Triggering precipitation type concrete crack repairing agent as well as preparation method and application thereof

Abstract

The invention relates to the technical field of concrete crack repairing agents, in particular to a triggered precipitation type concrete crack repairing agent as well as a preparation method and application thereof. The repairing agent is prepared from the following raw materials: sulphoaluminate cement, nano anhydrite with a core-shell structure, fluorine-phosphorus composite seed crystal, magnesium oxide, metakaolin and a water reducing agent, wherein the core-shell structure nano anhydrite is prepared by coating mesoporous silica with nano anhydrite, and the fluorine-phosphorus composite seed crystal is prepared from calcium fluoride and monocalcium phosphate through a hydrothermal reaction. According to the trigger precipitation type concrete crack repairing agent prepared by the invention, sulfate ions are slowly released through the core-shell structure nano anhydrite, the crystal form of ettringite is regulated and controlled through the fluorine-phosphorus composite seed crystal, the problems that a traditional repairing agent shrinks and falls off in the later period, the high-temperature strength is suddenly reduced, and the binding power is insufficient are solved, and the trigger precipitation type concrete crack repairing agent is suitable for concrete structure crack repairing and high-temperature environment engineering.

Description

Technical Field

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

[0002] Concrete structure cracks are common diseases in construction projects, which not only affect the structural aesthetics, but may also lead to potential safety hazards such as steel bar corrosion, water seepage and reduction of bearing capacity. As a key material for repair, the long-term stability and environmental adaptability of crack repair agents are crucial. However, traditional sulfoaluminate concrete crack repair agents generally have two major technical bottlenecks: First, during the hydration process of sulfoaluminate cement, tricalcium aluminate (C3A) reacts with gypsum to form ettringite (AFt). However, when the gypsum is exhausted, AFt is liable to further react with the unreacted C3A to form monosulfate calcium sulfoaluminate (AFm). AFm has poor stability, and its volume is reduced by about 12% compared with AFt, resulting in shrinkage and cracking of the repair agent in the later stage; moreover, the structure of AFm is liable to adsorb erosive ions such as CO2, accelerating the carbonation reaction, causing interfacial bonding failure and peeling of the repair layer. Second, in a high-temperature environment, traditional sulfoaluminate repair agents face the problem of sudden strength drop. When the temperature is above 70°C, as the SO4 in the cement 2- is exhausted, AFt will irreversibly transform into AFm, accompanied by volume shrinkage and gelling phase failure; on the other hand, traditional ettringite is in the form of needle-like crystals, and intercrystalline stress concentration is likely to occur at high temperatures, resulting in the generation of microcracks, leading to a reduction in the strength retention rate above 70°C. In addition, after the structural water of AFm is lost at high temperatures, a porous phase is formed, further exacerbating the strength attenuation, making it difficult to meet the long-term service requirements. At the same time, the bonding interface of existing sulfoaluminate cement repair agents relies on physical occlusion and lacks chemical anchoring effect, and the tensile bonding strength is insufficient, unable to effectively resist crack propagation under dynamic loads. Summary of the Invention

[0003] In view of this, the purpose of the present invention is to provide a triggered precipitation type concrete crack repair agent, its preparation method and application, so as to solve the problems of shrinkage and peeling caused by the generation of AFm in the later stage of sulfoaluminate cement repair agents, and at the same time improve the high-temperature strength retention rate, significantly improving the service performance in a high-temperature environment.

[0004] Based on the above purpose, the present invention provides a triggered precipitation type concrete crack repair agent, which comprises the following raw materials in parts by weight: sulfoaluminate cement: 50-60 parts, core-shell structured nano-gypsum: 8-12 parts, fluorophosphate composite crystal seeds: 2-4 parts, magnesium oxide: 1-3 parts, metakaolin: 10-15 parts, water reducing agent: 0.3-0.5 parts; The preparation process of the core-shell structured nano-gypsum is as follows: (1) Add cetyltrimethylammonium bromide, ethanol, and ammonia water into deionized water, heat up to 50 - 70 °C, stir for 30 - 60 min, and after cooling to room temperature, obtain the pre - liquid. (2) Add nano - anhydrite into the pre - liquid obtained in (1), ultrasonicate for 30 - 60 min while stirring synchronously to obtain a suspension. (3) Under nitrogen protection, add tetraethyl orthosilicate into the suspension obtained in (2), stir at 1000 - 1200 rpm at room temperature for 1 - 3 min, then stir at 300 - 500 rpm for 8 - 12 h, stand for 4 - 8 h, filter, wash, calcine in a muffle furnace at 500 - 600 °C for 3 - 5 h, and after cooling to room temperature, sieve to obtain core - shell structured nano - anhydrite. The preparation process of the fluorophosphate composite crystal seeds is as follows: Add calcium fluoride, calcium dihydrogen phosphate, and deionized water into a hydrothermal reactor, adjust the pH to 4.5 - 5.5, heat up to 180 - 220 °C, stir and react for 12 - 24 h. After cooling to room temperature, the reaction product is centrifuged and washed, and then ball - milled at 300 - 500 rpm in a planetary ball mill for 1 - 3 h to obtain fluorophosphate composite crystal seeds.

[0005] Preferably, the water - reducing agent is one of polycarboxylate superplasticizer or naphthalene - based high - efficiency water - reducing agent.

[0006] Preferably, in (1), the weight ratio of cetyltrimethylammonium bromide, ethanol, ammonia water, and deionized water is 0.01 - 0.05:0.3 - 0.5:0.01 - 0.025:0.6 - 1.

[0007] Preferably, in (2), the weight ratio of nano - anhydrite to the pre - liquid is 1:8 - 12.

[0008] Preferably, in (3), the weight ratio of tetraethyl orthosilicate to the suspension is 1:5 - 9, the particle size of the core - shell structured nano - anhydrite is 300 - 500 nm. Tricalcium aluminate (C3A) in Portland cement reacts with gypsum (CaSO4·2H2O) during the hydration reaction to form ettringite (AFt), and its reaction formula is: 3C3A + 3CaSO4·2H2O + 26H2O → 3CaO·Al2O3·3CaSO4·32H2O. When the gypsum (SO4 2- ) in the cement is exhausted, ettringite will further react with the unreacted tricalcium aluminate (C3A) to be converted into monosulfate - type hydrated calcium aluminate (AFm), and its reaction formula is: 3CaO·Al2O3·3CaSO4·32H2O + 2C3A + 4H2O → 3CaO·Al2O3·CaSO4·12H2O, with poor stability. The volume of AFm is reduced compared to AFt. However, if AFm reabsorbs water to form ettringite (delayed ettringite, DEF) due to changes in environmental conditions at a later stage, it will cause secondary expansion and damage the cement matrix. The structure of AFm is prone to adsorbing aggressive ions, accelerating carbonation and erosion. The SO4 slowly released by the core-shell nano-anhydrite 2- By maintaining sufficient SO4 2- concentration in the environment, the conversion of ettringite to AFm is inhibited.

[0009] Preferably, during the preparation of the fluorophosphate composite seed crystal, calcium fluoride, calcium dihydrogen phosphate and deionized water are in a weight ratio of 1:1:5 - 7. The traditional needle-like growth of ettringite originates from the chain polymerization of [Al(OH)6] 3- octahedrons along the c-axis, while the fluorophosphate composite system blocks chain extension through F - and provides lateral connection sites through PO4 3- forcing the crystal to expand laterally and form a flake structure with a lower aspect ratio.

[0010] Furthermore, the present invention also provides a preparation method for the above-mentioned trigger precipitation type concrete crack repair agent, specifically including the following steps: Mix the sulfoaluminate cement, core-shell structure nano-anhydrite, fluorophosphate composite seed crystal, magnesium oxide, metakaolin, and water reducer and add them to a dry powder mixer. Stir at 1000 - 1500 rpm for 20 - 30 min to make it fully homogenized, obtaining the trigger precipitation type concrete crack repair agent.

[0011] Preferably, magnesium oxide hydrates to form magnesium hydroxide, filling the pores of the cement paste and reducing pore connectivity, thereby reducing the CO2 penetration path and delaying the AFm carbonation reaction process. In addition, the dense structure of magnesium hydroxide can hinder the direct contact between CO2 and AFm. Magnesium oxide is combined with the slow-release SO4 2- to further reduce the negative impact brought by the generated AFm.

[0012] Furthermore, the present invention also provides the application of the above-mentioned trigger precipitation type concrete crack repair agent in the repair of concrete structure cracks and high-temperature environment engineering, specifically including the following steps: S1. Crack assessment: Measure the crack width with a crack width gauge (applicable for 0.05 - 10 mm), remove the loose concrete around the crack until the solid base layer is exposed (no hollow sound when knocking). For projects in high-temperature environments, the base layer temperature needs to be detected (the base layer temperature during construction should be ≤ 35°C). For micro-cracks (≤ 0.3 mm): Blow the dust in the crack with high-pressure air, wipe the surface oil stain with alcohol, and ensure it is dry without floating slag. For wide cracks (> 0.3 mm): Cut a "V" groove along the crack (depth 5 - 10 mm, width 3 - 8 mm). After removing the floating slurry in the groove, apply a neat slurry of the repair agent with a water-cement ratio of 0.16 as an interface agent to enhance the bonding force; S2. Mix the repair agent powder and deionized water according to a water-cement ratio of 0.16, and mechanically stir until it is uniform and free of lumps to obtain the repair slurry. Depending on the crack situation, different repair methods are adopted: Narrow cracks (≤ 0.3 mm): Adopt the low-pressure grouting method (pressure 0.2 - 0.4 MPa), inject the repair slurry from the lower end of the crack until the slurry overflows from the upper end, and smooth the surface with a spatula; Wide cracks (> 0.3 mm): Embed the repair slurry in 2 - 3 layers, with an interval of 30 minutes between each layer, and the last layer is polished until it is flush with the base body.

[0013] Construction in high-temperature environment: The temperature of the mixing water is ≤ 30°C. Avoid a sudden increase in the base layer temperature within 2 hours after construction to prevent early water loss and cracking.

[0014] After the addition of the repair agent, carry out curing, cover with a plastic film or a wet cloth, keep it in a moist state for ≥ 7 days, spray water 2 - 3 times a day, and complete one repair process after curing.

[0015] Advantages of the present invention: 1. Through the controlled release technology of core-shell structured nano-anhydrite in the present invention, the continuous release of sulfate ions is realized, effectively maintaining the SO4 concentration in the cement-based system, fundamentally inhibiting the transformation of ettringite (AFt) into monosulfate calcium sulfoaluminate (AFm), and avoiding the volume shrinkage and subsequent cracking problems caused by the formation of AFm. At the same time, the fluorophosphate composite crystal seeds block the chain-like growth of crystals through fluoride ions and promote lateral connection through phosphate ions, transforming ettringite from a needle-like structure to a flake-like structure, reducing the anisotropic shrinkage during crystal growth; magnesium oxide hydrates to form magnesium hydroxide and is accompanied by a certain volume expansion, filling the pores of the cement stone and reducing the pore connectivity. Under the synergistic action of multiple components, the repair agent maintains excellent volume stability during long-term service, significantly improving the anti-shrinkage performance and structural durability. 2- 2. Aiming at the problem of high-temperature failure of traditional repair agents, in the present invention, the crystal morphology of ettringite is optimized through fluorophosphate composite crystal seeds. The specific surface area of the flake-like structure is lower and the intercrystalline stress is smaller compared with the needle-like crystal. Combining F with Al

[0016] and Al - with Al3+ The formed high bond energy coordination bonds enhance the thermal stability of the crystal and reduce the generation of microcracks at high temperatures. In addition, metakaolin and magnesium oxide synergistically improve the matrix structure, forming a high-temperature resistant aluminosilicate network and rigid support points, enabling the repair agent to maintain high strength in high-temperature environments and meet the repair requirements of harsh scenarios such as industrial high temperatures and bridge tunnels.

[0017] 3. Through the synergistic effect of core-shell anhydrite and fluorophosphate composite seeds, the present invention realizes the dual strengthening of "physical anchoring" and "chemical bonding" at the crack interface: Core-shell anhydrite promotes the preferential deposition of AFt at the interface, forming a dense crystal layer and interpenetrating into the concrete pores to enhance the mechanical bite force; the plate-like AFt crystals regulated by fluorophosphate seeds increase the interface contact area, and F - and PO4 3- respectively form coordination bonds and calcium phosphate salts with Al 3+ and Ca 2+ in the concrete to construct chemical anchoring points. At the same time, the pozzolanic reaction of metakaolin generates C-S-H gel that penetrates into the matrix micropores, forming a "nano-bridging" effect, significantly enhancing the tensile bond strength. In addition, the alkaline barrier of magnesium oxide and the inhibitory effect of core-shell anhydrite on AFm effectively delay carbon dioxide penetration and carbonation reaction, reduce the adsorption of erosive ions, and improve the multi-dimensional durability from interface bonding to matrix protection, ensuring the long-term reliable bonding of the repair layer and the matrix and extending the service life of the concrete structure. Description of the Drawings

[0018] Figure 1 SEM image of the plate-like ettringite generated in the concrete crack repair agent prepared in Example 2. Detailed Embodiment

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to specific embodiments.

[0020] The sources of the reagent raw materials used in the embodiments of the present invention are as follows: Sulfoaluminate cement was purchased from Jinzhou Baixin Trading Co., Ltd.; magnesium oxide was purchased from Shanghai Macklin Biochemical Co., Ltd., with a purity of 98%; metakaolin was purchased from Langfang Shuangma Chemical Co., Ltd.; cetyltrimethylammonium bromide was purchased from Shanghai Macklin Biochemical Co., Ltd., with a purity of 99%; nano-anhydrite was purchased from Henan Baifeng Chemical Products Co., Ltd.; ethyl silicate was purchased from Shanghai Macklin Biochemical Co., Ltd., with a purity of 98%; calcium fluoride was purchased from Shanghai Macklin Biochemical Co., Ltd., with a purity of 99%; calcium dihydrogen phosphate was purchased from Shanghai Macklin Biochemical Co., Ltd., with a purity of 98%; polycarboxylate superplasticizer was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd.; naphthalene-based superplasticizer was purchased from Anhui Shengyuan Chemical Co., Ltd.

[0021] Example 1: A specific preparation method of a triggered precipitation type concrete crack repair agent, comprising the following steps: (1) Add 8 g of cetyltrimethylammonium bromide, 240 g of ethanol, and 8 g of ammonia water to 480 g of deionized water, heat up to 50 °C, stir for 30 min, and after cooling to room temperature, obtain a pre-liquid; (2) Add 60 g of nano-anhydrite to the pre-liquid obtained in (1) of 480 g, ultrasonically treat for 30 min, and stir synchronously to obtain a suspension; (3) Under nitrogen protection, add 108 g of tetraethyl orthosilicate to the suspension obtained in (2) of 540 g, stir at 100 rpm at room temperature for 1 min, then stir at 300 rpm for 8 h, stand for 4 h, filter, wash, calcine in a muffle furnace at 500 °C for 3 h, and after cooling to room temperature, sieve to obtain core-shell structured nano-anhydrite; (4) Add 15 g of calcium fluoride, 15 g of calcium dihydrogen phosphate, and 75 g of deionized water to a hydrothermal autoclave, adjust the pH to 4.5, heat up to 180 °C, stir and react for 12 h, after cooling to room temperature, the reaction product is centrifuged, washed, and ball milled at 300 rpm in a planetary ball mill for 1 h to obtain a fluorophosphate composite seed crystal.

[0022] (5) Mix 500 g of sulfoaluminate cement, 80 g of core-shell structured nano-anhydrite, 20 g of fluorophosphate composite seed crystal, 10 g of magnesium oxide, 100 g of metakaolin, and 3 g of polycarboxylate superplasticizer and add them to a dry powder mixer, stir at 1000 rpm for 20 min to make it fully homogenized to obtain a triggered precipitation type concrete crack repair agent.

[0023] Example 2: A specific preparation method of a triggered precipitation type concrete crack repair agent, comprising the following steps: (1) Add 30 g of cetyltrimethylammonium bromide, 400 g of ethanol, and 20 g of ammonia water to 800 g of deionized water, heat up to 60 °C, stir for 45 min, and after cooling to room temperature, obtain a pre-liquid; (2) Add 80 g of nano-anhydrite to the pre-liquid obtained in (1) of 800 g, ultrasonically treat for 45 min, and stir synchronously to obtain a suspension; (3) Under nitrogen protection, add 126 g of tetraethyl orthosilicate to the suspension obtained in (2) of 880 g, stir at 1100 rpm at room temperature for 2 min, then stir at 400 rpm for 10 h, stand for 6 h, filter, wash, calcine in a muffle furnace at 550 °C for 4 h, and after cooling to room temperature, sieve to obtain core-shell structured nano-anhydrite; (4) Add 20 g of calcium fluoride, 20 g of calcium dihydrogen phosphate, and 120 g of deionized water into a hydrothermal autoclave, adjust the pH to 5, heat up to 200 °C, stir and react for 18 h. After cooling to room temperature, the reaction product is centrifuged and washed, and then ball-milled in a planetary ball mill at 400 rpm for 2 h to obtain a fluorophosphorus composite seed crystal.

[0024] (5) Mix 550 g of sulfoaluminate cement, 100 g of core-shell structured nanoanhydrite, 30 g of fluorophosphorus composite seed crystal, 20 g of magnesium oxide, 120 g of metakaolin, and 4 g of polycarboxylate superplasticizer and add them to a dry powder mixer. Stir at 1200 rpm for 25 min to make it fully homogenized, and obtain a trigger precipitation type concrete crack repair agent.

[0025] Example 3: A specific preparation method of a trigger precipitation type concrete crack repair agent, comprising the following steps: (1) Add 35 g of cetyltrimethylammonium bromide, 350 g of ethanol, and 17.5 g of ammonia water into 700 g of deionized water, heat up to 70 °C, stir for 60 min, and after cooling to room temperature, obtain a pre-liquid. (2) Add 80 g of nanoanhydrite into 960 g of the pre-liquid obtained in (1), perform ultrasonic treatment for 60 min while stirring synchronously to obtain a suspension. (3) Under nitrogen protection, add 116 g of tetraethyl orthosilicate into the suspension obtained in (2) with a mass of 1.04 Kg, stir at 1200 rpm at room temperature for 3 min, then stir at 500 rpm for 12 h, stand for 8 h, filter, wash, calcine in a muffle furnace at 500 - 600 °C for 3 - 5 h, and after cooling to room temperature, sieve to obtain core-shell structured nanoanhydrite. (4) Add 30 g of calcium fluoride, 30 g of calcium dihydrogen phosphate, and 210 g of deionized water into a hydrothermal autoclave, adjust the pH to 5.5, heat up to 220 °C, stir and react for 24 h. After cooling to room temperature, the reaction product is centrifuged and washed, and then ball-milled in a planetary ball mill at 500 rpm for 3 h to obtain a fluorophosphorus composite seed crystal.

[0026] (5) Mix 600 g of sulfoaluminate cement, 120 g of core-shell structured nanoanhydrite, 40 g of fluorophosphorus composite seed crystal, 30 g of magnesium oxide, 150 g of metakaolin, and 5 g of polycarboxylate superplasticizer and add them to a dry powder mixer. Stir at 1500 rpm for 30 min to make it fully homogenized, and obtain a trigger precipitation type concrete crack repair agent.

[0027] Example 4: A specific preparation method of a trigger precipitation type concrete crack repair agent, comprising the following steps: (1) Add 30 g of cetyltrimethylammonium bromide, 400 g of ethanol, and 20 g of ammonia water into 800 g of deionized water, heat up to 60 °C, stir for 45 min, and after cooling to room temperature, obtain a pre-liquid. (2) Add 80 g of nano-anhydrite to the pre-treatment solution obtained in (1), ultrasonic for 45 min while stirring synchronously to obtain a suspension; (3) Under nitrogen protection, add 126 g of tetraethyl orthosilicate to the suspension obtained in (2), stir at 1100 rpm at room temperature for 2 min, then stir at 400 rpm for 10 h, let it stand for 6 h, then filter, wash, calcine in a muffle furnace at 550 °C for 4 h, and after cooling to room temperature, sieve to obtain core-shell structured nano-anhydrite; (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 up to 200 °C, stir and react for 18 h, after cooling to room temperature, the reaction product is centrifuged and washed, and then ball milled at 400 rpm for 2 h in a planetary ball mill to obtain fluorophosphate composite seeds.

[0028] (5) Mix 550 g of sulfoaluminate cement, 100 g of core-shell structured nano-anhydrite, 30 g of fluorophosphate composite seeds, 20 g of magnesium oxide, 120 g of metakaolin, and 4 g of naphthalene-based superplasticizer and add them to a dry powder mixer, stir at 1200 rpm for 25 min to make it fully homogenized to obtain a trigger precipitation type concrete crack repair agent.

[0029] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the core-shell structured nano-anhydrite is replaced by nano-anhydrite.

[0030] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that no fluorophosphate composite seeds are added.

[0031] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the fluorophosphate composite seeds are replaced by calcium fluoride.

[0032] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that no magnesium oxide is added.

[0033] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that no kaolin is added.

[0034] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that only sulfoaluminate cement is used as the concrete crack repair agent.

[0035] Specific application: Application scenario: Repair of concrete cracks in the steam turbine foundation of a thermal power plant (crack width 0.3 - 0.8 mm, 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 according to a water-binder ratio of 0.16 to make repair neat cement. A "V"-shaped groove (depth 8 mm, width 5 mm) was chiseled at the crack. After removing the loose concrete, the repair agent neat cement was applied. After waiting for 30 min, the repair mortar was filled in 2 - 3 layers at intervals of 30 min for each layer. The last layer was troweled to be flush with the substrate. After the addition of the repair agent was completed, curing was carried out by covering with a plastic film or a wet cloth to keep it in a moist state for ≥7 days, and water was sprayed 2 - 3 times a day. After the curing was completed, a repair process was completed. After 90 days of service, the changes in the repair interface were observed.

[0036] Changes in the repair interface: Example 1: The interface is densely bonded without peeling, and the volume stability is excellent without shrinkage marks; Example 2: The interface is densely bonded without peeling, and the volume stability is excellent without shrinkage marks; Example 3: The interface is densely bonded without peeling, and the volume stability is excellent without shrinkage marks; Example 4: The interface is densely bonded without peeling, and the volume stability is excellent without shrinkage marks; Comparative Example 1: Slightly shrinks and cracks, and slight interface peeling appears, and the surface of the repair layer turns white; Comparative Example 2: Microcracks appear, and hollowing is found by knocking inspection; Comparative Example 3: Irregular pits or protrusions appear on the surface of the repair layer, indicating that the local bonding is not dense. When knocking, a clear fracture sound is heard in some areas, showing that there are weak points at the interface; Comparative Example 4: The edge of the repair interface turns white, forming a white carbonized layer, and powder falls off when gently wiped. The whole repair layer is slightly lower than the substrate surface; Comparative Example 5: Continuous fine cracks appear along the crack direction of the repair interface. The repair layer and the substrate are delaminated in the crack depth direction. Water seeps out after watering, showing that the interface bonding is discontinuous. When knocking, the sound is dull, indicating that the internal structure is loose.

[0037] Comparative Example 6: The repair interface completely loses its bonding ability, part of the repair layer falls off, the falling surface is rough and adheres to a small amount of loose cement slurry, the matrix cracks are re-exposed and further expanded, the repair layer is grayish-white, brittle in texture, and can be easily scraped off with a tool.

[0038] Performance test: 1. Crack repair shrinkage rate experiment: 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 min to prepare a repair paste, which was poured into a mold of 250 mm×25 mm×25 mm to make test specimens. The height of the specimens at 7 days of hydration (denoted as A) was measured, and the height of the specimens at 90 days (denoted as B) was measured. Calculate the shrinkage rate Y at each stage = %, and the experimental results are shown in Table 1.

[0039] 2. Compressive and flexural strength experiments: 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 min to prepare a repair paste, and prismatic specimens of 40 mm×40 mm×160 mm were made. According to the standard of GB / T17671-2021, the compressive and flexural strengths of the specimens 3 days after demolding and the compressive and flexural strengths 28 days after demolding were tested. The experimental results are shown in Table 1.

[0040] 3. Carbonation resistance 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 min to prepare a repair paste, and cylindrical specimens of 100 mm×50 mm were made. They were dried in an oven at 60 °C for 48 h, and then the specimens were placed in a carbonation chamber (CO2 concentration 20±3%, humidity 70±5%, temperature 20±2 °C). After carbonation for 90 days, the specimens were taken out, split, and sprayed with a 1% phenolphthalein alcohol solution to measure the carbonation layer thickness (the area where phenolphthalein turns red is the non-carbonated part). The experimental results are shown in Table 1.

[0041] 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 min to prepare a repair paste, and prismatic specimens of 40 mm×40 mm×160 mm were made. They were placed in a high-temperature box and heated from room temperature to 70 °C, 90 °C, 105 °C, and 115 °C at a heating rate of 5 °C / min, and kept warm for 24 h. According to the compressive strength test method, the compressive strength of the specimens after high-temperature treatment was measured, and the strength retention rate was calculated: Q= , and the experimental results are shown in Table 1.

[0042] 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-binder ratio of 0.16 for 10 min to prepare a repair paste, and the tensile bond strength of the specimens was tested according to the test standard of GB / T 50344-2019. The experimental results are shown in Table 1.

[0043] Table 1 Performance test

[0044] Data analysis: From the data in Table 1, it can be seen that the concrete crack repair agents prepared in Examples 1-4 using the present invention have good anti-shrinkage properties and compressive and flexural strengths. At the same time, they can better resist the erosion of carbon dioxide and have a higher strength retention rate when facing high temperatures. Among them, the comprehensive performance of Example 2 is the best.

[0045] In terms of the shrinkage rate of crack repair, this may be because the core-shell structured nano-anhydrite used in the concrete crack repair agent prepared in Example 2 has a slow-release SO4 2- function. Through the coating of tetraethyl orthosilicate to form a "shell layer", the controlled slow release of SO4 2- is realized. In the initial stage of cement hydration, the gypsum in the cement provides a relatively high SO4 2- concentration, inhibiting the release of SO4 2- inside the shell and avoiding the excessive formation of ettringite (AFt) caused by too high a concentration of SO4 2- . As time goes by, the gypsum in the cement is exhausted, and the SO4 2- concentration in the cement decreases, and the SO4 2- inside the shell begins to be slowly released to ensure that a sufficient concentration is maintained for a long time, inhibiting the transformation of AFt to monosulfate calcium sulfoaluminate (AFm) (the formation of AFm is accompanied by a volume shrinkage of about 12%). In Comparative Example 1, nano-anhydrite is directly used, and SO4 2- is rapidly released, resulting in too high a concentration in the early stage, leading to excessive formation of AFt. After the SO4 2- is exhausted in the later stage, a large amount of AFm is generated, and the shrinkage rate increases significantly. At the same time, in Example 2, the F - in the fluorophosphorus composite seed blocks the chain polymerization of [Al(OH)6] 3- octahedrons along the c-axis, and PO4 3- provides transverse connection sites, converting ettringite from needle-like to flake-like, reducing the anisotropic shrinkage of crystal growth. In Comparative Example 2, this seed is not added, and ettringite is needle-like, and internal stress is generated during the cross-growth of crystals, resulting in an increase in the shrinkage rate. Finally, in Example 2, the hydration of magnesium oxide generates magnesium hydroxide, accompanied by a certain volume expansion, filling the pores of the cement stone, reducing the pore connectivity, and inhibiting the drying shrinkage. After removing magnesium oxide in Comparative Example 4, the porosity increases, and the shrinkage rate is higher than that of Example 2.

[0046] In terms of compressive and flexural strengths, this may be because the core-shell nano-anhydrite and fluorophosphorus seeds used in Example 2 have a synergistic effect. The SO4 2-Ensure the continuous generation of AFt, and the flaky AFt crystals overlap with each other to form a three-dimensional network structure. Compared with needle-like crystals, the contact area between crystals increases, and the flexural strength is improved. The fluorophosphate seed promotes the hydration of tricalcium aluminate (C3A), generating more AFt and calcium silicate hydrate (C-S-H) gel at an early stage, enhancing the compressive strength. At the same time, metakaolin undergoes a secondary reaction with the cement hydration product Ca(OH)2 to form C-S-H gel and calcium aluminosilicate (C-A-S-H). The reaction formula is: Al2O3・2SiO2 + xCa(OH)2 + yH2O → xCaO・Al2O3・2SiO2・(x + y)H2O, filling the pores and strengthening the interfacial transition zone, thus increasing the compressive strength.

[0047] In terms of carbonation resistance, this may be due to the alkaline barrier effect of magnesium oxide in Example 2. The solubility of magnesium oxide to form magnesium hydroxide is lower than that of calcium hydroxide, forming a more stable alkaline environment and delaying the diffusion of CO2. Its dense layer covers the surface of the cement, increasing the diffusion path of CO2. After removing magnesium oxide in Comparative Example 4, the thickness of the carbonation layer increases because calcium hydroxide is preferentially carbonated. The reaction formula is: Ca(OH)2 + CO2 → CaCO3 + H2O, resulting in the direct exposure of AFm to CO2 and accelerating its decomposition. At the same time, the core-shell nano-anhydrite inhibits the formation of AFm. The AFm structure contains [Al(OH)4] - , which is prone to adsorb CO3 2- and undergoes a carbonation reaction: 3CaO・Al2O3・CaSO4・12H2O + 3CO2 → 3CaCO3 + Al(OH)3 + CaSO4 + 9H2O, generating loose CaCO3 and Al(OH)3 and increasing the porosity. In Example 2, the core-shell anhydrite maintains a high SO4 2- concentration, inhibits the formation amount of AFm, thereby reducing the carbonation sites. Finally, metakaolin consumes Ca(OH)2 through the pozzolanic reaction. Metakaolin reacts with Ca(OH)2 to form C-S-H gel with a low Ca / Si ratio, decreasing the content of free Ca(OH)2 in the cement stone, reducing the reaction sites of CO2 with Ca(OH)2, and indirectly delaying the carbonation process.

[0048] In terms of maintaining high-temperature strength, this may be because the core-shell structured nano-anhydrite used in Example 2 has a high-temperature stability mechanism. By slowly releasing SO4 2- to maintain the stability of AFt and continuously release SO4 2- , it inhibits the transformation of AFt into AFm, avoiding a sudden drop in strength due to the transformation. At the same time, the fluorophosphate composite seed used induces the flaky growth of AFt. Flaky AFt has the advantage of thermal shock resistance. F - and Al 3+ form a [AlF6] 3- coordination ion with a coordination number of 6, replacing the [Al(OH)6] extending along the c-axis in traditional ettringite3- Chain-like structure, blocking the one-dimensional growth path of crystals, PO4 3- Provide lateral connection sites through P-O-Al bonds, forcing ettringite to transform from needle-like to flaky, reducing the specific surface area, reducing the intercrystalline stress at high temperatures, inhibiting the generation of microcracks, and the Al-F bond energy enhances the structural stability, F - Combines with Al 3+ To form a coordination bond with high bond energy, which is more resistant to high temperatures than the Al-OH bond, thus enhancing the high-temperature stability, PO4 3- The formed P-O-Al bond enhances the interlayer binding force of the crystal, enabling the flaky ettringite to maintain a complete skeleton at high temperatures, avoiding a sudden drop in strength caused by crystal decomposition. Finally, metakaolin reacts with Ca(OH)2 to form a low Ca / Si ratio gel, forming a three-dimensional network resistant to high temperatures and stabilizing the matrix strength.

[0049] In terms of the tensile bond strength, this may be because the core-shell nano-anhydrite used in Example 2 promotes the preferential deposition of AFt at the crack interface, forming an "anchoring" structure. The crystal density of AFt at the bonding interface increases, and the mechanical biting force between the crystals and the concrete matrix is enhanced. The fluorophosphate seed regulates the transformation of AFt from needle-like to flaky. Although the specific surface area decreases slightly, the contact area between the flaky crystals and the concrete matrix will increase, and the van der Waals force and hydrogen bond interaction sites increase, F - / PO4 3- Promote chemical bonding: F - Combines with Al in the concrete 3+ 、Fe 3+ To form stable coordination bonds, PO4 3- Combines with Ca 2+ To generate calcium phosphate salts, forming "chemical anchoring points" in the interface region. The chemical binding energy at the bonding interface increases. Finally, the C-S-H gel generated by the pozzolanic reaction of metakaolin penetrates into the pores of the concrete, forming a "nano-bridging" effect, which improves the bond strength.

[0050] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A trigger-precipitation type concrete crack repair agent, characterized in that, It includes the following raw materials in parts by weight: sulfoaluminate cement: 50 - 60 parts, core - shell structured nano - anhydrite: 8 - 12 parts, fluorophosphate composite seed crystal: 2 - 4 parts, magnesium oxide: 1 - 3 parts, metakaolin: 10 - 15 parts, water - reducing agent: 0.3 - 0.5 part; The preparation process of the core - shell structured nano - anhydrite is as follows: (1) Add cetyltrimethylammonium bromide, ethanol, and ammonia water into deionized water, heat up to 50 - 70 °C, stir for 30 - 60 min, and after cooling to room temperature, obtain a pre - liquid; (2) Add nano - anhydrite into the pre - liquid obtained in (1), ultrasonicate for 30 - 60 min while stirring synchronously to obtain a suspension; (3) Under nitrogen protection, add tetraethyl orthosilicate into the suspension obtained in (2), stir at 1000 - 1200 rpm at room temperature for 1 - 3 min, then stir at 300 - 500 rpm for 8 - 12 h, stand for 4 - 8 h, filter, wash, calcine in a muffle furnace at 500 - 600 °C for 3 - 5 h, and after cooling to room temperature, sieve to obtain the core - shell structured nano - anhydrite; The preparation process of the fluorophosphate composite seed crystal is as follows: Add calcium fluoride, calcium dihydrogen phosphate, and deionized water into a hydrothermal reactor, adjust the pH to 4.5 - 5.5, heat up to 180 - 220 °C, stir and react for 12 - 24 h, after cooling to room temperature, the reaction product is centrifuged and washed, and then ball - milled at 300 - 500 rpm in a planetary ball mill for 1 - 3 h to obtain the fluorophosphate composite seed crystal.

2. The trigger precipitation type concrete crack repair agent according to claim 1, characterized in that, The water - reducing agent refers to one of polycarboxylate superplasticizer or naphthalene - based high - efficiency water - reducing agent.

3. The triggered precipitation type concrete crack repair agent according to claim 1, wherein In (1), the weight ratio of cetyltrimethylammonium bromide, ethanol, ammonia water, and deionized water is 0.01 - 0.05:0.3 - 0.5:0.01 - 0.025:0.6 - 1.

4. The trigger precipitation type concrete crack repair agent according to claim 1, characterized in that In (2), the weight ratio of nano - anhydrite and the pre - liquid is 1:8 - 12.

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

6. The trigger precipitation type concrete crack repair agent according to claim 1, wherein In the preparation process of the fluorophosphate composite seed crystal, the weight ratio of calcium fluoride, calcium dihydrogen phosphate, and deionized water is 1:1:5 - 7.

7. The preparation method of the trigger precipitation type concrete crack repair agent according to any one of claims 1-6, characterized in that, It includes the following steps: Mix sulfoaluminate cement, core - shell structured nano - anhydrite, fluorophosphate composite seed crystal, magnesium oxide, metakaolin, and water - reducing agent and add them into a dry - powder mixer, stir at 1000 - 1500 rpm for 20 - 30 min to make them fully homogenized, and obtain a trigger - precipitation type concrete crack repair agent.

8. Application of the trigger - precipitation type concrete crack repair agent obtained by the preparation method described in claim 7 in the repair of concrete structure cracks and high - temperature environment engineering.

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