monomers.
Main types of commonly used reinforcing agents
Inorganic class:
(1) The strengthening of limewater is realized by the reaction between calcium hydroxide and carbon dioxide, and calcium carbonate formed by chemical reaction remains in the cracks of rocks. Reaction: calcium hydroxide+carbon dioxide = calcium carbonate +H2O
(2) The principle of barium hydroxide is similar to lime water: Ba (OH) 2+CO 2 = BacO3+H2O.
Organic class:
(1) epoxy resin is favored by people, especially some new modified epoxy resin materials, because they have no by-products, no bubbles, small volume shrinkage and no deformation during curing, and can penetrate into porous materials to form a network structure, which has good durability, viscosity and mechanical properties.
Epoxy resin is a typical in vivo polymerization method. The molecular end of epoxy resin contains more than two epoxy groups. By adding curing agent and relying on epoxy ring-opening polymerization or addition polymerization, the long-chain network structure with certain kneading, viscosity and chemical corrosion resistance is formed after polymer quantification. The epoxy resin used for cultural relics protection is a kind of bisphenol A component (bisphenol A diglycidyl ether) with more than two epoxy groups at the molecular end. The commonly used curing agent is amine. The characteristics of amine determine the curing speed, while the curing time affects the penetration depth. In order to enhance the effect, active diluents and toughening agents are often added.
The success of epoxy resin reinforcement is related to the reasonable selection of resin, infiltration mode and porosity of rock. Vacuum low-pressure infiltration can obtain better penetration depth, which is the best goal of epoxy resin reinforcement after weathering of porous rock itself. For successful treatment, the porosity of rocks is required to be 14% and 28%.
Although epoxy resin has obvious reinforcement effect, it also has some shortcomings, such as poor permeability and air tightness. Under ultraviolet radiation, the color will turn yellow
(2) Acrylic resin
Acrylic acid is also a kind of resin material widely used for the reinforcement and protection of porous cultural relics. Polymethyl methacrylate is called plexiglass, which can prevent the weathering of cultural relics and outdoor ultraviolet radiation, but the rocks treated with plexiglass prevent the movement of water. Paraloid B72 is a kind of acrylic resin that people study most. It has a white glassy structure and is soluble in many organic solvents. It is a typical example of film formation after solvent volatilization, which plays a reinforcing role. Usually the concentration of acetone or xylene is 2%- 10%. The biggest disadvantage of B72 is that the formed film is very brittle, which is neither resistant to alkali corrosion nor ultraviolet radiation, and the color will become darker. Now acrylic acid is being modified, such as epoxy acrylic acid, silicon-containing acrylic acid and so on.
(3) Silicones
Silicone reinforcing agent mainly includes ethyl silicate, alkyl silicate, silane, siloxane and silicate. For example, Remmers products from Germany and trimethyltetraethoxysilane from the United States. Remmers 300 (ethyl silicate) commonly used in China and silicone products produced by Wuhan University, such as WD- 10 surface sealant.
Characteristics of silicon-like materials: Silicon materials are the link between silicate chemistry and organic chemistry, and the structural characteristics of organic polymers make them have the functions of both organic materials and organic polymers. It not only has excellent high and low temperature resistance, electrical insulation, chemical stability and aging resistance. Silicone materials used for cultural relics protection have the characteristics of low viscosity, good air permeability, no discoloration after curing, no reflection and no oily feeling, which endows weathered stone with certain strength, excellent hydrophobicity and good air permeability.
3. Protection of Grottoes and Cliffs (Protection Project)
(1) Rock mass stability evaluation The protection of cliff-like cultural relics in grottoes often involves protection projects. First of all, it is necessary to evaluate the stability of the rock mass attached to the cliff of the grottoes. This package includes close-range photogrammetry, hydrogeologic engineering surveying and mapping, geophysical exploration, physical and mechanical properties experiment, material structure composition and chemical composition analysis, environmental detection and so on.
(2) Reinforcement treatment
Reinforcement method:
1) Use retaining walls, retaining walls and massive masonry to prevent the development of cracks in rock mass, resist rock mass cracking and prevent hanging rock from collapsing.
2) shotcrete reinforcement
3) Comprehensive methods of joint filling, anchoring, surface infiltration reinforcement and chemical reagent sealing.
③ Waterproof
Besides earthquakes, volcanoes and man-made destructive damage, water is the most common and serious destructive factor to stone cultural relics. Therefore, the waterproof treatment of stone cultural relics is very important.
1) Small stone cultural relics and large stone cultural relics (cave temples, cliffs, large stone carvings, etc. ) part is coated with sealant to form a protective film to prevent water from invading. Long-chain silicone sealants is commonly used now. For example, WD- 10 (dodecyl trimethoxysilane) produced by Wuhan University is a very good common sealant. One end of it is an alkoxy group as a coupling group, which is closely combined with the stone body and adjacent alkoxy groups, and the other end is a long-chain alkyl group, which forms a film and plays a hydrophobic role. The film made of WD- 10 is dense and thick, but it has good air permeability, corrosion resistance and sealing effect.
2) Engineering waterproofing
A. Prevention and control of precipitation
A. blocking sinkholes and big gaps. According to the characteristics of sinkholes and cracks on the top of the mountain, the mechanical properties of rock mass, material price and construction technology, etc. After filling loam or loess, polyethylene plastic film can be laid layer by layer to set up impervious layer. Considering the relatively developed cracks in the rock mass, cement mortar can also be used for grouting, which can not only prevent water infiltration, but also reinforce the rock mass. For example, when the cracks in the east cliff of Yulin Cave in Gansu Province are reinforced, PS materials are first infiltrated into the weathered cliffs on both sides of the cracks; Secondly, seal the cracks with cement mortar and insert the grouting pipe. Firstly, proper amount of PS slurry is injected to reinforce the two walls of the crack, and finally modified PS-F slurry is injected. After one week, it was observed that PS slurry penetrated into the two walls of the crack 15 cm, and PS-F adhered closely to the two walls of the crack.
B Digging a deep well In order to drain the water in the rock mass, a deep well can be dug at the top of the mountain far away from the cliff surface, and the location is selected at the intersection of various cracks, so that the water in the rock mass can be discharged into the well through the cracks, which can reduce the movement of water to the cliff surface. You can also dig a drainage tunnel parallel to the vertical wall of the cliff face. For example, the inner slope of the concave at the top of Beifo Bay in Dazu Beishan stone carving and the weathering layer of the cliff form a phreatic layer. When the steep wall of the stone carving cuts these water-bearing cracks, it will cause water seepage damage to the stone carving. In order to drain water, a trapezoidal tunnel with a height of 4m, a span of 2m and a section parallel to the steep wall was excavated 8m away from the front edge of the steep wall. It is observed that the drainage tunnel has drained away most of the leakage.
C. Aqueduct excavation According to the topography, a crisscross aqueduct is excavated at the top of the mountain, and then two main aqueducts are excavated on both sides of the vertical wall. The open aqueduct at the top of the mountain is connected with the two main aqueducts, so that rainwater can be quickly discharged from the top of the mountain, and the retention time of rainwater at the top of the mountain is reduced as much as possible, thereby reducing the amount of water entering the rock mass.
D After the above treatment, some large cracks have been blocked with clay or cement mortar, but the cracks below 0.25mm cannot be filled with mud. Even if the cracks with a width of more than 0.25mm have been filled with cement, the rock mass cannot be guaranteed to form a whole due to the poor shear force. Grouting can prevent these micro-cracks from becoming channels for water movement, and grouting must be carried out with suitable materials. There are many grouting materials used at present, which should be decided according to the mechanical characteristics of rock mass, local climate and environmental characteristics. For example, in the gravel sandstone grottoes located in the arid area of northwest China, inorganic PS material can be used as grouting material for waterproof reinforcement of rock mass, and the effect is ideal. Silicone furan modified epoxy resin can be used as the main agent for limestone stone carvings and rock paintings in high temperature and humid environment in South China.
B. Prevention and treatment of groundwater leakage
Groundwater mainly rises to the rock mass or building wall through capillary action, so the comprehensive scheme of blocking and guiding should be adopted to control water. Blocking refers to cutting off the capillary channel. The solution is to set a moistureproof layer at the bottom of the vertical wall or wall foundation, or to make a moistureproof layer next to it to isolate other walls in contact with it. However, to completely solve the problem of capillary seepage, the most fundamental method is to completely cut off the bottom of rock mass or the building itself from the source as far as possible. Diversion refers to burying underground pipelines, digging underground ditches to dredge diversion seepage, and combining with the method of cutting off seepage to achieve the purpose of water control. Capillary seepage is common, and when the amount of water-retaining agent is large, water corridors can be dug in the lower or rear part to concentrate drainage. Its function is to reduce the groundwater level and cut off the groundwater source flowing into the rock mass.
The barrier layer is made by drilling holes at regular intervals along the horizontal direction in the proper position of the vertical wall or wall, putting special lead plates at the bottom of each hole, and then connecting the holes together. This is the moisture-proof layer, which can block the passage of water rising under the capillary action.
For the waterproofing of the tomb, a water cutoff wall, that is, an underground wall made of reinforced concrete, can be set up, and its main function is to block water. For example, a waterproof reinforced concrete wall with a bottom width of 40cm and a top width of 20cm was built around the tomb of Nanyue King, and the floor of the tomb at the bottom of the wall was 50cm lower. Chemical grouting or jet grouting can also be used, that is, holes with a certain density are drilled around the tomb, and high-pressure epoxy resin or acrylic acid slurry is injected into the holes to make them penetrate into all cracks and micro-cracks in the formation, forming a closed water diversion area and blocking the penetration of water.
C. prevention and control of surface water
For surface water, in order to prevent water infiltration, a unified drainage system consisting of trunk, trunk and branch ditch should be adopted to discharge water as soon as possible, mainly culverts.
In a word, the comprehensive management scheme of "painting", "blocking", "guiding" and "discharging" should be adopted for the waterproofing of stone cultural relics, and the waterproofing work should be done well according to local conditions.
Through a comprehensive understanding of the present situation of stone cultural relics protection, we put forward the following ideas:
1. Adding titanium dioxide nano-materials to organic cultural relics protection materials. Organic materials are not anti-aging, especially ultraviolet rays in the air. Nano-titanium dioxide is the best ultraviolet absorber, so it is necessary to add it.
Second, there is an urgent need to produce a kind of reinforced sealing material for limestone cultural relics. Silicone is very successful in protecting siliceous cemented sandstone and mudstone, but when it is used for limestone reinforcement and plugging, there is a disadvantage that the protective material is not firmly bonded to the stone body. Nano-calcium carbonate composites show many advantages, such as good elasticity, good filling and good economy. From the point of view of the same kind of materials, it is considered whether nano-CaCO _ 3 composite can be produced for limestone treatment to overcome the shortage of limestone treatment in silicone.
Third, make full use of mixed materials. A single composite material can't adapt to the change of environmental conditions because of its single performance. At present, when selecting protective materials, the protective materials with outstanding performance are almost always selected according to the environmental conditions at that time. At that time, it seemed that the protection effect was good, but once the environmental conditions changed, it immediately showed the shortcomings of inadaptability. For example, the original demand for strength has now become a demand for toughness. It used to be good in dry conditions, but now it is very poor in wet environment. Mixed materials have the advantages of many materials, not only have excellent performance at that time, but also show certain adaptability and buffering ability even if environmental conditions change.
Here, we tested the enhancement and hydrophobicity of ASO-B, a new mixture material for cultural relics protection.