Concrete with frost resistance and preparation method thereof

Abstract

The invention discloses concrete with anti-freezing performance and a preparation method thereof, and belongs to the technical field of concrete. The invention aims to solve the problems that the porosity of an interface transition area is high, hydration products are easy to degrade and the anti-freezing and anti-permeation performances are insufficient under the working conditions of high altitude, low air pressure, strong ultraviolet rays and salt-freezing synergy of the existing large-dosage fly ash concrete. The concrete is prepared from the following components in parts by weight: 100 parts of cement, 100 to 150 parts of coal ash, 280 to 320 parts of fine aggregate, 1000 to 1200 parts of modified basalt coarse aggregate, 1.5 to 3.5 parts of polycarboxylic acid water reducing agent, 0.2 to 0.6 part of PEG (Polyethylene Glycol) coated chitosan quaternary ammonium salt microspheres, 0.05 to 0.25 part of chitosan grafted nano cerium oxide composite particles, 1.0 to 3.0 parts of bacillus pseudofirmus microcapsule-polyvinylpyrrolidone mixture and 0.03 to 0.12 part of trisodium citrate. Through the synergistic effect of all the components, the interface structure of the concrete is effectively optimized, the ultraviolet aging resistance, the salt-freezing synergistic damage resistance and the chloride ion infiltration resistance are improved, the high-altitude severe service condition is adapted, and reliable material support is provided for high-altitude infrastructure construction.

Description

Technical Field

[0001] This invention belongs to the field of concrete technology, specifically a type of concrete with frost resistance and its preparation method. Background Technology

[0002] With the large-scale advancement of infrastructure construction in high-altitude areas (≥3000m) such as the Qinghai-Tibet Plateau and the Qilian Mountains, the low-carbonization of engineering materials and the resource utilization of solid waste have become important guiding principles. High-volume fly ash concrete (≥50% fly ash content) can help reduce cement usage and carbon emissions, and can efficiently dispose of industrial solid waste such as fly ash, meeting the actual needs of solid waste disposal in high-altitude areas. Its application in highways, railways, water conservancy facilities, and photovoltaic base stations in these regions is gradually expanding, making it one of the important development directions for concrete used in high-altitude engineering.

[0003] However, the material properties of high-volume fly ash concrete, combined with the harsh environment of high-altitude regions, create a synergistic effect, making its durability a prominent issue. From the perspective of the materials themselves, the interfacial transition zone of high-volume fly ash concrete is a natural weak point: the reaction rate of fly ash and pozzolanic material is relatively slow, and the reactivity is low after 28 days of curing. A large number of unreacted fly ash particles are dispersed in the interfacial area, resulting in a higher porosity in the interfacial transition zone than in traditional concrete. At the same time, the hydration products in the interfacial transition zone are mainly calcium hydroxide (CH) crystals and calcium silicate hydrate (CSH) gel. Among them, CH crystals are mostly in a directional growth state with a relatively loose structure, and the amount of CSH gel generated is insufficient and unevenly distributed, which weakens the interfacial bond strength to a certain extent.

[0004] The unique natural environment of high-altitude areas can have a targeted effect on this vulnerable area, and the coupling of multiple factors further exacerbates the degree of degradation: Firstly, the air pressure at high altitudes is about 60%-70% of that at plains. The low air pressure environment accelerates the non-uniform evaporation of water during the hardening process of concrete, resulting in insufficient hydration reaction in the interface transition zone. This further reduces the amount of CSH gel generated, making it difficult for unhydrated cement particles to form a dense hydration product bonding layer with the aggregate surface. At the same time, the capillary channels left by the rapid migration of water are connected with the original pores, forming a non-uniform pore structure, which provides a channel for the penetration of corrosive media. Secondly, the intensity of ultraviolet radiation in this region is significantly higher than that in the plain. The UV-B band can directly act on the hydration products in the interface transition zone, destroying the lattice integrity of CH crystals and causing a decrease in their crystal regularity. At the same time, it may cause the breakage of Si-OT chains (T is Si or Al) in CSH gel, making the gel network structure loose and thus weakening the elastic modulus and bonding performance of the interface transition zone. Third, the diurnal temperature range in high-altitude areas can reach 20-30℃, easily leading to short-term freeze-thaw cycles of daytime thawing and nighttime freezing. Soil / groundwater salinization or the use of de-icing salt in some areas can further increase the concentration of chloride ions (Cl). - SO4 2- Salt ions penetrate into the interfacial transition zone through pore channels and react with CH crystals to generate expansive products such as calcium oxychloride and ettringite. The volume expansion force generated by the freezing of pore water during freeze-thaw cycles, combined with the salt crystallization expansion force, easily leads to the rupture of the CSH gel network and the shedding of CH crystals in the interfacial transition zone, accelerating the initiation and propagation of microcracks.

[0005] The combined effects of these three factors ultimately result in concrete surface spalling and a significant decrease in compressive strength, impacting the long-term service safety of engineering structures. Therefore, providing a type of concrete with frost-resistant properties is of great significance. Summary of the Invention

[0006] The purpose of this invention is to provide a concrete with antifreeze properties and its preparation method, so as to effectively solve the problems of high porosity in the interfacial transition zone, easy degradation of hydration products by ultraviolet radiation, and insufficient antifreeze performance under the synergistic effect of salt and freeze in traditional high-volume fly ash concrete under high-altitude multi-coupling conditions.

[0007] The objective of this invention is achieved through the following technical solution: A type of concrete with frost-resistant properties, comprising the following components in parts by weight: 100 parts cement, 100-150 parts fly ash, 280-320 parts fine aggregate, 1000-1200 parts modified basalt coarse aggregate, 1.5-3.5 parts polycarboxylate superplasticizer, 0.2-0.6 parts PEG-coated chitosan quaternary ammonium salt microspheres, 0.05-0.25 parts chitosan-grafted nano-cerium oxide composite particles, 1.0-3.0 parts Bacillus pseudostrongylus microcapsule-polyvinylpyrrolidone mixture, and 0.03-0.12 parts trisodium citrate; The modified basalt coarse aggregate is obtained by modifying basalt coarse aggregate with a composite modifier composed of ultrafine slag and nano-cerium oxide; In the Bacillus pseudostrongylus microcapsule-polyvinylpyrrolidone mixture, the wall material of the Bacillus pseudostrongylus microcapsule is a composite wall material of polylactic acid-glycolic acid copolymer and dimethylaminoethyl methacrylate (DMAEMA). The chitosan-grafted cerium oxide nanocomposite particles were prepared by grafting chitosan quaternary ammonium salt onto the surface of cerium oxide nanoparticles using the silane coupling agent KH550.

[0008] As one possible implementation of this application, in the composite modifier, the mass ratio of ultrafine slag and nano-cerium oxide is (25-35):(0.5-1.5).

[0009] As some possible implementations of this application, the particle size of the PEG-coated chitosan quaternary ammonium salt microspheres is 40-120 μm.

[0010] As some possible embodiments of this application, in the chitosan-grafted cerium oxide nanocomposite particles, the particle size of the cerium oxide nanoparticles is 40-120 nm; the mass ratio of chitosan quaternary ammonium salt to cerium oxide nanoparticles is 1:(4-9).

[0011] As some possible implementations of this application, the particle size of the *Bacillus pseudostrongylus* microcapsules is 150-600 μm, and the spore content is ≥109 spores / g.

[0012] As some possible embodiments of this application, the wall material of the microcapsule further includes nano-titanium dioxide, wherein the nano-titanium dioxide has a particle size of 15-60 nm.

[0013] In practice, high-altitude environments with strong ultraviolet radiation can accelerate the aging and degradation of microcapsule wall materials, leading to premature damage and spore inactivation, which affects the crack repair effect. This invention introduces nano-titanium dioxide into the microcapsule wall material, utilizing its ultraviolet shielding effect to slow down the aging process of the wall material, ensuring the structural stability of the wall material during the service life of concrete, and providing support for the stable storage and release of spores.

[0014] As one possible implementation of this application, the polycarboxylate superplasticizer is a β-cyclodextrin modified polycarboxylate superplasticizer.

[0015] In practice, high altitude, low air pressure, and large temperature differences can easily lead to a decrease in the dispersion stability of ordinary polycarboxylate superplasticizers and a rapid loss of slurry fluidity, affecting the pouring and construction effect. This invention uses β-cyclodextrin to modify polycarboxylate superplasticizers, which enhances the stability of superplasticizer molecules through the inclusion effect of β-cyclodextrin, reduces the impact of environmental factors on dispersion performance, and ensures the workability stability of concrete in high-altitude on-site construction.

[0016] Furthermore, to achieve the above objectives, the present invention also provides a method for preparing concrete with frost-resistant properties, comprising the following steps: S1. Component pre-dispersion: A mixed solution A was prepared by adding water to PEG-coated chitosan quaternary ammonium salt microspheres and chitosan-grafted cerium oxide nanocomposite particles; Separately, a mixture of Bacillus pseudostearate microcapsules-polyvinylpyrrolidone and trisodium citrate was prepared by mixing and adding water to form mixture B; S2. Concrete mixing: First, mix cement, fly ash and fine aggregate evenly, add mixed solution A, stir evenly, then add polycarboxylate superplasticizer and mixed solution B, stir evenly, then add modified coarse aggregate, stir evenly to obtain mixture C; S3. Casting and curing: Cast mixture C into shape, first place it in an environment of 18-22℃ and relative humidity ≥90% for 7-10 days, and then transfer it to a high-altitude simulated environment for 18-24 days to obtain the finished product.

[0017] Compared with the prior art, the beneficial effects of the present invention are: The frost-resistant concrete provided by this invention can effectively solve the problems of high porosity in the interfacial transition zone, easy degradation of hydration products by ultraviolet radiation, and insufficient frost resistance under the synergistic effect of salt and frost in traditional high-volume fly ash concrete under high-altitude and multi-coupling conditions. Specifically, it is as follows: In the matrix paste: a large amount of fly ash and cement synergistically construct a cementitious system. Fly ash particles can fill the pores formed by cement hydration and synergistically improve the internal pore structure of concrete with the calcium silicate hydrate (CSH) gel generated by cement hydration, alleviating the problem of insufficient hydration in the interface transition zone caused by rapid evaporation of moisture under high altitude and low air pressure environment.

[0018] In modified basalt coarse aggregate: ultrafine slag can refine hydration products and interweave with the hydration products of the matrix slurry, which helps to improve the interfacial bonding strength between aggregate and slurry; nano-cerium oxide can inhibit the breakage of Si-OT chains in hydration products by removing free radicals generated by ultraviolet irradiation, and reduce the damage of ultraviolet rays to hydration products in the interfacial transition zone. The two work together to optimize the density of the modified layer and reduce the risk of excessive porosity of the modified layer under low pressure environment.

[0019] Polycarboxylate superplasticizers can improve the fluidity of concrete slurry, making them suitable for on-site mixing and pouring at high altitudes.

[0020] In chitosan-grafted cerium oxide nanocomposite particles, chitosan quaternary ammonium salt is grafted onto the surface of cerium oxide nanoparticles using the silane coupling agent KH550. This reduces the surface energy of the cerium oxide nanoparticles, effectively solving the problem of easy agglomeration of single cerium oxide nanoparticles in the alkaline environment of cement paste. At the same time, the amino and hydroxyl groups on the chitosan quaternary ammonium salt molecular chain can form hydrogen bonds with Si-OH in CSH gel; its hydroxyl groups (-OH) can also form hydrogen bonds with the hydroxyl groups on the surface of calcium hydroxide (CH) crystals. The amino groups can be adsorbed onto the crystal surface through electrostatic interaction. This dual effect helps to stabilize the CH crystal structure and inhibit its directional growth. Synergistically with the ultraviolet shielding effect of cerium oxide nanoparticles, it further enhances the resistance of concrete to ultraviolet degradation.

[0021] In polyethylene glycol (PEG) coated chitosan quaternary ammonium salt microspheres, the chitosan quaternary ammonium salt itself has a certain broad-spectrum antibacterial activity. If added directly, it is easy to inhibit the activity of Bacillus pseudostrongylus due to excessively high local concentration, resulting in the failure of subsequent mineralization and repair functions. PEG film can prevent the release of chitosan quaternary ammonium salt in the early stage of concrete (3-14 days), avoiding the above-mentioned antibacterial conflict. As the concrete hydration process progresses, PEG gradually degrades, and chitosan quaternary ammonium salt is slowly released. It can not only exert a broad-spectrum antibacterial effect and inhibit the growth of miscellaneous bacteria to ensure the activity of Bacillus pseudostrongylus, but its film-forming property can also delay water penetration and provide a stable environment for the spore mineralization reaction.

[0022] pH-responsive Bacillus pseudostrongylus microcapsules-polyvinylpyrrolidone mixture can address two types of problems under high-altitude multi-coupled working conditions, specifically as follows: On the one hand, concrete under high-altitude multi-coupled working conditions is prone to microcracks due to low-temperature freeze-thaw cycles and drying shrinkage. These cracks can become penetration channels for corrosive media such as salt ions and moisture, exacerbating concrete deterioration. Based on this, Bacillus pseudostrongylus can produce calcium carbonate precipitates through metabolism, which can specifically fill these microcracks, block erosion channels, and achieve active repair of concrete microcracks. On the other hand, in high-altitude, low-pressure environments, the drying shrinkage stress generated by the rapid evaporation of moisture in the early stages of concrete pouring can easily lead to premature rupture of the microcapsule wall material and premature activation of spores, which then become inactive due to lack of moisture and nutrients. Therefore, the microcapsule wall material is prepared using a composite of polylactic acid-glycolic acid copolymer and dimethylaminoethyl methacrylate (DMAEMA), which can adapt to the pH environment at different stages of concrete pouring: the wall material remains stable in the early, highly alkaline environment of the slurry, avoiding the aforementioned problems of premature rupture and spore inactivation; when microcracks appear inside the concrete (due to moisture penetration causing a drop in pH), the microcapsule wall material gradually degrades under the combined action of moisture and carbon dioxide at the microcracks during the later stages of concrete hydration (when the pH of the pore fluid slowly decreases to around 11), releasing spores. Furthermore, polyvinylpyrrolidone can improve the uniformity of microcapsule dispersion in the slurry, preventing microcapsule aggregation, and synergistically with *Bacillus pseudostrongylus* to ensure pore filling and microcrack repair effects, further blocking the penetration channels of corrosive media.

[0023] In the hydration process under high-altitude multi-coupled working conditions, the hydration rate of the coarse aggregate modified layer and the hydration rate of the matrix slurry are prone to mismatch, leading to the formation of microcracks at the interface and weakening the performance of the interface transition zone. Trisodium citrate can selectively complex free calcium ions in cement slurry, moderately delaying the hydration rate of the matrix slurry, and at the same time has a slight promoting effect on the pozzolanic reaction of ultrafine slag. This makes the hydration process of the coarse aggregate modified layer and the matrix slurry more synergistic, effectively alleviating the above-mentioned interface compatibility imbalance problem and reducing the generation of interface microcracks.

[0024] In summary, this invention, through the synergistic combination of its components, not only helps to improve the interfacial transition zone performance of concrete under high-altitude multi-coupling conditions, stabilize the structure of hydration products, and enhance frost resistance, but also alleviates derivative problems caused by the synergistic effect of multiple components during actual implementation. The final concrete prepared is suitable for the use needs of infrastructure construction in high-altitude areas. Detailed Implementation

[0025] Example 1 S1. Raw material preparation: (1) Preparation of modified basalt coarse aggregate: ① Preparation of composite slurry: Weigh ultrafine slag (200 mesh) and nano-cerium oxide (80 nm particle size) at a mass ratio of 30:1, place them in a planetary ball mill, mix and grind at 300 rpm for 20 min to obtain composite modifier, then add 10 times the mass of deionized water to composite modifier, and ultrasonically disperse for 15 min (power 200W) to obtain composite modifier slurry; ② Modification treatment: Take basalt coarse aggregate (particle size 5-25mm), rinse it with clean water and dry it in an oven at 105℃ for 2 hours. After cooling, spray the composite modifier slurry evenly, with the spray amount being 5% of the mass of the basalt coarse aggregate. Then place it in an oven at 60℃ for 4 hours, turning it over once every 1 hour. After drying, pass it through a 20-mesh standard sieve (to remove unattached modifier powder) to obtain modified basalt coarse aggregate.

[0026] (2) Preparation of PEG-coated chitosan quaternary ammonium salt microspheres: ① Core material solution preparation: Weigh chitosan quaternary ammonium salt (degree of deacetylation ≥85%, degree of quaternization substitution ≥55%), add deionized water to prepare a 5% mass fraction solution, place it in a 40℃ constant temperature water bath, stir at 200 rpm for 30 min and assist in ultrasonic dispersion for 10 min (power 200W) to obtain the core material solution; ② Preparation of wall material solution: Weigh polyethylene glycol (PEG-6000), add deionized water, heat to 60℃ and stir at 200rpm until dissolved, to prepare a wall material solution with a mass fraction of 8%; ③ Coating molding: Mix the core material solution and the wall material solution at a volume ratio of 1:3, place them in a high-speed emulsifier, emulsify at 8000 rpm for 30 min, and then transfer them to a spray dryer (inlet air temperature 120℃, outlet air temperature 60℃, feed rate 8mL / min) for drying. Collect microspheres with a particle size of 50-100μm to obtain PEG-coated chitosan quaternary ammonium salt microspheres.

[0027] (3) Preparation of chitosan-grafted cerium oxide nanocomposite particles: ① Pretreatment: Weigh out nano-cerium oxide (particle size 80nm), add ethanol solution (ethanol:water = 1:1, v / v), and ultrasonically disperse for 30min (power 200W) to obtain nano-cerium oxide dispersion; ② Grafting reaction: Add silane coupling agent KH550 (purity ≥97%, addition amount is 5% of the mass of nano-cerium oxide) to the nano-cerium oxide dispersion, stir at 50℃ for 2h (speed 250 rpm), filter and wash, then add chitosan quaternary ammonium salt solution (mass fraction 3%, solvent water, mass ratio of chitosan quaternary ammonium salt to nano-cerium oxide 1:6), stir at 60℃ for 4h (speed 300 rpm); ③ Post-processing: After the reaction is completed, centrifuge (5000 rpm, 10 min), wash the precipitate three times with deionized water, dry it in a vacuum drying oven at 80℃ for 6 h, pulverize it and pass it through a 200-mesh sieve to obtain chitosan-grafted cerium oxide nanocomposite particles.

[0028] (4) Preparation of pH-responsive Bacillus pseudostrongylus microcapsules-polyvinylpyrrolidone mixture: ① Preparation of wall material solution: Weigh polylactic acid-glycolic acid copolymer and dimethylaminoethyl methacrylate (DMAEMA) at a mass ratio of 1:1, and add dichloromethane to prepare a wall material solution with a total solute mass fraction of 10% (solute: polylactic acid-glycolic acid copolymer and dimethylaminoethyl methacrylate). ② Preparation of core material dispersion: Weigh out Bacillus pseudostrongylus spore powder (spore content 10) 9 Mix (each piece / g) with polyvinylpyrrolidone (mass ratio 4:1), add 0.9% physiological saline [solid-liquid ratio 1:5 (m / v)] to prepare a suspension, and ultrasonically disperse for 15 min (power 200W) to obtain the core material dispersion; ③ Microcapsule preparation: The core material dispersion was slowly added to the wall material solution and emulsified at 10,000 rpm for 40 min to form an emulsion. Then, it was slowly added dropwise to a 1% polyvinyl alcohol solution and stirred at 30℃ for 6 h (200 rpm) to evaporate the solvent. After filtration and washing, it was placed in a vacuum drying oven (50℃) and dried for 8 h. The Bacillus pseudostrongylus microcapsules with a particle size of 200-400 μm were collected and then mixed evenly with 25% polyvinylpyrrolidone by the mass of the microcapsules to obtain the mixture.

[0029] S2. Component pre-dispersion: (1) Preparation of mixed solution A (parts by weight, the same below): Weigh 0.4 parts of PEG-coated chitosan quaternary ammonium salt microspheres and 0.15 parts of chitosan grafted nano-cerium oxide composite particles, add 50 parts of deionized water, ultrasonically disperse for 20 min (power 200W), and then stir at 200 rpm for 10 min to obtain mixed solution A; (2) Preparation of mixed solution B: Weigh 2.0 parts of Bacillus pseudostrongylus microcapsule-polyvinylpyrrolidone mixture and 0.08 parts of trisodium citrate, add 30 parts of deionized water, stir to dissolve (150 rpm, 15 min) to obtain mixed solution B.

[0030] S3. Concrete mixing: ① First, add 100 parts of cement (P·O 42.5 grade ordinary Portland cement), 120 parts of fly ash (F type, grade II), and 300 parts of fine aggregate (manufactured sand, fineness modulus 2.8, mud content ≤1.5%) to a forced concrete mixer and mix at low speed for 3 minutes (150 rpm) until evenly mixed. ② Add mixed solution A, stir at medium speed for 5 minutes (300 rpm), and after stirring evenly, add 2.5 parts of polycarboxylate superplasticizer (40% solid content, commercially available) and mixed solution B, and stir at medium speed for 6 minutes (300 rpm). ③ Finally, add 1100 parts of modified basalt coarse aggregate and stir at high speed for 8 minutes (450 rpm) to obtain a homogeneous mixture C.

[0031] S4. Pouring and Curing: ① Pouring: Pour mixture C into a mold of the specified size (e.g., a 200mm×200mm×200mm mold), and vibrate it for 40 seconds using an immersion vibrator (vibration frequency 50Hz, amplitude 2mm). After vibration, scrape the surface of the mold smooth. ② Pre-curing: Place in an environment with a temperature of 20℃ and a relative humidity of ≥90% for 8 days; ③ High-altitude simulated curing: Transfer to a high-altitude simulated environment [i.e., a multi-coupled environment: low air pressure 65kPa, strong ultraviolet radiation in the UV-B band (intensity 80W / m², continuous irradiation), salt corrosion (spraying 3wt% NaCl solution once every 24 hours to keep the surface moist), freeze-thaw cycle (24 hours / cycle, low temperature -15℃ and high temperature 5℃ for 12 hours each)] for 21 days to obtain the finished product.

[0032] Example 2 Compared to Example 1, rutile nano-titanium dioxide with a particle size of 30 nm was added to the microcapsule wall material of the pH-responsive Bacillus pseudostrongylus microcapsule-polyvinylpyrrolidone mixture.

[0033] Wall material solution preparation: Weigh polylactic acid-glycolic acid copolymer, dimethylaminoethyl methacrylate (DMAEMA), and nano-titanium dioxide (rutile type, particle size 30nm) at a mass ratio of 1:1:0.2, add dichloromethane (operate at low temperature in an ice-water bath to reduce dichloromethane volatilization), stir to dissolve and prepare a solution with a total solute mass fraction of 10% (solute: polylactic acid-glycolic acid copolymer, dimethylaminoethyl methacrylate, nano-titanium dioxide); the subsequent core material dispersion, microcapsule preparation and mixing steps are the same as in Example 1.

[0034] Example 3 Compared to Example 2, the addition of a step to prepare the β-cyclodextrin-modified polycarboxylate superplasticizer, while the remaining steps are the same as in Example 2: Preparation of β-cyclodextrin-modified polycarboxylate superplasticizer: Weigh out a polycarboxylate superplasticizer (40% solid content), dilute with deionized water to a solid content of 20%, add β-cyclodextrin (8% of the solid mass of the polycarboxylate superplasticizer), and stir at 60℃ for 3 hours (250 rpm). Then add ammonium persulfate (0.5% of the total mass) and continue stirring for 2 hours. After the reaction is complete, cool to room temperature, adjust the pH to 7-8 with sodium hydroxide solution, and concentrate by vacuum distillation to obtain the β-cyclodextrin-modified polycarboxylate superplasticizer. When using, the dosage of β-cyclodextrin-modified polycarboxylate superplasticizer is 2.5 parts.

[0035] Comparative Example 1 Compared to Example 1, the PEG-coated chitosan quaternary ammonium salt microspheres were removed.

[0036] The preparation of other raw materials, pouring and curing steps, and the amount of each component added are all the same as in Example 1.

[0037] Comparative Example 2 Compared to Example 1, the "modified basalt coarse aggregate" is replaced with an equal amount of "unmodified basalt coarse aggregate".

[0038] The preparation of other raw materials, pouring and curing steps, and the amount of each component added are all the same as in Example 1.

[0039] Comparative Example 3 Compared to Example 1, the chitosan-grafted cerium oxide nanocomposite particles were removed and replaced with an equal amount of ordinary cerium oxide nanoparticles.

[0040] The preparation of other raw materials, pouring and curing steps, and the amount of other components added are all the same as in Example 1.

[0041] Comparative Example 4 Compared to Example 1, the "pH-responsive Bacillus pseudostrongylus microcapsules" were replaced with an equal amount of "Bacillus pseudostrongylus spore powder (spore content 109 spores / g)".

[0042] The preparation of other raw materials, pouring and curing steps, and the amount of other components added are all the same as in Example 1.

[0043] Comparative Example 5 Compared to Example 1, trisodium citrate was removed.

[0044] The preparation of other raw materials, pouring and curing steps, and the amount of other components added are all the same as in Example 1.

[0045] Experimental Example The finished test blocks obtained from Examples 1-3 and Comparative Examples 1-5 (finished products after 8 days of pre-curing + 21 days of curing in a high-altitude multi-coupling environment) were subjected to the following relevant performance tests.

[0046] 1.28d compressive strength: The finished test block (test block size: 200mm×200mm×200mm) cured for 28 days was directly pressed with a press at a rate of 2.5kN / s. The maximum pressure when the test block failed was recorded, and the compressive strength was calculated.

[0047] 2. Mass loss rate and strength loss rate after 300 freeze-thaw cycles: The finished test blocks (test block size: 200mm×200mm×200mm) cured for 28 days were subjected to 300 freeze-thaw cycles under the same complete multi-coupling conditions as in Example 1 (24h / cycle, 12h at -15℃ and 12h at 5℃, with 3% NaCl solution sprayed every 24h during the cycle); the mass of the test blocks was weighed with an electronic balance before and after the cycle, and the mass loss rate (mass reduction after cycle / initial mass × 100%) and the strength loss rate (strength reduction after cycle / initial strength × 100%) were calculated.

[0048] 3.1000h multi-coupling aging strength retention rate: The finished test block (test block size: 200mm×200mm×200mm) cured for 28 days was placed under the same complete multi-coupling conditions as in Example 1 for another 1000 hours. The compressive strength after aging was tested, and the retention rate (strength after aging / initial strength × 100%) was calculated.

[0049] 4. Porosity of the interface transition zone: Samples were taken from the finished test blocks (test block size: 200mm×200mm×200mm) after 28 days of curing; the sampling location was the slurry area extending outward from the surface of the modified basalt coarse aggregate (the core area of ​​aggregate-slurry bonding); small test blocks of 5mm×5mm×5mm were drilled with a diamond core drill, and after removing surface dust, they were placed in a mercury intrusion porosimeter; mercury was injected by gradually increasing the pressure (0.001-414MPa), and the mercury injection volume under different pressures was recorded. The pore size range of 0.00110μm was screened, and the proportion of the pore volume in this range to the total volume of the test block was calculated.

[0050] 5.28d Chloride Ion Permeability Coefficient: Referring to the Rapid Chloride Ion Migration Method (RCM Method) in the "Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete", the finished test block (test block size: 100mm×100mm×100mm) cured for 28 days was directly fixed in the permeation device, with one end immersed in 3% NaCl solution and the other end immersed in distilled water; a constant voltage of 60V was applied for continuous testing for 24 hours, and the chloride ion concentration on the distilled water side was determined by silver nitrate titration. The permeability coefficient was calculated based on the migration amount, voltage, time and test block size.

[0051] The performance test results are shown in Table 1.

[0052] Table 1: As can be seen from Table 1: Example 1 effectively improves the problems of high porosity in the interfacial transition zone of high-volume fly ash concrete under high-altitude multi-coupling conditions, susceptibility of hydration products to UV degradation, and insufficient freeze-thaw resistance under the synergistic effect of salt and freeze. Its 28-day compressive strength reaches 43.8 MPa, ensuring the basic load-bearing requirements of high-altitude engineering structures; the mass loss rate after 300 freeze-thaw cycles is 3.1%, and the strength loss rate is 14.2%, demonstrating a certain degree of salt-freezing resistance; the strength retention rate after 1000 hours of multi-coupling aging is 82.5%, adapting to the deteriorating environment of long-term service at high altitudes; the porosity in the interfacial transition zone is 22.5%, alleviating the problem of insufficient hydration caused by low air pressure; and the 28-day chloride ion permeability coefficient is 5.8 × 10⁻⁶. -12 m² / s reduces the risk of salt ion penetration and corrosion, making it suitable for high-altitude service scenarios. Example 2, based on Example 1, further enhances the UV aging resistance of concrete by introducing nano-titanium dioxide into the microcapsule wall material, making it more suitable for engineering scenarios with prominent high-altitude and strong UV radiation. Example 3, based on Example 2, uses β-cyclodextrin-modified polycarboxylate superplasticizer to improve the density of concrete paste and its resistance to salt-freezing and impermeability, making it more suitable for engineering scenarios in high-altitude salinized areas or where the salt-freezing synergistic effect is significant. In Comparative Example 1, the lack of PEG-coated chitosan quaternary ammonium salt microspheres made it difficult to avoid the inhibitory effect of early chitosan quaternary ammonium salt release on Bacillus pseudostrongylus, resulting in slight fluctuations in related properties and making it difficult to ensure the long-term performance stability of concrete under high-altitude conditions. In Comparative Example 2, the use of unmodified basalt coarse aggregate lacked the optimization effect of composite modifiers on the interfacial transition zone, leading to a significant decrease in the overall performance of the concrete and making it difficult to solve the core problem of a weak interfacial transition zone under high-altitude multi-coupled conditions. In Comparative Example 3, the use of ordinary nano-cerium oxide instead of chitosan-grafted nano-cerium oxide composite particles resulted in poor nanoparticle dispersion, failing to effectively stabilize the structure of hydration products and optimize interfacial properties, making it difficult to adapt to the coupled deteriorating environment of high-altitude strong ultraviolet radiation and low air pressure. In Comparative Example 4, the use of ordinary Bacillus pseudostrongylus spore powder instead of pH-responsive microcapsules made the spores prone to inactivation in the early high-alkaline environment, failing to achieve effective repair of microcracks and making it difficult to solve the problems of crack propagation and erosion media penetration caused by high-altitude freeze-thaw cycles. In Comparative Example 5, due to the lack of trisodium citrate, the hydration process of the coarse aggregate modified layer and the matrix paste is difficult to coordinate, and microcracks are easily generated at the interface, resulting in a decrease in the concrete's resistance to salt-freeze synergistic damage and making it difficult to adapt to working conditions with large temperature differences at high altitudes and obvious salt erosion.

Claims

1. A type of concrete with frost-resistant properties, characterized in that, Includes the following components by weight: 100 parts cement, 100-150 parts fly ash, 280-320 parts fine aggregate, 1000-1200 parts modified basalt coarse aggregate, 1.5-3.5 parts polycarboxylate superplasticizer, 0.2-0.6 parts PEG-coated chitosan quaternary ammonium salt microspheres, 0.05-0.25 parts chitosan-grafted nano-cerium oxide composite particles, 1.0-3.0 parts Bacillus pseudostrongylus microcapsule-polyvinylpyrrolidone mixture, and 0.03-0.12 parts trisodium citrate; The modified basalt coarse aggregate is obtained by modifying basalt coarse aggregate with a composite modifier composed of ultrafine slag and nano-cerium oxide; In the Bacillus pseudostrongylus microcapsule-polyvinylpyrrolidone mixture, the wall material of the Bacillus pseudostrongylus microcapsule is a composite wall material of polylactic acid-glycolic acid copolymer and dimethylaminoethyl methacrylate. The chitosan-grafted cerium oxide nanocomposite particles were prepared by grafting chitosan quaternary ammonium salt onto the surface of cerium oxide nanoparticles using the silane coupling agent KH550.

2. The concrete with frost-resistant properties according to claim 1, characterized in that, In the composite modifier, the mass ratio of ultrafine slag and nano-cerium oxide is (25-35):(0.5-1.5).

3. The concrete with frost-resistant properties according to claim 1, characterized in that, The particle size of the PEG-coated chitosan quaternary ammonium salt microspheres is 40-120 μm.

4. The concrete with frost-resistant properties according to claim 1, characterized in that, In the chitosan-grafted cerium oxide nanocomposite particles, the particle size of the cerium oxide nanoparticles is 40-120 nm; the mass ratio of chitosan quaternary ammonium salt to cerium oxide nanoparticles is 1:(4-9).

5. The concrete with frost-resistant properties according to claim 1, characterized in that, The microcapsules of *Bacillus pseudostrongylus* have a particle size of 150-600 μm and a spore content ≥10%. 9 per g.

6. The concrete with frost-resistant properties according to claim 1, characterized in that, The wall material of the microcapsule also includes nano-titanium dioxide, the nano-titanium dioxide having a particle size of 15-60 nm.

7. The concrete with frost-resistant properties according to claim 1, characterized in that, The polycarboxylate superplasticizer is a β-cyclodextrin modified polycarboxylate superplasticizer.

8. A method for preparing concrete with frost-resistant properties as described in any one of claims 1-7, characterized in that, Includes the following steps: S1. Component pre-dispersion: A mixed solution A was prepared by adding water to PEG-coated chitosan quaternary ammonium salt microspheres and chitosan-grafted cerium oxide nanocomposite particles; Separately, a mixture of Bacillus pseudostearate microcapsules-polyvinylpyrrolidone and trisodium citrate was prepared by mixing and adding water to form mixture B; S2. Concrete mixing: First, mix cement, fly ash and fine aggregate evenly, add mixed solution A, stir evenly, then add polycarboxylate superplasticizer and mixed solution B, stir evenly, then add modified coarse aggregate, stir evenly to obtain mixture C; S3. Casting and curing: Cast mixture C into shape, first place it in an environment of 18-22℃ and relative humidity ≥90% for 7-10 days, and then transfer it to a high-altitude simulated environment for 18-24 days to obtain the finished product.

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