High-stability concrete in low-vacuum pipeline environment and preparation method thereof

Abstract

The invention discloses high-stability concrete in a low-vacuum pipeline environment and a preparation method thereof, and the high-stability concrete is prepared from the following raw materials in parts by weight: 300 parts of cement, 100 parts of ground mineral powder, 50 parts of fly ash floating beads, 25-50 parts of silica fume, 0-25 parts of rice hull ash, 5-10 parts of redispersible latex powder, 5-10 parts of modified emulsified asphalt, 1050 parts of coarse aggregate and 750 parts of fine aggregate, the concrete is prepared from the following components in parts by weight: 5 parts of silane coupling agent modified polyethylene fiber, 150 parts of water, 5 parts of polycarboxylate-type water reducing agent, 1 part of defoaming agent, 15-25 parts of nano dense reinforcing powder and 0.15-0.25 part of nano cross-linking reinforcing agent. According to the prepared high-stability concrete, the problem that current concrete is insufficient in service performance in a low-vacuum environment is solved, the mechanical strength, the deformation performance and the air tightness of the concrete are effectively improved, use of magnetic substances such as reinforcing steel bars, steel strands and steel fibers is avoided, and the comprehensive performance is outstanding.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete, and particularly relates to a high-stability concrete under a low-vacuum pipeline environment and a preparation method thereof. Background Art

[0002] The new ultra-high-speed low-vacuum pipeline maglev transportation system combining low-vacuum pipelines and maglev technology enables trains to break through the limitations of air resistance, noise, and wheel-rail friction resistance during operation, and has advantages such as fast speed, safety, environmental protection, and high efficiency, which can meet the more efficient and safer passenger transportation needs of modern society.

[0003] As the most abundant and low-cost non-metallic building material, concrete not only has the characteristics of low magnetic resistance, but also is convenient and flexible for construction and has a low cost. Therefore, it is still the preferred main building material for constructing the infrastructure of the low-vacuum pipeline maglev transportation system. However, when exposed to a low-vacuum environment for a long time, concrete will be in a state of rapid drying or dehydration, and the shrinkage behavior will be aggravated, making the brittleness of concrete increase and there is a potential risk of cracking. In addition, the low vacuum will also cause the coarsening of the internal pores of concrete and the reduction of the stability of hydration products, making it difficult to maintain the stability of its internal microstructure, which leads to the difficulty in ensuring the airtightness, mechanical properties, and durability of the low-vacuum pipeline concrete structure.

[0004] In summary, the low-vacuum pipeline environment poses a severe challenge to the stability of the service performance of concrete. Whether some existing technical measures to improve the performance stability of concrete can withstand the test of the harsh low-vacuum pipeline environment and play their due roles has not been reported. CN114573288A discloses a high-airtightness concrete and a preparation method thereof, and the gas permeability coefficient of concrete in a low-vacuum environment is reduced by adding components such as superabsorbent resin and dense nano-enhanced powder. However, the problem of lack of toughness faced by concrete still cannot be effectively solved, and there is also a problem that the water release by using the internal curing method of superabsorbent resin introduces voids and damages the strength and compactness of concrete. CN113264728A discloses a high-strength and high-toughness concrete and a preparation method thereof, and a high-viscosity cross-linked structure is formed by adding epoxy-modified phenolic resin, coupling agent, and cellulose to improve the strength and toughness of concrete and inhibit its cracking. However, due to the lack of toughening of fiber components and the nanoscale dense filling of nanomaterials, the strength, toughness, and airtightness of concrete are still relatively limited. In particular, due to the generation of a large magnetic field during the actual operation of the low-vacuum pipeline maglev train, the method of using traditional steel bars, steel strands, and steel fibers for toughening will cause a large magnetic resistance problem.

[0005] Therefore, on the basis of the refined design of the components of concrete raw materials, finding a suitable toughening method to prepare a type of concrete with high strength, high toughness and high airtightness is the key to ensuring its long-term service in a low-vacuum environment, and it is also an important measure to solve the bottleneck in the construction of a new type of maglev transportation infrastructure with ultra-high speed and low vacuum pipelines. Summary of the Invention

[0006] Aiming at the problem of insufficient service performance of concrete in the current low-vacuum environment, the present invention provides a highly stable concrete in a low-vacuum pipeline environment and a preparation method thereof. Through the concrete material provided by the present invention, on the basis of ensuring the basic performance of the concrete, the problems of mechanical property damage, microstructure degradation and durability decline of the concrete in the current low-vacuum environment can be solved, and the research and application of concrete materials in special environments such as low vacuum can be expanded.

[0007] To solve the above technical problems, the technical solution provided by the present invention is:

[0008] A highly stable concrete in a low-vacuum pipeline environment, comprising the following raw material components in parts by weight: 300 parts of cement, 100 parts of ground granulated blast-furnace slag, 50 parts of fly ash cenospheres, 25-50 parts of microsilica, 0-25 parts of rice husk ash, 5-10 parts of redispersible latex powder, 5-10 parts of modified emulsified asphalt, 1050 parts of coarse aggregate, 750 parts of fine aggregate, 5 parts of silane coupling agent-modified polyethylene fiber, 150 parts of water, 5 parts of polycarboxylate superplasticizer, 1 part of defoaming agent, 15-25 parts of nano-dense strengthening powder, 0.15-0.25 parts of nano-crosslinking strengthening agent;

[0009] The nano-dense strengthening powder is composed of nano-SiO 2 and nano-CaCO 3 ;

[0010] The nano-crosslinking strengthening agent is composed of polyethylene glycol grafted graphene oxide and multi-walled carbon nanotubes.

[0011] The highly stable concrete of the present invention solves the problem of insufficient service performance of the current concrete in a low-vacuum environment, effectively improves the mechanical strength, deformation performance and airtightness of the concrete, and avoids the use of magnetic substances such as steel bars, steel strands and steel fibers, and the comprehensive performance is relatively prominent.

[0012] Preferably, the cement is ordinary Portland cement with a strength grade ≥ P·O 42.5.

[0013] Preferably, the grade of the ground granulated blast-furnace slag ≥ S95 grade.

[0014] Preferably, the particle size of the fly ash cenospheres is 1-200 μm and the sphericity > 95%.

[0015] Preferably, the particle size of the microsilica powder is 100 - 300 nm, and the SiO 2 content is ≥ 96%.

[0016] Preferably, the particle size of the rice husk ash is 8 - 20 μm, and the water absorption per gram of the rice husk ash is 0.3 - 0.5 g.

[0017] Preferably, the redispersible latex powder is made by drying the VAE emulsion, with a volatility < 2% and an average particle size of 85 μm.

[0018] Preferably, the solid content of the emulsified asphalt in the modified emulsified asphalt is 60%, and the modifier is styrene - butadiene rubber emulsion. The specific preparation process is as follows: Add styrene - butadiene rubber emulsion accounting for 4% of the mass fraction of the emulsified asphalt to the emulsified asphalt, and mix and stir to obtain the modified emulsified asphalt.

[0019] Preferably, the coarse aggregate is impact - crushed limestone gravel with a particle size of 5 - 20 mm.

[0020] Preferably, the fine aggregate is washed river sand with a fineness modulus of 2.65.

[0021] Preferably, the length of the silane - coupling - agent - modified polyethylene fiber is 24 mm, the diameter is 25 μm, the elastic modulus is ≥ 122 MPa, and the breaking strength is ≥ 3100 MPa. The specific preparation process is as follows:

[0022] Soak and wash the polyethylene fiber to remove surface stains; then immerse it in a KH - 570 type silane - coupling - agent solution with a mass fraction of 5%, perform ultrasonic dispersion, and dry to obtain the silane - coupling - agent - modified polyethylene fiber.

[0023] Preferably, the polycarboxylate - based water - reducing agent is a pure water - reducing type, and the water - reducing rate > 30%.

[0024] Preferably, the defoaming agent is a polyether - modified organosilicon defoaming agent.

[0025] Preferably, in the nano - dense strengthening powder, the weight ratio of nano - SiO 2 and nano - CaCO 3 is 2:3; the average particle size of SiO 2 is 50 nm, and the average particle size of nano - CaCO 3 is 30 nm.

[0026] Preferably, the specific preparation process of the dense cross - linked nano - enhancer is as follows:

[0027] (1) Ultrasonically disperse graphene oxide in deionized water, and then centrifuge at a speed of 10000 rpm to obtain the graphene - oxide supernatant;

[0028] (2) Add hydroxy poly(ethylene glycol) carboxylic acid to the graphene oxide supernatant, ultrasonically disperse it in a hot water bath environment, and then centrifuge it at a speed of 12,000 rpm to obtain a stably dispersed polyethylene glycol grafted graphene oxide supernatant;

[0029] (3) Mix multi-walled carbon nanotubes and the polyethylene glycol grafted graphene oxide supernatant, and ultrasonically disperse to obtain a mixed solution; centrifuge the mixed solution at a speed of 12,000 rpm to obtain a mixed supernatant; the mixed supernatant is centrifuged 2 - 3 times repeatedly to obtain a well-dispersed graphene oxide - multi-walled carbon nanotube dense cross-linked nano-reinforcing agent;

[0030] The weight ratio of graphene oxide, multi-walled carbon nanotubes and hydroxy poly(ethylene glycol) carboxylic acid is 2:1:0.1; the purity of hydroxy poly(ethylene glycol) carboxylic acid > 95%; the multi-walled carbon nanotubes are carboxylated multi-walled carbon nanotubes, with a purity > 95%, an inner diameter of 3 - 5 nm, and an outer diameter of 8 - 15 nm.

[0031] The present invention also provides a method for preparing the high-stability concrete in the low-vacuum pipeline environment, including the following steps:

[0032] (1) Weigh the required raw materials according to the designed ratio described above. First, put the fine aggregate and coarse aggregate into a double-shaft horizontal concrete mixer, and then put cement, ground granulated blast-furnace slag, fly ash cenospheres, microsilica, rice husk ash and nano-dense reinforcing powder into the mixer, and mix and stir the dry materials for 2 - 3 min to obtain a mixed dry material;

[0033] (2) Add water, polycarboxylate superplasticizer and defoamer to the mixed dry material and continue stirring for 0.5 - 1 min, then add redispersible latex powder, modified emulsified asphalt, silane coupling agent modified polyethylene fiber and dense cross-linked nano-reinforcing agent and continue stirring for 2 - 3 min, pour it into a mold, and cure it under standard conditions until the age to obtain high-stability concrete.

[0034] The concrete prepared by the present invention is dense and uniform, and has good workability. Each raw material component cooperates with each other to improve the mechanical properties and durability of the concrete in a low-vacuum environment. Ground granulated blast-furnace slag, fly ash cenospheres, microsilica and rice husk ash are used as supplementary cementitious materials. Their particle filling effect and pozzolanic effect help the matrix form a denser structure, thereby improving the impermeability of the concrete, reducing the water loss of the concrete under low-vacuum drying, and reducing its shrinkage cracking. In addition, the particle structures of fly ash cenospheres and rice husk ash are loose and porous, and have high water absorption. Therefore, they have the effect of internal curing. Different from superabsorbent resins, there is no problem of volume change of water absorption and release, and the dry shrinkage cracking problem in a low-vacuum environment can be effectively solved. The redispersible latex powder can form a hydrophobic and highly flexible polymer film when it meets water in the mortar, which helps to overcome the problems of low bond strength, low tensile strength and low chemical resistance of cement-based materials. The emulsified asphalt modified by styrene-butadiene latex has better high-temperature stability and low-temperature crack resistance. It can form a continuous flexible film at the interface transition zone between the cement paste and the aggregate, improving the bond strength and deformation ability of the interface transition zone. It cooperates with the redispersible latex powder to form a continuous microscopic cross-linked structure with the hydration products in the matrix, thereby effectively improving the brittleness problem caused by the low-vacuum environment. The polyethylene fiber modified by silane coupling agent, as a non-metallic fiber, has a high elastic modulus and fracture strength, and is better than steel fiber in improving ductility. It can significantly improve the tensile strength, strain capacity and energy absorption capacity of the concrete, and is the key material to improve the fracture toughness of the concrete. Further, the polyethylene fiber modified by silane coupling agent overcomes the problem of its own surface inertness, and the bonding performance with the cement matrix is significantly improved. Moreover, the methacryloyl group in the silane coupling agent can react with the unsaturated double bonds in the latex, thereby enhancing the bonding force between the fiber and the polymer latex-modified cement matrix and realizing further strengthening and toughening of the matrix. The nano-dense strengthening powder can achieve dense packing of the concrete at the nano-scale, which is beneficial to further improving the denseness of the matrix. The nano-crystal nuclei also help to provide nucleation sites for the growth of C-S-H gel, promoting the formation of high-density C-S-H gel, thereby improving the strength of the matrix and the interface transition zone. The preparation process of the dense cross-linked nano-strengthening agent involves the grafting of the groups of graphene oxide and the groups of multi-walled carbon nanotubes. The graphene oxide modified by hydroxy poly(ethylene glycol) carboxylic acid can better combine with the groups of multi-walled carbon nanotubes, which helps the dispersion and stability of the two nano-materials in the slurry, and can react with the hydration product C-S-H gel and Ca(OH) 2A cross-linked structure in space is formed. Graphene oxide and multi-walled carbon nanotubes themselves have excellent mechanical and physical properties (high Young's modulus, large specific surface area). The formation of this nano-scale cross-linked structure helps to improve the stress transfer efficiency, change the propagation path of microcracks, and inhibit their development. Each of the above raw material components can work together during the plastic and hardening stages of the paste, thereby improving the mechanical properties, deformation properties, and durability of concrete at the meso-micro-nano scale in a low-vacuum environment, ensuring the service life of concrete in a low-vacuum pipeline environment.

[0035] The present invention has the following beneficial effects:

[0036] 1. In the present invention, fly ash cenospheres, ground granulated blast furnace slag, microsilica, and rice husk ash are used as supplementary cementitious materials, and nano-dense strengthening powder is used as a strengthening material. Its average particle size is smaller than that of cement, and each component fills each other, enabling dense packing with cement particles. On the other hand, the supplementary cementitious materials can undergo a pozzolanic reaction with the calcium hydroxide in the cement hydration products to form C-S-H gel. These gels can not only improve the mechanical properties and durability of concrete but also reduce the cement consumption, making the prepared concrete have the characteristics of low carbon and environmental protection. Further, fly ash cenospheres and rice husk ash have good pozzolanic effects and internal curing effects, achieving the dual effects of dense filling and shrinkage reduction and crack resistance.

[0037] 2. In the present invention, the redispersible latex powder can hydrolyze to form a flexible polymer film and form a continuous network structure with the hydration products. The addition of modified emulsified asphalt can promote the formation of a flexible layer in the interfacial transition zone. The synergistic effect of the two helps to improve the problem of increased brittleness of concrete in a low-vacuum environment. On the other hand, the polymer network formed by the two has irreversible characteristics and strong water retention, which can alleviate the shrinkage and cracking problems caused by water loss.

[0038] 3. In the present invention, the silane coupling agent-modified polyethylene fiber is non-magnetic, which can play a role in aggregate confinement and crack bridging, thereby resisting the dry shrinkage behavior caused by low vacuum, further strengthening the internal structure of concrete, and improving the overall toughness and impermeability of concrete. On the other hand, the polyethylene fiber modified by the silane coupling agent can not only have good adhesion with the cement matrix but also have a tight bonding effect with the cement matrix modified by polymer latex, jointly realizing the enhancement and toughening of the concrete matrix. Since the fiber will cause a decrease in the fluidity of concrete, adding a polycarboxylate-based water reducer can improve the workability of the concrete mixture and enhance the work performance and durability of concrete.

[0039] 4. In the present invention, the dense cross-linked nano-reinforcer combines the advantages of graphene oxide and multi-walled carbon nanotubes, breaks through the limitations of their own dimensions, can promote the formation of a tight three-dimensional cross-linked structure in the slurry, improves the strength and toughness of the matrix from the micro-nano scale, and inhibits the initiation and propagation of microcracks.

[0040] In summary, the present invention uses cement, ground granulated blast-furnace slag, fly ash cenospheres, microsilica, coarse and fine aggregates, and water as the basic raw materials of concrete, uses redispersible latex powder, modified emulsified asphalt, and silane coupling agent-modified polyethylene fibers as the toughness adjustment components of concrete, uses rice husk ash as the internal curing material of concrete, uses polycarboxylate superplasticizer and defoamer as the workability regulators of fresh slurry, and uses nano-dense reinforcing powder and nano-cross-linked reinforcer as the characteristic components for enhancing the stability of concrete, and prepares a high-strength, high-toughness, and high-airtightness concrete under a low-vacuum pipeline environment. It not only solves the problem of insufficient stability of concrete under a low-vacuum environment, but also saves the amount of cement used, reduces the carbon emissions of concrete, and reduces the magnetism of concrete structures, and its comprehensive performance is relatively prominent. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a Fourier transform infrared spectroscopy characterization diagram of the dense cross-linked nano-reinforcer.

[0042] As Figure 1 shown, compared with the multi-walled carbon nanotube monomer, there is an absorption peak of hydroxyl groups (3430 cm -1 ) in graphene oxide in the dense cross-linked nano-reinforcer, which confirms that the prepared dense cross-linked nano-reinforcer is a copolymer of graphene oxide and multi-walled carbon nanotubes.

[0043] Figure 2 It is a SEM diagram of the polymer network structure of the high-stability concrete prepared in Example 2 when exposed to the low-vacuum environment for 56 days.

[0044] Figure 3 It is a SEM diagram of the fiber constraint mechanism (a) and crack bridging mechanism (b) of the high-stability concrete prepared in Example 2 under the low-vacuum environment. DETAILED DESCRIPTION OF THE INVENTION

[0045] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0046] Example 1

[0047] In this example, the high-stability concrete and its preparation method under a low-vacuum pipeline environment include the following steps:

[0048] S1. By weight, weigh 300 parts of cement (P·O 42.5 ordinary Portland cement), 100 parts of ground granulated blast-furnace slag (S95 grade), 50 parts of fly ash cenospheres (particle size 1 - 200 μm, sphericity > 95%), 50 parts of microsilica (particle size 100 - 300 nm, SiO 2 content ≥ 96%), 5 parts of redispersible polymer powder (made by drying VAE emulsion, volatility < 2%, average particle size 85 μm), 5 parts of modified emulsified asphalt (solid content 60%, modifier is styrene-butadiene rubber emulsion, specific preparation process: add styrene-butadiene rubber emulsion accounting for 4% of the mass fraction of emulsified asphalt into the emulsified asphalt, mix and stir to obtain modified emulsified asphalt), 1050 parts of coarse aggregate (impact-crushed limestone gravel, particle size 5 - 20 mm), 750 parts of fine aggregate (washed river sand, fineness modulus 2.65), 5 parts of silane coupling agent modified polyethylene fiber (length 24 mm, diameter 25 μm, elastic modulus ≥ 122 MPa, fracture strength ≥ 3100 MPa, specific preparation process: soak and clean the original polyethylene fiber to remove surface stains; then immerse it in a 5% mass fraction KH-570 type silane coupling agent solution, ultrasonically disperse, and dry to obtain silane coupling agent modified polyethylene fiber), 150 parts of water, 5 parts of polycarboxylate superplasticizer (pure water-reducing type, water reduction rate > 30%), 1 part of defoamer (polyether-modified silicone defoamer), 15 parts of nano-dense strengthening powder (nano SiO 2 and nano CaCO 3 weight ratio is 2:3; average particle size of SiO 2 is 50 nm, average particle size of nano CaCO 3 is 30 nm), 0.15 parts of nano crosslinking strengthening agent (specific preparation process: (1) ultrasonically disperse graphene oxide in deionized water, then centrifuge at a speed of 10000 rpm to obtain graphene oxide supernatant; (2) add hydroxy polyethyleneglycol carboxylic acid to the graphene oxide supernatant, ultrasonically disperse in a hot water bath environment, then centrifuge at a speed of 12000 rpm to obtain a stably dispersed polyethylene glycol grafted graphene oxide supernatant; (3) mix multi-walled carbon nanotubes and polyethylene glycol grafted graphene oxide supernatant, ultrasonically disperse to obtain a mixture; centrifuge the mixture at a speed of 12000 rpm to obtain a mixed supernatant; the mixed supernatant is centrifuged 2 - 3 times repeatedly to obtain a well-dispersed graphene oxide-multi-walled carbon nanotube dense crosslinked nano-strengthening agent; where the weight ratio of graphene oxide, multi-walled carbon nanotubes and hydroxy polyethyleneglycol carboxylic acid is 2:1:0.1; purity of hydroxy polyethyleneglycol carboxylic acid > 95%; multi-walled carbon nanotubes are carboxylated multi-walled carbon nanotubes, purity > 95%, inner diameter 3 - 5 nm, outer diameter 8 - 15 nm).

[0049] S2. First, put the weighed river sand and crushed stones into a twin-shaft horizontal concrete mixer, and then put the weighed cement, supplementary cementitious materials and nano-dense strengthening powder into the mixer, and mix and stir the dry materials for 2 - 3 minutes;

[0050] S3. Add water, polycarboxylate superplasticizer and defoamer to the mixed dry materials and continue stirring for 30 seconds - 1 minute. Then add redispersible latex powder, modified emulsified asphalt, silane coupling agent modified polyethylene fiber and nano-crosslinked strengthening agent to the slurry and stir for 2 - 3 minutes. After mixing and stirring evenly, pour it into a mold and cure it under standard conditions until the age of the specimen to obtain high-stability concrete.

[0051] Example 2

[0052] In this example, the high-stability concrete under low-vacuum pipeline environment and its preparation method are different from those in Example 1 in that 25 parts by weight of rice husk ash is used to replace microsilica powder in equal amount.

[0053] Example 3

[0054] In this example, the high-stability concrete under low-vacuum pipeline environment and its preparation method are different from those in Example 1 in that the dosages of redispersible latex powder and modified emulsified asphalt are increased from 5 parts by weight to 10 parts by weight respectively.

[0055] Example 4

[0056] In this example, the high-stability concrete under low-vacuum pipeline environment and its preparation method are different from those in Example 3 in that the dosage of nano-dense strengthening powder is increased from 15 parts by weight to 25 parts by weight.

[0057] Example 5

[0058] In this example, the high-stability concrete under low-vacuum pipeline environment and its preparation method are different from those in Example 4 in that the dosage of nano-crosslinked strengthening agent is increased from 0.15 parts by weight to 0.25 parts by weight.

[0059] Comparative Example 1

[0060] Compared with Example 1, the difference is only that redispersible latex powder and modified emulsified asphalt are not used.

[0061] Comparative Example 2

[0062] Compared with Example 1, the difference is only that the polyethylene fiber is not modified with silane coupling agent.

[0063] Comparative Example 3

[0064] Compared with Example 1, the difference is only that nano-dense strengthening powder and nano-crosslinked strengthening agent are not used.

[0065] Comparative Example 4

[0066] Compared with Example 1, the difference is only that graphene oxide in the nano-crosslinked enhancer is not grafted with hydroxy poly(ethylene glycol) carboxylic acid.

[0067] Performance test

[0068] In order to prove the effects of the highly stable concrete prepared in the examples and comparative examples of the present invention, the highly stable concrete in Examples 1-5 and Comparative Examples 1-4 was subjected to performance tests as follows:

[0069] The highly stable concrete prepared in Examples 1-5 and Comparative Examples 1-4 was subjected to standard curing for 28 days, and then the specimens were transferred to a low-vacuum environmental simulation chamber with a vacuum pressure of 100 Pa for exposure treatment. The compressive strength, flexural strength, flexural toughness index (I 5 ) and gas permeability coefficient of the concrete were respectively tested at 28 days of standard curing and 56 days of low vacuum.

[0070] The compressive strength and flexural strength were tested according to the national standard GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". The size of the compressive strength specimen was 100 mm×100 mm×100 mm, and the size of the flexural strength specimen was 100 mm×100 mm×400 mm.

[0071] The flexural toughness index (I 5 ) was calculated according to CECS13-2009 "Standard for Test Methods of Fiber Reinforced Concrete".

[0072] The gas impermeability was tested and calculated according to JTJ 270-1998 "Code for Concrete Tests in Water Transportation Engineering" and Darcy's formula. The test equipment was the NELD-AV037 gas permeameter produced by Beijing Naird Intelligent Technology Co., Ltd. The specimens were cylindrical specimens with a diameter of 100 mm and a height of 50 mm. Before the test, the upper and lower surfaces of the formed specimens were polished flat, and the sides and the rings at 20 mm from the edges of the upper and lower surfaces of the specimens were cured and sealed with epoxy resin. The air permeability area of the specimens was a circle with a diameter of 80 mm. The calculation formula for the gas permeability coefficient is as follows:

[0073]

[0074] where k is the gas permeability coefficient, m 2 ; μ is the air dynamic viscosity, taking 1.78×10 -5 Pa·s; L is the thickness of the specimen, taking 50 mm; P 1 is the inlet pressure, taking 300 kPa; P 2 is the outlet pressure, i.e., the atmospheric pressure, taking 101 kPa; Q is the flow rate measured at the outlet of the sample, m3 / s; A is the cross-sectional area of the test piece, m 2 。

[0075] The test results are shown in Table 1 as follows:

[0076] Table 1 Performance test data of Examples 1-5 and Comparative Examples 1-4

[0077]

[0078] It can be seen from Table 1 that the mechanical strength and flexural toughness index of the concrete prepared in the examples of the present invention under the action of a low-vacuum pipeline environment are significantly greater than those of the concrete prepared in the comparative examples; further, the gas permeability coefficient of the example group is less than that of the comparative example group. The above phenomena indicate that the concrete prepared by the present invention has the characteristics of weak magnetism, high strength, high toughness and high airtightness, and has good stability in a low-vacuum pipeline environment.

[0079] Combining Example 1 and Comparative Example 1, it can be obtained that the combined addition of redispersible latex powder and modified emulsified asphalt will cause varying degrees of decline in the compressive strength and flexural strength of the concrete after standard curing. This is mainly because the redispersible latex powder has an air-entraining effect, and its synergistic effect with the modified emulsified asphalt will inhibit the hydration of cement, resulting in an increase in the porosity of the matrix and the interfacial transition zone, thereby delaying the strength development of the concrete. However, the incorporation of redispersible latex powder and modified emulsified asphalt improves the flexural toughness of the concrete and enhances its deformation ability. In particular, in a low-vacuum environment, the redispersible latex powder and polyethylene fibers promote the improvement of the mechanical properties of the concrete and alleviate the increase in gas permeability. This is mainly attributed to the fact that the redispersible latex powder and modified emulsified asphalt can form highly flexible polymer cross-links inside the concrete, as Figure 2 shown. This helps to overcome the problems of low bond strength and tensile strength of cement-based materials, and the hydrolysis of the molecular groups of the redispersible latex powder will release carboxyl groups and react with the hydration product calcium hydroxide to generate water molecules, increasing the wet density of the concrete. The hydrophobic polymer film formed by its cross-linking with the modified emulsified asphalt and its hydration products has good water retention ability, which is beneficial to maintaining the matrix saturation, and this is an important reason for improving the gas permeability resistance of the concrete.

[0080] Combining Example 1 and Comparative Example 2, it can be obtained that the polyethylene fibers modified by silane coupling agent can improve the compressive strength, flexural strength and flexural toughness of the concrete before and after the action of low vacuum, reduce its compressive-to-flexural ratio, and alleviate the problem of increased brittleness of the concrete under low-vacuum environment. The aggregate confinement effect and crack bridging mechanism of the polyethylene fibers effectively resist the shrinkage cracking caused by low vacuum, as Figure 3As shown in (a) and (b), this helps to improve the mechanical behavior and pore size distribution of concrete, and also has a positive impact on its impermeability. On the other hand, the polyethylene fibers modified with silane coupling agent have a higher bonding force with the matrix, enhancing the toughening effect of the fibers on the matrix.

[0081] Combining Example 1 and Comparative Example 3, it can be concluded that the nano-dense reinforcement powder and nano-crosslinking enhancer can significantly improve the mechanical strength and flexural toughness of concrete before and after low-vacuum treatment and reduce its gas permeability coefficient. On the one hand, the nano-materials have a small spatial scale. They can not only achieve dense filling at the nano-scale in cooperation with cement, supplementary cementitious materials and aggregates, but also provide nucleation sites for hydration products, reducing the nucleation barrier of the hydration reaction, thereby promoting the growth of high-density C-S-H gel on their surface and improving the strength and compactness of the matrix. On the other hand, the graphene oxide in the nano-crosslinking enhancer has a two-dimensional nano-sheet structure, and the multi-walled carbon nanotubes have a one-dimensional nano-columnar structure. The graphene oxide-multi-walled carbon nanotube mixture grafted with multi-walled carbon nanotubes breaks through the limitations of one-dimensional and two-dimensional structures and crosslinks with the hydration product C-S-H gel to form a three-dimensional nano-copolymer with a high elastic modulus and a large specific surface area. This can not only effectively inhibit the initiation and propagation of microcracks in the matrix, but also work synergistically with the polymer crosslinking body composed of redispersible latex powder and modified emulsified asphalt to jointly improve the load transfer efficiency and relieve the stress concentration phenomenon caused by drying shrinkage.

[0082] Combining Example 1 and Comparative Example 4, it can be concluded that the contribution of the graphene oxide-multi-walled carbon nanotube crosslinking body without being modified by hydroxy poly(ethylene glycol) carboxylic acid to the mechanical properties of concrete decreases, and the strength, toughness and gas impermeability of concrete in a low-vacuum environment all decrease. This is because the graphene oxide grafted with hydroxy poly(ethylene glycol) carboxylic acid has more branched chains and a more complex spatial structure compared with the single-layer graphene oxide. The abundant oxygen-containing functional groups on the branched chains can provide binding sites for multi-walled carbon nanotubes, thus forming a nano-crosslinking body with a higher crosslinking degree, and then improving the stability of concrete in a low-vacuum environment.

[0083] Combining Example 1 and Example 2, it can be found that replacing microsilica powder with an equal amount of rice husk ash will weaken the compressive strength and flexural strength of concrete after standard curing, but it is beneficial to improve the mechanical properties of concrete under low vacuum exposure environment and reduce the resistance to gas permeability. This is related to the unique internal curing effect of rice husk ash material. The pores in the rice husk ash will first absorb moisture from the fresh slurry, and then release it again in the low humidity environment caused by low vacuum, thereby alleviating the reduction in wet density of concrete and shrinkage cracking caused by the drying effect. Furthermore, the internal curing effect of rice husk ash and fly ash floating beads and the hydrophobic effect of the above-mentioned polymer cross-linked body can jointly provide moisture for the hydration of cement under low vacuum environment, ensuring the stability of concrete microstructure, mechanical properties, deformation properties, air tightness and durability.

[0084] Combining Examples 2 and 3, it can be concluded that increasing the amount of redispersible latex powder and modified emulsified asphalt will weaken the mechanical properties of concrete after standard curing, but has little effect on the mechanical properties of concrete after low vacuum, and can reduce the gas permeability coefficient of concrete to a certain extent.

[0085] Combining Examples 4 and 5, it can be concluded that increasing the amount of nano-compacting reinforcing powder and nano-crosslinking reinforcing agent can further improve the mechanical strength and gas permeability resistance of concrete, but will reduce its toughness. This further enhancement at the nanoscale will lead to an increase in matrix stiffness, which will increase the brittleness of concrete components and thus create the risk of cracking, which is not conducive to long-term service in a low vacuum pipeline environment.

[0086] In general, it can be concluded from Examples 3-5 that increasing the amount of redispersible latex powder, modified emulsified asphalt, nano-compacted reinforcing powder and nano-crosslinking reinforcing agent can further enhance the effects of each component, but the additional promoting effect of increasing each component is relatively limited.

Claims

1. A high stability concrete in a low vacuum pipeline environment, characterized in that: The invention comprises the following raw material components in parts by weight: 300 parts of cement, 100 parts of ground mineral powder, 50 parts of fly ash floating beads, 25-50 parts of microsilica powder, 0-25 parts of rice husk ash, 5-10 parts of redispersible latex powder, 5-10 parts of modified emulsified asphalt, 1050 parts of coarse aggregate, 750 parts of fine aggregate, 5 parts of silane coupling agent modified polyethylene fiber, 150 parts of water, 5 parts of polycarboxylic acid series water reducing agent, 1 part of defoaming agent, 15-25 parts of nano-densifying reinforcing powder, and 0.15-0.25 parts of nano-cross-linking reinforcing agent. The nano-densified and reinforced powder is composed of nano-SiO2 and nano-CaCO3; The nano cross-linking enhancer consists of polyethylene glycol-grafted graphene oxide and multi-walled carbon nanotubes.

2. The high stability concrete in low vacuum pipeline environment according to claim 1, characterized in that: The cement is ordinary Portland cement with a strength grade ≥ P·O 42.5; The grade of the ground mineral powder is ≥ S95; The particle size of the fly ash floating beads is 1-200 μm, and the sphericity rate is >95%; The microsilica powder has a particle size of 100-300nm and a SiO2 content of ≥96%; The rice husk ash has a particle size of 8-20 μm and a water absorption capacity of 0.3-0.5 g per gram of rice husk ash; The coarse aggregate is impact crushed limestone gravel with a particle size of 5-20 mm; The fine aggregate is washed river sand with a fineness modulus of 2.

65.

3. The high stability concrete in low vacuum pipeline environment according to claim 1, characterized in that: The redispersible latex powder is made by drying the VAE emulsion, with a volatility of <2% and an average particle size of 85 μm; The solid content of the emulsified asphalt in the modified emulsified asphalt is 60%, and the modifier is styrene-butadiene rubber emulsion. The specific preparation process is: adding styrene-butadiene rubber emulsion accounting for 4% of the mass fraction of the emulsified asphalt into the emulsified asphalt, mixing and stirring, and preparing the modified emulsified asphalt.

4. The high stability concrete in low vacuum pipeline environment according to claim 1, characterized in that: The silane coupling agent modified polyethylene fiber has a length of 24 mm, a diameter of 25 μm, an elastic modulus of ≥122 MPa, and a breaking strength of ≥3100 MPa. The specific preparation process is as follows: The polyethylene fiber is soaked and cleaned to remove surface stains; then it is immersed in a 5 mass percent KH-570 silane coupling agent solution, ultrasonically dispersed, and dried to obtain the silane coupling agent modified polyethylene fiber.

5. The high stability concrete in low vacuum pipeline environment according to claim 1, characterized in that: The polycarboxylic acid water reducer is a pure water reducing type, with a water reducing rate of >30%; The defoamer is a polyether-modified silicone defoamer.

6. The high stability concrete in low vacuum pipeline environment according to claim 1, characterized in that: The weight ratio of nano-SiO2 to nano-CaCO3 in the nano-dense reinforced powder is 2:3; the average particle size of SiO2 is 50nm, and the average particle size of nano-CaCO3 is 30nm.

7. The high stability concrete in low vacuum pipeline environment according to claim 1, characterized in that: The specific preparation process of the dense cross-linked nano-enhancer is as follows: (1) ultrasonically dispersing graphene oxide in deionized water, and then centrifuging at a speed of 10000 rpm to obtain a graphene oxide supernatant; (2) adding hydroxy polyethylene glycol carboxylic acid to the graphene oxide supernatant, dispersing the supernatant by ultrasonication in a hot water bath, and then centrifuging the supernatant at 12000 rpm to obtain a stably dispersed polyethylene glycol-grafted graphene oxide supernatant; (3) mixing the supernatant of multi-walled carbon nanotubes and polyethylene glycol grafted graphene oxide, and ultrasonically dispersing the mixture to obtain a mixed solution; centrifuging the mixture at a speed of 12000 rpm to obtain a mixed supernatant; and centrifuging the mixed supernatant 2 to 3 times to obtain a well-dispersed graphene oxide-multi-walled carbon nanotube dense cross-linked nano-enhancer; The weight ratio of graphene oxide, multi-walled carbon nanotubes and hydroxyl polyethylene glycol carboxylic acid is 2:1:0.1; the purity of hydroxyl polyethylene glycol carboxylic acid is >95%; the multi-walled carbon nanotubes are carboxylated multi-walled carbon nanotubes with a purity of >95%, an inner diameter of 3-5nm, and an outer diameter of 8-15nm.

8. The method for preparing high-stability concrete in a low-vacuum pipeline environment according to any one of claims 1 to 7, characterized in that: The steps include: (1) Weigh the required raw materials according to the above-mentioned design ratio, first put the fine aggregate and coarse aggregate into a double-shaft horizontal concrete mixer, then put cement, ground mineral powder, fly ash floating beads, microsilica powder, rice husk ash and nano-densifying and reinforcing powder into the mixer, mix and stir the dry materials for 2-3 minutes to obtain mixed dry materials; (2) Add water, polycarboxylic acid water reducer and defoamer to the mixed dry materials and continue stirring for 0.5-1 min. Then add redispersible latex powder, modified emulsified asphalt, silane coupling agent modified polyethylene fiber and dense cross-linked nano-reinforcement agent and continue stirring for 2-3 min. Pour into a mold and perform standard curing until the age to obtain high-stability concrete.

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