Preparation method of building solid waste modified recycled aggregate

Abstract

The invention discloses a preparation method of building solid waste modified recycled aggregate, which comprises the following steps: crushing and screening building solid waste into 5-31.5 mm recycled aggregate, and pre-curing for 15-25 minutes in a 0.3-0.5 MPa carbon dioxide environment; the content of SiO2 is 20-30%, the pH value is 8-10, the doping amount of nano Al2O3 is 0.5-1%, and the aggregate is soaked in the silicate sol for 30-60 min and then dried at the temperature of 80-100 DEG C for 2-4 h to form a silica gel layer; spraying and coating by using a gradient composite mineral admixture, and finally curing for 7-14 days in an environment with the temperature of 20-25 DEG C and the relative humidity of more than or equal to 90% for curing. The method comprises the following steps: activating chemical active sites on the surface and in the recycled aggregate through CO2, and activating Ca (OH) 2 microcrystals and unsaturated Si-O bonds which are not completely carbonized; dipping in silicate sol containing nano Al2O3, and forming a compact silica gel layer by utilizing a chemical reaction between the sol and active sites; and finally, through gradient mineral coating of the inner-layer silica fume and the outer-layer slag powder-fly ash composite material, the problems of high porosity, weak interface bonding and insufficient durability of the recycled aggregate are effectively solved.

Description

Technical Field

[0001] This invention relates to the field of construction solid waste resource utilization technology, specifically a method for preparing modified recycled aggregate from construction solid waste. Background Technology

[0002] With the acceleration of urbanization and the upgrading of infrastructure, the generation of construction solid waste (such as waste concrete and waste bricks) has continued to surge. This not only occupies a large amount of land resources but also easily causes environmental problems such as dust pollution, soil and water pollution. The harmless treatment and resource utilization of construction solid waste has become an urgent problem to be solved in the industry. Recycled aggregate, as the core product of construction solid waste resource utilization, can replace natural aggregate in concrete preparation and effectively alleviate the pressure of natural aggregate shortage. However, recycled aggregate prepared by existing technologies has many performance defects, which limits its large-scale application.

[0003] Recycled aggregates are formed from the crushing and screening of construction solid waste. Their surfaces are rough and riddled with irregular pores, containing a large amount of incompletely detached hardened cement paste and numerous microcracks. These structural defects directly lead to high water absorption and insufficient density in recycled aggregates. More importantly, when recycled aggregates bond with subsequent cement paste, a weak layer easily forms in the interface transition zone, resulting in insufficient bond strength and consequently affecting the overall mechanical properties of concrete. Furthermore, the presence of pores and microcracks accelerates the penetration of external moisture and corrosive media, significantly reducing the durability of recycled aggregates and products, making them prone to cracking, spalling, and other failures. Existing modification technologies have failed to fundamentally solve these structural defects and interface problems, resulting in limited performance improvements and failing to meet the comprehensive requirements of practical engineering projects for recycled aggregates with high strength, low water absorption, high durability, and good interfacial compatibility.

[0004] Therefore, it is necessary to provide a method for preparing modified recycled aggregates from construction solid waste to solve the above-mentioned technical problems. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing modified recycled aggregate from construction solid waste, so as to solve the problems of high porosity, weak interfacial bonding and insufficient durability of existing recycled aggregates.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing modified recycled aggregate from construction solid waste, comprising the following steps: 1) Crush and screen construction solid waste to obtain recycled aggregate with a particle size of 5-31.5mm; 2) Place the recycled aggregate obtained in step 1) in a CO2 environment with a pressure of 0.3-0.5 MPa for pre-curing for 15-25 minutes; 3) Prepare a silicate sol containing an activity regulator, wherein the activity regulator is nano-Al2O3 with a dosage of 0.5-1%; the silicate sol is a nano-silica sol with a SiO2 content of 20-30% and a pH value of 8-10. 4) Immerse the recycled aggregate treated in step 2) in the silicate sol containing the active modifier prepared in step 3) for 30-60 minutes; 5) Dry the impregnated recycled aggregate at 80-100℃ for 2-4 hours to solidify it and form a surface silica gel layer; 6) Prepare gradient composite mineral admixture, which consists of an inner layer of silica fume and an outer layer of slag powder-fly ash composite material, with a total coating amount of 5-10% of the mass of recycled aggregate; the inner layer of silica fume accounts for 30-40% of the total coating amount, the outer layer of slag powder-fly ash composite material accounts for 60-70%, and the mass ratio of slag powder to fly ash is 1:1-2:1; 7) The recycled aggregate treated in step 5) is coated with a gradient coating by spraying. First, the inner layer of silica fume is coated on the surface of the aggregate, and then the outer layer of slag powder-fly ash composite material is coated on the outer layer of silica fume. 8) Place the gradient-coated recycled aggregate in an environment with a temperature of 20-25℃ and a relative humidity of ≥90% for 7-14 days to cure.

[0007] In this invention, CO2-directed activation pre-curing, by controlling pressure and time, causes CO2 to react with Ca(OH)2 on the surface and inside of the recycled aggregate, resulting in CaCO3 crystals that initially fill surface microcracks and open pores, reducing the initial water absorption rate of the aggregate. At the same time, it can retain incompletely carbonized active sites, providing sufficient chemical anchoring points for subsequent silicate sol, avoiding the problem of unstable bonding caused by the sol only adsorbing through physical adsorption, and improving the interfacial bonding strength between the silica gel layer and the aggregate matrix from the source. When silicate sol containing nano-Al2O3 comes into contact with recycled aggregate, the Si-OH groups in the sol undergo a directional chemical reaction with the reserved active sites to generate Ca-O-Si chemical bonds, causing the gel layer to form a chemical fusion with the aggregate rather than a physical cover. The nano-Al2O3 reacts with the residual Ca(OH)2 cured by CO2 to generate calcium aluminum stone, which fills the nanoscale gaps between CaCO3 crystals, further improving the density and structural stability of the gel layer. At the same time, the introduction of nano-Al2O3 forms a large number of Al-OH active groups on the surface of the gel layer, providing conditions for the subsequent pozzolanic reaction of mineral admixtures. In the gradient composite mineral coating process, the inner layer of highly active silica fume, with its high specific surface area, rapidly undergoes a pozzolanic reaction with the Al-OH groups on the surface of the gel layer to generate a high-density CSH gel. This eliminates the interfacial gaps between the gel layer and the mineral coating layer, optimizing interfacial compatibility. The outer layer of slag powder-fly ash composite material continuously fills the residual pores in the interfacial transition zone through a slow pozzolanic reaction, forming a gradient dense structure with rapid inner strengthening and long-term outer filling. This not only locks in the densification effect formed by the previous carbonization and sol modification, but also further blocks the penetration channels of corrosive media, while avoiding the problems of reaction rate imbalance or interfacial stress concentration caused by single mineral coating.

[0008] Preferably, in step 1), the construction solid waste is at least one of waste concrete and waste bricks.

[0009] Preferably, in step 2), the recycled aggregate is dried at 60-80℃ until the moisture content is ≤2% before CO2 pre-curing.

[0010] In this invention, the carbonization reaction of CO2 with Ca(OH)2 in recycled aggregate needs to be carried out efficiently in an anhydrous or low-moisture environment. Drying at 60-80℃ to a moisture content ≤2% completely removes adsorbed water from the aggregate surface and free water from internal capillary pores, preventing moisture from occupying reaction sites and hindering CO2 penetration. The low-moisture environment increases the diffusion rate of CO2 within the aggregate, allowing the carbonization reaction to occur more uniformly in the surface and shallow layers, ensuring that the generated CaCO3 crystals uniformly cover the pore walls. Simultaneously, the dry environment reduces the erosion of active sites by moisture, ensuring the stability and reactivity of incompletely carbonized active sites, providing more sufficient and purer anchoring points for the subsequent chemical bonding of the sol and aggregate.

[0011] Preferably, in step 3), the particle size of the nano-Al2O3 is 50-100 nm.

[0012] Preferably, in step 3), the specific steps for preparing the silicate sol containing the activity regulator include: 3a) Take a nano-silica sol with a SiO2 content of 20-30% and a pH value of 8-10 and place it in a stirring device to form a base material; 3b) Weigh out 0.5-1% of nano Al2O3 and slowly add it to the base material while stirring at 300-500 r / min for 15-20 min. 3c) After stirring, let stand for 5-10 minutes.

[0013] In this invention, step 3a) clearly defines the selection criteria for the base material: a nano-silica sol with a SiO2 content of 20-30% and a pH value of 8-10. This ensures the stability of the sol and provides a suitable chemical environment for the reaction between Si-OH groups and active sites, avoiding deviations in modification effects due to fluctuations in the base material properties. Step 3b) controls the stirring speed to 300-500 r / min and the time to 15-20 min, achieving uniform dispersion of nano-Al2O3 in the silica sol, preventing the agglomeration of nanoparticles and the formation of local defects, ensuring that all recycled aggregates come into contact with the sol containing the activity regulator, and guaranteeing uniform modification. The slow feeding method avoids sol agglomeration caused by excessively high local concentrations, further improving the stability of the system. Step 3c) allows the system to stand for 5-10 min, eliminating bubbles generated during stirring, making the sol system more homogeneous, and allowing the nano-Al2O3 to fully contact the silica sol, preparing it for subsequent chemical reactions.

[0014] Preferably, in step 4), the impregnation is performed with ultrasonic assistance, with an ultrasonic power of 100-200W and an ultrasonic time of 10-15min.

[0015] Preferably, in step 6), the specific surface area of ​​the inner layer of silica fume is ≥20000 m². 2 / kg; the specific surface area of ​​the outer layer slag powder is 400-500m². 2 / kg; the specific surface area of ​​the outer layer of fly ash is 350-450m². 2 / kg.

[0016] Preferably, step 6) involves the following specific operations for preparing the gradient composite mineral admixture: 6a) Calculate the total material mass based on the total coating amount being 5-10% of the recycled aggregate mass, and then calculate the amount of each component based on the inner layer of silica fume being 30-40% and the outer layer of slag powder-fly ash composite material being 60-70%. 6b) Weigh the corresponding components according to the mass ratio of slag powder to fly ash of 1:1-2:1, place them in a mixer and stir at 200-300 r / min for 10-15 min to obtain the outer layer of slag powder-fly ash composite material; 6c) Weigh out the calculated amount of inner layer silica fume separately and seal it separately from the outer layer slag powder-fly ash composite material obtained in step 6b).

[0017] In this invention, step 6a) ensures that the ratio of the inner layer silica fume to the outer layer composite material strictly conforms to the gradient design of 30-40% and 60-70% by precisely calculating the dosage of each component. This guarantees the complementary functions of the highly active and rapid reaction of the inner layer and the slowly active reaction of the outer layer, avoiding insufficient interface strengthening or poor structural density due to ratio imbalance. Step 6b) controls the stirring speed to 200-300 r / min and the time to 10-15 min, which allows the slag powder and fly ash to be fully mixed to form a uniform outer layer composite material, avoiding the local aggregation of a single component that would affect the reaction rate and filling effect. Step 6c) stores the inner layer silica fume separately in a sealed container to prevent it from prematurely contacting and reacting with the outer layer composite material, ensuring the high activity of the silica fume and avoiding the chemical reaction with the gel layer due to activity decay.

[0018] Preferably, in step 7), the inner layer of silica fume coating has a thickness of 50-100 μm, and the outer layer of slag powder-fly ash composite material coating has a thickness of 150-200 μm.

[0019] Preferably, in step 7), the specific operations of the spraying method include: 7a) Take the inner layer of silica fume, add deionized water at a silica fume to water mass ratio of 1:2-1:3, and stir to make a slurry; 7b) Place the recycled aggregate processed in step 5) into a rotary spraying equipment, adjust the spraying pressure to 0.2-0.3MPa and the spraying distance to 15-20cm, spray the slurry onto the aggregate surface, and control the coating thickness to 50-100μm; 7c) Pre-dry the recycled aggregate after spraying with silica fume in an environment of 50-60℃ for 30-40 minutes; 7d) Take the outer layer of slag powder-fly ash composite material, add deionized water at a mass ratio of composite material to water of 1:1.5-1:2.5, and stir to make a slurry; 7e) Keeping the parameters of the rotary spraying equipment unchanged in step 7b), spray the slurry obtained in step 7d) onto the outer layer of the silica fume coating, and control the coating thickness to 150-200μm.

[0020] In this invention, step 7a) involves preparing the slurry with a silica fume to water mass ratio of 1:2 to 1:3. This ensures good flowability and sprayability of the slurry while preventing excessive moisture from causing coating cracking or insufficient moisture from resulting in uneven coverage. Step 7b) controls the spraying pressure to 0.2-0.3 MPa and the distance to 15-20 cm, ensuring uniform atomization of the silica fume slurry and its adhesion to the aggregate surface. Controlling the inner layer thickness to 50-100 μm ensures that the highly reactive silica fume can fully contact the Al-OH groups on the gel layer surface, guaranteeing rapid reaction and the formation of a high-density CSH gel. Step 7c) involves pre-drying at 50-60℃ for 30-40 minutes to remove excess moisture from the inner layer slurry, preventing peeling during subsequent outer layer spraying. It also pre-cures the inner layer coating, improving its bonding stability with the aggregate. Step 7d) involves preparing the slurry with a composite material to water mass ratio of 1:1.5 to 1:2.5, adapting to the particle characteristics of the outer composite material and ensuring effective slurry spraying. Step 7e) Keeping the equipment parameters unchanged and controlling the outer layer thickness to 150-200μm can form a gradient coating layer with uniform thickness and tight bonding, ensuring that the outer composite material can fully fill the residual pores.

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention provides reaction sites for sol-gel modification through carbonization pre-curing, which determines the basic effectiveness of subsequent modification. Sol-gel modification not only consolidates the sealing effect of carbonization pre-curing but also provides an active interface for gradient coating, achieving chemical connection between the preceding and following processes. Gradient coating continues to exert the effects of the preceding modification steps, resulting in recycled aggregates with a dense structure, stable interface, and excellent durability. Through the directional retention and transfer of active sites, the sequential triggering of chemical reactions, and the progressive densification of the structure, it fundamentally solves the core problems of high porosity, weak interfacial bonding, and insufficient durability in recycled aggregates, simultaneously improving mechanical strength, water resistance, and interfacial bonding performance, thus enhancing the quality and efficiency of the resource utilization process of construction solid waste.

[0022] 2. In the carbon dioxide-directed activation pre-curing process, the present invention generates calcium carbonate crystals through carbonation reaction to initially seal the pores, while directionally retaining the incompletely carbonized active sites, providing a chemical reaction basis for subsequent sol modification, so that the sol and aggregate change from physical adsorption to chemical bonding, forming the basic support of the modified system.

[0023] 3. This invention utilizes a silicate sol containing nano-Al2O3 to inherit the pre-curing effect of the preceding carbonization process. The silanol groups in the silicate sol containing nano-Al2O3 react with active sites to form calcium-oxy-silicon chemical bonds. The nano-Al2O3 reacts with residual calcium hydroxide to form calcium aluminum oxide, filling the nano-intervals and improving density. The aluminum hydroxyl active groups formed on the surface of the gel layer activate the pozzolanic reaction of subsequent mineral admixtures.

[0024] 4. This invention fully utilizes the preceding active interface and dense foundation through gradient composite mineral coating. The inner layer of highly active silica fume reacts rapidly to generate high-density hydrated calcium silicate gel, eliminating interfacial gaps; the outer layer of composite material reacts slowly to continuously fill residual pores, constructing a gradient dense structure and blocking the penetration of corrosive media. Attached Figure Description

[0025] Figure 1 Line graphs showing the comparison of crushing values ​​of the recycled aggregates prepared in Examples 1-3 and Comparative Examples 1-6; Figure 2 Line graphs showing the 24-hour water absorption rates of the recycled aggregates prepared in Examples 1-3 and Comparative Examples 1-6; Figure 3 The bar chart shows the comparison of interfacial bond strength between the recycled aggregates prepared in Examples 1-3 and Comparative Examples 1-6. Figure 4 The bar chart shows the comparison of chloride ion permeability coefficients of the recycled aggregates prepared in Examples 1-3 and Comparative Examples 1-6. Figure 5 Line graphs showing the comparison of mass loss rates after 50 freeze-thaw cycles for the recycled aggregates prepared in Examples 1-3 and Comparative Examples 1-6; Figure 6 Line graphs showing the strength loss rate after 50 freeze-thaw cycles for the recycled aggregates prepared in Examples 1-3 and Comparative Examples 1-6. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Example 1: This embodiment provides a method for preparing modified recycled aggregate from construction solid waste, including the following steps: 1) Crush and screen the waste concrete to obtain recycled aggregate with a particle size of 10-20mm; 2) Dry the recycled aggregate at 60℃ until the moisture content is ≤2%, and then pre-cur it in a CO2 environment with a pressure of 0.3MPa for 25 minutes; 3) Preparation of silicate sol containing active regulator: Take nano silica sol with 20% SiO2 content and pH value of 8 as base material, weigh nano Al2O3 with a particle size of 50nm at a dosage of 0.5%, slowly add it to the base material and stir at 300r / min for 20min, let it stand for 10min and set aside. 4) The CO2-pre-cured recycled aggregate is immersed in the above silicate sol and ultrasonic-assisted impregnation is used (power 100W, time 15min), with a total impregnation time of 30min; 5) The impregnated recycled aggregate is dried at 80℃ for 4 hours to solidify and form a surface silica gel layer; 6) Preparation of gradient composite mineral admixture: The total material usage is determined based on 5% of the recycled aggregate mass, of which the inner layer of silica fume is high-activity silica fume accounting for 30% (specific surface area ≥ 20000 m²). 2 / kg), the outer layer of slag powder-fly ash composite material accounts for 70% (slag powder to fly ash mass ratio 1:1, slag powder specific surface area 400m²). 2 / kg, fly ash specific surface area 350m² 2 / kg); put the slag powder and fly ash into the mixer in proportion, stir at 200 r / min for 15 min, and store the silica fume separately in a sealed container; 7) Gradient coating: Take the inner layer silica fume and add water at a mass ratio of 1:2 to make a slurry. Place the recycled aggregate in a rotary spraying device (pressure 0.2MPa, distance 15cm) and spray the silica fume slurry to a thickness of 50μm. Then pre-dry at 50℃ for 40min. Then take the outer layer composite material and add water at a mass ratio of 1:1.5 to make a slurry. Keep the equipment parameters unchanged and spray to a thickness of 150μm. 8) Place the gradient-coated recycled aggregate in an environment with a temperature of 20℃ and a relative humidity of ≥90% for 14 days to cure.

[0028] Example 2: This embodiment provides a method for preparing modified recycled aggregate from construction solid waste, including the following steps: 1) Crush and screen the waste bricks and stones to obtain recycled aggregate with a particle size of 15-25mm; 2) Dry the recycled aggregate at 70℃ until the moisture content is ≤2%, and then pre-cur it in a CO2 environment with a pressure of 0.4MPa for 20 minutes; 3) Preparation of silicate sol containing active regulator: Take nano silica sol with SiO2 content of 25% and pH value of 9 as base material, weigh nano Al2O3 with a particle size of 80nm at a dosage of 0.8%, slowly add it to the base material and stir at 400r / min for 18min, let it stand for 8min and set aside. 4) The CO2-pre-cured recycled aggregate was immersed in the above silicate sol and ultrasonic-assisted impregnation was performed (power 150W, time 12min), with a total impregnation time of 45min; 5) The impregnated recycled aggregate is dried at 90℃ for 3 hours to solidify and form a surface silica gel layer; 6) Preparation of gradient composite mineral admixture: The total material usage is determined based on 8% of the recycled aggregate mass, of which the inner layer of silica fume is high-activity silica fume accounting for 35% (specific surface area ≥ 20000 m²). 2 / kg), the outer layer of slag powder-fly ash composite material accounts for 65% (slag powder to fly ash mass ratio 1.5:1, slag powder specific surface area 450m²). 2 / kg, fly ash specific surface area 400m² 2 / kg); put the slag powder and fly ash into the mixer in proportion and stir at 250 r / min for 12 min. Store the silica fume separately in a sealed container. 7) Gradient coating: Take the inner layer silica fume and add water at a mass ratio of 1:2.5 to prepare a slurry. Place the recycled aggregate in a rotary spraying device (pressure 0.25MPa, distance 18cm) and spray the silica fume slurry to a thickness of 80μm. Then pre-dry at 55℃ for 35min. Then take the outer layer composite material and add water at a mass ratio of 1:2.0 to prepare a slurry. Keep the equipment parameters unchanged and spray to a thickness of 180μm. 8) Place the gradient-coated recycled aggregate in an environment with a temperature of 23℃ and a relative humidity of ≥90% for 10 days to cure.

[0029] Example 3: This embodiment provides a method for preparing modified recycled aggregate from construction solid waste, including the following steps: 1) Mix waste concrete and waste bricks and stones in a 1:1 ratio, crush and screen them to obtain recycled aggregate with a particle size of 20-31.5mm; 2) Dry the recycled aggregate at 80℃ until the moisture content is ≤2%, and then pre-cur it in a CO2 environment with a pressure of 0.5MPa for 15 minutes; 3) Preparation of silicate sol containing active regulator: Take nano silica sol with SiO2 content of 30% and pH value of 10 as base material, weigh nano Al2O3 with a particle size of 100nm at a dosage of 1.0%, slowly add it to the base material and stir at 500r / min for 15min, let it stand for 5min and set aside. 4) The CO2-pre-cured recycled aggregate was immersed in the above silicate sol and ultrasonic-assisted impregnation was performed (power 200W, time 10min), with a total impregnation time of 60min; 5) Dry the impregnated recycled aggregate at 100℃ for 2 hours to solidify it and form a surface silica gel layer; 6) Preparation of gradient composite mineral admixture: The total material usage is determined based on 10% of the recycled aggregate mass, of which the inner layer of silica fume is high-activity silica fume accounting for 40% (specific surface area ≥ 20000 m²). 2 / kg), the outer layer of slag powder-fly ash composite material accounts for 60% (slag powder to fly ash mass ratio 2:1, slag powder specific surface area 500m²). 2 / kg, fly ash specific surface area 450m² 2 / kg); put the slag powder and fly ash into the mixer in proportion and stir at 300 r / min for 10 min. Store the silica fume separately in a sealed container. 7) Gradient coating: Take the inner layer silica fume and add water at a mass ratio of 1:3 to prepare a slurry. Place the recycled aggregate in a rotary spraying device (pressure 0.3MPa, distance 20cm) and spray the silica fume slurry to a thickness of 100μm. Then pre-dry at 60℃ for 30min. Next, take the outer layer composite material and add water at a mass ratio of 1:2.5 to prepare a slurry. Keep the equipment parameters unchanged and spray to a thickness of 200μm. 8) Place the gradient-coated recycled aggregate in an environment with a temperature of 25℃ and a relative humidity of ≥90% for 7 days to cure.

[0030] Comparative Example 1: The only difference from Example 2 is that the CO2 pre-curing treatment in step 2 is missing, and the dried recycled aggregate is directly immersed in silicate sol for impregnation.

[0031] Expected performance: CaCO3 crystals cannot be generated on the surface of recycled aggregates, nor are there any incompletely carbonized active sites retained in a specific direction. The sol only adheres to the aggregate surface through physical adsorption. The bond between the silica gel layer and the aggregate matrix is ​​weak and easy to fall off. The pores inside the aggregates are not initially sealed, and the water absorption rate is relatively high. The interface strengthening effect of the subsequent mineral coating layer is limited. The overall compressive strength and interface bonding performance are significantly weaker than those of the example group.

[0032] Comparative Example 2: The only difference from Example 2 is that: in step 3, no nano Al2O3 activity modifier was added, and pure nano silica sol was used directly for impregnation treatment.

[0033] Expected performance: Silica sol can only form a gel layer that physically seals pores. It cannot react with the Ca(OH)2 remaining after CO2 curing to generate calcium aluminum stone to fill the nano gaps, resulting in a limited sealing effect. The surface of the gel layer lacks Al-OH active groups, which cannot activate the pozzolanic activity of subsequent mineral admixtures. The mineral coating layer is only physically attached, with poor compatibility with the gel layer interface, making it prone to peeling. The impermeability and long-term stability of the aggregate are insufficient.

[0034] Comparative Example 3: The only difference from Example 2 is that in step 6, only a single slag powder is used as the coating material (total coating amount 8%), and in step 7, the coating is applied in one go without layering of inner silica fume and outer composite material.

[0035] Expected performance: Without the rapid reaction of highly active silica fume and gel layer, it is impossible to form a high-density CSH gel-reinforced interface, and there is no gradient effect between fast and slow reactions; the reaction rate of single slag powder is uniform, the pores in the interface transition zone cannot be fully filled, the interface structure is loose, the stress transfer effect between aggregate and cement paste is poor, and the freeze-thaw resistance and crack resistance are far lower than those of the example group.

[0036] Comparative Example 4: The only difference from Example 2 is that the sol impregnation in step 4 is performed first, followed by the CO2 pre-curing treatment in step 2. The other steps and parameters remain unchanged.

[0037] Expected performance: The sol first adheres to the surface of the aggregate to form a gel layer. During subsequent CO2 pre-curing, the gas cannot penetrate into the interior of the aggregate to undergo a carbonization reaction. This results in the inability to seal internal pores or generate active sites that chemically bond with the gel layer. The gel layer also hinders the contact between CO2 and the aggregate, leading to incomplete carbonization. There is no synergistic effect between the processes, and the performance is only close to the effect of single sol modification, far inferior to the synergistic strengthening effect of the example group.

[0038] Comparative Example 5: The only difference from Example 2 is that in step 6, the inner layer of highly active silica fume is replaced with ordinary low-activity silica fume (with a specific surface area much lower than the ≥20000m² specified in the example). 2 / kg), the rest of the steps and parameters remain unchanged.

[0039] Expected performance: Low-activity silica fume cannot react quickly with the Al-OH groups of the silica gel layer, making it difficult to generate high-density CSH gel and effectively strengthen the interfacial bond between the gel layer and the coating layer; the interface remains a weak link, stress transmission is hindered, and the improvement in the compressive strength and interfacial bonding performance of the aggregate is limited, failing to achieve the interfacial strengthening effect of the example group.

[0040] Comparative Example 6: The only difference from Example 2 is that in step 7, the inner layer of silica fume and the outer layer of slag powder-fly ash composite material are mixed to form a slurry, which is then sprayed onto the surface of the aggregate in one go, without layered spraying and pre-drying treatment.

[0041] Expected performance: The rapid reaction of the highly active silica fume and the slow reaction of the composite material interfered with each other, resulting in a disordered structure of the hydration products. The lack of a pre-drying step caused moisture retention inside the coating, which easily led to shrinkage cracks and the inability to form a gradient dense interface structure. The porosity of the aggregate was high, and the impermeability and durability were significantly inferior to those of the example group.

[0042] To compare the performance differences between the preparation methods of modified recycled aggregates from construction solid waste provided in Examples 1-3 and Comparative Examples 1-6, the present invention provides the following test methods: 1. Compressive strength test: The crushing value of recycled aggregate was determined according to GB / T14685-2022 "Construction Gravel and Crushed Stone"; 2. Water absorption test: The water absorption rate of recycled aggregate after 24 hours was determined using the saturation method; 3. Interfacial bond strength test: Modified recycled aggregate and cement paste were made into φ50mm×50mm interfacial bond specimens. After curing for 28 days, a splitting test was performed using a universal testing machine to test the interfacial bond strength between aggregate and cement paste. 4. Permeability test: The chloride ion permeability coefficient of recycled aggregate concrete was determined by the Rapid Chloride Migration Method (RCM method).

[0043] 5. Freeze-thaw resistance test: According to GB / T50082-2024 "Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete", a rapid freeze-thaw cycle test was conducted, and the mass loss rate and strength loss rate of aggregate were recorded after 50 freeze-thaw cycles.

[0044] The experimental data are as follows:

[0045] Based on the experimental data, Examples 1-3 and Comparative Examples 1-6 showed significant differences in six key indicators: crushing value, 24-hour water absorption rate, interfacial bond strength, chloride ion permeability coefficient, and mass loss rate and strength loss rate after freeze-thaw cycles. Among them, Example 2 (CO2 pressure 0.4 MPa, nano-Al2O3 content 0.8%, gradient coating amount 8%) showed the best performance, with a crushing value of 7.5%, a 24-hour water absorption rate of 4.1%, an interfacial bond strength of 4.2 MPa, and a chloride ion permeability coefficient of 1.5 × 10⁻⁶. -12 m 2 / s, after 50 freeze-thaw cycles, the mass loss rate was 0.2% and the strength loss rate was 3.5%. Although there were slight differences in various indicators between Example 1 and Example 3, they were all at the same excellent level. The crushing value was less than 8.5%, the 24-hour water absorption rate was less than 5%, the interfacial bonding strength was higher than 3.6 MPa, and the durability index was also significantly better than the comparative group.

[0046] In terms of mechanical strength and density, the crushing value of the example group was concentrated in 7.5%-8.2%, and the water absorption rate in 24 hours was controlled at 4.1%-4.8%, which was much lower than that of the comparative group (11.5%-14.5% and 7.9%-11.2%). This is due to the CaCO3 crystals generated by CO2 pre-curing initially blocking the pores, the nano-Al2O3 and silicate sol synergistically filling the nano gaps, and then the formation of a dense structure through gradient mineral coating. The three layers progressively reduce the porosity. The comparative groups either lacked core processes (such as Comparative Example 1 without CO2 pre-curing), or reversed the process sequence (such as Comparative Example 4 with sol-gel impregnation followed by carbonization), or used a single mineral coating (such as Comparative Example 3), resulting in incomplete pore sealing, easy water penetration, and a significant decrease in mechanical strength and density.

[0047] Regarding interfacial bonding performance, the interfacial bonding strength of the example group was 1.5-2.3 times that of the comparative group. The core reason is that the Al-OH active groups on the surface of the sol layer activated the pozzolanic reaction of the highly active silica fume, generating a high-density CSH gel to eliminate interfacial gaps. The outer slag powder-fly ash composite material reacted slowly and continuously optimized the interfacial structure, achieving dual reinforcement of chemical bonding and physical filling. In contrast, comparative example 5 used low-activity silica fume, and comparative example 6 mixed spraying without layering and pre-drying destroyed the interfacial synergistic effect, resulting in a significant decrease in bonding strength.

[0048] In terms of durability, the chloride ion permeability coefficients of the example group were all below 2.1 × 10⁻⁶. -12 m 2 / s, after 50 freeze-thaw cycles, the mass loss rate was ≤0.3% and the strength loss rate was ≤4.2%, demonstrating excellent impermeability and freeze-thaw resistance. This is because the dense gradient coating layer and gel layer form multiple corrosive media barrier, and the chemically bonded interface structure improves the stability of freeze-thaw cycle resistance. In contrast, the comparative group has a loose interface and high porosity, making it easy for corrosive media to penetrate. After freeze-thaw cycles, the structure was severely damaged, and the strength loss rate of comparative example 1 was as high as 18.6%.

[0049] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing modified recycled aggregate from construction solid waste, characterized in that, Includes the following steps: 1) Crush and screen construction solid waste to obtain recycled aggregate with a particle size of 5-31.5mm; 2) Place the recycled aggregate obtained in step 1) in a CO2 environment with a pressure of 0.3-0.5 MPa for pre-curing for 15-25 minutes; 3) Prepare a silicate sol containing an activity regulator, wherein the activity regulator is nano-Al2O3 with a dosage of 0.5-1%; the silicate sol is a nano-silica sol with a SiO2 content of 20-30% and a pH value of 8-10. 4) Immerse the recycled aggregate treated in step 2) in the silicate sol containing the active modifier prepared in step 3) for 30-60 minutes; 5) Dry the impregnated recycled aggregate at 80-100℃ for 2-4 hours to solidify it and form a surface silica gel layer; 6) Prepare gradient composite mineral admixture, which consists of an inner layer of silica fume and an outer layer of slag powder-fly ash composite material, with a total coating amount of 5-10% of the mass of recycled aggregate; the inner layer of silica fume accounts for 30-40% of the total coating amount, the outer layer of slag powder-fly ash composite material accounts for 60-70%, and the mass ratio of slag powder to fly ash is 1:1-2:1; 7) The recycled aggregate treated in step 5) is coated with a gradient coating by spraying. First, the inner layer of silica fume is coated on the surface of the aggregate, and then the outer layer of slag powder-fly ash composite material is coated on the outer layer of silica fume. 8) Place the gradient-coated recycled aggregate in an environment with a temperature of 20-25℃ and a relative humidity of ≥90% for 7-14 days to cure.

2. The preparation method according to claim 1, characterized in that, In step 1), the construction solid waste is at least one of waste concrete and waste bricks.

3. The preparation method according to claim 1, characterized in that, In step 2), before CO2 pre-curing, the recycled aggregate is dried at 60-80℃ until the moisture content is ≤2%.

4. The preparation method according to claim 1, characterized in that, In step 3), the particle size of nano-Al2O3 is 50-100nm.

5. The preparation method according to claim 4, characterized in that, Step 3) involves the following specific steps for preparing the silicate sol containing the activity modifier: 3a) Take a nano-silica sol with a SiO2 content of 20-30% and a pH value of 8-10 and place it in a stirring device to form a base material; 3b) Weigh out 0.5-1% of nano Al2O3 and slowly add it to the base material while stirring at 300-500 r / min for 15-20 min. 3c) After stirring, let stand for 5-10 minutes.

6. The preparation method according to claim 1, characterized in that, In step 4), the impregnation is performed with ultrasonic assistance, with an ultrasonic power of 100-200W and an ultrasonic time of 10-15min.

7. The preparation method according to claim 1, characterized in that, In step 6), the specific surface area of ​​the inner layer of silica fume is ≥20000 m². 2 / kg; the specific surface area of ​​the outer layer slag powder is 400-500m². 2 / kg; the specific surface area of ​​the outer layer of fly ash is 350-450m². 2 / kg.

8. The preparation method according to claim 7, characterized in that, Step 6) involves the following specific operations for preparing the gradient composite mineral admixture: 6a) Calculate the total material mass based on the total coating amount being 5-10% of the recycled aggregate mass, and then calculate the amount of each component based on the inner layer of silica fume being 30-40% and the outer layer of slag powder-fly ash composite material being 60-70%. 6b) Weigh the corresponding components according to the mass ratio of slag powder to fly ash of 1:1-2:1, place them in a mixer and stir at 200-300 r / min for 10-15 min to obtain the outer layer of slag powder-fly ash composite material; 6c) Weigh out the calculated amount of inner layer silica fume separately and seal it separately from the outer layer slag powder-fly ash composite material obtained in step 6b).

9. The preparation method according to any one of claims 1-8, characterized in that, In step 7), the inner layer of silica fume coating is 50-100μm thick, and the outer layer of slag powder-fly ash composite material coating is 150-200μm thick.

10. The preparation method according to claim 9, characterized in that, In step 7), the specific operations of the spraying method include: 7a) Take the inner layer of silica fume, add deionized water at a silica fume to water mass ratio of 1:2-1:3, and stir to make a slurry; 7b) Place the recycled aggregate processed in step 5) into a rotary spraying equipment, adjust the spraying pressure to 0.2-0.3MPa and the spraying distance to 15-20cm, spray the slurry onto the aggregate surface, and control the coating thickness to 50-100μm; 7c) Pre-dry the recycled aggregate after spraying with silica fume in an environment of 50-60℃ for 30-40 minutes; 7d) Take the outer layer of slag powder-fly ash composite material, add deionized water at a mass ratio of composite material to water of 1:1.5-1:2.5, and stir to make a slurry; 7e) Keeping the parameters of the rotary spraying equipment unchanged in step 7b), spray the slurry obtained in step 7d) onto the outer layer of the silica fume coating, and control the coating thickness to 150-200μm.

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