A building waste recycled aggregate gradient functionalized seedling planting substrate in urban renewal and a preparation method thereof

Abstract

The present application belongs to the technical field of landscaping planting substrate and the technical field of solid waste resource utilization, in particular relates to a kind of construction waste recycled aggregate gradient functional modified nursery stock planting substrate in urban renewal and preparation method thereof. By mass parts, the following components are included: gradient functional recycled coarse aggregate 40~60 parts, gradient functional recycled fine aggregate 25~35 parts, biochar 3~8 parts, mature organic matter 15~25 parts, bentonite 5~10 parts, double-response water-retaining agent 2~5 parts, composite microbial inoculant 1~3 parts, slow-release nutrient precursor 3~8 parts, calcium source 1~3 parts, chitosan 0.5~2 parts. The present application solves the problems of high alkalinity, insufficient organic matter content, poor water and fertilizer retention capacity of recycled aggregate, and utilizes the porous structure of recycled aggregate to make it a carrier for nutrients, which is slowly released during the growth of nursery stock. The survival rate of nursery stock reaches 96.8%, and the plant height growth rate and new root dry weight are both improved.

Description

Technical Field

[0001] This invention belongs to the fields of landscaping planting substrate technology and solid waste resource utilization technology, and particularly relates to a graded functionalized seedling planting substrate made from recycled aggregates of construction waste in urban renewal and its preparation method. Background Technology

[0002] With the continuous acceleration of urbanization in my country, the pace of urban renewal, old city renovation, and urban village demolition projects is accelerating, leading to a year-on-year increase in construction waste. Statistics show that my country generates over 2 billion tons of construction waste annually, of which approximately 30% can be processed into recycled concrete aggregate (RCA). If construction waste is directly landfilled or dumped in the open without any treatment, it not only consumes a large amount of land resources and incurs significant waste disposal costs, but also disrupts the soil's food chain, causing serious environmental pollution.

[0003] On the other hand, a large amount of existing green space is removed during urban renewal, while the construction of new urban areas urgently requires a large amount of planting substrate for seedling planting. Traditional planting substrates mostly rely on non-renewable resources such as peat moss and sphagnum moss, or use large amounts of natural sand and gravel, which not only leads to the over-exploitation of natural resources, but also incurs high costs.

[0004] Using construction waste as a substrate for seedling planting can achieve the recycling and reuse of construction waste and reduce the over-exploitation of natural resources in seedling planting, thereby reducing costs. However, construction waste, such as recycled concrete aggregate, has high alkalinity, less than 1% organic matter content, and poor water and fertilizer retention capacity. Further improvements are needed to prepare a seedling planting substrate suitable for rapid conversion in urban renewal sites, thereby improving its environmental benefits and practical value. Summary of the Invention

[0005] The purpose of this invention is to provide a gradient functionalized seedling planting substrate made from recycled aggregates of construction waste for urban renewal and its preparation method, so as to solve the problems existing in the prior art.

[0006] The technical solution adopted by this invention to solve its technical problem is:

[0007] A graded functionalized seedling planting substrate made from recycled construction waste aggregate for urban renewal, characterized in that it comprises the following components by weight:

[0008] The mixture consists of 40-60 parts of graded functionalized recycled coarse aggregate, 25-35 parts of graded functionalized recycled fine aggregate, 3-8 parts of biochar, 15-25 parts of decomposed organic matter, 5-10 parts of bentonite, 2-5 parts of dual-response water-retaining factor, 1-3 parts of compound microbial agent, 3-8 parts of slow-release nutrient precursor, 1-3 parts of calcium source, and 0.5-2 parts of chitosan.

[0009] Among them, both the gradient functionalized recycled coarse aggregate and the gradient functionalized recycled fine aggregate have a three-layer gradient functionalized structure from the inside out:

[0010] The inner layer is a porous nutrient loading layer, which includes recycled aggregate and its internal pores loaded with slow-release nutrient precursors;

[0011] The middle layer is a pH buffer control layer, which includes an acidic slow-release material layer coated on the surface of the aggregate;

[0012] The outer layer is a bioactive interface layer, including a biopolymer-biochar composite gel layer sprayed on the outermost layer.

[0013] Furthermore, the slow-release nutrient precursor is a urea-formaldehyde condensate or ammonium polyphosphate.

[0014] Furthermore, the calcium source is calcium lactate or calcium chloride.

[0015] Furthermore, the dual-responsive water-retaining factor is a copolymer of N-isopropylacrylamide and acrylic acid, exhibiting both temperature-responsive and pH-responsive properties.

[0016] Furthermore, the compound microbial agent contains urease-producing strains with an effective viable count ≥5×10⁻⁶. 8 CFU / g.

[0017] Furthermore, the acidic slow-release material of the pH buffer control layer is a composite of sulfur powder and gypsum powder, with a mass ratio of sulfur powder to gypsum powder of (1:2) to (1:3).

[0018] Furthermore, the biopolymer of the bioactive interface layer is chitosan, and the mass ratio of chitosan to biochar in the biopolymer-biochar composite gel layer is (1:2) to (1:3).

[0019] A method for preparing a graded functionalized seedling planting substrate made from recycled aggregates of construction waste in urban renewal includes the following steps:

[0020] S1. Sorting and removing impurities from construction waste generated during urban renewal, crushing and screening out recycled coarse aggregate with a particle size of 5~15mm and recycled fine aggregate with a particle size of 0.5~5mm, rinsing and drying until the moisture content is ≤5%;

[0021] S2. The recycled coarse aggregate and recycled fine aggregate are subjected to three-layer gradient functionalization modification, and then cured at 25~35℃ and 60~80% relative humidity for 2~3 days to obtain gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate.

[0022] S3. Sieve the decomposed organic matter to remove impurities, crush the bentonite and pass it through a 100-mesh sieve, and add water to the dual-response water-retaining factor at a mass ratio of (1:50) to (1:80) to fully absorb water and expand.

[0023] S4. Add the gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate obtained in step S2, as well as the decomposed organic matter, bentonite, dual-response water-retaining factor, slow-release nutrient precursor, calcium source and chitosan treated in step S3 into a mixer according to the ratio, and dry mix for 5-8 minutes; add the compound microbial agent, continue to mix for 8-12 minutes and spray water to the mixture until the moisture content is 30-40% to obtain the mixed matrix;

[0024] S5. Place the mixed substrate in a ventilated stacking area and mature it for 3-5 days under forced ventilation. Control the stack temperature to 30-40℃. Turn the stack over once a day during this period to obtain the seedling planting substrate.

[0025] Furthermore, the specific methods for gradient functionalization modification are as follows:

[0026] a. Place the recycled coarse aggregate and recycled fine aggregate separately in a vacuum impregnation tank, evacuate to -0.08~-0.095MPa, and maintain for 10~15 minutes; inject a slow-release nutrient precursor solution with a mass concentration of 15~25%, and maintain vacuum at room temperature (20~30℃) for 5~10 minutes; restore normal pressure and soak for 2~4 hours; remove and drain, and dry and solidify at 40~60℃ to obtain the inner layer load aggregate;

[0027] b. Intermediate layer coating: Mix sulfur powder and gypsum powder in a mass ratio of (1:2) to (1:3), add water to make a slurry, spray it on the surface of the inner layer load aggregate, control the coating thickness to 0.5 to 2 mm, and dry and cure at 50 to 70°C to obtain the intermediate layer coated aggregate.

[0028] c. Outer layer spraying: Dissolve chitosan in 1-2% acetic acid solution to prepare 1-2% chitosan solution, add biochar, the mass ratio of chitosan to biochar is (1:2)~(1:3), stir to form a composite gel, spray it on the surface of the middle layer of coated aggregate, control the spraying thickness to 0.1~0.5mm, and air dry at room temperature to obtain gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate.

[0029] The present invention has the following beneficial effects:

[0030] 1. This invention solves the problems of high alkalinity, insufficient organic matter content, and poor water and fertilizer retention capacity of recycled aggregate. It utilizes the porous structure of recycled aggregate to make it a nutrient carrier, which is released slowly during the seedling growth process. The seedling survival rate reaches 96.8%, and the growth rate of plant height and the dry weight of new roots are improved.

[0031] 2. Through three-layer gradient functionalization modification, the dissolution channels of alkaline substances are partially blocked and the dissolved alkaline substances are continuously neutralized, thereby achieving the reduction of alkalinity and pH stability of the matrix.

[0032] 3. It adopts a dual-response water-retaining factor that is sensitive to both temperature and pH, releasing water under high temperature or alkaline conditions, thereby achieving automatic water replenishment at high temperatures and water dilution at high pH, ​​avoiding water stress.

[0033] 4. This invention can dispose of large quantities of construction waste, enabling rapid in-situ transformation in urban renewal scenarios. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the method steps of the present invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0036] Example 1:

[0037] A graded functionalized seedling planting substrate made from recycled construction waste aggregate for urban renewal, comprising the following components:

[0038] 50 kg of graded functionalized recycled coarse aggregate, 30 kg of graded functionalized recycled fine aggregate, 5 kg of corn straw biochar, 20 kg of decomposed organic matter, 6 kg of bentonite, 3 kg of N-isopropylacrylamide-acrylic acid copolymer (dual-responsive water-retaining factor), 1.5 kg of compound microbial agent, 4 kg of urea-formaldehyde condensate (slow-release nutrient precursor), 2 kg of calcium lactate (calcium source), and 1 kg of chitosan.

[0039] Among them, both the gradient functionalized recycled coarse aggregate and the gradient functionalized recycled fine aggregate have a three-layer gradient functionalized structure from the inside out:

[0040] The inner layer is a porous nutrient-loaded layer, consisting of recycled aggregate and slow-release nutrient precursors loaded in its internal pores. The organic matter content of the recycled aggregate itself is less than 1%, lacking nutrient supply capacity. However, the recycled aggregate has a porous structure. By extracting the air from its pores and pressing in the slow-release nutrient precursors, the slow-release nutrient precursors can be stored in the pores of the aggregate and released slowly during the seedling growth process.

[0041] The middle layer is a pH buffer layer, including an acidic slow-release material layer coated on the aggregate surface. This acidic slow-release material is a composite of sulfur powder and gypsum powder in a 1:2 mass ratio. Components such as cement in recycled aggregates contain alkaline substances like calcium hydroxide, which slowly dissolve upon contact with water, leading to a persistently high pH value in the matrix. Simply mixing acidic materials like sulfur powder into the matrix for one-time neutralization does not solve the problem of continuous alkaline release. By directly coating the aggregate surface with the sulfur powder and gypsum powder composite, a 0.5-2mm thick physical barrier is formed, preventing the diffusion of alkaline substances. Simultaneously, the sulfur powder slowly oxidizes to sulfuric acid under microbial action, while the gypsum powder slowly hydrolyzes, releasing calcium and sulfate ions. Together, these provide a continuous supply of hydrogen and sulfate ions, neutralizing the alkaline substances seeping from the aggregate over a long period.

[0042] The outer layer is a bioactive interface layer, including a chitosan-biochar composite gel layer sprayed on the outermost layer. Chitosan is a natural cationic polysaccharide with good biocompatibility, film-forming properties, and antibacterial activity, which can promote root cell adhesion and extension. Biochar has a huge specific surface area and abundant surface functional groups (carboxyl, hydroxyl, phenolic hydroxyl, etc.), which can efficiently adsorb cationic nutrients and prevent nutrient leaching. The gel layer formed by the chitosan and biochar composite has three functions: adsorption and fertilizer retention, microbial colonization carrier, and root affinity interface.

[0043] The compound microbial agent contains urease-producing strains with an effective viable count ≥5×10⁻⁶. 8 CFU / g. Other strains of the compound microbial agent can be combined as needed, such as a combination of Bacillus subtilis + Bacillus amyloliquefaciens + Bacillus belyceae + Bacillus pasteurellii (urease-producing strain), which can prevent diseases and promote root growth. Bacillus pasteurellii produces urease through metabolism. Urease hydrolyzes and slowly releases urea-formaldehyde condensate to release urea. Urea is further hydrolyzed to generate carbonate ions. The carbonate ions combine with calcium ions in the calcium source to form calcite precipitate. The calcite precipitate fills some of the microcracks on the surface of the recycled aggregate and encapsulates alkaline substances, realizing in-situ mineralization and alkali reduction of MIP, and preventing alkaline substances from dissolving too quickly.

[0044] The three-layered gradient functional structure utilizes the concentration gradient as a natural driving force for nutrient release. When the outer root system absorbs nutrients, causing a decrease in the concentration of nutrients in the outer layer, nutrients from the inner layer will automatically, slowly, and continuously replenish the outer layer. Furthermore, the release rate automatically matches the plant's absorption rate; the faster the plant absorbs, the greater the concentration difference, and the faster the release; conversely, as plant absorption slows, the concentration difference decreases, and the release slows. This slow-release mechanism achieves precise nutrient supply.

[0045] A copolymer of N-isopropylacrylamide and acrylic acid is used as a dual-responsive water-retaining agent, exhibiting dual temperature and pH response characteristics. When the temperature is below 25°C, it fully absorbs water and swells to store water, while when the temperature is above 30°C and water replenishment is needed, it moderately shrinks and releases water. Simultaneously, it fully absorbs water in neutral / slightly acidic environments and shrinks and releases water in alkaline environments to reduce water stress on the roots at high pH levels.

[0046] Example 2:

[0047] A method for preparing a graded functionalized seedling planting substrate made from recycled aggregates of construction waste in urban renewal includes the following steps:

[0048] S1. Sorting the construction waste generated during urban renewal to remove impurities such as metal, plastic, wood, and gypsum; after crushing, separating recycled coarse aggregate with a particle size of 5~15mm and recycled fine aggregate with a particle size of 0.5~5mm, rinsing and drying until the moisture content is ≤5%.

[0049] Powder with a particle size of less than 0.5 mm can be further pulverized into recycled micro-powder with a mesh size of less than 100 to replace part of the bentonite. This recycled micro-powder has certain water retention and binding properties, although slightly weaker than bentonite, but it is less expensive. The recycled micro-powder has a certain alkalinity, so it needs to be de-alkaliened. Biochar can be mixed with the recycled micro-powder, then soaked in water and piled up for 3-5 days to de-alkalienate it.

[0050] S2. The recycled coarse aggregate and recycled fine aggregate are subjected to three-layer gradient functionalization modification, and then cured for 2-3 days at 25-35℃ and 60-80% relative humidity to obtain gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate.

[0051] The specific methods for gradient functionalization modification are as follows:

[0052] a. Inner Layer Loading: Recycled coarse aggregate and recycled fine aggregate are placed in vacuum impregnation tanks, and a vacuum is drawn to -0.09 MPa and maintained for 12 minutes to remove air from the pores inside the aggregate. A 20% urea-formaldehyde condensate solution is injected under vacuum and maintained at 20~30℃ for 8 minutes. The pressure is restored to normal and the impregnation continues for 3 hours to allow the urea-formaldehyde condensate solution to fully penetrate into the pores inside the aggregate under pressure difference. The excess liquid on the surface is drained, and the aggregate is dried and solidified at 50℃, so that the urea-formaldehyde condensate is loaded in solid form into the pores inside the aggregate, resulting in inner layer loaded aggregate.

[0053] b. Intermediate coating: Mix sulfur powder and gypsum powder at a mass ratio of 1:2.5, add water to make a slurry (solid content of about 50%), put the inner layer loaded aggregate into a coating machine, and spray the intermediate coating slurry evenly on the surface of the aggregate at a speed of 50 r / min, with a coating thickness of about 1 mm. Dry and cure at 60℃ to obtain intermediate coated aggregate;

[0054] c. Outer layer spraying: Chitosan is dissolved in a 1.5% acetic acid solution to prepare a 1.5% chitosan solution. Biochar is added to the chitosan solution at a chitosan to biochar mass ratio of 1:2.5. The mixture is stirred to form a composite gel. The middle layer of aggregate is placed in a coating machine and the composite gel is evenly sprayed onto the aggregate surface at a speed of 40 r / min, with the coating thickness controlled at about 0.3 mm. The aggregate is then air-dried at room temperature to obtain gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate.

[0055] S3. Raw material processing: The decomposed organic matter is sieved to remove impurities, and the bentonite is crushed and passed through a 100-mesh sieve. Take 3 kg of the copolymer of N-isopropylacrylamide and acrylic acid, add water at a mass ratio of 1:60, and allow it to fully absorb water and swell to obtain a water-retaining gel;

[0056] S4. Take 50 kg of gradient functionalized recycled coarse aggregate and 30 kg of gradient functionalized recycled fine aggregate obtained in step S2, as well as 20 kg of decomposed organic matter, 6 kg of bentonite, water-retaining gel, 4 kg of urea-formaldehyde condensate, 2 kg of calcium lactate and 1 kg of chitosan treated in step S3, and put them into a mixer. Dry mix for 6 minutes. Add 1.5 kg of compound microbial agent, continue mixing for 10 minutes and spray water to add water until the moisture content is 35% to obtain a mixed matrix.

[0057] S5. Place the mixed substrate in a well-ventilated stacking area and pile it up. Use bottom perforated pipes for forced ventilation, with a ventilation volume of 0.8m³. 3 / min·m 3 The substrate is controlled at a temperature of 35℃ and matured for 4 days under forced ventilation, with the pile turned over once a day during the period. After the maturation is completed, the seedling planting substrate is obtained.

[0058] Example 3:

[0059] This embodiment provides a gradient functionalized seedling planting substrate made from recycled aggregates of construction waste for urban renewal and its preparation method. The composition of the seedling planting substrate is basically the same as that in Example 1, and the preparation method is basically the same as that in Example 2. The difference is that the slow-release nutrient precursor is ammonium polyphosphate and the calcium source is calcium chloride.

[0060] Comparative Example 1:

[0061] The seedling planting substrate of Example 1 was used for planting London plane trees, and a comparative experiment was conducted. Each group of experimental seedlings consisted of 30 plants, and the experimental period was 6 months.

[0062] Experimental group: The seedling planting substrate was the same as in Example 1;

[0063] Control group 1: Conventional substrate was used as the seedling planting substrate, with the composition of garden soil: peat moss: river sand: decomposed organic matter = 5:3:2:2;

[0064] Control group 2: The seedling planting substrate used was recycled coarse aggregate and recycled fine aggregate that had not undergone three-layer gradient functionalization modification. Other components of the substrate were the same as in Example 1.

[0065] Table 1: Comparative Experiment of Planting in Different Substrates

[0066]

[0067] As shown in Table 1, the survival rates of the seedlings in the three groups were similar. The recycled aggregate in control group 2 did not undergo three-layer gradient functionalization modification, resulting in the continuous dissolution of alkaline substances and large pH fluctuations. The pH was alkaline at 6 months, and the slow-release properties of nutrients were low, with an effective nutrient slow-release period of only 28 days, which led to a slightly lower survival rate than the other two groups.

[0068] All indicators in the experimental group were superior to those in control group 1. The slow-release nutrient performance of the experimental group ensured that the seedlings could continuously obtain nutrients. In contrast, control group 1 experienced insufficient nutrient supply after about 30 days, leading to slowed seedling growth. The outer chitosan-biochar gel layer of the experimental group provided a good adhesion interface for the roots, and coupled with a suitable pH environment, the number of new roots was significantly higher than that of control group 1. The experimental group used recycled coarse aggregate and recycled fine aggregate that underwent three-layer gradient functionalization modification to neutralize the release of alkaline substances in the aggregate, thereby achieving pH stability at the level of control group 1.

[0069] The above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the design concept of the present invention should fall within the protection scope of the present invention.

[0070] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A graded functionalized seedling planting substrate made from recycled construction waste aggregate for urban renewal, characterized in that, Based on parts by weight, it includes the following components: The mixture consists of 40-60 parts of graded functionalized recycled coarse aggregate, 25-35 parts of graded functionalized recycled fine aggregate, 3-8 parts of biochar, 15-25 parts of decomposed organic matter, 5-10 parts of bentonite, 2-5 parts of dual-response water-retaining factor, 1-3 parts of compound microbial agent, 3-8 parts of slow-release nutrient precursor, 1-3 parts of calcium source, and 0.5-2 parts of chitosan. Among them, both the gradient functionalized recycled coarse aggregate and the gradient functionalized recycled fine aggregate have a three-layer gradient functionalized structure from the inside out: The inner layer is a porous nutrient loading layer, which includes recycled aggregate and its internal pores loaded with slow-release nutrient precursors; The middle layer is a pH buffer control layer, which includes an acidic slow-release material layer coated on the surface of the aggregate; The outer layer is a bioactive interface layer, including a biopolymer-biochar composite gel layer sprayed on the outermost layer.

2. The matrix according to claim 1, characterized in that, The slow-release nutrient precursor is a urea-formaldehyde condensate or ammonium polyphosphate.

3. The matrix according to claim 1, characterized in that, The calcium source is calcium lactate or calcium chloride.

4. The matrix according to claim 1, characterized in that, The dual-responsive water-retaining factor is a copolymer of N-isopropylacrylamide and acrylic acid, exhibiting both temperature and pH responsiveness.

5. The matrix according to claim 1, characterized in that, The compound microbial agent contains urease-producing strains with an effective viable count ≥5×10⁻⁶. 8 CFU / g.

6. The matrix according to claim 1, characterized in that, The acidic slow-release material of the pH buffer control layer is a composite of sulfur powder and gypsum powder, with a mass ratio of sulfur powder to gypsum powder of (1:2) to (1:3).

7. The matrix according to claim 1, characterized in that, The biopolymer of the bioactive interface layer is chitosan, and the mass ratio of chitosan to biochar in the biopolymer-biochar composite gel layer is (1:2) to (1:3).

8. A method for preparing a graded functionalized seedling planting substrate made from recycled construction waste aggregate in urban renewal as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Sorting and removing impurities from construction waste generated during urban renewal, crushing and screening out recycled coarse aggregate with a particle size of 5~15mm and recycled fine aggregate with a particle size of 0.5~5mm, rinsing and drying until the moisture content is ≤5%; S2. The recycled coarse aggregate and recycled fine aggregate are subjected to three-layer gradient functionalization modification, and then cured at 25~35℃ and 60~80% relative humidity for 2~3 days to obtain gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate. S3. Sieve the decomposed organic matter to remove impurities, crush the bentonite and pass it through a 100-mesh sieve, and add water to the dual-response water-retaining factor at a mass ratio of (1:50) to (1:80) to fully absorb water and expand. S4. Add the gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate obtained in step S2, as well as the decomposed organic matter, bentonite, dual-response water-retaining factor, slow-release nutrient precursor, calcium source and chitosan treated in step S3 into a mixer according to the ratio, and dry mix for 5-8 minutes; add the compound microbial agent, continue to mix for 8-12 minutes and spray water to the mixture until the moisture content is 30-40% to obtain the mixed matrix; S5. Place the mixed substrate in a ventilated stacking area and mature it for 3-5 days under forced ventilation. Control the stack temperature to 30-40℃. Turn the stack over once a day during this period to obtain the seedling planting substrate.

9. The preparation method according to claim 8, characterized in that, The specific method for gradient functionalization modification is as follows: a. Place the recycled coarse aggregate and recycled fine aggregate separately in a vacuum impregnation tank, evacuate to -0.08~-0.095MPa, and maintain for 10~15 minutes; inject a slow-release nutrient precursor solution with a mass concentration of 15~25%, and maintain vacuum at room temperature (20~30℃) for 5~10 minutes; restore normal pressure and soak for 2~4 hours; remove and drain, and dry and solidify at 40~60℃ to obtain the inner layer load aggregate; b. Intermediate layer coating: Mix sulfur powder and gypsum powder in a mass ratio of (1:2) to (1:3), add water to make a slurry, spray it on the surface of the inner layer load aggregate, control the coating thickness to 0.5 to 2 mm, and dry and cure at 50 to 70°C to obtain the intermediate layer coated aggregate. c. Outer layer spraying: Dissolve chitosan in 1-2% acetic acid solution to prepare 1-2% chitosan solution, add biochar, the mass ratio of chitosan to biochar is (1:2)~(1:3), stir to form a composite gel, spray it on the surface of the middle layer of coated aggregate, control the spraying thickness to 0.1~0.5mm, and air dry at room temperature to obtain gradient functionalized recycled coarse aggregate and gradient functionalized recycled fine aggregate.

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