A triggered precipitation type concrete crack repair agent and its preparation method and application

Abstract

The present invention relates to the technical field of concrete crack repair agents, and specifically to a triggered precipitation type concrete crack repair agent, its preparation method and application. The repair agent is composed of the following raw materials: sulphoaluminate cement, core-shell structure nano-hard gypsum, fluorine-phosphorus composite crystal seeds, magnesium oxide, metakaolin, and a water reducer; wherein the core-shell structure nano-hard gypsum is made by coating mesoporous silica with nano-hard gypsum, and the fluorine-phosphorus composite crystal seeds are made by hydrothermal reaction of calcium fluoride and calcium dihydrogen phosphate. The triggered precipitation type concrete crack repair agent prepared by the present invention regulates the crystal morphology of calcium aluminate by slowly releasing sulfate ions through the core-shell structure nano-hard gypsum and regulating the fluorine-phosphorus composite crystal morphology, thereby solving the problems of late shrinkage and shedding, sudden drop in high-temperature strength and insufficient bonding strength of traditional repair agents, and is suitable for concrete structure crack repair and high-temperature environment engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete crack repair agents, and in particular to a triggered precipitation type concrete crack repair agent, a preparation method thereof and an application thereof. Background Art

[0002] Cracks in concrete structures are a common problem in construction projects. They not only affect the aesthetics of the structure but can also lead to safety hazards such as steel corrosion, water seepage, and reduced bearing capacity. As a key repair material, crack repair agents are crucial for their long-term stability and environmental adaptability. However, traditional sulfoaluminate concrete crack repair agents generally have two major technical bottlenecks:

[0003] First, during the hydration of sulfoaluminate cement, tricalcium aluminate (CA) reacts with gypsum to form ettringite (AFt). However, once the gypsum is depleted, AFt readily reacts with unreacted CA to form monosulfide-type calcium sulfoaluminate hydrate (AFm). AFm exhibits poor stability, with a volume approximately 12% smaller than AFt, leading to shrinkage and cracking of the patch during later stages. Furthermore, the AFm structure readily adsorbs aggressive ions such as CO2, accelerating carbonation reactions and causing interfacial bond failure and patch shedding. Second, at high temperatures, traditional sulfoaluminate patching agents face a sudden drop in strength. Above 70°C, as the SO4²⁻ in the cement is depleted, AFt irreversibly converts to AFm, resulting in volume shrinkage and cementitious phase failure. Furthermore, traditional ettringite exhibits needle-shaped crystals, which, at high temperatures, lead to intergranular stress concentration and microcracks, resulting in reduced strength retention above 70°C. Furthermore, at high temperatures, AFm forms a porous phase after structural water loss, further exacerbating strength degradation and making it difficult to meet long-term service requirements. At the same time, the bonding interface of existing sulfoaluminate cement repair agents relies on physical biting, lacks chemical anchoring effect, and has insufficient tensile bonding strength, which cannot effectively resist crack expansion under dynamic loads. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to propose a triggered precipitation concrete crack repair agent and its preparation method and application, so as to solve the shrinkage and shedding problem caused by the formation of AFm in the later stage of sulfoaluminate cement repair agent, while improving the high-temperature strength retention rate and significantly improving the service performance in high-temperature environment.

[0005] Based on the above purpose, the present invention provides a triggered precipitation type concrete crack repair agent, comprising the following raw materials in parts by weight: 50-60 parts of sulfoaluminate cement, 8-12 parts of core-shell structured nano-anhydrite, 2-4 parts of fluorine-phosphorus composite seed crystals, 1-3 parts of magnesium oxide, 10-15 parts of metakaolin, and 0.3-0.5 parts of water reducer;

[0006] The preparation process of the core-shell structured nano anhydrite is as follows:

[0007] (1) Add hexadecyltrimethylammonium bromide, ethanol, and ammonia water to deionized water, heat to 50-70°C, stir for 30-60 minutes, and cool to room temperature to obtain a pre-solution;

[0008] (2) Adding nano-hard gypsum to the pre-liquid obtained in (1), ultrasonicating for 30-60 minutes, and stirring simultaneously to obtain a suspension;

[0009] (3) Under nitrogen protection, ethyl silicate is added to the suspension obtained in (2), stirred at room temperature at 1000-1200 rpm for 1-3 min, then stirred at 300-500 rpm for 8-12 h, allowed to stand for 4-8 h, filtered, washed, calcined in a muffle furnace at 500-600 ° C for 3-5 h, cooled to room temperature, and sieved to obtain core-shell structured nano-hard gypsum;

[0010] The preparation process of the fluorine-phosphorus composite seed crystal is as follows:

[0011] Calcium fluoride, calcium dihydrogen phosphate and deionized water are added to a hydrothermal kettle, the pH is adjusted to 4.5-5.5, the temperature is raised to 180-220° C., the reaction is stirred for 12-24 hours, and after cooling to room temperature, the reaction product is centrifuged and washed, and then ball-milled in a planetary ball mill at 300-500 rpm for 1-3 hours to obtain fluorine-phosphorus composite seed crystals.

[0012] Preferably, the water reducer is a polycarboxylic acid high performance water reducer or a naphthalene high efficiency water reducer.

[0013] Preferably, the weight ratio of hexadecyltrimethylammonium bromide, ethanol, ammonia water and deionized water in (1) is 0.01-0.05:0.3-0.5:0.01-0.025:0.6-1.

[0014] Preferably, the weight ratio of the nano-hard gypsum to the pre-fluid in (2) is 1:8-12.

[0015] Preferably, the weight ratio of ethyl orthosilicate to the suspension in (3) is 1:5-9, the particle size of the core-shell structured nano-anhydrite is 300-500 nm, and tricalcium aluminate (C3A) in silicate cement and gypsum (CaSO4·2H2O) generate ettringite (AFt) in the hydration reaction, and the reaction formula is:

[0016] 3C3A+3CaSO4·2H2O+26H2O→3CaO·Al2O3·3CaSO4·32H2O, when gypsum (SO4 2- ) is exhausted, the ettringite will further react with the unreacted tricalcium aluminate (C3A) to convert into monosulfide type hydrated calcium sulfoaluminate (AFm), and the reaction formula is:

[0017] 3CaO·Al2O3·3CaSO4·32H2O+2C3A+4H2O→3CaO·Al2O3·CaSO4·12H2O, the stability is poor, the volume of AFm is smaller than that of AFt, but if the environmental conditions change later and AFm reabsorbs water to form ettringite (delayed ettringite, DEF), it will cause secondary expansion and destroy the cement matrix. The structure of AFm is easy to adsorb corrosive ions, accelerating carbonization and erosion. The SO4 released by core-shell nano-anhydrite is 2- By maintaining sufficient SO4 in the environment 2- concentration, inhibiting the conversion of ettringite to AFm.

[0018] Preferably, in the preparation process of the fluorine-phosphorus composite seed crystal, the weight ratio of calcium fluoride, calcium dihydrogen phosphate and deionized water is 1:1:5-7. The needle-like growth of traditional ettringite originates from [Al(OH)6] 3- Octahedrons are chain-polymerized along the c-axis, while the fluorine-phosphorus composite system is polymerized by F - Blocking chain extension, PO4 3- Providing lateral connection sites forces the crystal to expand laterally, forming a sheet-like structure with a lower aspect ratio.

[0019] Furthermore, the present invention also provides a method for preparing the above-mentioned triggered precipitation type concrete crack repair agent, which specifically comprises the following steps:

[0020] Sulphoaluminate cement, core-shell structured nano-hard gypsum, fluorine-phosphorus composite seed crystals, magnesium oxide, metakaolin, and water reducer are mixed and added into a dry powder mixer, and stirred at 1000-1500 rpm for 20-30 minutes to fully homogenize the mixture to obtain a triggered precipitation type concrete crack repair agent.

[0021] Preferably, magnesium oxide is hydrated to form magnesium hydroxide, which fills the pores of cement paste and reduces pore connectivity, thereby reducing the permeation path of CO2 and delaying the carbonization reaction process of AFm. In addition, the dense structure of magnesium hydroxide can hinder the direct contact between CO2 and AFm. Magnesium oxide is combined with slow-release SO4 2- , further reducing the negative impact of the generated AFm.

[0022] Furthermore, the present invention also provides the use of the above-mentioned triggered precipitation concrete crack repair agent in concrete structure crack repair and high temperature environment engineering, which specifically includes the following steps:

[0023] S1. Crack Assessment: Measure the crack width with a crack gauge (applicable to 0.05-10mm). Remove loose concrete around the crack until a solid base layer is exposed (no hollowing sound when tapped). For projects in high-temperature environments, test the base layer temperature (the base layer temperature should be ≤35°C during construction). For micro-cracks (≤0.3mm): Blow away dust from the crack with high-pressure air and wipe the surface with alcohol to ensure it is dry and free of scum. For wide cracks (>0.3mm): Create a V-shaped groove along the crack (5-10mm deep, 3-8mm wide). After removing the slurry from the groove, apply a layer of repair paste with a water-cement ratio of 0.16 as an interface agent to enhance adhesion.

[0024] S2. Mix the repair agent powder and deionized water at a water-binder ratio of 0.16, and mechanically stir until uniform and free of lumps to obtain a repair slurry. Use different repair methods depending on the crack condition:

[0025] Narrow cracks (≤0.3mm): Use low-pressure grouting method (pressure 0.2-0.4MPa), inject repair slurry from the lower end of the crack until the slurry overflows from the upper end, and smooth the surface with a scraper;

[0026] Wide cracks (>0.3mm): Fill with repair slurry in 2-3 layers, with an interval of 30 minutes between each layer, and finish the last layer until it is flush with the substrate.

[0027] Construction in high temperature environment: Mixing water temperature ≤30℃, avoid sudden temperature rise of base layer within 2 hours after construction to prevent early water loss and cracking.

[0028] S3. After the repair agent is added, carry out maintenance, cover with plastic film or wet cloth, keep it moist for ≥7 days, spray water 2-3 times a day, and complete the repair process after maintenance.

[0029] Beneficial effects of the present invention:

[0030] 1. The present invention realizes the sustained release of sulfate ions through the controlled sustained release technology of core-shell structure nano anhydrite, effectively maintaining SO4 in the cement-based system. 2- The fluorine-phosphorus composite seed crystals, which inhibit the conversion of ettringite (AFt) to monosulfide calcium sulfoaluminate hydrate (AFm), prevent the volume shrinkage and subsequent cracking caused by AFm formation. Furthermore, the fluorine-phosphorus composite seed crystals transform the ettringite from a needle-like structure to a flake-like structure by blocking crystal chain growth with fluoride ions and promoting lateral connection with phosphate ions, thus reducing anisotropic shrinkage during crystal growth. The hydration of magnesium oxide to magnesium hydroxide, accompanied by a certain volume expansion, fills the pores of the cement paste and reduces pore connectivity. The synergistic effect of these multiple components ensures that the repair agent maintains excellent volume stability over long-term service, significantly improving shrinkage resistance and structural durability.

[0031] 2. Aiming at the high temperature failure problem of traditional repair agents, the present invention optimizes the crystal morphology of ettringite by using fluorine-phosphorus composite crystal seeds. The flake structure has a lower specific surface area and reduced intercrystalline stress than the needle-shaped crystal. - With Al 3+ The high-energy coordination bonds formed enhance the thermal stability of the crystal and reduce the occurrence of high-temperature microcracks. Furthermore, metakaolin and magnesium oxide synergistically improve the matrix structure, forming a high-temperature resistant aluminum-silicon network and rigid support points, allowing the repair compound to maintain high strength even in high-temperature environments, meeting the repair needs of harsh scenarios such as industrial high-temperature environments and bridges and tunnels.

[0032] 3. The present invention achieves dual strengthening of "physical anchoring" and "chemical bonding" of the crack interface through the synergistic effect of core-shell anhydrite and fluorine-phosphorus composite crystal seeds: the core-shell anhydrite promotes the preferential deposition of AFt at the interface, forming a dense crystal layer that penetrates into the pores of the concrete, enhancing the mechanical bite force; the flaky AFt crystals regulated by the fluorine-phosphorus crystal seeds increase the interface contact area, and the F - With PO4 3- Respectively with Al in concrete 3+ , Ca 2+ Coordinate bonds are formed with calcium phosphates, creating chemical anchoring points. Simultaneously, the volcanic ash of the metakaolin reacts to form CSH gel, which penetrates the micropores of the matrix, creating a "nano-bridging" effect and significantly improving tensile bond strength. Furthermore, the alkaline barrier of magnesium oxide and the inhibitory effect of core-shell anhydrite on AFm effectively delay carbon dioxide penetration and carbonization reactions, reducing the adsorption of aggressive ions. This multi-dimensional durability improvement, from interfacial bonding to matrix protection, ensures a long-term and reliable bond between the repair layer and the matrix, extending the service life of the concrete structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the SEM image of the flaky ettringite generated in the concrete crack repair agent prepared in Example 2. DETAILED DESCRIPTION

[0034] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to specific embodiments.

[0035] The sources of the reagents and raw materials used in the examples of the present invention are as follows:

[0036] Sulphoaluminate cement was purchased from Jinzhou Baixin Trading Co., Ltd.; magnesium oxide was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a purity of 98%; metakaolin was purchased from Langfang Shuangma Chemical Co., Ltd.; hexadecyltrimethylammonium bromide was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a purity of 99%; nano-anhydrite was purchased from Henan Baifeng Chemical Products Co., Ltd.; ethyl silicate was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a purity of 98%; calcium fluoride was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a purity of 99%; calcium dihydrogen phosphate was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a purity of 98%; polycarboxylic acid high-efficiency water-reducing agent was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd.; and naphthalene-based high-efficiency water-reducing agent was purchased from Anhui Shengyuan Chemical Co., Ltd.

[0037] Example 1: A specific preparation method of a triggered precipitation concrete crack repair agent, comprising the following steps:

[0038] (1) Add 8 g of hexadecyltrimethylammonium bromide, 240 g of ethanol, and 8 g of ammonia water to 480 g of deionized water, heat to 50 °C, stir for 30 min, and cool to room temperature to obtain a pre-solution;

[0039] (2) Add 60 g of nano-hard gypsum to 480 g of the pre-liquid obtained in (1), ultrasonicate for 30 min, and stir simultaneously to obtain a suspension;

[0040] (3) Under nitrogen protection, 108 g of ethyl silicate was added to 540 g of the suspension obtained in (2), stirred at 100 rpm at room temperature for 1 min, then stirred at 300 rpm for 8 h, and allowed to stand for 4 h, filtered, washed, and calcined in a muffle furnace at 500 °C for 3 h. After cooling to room temperature, the suspension was sieved to obtain core-shell structured nano-hard gypsum;

[0041] (4) Add 15 g of calcium fluoride, 15 g of calcium dihydrogen phosphate and 75 g of deionized water to a hydrothermal reactor, adjust the pH to 4.5, heat to 180 °C, stir and react for 12 h, cool to room temperature, centrifuge and wash the reaction product, and then ball mill in a planetary ball mill at 300 rpm for 1 h to obtain fluorophosphorus composite seed crystals.

[0042] (5) 500 g of sulfoaluminate cement, 80 g of core-shell structured nano-hard gypsum, 20 g of fluorophosphorus composite seed crystals, 10 g of magnesium oxide, 100 g of metakaolin, and 3 g of polycarboxylic acid high-efficiency water reducer were mixed and added to a dry powder mixer. The mixture was stirred at 1000 rpm for 20 min to fully homogenize the mixture to obtain a triggered precipitation type concrete crack repair agent.

[0043] Example 2: A specific preparation method of a triggered precipitation concrete crack repair agent, comprising the following steps:

[0044] (1) Add 30 g of hexadecyltrimethylammonium bromide, 400 g of ethanol, and 20 g of ammonia water to 800 g of deionized water, heat to 60 °C, stir for 45 min, and cool to room temperature to obtain a pre-solution;

[0045] (2) Add 80 g of nano-hard gypsum to 800 g of the pre-liquid obtained in (1), ultrasonicate for 45 min, and stir simultaneously to obtain a suspension;

[0046] (3) Under nitrogen protection, 126 g of ethyl silicate was added to 880 g of the suspension obtained in (2), stirred at 1100 rpm at room temperature for 2 min, then stirred at 400 rpm for 10 h, and allowed to stand for 6 h, filtered, washed, and calcined in a muffle furnace at 550 °C for 4 h. After cooling to room temperature, the suspension was sieved to obtain core-shell structured nano-hard gypsum;

[0047] (4) Add 20 g of calcium fluoride, 20 g of calcium dihydrogen phosphate and 120 g of deionized water to a hydrothermal reactor, adjust the pH to 5, heat to 200 °C, stir and react for 18 h, cool to room temperature, centrifuge and wash the reaction product, and then ball mill in a planetary ball mill at 400 rpm for 2 h to obtain fluorophosphorus composite seed crystals.

[0048] (5) 550 g of sulfoaluminate cement, 100 g of core-shell structured nano-hard gypsum, 30 g of fluorophosphorus composite seed crystals, 20 g of magnesium oxide, 120 g of metakaolin, and 4 g of polycarboxylic acid high-efficiency water reducer were mixed and added to a dry powder mixer. The mixture was stirred at 1200 rpm for 25 min to fully homogenize the mixture to obtain a triggered precipitation type concrete crack repair agent.

[0049] Example 3: A specific preparation method of a triggered precipitation concrete crack repair agent, comprising the following steps:

[0050] (1) Add 35 g of hexadecyltrimethylammonium bromide, 350 g of ethanol, and 17.5 g of ammonia water to 700 g of deionized water, heat to 70 °C, stir for 60 min, and cool to room temperature to obtain a pre-solution;

[0051] (2) Add 80 g of nano-hard gypsum to 960 g of the pre-liquid obtained in (1), ultrasonicate for 60 min, and stir simultaneously to obtain a suspension;

[0052] (3) Under nitrogen protection, 116 g of ethyl silicate was added to 1.04 kg of the suspension obtained in (2), stirred at 1200 rpm at room temperature for 3 min, then stirred at 500 rpm for 12 h, and allowed to stand for 8 h, filtered, washed, and calcined in a muffle furnace at 500-600 °C for 3-5 h. After cooling to room temperature, the suspension was sieved to obtain core-shell structured nano-hard gypsum;

[0053] (4) Add 30 g of calcium fluoride, 30 g of calcium dihydrogen phosphate and 210 g of deionized water to a hydrothermal reactor, adjust the pH to 5.5, raise the temperature to 220 °C, stir and react for 24 h, cool to room temperature, centrifuge and wash the reaction product, and then ball mill in a planetary ball mill at 500 rpm for 3 h to obtain fluorophosphorus composite seed crystals.

[0054] (5) 600 g of sulfoaluminate cement, 120 g of core-shell structured nano-hard gypsum, 40 g of fluorophosphorus composite seed crystals, 30 g of magnesium oxide, 150 g of metakaolin, and 5 g of polycarboxylic acid high-efficiency water reducer were mixed and added to a dry powder mixer. The mixture was stirred at 1500 rpm for 30 min to fully homogenize the mixture to obtain a triggered precipitation concrete crack repair agent.

[0055] Example 4: A specific preparation method of a triggered precipitation concrete crack repair agent, comprising the following steps:

[0056] (1) Add 30 g of hexadecyltrimethylammonium bromide, 400 g of ethanol, and 20 g of ammonia water to 800 g of deionized water, heat to 60 °C, stir for 45 min, and cool to room temperature to obtain a pre-solution;

[0057] (2) Add 80 g of nano-hard gypsum to 800 g of the pre-liquid obtained in (1), ultrasonicate for 45 min, and stir simultaneously to obtain a suspension;

[0058] (3) Under nitrogen protection, 126 g of ethyl silicate was added to 880 g of the suspension obtained in (2), stirred at 1100 rpm at room temperature for 2 min, then stirred at 400 rpm for 10 h, and allowed to stand for 6 h, filtered, washed, and calcined in a muffle furnace at 550 °C for 4 h. After cooling to room temperature, the suspension was sieved to obtain core-shell structured nano-hard gypsum;

[0059] (4) Add 20 g of calcium fluoride, 20 g of calcium dihydrogen phosphate and 120 g of deionized water to a hydrothermal reactor, adjust the pH to 5, heat to 200 °C, stir and react for 18 h, cool to room temperature, centrifuge and wash the reaction product, and then ball mill in a planetary ball mill at 400 rpm for 2 h to obtain fluorophosphorus composite seed crystals.

[0060] (5) 550 g of sulfoaluminate cement, 100 g of core-shell structured nano-hard gypsum, 30 g of fluorophosphorus composite seed crystals, 20 g of magnesium oxide, 120 g of metakaolin, and 4 g of naphthalene-based high-efficiency water reducer were mixed and added to a dry powder mixer. The mixture was stirred at 1200 rpm for 25 min to fully homogenize the mixture to obtain a triggered precipitation concrete crack repair agent.

[0061] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the core-shell structured nano-hard gypsum is replaced by nano-hard gypsum.

[0062] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that no fluorine-phosphorus composite crystal seeds are added.

[0063] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the fluorine-phosphorus composite seed crystals are replaced by calcium fluoride.

[0064] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that magnesium oxide is not added.

[0065] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that no kaolin is added.

[0066] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that only sulphoaluminate cement is used as the concrete crack repairing agent.

[0067] Specific applications:

[0068] Application scenario: Repair of cracks in the foundation of a steam turbine in a thermal power plant (crack width 0.3-0.8mm, ambient temperature maintained at 60-80°C for a long time). The repair agents prepared in Examples 1-4 and Comparative Examples 1-6 were respectively mixed and stirred at a water-cement ratio of 0.16 to form a repair slurry. A "V"-shaped groove (depth 8mm, width 5mm) was chiseled at the crack. After removing the loose concrete, the repair slurry was applied. After waiting for 30 minutes, the repair slurry was embedded in 2-3 layers, with an interval of 30 minutes between each layer. The last layer was polished to be flush with the substrate. After the repair agent was added, it was cured and covered with plastic film or wet cloth to keep it moist for ≥7 days. Water was sprayed 2-3 times a day. After the curing was completed, the repair process was completed. After 90 days of service, the changes in the repair interface were observed.

[0069] Patch interface changes:

[0070] Example 1: The interface is tightly bonded without peeling, and the volume stability is excellent with no shrinkage marks;

[0071] Example 2: The interface is tightly bonded without peeling, and the volume stability is excellent with no shrinkage marks;

[0072] Example 3: The interface is tightly bonded without peeling, and the volume stability is excellent with no shrinkage marks;

[0073] Example 4: The interface is tightly bonded without peeling, and the volume stability is excellent with no shrinkage marks;

[0074] Comparative Example 1: Slight shrinkage and cracking, slight interface peeling, and whitening of the repair layer surface;

[0075] Comparative Example 2: Microcracks appeared, and hollows were found after knocking.

[0076] Comparative Example 3: Irregular pits or protrusions appeared on the surface of the repair layer, indicating that the local bonding was not dense. When knocked, some areas made a crisp cracking sound, indicating that there were weak points in the interface.

[0077] Comparative Example 4: The edge of the repair interface turned white, forming a white carbonized layer. Powder fell off when gently wiped, and the overall repair layer was slightly lower than the substrate surface;

[0078] Comparative Example 5: Continuous fine cracks appeared along the direction of the crack on the repair interface. Delamination of the repair layer and the substrate was visible in the depth direction of the crack. Water seeped out after watering, indicating that the interface bonding was discontinuous. The sound was dull when knocked, indicating that the internal structure was loose.

[0079] Comparative Example 6: The repair interface completely lost its bonding ability, part of the repair layer fell off, the peeling surface was rough and attached with a small amount of loose cement slurry, the matrix cracks were exposed again and further expanded, the repair layer was grayish white, brittle in texture, and could be easily scraped off with tools.

[0080] Performance testing:

[0081] 1. Crack repair shrinkage test: The repair agents prepared in Examples 1-4 and Comparative Examples 1-6 were mixed with water at a water-binder ratio of 0.16 for 10 minutes to prepare a repair slurry. The slurry was poured into a 250 mm × 25 mm × 25 mm mold to prepare a test mold. The height of the test piece at 7 days of hydration (denoted as A) and the height of the test piece at 90 days (denoted as B) were measured, and the shrinkage rate Y at each period was calculated. %, the experimental results are shown in Table 1.

[0082] 2. Compressive and flexural strength test: The repair agents prepared in Examples 1-4 and Comparative Examples 1-6 were added with water at a water-binder ratio of 0.16 and mixed for 10 minutes to prepare a repair slurry. 40 mm × 40 mm × 160 mm prism specimens were made. The compressive and flexural strengths of the specimens were tested 3 days after demolding and 28 days after demolding according to the standard of GB / T17671-2021. The experimental results are shown in Table 1.

[0083] 3. Anti-carbonization performance test: The repair agents prepared in Examples 1-4 and Comparative Examples 1-6 were added with water at a water-binder ratio of 0.16 and mixed for 10 minutes to prepare a repair slurry. A 100 mm × 50 mm cylindrical specimen was made and placed in a 60°C oven for drying for 48 hours. The specimen was placed in a carbonization box (CO2 concentration 20±3%, humidity 70±5%, temperature 20±2°C), taken out after carbonization for 90 days, split the specimen, sprayed with a 1% phenolphthalein alcohol solution, and measured the thickness of the carbonized layer (the area where the phenolphthalein turned red was the uncarbonized part). The experimental results are shown in Table 1.

[0084] 4. High temperature strength retention test: The repair agents prepared in Examples 1-4 and Comparative Examples 1-6 were mixed with water at a water-binder ratio of 0.16 for 10 minutes to prepare a repair slurry. A 40 mm × 40 mm × 160 mm prism specimen was made and placed in a high temperature box. The temperature was raised from room temperature to 70°C, 90°C, 105°C, and 115°C at a heating rate of 5°C / min. The specimens were kept warm for 24 hours. The compressive strength of the specimens after high temperature treatment was measured according to the compressive strength test method. The strength retention rate was calculated as: Q = , the experimental results are shown in Table 1.

[0085] 5. Tensile bond strength test: The repair agents prepared in Examples 1-4 and Comparative Examples 1-6 were mixed with water at a water-to-binder ratio of 0.16 for 10 minutes to prepare a repair slurry. The tensile bond strength of the samples was tested according to the test standard of GB / T 50344-2019. The experimental results are shown in Table 1.

[0086] Table 1 Performance test

[0087]

[0088] Data Analysis:

[0089] It can be seen from the data in Table 1 that the concrete crack repair agents prepared by the present invention in Examples 1-4 have good shrinkage resistance and compressive and flexural strength, and can also better resist the erosion of carbon dioxide. The strength retention rate is higher when facing high temperatures. Among them, Example 2 has the best comprehensive performance.

[0090] In terms of crack repair shrinkage, this may be because the core-shell structure nano-hard gypsum used in the concrete crack repair agent prepared in Example 2 has a slow release of SO4 2- The effect is achieved by coating with ethyl silicate to form a "shell" to achieve SO4 2- Controlled slow release, in the early stage of cement hydration, gypsum in cement provides higher SO4 2- concentration, suppressing SO4 in the shell 2- Release to avoid excessive concentration of SO4 2- Causes the rapid generation of ettringite (AFt); over time, the gypsum in the cement is consumed, and the SO4 in the cement 2- The concentration decreases, the SO4 in the shell 2- Start slow release to ensure that sufficient concentration is maintained for a long time, inhibit the conversion of AFt to monosulfur type calcium sulfoaluminate hydrate (AFm) (AFm generation is accompanied by about 12% volume shrinkage). Comparative Example 1 directly uses nano anhydrite, SO4 2- Rapid release, high concentration in the early stage, leading to excessive generation of AFt, and SO4 2- After the exhaustion, AFm is generated in large quantities, and the shrinkage rate is significantly increased; at the same time, in Example 2, F in the fluorine-phosphorus composite seed is also- Blocking [Al (OH)6] 3- Octahedral chain polymerization along the c-axis, PO4 3- Providing lateral connection sites allows the ettringite to transform from needle-shaped to flake-shaped, reducing the anisotropic shrinkage of crystal growth. In Comparative Example 2, no seed crystals were added, and the ettringite was needle-shaped. Internal stress was generated when the crystals grew crosswise, resulting in an increase in shrinkage. Finally, in Example 2, magnesium oxide was hydrated to form magnesium hydroxide, accompanied by a certain volume expansion, filling the pores of the cement stone, reducing pore connectivity, and inhibiting drying shrinkage. After removing magnesium oxide in Comparative Example 4, the porosity increased and the shrinkage was higher than that of Example 2.

[0091] In terms of compressive and flexural strength, this may be because the core-shell nano-anhydrite used in Example 2 has a synergistic effect with the fluorine phosphorus seeds, and the core-shell anhydrite slowly releases SO4 2- The continuous generation of AFt is ensured, and the lamellar AFt crystals overlap to form a three-dimensional network structure. Compared with needle-shaped crystals, the contact area between the crystals is increased, and the flexural strength is improved. Fluorophosphorus seed crystals promote the hydration of tricalcium aluminate (C3A), generating more AFt and calcium silicate hydrate (CSH) gel at an early stage, thereby improving the compressive strength. At the same time, metakaolin reacts secondary with the cement hydration product Ca (OH)2 to generate CSH gel and calcium aluminosilicate (CASH). The reaction formula is: Al2O3・2SiO2+ xCa (OH)2+ yH2O → xCaO・Al2O3・2SiO2・(x+y) H2O, which fills the pores and strengthens the interface transition zone, thereby improving the compressive strength.

[0092] In terms of carbonization resistance, this may be due to the alkaline barrier effect of magnesium oxide in Example 2. The solubility of magnesium hydroxide generated by magnesium oxide is lower than that of calcium hydroxide, forming a more stable alkaline environment, delaying the diffusion of CO2. Its dense layer covers the cement surface, extending the CO2 penetration path. After removing magnesium oxide in Comparative Example 4, the thickness of the carbonized layer increases because calcium hydroxide is preferentially carbonized. The reaction formula is: Ca (OH)2+CO2→CaCO3+ H2O, resulting in AFm being directly exposed to CO2, accelerating its decomposition. At the same time, the core-shell nano-hard gypsum inhibits the generation of AFm. The AFm structure contains [Al (OH)4] - , easy to adsorb CO3 2- Carbonization reaction occurs: 3CaO・Al2O3・CaSO4・12H2O + 3CO2→3CaCO3+ Al (OH)3+ CaSO4+ 9H2O, generating loose CaCO3 and Al(OH)3, increasing porosity. In Example 2, the core-shell anhydrite maintains high SO4 2-concentration, inhibiting the generation of AFm, thereby reducing the carbonation sites. Finally, metakaolin consumes Ca (OH)2 through pozzolanic reaction. Metakaolin reacts with Ca (OH)2 to form CSH gel with a low Ca / Si ratio, which reduces the free Ca (OH)2 content in cement paste, reduces the reaction sites between CO2 and Ca (OH)2, and indirectly delays the carbonation process.

[0093] In terms of high temperature strength maintenance, this may be because the core-shell structure nano-hard gypsum used in Example 2 has a high temperature stability mechanism, which can be achieved by slowly releasing SO4 2- Maintain AFt stability and continuously release SO4 2- , inhibiting the transformation of AFt to AFm, avoiding the sudden drop in strength due to the transformation, at the same time, the fluorine-phosphorus composite crystal seeds used induce the flake growth of AFt, and the flake AFt has the advantage of thermal shock resistance, F - With Al 3+ Forming [AlF6] with a coordination number of 6 3- Coordination ions, replacing the [Al (OH)6] extending along the c-axis in traditional ettringite 3- Chain structure, blocking the one-dimensional growth path of the crystal, PO4 3- The PO-Al bond provides a lateral connection site, forcing the ettringite to transform from needle-shaped to flake-shaped, reducing the specific surface area, reducing the intergranular stress at high temperature, and inhibiting the formation of microcracks. The Al-F bond can improve the structural stability. - With Al 3+ Forming high bond energy coordination bonds, which are more resistant to high temperatures than Al-OH bonds, thereby improving high temperature stability, PO4 3- The formed PO-Al bonds enhance the inter-crystalline bonding force, allowing the flaky ettringite to maintain an intact skeleton at high temperatures, thus avoiding a sudden drop in strength due to crystal decomposition. Finally, metakaolin reacts with Ca (OH)2 to generate a low Ca / Si ratio gel, forming a high-temperature resistant three-dimensional network and stabilizing the matrix strength.

[0094] In terms of tensile bond strength, this may be because the core-shell nano-hard gypsum used in Example 2 promotes the preferential deposition of AFt at the crack interface, forming an "anchor" structure, increasing the density of AFt crystals at the bonding interface, and enhancing the mechanical bite force between the crystals and the concrete matrix. Fluorophosphorus seeds regulate the transformation of AFt from needle-shaped to flake-shaped. Although the specific surface area is slightly reduced, the contact area between the flake crystals and the concrete matrix increases, and the number of van der Waals force and hydrogen bond sites increases. - / PO4 3- Promote chemical bonding: F - Al in concrete 3+ 、Fe 3+ Forming a stable coordination bond, PO4 3- With Ca 2+Calcium phosphate salts are generated, forming "chemical anchor points" in the interface area, and the chemical binding energy of the bonding interface increases. Finally, the CSH gel generated by the volcanic ash reaction of kaolin penetrates into the pores of the concrete, forming a "nano-bridging" effect, which improves the bonding strength.

[0095] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present invention is limited to these examples. Within the scope of the present invention, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity.

Claims

1. A triggered precipitation concrete crack repair agent, characterized in that: The invention comprises the following raw materials in parts by weight: 50-60 parts of sulphoaluminate cement, 8-12 parts of core-shell structured nano-hard gypsum, 2-4 parts of fluorine-phosphorus composite seed crystals, 1-3 parts of magnesium oxide, 10-15 parts of metakaolin, and 0.3-0.5 parts of water reducer. The preparation process of the core-shell structured nano anhydrite is as follows: (1) Add hexadecyltrimethylammonium bromide, ethanol, and ammonia water to deionized water, heat to 50-70°C, stir for 30-60 minutes, and cool to room temperature to obtain a pre-solution; (2) Adding nano-hard gypsum to the pre-liquid obtained in (1), ultrasonicating for 30-60 minutes, and stirring simultaneously to obtain a suspension; (3) Under nitrogen protection, ethyl silicate is added to the suspension obtained in (2), stirred at room temperature at 1000-1200 rpm for 1-3 min, then stirred at 300-500 rpm for 8-12 h, allowed to stand for 4-8 h, filtered, washed, calcined in a muffle furnace at 500-600 ° C for 3-5 h, cooled to room temperature, and sieved to obtain core-shell structured nano-hard gypsum; The preparation process of the fluorine-phosphorus composite seed crystal is as follows: Calcium fluoride, calcium dihydrogen phosphate and deionized water are added to a hydrothermal kettle, the pH is adjusted to 4.5-5.5, the temperature is raised to 180-220° C., the reaction is stirred for 12-24 hours, and after cooling to room temperature, the reaction product is centrifuged and washed, and then ball-milled in a planetary ball mill at 300-500 rpm for 1-3 hours to obtain fluorine-phosphorus composite seed crystals.

2. The triggered precipitation concrete crack repair agent according to claim 1, characterized in that: The water reducer is a polycarboxylic acid high performance water reducer or a naphthalene high efficiency water reducer.

3. The triggered precipitation concrete crack repair agent according to claim 1, characterized in that: The weight ratio of hexadecyltrimethylammonium bromide, ethanol, ammonia water and deionized water in (1) is 0.01-0.05:0.3-0.5:0.01-0.025:0.6-1.

4. The triggered precipitation concrete crack repair agent according to claim 1, characterized in that: The weight ratio of the nano-hard gypsum to the pre-fluid in (2) is 1:8-12.

5. The triggered precipitation concrete crack repair agent according to claim 1, characterized in that: In the (3), the weight ratio of ethyl orthosilicate to the suspension is 1:5-9, and the particle size of the core-shell structured nano-hard gypsum is 300-500nm.

6. The triggered precipitation concrete crack repair agent according to claim 1, characterized in that: In the preparation process of the fluorine-phosphorus composite seed crystal, the weight ratio of calcium fluoride, calcium dihydrogen phosphate and deionized water is 1:1:5-7.

7. The method for preparing the triggered precipitation concrete crack repair agent according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: mixing sulphoaluminate cement, core-shell structure nano-hard gypsum, fluorine-phosphorus composite crystal seeds, magnesium oxide, metakaolin and a water reducer, adding the mixture into a dry powder mixer, stirring at 1000-1500 rpm for 20-30 minutes to fully homogenize the mixture, and obtaining a triggered precipitation type concrete crack repair agent.

8. Use of the triggered precipitation concrete crack mending agent obtained by the preparation method according to claim 7 in concrete structure crack repair and high temperature environment engineering.

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