Concrete for underground structure and preparation method thereof

Abstract

The invention provides concrete for an underground structure. The concrete is prepared from the following raw materials in parts by weight: 15-18 parts of cement, 3-5 parts of plant residue charcoal, 45-50 parts of fine aggregate, 50-55 parts of coarse aggregate, 8-10 parts of fly ash, 0.2-0.4 part of biogel, 1-2 parts of micron-sized calcium carbonate, 0.3-0.6 part of a water reducing agent and 10-15 parts of water. The invention further provides a preparation method of the concrete for the underground structure. According to the concrete for the underground structure, the segregation resistance and the crack resistance can be remarkably improved under the high free inclination and falling condition, and meanwhile the environment-friendly requirement of green buildings is met.

Description

Technical Field

[0001] The invention relates to concrete, in particular to concrete for underground structures and a preparation method thereof. Background Art

[0002] Concrete is one of the most widely used building materials. Its excellent strength and durability make it play an important role in engineering construction. In underground engineering construction, the free fall height of concrete mixture is usually required to be no more than 2m. This is because the density of each component in the concrete mixture is different. If the free fall height is not strictly controlled, concrete is prone to segregation, that is, the coarse aggregate and cement paste separate during the free fall process, resulting in uneven components of cast concrete components, causing problems such as insufficient strength and cracking. In addition, excessive free fall height will cause a large amount of air to be mixed in the concrete during the rapid fall process, forming bubbles, thereby reducing the density of the concrete and affecting the structural strength, impermeability and apparent quality. However, underground structures usually have complex construction conditions, and a free fall height of less than 2m is difficult to guarantee.

[0003] At present, in order to reduce the impact of free fall height on concrete quality, construction sites often use the following methods to control it:

[0004] (1) Conduit or chute: Concrete is guided to the pouring position by using auxiliary equipment such as conduits or chutes to reduce the free fall height and segregation. However, these auxiliary equipment increase the complexity and cost of construction, especially in deep underground structural projects. The arrangement and use of conduits are limited by space and are difficult to flexibly adapt to various construction environments.

[0005] (2) Layered pouring: In order to reduce the pouring height of concrete, the construction party usually adopts the layered pouring method, pouring the concrete in batches and layers, thereby reducing the height of a single free fall. However, this method prolongs the construction time and increases the on-site management cost, especially for projects that require large-scale continuous pouring. It not only affects the construction efficiency, but also brings about the problem of poor bonding between concrete layers.

[0006] (3) Concrete modification: Conventional solutions also include the use of chemical admixtures in concrete, such as polycarboxylate water reducers and viscosity modifiers, to improve its fluidity and viscosity. Although these admixtures can reduce the risk of segregation to a certain extent, they cannot completely eliminate the problems of air inclusion and reduced density caused by high free fall heights.

[0007] In addition, modern engineering construction has increasing requirements for environmentally friendly and sustainable building materials. Most current concrete materials use cement as the main component, and a large amount of carbon dioxide is produced during the production process, which has a great impact on the environment. At the same time, traditional chemical admixtures may also have adverse effects on the ecosystem. With the promotion of the concept of green building, people expect to use more eco-friendly materials to replace traditional ingredients to reduce carbon emissions and improve the overall sustainability of materials. However, most of the existing eco-friendly concretes cannot meet the requirements of anti-segregation and bubble control under high free fall conditions, and therefore their application in underground structure construction is limited.

[0008] Therefore, in order to meet the special requirements of underground structure projects for concrete performance under the condition of a large free fall height, there is an urgent need for a concrete that has high resistance to segregation and low air inclusion rate and conforms to the concept of eco-friendliness and sustainable development. This innovative eco-friendly concrete material will effectively improve the overall quality and durability of underground structure projects in terms of construction convenience and environmental protection performance. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a concrete for underground structure, which can significantly improve the segregation resistance and crack resistance under high free fall conditions and meet the environmental protection requirements of green buildings.

[0010] In order to solve the above technical problems, the technical solution of the present invention is:

[0011] A concrete for underground structure is made of the following raw materials in parts by weight: 15-18 parts of cement, 3-5 parts of plant residue biochar, 45-50 parts of fine aggregate, 50-55 parts of coarse aggregate, 8-10 parts of fly ash, 0.2-0.4 parts of bio-glue, 1-2 parts of micron-grade calcium carbonate, 0.3-0.6 parts of water reducer and 10-15 parts of water.

[0012] Among them, cement, as the main cementitious material, provides basic structural strength for concrete; plant residue biochar has a porous structure, which can effectively absorb moisture and reduce the separation tendency of aggregate and cement slurry, thereby improving the water retention and cohesion of concrete, reducing segregation, and complying with the eco-friendly concept; fine aggregate is used to improve the density and overall fluidity of concrete; coarse aggregate, as the skeleton material of concrete, is used to improve the compressive strength of concrete; fly ash is used as an admixture to fill the gaps between aggregates, which is used to enhance the stability of concrete and reduce the heat of hydration; bio-glue is used to improve the cohesion and water retention of concrete, which helps to maintain consistency under free fall; micron-sized calcium carbonate is used to enhance crack resistance and pore filling effects, and improve the durability of concrete; water reducer is used to optimize the water-cement ratio and improve fluidity, ensuring that the concrete maintains high strength and density under low water-cement ratio conditions.

[0013] Furthermore, the cement described in the present invention is P·O 42.5 ordinary Portland cement or P·O 52.5 ordinary Portland cement.

[0014] Furthermore, the plant residue biochar described in the present invention is one of bagasse biochar, corn straw biochar, coffee grounds biochar or rice husk biochar.

[0015] Furthermore, the fine aggregate of the present invention is river sand or machine-made sand with a water absorption rate of ≤0.2% and a 4.75 mm square hole sieve pass rate of ≥90%.

[0016] Furthermore, the coarse aggregate of the present invention is pebbles or crushed stones with a particle size of 5-20 mm.

[0017] Furthermore, the bio-glue of the present invention is calcium alginate or chitosan, which can be evenly dispersed in the concrete dry material to enhance the stability of the concrete during the pouring process and effectively control air inclusion and segregation phenomena.

[0018] Furthermore, the particle size of the micron-sized calcium carbonate described in the present invention is 5-20 mm, and calcium carbonate deposits (CaCO3) produced by microbial metabolism can be preferably used, specifically calcium carbonate (CaCO3) particles induced by microbial metabolic processes. This calcium deposit is naturally deposited in the concrete matrix through the biomineralization process, and its microscopic bridging structure can effectively enhance the crack resistance and long-term density of concrete, and has stronger durability and environmental protection effects compared with traditional industrial calcium carbonate powder.

[0019] Furthermore, the water reducer of the present invention is a polycarboxylate water reducer.

[0020] Another technical problem to be solved by the present invention is to provide a method for preparing the above-mentioned concrete for underground structure.

[0021] To solve the above technical problems, the technical solution is:

[0022] A method for preparing concrete for underground structures comprises the following steps:

[0023] S1. Weigh each raw material by weight, add plant residue biochar, fine aggregate, coarse aggregate into a mixer, and stir at low speed for 2-4 minutes to obtain a mixture;

[0024] S2. Add cement and fly ash to the mixture obtained in step S1, and stir at low speed for 1-3 minutes to obtain a mixture II;

[0025] S3. Add the bio-glue to the mixture obtained in step S2 and stir at low speed for 1-2 minutes to obtain a mixture of three;

[0026] S4. The water reducing agent and water are added to the mixture obtained in step S3, and stirred at medium speed for 1-3 minutes to obtain a mixture of four;

[0027] S5. Add micron-sized calcium carbonate to the mixture obtained in step S4, and stir at medium speed for 1-2 minutes to obtain concrete for underground structures.

[0028] Furthermore, in steps S1, S2, and S3 of the present invention, the stirring speed of the low-speed stirring is 60-90 rpm; in steps S4 and S5, the stirring speed of the medium-speed stirring is 90-120 rpm.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) The present invention not only significantly improves the performance of concrete under free fall conditions, but also effectively reduces aggregate segregation, thereby ensuring the construction quality of concrete under high free fall conditions.

[0031] (2) The present invention utilizes plant residue biochar, bio-gum and micron-sized calcium carbonate sediments produced by microbial metabolism, which significantly reduces the carbon footprint and environmental impact of concrete.

[0032] (3) The present invention can effectively enhance the crack resistance of concrete and extend its service life in underground structures. It is particularly suitable for engineering applications with high durability requirements, especially underground structures.

[0033] (4) While achieving the improvement of concrete performance under high free fall conditions, the present invention provides a green and eco-friendly technical solution that is suitable for the sustainable development needs of modern construction and infrastructure construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute an improper limitation of the present invention. In the drawings:

[0035] Figure 1 This is a comparison chart of the segregation rates before and after free fall of Example 1, Example 2, and Comparative Example of the present invention;

[0036] Figure 2 It is a comparison chart of the gas content before and after free fall of Example 1, Example 2 and the comparative example of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be described in detail below in conjunction with specific embodiments. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention but are not intended to limit the present invention.

[0038] Example 1

[0039] Concrete for underground structures is made of the following raw materials in parts by weight: 16 parts of cement, 5 parts of plant residue biochar, 48 parts of fine aggregate, 52 parts of coarse aggregate, 8 parts of fly ash, 0.3 parts of bio-glue, 1.5 parts of micron-grade calcium carbonate, 0.5 parts of water reducer, and 11 parts of water. Among them, the cement is ordinary Portland cement with a P·O 42.5; the plant residue biochar is sugarcane bagasse biochar; the fine aggregate is river sand with a water absorption rate of ≤0.2% and a 4.75mm square hole sieve passing rate of ≥90%; the coarse aggregate is pebbles with a particle size of 10mm; the bio-glue is calcium alginate; the micron-grade calcium carbonate is calcium carbonate (CaCO3) particles with a particle size of 10mm, induced by microbial metabolism; and the water reducer is a polycarboxylate water reducer.

[0040] The preparation steps of bagasse biochar are as follows:

[0041] ① Raw material drying: Dry the bagasse at 105°C for 24 hours until its moisture content drops below 10%. This step ensures the low moisture content of the raw material, which is beneficial to the subsequent pyrolysis process;

[0042] ② Crushing and screening: crush the dried bagasse in step ① to a particle size of 2 mm, and remove larger or irregular particles by screening to obtain bagasse particles, so as to ensure uniformity and consistency of particle structure in the subsequent pyrolysis process;

[0043] ③ Pyrolysis process: The bagasse particles obtained in step ② are placed in a pyrolysis furnace and pyrolyzed at 500°C for 2 hours under anaerobic conditions to obtain biochar particles. During the pyrolysis process, the carbon structure of the bagasse is retained, and abundant micropores and mesopores are formed, which give the biochar good adsorption and water retention properties;

[0044] ④ Powdering treatment: The biochar particles obtained in step ③ are further crushed into fine powder to obtain bagasse biochar with a particle size of less than 50 microns, which is convenient for uniform mixing with other components of concrete. This process retains the porous structure of biochar and ensures its water absorption and filling effect in concrete;

[0045] ⑤ Acid washing and neutralization: In order to remove impurities that may remain on the surface of biochar, the bagasse biochar obtained in step ④ can be soaked in a dilute hydrochloric acid solution with a mass concentration of 5% for 30 minutes, then repeatedly washed with distilled water until neutral, and dried at 105°C to constant weight. This step enhances the chemical stability of the bagasse biochar and its bonding properties with the cement matrix.

[0046] The preparation method of Example 1 comprises the following steps:

[0047] S1. Weigh each raw material by weight, add plant residue biochar, fine aggregate and coarse aggregate into a mixer, and stir at a low speed of 80 rpm for 3 minutes to obtain a mixture;

[0048] S2. Cement and fly ash are added to the mixture obtained in step S1, and stirred at a low speed of 80 rpm for 2 minutes to obtain a mixture II;

[0049] S3. Add the bio-gum to the mixture obtained in step S2, and stir at a low speed of 80 rpm for 1 minute to obtain a mixture of three;

[0050] S4. The water reducing agent and water were added to the mixture obtained in step S3, and stirred at a stirring speed of 110 rpm for 2 minutes to obtain a mixture of four;

[0051] S5. Add micron-sized calcium carbonate to the mixture obtained in step S4, and stir at a medium speed of 110 rpm for 1 minute to obtain concrete for underground structures.

[0052] Example 2

[0053] Concrete for underground structures is made of the following raw materials in parts by weight: 17 parts of cement, 4 parts of plant residue biochar, 46 parts of fine aggregate, 54 parts of coarse aggregate, 10 parts of fly ash, 0.3 parts of bio-glue, 1.5 parts of micron-grade calcium carbonate, 0.4 parts of water reducer, and 12 parts of water. Among them, the cement is ordinary Portland cement with a P·O 42.5; the plant residue biochar is rice husk biochar; the fine aggregate is river sand with a water absorption rate of ≤0.2% and a 4.75mm square hole sieve passing rate of ≥90%; the coarse aggregate is pebbles with a particle size of 15mm; the bio-glue is chitosan; the micron-grade calcium carbonate is calcium carbonate (CaCO3) particles with a particle size of 15mm, induced by microbial metabolism; and the water reducer is a polycarboxylic acid water reducer.

[0054] The preparation steps of rice husk biochar are as follows:

[0055] ① Drying of raw materials: Dry the rice husk at 105°C for 24 hours until its moisture content drops below 8%. This step helps reduce the amount of steam generated during subsequent pyrolysis;

[0056] ② Crushing and screening: Crushing the dried rice husks in step ① to a particle size of 2 mm, and removing larger or irregular particles by screening to obtain rice husk particles, to ensure uniformity and consistency of particle structure in the subsequent pyrolysis process;

[0057] ③ Pyrolysis process: The rice husk particles obtained in step ② are placed in a pyrolysis furnace and pyrolyzed at 600°C for 1.5 hours under anaerobic conditions to obtain biochar particles. During the pyrolysis process, the rice husk forms a carbonaceous porous structure with a high specific surface area, which can significantly improve the water retention and cohesion of concrete;

[0058] ④ Powdering treatment: The biochar particles obtained in step ③ are further crushed into fine powder to obtain rice husk biochar with a particle size of less than 50 microns, which is convenient for uniform mixing with other components of concrete to ensure uniform distribution and enhance performance;

[0059] ⑤ Alkaline washing and neutralization: In order to remove impurities that may remain on the surface of biochar, the rice husk biochar obtained in step ④ can be soaked in a sodium hydroxide solution with a mass concentration of 5% for 20 minutes, then repeatedly washed with distilled water until neutral, and dried at 105°C to constant weight. This step enhances the surface activity of rice husk biochar and its binding force with cement slurry.

[0060] The preparation method of Example 2 comprises the following steps:

[0061] S1. Weigh each raw material by weight, add plant residue biochar, fine aggregate and coarse aggregate into a mixer, and stir at a low speed of 60 rpm for 4 minutes to obtain a mixture;

[0062] S2. Cement and fly ash are added to the mixture obtained in step S1, and stirred at a low speed of 60 rpm for 2 minutes to obtain a mixture II;

[0063] S3. Add the bio-gum to the mixture obtained in step S2, and stir at a low speed of 60 rpm for 1 minute to obtain a mixture of three;

[0064] S4. The water reducing agent and water were added to the mixture obtained in step S3, and the mixture was stirred at a medium speed of 100 rpm for 2 minutes to obtain a mixture of four;

[0065] S5. Add micron-sized calcium carbonate to the mixture obtained in step S4, and stir at a medium speed of 100 rpm for 1 minute to obtain a concrete for underground structure.

[0066] Comparative Example

[0067] Concrete for underground structure is made of the following raw materials in parts by weight: 16.5 parts of cement, 48 parts of fine aggregate, 52 parts of coarse aggregate, 9 parts of fly ash, 0.4 parts of water reducer, and 10 parts of water. Among them, the cement is P·O 42.5 ordinary Portland cement; the fine aggregate is river sand with a water absorption rate of ≤0.2% and a passing rate of 4.75mm square hole sieve of ≥90%; the coarse aggregate is pebbles with a particle size of 10mm; and the water reducer is a polycarboxylate water reducer.

[0068] The preparation method of the comparative example comprises the following steps:

[0069] S1. Weigh each raw material by weight, add fine aggregate and coarse aggregate to the mixer, stir at a low speed of 80 rpm for 4 minutes to obtain a mixture;

[0070] S2. Cement and fly ash are added to the mixture obtained in step S1, and stirred at a low speed of 80 rpm for 2 minutes to obtain a mixture II;

[0071] S3. Add the water reducing agent and water to the mixture obtained in step S2, and stir at a medium speed of 110 rpm for 2 minutes to obtain concrete for underground structures.

[0072] The main difference between the comparative example and the present invention is that the raw materials do not contain plant residue biochar, bio-gum, and micron-sized calcium carbonate.

[0073] The segregation rate and air content of the concrete obtained in Example 1, Example 2 and the comparative example were tested before and after free fall. The test results are as follows: Figure 1 , Figure 2 As shown. The test method of segregation rate refers to "17. Anti-segregation performance test" in GB / T 50080-2016 "Standard for test methods of ordinary concrete mixture performance", and the test method of air content refers to "15. Air content test" in GB / T 50080-2016 "Standard for test methods of ordinary concrete mixture performance".

[0074] from Figure 1 From the above, although the segregation rate of the comparative example before free fall is acceptable (<8%), the segregation is more serious after free fall. In comparison, although the initial segregation rates of Examples 1 and 2 are slightly higher than that of the comparative example, the segregation rates after free fall decrease by 4 and 5 percentage points respectively.

[0075] Similar results have been achieved in gas content control: Figure 2 From the above, it can be seen that the initial gas content of the comparative example, Example 1 and Example 2 is not much different, all of which are about 3%. However, after free falling from a high altitude, the gas content of the comparative example has more than doubled, while that of Example 1 has only a slight increase, and that of Example 2 has even decreased slightly.

[0076] The above test results show that the present invention effectively solves the problem of easy segregation and air inclusion of concrete after free fall from a height by using plant residue biochar, bio-gum and micron-sized calcium carbonate, and is suitable for underground engineering construction.

[0077] Example 3

[0078] Concrete for underground structures is made of the following raw materials in parts by weight: 15 parts of cement, 3 parts of plant residue biochar, 45 parts of fine aggregate, 50 parts of coarse aggregate, 9 parts of fly ash, 0.2 parts of biological glue, 1 part of micron-grade calcium carbonate, 0.3 parts of water reducer, and 10 parts of water. Among them, the cement is ordinary Portland cement with a P·O 52.5; the plant residue biochar is coffee grounds biochar; the fine aggregate is machine-made sand with a water absorption rate of ≤0.2% and a 4.75mm square hole sieve pass rate of ≥90%; the coarse aggregate is crushed stone with a particle size of 5mm; the biological glue is calcium alginate; the micron-grade calcium carbonate is calcium carbonate (CaCO3) particles with a particle size of 5mm, induced by microbial metabolism; and the water reducer is a polycarboxylate water reducer.

[0079] The preparation steps of coffee grounds biochar are:

[0080] ① Drying of raw materials: Dry the coffee grounds at 105°C for 12 hours until the moisture content drops below 8%. This step removes the residual moisture in the coffee grounds to ensure the efficiency of pyrolysis.

[0081] ② Crushing and sieving: The coffee grounds dried in step ① are crushed to a particle size of 2 mm, and larger or irregular particles are removed by sieving to obtain coffee ground particles, so as to ensure uniformity and consistency of particle structure in the subsequent pyrolysis process;

[0082] ③ Pyrolysis process: The coffee grounds particles obtained in step ② are placed in a pyrolysis furnace and pyrolyzed at 450°C for 1.5 hours under anaerobic conditions to obtain biochar particles. The high porosity characteristics of sugarcane bagasse are retained during the pyrolysis process, ensuring its good adsorption and water retention in concrete;

[0083] ④ Powdering treatment: The biochar particles obtained in step ③ are further crushed into fine powder to obtain coffee grounds biochar with a particle size of less than 50 microns, ensuring that it is evenly mixed with cement, aggregate and other ingredients in concrete;

[0084] ⑤ Alkali washing and neutralization: In order to remove impurities that may remain on the surface of biochar, the coffee grounds biochar obtained in step ④ can be soaked in a 5% sodium hydroxide solution for 20 minutes, then repeatedly washed with distilled water until neutral, and dried at 105°C to constant weight. This step improves the bonding strength between the coffee grounds biochar and the cement matrix.

[0085] The preparation method of Example 3 comprises the following steps:

[0086] S1. Weigh each raw material by weight, add plant residue biochar, fine aggregate and coarse aggregate into a mixer, and stir at a low speed of 90 rpm for 2 minutes to obtain a mixture;

[0087] S2. Cement and fly ash are added to the mixture obtained in step S1, and stirred at a low speed of 90 rpm for 1 minute to obtain a mixture II;

[0088] S3. The bio-gum was added to the mixture obtained in step S2, and stirred at a low speed of 90 rpm for 1.5 minutes to obtain a mixture of three;

[0089] S4. The water reducing agent and water were added to the mixture obtained in step S3, and stirred at a stirring speed of 120 rpm for 1 minute to obtain a mixture of four;

[0090] S5. Add micron-sized calcium carbonate to the mixture obtained in step S4, and stir at a medium speed of 120 rpm for 1 minute to obtain a concrete for underground structure.

[0091] Example 4

[0092] Concrete for underground structures is made of the following raw materials in parts by weight: 18 parts of cement, 5 parts of plant residue biochar, 50 parts of fine aggregate, 55 parts of coarse aggregate, 10 parts of fly ash, 0.4 parts of bio-glue, 2 parts of micron-grade calcium carbonate, 0.6 parts of water reducer, and 15 parts of water. Among them, the cement is ordinary Portland cement with a P·O 52.5; the plant residue biochar is corn straw biochar; the fine aggregate is machine-made sand with a water absorption rate of ≤0.2% and a 4.75mm square hole sieve passing rate of ≥90%; the coarse aggregate is crushed stone with a particle size of 20mm; the bio-glue is chitosan; the micron-grade calcium carbonate is calcium carbonate (CaCO3) particles with a particle size of 20mm, induced by microbial metabolism; and the water reducer is a polycarboxylate water reducer.

[0093] The preparation steps of corn straw biochar are as follows:

[0094] ① Raw material drying: Dry the corn stalks at 105°C for 24 hours until their moisture content drops below 10%. This step removes moisture from the corn stalks to ensure efficient production of biochar during pyrolysis.

[0095] ② Crushing and screening: crushing the corn stalks dried in step ① to a particle size of 2 mm, and removing larger or irregular particles by screening to obtain corn stalk particles, so as to ensure the uniformity and consistency of the corn stalk particles, as well as the uniformity of subsequent pyrolysis;

[0096] ③ Pyrolysis process: The corn stalk particles obtained in step ② are placed in a pyrolysis furnace and pyrolyzed at 500°C for 2 hours under anaerobic conditions to obtain biochar particles. The porous structure of the corn stalks is retained during the pyrolysis process, giving the biochar good water absorption and adhesion properties;

[0097] ④ Powdering treatment: The biochar particles obtained in step ③ are further crushed into fine powder to obtain corn straw biochar with a particle size of less than 50 microns, ensuring that the corn straw biochar can be evenly distributed in the concrete and maintain its porous structure;

[0098] ⑤ Acid washing and neutralization: In order to remove impurities that may remain on the surface of biochar, the corn straw biochar obtained in step ④ can be soaked in a dilute hydrochloric acid solution with a mass concentration of 5% for 30 minutes, then repeatedly washed with distilled water until neutral, and dried at 105°C to constant weight. This step enhances the stability of corn straw biochar and its compatibility with cement.

[0099] The preparation method of Example 4 comprises the following steps:

[0100] S1. Weigh each raw material by weight, add plant residue biochar, fine aggregate and coarse aggregate into a mixer, and stir at a low speed of 70 rpm for 3 minutes to obtain a mixture;

[0101] S2. Cement and fly ash are added to the mixture obtained in step S1, and stirred at a low speed of 70 rpm for 3 minutes to obtain a mixture II;

[0102] S3. Add the bio-gum to the mixture obtained in step S2, and stir at a low speed of 70 rpm for 2 minutes to obtain a mixture of three;

[0103] S4. The water reducing agent and water were added to the mixture obtained in step S3, and stirred at a stirring speed of 90 rpm for 3 minutes to obtain a mixture of four;

[0104] S5. Add micron-sized calcium carbonate to the mixture obtained in step S4, and stir at a medium speed of 90 rpm for 2 minutes to obtain a concrete for underground structure.

[0105] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A concrete for underground structure, characterized in that: The invention is made of the following raw materials in parts by weight: 15-18 parts of cement, 3-5 parts of plant residue biochar, 45-50 parts of fine aggregate, 50-55 parts of coarse aggregate, 8-10 parts of fly ash, 0.2-0.4 parts of bio-glue, 1-2 parts of micron-sized calcium carbonate, 0.3-0.6 parts of water reducing agent and 10-15 parts of water.

2. The concrete for underground structure according to claim 1, characterized in that: The cement is P·O 42.5 ordinary Portland cement or P·O 52.5 ordinary Portland cement.

3. The concrete for underground structure according to claim 1, characterized in that: The plant residue biochar is one of bagasse biochar, corn straw biochar, coffee grounds biochar or rice husk biochar.

4. The concrete for underground structure according to claim 1, characterized in that: The fine aggregate is river sand or machine-made sand with a water absorption rate of ≤0.2% and a 4.75 mm square hole sieve passing rate of ≥90%.

5. The concrete for underground structure according to claim 1, characterized in that: The coarse aggregate is pebbles or crushed stones with a particle size of 5-20 mm.

6. The concrete for underground structure according to claim 1, characterized in that: The biological glue is calcium alginate or chitosan.

7. The concrete for underground structure according to claim 1, characterized in that: The particle size of the micron-sized calcium carbonate is 5-20 mm.

8. The concrete for underground structure according to claim 1, characterized in that: The water reducer is a polycarboxylate water reducer.

9. A method for preparing concrete for underground structure according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1. Weigh each raw material by weight, add plant residue biochar, fine aggregate, coarse aggregate into a mixer, and stir at low speed for 2-4 minutes to obtain a mixture; S2. Add cement and fly ash to the mixture obtained in step S1, and stir at low speed for 1-3 minutes to obtain a mixture II; S3. Add the bio-glue to the mixture obtained in step S2 and stir at low speed for 1-2 minutes to obtain a mixture of three; S4. The water reducing agent and water are added to the mixture obtained in step S3, and stirred at medium speed for 1-3 minutes to obtain a mixture of four; S5. Add micron-sized calcium carbonate to the mixture obtained in step S4, and stir at medium speed for 1-2 minutes to obtain concrete for underground structures.

10. The method for preparing concrete for underground structure according to claim 9, characterized in that: In the steps S1, S2 and S3, the stirring speed of the low-speed stirring is 60-90 rpm; in the steps S4 and S5, the stirring speed of the medium-speed stirring is 90-120 rpm.

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