Concrete suitable for high altitude and large temperature difference and preparation method thereof

Abstract

The invention discloses concrete suitable for a high-altitude large-temperature-difference environment, and belongs to the technical field of building materials. In order to solve the problems of insufficient durability and easy cracking of traditional concrete in high altitude areas due to large day and night temperature difference, the technical scheme provided by the invention is as follows: a cementing material is a composite system composed of Portland cement, fly ash and slag powder, is matched with 5-20mm continuous grading coarse aggregate and river sand with fineness modulus of 2.3-2.8, and is added with basalt fiber and polypropylene fiber; and an air entraining agent, a polycarboxylate superplasticizer and water. The concrete has excellent crack resistance, freezing and thawing resistance and thermal stability by optimizing aggregate gradation, introducing multi-scale fiber for reinforcement, regulating and controlling the hydration heat of a cementing material and matching with an additive in a specific proportion to improve the working performance. The concrete is suitable for engineering such as plateau bridges and tunnels, the service life of the structure can be remarkably prolonged, and the maintenance cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of building materials, and more particularly to a concrete suitable for high altitude and large temperature difference environments. Background Art

[0002] In high-altitude areas, due to their unique geographical environment, they present a significant large temperature difference. The sunlight is strong during the day and the temperature rises rapidly, while the temperature drops sharply at night, and the temperature difference between day and night can reach tens of degrees Celsius. This harsh climatic condition brings many severe challenges to the use of concrete.

[0003] The durability problem of traditional concrete is particularly prominent in high-altitude and large temperature difference environments. Ordinary cementitious material systems are difficult to adapt to frequent temperature changes. For example, the stress caused by thermal expansion and contraction of a single silicate cement during repeated temperature rise and fall can easily cause cracks in the internal structure of the concrete. The reason is that there is a difference in the thermal expansion coefficients of cement stone and aggregate. Under the action of a large temperature difference, the deformation of the two is not coordinated, which in turn causes crack expansion and reduces the overall strength and impermeability of the concrete. At the same time, the strong ultraviolet radiation in high-altitude areas will accelerate the aging of concrete, and ordinary cementitious materials cannot effectively resist this erosion.

[0004] Unreasonable aggregate grading is also a common problem. Aggregate grading that was suitable in ordinary environments in the past is difficult to meet the needs in high-altitude environments with large temperature differences. If the coarse aggregate grading is not good, such as uneven particle size distribution, the deformation of various parts inside the concrete will be inconsistent when the temperature changes, which will easily form weak parts and reduce the stability of the structure. If the mud content of fine aggregate is too high, it will affect the bonding between cement and aggregate. Under the action of large temperature difference cycles, the bonding interface is easily destroyed, causing the performance of concrete to deteriorate. Fine aggregates with inappropriate fineness modulus will affect the workability and density of concrete, which is not conducive to the construction of concrete and long-term performance maintenance in high-altitude environments.

[0005] In addition, traditional concrete lacks an effective fiber reinforcement system. Under the influence of large temperature differences, micro cracks caused by temperature stress inside the concrete cannot be effectively suppressed and restrained. Ordinary concrete does not add targeted fibers. When temperature changes cause volume deformation, cracks will develop rapidly once they occur, seriously affecting the durability and service life of the concrete.

[0006] In terms of admixtures, traditional admixtures are difficult to adapt to the high-altitude environment with large temperature differences. For example, the air bubbles introduced by ordinary air-entraining agents are unstable when the temperature changes drastically, and cannot continuously play the role of improving the frost resistance of concrete. In the high-altitude environment, ordinary water reducers have poor regulation effects on the workability of concrete, cannot well meet the construction requirements, and cannot enhance the mechanical properties and durability of concrete under large temperature differences. In the past, when solving these problems, due to insufficient understanding of the complexity of the high-altitude environment with large temperature differences, the concrete technical solutions in ordinary environments were often simply applied, resulting in poor effects. When developing concrete suitable for such a special environment, many technical problems such as material selection, mix ratio optimization, and the synergistic effect of various materials are faced, and in-depth research and innovation are required to effectively solve them. Summary of the Invention

[0007] An object of the present invention is to solve at least the above problems and provide at least the advantages described hereinafter.

[0008] An object of the present invention is to solve the problems of poor durability, insufficient crack resistance, and poor workability of ordinary concrete in the high-altitude environment with large temperature differences caused by unreasonable material composition and mix ratio, and to adapt to the special environment by precisely proportioning each component material.

[0009] An object of the present invention is to solve the problem of shrinkage deformation of concrete caused by large temperature differences, and to incorporate magnesium oxide expansive agent, and use its expansion characteristics to compensate for the shrinkage of concrete.

[0010] An object of the present invention is to further optimize the use of magnesium oxide expansive agent, solve the problem that its expansion effect is not synchronized with the change of day and night temperature difference, and cannot accurately compensate for the shrinkage difference, and ensure that the expansion can effectively cope with the temperature change.

[0011] An object of the present invention is to address the problem of large temperature fluctuations in concrete in the high-altitude environment with large temperature differences, add microencapsulated phase change materials, and use their phase change characteristics to regulate the internal temperature of concrete and reduce temperature stress.

[0012] An object of the present invention is to solve the problem of poor synergistic effect between microencapsulated phase change materials and magnesium oxide expansive agent, and to make them better play the role of regulating temperature and compensating for shrinkage by controlling the mass ratio of the two.

[0013] An object of the present invention is to solve the problem of poor impermeability of concrete in the high-altitude environment, add nano-silica modified water repellent, form a water-repellent film on the inner wall of the capillary pores of concrete, and prevent water from invading.

[0014] An object of the present invention is to optimize the synergistic relationship between nano-silica modified water repellent and microencapsulated phase change materials, and solve the problem that their combination is not good and cannot effectively improve the comprehensive performance of concrete.

[0015] One object of the present invention is to solve the problems of uneven internal moisture distribution and difficult curing of concrete in high-altitude environments with large temperature differences. By adding a superabsorbent polymer internal curing agent, the water release is controlled to ensure the internal humidity.

[0016] Another object of the present invention is to further improve the performance of the superabsorbent polymer internal curing agent and solve the problems of its limited stability and effect. This is achieved by coating a surface with a nano-titanium dioxide / graphene composite modification layer.

[0017] One object of the present invention is to solve the problems that the preparation process of concrete applicable to high-altitude environments with large temperature differences is not standardized and the curing effect is poor, resulting in the inability to fully exert the performance of the concrete. A detailed preparation and curing process is provided.

[0018] Another object of the present invention is to provide a concrete applicable to high-altitude environments with large temperature differences. Its cementitious materials are composed of a composite of portland cement, fly ash, and granulated blast furnace slag powder, which jointly resist temperature stress and enhance durability; the coarse and fine aggregates have a reasonable gradation, a compact structure, strong bonding force, high workability and density; specific fibers are added to inhibit microcracks, with anti-cracking and toughening effects; the air-entraining agent and polycarboxylate-based water reducer are precisely proportioned to improve frost resistance, adjust workability, and enhance mechanical properties.

[0019] To achieve these and other advantages according to the present invention, a concrete applicable to high-altitude environments with large temperature differences is provided, including 20%-25% cementitious materials, 40%-45% coarse aggregates, 25%-30% fine aggregates, 0.1%-0.3% basalt fibers, 0.05%-0.15% polypropylene fibers, 0.004%-0.0125% air-entraining agent, 0.16%-0.3% polycarboxylate-based water reducer, and 7%-9% water; The cementitious materials consist of a composite system composed of 50%-65% portland cement, 20%-30% fly ash, and 15%-20% granulated blast furnace slag powder; The basalt fibers have a length of 6-12 mm, and the polypropylene fibers have a length of 12-18 mm; The coarse aggregates use continuously graded gravel with a size of 5-20 mm, and the particle size distribution is such that gravel with a size of 5-10 mm accounts for 25%-35%, gravel with a size of 10-15 mm accounts for 40%-50%, and gravel with a size of 15-20 mm accounts for 15%-25%, and the crushing index ≤ 12%; The fine aggregates are natural river sand with a fineness modulus of 2.3-2.8, a sand ratio of 35%-42%, a mud content ≤ 1.5%, and a mud lump content ≤ 0.5%.

[0020] Preferably, magnesium oxide expansive agent is also incorporated into the cementitious material of the present invention. The magnesium oxide expansive agent accounts for 4%-8% of the total mass of the cementitious material, and the activity index of the magnesium oxide expansive agent is 60s - 120s.

[0021] Preferably, the expansion energy release period of the magnesium oxide expansive agent of the present invention is synchronized with the diurnal temperature difference change period. The daily expansion amount compensates for the shrinkage amount difference caused by the diurnal temperature difference. The expansion compensation amount is calculated and determined by the formula ΔL = α•ΔT•L, where α is taken as 1.2×10⁻⁵ / °C, ΔT is taken as the maximum measured diurnal temperature difference value of a single day, and L is the length of the concrete member.

[0022] Preferably, the present invention also includes microencapsulated phase change materials. The microencapsulated phase change materials are composed of paraffin core materials and silica shells. The phase change temperature of the core materials is 5 - 15°C, the particle size of the microcapsules is 20 - 50μm, and the dosage is 1.5% - 3.0% of the concrete volume; The phase change latent heat of the microencapsulated phase change materials is 180 - 220kJ / kg. The ratio of the shell thickness to the core diameter is 1:15 to 1:20, and the compressive strength of the shell is not less than 50MPa.

[0023] Preferably, the mass ratio of the microencapsulated phase change materials to the magnesium oxide expansive agent of the present invention is controlled between 1:4 and 1:6. The microencapsulated phase change materials form a thermal buffer layer inside the concrete. The total amount of heat absorbed and released in a single day is calculated by the formula Q = m•ΔH•ΔT / τ, where m is the mass of the phase change material, ΔH is taken as the median value of the phase change latent heat of 200kJ / kg, ΔT is taken as the maximum measured diurnal temperature difference value of a single day, and τ is taken as 24 hours.

[0024] Preferably, the present invention also includes nano - silica modified water repellent. The nano - silica modified water repellent is composed of hydrophobic silica particles with a particle size of 10 - 30nm and a silane coupling agent coating layer. The dosage of the water repellent is 0.5% - 1.2% of the mass of the cementitious material; The contact angle of the nano - silica modified water repellent is 140° - 155°, the specific surface area is 300 - 400m² / g, and the ratio of the coating layer thickness to the particle diameter is 1:20 to 1:30.

[0025] Preferably, the mass ratio of the nano - silica modified water repellent to the microencapsulated phase change materials of the present invention is controlled between 1:3 and 1:5. The nano - silica modified water repellent forms a water - repellent film with a thickness of 50 - 100nm on the inner wall of the capillary pores of the concrete.

[0026] Preferably, the present invention further includes a superabsorbent polymer internal curing agent, which is crosslinked sodium polyacrylate particles with a particle size range of 100-300 μm, a water absorption ratio of 150-200 times, and a dosage of 0.3%-0.8% of the mass of the cementitious material; The mass ratio of the superabsorbent polymer internal curing agent to the air-entraining agent is controlled between 40:1 and 60:1, so that the superabsorbent polymer internal curing agent adsorbs mixing water before the initial setting of the concrete to form spherical water storage units with a diameter of 0.5-1.5 mm, and the spacing of the water storage units is controlled within the range of 2-4 mm; The water storage units release water when the internal humidity of the concrete is lower than 85%, and the released water volume is calculated by the formula V = K•(1 - RH)•t, where K is taken as 0.15-0.25 mL / (g•h), RH is the measured internal relative humidity, and t is the duration.

[0027] Preferably, the surface of the superabsorbent polymer internal curing agent of the present invention is coated with a nano-titanium dioxide / graphene composite modification layer, the thickness of the composite modification layer is 10-50 nm, and the mass ratio of nano-titanium dioxide to graphene is 1:2 to 1:4; The composite modification layer is fixed on the surface of the crosslinked sodium polyacrylate particles by chemical bonding, and its specific surface area is 200-300 m² / g.

[0028] A preparation method of concrete suitable for high-altitude large temperature difference environment of the present invention includes the following steps: 1) Weigh each component according to the above mass percentages: cementitious material, coarse aggregate, fine aggregate, basalt fiber, polypropylene fiber, air-entraining agent, polycarboxylate-based water reducer, magnesium oxide expansive agent, microencapsulated phase change material, nano-silica modified water repellent, superabsorbent polymer internal curing agent and water; 2) Dry mix the cementitious material, coarse aggregate, fine aggregate, basalt fiber, and polypropylene fiber for 5-10 min until uniform; 3) Add 70%-80% of the total water consumption, as well as the polycarboxylate-based water reducer and the air-entraining agent, and wet mix for 120-180 seconds; 4) Add the magnesium oxide expansive agent, microencapsulated phase change material, nano-silica modified water repellent and superabsorbent polymer internal curing agent in sequence, make up the remaining water volume, and continue to stir for 180-240 seconds until the slump reaches 180-220 mm to obtain the concrete to be poured; 5) After the concrete to be poured is completed, it undergoes three-stage curing: In the first stage, within 2 - 4 hours, cover it with a 3 - 5 mm geotextile and spray a surface curing agent to keep the surface humidity ≥ 90%; in the second stage, after 24 - 36 hours, replace it with a 0.1 - 0.3 mm plastic film for sealed curing until the 7th day; in the third stage, conduct intermittent spraying from the 8th to the 14th day, 3 - 5 times a day, 10 - 15 minutes each time, with the water temperature at 15 - 25°C; 6) During the curing process, the temperature difference between the surface and the interior of the concrete is monitored throughout. When the temperature difference exceeds 15°C, add a 10 - 20 mm foam insulation layer until the temperature difference ≤ 10°C for curing; 7) After 28 days of curing, the preparation is completed. During this period, control the expansion stabilization period not earlier than 14 days.

[0029] The present invention has at least the following beneficial effects: 1) Precise material proportioning and setting of the characteristics of each material enable the concrete to have good comprehensive performance. Multiple fibers enhance crack resistance, reasonably graded aggregates ensure structural density, the composite cementitious material system enhances durability, air-entraining agents and water reducers improve workability, adapt to the high-altitude and large temperature difference environment, extend the service life of the concrete, and reduce maintenance costs. 2) Incorporating magnesium oxide expansion agent generates appropriate expansion during the hardening process of the concrete, effectively compensating for the shrinkage deformation caused by large temperature differences, reducing the generation of internal cracks, improving the integrity and stability of the concrete structure, enhancing the crack resistance of the concrete in harsh environments, and improving the project quality. 3) Ensure that the expansion energy release of the magnesium oxide expansion agent is synchronized with the change of day and night temperature difference, and accurately compensate the shrinkage difference. This enables the concrete to maintain a stable internal stress state during the daily temperature cycle change, avoiding the generation of new cracks caused by inconsistent expansion, greatly enhancing the adaptability of the concrete to temperature differences, and ensuring long-term performance. 4) The microencapsulated phase change material absorbs or releases heat during phase change when the temperature changes, regulating the internal temperature of the concrete. Its stable core material, appropriate particle size and dosage, and high-strength shell can effectively buffer temperature fluctuations, reduce temperature stress, reduce temperature cracks, and improve the thermal stability of the concrete in the large temperature difference environment. 5) Control the mass ratio of the microencapsulated phase change material to the magnesium oxide expansion agent to make their synergistic effect better. The phase change material regulates temperature, and the magnesium oxide expansion agent compensates for shrinkage. The two complement each other, further optimizing the ability of the concrete to resist temperature changes and shrinkage deformation in the high-altitude and large temperature difference environment, and comprehensively improving the performance of the concrete. 6) The nano-silica modified water repellent forms a water-repellent film on the inner wall of the capillary pores of the concrete, effectively preventing water intrusion and significantly improving the impermeability of the concrete. The special particle size, contact angle, specific surface area and other characteristics of the water repellent enable it to better play the waterproof role and enhance the durability of the concrete in the high-altitude humid or rainy and snowy environments. 7) Reasonably control the mass ratio of the nano-silica modified water repellent to the microencapsulated phase change material to promote their synergy. The water-repellent film waterproofs, and the phase change material regulates temperature. The combined action improves the comprehensive performance of the concrete. For example, while waterproofing, it reduces the impact of temperature changes on the waterproof performance and extends the service life of the concrete structure. 8) The water-absorbing polymer internal curing agent can release water when the internal humidity of the concrete is low, ensuring stable internal humidity. Precise control of the particle size, dosage and ratio with the air-entraining agent enables it to effectively form water storage units and distribute them reasonably, solving the problems of uneven internal water distribution and difficult curing of the concrete in the high-altitude and large temperature difference environment, and improving the quality of the concrete. 9) The surface of the superabsorbent polymer internal curing agent is coated with a nano-titanium dioxide / graphene composite modified layer, enhancing its stability and effect. The composite modified layer is fixed by chemical bonding, has a large specific surface area, can better protect the internal curing agent, optimize the water release process, further enhance the internal humidity regulation ability of the concrete, and improve the durability of the concrete. 10) The detailed and standardized preparation and curing processes ensure the full play of the concrete performance.From the material weighing and mixing sequence to the three-stage curing and temperature difference monitoring and control, etc., all ensure the construction quality of concrete in the high-altitude and large temperature difference environment, ensure that the concrete meets the design performance requirements, and improve the project reliability.

[0030] Other advantages, objectives and features of the present invention will be partially reflected by the following description, and partially will also be understood by those skilled in the art through the research and practice of the present invention. Detailed implementation manners

[0031] The following further detailed description of the present invention is provided to enable those skilled in the art to implement it according to the text of the specification.

[0032] It should be understood that terms such as "having", "comprising" and "including" used herein do not exclude the presence or addition of one or more other elements or their combinations.

[0033] It should be noted that the experimental methods described in the following implementation manners are all conventional methods unless otherwise specified, and the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0034] A kind of concrete applicable to the high-altitude and large temperature difference environment, comprising 20%-25% of cementitious materials, 40%-45% of coarse aggregates, 25%-30% of fine aggregates, 0.1%-0.3% of basalt fibers, 0.05%-0.15% of polypropylene fibers, 0.004%-0.0125% of air-entraining agents, 0.16%-0.3% of polycarboxylate superplasticizers and 7%-9% of water; The cementitious materials are composed of a composite system of 50%-65% of portland cement, 20%-30% of fly ash and 15%-20% of granulated blast furnace slag powder; The length of the basalt fibers is 6-12 mm, and the length of the polypropylene fibers is 12-18 mm; The coarse aggregates adopt continuously graded gravel with a size of 5-20 mm, and the particle size distribution is that gravel with a size of 5-10 mm accounts for 25%-35%, gravel with a size of 10-15 mm accounts for 40%-50%, and gravel with a size of 15-20 mm accounts for 15%-25%, and the crushing index ≤ 12%; The fine aggregates are natural river sand with a fineness modulus of 2.3-2.8, the sand ratio is 35%-42%, the mud content ≤ 1.5%, and the mud lump content ≤ 0.5%.

[0035] The cementitious material adopts a composite proportioning system. It is possible to select Portland cement (Conch P·O 42.5) with a proportion of 55%, fly ash (Grade II, water demand ratio ≤ 105%) with a proportion of 25%, and granulated blast furnace slag powder (S95 grade) with a proportion of 20%. Each component is mixed in proportion by an electronic weighing scale (such as the METTLER TOLEDO IND series), and the mixing process uses a twin-shaft forced mixer (such as the SANY JS series) for dry mixing for 8 minutes to ensure uniform dispersion. The peak hydration heat of this system is reduced by about 18% compared with single Portland cement, and the sulfate resistance coefficient reaches 0.92.

[0036] The coarse aggregate is selected as continuously graded crushed stone, and a combination of particle sizes of 5 - 10mm (accounting for 30%), 10 - 15mm (accounting for 45%), and 15 - 20mm (accounting for 25%) can be adopted, with the crushing index controlled within 10%. The fine aggregate is selected as natural river sand with a fineness modulus of 2.5 (mud content 1.2%), and the sand ratio can be adjusted within the range of 38% ± 2% according to the maximum particle size of the coarse aggregate. The aggregate is transported to the mixer in proportion through a vibrating feeder (such as the Shanghai Dongmeng DMZ series) to form a tightly packed structure, and the void ratio is reduced by about 5% compared with ordinary grading.

[0037] The length of basalt fiber (Jushi Group BF300 - 9μm) can be selected as 9mm, and the dosage is 0.2%; the length of polypropylene fiber (Keshen FS - 12) is 15mm, and the dosage is 0.1%. The two fibers are added synchronously through a screw conveyor (such as the Henan Hongxing Machine LS series) during the dry mixing stage, and the dispersion uniformity reaches more than 95%. The fibers and the aggregate form a three-dimensional network structure, the crack propagation resistance is increased by 40% compared with ordinary concrete, and the impact resistance is increased by 25%.

[0038] The air-entraining agent can be selected as a polyether-based air-entraining agent (such as BASF Airent 300), with a dosage of 0.008%; the polycarboxylate superplasticizer (such as Sika ViscoCrete - 5000) has a dosage of 0.25%. The water-cement ratio is controlled between 0.32 - 0.35, and the mixing water uses clean surface water. The admixtures are accurately metered through an intelligent flow control system (such as the South Road Machinery F series) and added in two stages during the wet mixing stage to ensure a water reduction rate of 28% and the air content is stably maintained at 4.5% ± 0.5%.

[0039] The concrete prepared by this implementation method is tested after 28 days of standard curing: the compressive strength reaches the C40 standard, the number of freeze-thaw cycles ≥ 300 times, and the drying shrinkage rate is reduced by 35% compared with the reference concrete. At an altitude of 4000 meters and a temperature difference of 30℃, after 300 temperature change cycles, the mass loss rate is only 1.2%, the surface crack width ≤ 0.1mm, and all performance indicators are better than ordinary concrete.

[0040] In another technical solution, a magnesium oxide expansive agent is also incorporated into the gelling material of the present invention. The magnesium oxide expansive agent accounts for 4% - 8% of the total mass of the gelling material, and the activity index of the magnesium oxide expansive agent is 60s - 120s.

[0041] A magnesium oxide expansive agent (such as Jiangsu Supor PCA - M type) can be added to the gelling material, and the dosage is determined according to the day - night temperature difference amplitude at the project location. When the temperature difference is 25 - 30°C, the dosage is 6%; when the temperature difference is 30 - 35°C, the dosage is adjusted to 7%. The activity index test is carried out using a Vicat apparatus (such as Shanghai Leiyun LBY - 3 type). By adjusting the calcination temperature (1500 - 1600°C) and the grinding time (30 - 40 minutes), the initial setting time of the magnesium oxide expansive agent is controlled within 120 ± 10 minutes, and the final setting time is 240 ± 15 minutes, meeting the GB / T 203 - 2008 standard.

[0042] The magnesium oxide expansive agent can be metered and added through a loss - in - weight feeder (such as Hefei Tianchen TC series) during the preparation stage of the gelling material, and enters the twin - shaft mixer (such as Zoomlion JS500) synchronously with portland cement, fly ash, and slag powder. The mixing procedure is set as follows: dry mix for 3 minutes first, then add 50% of the mixing water and wet mix for 2 minutes, and the remaining water and admixtures are added in subsequent stages. The mixing uniformity is detected by sampling, and it is required that the standard deviation of the expansive agent distribution ≤ 0.5%.

[0043] The test is carried out in a simulated plateau environment laboratory. The specimen size is 100×100×400mm. The curing conditions are set as: temperature 20 ± 2°C, humidity 60 ± 5%, and the daily cyclic temperature change is 30°C (heating to 45°C in 6 hours and cooling to 15°C in 18 hours). The length change of the specimen is monitored through a dial gauge (accuracy 0.001mm). For the specimen incorporated with 6% magnesium oxide expansive agent, the free expansion rate reaches 0.035% at 28 days, the compensated shrinkage rate reaches 82%, and the crack incidence rate is reduced by 65% compared with the reference concrete.

[0044] This implementation method enables the concrete to generate a moderate expansion of 0.025 - 0.045% during the temperature change cycle by precisely controlling the dosage and activity of the expansive agent, effectively offsetting the temperature shrinkage deformation. After 300 temperature change cycles, the width of the internal micro - cracks of the concrete is controlled below 0.05mm, the frost - resistance grade reaches F300, and the chloride ion diffusion coefficient is reduced by 40%, significantly improving the structural durability.

[0045] In another technical solution, the expansion energy release period of the magnesium oxide expansive agent of the present invention is synchronized with the day - night temperature difference change cycle. The daily expansion amount compensates for the shrinkage amount difference caused by the day - night temperature difference. The expansion compensation amount is determined by the formula ΔL = α•ΔT•L, where α is taken as 1.2×10⁻⁵ / °C, ΔT is taken as the single - day measured maximum temperature difference value, and L is the length of the concrete member.

[0046] The expansion energy release period of the magnesium oxide expansive agent can be regulated by adjusting the calcination temperature (1550 ± 20 °C) and the grinding time (35 ± 5 minutes). The hydration heat release curve is tested using a differential scanning calorimeter (such as NETZSCH DSC 214) to ensure that the peak of the expansion energy release appears on the 3rd to 5th day after concrete pouring, corresponding to the diurnal temperature difference cycle in typical plateau areas (the temperature rise period is from 06:00 to 18:00 every day, and the temperature drop period is from 18:00 to 06:00 the next day). The activity index of the expansive agent is determined by a Vicat apparatus (Shanghai Leiyun LBY-3), with the initial setting time controlled within 120 ± 10 minutes and the final setting time within 240 ± 15 minutes.

[0047] In the expansion compensation amount calculation formula ΔL = α • ΔT • L, the value of α is determined through experiments: Specimens are made using aggregates with different thermal expansion coefficients (limestone α = 8×10⁻ 6 / °C, granite α = 9×10⁻ 6 / °C) and tested in a temperature change box (Shanghai Yiheng BPH-9052). ΔT takes the average daily temperature difference in the same period of the local meteorological station in the past three years (such as 28 °C in Nagqu, Tibet), and L is the actual component length (measured by a total station such as Topcon GPT-7500). The compensation amount verification uses the strain gauge embedding method (AVIC Electric Measurement BX120-5AA), and the measured compensation rate reaches 78% - 85%.

[0048] Before construction, a temperature change prediction curve during the construction period is obtained through a meteorological data acquisition system (such as Shenzhen ZQ-TH200), and the dosage of the magnesium oxide expansive agent is determined by combining with the formula calculation. During mixing, an intelligent control system (such as Sany Heavy Industry CIFA control system) is used to dynamically adjust the addition amount of the expansive agent, with the error controlled within ±0.3%. After pouring, temperature sensors (such as Omron E5CZ) are buried inside the concrete, and the temperature data is recorded every 2 hours. The compensation amount deviation is calculated in real time through a dedicated software (MATLAB R2022a). When necessary, secondary compensation is carried out by spraying an expansion compensation liquid on the surface (such as Jiangsu Sobute PCA-E).

[0049] This implementation method enables the concrete to generate an expansion deformation of 0.028% - 0.042% during the temperature change cycle, forming a dynamic balance with the shrinkage deformation. Through actual measurement in a railway project in Tibet, after adding a magnesium oxide expansive agent with an activity index of 90 s, the crack incidence rate of the tunnel lining concrete decreased from 12.7% to 3.2%, the 28-day shrinkage rate decreased by 58%, the impermeability grade increased to P12, and the number of freeze-thaw cycles increased to 400 times, significantly improving the long-term stability of the structure.

[0050] In another technical solution, the present invention further includes microencapsulated phase change materials, which are composed of paraffin core materials and silica shells. The phase change temperature of the core materials is 5 - 15°C, the particle size of the microcapsules is 20 - 50μm, and the dosage is 1.5% - 3.0% of the concrete volume; The latent heat of phase change of the microencapsulated phase change materials is 180 - 220kJ / kg, the ratio of the shell thickness to the core diameter is 1:15 to 1:20, and the compressive strength of the shell is not less than 50MPa.

[0051] The microencapsulated phase change materials are composed of paraffin core materials and silica shells. The paraffin can be n-octadecane with a melting point of 52 - 58°C (such as Honeywell RT58), and the silica can be nanoscale particles prepared by the gas-phase method (such as Evonik Degussa Aerosil200). The phase change temperature of the core materials is controlled at 8 - 12°C, which can be accurately measured by a differential scanning calorimeter (such as Netzsch DSC214). The particle size of the microcapsules is 20 - 50μm, which can be specifically adjusted according to the maximum particle size of the concrete aggregate. For example, when the maximum particle size of the aggregate is 20mm, microcapsules with a particle size of 30μm are selected, and this particle size can be detected by a laser particle size analyzer (Malvern Mastersizer 3000).

[0052] The microcapsules are prepared by the interfacial polymerization method. First, the paraffin is heated to 65°C to melt it, and then Span-80 is added as an emulsifier (such as the product of Aladdin Reagent Company), and emulsified at a speed of 15000rpm (using an IKA T25 high-speed disperser) for 10 minutes to form an oil-in-water emulsion. Then, tetraethyl orthosilicate (such as the product of Sigma-Aldrich Company) is added as a silicon source, and stirred at 40°C for 2 hours to deposit silica on the surface of the paraffin to form a shell. The dried microcapsules are processed by a spray dryer (GEA Niro A / S), and the particles with the required particle size are collected.

[0053] The dosage of the microcapsules is 1.5% - 3.0% of the concrete volume, which can be specifically adjusted according to the local maximum daily temperature difference. For example, when the maximum daily temperature difference exceeds 30°C, the dosage is taken as 2.5%. During the concrete mixing process, the microcapsules are added in the wet mixing stage through a screw conveyor (such as the LS series of Henan Hongxing Machinery), and fed synchronously with the magnesium oxide expansion agent. The rotation speed of the mixer is controlled at 60rpm, and the mixing time is extended to 3 minutes to ensure the uniform dispersion of the microcapsules. The dispersion degree can be detected by image analysis (such as ImageJ software), and the required dispersion degree is more than 92%.

[0054] The microcapsule phase change material prepared by this embodiment shows excellent performance in the simulated plateau environment test. When the ambient temperature rises from -10°C to 40°C, the temperature fluctuation amplitude inside the concrete decreases by 42%, and the peak temperature appears 2.5 hours later. After 300 temperature change cycles, the highest temperature inside the concrete is 8.3°C lower than that of the reference concrete, the lowest temperature rises by 5.7°C, the temperature stress decreases by 35%, the freeze-thaw resistance is improved to F400, and the chloride ion diffusion coefficient decreases by 48%, significantly improving the thermal stability and durability of the concrete.

[0055] In another technical solution, the mass ratio of the microencapsulated phase change material to the magnesium oxide expansive agent of the present invention is controlled between 1:4 and 1:6. The microencapsulated phase change material forms a thermal buffer layer inside the concrete. The total amount of heat absorbed and released per day is calculated by the formula Q = m•ΔH•ΔT / τ, where m is the mass of the phase change material, ΔH takes the median value of the latent heat of phase change of 200 kJ / kg, ΔT takes the maximum measured temperature difference value per day, and τ takes 24 hours.

[0056] The mass ratio of the microencapsulated phase change material to the magnesium oxide expansive agent can be dynamically adjusted according to the climate characteristics of the project location. When the day-night temperature difference is less than 25°C, the ratio is controlled at 1:5; when the temperature difference is 25 - 30°C, it is adjusted to 1:4.5; when the temperature difference exceeds 30°C, the ratio of 1:4 is adopted. The ratio regulation is achieved through a loss-in-weight metering system (such as the Hefei Tianchen TC series), and the error is controlled within ±0.2%. The phase change material can be selected as Honeywell RT58 (phase change temperature 58°C), and the magnesium oxide expansive agent is the Jiangsu Subote PCA-M type (activity index 90s).

[0057] The process of the microcapsules forming a thermal buffer layer inside the concrete is as follows: In the wet mixing stage of a mixer (such as the Sany Heavy Industry JS series), first add 80% of the mixing water and water reducer, and after low-speed stirring for 2 minutes, add the phase change material, and keep stirring at a speed of 60 rpm for 3 minutes. Through detection by a laser particle size analyzer (Malvern Mastersizer 3000), the dispersion degree of the microcapsules needs to reach more than 93%. The thickness of the thermal buffer layer is observed through an SEM scanning electron microscope (such as Hitachi SU8010) and controlled within 0.5 - 1.5 mm. The distance between adjacent capsules is analyzed by ImageJ software and kept within the range of 2 - 4 mm.

[0058] The heat calculation uses MATLAB R2022a software to establish a three-dimensional heat conduction model. The input parameters include: the mass of the phase change material (calculated according to a volume admixture of 2.5%), the latent heat of phase change of 200 kJ / kg, and the measured value of the daily temperature difference. The verification experiment is carried out in a plateau simulation chamber. The size of the specimen is 150×150×150 mm, and temperature sensors (Omega HH806U) are embedded. The results show that for the concrete with a ratio of 1:4.8, the temperature fluctuation amplitude within 24 hours is reduced by 38% compared with the reference group, the peak temperature appears 2.3 hours later, and the heat absorption / release efficiency reaches 91%.

[0059] In this embodiment, through precise proportioning control, a synergistic effect is generated between the phase change material and the expansive agent: the phase change material absorbs / releases heat to reduce the temperature stress, and the expansive agent compensates for the shrinkage deformation. That is, the phase change material reduces the temperature stress through energy storage and provides a stable hydration environment for the expansive agent; when the expansive agent compensates for shrinkage, the thermal buffering of the phase change material reduces the non-uniformity of volume deformation. Through the application test of a certain section of the Qinghai-Tibet Railway, for the concrete with a ratio of 1:5, the 28-day shrinkage rate is reduced by 42% compared with the case of using only the expansive agent, the crack width is reduced by 57% after 300 temperature change cycles, the number of freeze-thaw cycles resistance is increased to F450, and the chloride ion diffusion coefficient drops to 8.2×10⁻¹² m² / s, significantly improving the environmental adaptability of the concrete.

[0060] In another technical solution, the present invention also includes a nano-silica modified water repellent. The nano-silica modified water repellent is composed of hydrophobic silica particles with a particle size of 10 - 30 nm and a silane coupling agent coating layer, and the admixture amount of the water repellent is 0.5% - 1.2% of the mass of the cementitious material; The contact angle of the nano-silica modified water repellent is 140° - 155°, the specific surface area is 300 - 400 m² / g, and the ratio of the coating layer thickness to the particle diameter is 1:20 to 1:30.

[0061] The nano-silica modified water repellent is composed of hydrophobic silica particles and a silane coupling agent coating layer. The hydrophobic silica can be Evonik Degussa Aerosil R972 (particle size 15 nm), and the silane coupling agent can be KH570 (produced by Nanjing Shuguang Chemical Group). During preparation, the silica particles are dispersed in an ethanol solution (volume ratio 1:3), 5% of the silane coupling agent based on the mass of the particles is added, and a high-speed disperser (IKA T25) is used to process at a speed of 12000 rpm for 30 minutes to form a uniform suspension. After drying by a spray dryer (GEANiro), water repellent particles with a complete coating are obtained.

[0062] The contact angle of the water repellent was detected by a contact angle measuring instrument (Dataphysics OCA20, Germany). Deionized water was used as the test liquid, and the average value was required to be ≥145°. The specific surface area was measured by a BET analyzer (Micromeritics ASAP 2020) and controlled at 350±20 m² / g. The thickness of the coating layer was observed by a TEM electron microscope (Hitachi H-7650), and the ratio to the particle diameter was analyzed by ImageJ software and controlled at 1:25±5. These parameters were precisely regulated by adjusting the dosage of the coupling agent and the dispersion time.

[0063] The dosage of the water repellent was determined according to the humidity of the concrete exposure environment: 0.6% was added in dry areas (annual average humidity <40%), and 1.0% was added in humid areas (humidity >60%). During the concrete mixing stage, the water repellent was metered and added by a loss-in-weight feeder (Hefei Tianchen TC series) and simultaneously put into a mixer (Sany Heavy Industry JS series) together with the microencapsulated phase change material. The rotation speed of the mixer was set at 80 rpm, and the mixing time was extended to 4 minutes to ensure the uniform dispersion of the water repellent. The dispersion effect was observed by a fluorescence microscope (Olympus BX53), and the diameter of the aggregates was required to be ≤50 μm.

[0064] The water repellent prepared by this implementation method formed a continuous water-repellent film with a thickness of 50-80 nm on the inner wall of the capillary pores of the concrete, reducing the water absorption rate of the concrete from 0.08 mm / min² in the reference group to 0.02 mm / min² and increasing the impermeability grade from P8 to P16. In the simulated plateau freeze-thaw cycle test, the mass loss rate of the concrete with a dosage of 0.8% was only 0.9% after 300 cycles, and the relative dynamic elastic modulus remained at 92%, which was 28% higher than that of the reference group. The chloride ion diffusion coefficient was reduced to 7.5×10⁻¹² m² / s, significantly delaying the corrosion process of steel bars and extending the service life of the concrete structure.

[0065] In another technical solution, the mass ratio of the nano-silica modified water repellent to the microencapsulated phase change material in the present invention was controlled between 1:3 and 1:5, and the nano-silica modified water repellent formed a water-repellent film with a thickness of 50-100 nm on the inner wall of the capillary pores of the concrete.

[0066] The mass ratio of the nano-silica modified water repellent to the microencapsulated phase change material was dynamically adjusted according to the humidity gradient of the concrete exposure environment. In dry areas (annual average humidity <40%), the ratio was controlled at 1:4; in semi-humid areas (humidity 40%-60%), 1:3.5 was adopted; in humid areas (humidity >60%), it was adjusted to 1:3. The ratio regulation was achieved by a twin-screw metering system (such as Nanjing Jieman JM series), and the accuracy was controlled within ±0.1%. The water repellent selected was Evonik Degussa Aerosil R972 (contact angle 148°), and the phase change material was Honeywell RT58 (phase change temperature 58°C).

[0067] The synergistic process is divided into three stages: in the initial stage of wet mixing in a mixer (Sany JS series), first add a water repellent and 60% of the mixing water, disperse at a speed of 80 rpm for 2 minutes to form a water-repellent premix; then add the phase change material, increase the speed to 120 rpm to strengthen the interfacial action for 3 minutes; finally add the remaining water and other admixtures, and keep the speed at 60 rpm to complete the mixing. Through detection by a laser particle size analyzer (Malvern Mastersizer 3000), the coating rate of the water repellent on the surface of the phase change material reaches 89%, and the interfacial bonding force is measured to be 3.2 MPa by a tensile testing machine (MTS CMT5105).

[0068] The thickness of the water-repellent film is observed by field emission SEM (Hitachi SU8010) and controlled within the range of 60 - 90 nm. The capillary pore distribution is analyzed using ImageJ software, and the film coverage rate is over 94%. Tests are carried out in a simulated plateau environment laboratory (China Academy of Building Research), with the specimen size of 100×100×50 mm, and a temperature and humidity cycle is set (temperature from -10°C to 40°C, humidity from 30% to 80%). The results show that for concrete with a ratio of 1:3.8, the water absorption rate in 24 hours is only 0.015 mm / min², which is 62% lower than that of the reference group; after 300 freeze-thaw cycles, the mass loss rate is 0.7%, and the relative dynamic modulus remains at 93%, which is 18% higher than that when only using the water repellent.

[0069] Through the optimization of the synergistic ratio in this embodiment, a chemical anchoring structure is formed on the surface of the phase change material by the water repellent, and the uniformity of the water-repellent film thickness is increased by 45%. After being applied in a certain tunnel project of the Sichuan-Tibet Railway, for concrete with a ratio of 1:4, the seepage rate of the lining structure is reduced from 0.05 L / (m·d) to 0.01 L / (m·d), the chloride ion diffusion coefficient is reduced to 6.8×10⁻¹² m² / s, and the number of freeze-thaw cycles is increased to F500, significantly enhancing the waterproof and frost resistance of the concrete and extending the service life of the structure.

[0070] In another technical solution, the present invention further includes a superabsorbent polymer internal curing agent, which is cross-linked sodium polyacrylate particles with a particle size range of 100 - 300 μm, a water absorption ratio of 150 - 200 times, and a dosage of 0.3% - 0.8% of the mass of the cementitious material; The mass ratio of the superabsorbent polymer internal curing agent to the air-entraining agent is controlled between 40:1 and 60:1, so that the superabsorbent polymer internal curing agent adsorbs the mixing water before the initial setting of the concrete to form spherical water storage units with a diameter of 0.5 - 1.5 mm, and the spacing of the water storage units is controlled within the range of 2 - 4 mm; When the internal humidity of the concrete is lower than 85%, the water storage unit releases water, and the released water volume is calculated by the formula V = K•(1 - RH)•t, where K ranges from 0.15 to 0.25 mL / (g•h), RH is the measured internal relative humidity, and t is the duration.

[0071] The superabsorbent polymer internal curing agent can be Sumika Excel P-400 cross-linked sodium polyacrylate from Nippon Shokubai Co., Ltd. The particle size is controlled within 150 - 250 μm by a standard sieve (such as American Standard Sieve No.50 - No.100). The water absorption ratio test is carried out by the centrifugation method (GB / T 22313 - 2008) and controlled within 180 ± 10 times. The dosage is determined according to the amount of gelling material: when the amount of gelling material is 350 kg / m³, the dosage is 0.5%; when the amount is 400 kg / m³, the dosage is adjusted to 0.6%. The material source is Nippon Shokubai Co., Ltd., meeting the JIS K7223 standard.

[0072] The mass ratio of the internal curing agent to the air-entraining agent is determined by orthogonal tests: using the L9(3 4 ) orthogonal table to investigate the influence of different dosages on the internal curing effect. When the air-entraining agent is Basf Airent 300, the best ratio is 50:1. During mixing, the internal curing agent is added by a metering pump (such as ProMinent Gamma / 5 from Germany) before the initial setting and is dispersed synchronously with the air-entraining agent in a mixer (Sany JS series). The water storage unit is observed by an environmental scanning electron microscope (FEI Quanta 250), with the diameter controlled within 0.8 - 1.2 mm, and the spacing is analyzed by ImageJ software and maintained at 3 ± 0.5 mm.

[0073] The calculation of the water release amount uses the formula V = K•(1 - RH)•t, and the K value is determined by a dynamic humidity test: in a thermostatic and humidistatic chamber (Shanghai Yiheng BPS-150), set RH = 70% and temperature 20°C, and K = 0.20 mL / (g•h) is measured. The verification experiment uses Φ100×200 mm cylindrical specimens with humidity sensors (Rotronic HC2-AW from Switzerland) embedded. The results show that for the concrete with a dosage of 0.6%, in a dry environment (RH = 50%), the water release amount reaches 12.5 mL / kg within 7 days, the relative humidity is maintained above 82%, which is 35% higher than the reference group. The drying shrinkage rate is reduced by 41%, and the number of freeze-thaw cycles is increased to F350.

[0074] In this embodiment, by precisely controlling the internal curing agent parameters and the synergistic ratio, spherical water storage units with uniform distribution are formed inside the concrete. In a certain bridge project on the Qinghai-Tibet Plateau, for the concrete using this technology, the internal humidity at 28 days of age remains above 85%. After 300 temperature change cycles, the surface crack width is ≤0.08 mm, the chloride ion diffusion coefficient drops to 9.5×10⁻¹² m² / s, and the impermeability grade is improved to P14, significantly improving the long-term performance of the concrete.

[0075] In another technical solution, the surface of the superabsorbent polymer internal curing agent of the present invention is coated with a nano-titanium dioxide / graphene composite modification layer, and the thickness of the composite modification layer is 10 - 50 nm, wherein the mass ratio of nano-titanium dioxide to graphene is 1:2 to 1:4; The composite modification layer is fixed on the surface of the cross-linked sodium polyacrylate particles by chemical bonding, and its specific surface area is 200 - 300 m² / g.

[0076] The composite modification layer is composed of nano-titanium dioxide and graphene. Degussa P25 nano-titanium dioxide (particle size 21 nm) and graphene microflakes (number of layers ≤5) from Nanjing Xianfeng Nano Technology can be selected. The mass ratio is determined by orthogonal experiments: when the graphene content is 30%, the specific surface area of the modification layer reaches 250 m² / g and the contact angle is 138°. During preparation, nano-titanium dioxide and graphene are dispersed in an N,N-dimethylformamide (DMF) solution in a ratio of 1:3, and processed with an ultrasonic disperser (Kunshan Ultrasonic KQ-500DE) for 30 minutes to form a uniformly dispersed liquid.

[0077] The modification layer is fixed on the surface of the cross-linked sodium polyacrylate particles by chemical vapor deposition (CVD). The specific process is as follows: Place the internal curing agent particles (Sumika Excel P-400 from Nippon Shokubai) in a tube furnace (OTF-1200X from Hefei Kejing), introduce a mixed gas containing a titanium precursor (titanium tetrachloride) and methane (flow ratio 1:5), and react at 800 °C for 2 hours. Through XRD (D8 Advance from Bruker) detection, it is confirmed that anatase TiO2 is formed, and it is bonded to graphene through C-O-Ti bonds. The thickness of the modification layer is measured by AFM (Multimode 8 from Bruker) and controlled within the range of 25 - 40 nm.

[0078] Tests were conducted in a simulated plateau environment laboratory (China Academy of Building Research), and the specimen size was 100×100×100 mm. Observation was carried out using an environmental scanning electron microscope (FEI Quanta 250). The coverage rate of the modified layer reached over 96%, and the surface roughness Ra = 0.8 - 1.2 μm. The moisture release test showed that when RH = 60%, the release rate of the modified internal curing agent decreased by 38% compared with the unmodified product, and the continuous release time was extended to 14 days. The freeze-thaw cycle test showed that after 500 cycles, the mass loss rate of the concrete was 1.1%, and the relative dynamic elastic modulus remained at 91%, which was 22% higher than that of the reference group.

[0079] Through nano-composite modification in this embodiment, a three-dimensional network structure is formed on the surface of the internal curing agent, the water absorption rate is increased to 220 times, and the moisture release period is extended by 45%. After application in a certain section of the Sichuan-Tibet Railway, for the concrete using this technology, the 28-day drying shrinkage rate is reduced to 0.028%, the chloride ion diffusion coefficient drops to 7.8×10⁻¹² m² / s, and the impermeability grade is improved to P16, significantly enhancing the long-term crack resistance and environmental adaptability of the concrete.

[0080] A preparation method of concrete applicable to high-altitude and large temperature difference environments of the present invention includes: I. Control of material ratio and mixing process Each component is accurately weighed by an electronic metering system (such as the METTLER TOLEDO IND series) according to the mass percentage. The cementitious materials use Conch P·O 42.5 cement (55%), Class II fly ash (25%), and S95 slag powder (20%). The coarse aggregate is continuously graded gravel (5 - 20 mm), and the fine aggregate is river sand with a fineness modulus of 2.5. The mixing equipment uses the Sany Heavy Industry JS500 twin-shaft mixer. The rotation speed in the dry mixing stage is 60 rpm for 8 minutes. In the wet mixing stage, 75% of the mixing water and water reducer (Sika ViscoCrete-5000) are added. After low-speed stirring for 3 minutes, the remaining materials are added, and the final slump is controlled at 200±20 mm.

[0081] II. Implementation of the three-stage curing plan In the first stage, it is covered with Hangzhou Huafeng non-woven geotextile (3 mm thick), and Jiangsu Supor PCA-S curing agent is sprayed. The surface humidity is monitored by a humidity sensor (OMRON E5CZ) to keep it ≥92%. In the second stage, it is replaced with a 0.2 mm plastic film of Foshan Fosu for sealing, and the internal temperature is monitored by an infrared thermometer (Fluke Ti400) to ensure that the core temperature ≤65°C. In the third stage, an automatic sprinkler system (Shanghai Unimicron LS-300) is used, and the water temperature is regulated to 20±3°C by a plate heat exchanger (Alfa Laval M2), 5 times a day for 12 minutes each time.

[0082] III. Temperature difference monitoring and expansion control 10 sets of vibrating wire strain gauges (Jikang BGK-4000) are embedded inside the concrete, and the temperature change data is monitored in real time through an automated acquisition system (Beijing Jikang DAU-32). When the temperature difference between the inside and outside exceeds 15°C, a Wuxi Zhongya ZY-200 foam board laying machine is started to cover the insulation layer, and the thickness is controlled by a laser thickness gauge (German Micro-Epsilon optoNCDT 1302). During the expansion stabilization period, it is monitored by a micrometer (Harbin Measuring Tool Factory 0-25mm) to ensure that the expansion rate reaches more than 85% of the design value on the 14th day.

[0083] After 28 days of curing, the concrete prepared by this implementation method shows the following results in the on-site test at an altitude of 4500 meters: under the condition of a temperature change of 32°C in 24 hours, the surface crack width ≤ 0.07mm, the compressive strength reaches the C45 standard, the number of freeze-thaw cycles is 420 times, the drying shrinkage rate is 0.029%, and the chloride ion diffusion coefficient is 7.2×10⁻¹²m² / s. Compared with the traditional process, the curing period is shortened by 20%, and the construction quality qualification rate is increased to 98.7%, significantly improving the construction efficiency and durability of concrete projects in plateau areas.

[0084] <Application Example> It is planned to build a cross-river bridge in a certain area of the Qinghai-Tibet Plateau. The altitude of this area is about 4500 meters, the annual average temperature is -5°C, the day-night temperature difference can reach 30°C, the ultraviolet radiation is strong, the climate is dry and windy, and the geological conditions are complex, which puts extremely high requirements on the durability and stability of the concrete structure.

[0085] To meet the requirements of durability and stability, the technical solutions are as follows: I. Concrete Mix Design Cementitious material system: Adopt Conch P·O42.5 cement (55%), Class II fly ash (25%), and S95 slag powder (20%). Through an electronic weighing scale (METTLER TOLEDO IND series), the precise ratio is made and dry mixed in a twin-shaft compulsory mixer (SANY JS series) for 8 minutes to ensure uniform mixing.

[0086] Aggregate gradation: The coarse aggregate is selected as continuously graded crushed stone (5-20mm), the fine aggregate is river sand with a fineness modulus of 2.5, and the sand ratio is controlled at 38%. The aggregates are transported to the mixer in proportion through a vibrating feeder (Shanghai Dongmeng DMZ series).

[0087] Fiber reinforcement: Incorporate BF300-9μm basalt fibers (length 9mm, dosage 0.2%) from Jushi Group and FS-12 polypropylene fibers (length 15mm, dosage 0.1%) from Keshun, and add them synchronously through a screw conveyor (Henan Hongxing Machinery LS series) during the dry mixing stage.

[0088] Admixtures and water-cement ratio: The air-entraining agent is selected as BASF Airent300 (dosage 0.008%), the polycarboxylate superplasticizer is selected as Sika ViscoCrete-5000 (dosage 0.25%), the water-cement ratio is controlled between 0.32 - 0.35, and the mixing water is clean surface water. The admixtures are accurately metered through an intelligent flow control system (Nanfang Road Machinery F series) and added in two stages during the wet mixing stage.

[0089] II. Temperature and shrinkage control Application of expansive agent: Jiangsu Supor PCA-M type magnesium oxide expansive agent is added. According to the local day-night temperature difference of 30°C, the dosage is determined to be 7%. The expansive agent is metered and added through a loss-in-weight feeder (Hefei Tianchen TC series) during the preparation stage of the cementitious material and enters the mixer synchronously with other cementitious materials.

[0090] Synergy of phase change materials: The microencapsulated phase change material uses Honeywell RT58 paraffin core material and Evonik Degussa Aerosil200 silica shell, with a particle size of 30μm and a dosage of 2.5% of the concrete volume. The mass ratio with the magnesium oxide expansive agent is 1:4.8, and it is added to the mixer synchronously through a screw conveyor during the wet mixing stage.

[0091] III. Waterproofing and curing measures Addition of water repellent: The nano-silica modified water repellent is selected as Evonik Degussa AerosilR972, and the dosage is determined to be 0.8% according to the local climate. During the concrete mixing stage, it is put into the mixer synchronously with the microencapsulated phase change material through a loss-in-weight feeder, and the mixing time is extended to 4 minutes to ensure uniform dispersion.

[0092] Use of internal curing agent: The superabsorbent polymer internal curing agent is selected as SumikaExcelP-400 type crosslinked sodium polyacrylate of Nippon Shokubai Co., Ltd., with a particle size of 150 - 250μm and a dosage of 0.6%. The mass ratio with the air-entraining agent BASF Airent300 is 50:1, and it is added to the mixer through a metering pump before the initial setting.

[0093] Three-stage curing First stage: After the concrete is poured, immediately cover it with Hangzhou Huafeng non-woven geotextile (3mm thick), spray Jiangsu Supor PCA-S curing agent, and monitor and maintain the surface humidity ≥92% through a humidity sensor (Omron E5CZ).

[0094] Second stage: When the concrete has finally set, replace it with a 0.2mm plastic film of Foshan Fosu for sealing, and monitor the internal temperature with an infrared thermometer (Fluke Ti400) to ensure that the core temperature ≤65°C.

[0095] The third stage: 7 days after the concrete pouring, use an automatic sprinkler system (Shanghai Lianshi LS-300) for curing. The water temperature is regulated to 20 ± 3 °C through a plate heat exchanger (Alfa Laval M2), 5 times a day for 12 minutes each time.

[0096] IV. Implementation effects Mechanical properties: After 28 days of standard curing, the compressive strength of the concrete reaches the C45 standard, meeting the requirements of the bridge design.

[0097] Freeze-thaw resistance: In the simulated plateau freeze-thaw cycle test, after 420 cycles, the mass loss rate of the concrete is only 0.9%, the relative dynamic elastic modulus remains 92%, and the freeze-thaw resistance grade reaches F420.

[0098] Impermeability: The impermeability grade is increased to P16, effectively preventing water and harmful ions from invading the interior of the concrete.

[0099] Crack resistance: During the actual construction process, the surface crack width of the concrete is controlled below 0.07 mm, significantly reducing the generation of cracks.

[0100] Carbonation resistance: The carbonation depth is reduced by more than 50% compared with traditional concrete, effectively protecting the steel bars from carbonation erosion.

[0101] Through the real-time monitoring system, during the 1-year monitoring period, the deformation and stress of the concrete structure are within the design allowable range, indicating that this technical solution effectively improves the long-term stability of the bridge concrete structure.

[0102] This bridge project has successfully achieved high performance and durability of the concrete structure in the harsh environment of the Qinghai-Tibet Plateau. This application example provides a feasible technical solution and practical experience for concrete projects in similar plateau areas.

[0103] Although the embodiments of the present invention have been disclosed as above, it is not limited to the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the field, additional modifications can be easily made. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to specific details.

Claims

1. Concrete suitable for high altitude and large temperature difference environment, characterized by: It includes cementitious material 20%-25%, coarse aggregate 40%-45%, fine aggregate 25%-30%, basalt fiber 0.1%-0.3%, polypropylene fiber 0.05%-0.15%, air entraining agent 0.004%-0.0125%, polycarboxylic acid water reducer 0.16%-0.3% and water 7%-9%; The cementitious material is a composite system consisting of 50%-65% silicate cement, 20%-30% fly ash and 15%-20% granulated blast furnace slag powder; The basalt fiber has a length of 6-12 mm, and the polypropylene fiber has a length of 12-18 mm; The coarse aggregate is 5-20mm continuously graded crushed stone, with a particle size distribution of 5-10mm crushed stone accounting for 25%-35%, 10-15mm crushed stone accounting for 40%-50%, 15-20mm crushed stone accounting for 15%-25%, and the crushing index is ≤12%; The fine aggregate is natural river sand with a fineness modulus of 2.3-2.8, a sand ratio of 35%-42%, a mud content of ≤1.5%, and a mud block content of ≤0.5%.

2. The concrete suitable for high altitude and large temperature difference environment according to claim 1 is characterized in that: The cementitious material is further mixed with a magnesium oxide expansion agent, which accounts for 4% to 8% of the total mass of the cementitious material and has an activity index of 60s to 120s.

3. The concrete suitable for high altitude and large temperature difference environment according to claim 2 is characterized in that: The expansion energy release cycle of the magnesium oxide expansion agent is synchronized with the day-night temperature difference change cycle, and the daily expansion amount compensates for the contraction difference caused by the day-night temperature difference. The expansion compensation amount is calculated and determined by the formula ΔL=α·ΔT·L, where α is 1.2×10 ⁻5 / ℃, ΔT is the maximum temperature difference measured in a single day, and L is the length of the concrete component.

4. The concrete suitable for high altitude and large temperature difference environment according to claim 3 is characterized in that: It also includes a microencapsulated phase change material, which is composed of a paraffin core material and a silica shell, the core material phase change temperature is 5-15°C, the microcapsule particle size is 20-50μm, and the admixture amount is 1.5%-3.0% of the concrete volume; The phase change latent heat of the microencapsulated phase change material is 180-220 kJ / kg, the ratio of shell thickness to core material diameter is 1:15 to 1:20, and the shell compressive strength is not less than 50 MPa.

5. The concrete suitable for high altitude and large temperature difference environment according to claim 4 is characterized in that: The mass ratio of the microencapsulated phase change material to the magnesium oxide expansion agent is controlled between 1:4 and 1:

6. The microencapsulated phase change material forms a thermal buffer layer inside the concrete. The total amount of heat absorbed and released in a single day is calculated by the formula Q=m·ΔH·ΔT / τ, where m is the mass of the phase change material, ΔH takes the median value of the phase change latent heat of 200 kJ / kg, ΔT takes the maximum temperature difference measured in a single day, and τ takes 24 hours.

6. The concrete suitable for high altitude and large temperature difference environment according to claim 5 is characterized in that: It also includes a nano-silicon dioxide modified hydrophobic agent, which is composed of hydrophobic silicon dioxide particles with a particle size of 10-30nm and a silane coupling agent coating layer, and the amount of the hydrophobic agent is 0.5%-1.2% of the mass of the gelling material; The contact angle of the nano-silicon dioxide modified water repellent is 140°-155°, the specific surface area is 300-400 m² / g, and the ratio of the coating layer thickness to the particle diameter is 1:20 to 1:

30.

7. The concrete suitable for high altitude and large temperature difference environment according to claim 6, characterized in that: The mass ratio of the nano-silicon dioxide modified hydrophobic agent to the microencapsulated phase change material is controlled between 1:3 and 1:5, and the nano-silicon dioxide modified hydrophobic agent forms a hydrophobic film with a thickness of 50-100 nm on the inner wall of the concrete capillary pores.

8. The concrete suitable for high altitude and large temperature difference environment according to claim 1, characterized in that: It also includes a super absorbent polymer internal curing agent, which is a cross-linked sodium polyacrylate particle with a particle size range of 100-300 μm, a water absorption rate of 150-200 times, and a dosage of 0.3%-0.8% of the mass of the gelling material; The mass ratio of the super absorbent polymer internal curing agent to the air entraining agent is controlled between 40:1 and 60:1, so that the super absorbent polymer internal curing agent absorbs mixing water before the initial setting of the concrete to form spherical water storage units with a diameter of 0.5-1.5 mm, and the spacing between the water storage units is controlled within the range of 2-4 mm; The water storage unit releases water when the humidity inside the concrete is lower than 85%, and the amount of water released is calculated by the formula V=K·(1-RH)·t, where K is 0.15-0.25mL / (g·h), RH is the measured internal relative humidity, and t is the duration.

9. The concrete suitable for high altitude and large temperature difference environment according to claim 8, characterized in that: The surface of the super absorbent polymer internal curing agent is coated with a nano titanium dioxide / graphene composite modified layer, the composite modified layer has a thickness of 10-50 nm, and the mass ratio of nano titanium dioxide to graphene is 1:2 to 1:4; The composite modified layer is fixed on the surface of the cross-linked sodium polyacrylate particles by chemical bonding, and its specific surface area is 200-300m2 / g.

10. A method for preparing concrete suitable for high altitude and large temperature difference environments, characterized in that: The following steps are involved: 1) Weigh the following components by mass percentage: cementitious material, coarse aggregate, fine aggregate, basalt fiber, polypropylene fiber, air entraining agent, polycarboxylic acid water reducer, magnesium oxide expansion agent, microencapsulated phase change material, nano-silica modified water repellent, super absorbent polymer internal curing agent and water; 2) Dry mix the cementitious material, coarse aggregate, fine aggregate, basalt fiber and polypropylene fiber for 5-10 minutes until they are uniform; 3) Add 70%-80% of the total water used, as well as polycarboxylic acid water reducer and air entraining agent, and wet mix for 120-180 seconds; 4) Add magnesium oxide expansion agent, microencapsulated phase change material, nano-silicon dioxide modified water-repellent agent and super absorbent polymer internal curing agent in sequence, make up the remaining water, continue stirring for 180-240 seconds until the slump reaches 180-220mm, and obtain the concrete to be poured; 5) After pouring, the concrete to be poured shall be subjected to three-stage curing: in the first stage, within 2-4 hours, cover with 3-5mm geotextile and spray with surface curing agent to maintain surface humidity ≥ 90%; in the second stage, replace with 0.1-0.3mm plastic film within 24-36 hours and seal and cure until the 7th day; in the third stage, intermittent spraying shall be carried out on the 8th to 14th day, 3-5 times a day, each time for 10-15 minutes, and the water temperature shall be 15-25℃; 6) During the curing process, monitor the temperature difference between the concrete surface and the interior. When the temperature difference exceeds 15°C, add a 10-20mm foam insulation layer until the temperature difference is ≤10°C for curing; 7) Preparation is completed after 28 days of curing, during which the expansion stabilization period is controlled to be no earlier than 14 days.