Method for improving performance of recycled aggregate concrete

Abstract

The invention discloses a method for improving the performance of recycled aggregate concrete, and belongs to the technical field of building materials. The preparation method comprises the following steps: preparing a modified solution which is compounded by double components of gamma-methacryloxy propyl trimethoxy silane and vinyl triethoxy silane; carrying out surface modification on the recycled aggregate by adopting a graded pre-wetting-spraying composite modification method; preparing a slag powder-fly ash-nano silicon dioxide ternary composite admixture; preparing a concrete mixture according to the mixing ratio; through surface modification of the silane coupling agent and cooperation of the composite admixture, the problems of poor workability and low strength of the concrete caused by high water absorption of the recycled aggregate are effectively solved, the 28d compressive strength of the prepared concrete is improved by 20-30%, the anti-freezing grade reaches F200, and the shrinkage rate is reduced by 15-25%.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and specifically to a method for improving the performance of recycled aggregate concrete. Background Technology

[0002] Traditional methods of construction waste disposal primarily involve dumping and landfilling, which not only consumes vast amounts of land resources but also causes environmental problems such as dust and groundwater pollution. Meanwhile, the natural sand and gravel resources needed for concrete production are becoming increasingly scarce, with some regions already facing a shortage of sand and gravel. Therefore, processing waste concrete into recycled aggregates for use in new concrete production can both realize the resource utilization of construction waste and alleviate the pressure on the supply of natural aggregates, resulting in significant economic and environmental benefits.

[0003] However, recycled aggregates have significant performance disadvantages compared to natural aggregates. The surface of recycled aggregates is covered with a large amount of old cement mortar, which has high porosity and low strength. This results in a significantly higher water absorption rate for recycled aggregates compared to natural aggregates, typically reaching 4-10%, while natural aggregates only absorb 0.5-2%. This high water absorption rate leads to a series of adverse effects: First, the aggregates absorb a large amount of mixing water during the mixing process, reducing the effective water-cement ratio of the paste and worsening the workability of the concrete. Second, the water stored inside the aggregates migrates outwards later, forming pores in the interface transition zone and weakening the bond strength between the aggregates and the paste. Third, the high water absorption rate increases the internal water content of the concrete, making it more susceptible to frost heave damage under freeze-thaw cycles.

[0004] To address the aforementioned shortcomings of recycled aggregates, researchers have proposed various modification methods. Physical modification methods include pre-wetting and mechanical strengthening. Pre-wetting stabilizes the effective water-cement ratio by pre-saturating the aggregate, but it cannot fundamentally solve the problem of insufficient interfacial bonding. Mechanical strengthening removes surface mortar through grinding, which reduces water absorption but increases fine powder content and leads to aggregate loss. Chemical modification methods include carbonation, slurry coating, and coupling agent treatment. Carbonation utilizes the reaction of CO2 with Ca(OH)2 in the old mortar to generate CaCO3, filling pores, but the process is time-consuming and requires significant equipment investment. Slurry coating uses cement slurry or silicate solution to cover the aggregate surface, but its effectiveness is limited and its cost is high.

[0005] In the prior art, Chinese invention patent CN117185747A discloses a high-performance fiber-reinforced recycled concrete, which uses 3-aminopropyltriethoxysilane to impregnate and modify recycled aggregates, and adds components such as zeolite powder and triisopropanolamine. Although this scheme improves the performance of recycled concrete to a certain extent, it has the following shortcomings: First, the amino functional groups have poor stability in alkaline cement paste and are prone to degradation under long-term action; second, the modification effect of a single silane coupling agent is limited, and it is difficult to form a dense organic layer on the surface of the aggregate; third, the impregnation method is inefficient in treating aggregates, and a large amount of coupling agent solution is absorbed inside the aggregates, resulting in waste; fourth, although zeolite powder has pozzolanic activity, its effect on improving the interfacial transition zone is not as significant as that of the slag-fly ash composite system. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a method for improving the performance of recycled aggregate concrete. Through a technical route of surface modification with silane coupling agents and synergistic effects with composite admixtures, it effectively solves the problems of poor workability, low strength, and insufficient durability of concrete caused by the high water absorption rate of recycled aggregates.

[0007] The technical solution adopted in this invention is as follows: A method for improving the performance of recycled aggregate concrete includes the following steps: Step 1: Grading and screening of recycled aggregates: After crushing the construction waste concrete, it is graded by vibrating screen to obtain recycled coarse aggregate with a particle size of 5-25mm and recycled fine aggregate with a particle size of 0.15-4.75mm.

[0008] Step 2: Preparation of silane coupling agent modified solution: Mix γ-methacryloxypropyltrimethoxysilane and vinyltriethoxysilane at a mass ratio of 1:0.3-0.8, add to a mixed solvent of anhydrous ethanol and deionized water, the total concentration of silane coupling agent is 1-3wt%, and the volume ratio of anhydrous ethanol to deionized water in the solvent is 4:1-6:1. Hydrolyze and activate at 40-60℃ for 20-40 min to obtain the modified solution.

[0009] Step 3: Surface modification treatment of recycled aggregate: The surface of recycled aggregate is modified by a graded pre-wetting-spraying composite modification method. First, the recycled aggregate is pre-wetted until the surface is saturated and the interior is dry. Then, the modification solution obtained in step 2 is uniformly sprayed onto the surface of the recycled aggregate. The spraying amount is 0.5-1.5% of the mass of the recycled aggregate. After spraying, it is dried at 60-80℃ for 30-60 minutes to obtain modified recycled aggregate.

[0010] Step 4: Preparation of composite admixture: Combine slag powder, fly ash and nano silica in a mass ratio of 60-70:25-35:2-5 to obtain composite admixture.

[0011] Step 5: Concrete Mixture Preparation: Prepare the concrete mixture according to the following mass proportions: 800-1100 parts modified recycled coarse aggregate, 400-600 parts modified recycled fine aggregate, 220-280 parts cement, 60-100 parts composite admixture, 3-6 parts polycarboxylate superplasticizer, 2-4 parts early-strength accelerator, and 140-180 parts water. First, dry mix the modified recycled aggregate with cement and composite admixture for 60-90 seconds. Then, dissolve the polycarboxylate superplasticizer and early-strength accelerator in water and add them to the dry mix. Wet mix for 120-180 seconds to obtain the recycled aggregate concrete mixture.

[0012] Step 6, Molding and Curing: Pour the mixture into shape, cover and cure for 24 hours, then demold and cure for 28 days to obtain recycled aggregate concrete with improved performance.

[0013] Compared with the prior art, the present invention has the following beneficial effects: This invention employs a two-component silane coupling agent system composed of γ-methacryloxypropyltrimethoxysilane and vinyltriethoxysilane. The two silane coupling agents undergo a condensation reaction on the aggregate surface to form an interpenetrating network structure. The methacryloxy functional group of γ-methacryloxypropyltrimethoxysilane exhibits good alkali resistance and remains stable in the strongly alkaline environment of cement paste; the vinyl functional group of vinyltriethoxysilane can undergo free radical polymerization with methacryloxy groups to form a three-dimensional cross-linked network, significantly improving the density and durability of the organic layer. This invention uses a graded pre-wetting-spraying composite modification method to replace the traditional impregnation method. The pre-wetting step wets the aggregate surface while keeping the interior dry, and the spraying method has the advantages of high processing efficiency and low coupling agent dosage. This invention uses a ternary composite admixture system of slag powder, fly ash, and nano-silica. This ternary composite system combines rapid early strength development, continuous later strength growth, and excellent workability. The recycled aggregate concrete prepared by the method of this invention has a 28-day compressive strength of 40-55 MPa, which is 20-30% higher than that of unmodified recycled aggregate concrete; the freeze-thaw resistance grade reaches F200; and the drying shrinkage rate is reduced by 15-25%. Detailed Implementation

[0014] The present invention will be further described in detail below with reference to specific embodiments.

[0015] The present invention provides a method for improving the performance of recycled aggregate concrete, comprising six main steps: Step 1: grading and screening the recycled aggregate to obtain recycled coarse aggregate and recycled fine aggregate; Step 2: preparing a two-component silane coupling agent modification solution; Step 3: surface modification of the recycled aggregate using a graded pre-wetting-spraying composite modification method; Step 4: preparing a ternary composite admixture of slag powder, fly ash, and nano-silica; Step 5: mixing the modified recycled aggregate with cement, composite admixture, water-reducing agent, early-strength agent, and water according to the mix proportion; Step 6: molding and curing, ultimately obtaining recycled aggregate concrete with improved performance.

[0016] The modification mechanism of silane coupling agents is as follows: After hydrolysis and activation, γ-methacryloxypropyltrimethoxysilane and vinyltriethoxysilane undergo dehydration condensation reaction with the hydroxyl groups of the old mortar layer on the surface of recycled aggregate to form Si-O-Si covalent bonds. At the same time, the organic functional groups of the two silane molecules form a cross-linked network structure through free radical copolymerization, forming a dense organic modified layer on the surface of the aggregate. This modified layer reduces the water absorption rate of the aggregate on the one hand, and enhances the interfacial adhesion between the aggregate and the fresh cement paste on the other hand.

[0017] The specific specifications and sources of the raw materials used in this invention are as follows: Recycled aggregates are derived from waste concrete generated during building demolition or road renovation. The original concrete design strength grade should be no less than C25. After coarse crushing by a jaw crusher and fine crushing by an impact crusher, the waste concrete is graded by a vibrating screen to obtain recycled aggregates of different particle sizes. The particle size range of the recycled coarse aggregate is 5-25mm. Referring to GB / T 25177-2010 "Recycled Coarse Aggregate for Concrete" standard, its apparent density should be no less than [amount missing]. The bulk density should not be less than The crushing index should not exceed 12%, the water absorption rate should range from 4-8%, the content of needle-like and flaky particles should not exceed 10%, the mud lump content should not exceed 0.5%, and the mud content should not exceed 2%. The particle size range of the recycled fine aggregate is 0.15-4.75mm. Referring to GB / T 25176-2010 "Recycled Fine Aggregate for Concrete and Mortar" standard, its apparent density should not be less than [missing value]. The bulk density should not be less than The fineness modulus ranges from 2.3 to 3.0, the water absorption rate ranges from 6 to 10%, and the mud content should not exceed 5%.

[0018] γ-Methacryloxypropyltrimethoxysilane, trade name KH570, molecular formula is It has a molecular weight of 248.35, is a colorless and transparent liquid, and has a density of [missing information]. (25℃), refractive index 1.429, boiling point 255℃, flash point 135℃, purity not less than 98%. The organic functional group of this silane coupling agent is methacryloyloxy, which can undergo free radical polymerization with organic compounds containing unsaturated double bonds; the inorganic functional group is trimethoxysilyl group, which, after hydrolysis, generates silanol groups that can undergo condensation reactions with hydroxyl groups on the surface of inorganic substrates to form covalent bonds. The methacryloyloxy functional group of this silane coupling agent exhibits better stability in alkaline environments than amino-type silane coupling agents, making it suitable for use in cement-based material systems.

[0019] Vinyltriethoxysilane, trade name A151, molecular formula is It has a molecular weight of 190.31, is a colorless and transparent liquid, and has a density of [missing information]. (25℃), refractive index 1.395, boiling point 161℃, flash point 41℃, purity not less than 97%. The organic functional group of this silane coupling agent is vinyl, exhibiting high reactivity and capable of addition polymerization with various compounds containing unsaturated double bonds; the inorganic functional group is triethoxysilane, with a slightly slower hydrolysis rate than trimethoxysilane, but the hydrolysis product is also a silanol group. The presence of vinyl groups gives this silane excellent crosslinking ability; when used in combination with KH570, the unsaturated double bonds of both can undergo free radical copolymerization to form a three-dimensional network structure.

[0020] Anhydrous ethanol is of analytical grade, with a purity of not less than 99.5% and a water content of not more than 0.5%. As a solvent for silane coupling agents, anhydrous ethanol can promote the uniform dispersion of silane molecules. At the same time, as a co-solvent in the hydrolysis reaction, it controls the rate of hydrolysis and avoids excessive hydrolysis and condensation of silane molecules to form insoluble polymers.

[0021] The conductivity of deionized water should not exceed [value missing]. The pH value is 6.5-7.5. Deionized water provides water molecules for the hydrolysis reaction of silane coupling agents, causing the alkoxy groups of silane molecules to hydrolyze and generate silanol groups.

[0022] The blast furnace slag powder is S95 grade granulated blast furnace slag powder, conforming to GB / T 18046-2017 standard "Granulated Blast Furnace Slag Powder for Cement, Mortar and Concrete". Its chemical composition mainly includes: Content 35-45%, Content 30-38%, Content 10-18%, Content not more than 14%, Content not exceeding 4%. Physical performance indicators: specific surface area not less than... The density is The fluidity ratio should be no less than 95%, the 7-day activity index no less than 75%, and the 28-day activity index no less than 95%. The glass content of the slag powder should be no less than 85%. Glass is the main source of activity in slag powder; it can undergo depolymerization under the action of an alkaline activator, releasing... , , Plasma participates in the hydration reaction.

[0023] The fly ash is Class II fly ash, conforming to GB / T 1596-2017 "Fly Ash for Cement and Concrete" standard. Its chemical composition mainly includes: Content not less than 38%, Content not less than 10%, Content not more than 15%, Content not more than 10% (low calcium ash). Content not exceeding 3%. Physical performance indicators: fineness (45μm sieve residue) not exceeding 25%, water requirement ratio not exceeding 105%, loss on ignition not exceeding 8%, and moisture content not exceeding 1%. Fly ash particles are mostly spherical glass microspheres. This spherical morphology allows them to exert a ball-bearing effect in concrete mixtures, improving the workability of concrete. The active components in fly ash... and In an alkaline environment and A volcanic ash reaction occurs, generating CSH gel and hydrated calcium aluminate, which enhances the density and durability of concrete.

[0024] Nano-silica is a white powder product prepared by gas-phase method or precipitation method, with the chemical formula [missing information]. The purity is not less than 99%. Its physical properties include: primary particle size of 15-30 nm and specific surface area of... The bulk density is The pH value is 3.5-4.5 (4% ​​aqueous suspension). Nano-silica, due to its extremely high specific surface area and surface energy, possesses extremely strong chemical activity and can react with... The rapid volcanic ash reaction generates CSH gel. Simultaneously, the nanoparticles act as nuclei for the CSH gel, accelerating the cement hydration process and filling the nanoscale pores between hydration products, significantly improving the microstructure of concrete.

[0025] The cement is P·O 42.5 ordinary Portland cement, conforming to GB 175-2007 "General Portland Cement" standard. Its chemical composition mainly includes: Content 60-67%, Content 19-24%, Content 4-7%, Content 2-4%, Content not more than 5%, Content not exceeding 3.5%, loss on ignition not exceeding 5%, insoluble matter not exceeding 1.5%. Physical performance indicators: specific surface area is... The standard consistency water requirement is 24-30%, the initial setting time is not less than 45 minutes, the final setting time is not more than 600 minutes, the 3-day compressive strength is not less than 17 MPa, and the 28-day compressive strength is not less than 42.5 MPa. The main mineral components of cement include: tricalcium silicate (… ) content 45-60%, dicalcium silicate ( ) content 15-30%, tricalcium aluminate ( ) content 6-12%, tetracalcium aluminoferrite ( Content 6-12%.

[0026] Polycarboxylate superplasticizer is a high-performance polycarboxylate superplasticizer, conforming to the requirements for high-performance superplasticizers in GB 8076-2008 "Concrete Admixtures". Its main technical indicators are: appearance: brownish-yellow liquid; pH value: 6-8; density: [not specified]. The solid content is 40-50%, the water reduction rate is not less than 25%, the bleeding rate is not greater than 100%, the air content is not greater than 6%, the setting time difference (initial setting and final setting) is within ±120 min, and the 28-day compressive strength ratio is not less than 100%. The molecular structure of polycarboxylate superplasticizer consists of a main chain and side chains. The main chain contains anionic groups such as carboxyl and sulfonic acid groups, which can be adsorbed on the surface of cement particles to generate electrostatic repulsion. The side chains are polyether segments such as polyethylene glycol or polypropylene glycol, which form a steric hindrance effect on the surface of cement particles. The two work together to disperse the cement particles.

[0027] The early-strength accelerator is a complex of sodium sulfate and triethanolamine. The sodium sulfate is anhydrous sodium sulfate with a purity of not less than 98%, a white powder with a density of [insert density here]. Triethanolamine is a chemically pure grade liquid with a purity of not less than 98%, a pale yellow viscous liquid with a density of [missing information]. The boiling point is 335℃. The mass ratio of sodium sulfate to triethanolamine in the complex is 8-12:1. Sodium sulfate can react with... and The reaction produces ettringite ( Sodium sulfate accelerates cement setting; at the same time, it can increase the concentration of sodium sulfate in the liquid phase. Ion concentration, promotes Early hydration. Triethanolamine molecules contain three hydroxyl groups and one tertiary amine group, enabling them to complex with... and Ions disrupt the coating layer formed on the surface of cement particles by hydration products, promoting... and Continuous hydration enhances early strength.

[0028] Example 1 A method for improving the performance of recycled aggregate concrete includes the following steps: Step 1: Grading and Screening of Recycled Aggregates: Waste concrete from building demolition is selected; the original concrete design strength grade is C30. The waste concrete is coarsely crushed using a jaw crusher (400mm × 600mm feed opening, output particle size 40-100mm); then finely crushed using an impact crusher, output particle size 5-40mm. The crushed aggregate is then graded using a three-layer vibrating screen with mesh sizes of 25mm, 5mm, and 0.15mm, obtaining recycled coarse aggregate with a particle size of 5-25mm and recycled fine aggregate with a particle size of 0.15-4.75mm. The recycled aggregate is then quality tested; the apparent density of the recycled coarse aggregate is... The bulk density is The crushing index was 10.2%, the water absorption rate was 5.8%, and the content of needle-like and flaky particles was 6.5%; the apparent density of the recycled fine aggregate was... The bulk density is It has a fineness modulus of 2.6 and a water absorption rate of 7.5%.

[0029] Step 2: Preparation of Silane Coupling Agent Modified Solution: Weigh 100g of γ-methacryloxypropyltrimethoxysilane and 50g of vinyltriethoxysilane and place them in a 5000mL beaker. Separately, mix 4000mL of anhydrous ethanol and 1000mL of deionized water to form a mixed solvent. Slowly add the mixed solvent to the beaker containing the silane coupling agent while stirring. The total mass of the silane coupling agent is 150g, the total volume of the solution is approximately 5150mL, the total concentration of the silane coupling agent is approximately 2.9wt%, and the volume ratio of anhydrous ethanol to deionized water in the solvent is 4:1. Transfer the prepared solution to a constant temperature water bath, set the water bath temperature to 50℃, and maintain it under magnetic stirring for 30min for hydrolysis activation. During the hydrolysis activation process, the alkoxy groups of the silane molecules (… and Stepwise hydrolysis generates silanol groups ( The reaction equation is: The hydrolyzed silanol groups exhibit high chemical activity and can undergo condensation reactions with hydroxyl groups on the surface of inorganic substrates. After hydrolysis and activation, the solution changes from clear to slightly turbid, and the pH value decreases slightly from neutral to 5.5-6.0, indicating that the hydrolysis reaction has occurred. The modified solution is then transferred to the storage tank of the spraying equipment for later use.

[0030] Step 3: Surface Modification Treatment of Recycled Aggregates: The recycled coarse aggregate and recycled fine aggregate obtained in Step 1 are separately loaded into a rotary drum-type modification device for surface modification treatment. The drum has an inner diameter of 800 mm, a length of 1500 mm, and a rotation speed of 15 rpm. The inner wall of the drum is equipped with spiral blades, which continuously tumble the aggregates during rotation, ensuring uniform surface treatment. Pre-wetting treatment is performed first: The deionized water atomizing nozzle is activated, with a working pressure of 0.15 MPa and an atomized particle size of 20-50 μm. Deionized water is sprayed to wet the aggregate surface for 4 minutes. By controlling the pre-wetting time, the aggregate surface is kept wet while the interior remains essentially dry, achieving a surface saturation and internal dryness state. At this point, the moisture content of the recycled coarse aggregate is approximately 1.5%, and the moisture content of the recycled fine aggregate is approximately 2.0%, both far below their saturation water absorption rate. A wet surface facilitates full contact between the silane coupling agent and the hydroxyl groups of the old mortar layer on the aggregate surface, while keeping the interior dry avoids waste caused by the coupling agent solution being absorbed by the aggregate interior.

[0031] After pre-wetting, switch to modified solution spraying. Apply the modified solution prepared in step two evenly to the surface of the recycled aggregate using a high-pressure spray gun. The spraying pressure is 0.3 MPa, and the spraying time is 8 minutes. During the spraying process, the roller rotates continuously to ensure uniform coverage of the aggregate surface. The amount of modified solution sprayed is approximately 1.0% of the dry weight of the recycled aggregate, i.e., approximately 10 kg of modified solution is sprayed per 1000 kg of dry aggregate. After spraying, remove the aggregate from the roller and spread it evenly on a stainless steel tray, with a thickness not exceeding 50 mm. Place the tray in a forced-air drying oven, setting the drying temperature to 70℃ and the drying time to 45 minutes. During the drying process, the solvent evaporates, and the silanol groups undergo a dehydration condensation reaction with the hydroxyl groups of the old mortar on the aggregate surface, forming Si-O-Si covalent bonds. The reaction equation is: Simultaneously, condensation polymerization occurs between the two-component silane molecules to form Si-O-Si crosslinks. Furthermore, the methacryloxy group of KH570 and the vinyl group of A151 can undergo free radical-initiated copolymerization under thermal conditions to form an organic crosslinking network. The combined result of these reactions is the formation of an organosilicon-modified layer with a thickness of approximately 100-500 nm on the surface of the recycled aggregate. This modified layer possesses a dual structure of Si-O-Si inorganic framework and organic crosslinking network, forming a strong covalent bond with the aggregate matrix while exhibiting good hydrophobicity and alkali resistance.

[0032] After modification, the recycled aggregates were re-tested for quality. The water absorption rate of the modified recycled coarse aggregate decreased from 5.8% to 3.2%, a reduction of 44.8%; the water absorption rate of the modified recycled fine aggregate decreased from 7.5% to 4.1%, a reduction of 45.3%. The significant reduction in water absorption rate indicates that the modified layer on the aggregate surface effectively sealed the surface pores and prevented water from penetrating into the aggregate interior. The water contact angle of the aggregate surface before and after modification was measured using a contact angle meter. The contact angle of the unmodified recycled aggregate surface was approximately 35°, while the contact angle of the modified aggregate surface increased to 78°, indicating that the aggregate surface changed from hydrophilic to hydrophobic.

[0033] Step 4: Preparation of Composite Admixture: Weigh 650g of S95 grade granulated blast furnace slag powder, 300g of Grade II fly ash, and 50g of nano-silica, and mix them according to a mass ratio of slag powder: fly ash: nano-silica = 65:30:5. Place the above raw materials in a ball mill, using ceramic balls as the grinding medium, with a ball-to-material ratio of 3:1, a rotation speed of 150 rpm, and a grinding and mixing time of 30 minutes. The purpose of grinding and mixing is to ensure that the three components are fully and evenly dispersed, especially to ensure that the nano-silica is evenly distributed between the slag powder and fly ash particles, avoiding agglomeration of nanoparticles due to van der Waals forces. After mixing, remove the mixture to obtain 1000g of composite admixture. Perform quality testing on the composite admixture: apparent density is... Specific surface area is (Due to the contribution of nano-silica, the specific surface area is significantly higher than that of single slag powder), the fluidity ratio is 98%, and the 28-day activity index is 108%.

[0034] Step 5: Preparation of concrete mix: Prepare 1 according to the following mix proportions. Concrete mix composition: 950 kg modified recycled coarse aggregate, 500 kg modified recycled fine aggregate, 250 kg P·O 42.5 ordinary Portland cement, 80 kg composite admixture, 4.5 kg polycarboxylate superplasticizer (45% solids content), 3 kg early-strength accelerator (sodium sulfate: triethanolamine = 10:1), 160 kg water. The calculated water-cement ratio is 160 / (250+80) = 0.48, the total cementitious material weight is 330 kg, and the sand ratio is 500 / (950+500)×100% = 34.5%.

[0035] The mixing process employs a two-stage mixing method: dry mixing followed by wet mixing. First, modified recycled coarse aggregate, modified recycled fine aggregate, cement, and composite admixtures are added to a 60L forced concrete mixer at a blade speed of 30 rpm. The mixer is then dry-mixed for 75 seconds to ensure the cement and composite admixtures evenly coat the aggregate surface, forming a pre-coating layer. This pre-coating layer allows for closer contact between the cement paste and the aggregate surface during subsequent water mixing, facilitating the formation of an interfacial transition zone. Next, polycarboxylate superplasticizer and early-strength accelerator are pre-dissolved in the mixing water and stirred thoroughly to form an admixture aqueous solution. This solution is then evenly poured into the dry mix, and the mixer is wet-mixed for 150 seconds. During wet mixing, water contacts the cementitious materials, initiating a hydration reaction. The polycarboxylate superplasticizer adsorbs onto the cement particle surface, creating a dispersion effect, while the early-strength accelerator promotes early cement hydration. After mixing is complete, the mixer is stopped, and samples are taken for workability testing.

[0036] Slump tests were conducted according to GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures". The measured slump was 180 mm, and the slump loss over time (60 min) was 15 mm. The concrete mixture exhibits good workability and meets the requirements for pumping construction. Observing the appearance of the mixture, the concrete has good cohesiveness, with no bleeding or segregation, uniform aggregate distribution, and a glossy surface.

[0037] Step Six: Molding and Curing: The prepared concrete mixture is poured into 100mm×100mm×100mm cube steel molds, 100mm×100mm×400mm prism steel molds, and 100mm×100mm×515mm prism steel molds respectively for testing compressive strength, frost resistance, drying shrinkage, and other properties. A release agent is applied to the inner wall of the steel mold before pouring. The concrete is poured in two layers, and after each layer, it is vibrated for 20 seconds on a vibrating table at a frequency of 50Hz and an amplitude of 0.5mm. The vibration time should not be too long; the standard is that the concrete surface becomes smooth and no obvious air bubbles rise. After pouring, the surface is smoothed with a scraper and covered with plastic film to prevent moisture evaporation. The specimens are then cured at room temperature (…). After standing at a temperature of ℃ for 24 hours, the specimens are demolded. Handle them gently during demolding to avoid damaging the edges and corners. Mark the demolded specimens with a number using a marker and transfer them to a standard curing room for curing. Standard curing conditions are: temperature... The temperature was set at ℃, and the relative humidity was not lower than 95%. The specimens were cured for 3 days, 7 days, and 28 days, respectively, before performance testing.

[0038] The performance testing methods and results are as follows: Compressive strength testing: The tests were conducted according to GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". A 2000kN microcomputer-controlled electro-hydraulic servo pressure testing machine was used, with a loading speed of 0.5MPa / s. Three specimens were tested at each age, and the arithmetic mean was taken as the compressive strength value at that age. Test results: The 3-day compressive strength was 28.5MPa, reaching 58.6% of the 28-day design strength; the 7-day compressive strength was 38.2MPa, reaching 78.6% of the 28-day design strength; and the 28-day compressive strength was 48.6MPa. The strength development pattern conforms to the strength growth curve of ordinary concrete, and the 28-day strength meets the requirements of C40 concrete strength grade.

[0039] Freeze-thaw resistance test: The rapid freeze-thaw method was used according to GB / T 50082-2009 "Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete". The specimen size was 100mm × 100mm × 400mm. After curing for 28 days, the specimens were removed, the surface moisture was wiped off, and the initial mass and initial dynamic modulus of elasticity were measured. The specimens were then placed in a rapid freeze-thaw testing machine for freeze-thaw cycles, with a freeze-thaw temperature range of -18℃ to +5℃, and each freeze-thaw cycle lasting approximately 4 hours. The specimens were removed every 25 freeze-thaw cycles, and their mass and dynamic modulus of elasticity were measured. Test results: After 100 freeze-thaw cycles, the mass loss rate was 0.8%, and the relative dynamic modulus of elasticity was 92%; after 200 freeze-thaw cycles, the mass loss rate was 1.5%, and the relative dynamic modulus of elasticity was 85%; after 300 freeze-thaw cycles, the mass loss rate was 2.3%, and the relative dynamic modulus of elasticity was 78%. According to the judgment criteria of GB / T 50082-2009, a specimen is considered to have failed when the mass loss rate exceeds 5% or the relative dynamic elastic modulus drops below 60%. In this embodiment, the mass loss rate and relative dynamic elastic modulus of the specimen after 300 freeze-thaw cycles both meet the conditions for continued testing, indicating that the freeze resistance grade reaches F200 or higher.

[0040] Drying shrinkage test: The contact method was used in accordance with GB / T 50082-2009. The specimen size was 100mm×100mm×515mm, and probes were embedded at both ends of the specimen. After curing for 3 days, the specimen was removed, and the initial length was measured as the reference length. Then the specimen was placed in a temperature... ℃, relative humidity Drying shrinkage tests were conducted in a constant temperature and humidity chamber. The specimen lengths were measured at 3, 7, 14, 28, 45, 60, and 90 days of age, and the drying shrinkage rate was calculated. Test results: The 7-day drying shrinkage rate was... The 14-day drying shrinkage rate is The 28-day drying shrinkage rate is The 60-day drying shrinkage rate is The 90-day drying shrinkage rate is Drying shrinkage gradually stabilizes with age.

[0041] Chloride ion penetration resistance test: The rapid chloride ion migration coefficient method (RCM method) was used according to GB / T 50082-2009. The specimen was a cylinder with a diameter of 100 mm and a thickness of 50 mm, which was cured for 28 days before testing. The test voltage was 30 V, and the test duration was 24 hours. After the test, the specimen was split along the axial direction, sprayed with 0.1 mol / L silver nitrate solution for color development, the chloride ion penetration depth was measured, and the chloride ion migration coefficient was calculated. Test results: The chloride ion migration coefficient after 28 days was... According to the evaluation criteria of CCES 01-2004 "Guidelines for Durability Design and Construction of Concrete Structures", the chloride ion migration coefficient is less than [value missing]. This indicates that the concrete has good resistance to chloride ion penetration.

[0042] Example 2 A method for improving the performance of recycled aggregate concrete differs from Example 1 mainly in the source of recycled aggregate, the proportion of silane coupling agent, and the composition of composite admixtures.

[0043] Step 1: Grading and Screening of Recycled Aggregates: Waste concrete from road renovation is selected, with an original concrete design strength grade of C35. After crushing and screening, recycled aggregates are obtained. The crushing index of the recycled coarse aggregate is 9.5%, and the water absorption rate is 4.6%; the fineness modulus of the recycled fine aggregate is 2.4, and the water absorption rate is 6.8%.

[0044] Step 2: Preparation of Silane Coupling Agent Modified Solution: Weigh 100g of γ-methacryloxypropyltrimethoxysilane and 80g of vinyltriethoxysilane, with a KH570 to A151 mass ratio of 1:0.8. Add to a mixed solvent consisting of 4500mL anhydrous ethanol and 900mL deionized water, resulting in a total silane coupling agent concentration of approximately 3.3wt%, and a 5:1 volume ratio of anhydrous ethanol to deionized water in the solvent. Activate the solution by hydrolysis at 55℃ for 35min to obtain the modified solution. A higher A151 ratio is beneficial for forming a denser cross-linked network structure.

[0045] Step 3: Surface modification treatment of recycled aggregate: A graded pre-wetting-spraying composite modification method was adopted. Pre-wetting time was 3 minutes, spraying pressure was 0.35 MPa, spraying time was 6 minutes, and the amount of modification solution sprayed was 0.8% of the dry weight of the recycled aggregate. Drying was carried out at 75℃ for 40 minutes. After treatment, the water absorption rate of recycled coarse aggregate decreased from 4.6% to 2.8%, a reduction of 39.1%; the water absorption rate of recycled fine aggregate decreased from 6.8% to 3.6%, a reduction of 47.1%.

[0046] Step 4: Preparation of composite admixture: Weigh 700g of S95 grade granulated blast furnace slag powder, 250g of Grade II fly ash, and 50g of nano-silica, and mix them evenly. The ratio of slag powder: fly ash: nano-silica is 70:25:5. A higher proportion of slag powder is beneficial to improving the early strength development rate.

[0047] Step 5: Preparation of concrete mix: Prepare 1 according to the following mix proportions. Concrete mix composition: 1000 kg modified recycled coarse aggregate, 450 kg modified recycled fine aggregate, 260 kg P·O 42.5 ordinary Portland cement, 90 kg composite admixture, 5 kg polycarboxylate superplasticizer, 3.5 kg early-strength accelerator, and 155 kg water. The water-cement ratio is 0.44, and the sand ratio is 31%. Dry mixing for 80 seconds and wet mixing for 160 seconds yields a recycled aggregate concrete mix with a slump of 175 mm.

[0048] Step 6, Molding and Curing: Cast into shape and cure for 28 days according to standard.

[0049] Performance test results: 3-day compressive strength is 31.2 MPa, 7-day compressive strength is 41.5 MPa, and 28-day compressive strength is 52.8 MPa. After 300 freeze-thaw cycles, the mass loss rate is 1.9%, the relative dynamic modulus of elasticity retention rate is 82%, and the freeze-thaw resistance grade reaches F250. The 28-day drying shrinkage rate is... The chloride ion migration coefficient at 28 days was... .

[0050] Example 3 A method for improving the performance of recycled aggregate concrete differs from Example 1 mainly in that the recycled aggregate has lower quality and requires a larger amount of modified solution to be sprayed.

[0051] Step 1: Grading and Screening of Recycled Aggregates: Waste concrete from building demolition is selected, with an original concrete design strength grade of C25. After crushing and screening, recycled aggregates are obtained. The crushing index of the recycled coarse aggregate is 11.8%, and the water absorption rate is 7.2%; the fineness modulus of the recycled fine aggregate is 2.8, and the water absorption rate is 9.2%.

[0052] Step 2: Preparation of silane coupling agent modified solution: Weigh 100g of γ-methacryloxypropyltrimethoxysilane and 30g of vinyltriethoxysilane, with a KH570 to A151 mass ratio of 1:0.3. Add to a mixed solvent consisting of 3600mL anhydrous ethanol and 900mL deionized water, with a volume ratio of anhydrous ethanol to deionized water of 4:1. Hydrolyze and activate the solution at 45℃ for 40min to obtain the modified solution.

[0053] Step 3: Surface Modification Treatment of Recycled Aggregate: Due to the high water absorption and large porosity of the raw aggregate, the amount of modification solution sprayed needs to be increased to obtain sufficient modification effect. Pre-wetting time: 5 min; spraying pressure: 0.25 MPa; spraying time: 10 min; modification solution spraying amount: 1.5% of the dry weight of the recycled aggregate. Drying at 65℃ for 60 min. After treatment, the water absorption rate of recycled coarse aggregate decreased from 7.2% to 3.8%, a reduction of 47.2%; the water absorption rate of recycled fine aggregate decreased from 9.2% to 4.9%, a reduction of 46.7%.

[0054] Step 4: Preparation of composite admixtures: Weigh 600g of S95 grade granulated blast furnace slag powder, 350g of Grade II fly ash, and 50g of nano-silica, and mix them evenly. The ratio of slag powder: fly ash: nano-silica is 60:35:5. A higher proportion of fly ash is beneficial to improving the workability of low-quality aggregate concrete.

[0055] Step 5: Preparation of concrete mix: Prepare 1 according to the following mix proportions. Concrete mix composition: 900 kg modified recycled coarse aggregate, 550 kg modified recycled fine aggregate, 240 kg P·O 42.5 ordinary Portland cement, 70 kg composite admixture, 4 kg polycarboxylate superplasticizer, 2.5 kg early-strength accelerator, and 170 kg water. The water-cement ratio is 0.55, and the sand ratio is 38%. The relatively high water-cement ratio and sand ratio are suitable for the characteristics of low-quality aggregates. Dry mixing for 70 seconds and wet mixing for 140 seconds yields a recycled aggregate concrete mix with a slump of 190 mm.

[0056] Step 6, Molding and Curing: Cast into shape and cure for 28 days according to standard.

[0057] Performance test results: 3-day compressive strength is 24.8 MPa, 7-day compressive strength is 33.6 MPa, and 28-day compressive strength is 42.5 MPa. After 300 freeze-thaw cycles, the mass loss rate is 2.8%, the relative dynamic modulus of elasticity retention rate is 75%, and the freeze-thaw resistance grade reaches F200. The 28-day drying shrinkage rate is... The chloride ion migration coefficient at 28 days was... .

[0058] Example 4 A method for improving the performance of recycled aggregate concrete differs from Example 1 in that the amount of composite admixture is adjusted to 100 kg, the amount of cement is adjusted to 230 kg, and the water-cement ratio is maintained at 0.48.

[0059] Performance test results: 3-day compressive strength is 26.2 MPa, 7-day compressive strength is 36.8 MPa, and 28-day compressive strength is 50.2 MPa. After 300 freeze-thaw cycles, the mass loss rate is 2.1%, and the relative dynamic modulus of elasticity retention rate is 80%. The 28-day drying shrinkage rate is... The chloride ion migration coefficient at 28 days was... .

[0060] Comparative Example 1 The difference from Example 1 is that in step three, no silane coupling agent modification treatment is performed, and the original recycled aggregate is used directly.

[0061] Performance test results: 3-day compressive strength is 21.5 MPa, 7-day compressive strength is 30.2 MPa, and 28-day compressive strength is 38.6 MPa. After 300 freeze-thaw cycles, the mass loss rate is 4.8%, and the relative dynamic modulus of elasticity retention rate is 62%. The 28-day drying shrinkage rate is... The chloride ion migration coefficient at 28 days was... .

[0062] The results of Comparative Example 1 show that without silane coupling agent modification, the 28-day compressive strength of the recycled aggregate concrete was only 38.6 MPa, a decrease of 20.6% compared to Example 1; the freeze-thaw resistance was significantly reduced, with the mass loss rate increasing by 109% after freeze-thaw cycles; the drying shrinkage rate increased by 28%; and the resistance to chloride ion penetration decreased by 64%. This is because the high water absorption rate of the unmodified recycled aggregate leads to high porosity and loose structure in the interfacial transition zone, weakening the bond strength between the aggregate and the paste, while also providing channels for chloride ion and water penetration.

[0063] Comparative Example 2 The difference from Example 1 is that in step two, only a single silane coupling agent, γ-methacryloyloxypropyltrimethoxysilane, is used, without the addition of vinyltriethoxysilane.

[0064] Performance test results: 3-day compressive strength is 25.8 MPa, 7-day compressive strength is 35.6 MPa, and 28-day compressive strength is 45.2 MPa. After 300 freeze-thaw cycles, the mass loss rate is 3.2%, and the relative dynamic modulus of elasticity retention rate is 72%. The 28-day drying shrinkage rate is... The chloride ion migration coefficient at 28 days was... .

[0065] The results of Comparative Example 2 show that the modification effect of a single silane coupling agent is not as good as that of the two-component compound system. The 28-day compressive strength is 7.0% lower than that of Example 1, and the antifreeze and antipermeability properties are also reduced. This is because the copolymerization reaction between the two-component silanes can form a denser cross-linked network structure, while a single silane can only form a linear or branched structure, resulting in poor density and durability of the modified layer.

[0066] Comparative Example 3 The difference from Example 1 is that in step two, 3-aminopropyltriethoxysilane is used instead of γ-methacryloyloxypropyltrimethoxysilane, while the other conditions remain the same.

[0067] Performance test results: 3-day compressive strength is 26.2 MPa, 7-day compressive strength is 36.0 MPa, and 28-day compressive strength is 44.8 MPa. After 300 freeze-thaw cycles, the mass loss rate is 3.5%, and the relative dynamic modulus of elasticity retention rate is 70%. The 28-day drying shrinkage rate is... The chloride ion migration coefficient at 28 days was... .

[0068] The results of Comparative Example 3 show that the modification effect of the amino-type silane coupling agent is not as good as that of the methacryloxy-type silane coupling agent, and the 28-day compressive strength is 7.8% lower than that of Example 1. This is because the amino functional group has poor stability in the alkaline cement paste environment and is easily affected by... The reaction of water molecules can lead to a gradual deterioration of the performance of the modified layer.

[0069] Comparative Example 4 The difference from Example 1 is that in step three, the traditional impregnation method is used instead of the graded pre-wetting-spraying composite modification method. The recycled aggregate is completely immersed in the modification solution for 30 minutes, then removed, drained, and dried at 70°C for 45 minutes.

[0070] Performance test results: 3-day compressive strength is 27.0 MPa, 7-day compressive strength is 37.2 MPa, and 28-day compressive strength is 46.8 MPa. After 300 freeze-thaw cycles, the mass loss rate is 2.8%, and the relative dynamic modulus of elasticity retention rate is 75%. The 28-day drying shrinkage rate is... .

[0071] Comparative Example 4, treated by impregnation, showed performance similar to Example 1, but suffered from the following problems: the consumption of silane coupling agent was approximately three times that of the spraying method, significantly increasing costs; the coupling agent solution absorbed inside the aggregate could not effectively participate in the surface modification reaction, resulting in waste; and the aggregate had a high moisture content after impregnation, requiring a longer drying time. Therefore, the graded pre-wetting-spraying composite modification method is superior to the traditional impregnation method in terms of economy and efficiency.

[0072] Comparative Example 5 The difference from Example 1 is that only slag powder is used as an admixture in step four, without the addition of fly ash and nano silica.

[0073] Performance test results: 3-day compressive strength is 23.5 MPa, 7-day compressive strength is 34.8 MPa, and 28-day compressive strength is 47.2 MPa. After 300 freeze-thaw cycles, the mass loss rate is 2.6%, and the relative dynamic modulus of elasticity retention rate is 76%. The 28-day drying shrinkage rate is... The chloride ion migration coefficient at 28 days was... .

[0074] Comparative Example 5 shows that when only slag powder is used, the 3-day early strength is reduced by 17.5% compared to Example 1. This is because the pozzolanic reaction of slag powder is relatively slow, lacking the nucleation and promoting effects of nano-silica, as well as the filling effect of fly ash. The ternary composite admixture system can balance early strength and later durability.

[0075] Comparative Example 6 The difference from Example 1 is that no early-strength accelerator is added in step five.

[0076] Performance test results: 3-day compressive strength is 22.8 MPa, 7-day compressive strength is 35.0 MPa, and 28-day compressive strength is 47.8 MPa. After 300 freeze-thaw cycles, the mass loss rate is 2.5%, and the relative dynamic modulus of elasticity retention rate is 77%.

[0077] Comparative Example 6 shows that without the addition of an early-strength accelerator, the 3-day early strength is reduced by 20% compared to Example 1, but the 28-day strength is essentially unaffected. For engineering applications requiring early strength, an early-strength accelerator is necessary.

[0078] Based on the results of the above embodiments and comparative examples, the mechanism of action of the present invention can be analyzed in depth.

[0079] The core of this invention lies in the organic combination of two major technical routes: surface modification with silane coupling agents and synergistic effects with composite admixtures. From a molecular level perspective, after hydrolysis and activation, the silanol groups of γ-methacryloyloxypropyltrimethoxysilane and vinyltriethoxysilane undergo a dehydration condensation reaction with the hydroxyl groups of the old mortar on the surface of recycled aggregate, forming Si-O-Si covalent bonds with a bond energy of approximately 460 kJ / mol, far exceeding van der Waals forces and hydrogen bonds. This covalent bonding ensures a strong bond between the modified layer and the aggregate matrix. Simultaneously, the organic functional groups of the two silanes can undergo free radical copolymerization to form a three-dimensional cross-linked network with a cross-linking density of approximately [missing information]. The hydrophobicity is significantly higher than that of linear or branched structures formed by a single silane. This cross-linked network endows the modified layer with excellent hydrophobicity and durability.

[0080] At the microstructural level, the formation of the silane coupling agent modified layer alters the structural characteristics of the interfacial transition zone between the recycled aggregate and the freshly mixed slurry. The interfacial transition zone of the unmodified recycled aggregate is approximately 50-80 μm thick, and this region has high porosity. Crystal enrichment and low CSH gel content are weak points in concrete. After modification with silane coupling agents, the organic layer on the aggregate surface reduces water absorption, decreasing porosity formed in the interfacial transition zone due to internal water migration. Furthermore, the organic functional groups in the modified layer can chemically or physically bond with cement hydration products, enhancing interfacial adhesion. Nano-silica in the composite admixture acts as nuclei for CSH gel, promoting the precipitation of hydration products in the interfacial transition zone and effectively filling pores. The pozzolanic reaction of slag powder and fly ash consumes the CSH gel enriched in the interfacial transition zone. This process generates more CSH gel, resulting in a denser structure in the interfacial transition zone. After modification, the thickness of the interfacial transition zone is reduced to 30-40 μm, and the porosity is reduced by approximately 40%.

[0081] Significant synergistic effects exist among the various technical characteristics. The synergistic effect of silane coupling agent modification and composite admixtures is manifested in the following ways: silane coupling agents improve aggregate surface properties, while composite admixtures improve the properties of the paste and interfacial transition zone. Together, they act on the aggregate-paste interface, achieving comprehensive optimization of the interfacial structure. The synergistic effect of two-component silane coupling agents is manifested in the following ways: KH570 provides alkali-resistant functional groups and copolymerization active sites, while A151 provides cross-linking structures and hydrophobicity. The network structure formed by their copolymerization exhibits superior performance compared to a single component. The synergistic effect of ternary composite admixtures is manifested in the following ways: slag powder provides high activity and low heat of hydration, fly ash provides improved workability and increased later-stage strength, and nano-silica provides nucleation and nano-filling effects. The synergistic effect of these three elements achieves a comprehensive improvement in concrete performance.

[0082] Compared with existing technologies, the main differences in the mechanism of this invention are reflected in the following aspects: A two-component silane coupling agent system is used instead of a single silane coupling agent, forming a cross-linked network structure through copolymerization, resulting in significantly better density and durability of the modified layer compared to existing technologies; a graded pre-wetting-spraying composite modification method is used instead of the traditional impregnation method, achieving a dual improvement in modification efficiency and economy; a slag-fly ash-nano silica ternary composite admixture system is used instead of a single admixture, achieving a comprehensive balance of early strength, later strength, and workability; and an early-strength accelerator is added to compensate for the influence of the admixture on early strength, ensuring early demolding requirements in engineering applications. These mechanistic innovations are the fundamental reasons why this invention achieves a 20-30% improvement in overall performance, a freeze-thaw resistance rating of F200, and a 15-25% reduction in shrinkage.

[0083] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A method for improving the performance of recycled aggregate concrete, characterized by, The method comprises the following steps: Step one, grading and screening of recycled aggregate: after crushing the construction waste concrete, the recycled coarse aggregate with a particle size of 5-25mm and the recycled fine aggregate with a particle size of 0.15-4.75mm are obtained by grading through a vibrating screen; Step two, preparation of silane coupling agent modification solution: γ-methacryloxypropyltrimethoxysilane and vinyltriethoxysilane are compounded at a mass ratio of 1:0.3-0.8, and then added into a mixed solvent of anhydrous ethanol and deionized water, the total concentration of the silane coupling agent is 1-3wt%, the volume ratio of anhydrous ethanol to deionized water in the solvent is 4:1-6:1, and the solution is hydrolyzed and activated at 40-60℃ for 20-40min to obtain the modified solution; Step three, surface modification treatment of recycled aggregate: the surface of the recycled aggregate is modified by using a hierarchical pre-wetting-spraying composite modification method, the recycled aggregate is pre-wetted to a surface-saturated and internal-dry state, and then the modified solution obtained in step two is uniformly sprayed on the surface of the recycled aggregate, the spraying amount is 0.5-1.5% of the mass of the recycled aggregate, and the modified recycled aggregate is obtained by drying at 60-80℃ for 30-60min after spraying is completed; Step four, preparation of composite admixture: the composite admixture is obtained by compounding the slag powder, fly ash and nano silicon dioxide at a mass ratio of 60-70:25-35:2-5; Step five, preparation of concrete mixture: the concrete mixture is prepared according to the following mass fractions: modified recycled coarse aggregate 800-1100 parts, modified recycled fine aggregate 400-600 parts, cement 220-280 parts, composite admixture 60-100 parts, polycarboxylic acid water reducing agent 3-6 parts, early strength type accelerator 2-4 parts, and water 140-180 parts; the modified recycled aggregate, cement and composite admixture are dry-mixed for 60-90s, the polycarboxylic acid water reducing agent and early strength type accelerator are dissolved in water and added to the dry-mixed material, and the wet-mixing is performed for 120-180s to obtain the recycled aggregate concrete mixture; Step six, molding and curing: the mixture is poured and molded, demolded after film curing for 24h, and standard cured for 28d to obtain the recycled aggregate concrete with improved performance.

2. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, In step one, the crushing index of the recycled coarse aggregate is not more than 12%, and the water absorption rate is 4-8%; the fineness modulus of the recycled fine aggregate is 2.3-3.0, and the water absorption rate is 6-10%.

3. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, In step two, the molecular formula of the γ-methacryloyloxypropyltrimethoxysilane is , and the purity is not less than 98%; the molecular formula of the vinyltriethoxysilane is , and the purity is not less than 97%.

4. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, In step three, the hierarchical pre-wetting-spraying composite modification method specifically comprises the following steps: the recycled aggregate is placed in a rotating drum, deionized water is sprayed on the surface of the aggregate by using an atomizing nozzle to make the aggregate surface wet, the pre-wetting time is controlled to be 3-5min, and the water content of the aggregate reaches a surface-saturated and internal-dry state; then the spraying of the modified solution is switched on, the spraying pressure is 0.2-0.4MPa, and the spraying time is 5-10min.

5. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, In step four, the slag powder is S95 grade granulated blast furnace slag powder, with specific surface area not less than , and 28d activity index not less than 95%; the fly ash is II grade fly ash, with fineness not more than 25% and loss on ignition not more than 8%; the nano-silica has particle size of 15-30 nm and specific surface area of .

6. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, In step five, the water reducing rate of the polycarboxylic acid water reducing agent is not less than 25%, and the solid content is 40-50%; the early strength type accelerator is a composite of sodium sulfate and triethanolamine, and the mass ratio of sodium sulfate to triethanolamine is 8-12:

1.

7. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, In step five, the cement is P·O 42.5 ordinary portland cement, with a specific surface area of , an initial setting time of not less than 45 min, and a final setting time of not more than 600 min.

8. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, In step six, the standard curing conditions are a temperature of 23°C and a relative humidity of not less than 95%. 23°C and a relative humidity of not less than 95%.

9. The method for improving the performance of recycled aggregate concrete according to claim 1, characterized by, The 28d compressive strength of the recycled aggregate concrete is 40-55MPa, which is increased by 20-30% compared with that of the unmodified recycled aggregate concrete; the frost resistance grade reaches F200, and the mass loss rate after 300 freeze-thaw cycles is not more than 3%; and the 28d dry shrinkage rate is reduced by 15-25% compared with that of the unmodified recycled aggregate concrete.

10. The method for improving the performance of recycled aggregate concrete according to any one of claims 1 to 9, characterized in that, The recycled aggregate concrete prepared by the method is used for building structure engineering or road engineering.

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