A nano-calcium-based inorganic carbonized coating, its preparation method and application

Abstract

This invention discloses a nano-lime-based inorganic carbonized coating, its preparation method, and its applications. The preparation method includes: providing nano-lime; wherein the nano-lime has a core-shell structure, with an outer shell of nano-calcium oxide and a core of calcium carbonate; mixing the nano-lime, a water-reducing agent, and water to prepare a slurry; then coating the slurry onto the surface of a substrate; subsequently subjecting it to natural air carbonization and high-concentration carbon dioxide atmosphere carbonization treatment to obtain the nano-lime-based inorganic carbonized coating. This invention uses nano-lime with a core-shell structure as the core raw material, utilizes a water-reducing agent to regulate the slurry's fluidity and water retention properties; after the slurry is coated onto the substrate surface, it undergoes natural air carbonization and high-concentration carbon dioxide atmosphere carbonization treatment to prepare a nano-lime-based inorganic carbonized coating with high bonding strength, high density, resistance to cracking, and strong durability. The preparation process of this invention is simple, low-cost, environmentally friendly, and pollution-free, making it suitable for applications such as building substrate protection and ancient building restoration.

Description

Technical Field

[0001] This invention relates to the field of inorganic coating technology, and in particular to a nano-calcium-based inorganic carbonization coating, its preparation method, and its application. Background Technology

[0002] Inorganic coatings, due to their weather resistance, fire resistance, and environmental friendliness, have broad application prospects in fields such as building substrate protection, ancient building restoration, and infrastructure corrosion prevention. However, existing inorganic coating technologies generally face the following prominent problems: First, the adhesion strength between the coating and the substrate is insufficient, making it prone to peeling and detachment under temperature and humidity changes or external forces after coating; second, the coating density is not ideal, with many micropores and microcracks inside, resulting in poor weather resistance, and it is prone to powdering, cracking, and failure under the influence of freeze-thaw cycles, acid rain erosion, and ultraviolet radiation; third, the preparation process is complex, often requiring high-temperature sintering, special atmosphere protection, or expensive nanomaterial precursors, resulting in high production costs and making it difficult to promote large-scale application.

[0003] Lime coating is a widely used inorganic coating material. However, existing lime coatings generally face the problem of easy cracking, specifically in the following aspects: First, existing technologies mostly use fully calcined calcium oxide or industrial-grade calcium hydroxide as raw materials. The reaction is violent and requires a large amount of water (calcium oxide hydration consumes water, and calcium hydroxide has a high specific surface area, requiring a large amount of water to form a fluid slurry). The carbonization reaction is difficult to control, resulting in rapid carbonization of the coating surface to form a dense shell, followed by insufficient internal carbonization, leading to defects such as uneven surface and internal carbonization, high internal residual stress, and easy cracking. Second, some technologies use inert fillers to reduce cracking during the reaction process. However, these inert fillers do not participate in the reaction process, resulting in interface problems and significantly reducing the mechanical properties of the coating. Third, existing lime coating crack-resistant reinforcing materials are mainly fibers and polymers. Although they can improve the crack resistance of lime coatings, the unit cost of the coating will also increase. Moreover, some reinforcing materials (such as paper pulp and straw fibers) will affect the durability of the coating material, limiting its application in modern engineering.

[0004] In summary, there is an urgent need to provide a new nano-lime-based inorganic carbonized coating to solve the technical problems of low bonding strength, insufficient density, easy cracking, and complex preparation process of existing inorganic coatings. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical deficiencies and propose a nano-lime-based inorganic carbonized coating, its preparation method and application, to solve the technical problems of low bonding strength, insufficient density, easy cracking and complex preparation process of inorganic coatings in the prior art.

[0006] In a first aspect, the present invention provides a method for preparing a nano-calcium-based inorganic carbonized coating, comprising the following steps: Nano-lime is provided; wherein, nano-lime has a core-shell structure, with an outer shell of nano-calcium oxide and a core of calcium carbonate; nano-lime is obtained by partial calcination of limestone; A slurry is prepared by mixing nano-lime, water-reducing agent and water, and then the slurry is coated on the surface of the substrate. Subsequently, it is subjected to natural air carbonization treatment and high-concentration carbon dioxide atmosphere carbonization treatment to obtain nano-lime-based inorganic carbonized coating.

[0007] In a second aspect, the present invention provides a nano-calcium-based inorganic carbonized coating, which is obtained by the preparation method of the nano-calcium-based inorganic carbonized coating provided in the first aspect of the present invention.

[0008] Thirdly, the present invention provides an application of a nano-lime-based inorganic carbonized coating, which is used for the protection of building substrates and the restoration of ancient buildings.

[0009] Compared with the prior art, the beneficial effects of the present invention include: This invention uses nano-lime with a core-shell structure as the core raw material and utilizes a water-reducing agent to regulate the fluidity and water retention of the slurry. After the slurry is coated onto the substrate surface, it undergoes natural air carbonization and high-concentration carbon dioxide atmosphere carbonization treatment to prepare a nano-lime-based inorganic carbonized coating with high bonding strength, high density, resistance to cracking, and high durability. The preparation process of this invention is simple, low-cost, environmentally friendly, and pollution-free, making it suitable for applications such as building substrate protection and ancient building restoration. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the core-shell structure of nano-lime used in this invention. Figure 2 Here is a SEM image of the natural limestone in Example 1 of this invention; Figure 3 This is a SEM image of the core-shell structured nano-lime in Example 1 of the present invention; Figure 4 This is a photograph of the coating after natural air carbonization treatment in Example 1 of the present invention. Figure 4 a) and a physical image of the coating after natural air carbonization treatment in Comparative Example 1 ( Figure 4 b). Detailed Implementation

[0011] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0012] Traditional lime coatings use fully calcined calcium oxide or industrial-grade calcium hydroxide as raw materials. The hydration of fully calcined calcium oxide to form calcium hydroxide slurry generates a significant amount of heat. Furthermore, after coating, the drying shrinkage rate is high during the strength-building process, easily leading to coating cracking. Simultaneously, after coating or masonry work, traditional lime slurry relies on the slow penetration and diffusion of atmospheric carbon dioxide (approximately 0.04% concentration) from the natural environment to gradually undergo a carbonization reaction from the coating surface inwards. This process can last for months or even years, during which calcium hydroxide slowly transforms into calcium carbonate calcite crystals. These crystals gradually grow, cross-link, and fill the pores, ultimately forming a high-density, high-bonding-strength carbonized coating. The advantages of this slow carbonization process are: uniform carbonation reaction, sufficient calcium carbonate crystal development, low internal stress in the coating, and high density. However, its drawbacks are also significant: the excessively long carbonization cycle severely restricts construction efficiency and project progress.

[0013] Based on this, the present invention is proposed.

[0014] In a first aspect, the present invention provides a method for preparing a nano-calcium-based inorganic carbonized coating, comprising the following steps: S1. Provide nano-lime; wherein, nano-lime has a core-shell structure, with an outer shell of nano-calcium oxide and a core of calcium carbonate; nano-lime is obtained by partially calcining limestone (the main component of which is calcium carbonate, CaCO3); S2. Prepare a slurry by mixing nano-lime, water-reducing agent and water, then coat the slurry onto the surface of the substrate, and then perform natural carbonization treatment in air and carbonization treatment in a high-concentration carbon dioxide atmosphere to obtain a nano-lime-based inorganic carbonized coating.

[0015] In this invention, nano-lime with a core-shell structure is used as the core raw material to replace traditional fully calcined lime. The core, calcium carbonate, serves as the inert framework and volume-stabilizing component, while the outer shell, nano-calcium oxide, acts as the active component for carbonization. A water-reducing agent is used to regulate the slurry's flowability and water retention properties to obtain the slurry. After the slurry is coated onto the substrate surface, it undergoes both natural air carbonization and high-concentration carbon dioxide atmosphere carbonization. Leveraging the core advantages of natural air carbonization (uniform carbonization and fully developed crystals), and combined with high-concentration carbon dioxide atmosphere carbonization, a nano-lime-based inorganic carbonized coating is prepared that is firmly bonded, highly dense, resistant to cracking, and highly durable. Compared to traditional lime coatings, this application significantly shortens the carbonization cycle.

[0016] Please refer to Figure 1The nano-lime selected in this invention has a unique core-shell structure. Its outer shell is a layer of nano-sized calcium oxide (CaO) generated during the decomposition of calcium carbonate in the calcination process, while the inner core is incompletely decomposed calcium carbonate. The formation mechanism of this core-shell structure is as follows: under conditions where the calcination temperature is lower than the complete decomposition temperature of calcium carbonate or the calcination time is insufficient for the complete decomposition of calcium carbonate, the limestone particles undergo gradual thermal decomposition from the outside to the inside. The outer layer of calcium carbonate decomposes first into calcium oxide and releases carbon dioxide, forming a calcium oxide shell with a nanoporous structure. Meanwhile, the inner calcium carbonate remains in an undecomposed state due to insufficient heat conduction, thus naturally forming a core-shell structure particle with a nano-sized calcium oxide shell and a calcium carbonate core.

[0017] Among them, the water-reducing agent, through its surface activity and steric hindrance effect, adsorbs onto the surface of nano-lime particles, releasing the free water trapped between the particles. This significantly reduces water consumption (water-cement ratio reduced by 20%-40%) while maintaining the fluidity of the slurry, reducing internal pores caused by water evaporation after coating curing and improving coating density. At the same time, the water-reducing agent's water retention properties can delay the evaporation of water on the slurry surface, providing the necessary moisture environment for the subsequent carbonization reaction. In addition, the dispersing effect of the water-reducing agent can make the nano-lime particles more evenly distributed in the slurry, ensuring the uniformity of the carbonization reaction and thus avoiding local over- or under-carbonization.

[0018] In the slurry preparation stage, the nano-calcium oxide of the nano-lime shell reacts with water to generate calcium hydroxide (hydration reaction). After the slurry is coated onto the substrate surface, it undergoes natural carbonization in air to achieve uniform carbonization and promote full crystal development. Finally, it undergoes carbonization in a high-concentration carbon dioxide atmosphere to prepare a nano-lime-based inorganic carbonized coating that is firmly bonded, not prone to cracking, highly dense, and durable. Compared with traditional lime coatings, this application can significantly shorten the carbonization cycle.

[0019] In this invention, the average grain size of nano-calcium oxide in the outer shell of nano-lime is 50nm-400nm, preferably 50nm-200nm.

[0020] In this invention, the mass ratio of the outer shell nano-calcium oxide to the core calcium carbonate (i.e., the shell-core mass ratio) is 1.5:8.5-6:4, including but not limited to 1.5:8.5, 1.8:8.2, 2.5:7.5, 3:7, 4:6, 5.5:4.5, 6:4, etc.

[0021] The shell-core mass ratio is calculated using the following formula: M CaO = (M1-M2)×56 / 44(1); Shell-core mass ratio = M CaO / (M2-M CaO (2); In the formula, M1 is the mass of limestone before calcination, M2 is the mass of nano-lime after calcination, and M... CaO It refers to the mass of CaO produced after calcination.

[0022] This invention precisely controls the proportion of active components participating in the carbonization reaction by adjusting the shell-core mass ratio of nano-lime, thereby controlling the drying shrinkage rate of the lime coating and ensuring that the lime coating does not crack during the drying process, achieving a bond strength higher than 1.5 MPa. Furthermore, the inventors discovered in experiments that when the shell-core mass ratio is 3:7, the ratio of active calcium oxide to inert calcium carbonate core in the system is most balanced. This provides sufficient precursors for the carbonization reaction while the residual calcium carbonate core acts as a volume stabilizer and micro-aggregate support, thus achieving optimal overall bond strength, freeze-thaw resistance, and weather resistance.

[0023] In this invention, the limestone is natural limestone.

[0024] In this invention, the purity of limestone is ≥80%, preferably ≥90%, and more preferably ≥95%.

[0025] In this invention, the limestone has a particle size of 5-75 μm.

[0026] In this invention, the partial calcination process includes: heating from room temperature to 750℃-950℃ (preferably 800℃-900℃, more preferably 850℃) at a heating rate of 8-12℃ / min (preferably 10℃ / min), and calcining at 750℃-950℃ (preferably 800℃-900℃, more preferably 850℃) for 1-10 minutes (preferably 3-7 minutes, more preferably 5 minutes), followed by stopping heating and allowing natural cooling to room temperature. By employing the above-mentioned calcination temperature and time, and matching it with the raw material particle size, this invention can obtain nano-lime that meets the above-mentioned core-shell mass ratio range.

[0027] In this invention, the mass ratio of nano-lime to water is 100:(25-60), preferably 100:(30-50), and more preferably 100:(35-45).

[0028] In this invention, the mass ratio of nano-lime to water-reducing agent is 100:(0.1-3.0), preferably 100:(0.3-1.5), and more preferably 100:(0.5-1).

[0029] This invention does not limit the type of water-reducing agent, and those skilled in the art can select one according to the actual situation. For example, a polycarboxylate-based high-efficiency water-reducing agent (PCA®-Ⅰ series polycarboxylate high-performance water-reducing agent from Subote New Materials Co., Ltd.) with a solid content of 40% can be selected, or a naphthalene-based water-reducing agent (SBTJM®-B naphthalene-based high-efficiency water-reducing agent from Subote New Materials Co., Ltd.) can be selected.

[0030] In this invention, the preparation of slurry by mixing nano-lime, water-reducing agent and water includes: first premixing water and water-reducing agent, then adding nano-lime and stirring evenly to obtain slurry.

[0031] Preferably, during the stirring process, planetary stirring or high-speed dispersion stirring is used, with a stirring speed of 300rpm-2000rpm, preferably 500rpm-1500rpm, and more preferably 1000rpm; the stirring time is 5 minutes-30 minutes, preferably 10 minutes-20 minutes, and more preferably 15 minutes.

[0032] In this invention, the fluidity of the slurry is 140mm-200mm (measured according to GB / T 2419 standard), preferably 160mm-180mm.

[0033] This invention does not limit the type of substrate, and those skilled in the art can select it according to the actual situation. For example, the substrate can be at least one of the following: mortar specimen surface, concrete component surface, brick and stone surface, or natural stone surface.

[0034] In this invention, the substrate surface is pretreated before coating, and the pretreatment includes: pre-wetting the substrate surface with water until it is saturated and surface dry, so as to improve the interfacial adhesion between the slurry and the substrate.

[0035] In this invention, before wetting the substrate surface with water to a saturated surface-dry state, the following pretreatment can be performed according to the surface condition of the substrate: removing floating dust, oil stains and / or loose layers.

[0036] This invention does not limit the coating method, and those skilled in the art can choose according to the actual situation, including but not limited to scraping, spraying, roller coating or brushing.

[0037] In this invention, the temperature during the coating process is 19℃-30℃ and the relative humidity is 55%-75%.

[0038] In this invention, the coating thickness is ≤1mm, further 0.2mm-0.8mm, and even further 0.5mm.

[0039] In this invention, the temperature during the natural air carbonization process is 19℃-30℃, and the relative humidity is 55%-75%. Natural air carbonization refers to carbonization performed in a natural air environment. This invention does not exert excessive control over CO2 concentration and air pressure, allowing them to be controlled by natural environmental conditions. However, relative humidity has a significant impact on natural air carbonization; if the relative humidity is too low, the coating is prone to drying and cracking; if the relative humidity is too high, the coating is difficult to harden.

[0040] In this invention, the air-carbonization process involves a natural carbonization time of 0.5 hours to 8 hours, preferably 3 hours to 8 hours, and more preferably 4 hours to 6 hours. The natural carbonization time refers to the interval between coating completion and high-concentration carbon dioxide atmosphere carbonization treatment. If the natural carbonization time is too short, a stable structure cannot be formed inside the coating, resulting in uneven distribution of carbonization precursors and uneven carbonization. If the natural carbonization time is too long, the slurry surface becomes excessively dry, hindering subsequent carbon dioxide penetration and diffusion, and reducing carbonization efficiency.

[0041] In this invention, the temperature during the high-concentration carbon dioxide atmosphere carbonization process is 19℃-30℃, and the pressure is atmospheric pressure.

[0042] In this invention, during the high-concentration carbon dioxide atmosphere carbonization process, a carbon dioxide-rich atmosphere is used, with a carbon dioxide volume concentration of 5%-50%, preferably 10%-40%, and more preferably 15%-20%. Compared to the prior art that relies solely on the atmospheric carbon dioxide concentration of approximately 0.04% for natural carbonization, this invention increases the carbon dioxide concentration in the carbonization atmosphere through high-concentration carbon dioxide atmosphere carbonization, thereby increasing the carbonization reaction rate by approximately 250-750 times while maintaining a gradual and uniform carbonization reaction, and significantly shortening the carbonization cycle.

[0043] In this invention, during the high-concentration carbon dioxide atmosphere carbonization process, the relative humidity is 50%-95%, preferably 60%-80%. Appropriate humidity ensures that carbon dioxide dissolves in the coating surface and internal pore water to form carbonic acid, promoting the liquid-phase reaction between carbonic acid and calcium hydroxide.

[0044] In this invention, during the high-concentration carbon dioxide atmosphere carbonization process, the carbonization time is 2-12 hours, preferably 3-8 hours, and most preferably 4-6 hours. The selection of the carbonization time is closely related to the core-shell mass ratio, the carbon dioxide concentration in the carbonization atmosphere, and the coating thickness. The higher the core-shell mass ratio (i.e., the higher the content of active calcium oxide), the longer the required carbonization time; the higher the carbon dioxide concentration, the faster the carbonization rate, and the shorter the required carbonization time; the thicker the coating, the longer the diffusion path of carbon dioxide from the surface to the interior, and the longer the required carbonization time.

[0045] In a second aspect, the present invention provides a nano-calcium-based inorganic carbonized coating, which is obtained by the preparation method of the nano-calcium-based inorganic carbonized coating provided in the first aspect of the present invention.

[0046] In this invention, the bonding strength between the nano-lime-based inorganic carbonized coating and the substrate is not less than 1.5 MPa, the water absorption rate is not higher than 8%, and the mass loss after 50 freeze-thaw cycles does not exceed 2%.

[0047] Thirdly, the present invention provides an application of a nano-lime-based inorganic carbonized coating, which is used for the protection of building substrates and the restoration of ancient buildings.

[0048] To avoid redundancy, the test methods in the following embodiments and comparative examples of this invention are as follows: Bond strength: determined by pull-out method, in accordance with JGJ / T 70 standard; Taber wear rate: in accordance with GB / T 1768-2006 standard; 50 freeze-thaw cycles: Performed according to the coating temperature resistance test in GB / T 9755-2014 standard; 1000-hour UV aging test: Performed according to GB / T 16422.3-2014 standard. A QUV accelerated UV aging test chamber was used, equipped with a UVA-340 fluorescent UV lamp (peak wavelength 340nm), and the irradiance was set to 0.89 W / m². 2 @340nm. The test cycle was as follows: alternating 8h of UV irradiation (black standard temperature 60℃±3℃) and 4h of condensation (black standard temperature 50℃±3℃), for a total exposure time of 1000h. After the test, the samples were allowed to recover in a standard environment (23℃, RH 50%) for 24h, and the coating adhesion strength was measured. The adhesion strength retention rate was the ratio of the adhesion strength after aging to the initial adhesion strength.

[0049] Example 1 (1) Select natural limestone with a calcium carbonate content of 97% and ground to a particle size of 5-75 μm (e.g. Figure 2 As shown), the temperature was increased from room temperature to 850℃ at a rate of 10℃ / min, and calcined at 850℃ for 5 minutes. Heating was then stopped, and the mixture was allowed to cool naturally to room temperature to obtain nano-lime with a core-shell mass ratio of 3:7. The average grain size of the outer shell nano-calcium oxide was 50-150 nm (as shown). Figure 3 (As shown).

[0050] (2) According to the mass ratio of nano-lime: water: water-reducing agent (polycarboxylic acid system, solid content 40%, water reduction rate 30%) = 100:40:0.8, first premix the water and water-reducing agent, then add the nano-lime powder, and stir at 1000 rpm for 15 minutes to obtain the slurry (flowability 170 mm).

[0051] (3) The slurry was applied to the surface of the mortar specimen that had been moistened with water by scraping. The temperature was 25℃, the relative humidity was 60%, and the coating thickness was 0.5mm. After the coating was completed, it was naturally cured in the air for 6 hours (i.e., natural air carbonization treatment). Then it was placed in a carbonization box for high-concentration carbon dioxide atmosphere carbonization treatment to obtain nano-lime-based inorganic carbonized coating. The conditions for natural air carbonization treatment were: temperature 25℃, relative humidity 65%, CO2 concentration and air pressure were natural environmental conditions. The conditions for high-concentration carbon dioxide atmosphere carbonization treatment were: normal pressure, CO2 concentration 20% (volume fraction), relative humidity 65%, temperature 25℃, and carbonization time 6 hours.

[0052] Performance test results: After natural air carbonization treatment (before high-concentration carbon dioxide atmosphere carbonization treatment), the coating surface is smooth and no cracks appear (e.g. Figure 4 (as shown in a) After carbonization treatment in a high-concentration carbon dioxide atmosphere, the bonding strength between the coating and the mortar specimen was 4.26 MPa, the Taber abrasion was 32 mg, and the mass loss after 50 freeze-thaw cycles was 1.1%; after 1000 hours of ultraviolet aging, the bonding strength retention rate was 91%, and there were no visible cracks, powdering, or peeling on the coating surface.

[0053] Example 2 (1) Select natural limestone with a calcium carbonate content of 97% and a particle size of 5-75 μm. Heat it from room temperature to 800℃ at a heating rate of 10℃ / min and calcine it at 800℃ for 5 minutes. Then stop heating and let it cool naturally to room temperature to obtain nano lime with a shell-core mass ratio of 1.8:8.2.

[0054] (2) According to the mass ratio of nano-lime: water: water-reducing agent (polycarboxylic acid system, solid content 40%, water reduction rate 30%) = 100:40:0.5, first premix the water and water-reducing agent, then add the nano-lime powder, and stir at 1000 rpm for 15 minutes to obtain the slurry (flowability 170 mm).

[0055] (3) The slurry was applied to the surface of the mortar specimen that had been moistened with water by scraping. The temperature was 25℃, the relative humidity was 60%, and the coating thickness was 0.5mm. After the coating was completed, it was naturally cured in the air for 6 hours (i.e., natural air carbonization treatment). Then it was placed in a carbonization box for high-concentration carbon dioxide atmosphere carbonization treatment to obtain nano-lime-based inorganic carbonized coating. The conditions for natural air carbonization treatment were: temperature 25℃, relative humidity 65%, CO2 concentration and air pressure were natural environmental conditions. The conditions for high-concentration carbon dioxide atmosphere carbonization treatment were: normal pressure, CO2 concentration 20% (volume fraction), relative humidity 65%, temperature 25℃, and carbonization time 6 hours.

[0056] Performance test results: bond strength 1.89MPa, Taber abrasion loss 48mg, mass loss 1.3% after 50 freeze-thaw cycles; bond strength retention rate 93% after 1000 hours of UV aging, and no visible cracks, chalking or peeling on the coating surface.

[0057] Example 3 (1) Select natural limestone with a calcium carbonate content of 97% and a particle size of 5-75 μm. Heat it from room temperature to 900℃ at a heating rate of 10℃ / min and calcine it at 900℃ for 5 minutes. Then stop heating and let it cool naturally to room temperature to obtain nano lime with a shell-core mass ratio of 5.5:4.5.

[0058] (2) According to the mass ratio of nano-lime: water: water-reducing agent (polycarboxylic acid system, solid content 40%, water reduction rate 30%) = 100:45:1.0, first premix the water and water-reducing agent, then add the nano-lime powder, and stir at 1000 rpm for 15 minutes to obtain the slurry (flowability 170 mm).

[0059] (3) The slurry was applied to the surface of the mortar specimen that had been moistened with water by scraping. The temperature was 25℃, the relative humidity was 60%, and the coating thickness was 0.5mm. After the coating was completed, it was naturally cured in the air for 6 hours (i.e., natural air carbonization treatment). Then it was placed in a carbonization box for high-concentration carbon dioxide atmosphere carbonization treatment to obtain nano-lime-based inorganic carbonized coating. The conditions for natural air carbonization treatment were: temperature 25℃, relative humidity 65%, CO2 concentration and air pressure were natural environmental conditions. The conditions for high-concentration carbon dioxide atmosphere carbonization treatment were: normal pressure, CO2 concentration 20% (volume fraction), relative humidity 65%, temperature 25℃, and carbonization time 6 hours.

[0060] Performance test results: bond strength 3.48MPa, Taber abrasion loss 36mg, mass loss 1.2% after 50 freeze-thaw cycles; bond strength retention rate 88% after 1000 hours of UV aging, and no visible cracks, chalking or peeling on the coating surface.

[0061] Example 4 Compared with Example 1, the only difference is that in step (3), the time for natural carbonization of air and the conditions for carbonization in a high-concentration carbon dioxide atmosphere are different, as follows: the time for natural carbonization of air is 4 hours; the conditions for carbonization in a high-concentration carbon dioxide atmosphere are: normal pressure, CO2 concentration 10% (volume fraction), relative humidity 65%, temperature 25°C, and carbonization time 8 hours.

[0062] The remaining steps and preparation conditions are the same as in Example 1.

[0063] Performance test results: bond strength 3.59MPa, Taber abrasion loss 34mg, mass loss 1.2% after 50 freeze-thaw cycles; bond strength retention rate 91% after 1000 hours of UV aging, and no visible cracks, chalking or peeling on the coating surface.

[0064] Example 5 Compared with Example 1, the only difference is that the conditions for carbonization treatment in step (3) are different, as follows: the conditions for carbonization treatment in high concentration carbon dioxide atmosphere are: atmospheric pressure, CO2 concentration 40% (volume fraction), relative humidity 65%, temperature 25℃, and carbonization time 3 hours.

[0065] The remaining steps and preparation conditions are the same as in Example 1.

[0066] Performance test results: bond strength 3.21MPa, Taber abrasion loss 39mg, mass loss 1.3% after 50 freeze-thaw cycles; bond strength retention rate 91% after 1000 hours of UV aging, and no visible cracks, chalking or peeling on the coating surface.

[0067] Comparative Example 1 Compared with Example 1, the only difference is that step (1) is omitted, and in step (2), industrial-grade fully calcined calcium oxide (calcium oxide content 95%) is used instead of nano-lime. Step (2) is as follows: (2) According to the mass ratio of lime:water:water-reducing agent (polycarboxylate-based, solid content 40%, water reduction rate 30%) = 100:150:0.8, water and water-reducing agent are premixed first, then lime powder is added, and the mixture is stirred at 1000 rpm for 15 minutes to obtain a slurry (flowability 170 mm). In this step, the water consumption is significantly increased because the reaction of calcium oxide with water consumes water, the reaction releases heat and evaporates water, and calcium hydroxide requires a large amount of water to ensure fluidity.

[0068] The remaining steps and preparation conditions are the same as in Example 1.

[0069] Performance test results: After natural air carbonization, the coating layer showed significant shrinkage and cracking (e.g. Figure 4 As shown in b), the relevant tests cannot be performed.

[0070] Comparative Example 2 Compared with Example 1, the only difference is that in step (3), carbonization in a high-concentration carbon dioxide atmosphere is performed immediately after coating (the time for natural carbonization in air is 0h).

[0071] The remaining steps and preparation conditions are the same as in Example 1.

[0072] Performance test results: Before carbonization, a stable structure could not be formed, the carbonization precursor was unevenly distributed, and the pores were still filled with a large amount of water, which hindered the carbonization of calcium hydroxide and the coating carbonization was uneven; the surface bond strength was 0.9 MPa, the Taber abrasion loss was 62 mg, and the mass loss after 50 freeze-thaw cycles was 3.8%; after 1000 hours of ultraviolet aging, the bond strength retention rate was 72%, and local powdering appeared on the coating surface. The performance uniformity and reliability were significantly worse than those of Example 1.

[0073] Comparative Example 3 Compared with Example 1, the only difference is that in step (3), after coating, the material is naturally cured in the air for 12 hours and then carbonized in a high-concentration carbon dioxide atmosphere.

[0074] The remaining steps and preparation conditions are the same as in Example 1.

[0075] Performance test results: Due to the excessive time spent in the air, the subsequent carbonization process was limited. The surface adhesion strength of the coating was 1.1 MPa, the Taber abrasion was 58 mg, and the mass loss after 50 freeze-thaw cycles was 3.2%. After 1000 hours of UV aging, the adhesion strength retention rate was 76%, and microcracks appeared on the surface. The performance uniformity and reliability were significantly worse than those of Example 1.

[0076] Comparative Example 4 Compared with Example 1, the only difference is that in step (2), the nano-lime of Example 1 is used, but no water-reducing agent is added, and the water content is increased to 100:65 to obtain a similar slurry fluidity.

[0077] The remaining steps and preparation conditions are the same as in Example 1.

[0078] Performance test results: Due to excessive water consumption, the coating has a large internal porosity after curing, the bonding strength is only 1.2 MPa, the Taber abrasion loss is 75 mg, and the mass loss after 50 freeze-thaw cycles is 5.3%; after 1000 hours of UV aging, the bonding strength retention rate is 65%, the coating surface shows obvious powdering and local peeling, and the durability is significantly worse than that of Example 1.

[0079] As can be seen from Examples 1-3, the ratio of calcium oxide shell to calcium carbonate core in the core-shell structure has a significant impact on coating performance. From the perspective of bond strength, there is an optimal range for the shell-core mass ratio. When the calcination degree is insufficient and the proportion of calcium oxide shell is low (as in Example 2), the amount of active calcium oxide that can participate in hydration and subsequent carbonation reactions is insufficient, resulting in limited calcium carbonate gel products. This makes it difficult to form a dense calcium carbonate network structure between particles and at the coating-substrate interface, leading to low bond strength. When the calcination degree is high and the proportion of calcium oxide shell is high (as in Example 3), although the content of active calcium oxide increases, the relative volume of the calcium carbonate core "skeleton" decreases. During hydration and carbonation, the internal stress caused by volume changes increases, potentially generating more microcracks in the coating's microstructure, which is detrimental to further improvement of mechanical properties. Furthermore, the bond strength retention rate after 1000 hours of UV aging in Example 3 was 88%, lower than 91% in Example 1 and 93% in Example 2, further supporting the inference that an excessively high proportion of calcium oxide shell may lead to increased internal defects in the coating and a decrease in long-term durability. From the perspective of mass loss during freeze-thaw cycles, the differences among the three sets of embodiments were small (1.1%-1.3%), all showing good freeze-thaw resistance. This indicates that within the shell-core mass ratio range covered by this invention, the overall density of the calcium carbonate coating formed by carbonization is at a high level, which can effectively resist the damage to the coating caused by freeze-thaw cycles.

[0080] Examples 1, 4, and 5 show that the efficiency and uniformity of the carbonization reaction are synergistically affected by CO2 concentration and carbonization time. When the CO2 concentration is low (Example 4), even with an extended carbonization time of 8 hours, the driving force for CO2 diffusion within the coating is insufficient, resulting in a slow carbonization reaction rate. Insufficient carbonization may exist in the deeper layers of the coating, leading to a decrease in overall strength. When the CO2 concentration is high (Example 5), the thermodynamic driving force for the carbonization reaction increases, and the surface layer of the coating rapidly carbonizes under high CO2 concentrations to form a dense calcium carbonate layer. This hinders further diffusion of CO2 within the coating, creating a gradient structure of "dense on the surface and sparse on the inside," resulting in relatively insufficient carbonization within the coating and a decrease in overall bonding strength. Furthermore, Example 4... The air-carbonization time was shortened to 4 hours, and the uniformity of calcium hydroxide precursor distribution and the degree of crystallization were not as good as in Example 1 (air-carbonization time of 6 hours), which is one of the reasons why its bonding strength was slightly lower than that in Example 1. From the perspective of mass loss after freeze-thaw cycles and the retention rate of bonding strength after UV aging, the three sets of examples showed similar performance (mass loss of 1.1%-1.3%, and retention rate of 91% for all), indicating that within the range of carbonization process parameters covered by this invention, the coating can obtain good durability. The change of carbonization process parameters has a relatively limited impact on long-term durability, but a more significant impact on short-term mechanical strength.

[0081] As demonstrated in Example 1 and Comparative Example 1, the coating formed in Comparative Example 1, which used industrial-grade fully calcined calcium oxide instead of the core-shell structured nano-lime in Example 1, exhibited significant shrinkage and cracking even before carbonization in a high-concentration carbon dioxide atmosphere. This is because the fully calcined calcium oxide particles lack a calcium carbonate core, resulting in a violent exothermic hydration reaction upon contact with water, accompanied by significant volume expansion. Furthermore, after hydration, the excess calcium hydroxide undergoes substantial shrinkage during strength formation and hardening, causing the internal stress of the coating to exceed its early tensile strength, leading to macroscopic cracking. In contrast, the core-shell structured nano-lime used in this invention does not participate in the hydration reaction, acting as an inert micro-aggregate and a volume-stabilizing framework. This effectively constrains and disperses the volume changes during the hydration and hardening of the outer calcium oxide shell, fundamentally inhibiting coating cracking. These results fully demonstrate that the core-shell structure is a key technical feature for achieving crack-free curing of the coating in this invention.

[0082] As can be seen from Example 1 and Comparative Example 2, in Comparative Example 2, the air-based natural carbonization curing stage was omitted after coating, and the coating was directly placed in a carbonization chamber for high-concentration carbon dioxide atmosphere carbonization treatment (air-based natural carbonization time was 0 h). The coating adhesion strength was only 0.9 MPa, far lower than the 4.26 MPa of Example 1, and the carbonization uniformity and reliability were significantly deteriorated. The reason for this is that the core role of the air-based natural carbonization treatment stage is mainly as follows: First, during natural carbonization, the coated slurry gradually dries, and its internal pores change from being filled with water to being filled with air; second, although the calcium carbonate inside the core-shell structure can inhibit most of the drying shrinkage during the drying process, shrinkage deformation will still lead to the rearrangement of the internal structure of the coating, thereby forming a uniform and stable network structure; finally, during the gradual evaporation of moisture and the reformation of the network structure, CO2 in the air will react with calcium hydroxide to form trace amounts of calcium carbonate microcrystals. These calcium carbonate crystals are uniformly distributed in the network structure, forming a continuous carbonization reaction precursor. If this stage is omitted and CO2 is introduced directly, on the one hand, the pores are filled with moisture, which hinders the diffusion of CO2 and affects the reaction process; on the other hand, excessively high concentrations of CO2 react with calcium hydroxide, and since the internal structure of the coating has not yet rearranged and lacks calcium carbonate microcrystals, the carbonized coating structure will be uneven, affecting the coating performance. The above results fully verify the indispensability of the natural air carbonization curing stage for ensuring subsequent carbonization efficiency, achieving carbonization uniformity, and high coating performance.

[0083] As shown in Example 1 and Comparative Example 3, extending the air-carbonization time to 12 hours in Comparative Example 3, far exceeding the 6 hours in Example 1, resulted in a coating adhesion strength of only 1.1 MPa, approximately 26% of that in Example 1. Microcracks appeared on the coating surface. This is because, although the CO2 concentration was low (approximately 0.04%) during the air-carbonization stage, the calcium hydroxide on the coating surface reacted with CO2 in the air during the 12-hour exposure, forming a dense calcium carbonate shell. This dense shell created a "sealing effect": on the one hand, it blocked the diffusion and penetration path of high-concentration CO2 into the coating interior during the subsequent high-concentration carbon dioxide atmosphere carbonization treatment, hindering the carbonization of calcium hydroxide in the deeper layers of the coating; on the other hand, the rigid shell formed by premature carbonization on the surface and the uncarbonized flexible substrate inside exhibited significant differences in stiffness and volume stability. Under the subsequent effects of drying shrinkage and temperature changes, stress concentration occurred at the interface, inducing the initiation and propagation of microcracks. These results fully verify the indispensability of appropriate air-carbonization curing time for the final mechanical properties of the coating.

[0084] As can be seen from Example 1 and Comparative Example 4, no water-reducing agent was added in Comparative Example 4. By significantly increasing the amount of water (the water-cement ratio was increased from 0.40 to 0.65), a slurry fluidity similar to that of Example 1 was obtained. The coating adhesion strength dropped to 1.2 MPa (only about 28% of that of Example 1), and the mass loss after 50 freeze-thaw cycles increased sharply to 5.3% (nearly 5 times that of Example 1). The fundamental reason is that the excessive amount of water left a large number of capillary pores and interconnected channels due to water evaporation during the coating curing process, which significantly increased the porosity inside the coating. The high porosity directly weakened the mechanical strength of the coating and the interfacial adhesion with the substrate. On the other hand, it provided a convenient channel for water intrusion. During the freeze-thaw cycle, the water repeatedly froze and expanded, accelerating the internal damage and surface peeling of the coating. The above results fully demonstrate the key role of the water-reducing agent in the system of this invention: through the dispersion and water-reducing effect of the water-reducing agent, the water consumption is controlled to the minimum necessary level while ensuring the workability of the slurry, thereby obtaining a coating structure with low porosity and high density, and achieving dual protection of mechanical properties and durability.

[0085] In summary, this invention uses nano-lime with a core-shell structure as the core raw material for the coating, utilizes a water-reducing agent to regulate the fluidity and water retention of the slurry, and then subjects it to natural air carbonization and high-concentration carbon dioxide atmosphere carbonization treatments. Through the coordinated control of key parameters such as the core-shell mass ratio of the nano-lime, the water-water-reducing agent ratio, the time of natural air carbonization, the carbonization atmosphere concentration, carbonization temperature, and carbonization time during high-concentration carbon dioxide atmosphere carbonization, a nano-lime-based inorganic carbonized coating with high bonding strength, high density, resistance to cracking, and strong durability is prepared. Compared with existing industrial inorganic coatings, the coating of this invention significantly improves bonding strength, density, and weather resistance. Furthermore, the preparation process is simple, requiring no high-temperature sintering or special atmosphere protection (the carbon dioxide in the carbonization atmosphere is an industrial byproduct, widely available and inexpensive), and can be completed under normal temperature and pressure conditions, exhibiting significant economic and scalability advantages. The specific advantages of this invention are as follows: (1) Adhesion strength: The adhesion strength between the coating of the present invention and the substrate is not less than 1.5 MPa, preferably not less than 2.0 MPa, and can reach more than 3.0 MPa, which is significantly better than the existing conventional inorganic coatings (usually 0.5-1.2 MPa).

[0086] (2) Weather resistance: After 1000 hours of accelerated UV aging test, the coating of the present invention retains an adhesion strength of not less than 85%, and there is no powdering, blistering and peeling on the coating surface.

[0087] (3) Preparation efficiency: Compared with traditional calcium carbonate coatings, the inorganic carbonized coating of the present invention shortens the carbonization cycle from several months to several years to several hours to several days, and the preparation efficiency is significantly improved.

[0088] (4) Raw material cost: The present invention replaces traditional crack-resistant reinforcing materials with cheap and readily available limestone and industrial water-reducing agents, which significantly reduces the cost.

[0089] (5) Environmental adaptability: The present invention avoids the risk of long-term performance degradation of the coating due to mold and degradation of the crack-resistant reinforcing fiber or polymer organic components in a humid environment. The coating is a pure inorganic system with better long-term durability and environmental stability.

[0090] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a nano-calcium-based inorganic carbonized coating, characterized in that, Includes the following steps: A nano-lime is provided; wherein the nano-lime has a core-shell structure, with an outer shell of nano-calcium oxide and a core of calcium carbonate; the nano-lime is obtained by partial calcination of limestone; The nano-lime, water-reducing agent and water are mixed to prepare a slurry, which is then coated onto the surface of a substrate. Subsequently, the substrate undergoes natural air carbonization treatment and high-concentration carbon dioxide atmosphere carbonization treatment to obtain a nano-lime-based inorganic carbonized coating.

2. The method for preparing the nano-calcium-based inorganic carbonized coating according to claim 1, characterized in that, In the outer shell of the nano-lime, the average grain size of the nano-calcium oxide is 50nm-400nm; and / or, The mass ratio of the outer shell nano-calcium oxide to the core calcium carbonate is 1.5:8.5-6:

4.

3. The method for preparing the nano-calcium-based inorganic carbonized coating according to claim 1, characterized in that, The limestone is natural limestone; and / or The purity of the limestone is ≥80%; and / or, The limestone has a particle size of 5-75 μm.

4. The method for preparing the nano-calcium-based inorganic carbonized coating according to claim 1, characterized in that, The partial calcination process includes: heating from room temperature to 750℃-950℃ at a heating rate of 8-12℃ / min, calcining at 750℃-950℃ for 1-10 minutes, then stopping heating and allowing it to cool naturally to room temperature.

5. The method for preparing the nano-calcium-based inorganic carbonized coating according to claim 1, characterized in that, The mass ratio of the nano-lime to water is 100:(25-60); and / or, The mass ratio of the nano-lime to the water-reducing agent is 100:(0.1-3.0).

6. The method for preparing the nano-calcium-based inorganic carbonized coating according to claim 1, characterized in that, The preparation of a slurry by mixing the nano-lime, water-reducing agent, and water includes: first premixing the water and water-reducing agent, then adding the nano-lime and stirring until homogeneous to obtain the slurry; wherein, During the stirring process, planetary stirring or high-speed dispersion stirring is used, with a stirring speed of 300 rpm-2000 rpm and a stirring time of 5 minutes-30 minutes; and / or, The fluidity of the slurry is 140mm-200mm.

7. The method for preparing the nano-lime-based inorganic carbonized coating according to claim 1, characterized in that, Before coating, the substrate surface is pretreated, and the pretreatment includes: pre-wetting the substrate surface with water until it is saturated and surface-dry; and / or, During the coating process, the temperature is 19℃-30℃, and the relative humidity is 55%-75%; and / or, The coating thickness is ≤1mm.

8. The method for preparing the nano-calcium-based inorganic carbonized coating according to claim 1, characterized in that, During the aforementioned natural carbonization treatment of air, the temperature is 19℃-30℃ and the relative humidity is 55%-75%; and / or, During the aforementioned air natural carbonization treatment, the air natural carbonization time is 0.5 hours to 8 hours; and / or, During the high-concentration carbon dioxide atmosphere carbonization process, the temperature is 19℃-30℃ and the pressure is atmospheric pressure; and / or, During the high-concentration carbon dioxide atmosphere carbonization process, a carbon dioxide-rich atmosphere is used, with a carbon dioxide volume concentration of 5%-50%; and / or, During the high-concentration carbon dioxide atmosphere carbonization process, the relative humidity is 50%-95%; and / or, During the high-concentration carbon dioxide atmosphere carbonization process, the carbonization time is 2 to 12 hours.

9. A nano-calcium-based inorganic carbonized coating, characterized in that, The nano-lime-based inorganic carbonized coating is obtained by the preparation method of the nano-lime-based inorganic carbonized coating according to any one of claims 1-8.

10. An application of the nano-lime-based inorganic carbonized coating as described in claim 9, characterized in that, The nano-lime-based inorganic carbonized coating is used for the protection of building substrates and the restoration of ancient buildings.

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