Preparation method of calcium oxalate protective film on limestone surface

Abstract

The application belongs to the technical field of open-pit limestone building and stone carving protection, and provides a preparation method of a calcium oxalate protective film on a limestone surface, which comprises the following steps: step one: a water solution of a reducing organic acid is brushed on the surface of a pretreated limestone and is sealed to keep water for a certain time, and a calcium film of the reducing organic acid is prepared on the surface of the limestone by reaction between the two; step two: a water solution of a peroxide is continuously brushed on the surface of the limestone covered with the calcium film of the reducing organic acid and is sealed to keep water for a certain time, and the calcium film of the organic acid is oxidized into a calcium oxalate film. The preparation method has mild and controllable reaction conditions, the calcium oxalate crystals can grow slowly and orderly, the obtained calcium oxalate protective film is denser and more uniform, and the limestone surface is more effectively protected.

Description

Technical Field

[0001] This application belongs to the field of open-air limestone architecture and stone carving protection technology, and relates to a method for preparing a calcium oxalate protective film on the surface of limestone. Background Technology

[0002] Limestone (mainly composed of calcium carbonate, CaCO3) is a natural stone widely used in architecture and stone carving. However, due to its carbonate material characteristics, limestone is easily eroded and weathered by acid rain in the natural environment [Tan Chaohong, Li Haiyan, Zhang Xiaoran, Zhang Ziyang. A brief analysis of the acid rain weathering mechanism and control technology of carbonate rock cultural relics. Research on Natural and Cultural Heritage, 2019, 4(8):33-38].

[0003] Calcium oxalate (CaC2O4) is chemically stable, with a solubility much lower than that of calcium carbonate, and exhibits excellent acid resistance. Therefore, calcium oxalate is considered an ideal protective material for limestone surfaces [Ceza T. Calcium Oxalate: A surface treatment for limestone. Journal of Conservation and Museum Studies, 1998, 4: 6-10]. Currently, there are two types of methods for preparing calcium oxalate protective films on limestone surfaces: biological methods and chemical methods. Biological methods typically involve inoculating the limestone surface with special microorganisms, utilizing the oxalic acid secreted during their metabolic processes to react with the limestone to prepare a calcium oxalate film. However, biological methods are too inefficient. Studies have shown that the calcium oxalate film produced by this method is difficult to completely cover the limestone surface within eight months [MONTE M. Oxalate film formation on marble specimens caused by fungus. Journal of Cultural Heritage, 2003, 4(3): 255-258]. Chemical methods involve directly reacting oxalic acid and its derivatives, such as ammonium oxalate, potassium oxalate, and sodium oxalate, with the surface of limestone to prepare a calcium oxalate protective film [Liu Yan et al. A review of the application of calcium oxalate materials in the protection of limestone cultural relics. Journal of Northwest University (Natural Science Edition), 2021, 51(3): 390-396]. In comparison, chemical methods are significantly more efficient than biological methods due to the rapid chemical reaction between oxalic acid and its derivatives and limestone. However, the calcium oxalate film generated by this rapid reaction is usually loose and porous with insufficient bonding to the underlying limestone, resulting in limited protective effect. Summary of the Invention

[0004] The purpose of this application is to overcome the shortcomings of the prior art and provide a method for preparing a calcium oxalate protective film on the surface of limestone. This method is based on a mild and controllable reaction, which allows calcium oxalate crystals to grow slowly and orderly, resulting in a denser and more uniform calcium oxalate protective film that provides more effective protection for the limestone surface.

[0005] To achieve the above objectives, the technical solution adopted in this application is: a method for preparing a calcium oxalate protective film on a limestone surface.

[0006] Step 1: Apply an aqueous solution of reducing organic acid to the pretreated limestone surface and seal it to retain water for a certain period of time to obtain limestone covered with a calcium film of reducing organic acid.

[0007] Step 2: Continue to brush an aqueous solution of peroxide onto the limestone surface covered with a reducing organic acid calcium film and seal it to retain water for a certain period of time to obtain limestone covered with a calcium oxalate protective film.

[0008] Furthermore, the reducing organic acid mentioned in step one is one of glycolic acid and glyoxylic acid.

[0009] Furthermore, the peroxide mentioned in step two is one of hydrogen peroxide, urea peroxide, and peracetic acid.

[0010] Further, an alkaline adjuster is added to the aqueous solution of the peroxide described in step two to adjust the pH of the solution to 7.8-9.0. The resulting solution is then brushed onto the surface of the limestone covered with the reducing organic acid calcium film and sealed to retain water for a certain period of time to obtain limestone covered with a calcium oxalate protective film.

[0011] Furthermore, the alkalinity regulator is one or more of ethylamine, ammonia, and ammonium carbonate.

[0012] Furthermore, the concentration of the alkaline regulator is from 1.6% to 5.0%.

[0013] Furthermore, the mass percentage concentration of the reducing organic acid solution is 4.5% to 7.1%; and the mass percentage concentration of the peroxide solution is 5.0% to 8.0%.

[0014] Furthermore, the molar ratio of the reducing organic acid in step one to the peroxide in step two is 1:1.1 to 1:2.3.

[0015] Furthermore, when preparing a calcium oxalate protective film in the aqueous solution of the peroxide described in step two without adding an alkaline regulator, the required reaction temperature is 15°C to 30°C, the ambient humidity is 30% to 60%, the required reaction time in step one is 12 hours to 24 hours, and the required reaction time in step two is 60 hours to 72 hours.

[0016] Furthermore, when preparing a calcium oxalate protective film by adding an alkaline regulator to the aqueous solution of the peroxide described in step two, the required reaction temperature is 15°C to 30°C, the ambient humidity is 30% to 60%, the required reaction time in step one is 12 hours to 24 hours, and the required reaction time in step two is 20 hours to 48 hours.

[0017] Compared with existing methods for preparing calcium oxalate membranes, the advantages of this application are as follows:

[0018] (1) A reducing organic acid solution is brushed onto the surface of limestone to generate a reducing organic acid calcium film. The reducing organic acid calcium reacts with peroxide to generate a calcium oxalate protective film. In this application, the reaction between the reducing organic acid calcium and peroxide is mild and controllable, and the calcium oxalate crystals can be deposited and grown slowly and orderly, making the obtained calcium oxalate protective film denser and more uniform. Compared with the process of forming a calcium oxalate protective film by reacting oxalate with limestone, the process is prolonged and the reaction is more complete.

[0019] (2) The reducing organic acid solution can penetrate and deposit inside the micro-cracks and pores of limestone, generating a reducing organic acid calcium film to improve the density of limestone. The calcium oxalate formed after its in-situ oxidation can also play a micro-anchor role to improve the bonding force between the protective film and limestone, and further provide more effective protection for the limestone surface.

[0020] (3) The addition of alkaline regulator in step two can make the reaction time more controllable. Without affecting the protective effect, the reaction process can be effectively controlled, the reaction time can be further regulated, and the controllability of the reaction cycle and the stability of the maintenance effect can be achieved.

[0021] (4) The preparation method of this application can be applied to limestone construction, stone carving and other fields. Without affecting the structure of the building itself, the generated calcium oxalate protective film has a good protective effect.

[0022] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0023] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure. It is obvious that the drawings described below are merely some embodiments of this disclosure, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0024] Figure 1The limestone surface photographs in Example 1 include: a. a photograph of the limestone surface after ultrasonic cleaning; b. a photograph of the limestone surface after being coated with glycolic acid solution for sealing and water retention; and c. a photograph of the limestone surface after being coated with hydrogen peroxide solution for sealing and water retention.

[0025] Figure 2 SEM images of calcium oxalate protective films prepared in the examples and comparative examples, wherein a. is the SEM image of comparative example 2, b. is the SEM image of example 1, and c. is the SEM image of example 4. Detailed Implementation

[0026] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided to make this disclosure more comprehensive and complete, and to fully convey the concept of the example embodiments to those skilled in the art. The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a full understanding of embodiments of this disclosure. However, those skilled in the art will recognize that the technical solutions of this disclosure can be practiced with one or more of the specific details omitted, or other methods, components, apparatus, steps, etc., can be employed. In other instances, well-known technical solutions are not shown or described in detail to avoid obscuring various aspects of this disclosure.

[0027] The following detailed explanation of this application is provided with specific examples.

[0028] Example 1

[0029] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times using ultrasonic power of 500W. After removal, it was air-dried naturally at 25℃ and 50% relative humidity until constant weight. A photograph of its surface is shown below. Figure 1 a. Then, under conditions of 15℃ and 30% humidity, 2 mL of a 5.0% glycolic acid solution was brushed onto the surface of the above-mentioned constant-weight limestone specimen. After a 12-hour static reaction, a photograph of the surface is shown below. Figure 1 b. Apply 2 mL of 5.0% hydrogen peroxide solution to the surface of the limestone specimen. Seal the limestone specimen with plastic wrap and leave for 60 hours. Remove it, rinse three times with 50 mL of distilled water, and air dry at 25°C and 50% relative humidity until constant weight. See the surface photograph below. Figure 1 The performance test results are shown in Table 1.

[0030] Example 2

[0031] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times with an ultrasonic power of 500W. After removal, it was naturally air-dried to constant weight at 25℃ and 50% relative humidity. Then, at 23℃ and 40% humidity, 2mL of a 4.5% glycolic acid solution was brushed onto the surface of the constant-weight limestone specimen. After a 24-hour static reaction, 2mL of an 8.0% urea peroxide solution was brushed onto the surface of the limestone specimen. The limestone specimen was sealed with plastic wrap and left for 66 hours. After removal, it was rinsed three times with 50mL of distilled water and naturally air-dried to constant weight at 25℃ and 50% relative humidity. Its performance was then tested, and the results are shown in Table 1.

[0032] Example 3

[0033] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times using an ultrasonic power of 500W. After removal, it was air-dried at 25℃ and 50% relative humidity until constant weight. Then, at 30℃ and 60% humidity, 2mL of a 7.1% glyoxylic acid solution was brushed onto the surface of the constant-weight limestone specimen. After a 24-hour static reaction, 2mL of an 8.0% peracetic acid solution was brushed onto the surface of the limestone specimen. The limestone specimen was sealed with plastic wrap and left for 72 hours. After removal, it was rinsed three times with 50mL of distilled water and air-dried at 25℃ and 50% relative humidity until constant weight. Its performance was then tested, and the results are shown in Table 1.

[0034] Example 4

[0035] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times with an ultrasonic power of 500W. After removal, it was naturally air-dried to constant weight at 25℃ and 50% relative humidity. Then, at 15℃ and 30% humidity, 2mL of a 5.0% glycolic acid solution was brushed onto the surface of the constant-weight limestone specimen. After standing for 12 hours, 2mL of a mixed solution containing 5.0% hydrogen peroxide and 1.6% ethylamine (pH 7.8) was brushed onto the surface of the limestone specimen. The limestone specimen was sealed with plastic wrap and left for 48 hours. After removal, it was rinsed three times with 50mL of distilled water and naturally air-dried to constant weight at 25℃ and 50% relative humidity. Its performance was then tested, and the results are shown in Table 1.

[0036] Example 5

[0037] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times with an ultrasonic power of 500W. After removal, it was air-dried at 25℃ and 50% relative humidity until constant weight. Then, at 15℃ and 30% humidity, 2mL of a 5.0% glycolic acid solution was brushed onto the surface of the constant-weight limestone specimen. After standing for 12 hours, 2mL of a mixed solution containing 5.0% hydrogen peroxide and 5.0% ammonium carbonate (pH 8.3) was brushed onto the surface of the limestone specimen. The limestone specimen was sealed with plastic wrap and left for 45 hours. After removal, it was rinsed three times with 50mL of distilled water and air-dried at 25℃ and 50% relative humidity until constant weight. Its performance was then tested, and the results are shown in Table 1.

[0038] Example 6

[0039] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times with an ultrasonic power of 500W. After removal, it was air-dried at 25℃ and 50% relative humidity until constant weight. Then, at 25℃ and 40% humidity, 2mL of a 4.5% glycolic acid solution was brushed onto the surface of the constant-weight limestone specimen. After standing for 24 hours, 2mL of an aqueous solution containing 8.0% urea peroxide and 2.0% ammonium carbonate (pH 9.0) was brushed onto the surface of the limestone specimen. The limestone specimen was sealed with plastic wrap for 42 hours, then removed, rinsed three times with 50mL of distilled water, and air-dried at 25℃ and 50% relative humidity until constant weight. Its performance was then tested, and the results are shown in Table 1.

[0040] Example 7

[0041] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times with an ultrasonic power of 500W. After removal, it was naturally air-dried to constant weight at 25℃ and 50% relative humidity. Then, at 30℃ and 60% humidity, 2mL of a 7.1% glyoxylic acid solution was brushed onto the surface of the constant-weight limestone specimen. After a 24-hour static reaction, 2mL of a solution containing 8.0% peracetic acid and 2.0% ammonia (pH 8.0) was brushed onto the surface of the limestone specimen. The limestone specimen was sealed with plastic wrap for 20 hours, then removed, rinsed three times with 50mL of distilled water, and naturally air-dried to constant weight at 25℃ and 50% relative humidity. Its performance was then tested, and the results are shown in Table 1.

[0042] Comparative Example 1

[0043] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and washed three times with ultrasonic power of 500W. It was then air-dried at 25℃ and 50% relative humidity until constant weight. The test results for surface adhesion, acid resistance, water absorption, and porosity are shown in Table 1.

[0044] Comparative Example 2

[0045] A limestone specimen measuring 50mm × 50mm × 20mm was placed in 200mL of deionized water and cleaned three times with an ultrasonic power of 500W. After removal, it was naturally air-dried to constant weight at 25℃ and 50% relative humidity. Then, 2mL of 5.0% ammonium oxalate solution was brushed onto the surface of the limestone specimen at 15℃ and 30% humidity, and the specimen was sealed and allowed to react for 24 hours [B. Dohertya. Efficiency and resistance of the artificial oxalate protection treatment on marble against chemical weathering. Applied Surface Science 253 (2007) 4477–4484]. After removal, it was rinsed three times with 50mL of distilled water, and then air-dried to constant weight at 25℃ and 50% relative humidity. The performance was then tested, and the results are shown in Table 1.

[0046] The properties of the test block, experimental conditions, and testing methods are as follows:

[0047] 1. The limestone specimen used in the example has the chemical composition of calcite-type calcium carbonate with a content of 93.5%, and the specimen size is 50mm×50mm×20mm. Other details are shown in Table 1.

[0048] 2. The protective effect, density, and bonding strength of the calcium oxalate protective film to the limestone substrate were tested for acid resistance, porosity, water absorption, and bonding strength.

[0049] 3. Acid Resistance Test: In the experiment, a dilute sulfuric acid solution with pH = 5.1 was used to simulate acid rain. The test blocks were immersed in the above dilute sulfuric acid solution, and the pH value of the solution was continuously monitored using a pH meter of model PHSJ-3F.

[0050] 4. Porosity and water absorption rate test: The porosity and water absorption rate of the samples were tested in accordance with the People's Republic of China National Standard GB / T 9966.3-2020 "Test Methods for Natural Stone Part 3".

[0051] 5. Adhesion Test: The adhesion was tested according to the international standard (ASTM D3359-17: Standard Test Methods for Measuring Adhesion by Tape Test, ASTM International, West Conshohocken, PA, 2017). The adhesion was assessed by the peel mass per unit area of ​​the test specimen. The method involves pressing a piece of tape of known mass (m1) and area (A) onto the surface of the specimen, peeling off the tape after 30 seconds, and weighing its mass (m2). The index DI reflects the peel mass of the film (layer) per unit area of ​​the surface, calculated as DI = (m2 - m1) / A. The index DI is inversely proportional to the film adhesion; that is, the higher the value, the weaker the film (layer) adhesion.

[0052] Table 1. Properties of limestone specimens before and after treatment

[0053]

[0054] In summary, when a reducing organic acid solution is brushed onto the surface of limestone, it simultaneously penetrates and deposits within the micro-fractures and pores of the limestone, forming a calcium reducing organic acid film. This calcium reducing organic acid then reacts with peroxides to form a calcium oxalate protective film. Compared to the process of forming a calcium oxalate protective film through the reaction of oxalate with limestone, this method is milder and more controllable, resulting in a more complete reaction. This allows for the slow and orderly deposition and growth of calcium oxalate crystals, leading to a denser and more uniform calcium oxalate protective film (see...). Figure 2 This achieves a more effective technical protection for limestone surfaces.

[0055] Furthermore, the reducing organic acid calcium solution can penetrate and deposit inside the micro-fractures and pores of limestone. The calcium oxalate formed after in-situ oxidation can also act as a micro-anchor, improving the bonding force between the protective film and the limestone. The results show that, using the blank limestone specimen in Comparative Example 1 and the ammonium oxalate-treated specimen in Comparative Example 2 as references, the limestone specimen with the protective film obtained using the method of this application exhibits improved acid resistance, decreased water absorption, and decreased porosity, indicating that the formed calcium oxalate film is more dense. The reduced surface peeling mass in the bonding force test indicates improved bonding force between the calcium oxalate film and the underlying limestone (see Table 1).

[0056] The addition of the alkaline regulator in step two makes the reaction time more controllable. It can effectively control the reaction process without significantly affecting the protective effect, further regulate the reaction time, and achieve controllability of the reaction cycle and stability of the effect.

[0057] The preparation method of this application can be applied to limestone construction, stone carving and other fields. Without affecting the structure of the building itself, the generated calcium oxalate protective film has a good protective effect.

[0058] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of this disclosure are indicated by the appended claims.

Claims

1. A method for preparing a calcium oxalate protective film on a limestone surface, characterized in that: Step 1: Apply an aqueous solution of reducing organic acid to the pretreated limestone surface and seal it to retain water for a certain period of time to obtain limestone covered with a calcium film of reducing organic acid. Step 2: Continue to brush an aqueous solution of peroxide with an alkaline adjuster adjusted to pH 7.8-9.0 onto the limestone surface covered with a reducing organic acid calcium film, and seal it to retain water for a certain period of time to obtain limestone covered with a calcium oxalate protective film. The reducing organic acid is one of glycolic acid and glyoxylic acid; the peroxide is one of hydrogen peroxide, urea peroxide, and peracetic acid.

2. The preparation method according to claim 1, characterized in that, The alkalinity regulator is one or more of ethylamine, ammonia, and ammonium carbonate.

3. The preparation method according to claim 2, characterized in that, The mass percentage concentration of the alkaline regulator is 1.6% to 5.0%.

4. The preparation method according to claim 1, characterized in that, The mass percentage concentration of the reducing organic acid solution is 4.5% to 7.1%; the mass percentage concentration of the peroxide solution is 5.0% to 8.0%.

5. The preparation method according to claim 1, characterized in that, The molar ratio of the reducing organic acid in step one to the peroxide in step two is 1:1.1 to 1:2.

3.

6. The preparation method according to claim 1, characterized in that, The reaction temperature is 15℃ to 30℃, the ambient humidity is 30% to 60%, the reaction time required for step one is 12 hours to 24 hours, and the reaction time required for step two is 20 hours to 48 hours.

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