Microwave-responsive phosphonic acid geopolymer modified recycled aggregate, method of making and use thereof

Abstract

The present application relates to solid waste resource utilization and functional building material technical field, specifically relates to a microwave response type phosphoric acid geopolymer modified recycled aggregate, its preparation method and application, the method comprises the following steps: mixing phosphoric acid, phosphate and water to prepare an activator; then dry mixing silicon-aluminum raw materials and composite wave absorber (graphite powder and core-shell structure Fe3O4@C) and adding the activator to prepare geopolymer slurry; the recycled aggregate is soaked in the slurry to form a coating layer, and is cured after curing.The present application strengthens the recycled aggregate through the chemical bonding and physical filling of phosphoric acid geopolymer, and simultaneously utilizes the magnetic-electric synergistic effect of the composite wave absorber to give it high-efficiency microwave response capability.The modified aggregate is used for asphalt mixture, can rapidly produce heat at the aggregate-asphalt interface under microwave irradiation, and realizes efficient self-healing of microcracks.

Description

Technical Field

[0001] This invention relates to the fields of solid waste resource utilization and functional building materials technology, specifically to a microwave-responsive phosphate geopolymer-modified recycled aggregate, its preparation method and application. Background Technology

[0002] The reconstruction and expansion of roads, bridges, and buildings generate a large amount of solid construction waste, such as waste cement concrete. Currently, most of this waste is disposed of through landfill or stockpiling, which not only occupies significant land resources but also pollutes and damages the environment. While waste concrete is crushed and used to prepare recycled aggregate (RCA), the surface of the crushed concrete retains a large amount of old cement paste, resulting in high porosity, high water absorption, and low strength, making it difficult to meet highway construction standards. Furthermore, asphalt pavements are prone to micro-cracks during service. Microwave heating self-healing technology involves directly adding microwave-absorbing materials to the mixture, using microwave radiation to rapidly heat it, thereby melting the asphalt around the cracks to achieve healing. Existing technologies often directly incorporate steel slag or carbon fiber into asphalt mixtures, which suffers from uneven dispersion and affects the original gradation and mechanical properties of the mixture.

[0003] Regarding the strengthening and modification of recycled aggregates, researchers have tried various methods: some researchers used a Los Angeles abrasion tester to abrade the recycled aggregates, and then soaked them in acid solutions (acetic acid, hydrochloric acid, etc.), using the H+ in the acidic solution... + Reaction with substances such as Ca(OH)2 in old mortar causes the aged cement mortar adhering to the surface of recycled aggregates to detach. While this effectively removes surface mortar, it does not effectively improve micro-cracks within the aggregates and carries the risk of corrosion. Other researchers have used organosilicon resin solutions to soak recycled aggregates, forming a polymer-coated shell on the aggregate surface that fills cracks and pores, thereby improving the density of the recycled aggregates. However, this method has high raw material costs and also reduces the surface texture of the aggregates, affecting the adhesion between the aggregates and asphalt, making large-scale production difficult. Still other researchers utilize hydrated substances such as calcium hydroxide and unhydrated substances such as dicalcium silicate present in the mortar on the surface of recycled aggregates, reacting with CO2 gas to generate CaCO3, which fills cracks and pores in the old mortar, reducing porosity and improving the performance of recycled aggregates. However, this method requires high reaction conditions (high temperature, high pressure), long reaction times, and sophisticated equipment and processes.

[0004] Chinese patent CN104628282B discloses a method for preparing a surface treatment agent for recycled aggregates. The method involves preparing a chemical slurry using acrylate, persulfate, triethanolamine, a crosslinking agent, a silane coupling agent, calcium lignosulfonate, and water. The recycled aggregate is then immersed in the surface modifier for 5-10 minutes. The acrylate aqueous solution polymerizes to form a high-strength network structure on the aggregate surface, which overlaps with the network structure formed by the cement paste, improving concrete performance. The inorganic ions of the acrylate form a good interfacial bond with the aggregate and cement paste, improving the interfacial structure and porosity between the aggregate and cement paste. Simultaneously, the high permeability and film-forming properties of the silane coupling agent allow it to penetrate into the cracks of the recycled aggregate and into the capillaries of residual cement paste on the surface, forming a film and reducing water absorption. The bonding between the chemical slurry and the recycled aggregate old cement mortar used in this patent mainly relies on physical adhesion and mechanical filling. It does not fundamentally eliminate the old cement mortar that causes poor aggregate performance. There is a clear physical boundary line at the interface. Under the action of complex road loads (shear and compression), the reinforcement layer is very easy to peel off from the surface of the old mortar.

[0005] Chinese patent CN114477873B discloses a self-compacting concrete made from recycled aggregate and its preparation method. It uses a phenolic resin composite gel to coat the recycled aggregate, improving its compactness, filling its capillary pores and microcracks, reducing its sharp edges and increasing its roundness, thereby improving the workability and compactness of the concrete, and enhancing its mechanical and performance properties. However, this method faces problems such as easy agglomeration of nanoparticles, difficulty in dispersing the paste, and shrinkage cracks. Furthermore, its strengthening mechanism is still mainly based on physical filling, resulting in low interfacial bonding energy.

[0006] Chinese patent CN120208570A discloses a high microwave thermal response aggregate based on graphitized carbon layer modification, its preparation method, and its application. The method involves coating ordinary aggregates with woody biomass material, and then carbonizing the woody biomass on the aggregate surface through a high-temperature pyrolysis process, transforming it into a graphitized microwave-absorbing shell, thereby giving the aggregate microwave response capability. This method avoids problems such as localized overheating caused by uneven dispersion of the conductive phase in traditional technologies. However, the microwave-absorbing shell used requires high-temperature pyrolysis carbonization under nitrogen protection, which is a complex process with extremely high equipment requirements, making it difficult to use for large-scale production. Furthermore, the direct carbonization process makes it difficult to further control the microwave absorption efficiency of the aggregate and the microwave self-healing efficiency of the subsequent mixture.

[0007] Therefore, there is an urgent need to develop a green modification technology that can synergistically remove old cement mortar from the surface of recycled aggregates, fill internal micro-cracks, and endow them with efficient microwave thermal response capabilities. This would improve the mechanical properties and compactness of the aggregates while achieving good compatibility with asphalt mixtures and microwave self-healing function of the pavement. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a microwave-responsive phosphate geopolymer-modified recycled aggregate, its preparation method and application.

[0009] The technical solution of this invention is implemented as follows: In a first aspect, the present invention proposes a method for preparing microwave-responsive phosphate geopolymer-modified recycled aggregate, comprising the following steps: S1. Mix phosphoric acid, phosphate, and water to prepare a phosphoric acid activator; S2. Dry mix the silica-alumina raw material and the composite microwave absorber evenly, add the phosphoric acid activator, stir and mix to obtain a microwave-responsive phosphoric acid geopolymer slurry; Specifically, the silica-alumina raw material and the composite microwave absorber are uniformly mixed by rotary dry mixing in a ball mill; the geopolymer slurry, as a chemical slurry for modification, has good fluidity and high reactivity at room temperature, and can rapidly generate a stable three-dimensional gel network structure under medium and low temperature conditions. S3. Immerse the recycled aggregate in the geopolymer slurry to form a coating layer on the surface of the aggregate; S4. Curing the coated recycled aggregate to solidify the coating layer, resulting in microwave-responsive phosphate geopolymer-modified recycled aggregate. The composite microwave absorbing agent comprises graphite powder and core-shell structured iron oxide (Fe3O4@C); the core of the core-shell structured iron oxide includes iron oxide, the shell is a carbon layer with a thickness of 15-30 nm, and the particle size of the core-shell structured iron oxide is 130 nm-160 nm.

[0010] More preferably, the preparation method of the core-shell structured iron(III) oxide includes: S1-1. Add Fe3O4 powder to sodium citrate solution, stir magnetically at 60℃ for 1.5h, centrifuge to obtain pretreated Fe3O4, which enhances the dispersibility of Fe3O4; S1-2. The above Fe3O4 is uniformly dispersed in a glucose solution and transferred to a hydrothermal reactor. The reaction is carried out in an oven at 160°C-200°C for 8-12 hours. Under high pressure and hydrothermal conditions, glucose undergoes dehydration aromatization to generate the active intermediate 5-hydroxymethylfurfural (5-HMF). Through heterogeneous nucleation mechanism, it undergoes in-situ polycondensation and crosslinking on the surface of magnetic iron oxide to form an organic carbon shell. The product is centrifuged and dried to obtain core-shell structured iron oxide (Fe3O4@C) with a carbon layer on the surface.

[0011] More preferably, the concentration of the glucose solution in steps S1-2 is 0.4 mol / L.

[0012] Preferably, the mass of the composite microwave absorbing agent accounts for 8% to 16% of the total mass of the geopolymer slurry.

[0013] Preferably, in the composite microwave absorbing agent of step S2, the mass ratio of core-shell structured iron(III) oxide (Fe3O4@C) to graphite powder is 1:(2-4).

[0014] Preferably, the particle size of the silicon-aluminum raw material in step S2 is 800~1200 mesh.

[0015] More preferably, in the geopolymer slurry of step S2, the liquid-solid mass ratio of the liquid phosphoric acid activator to the solid raw materials such as the aluminosilicate raw material, microwave absorber, and dispersant is 1.7, and the molar ratio of phosphorus in the phosphoric acid activator to aluminum in the aluminosilicate raw material is 1.3.

[0016] Preferably, the geopolymer slurry in step S2 further includes a dispersant, which includes lignin sulfonate, and its mass is 0.5% of the total mass of the phosphate geopolymer slurry.

[0017] Specifically, the acid-resistant lignin sulfonate dispersant is subsequently adsorbed onto the carbon shell or graphite surface in the slurry, preventing the agglomeration and sedimentation of the microwave absorber particles through charge repulsion or steric hindrance effects.

[0018] More preferably, in step S2, after adding the phosphoric acid activator, the mixture is magnetically stirred for 5-10 minutes and cured in a 40°C water bath for 1 hour to allow the silicon-aluminum raw material to initially dissolve in the acidic phosphoric acid system, releasing Si. 4+ And Al 3+ The monomer undergoes pre-hydrolysis and pre-condensation in the phosphoric acid system to form oligomeric aluminosilicate intermediates, which are beneficial for subsequent polycondensation reactions.

[0019] More preferably, in step S3, the recycled aggregate is immersed in the geopolymer slurry under water bath curing at 65℃-75℃, which accelerates the geopolymerization reaction of the slurry, thickens it, and increases the adhesion of the slurry to the surface of the recycled aggregate.

[0020] Preferably, the curing in step S4 includes: pre-curing in a 40°C oven for 12 hours, and then intermittently heating with an 80W microwave for 1 hour, wherein heating is performed for 1 minute every 10 minutes.

[0021] Specifically, the principle of phased, multi-mode curing is as follows: First, pre-curing in an oven (around 40℃) for 12 hours allows the geopolymer shell to initially solidify, giving the geopolymer slurry sufficient reaction time and avoiding expansion and cracking caused by excessive moisture loss due to direct microwave instantaneous heating. Then, intermittent microwave heating at 80W (low-power, gentle curing) for 1 hour (1 minute of microwave heating every 10 minutes) allows the geopolymer to complete the reaction, transforming the unstable component CaHPO4 formed in the geopolymer shell into stable AlHPO4, ensuring the stability of the modified aggregate. Traditional technology involves gradient high-temperature curing (curing at 60℃ for 24 hours, followed by curing at 150℃ for approximately 10 hours).

[0022] In addition, the modified recycled aggregate of this invention has a high efficiency of microwave response capability. By using microwave curing, the polymerization reaction process is accelerated by rapidly heating from the inside out, which greatly shortens the curing time of the modified aggregate.

[0023] Preferably, the phosphate in step S1 includes sodium phosphate, aluminum phosphate, aluminum hydrogen phosphate, or aluminum dihydrogen phosphate; the silicoaluminous raw material in step S2 includes one or more of metakaolin, fly ash, or slag.

[0024] In a second aspect, the present invention provides a microwave-responsive phosphate geopolymer-modified recycled aggregate obtained by the preparation method described in the first aspect.

[0025] Specifically, the microwave-responsive phosphate geopolymer-modified recycled aggregate exhibits a water absorption rate that decreases from 5%-6% in its original state to below 3%, and an apparent density that increases from 2.5 to above 2.7, meeting the requirements for road engineering aggregates. When used in asphalt mixtures, the modified recycled aggregate demonstrates excellent asphalt adhesion and anti-stripping properties, meeting the requirements for highway road materials. It also possesses highly efficient microwave heating response capabilities, enabling self-repair of asphalt pavements through microwave heating within the asphalt mixture, filling cracks and voids in aging pavements, and extending the service life of asphalt pavements.

[0026] The microwave-responsive phosphate geopolymer-modified recycled aggregate can be used to improve the microwave-induced self-healing efficiency of asphalt pavement. Under microwave irradiation, the asphalt pavement heats up rapidly to above the asphalt softening point, and under capillary action, the asphalt begins to flow and fill pavement cracks.

[0027] Thirdly, the present invention provides an application of microwave-responsive phosphate geopolymer-modified recycled aggregate as described in the second aspect in asphalt mixtures, which is applied to the upper, middle, and lower layers of asphalt mixtures involved in asphalt pavements, as well as flexible base layers.

[0028] Specifically, the microwave-responsive recycled aggregate of the present invention can be applied to asphalt pavement. When the pavement is damaged and cracks are generated, the cracked area is heated by microwave. The microwave-responsiveness of the modified recycled aggregate generates heat from the inside. When the temperature reaches the softening point of the asphalt, it promotes the softening and flow of the asphalt matrix. Under capillary action, the cracks are filled and healed.

[0029] Compared with the prior art, the advantages of the present invention are as follows: (1) In this invention, Fe3O4@C is prepared by a hydrothermal method. Fe3O4 reacts with phosphoric acid in the system to generate FePO4 and Fe3(PO4)2, losing its microwave responsiveness. A protective carbon layer is formed on the Fe3O4 surface by hydrothermal carbonization with glucose, which isolates the reaction between phosphoric acid and Fe3O4 and preserves the microwave responsiveness of Fe3O4. To ensure that the microwave performance of Fe3O4 is not affected, the shell thickness is controlled at 15-30 nm. If the shell thickness is too low, it cannot fully protect Fe3O4 from phosphoric acid corrosion. If the shell thickness is too high, it will lead to weakened microwave transmission, which will also affect the microwave performance of the material.

[0030] (2) The recycled aggregate of the present invention has a high efficiency microwave response capability. Under microwave irradiation, the surface of the aggregate can be heated to 103°C in only 60s. The composite microwave absorbing shell composed of PAG, Fe3O4@C and graphite powder, through the synergistic control impedance matching among the three, constructs a high-performance microwave absorbing shell that can effectively absorb microwave energy, convert the incident microwave energy into heat energy with high loss, and conduct heat energy efficiently.

[0031] (3) This invention employs microwave curing technology to rapidly harden the PAG shell layer on the surface of modified recycled aggregate, shortening the curing cycle and improving the performance of the aggregate-PAG shell interface. Utilizing the selective heating characteristics of microwaves on polar groups in PAG slurry, combined with the synergistic effect of magnetic and dielectric loss of the composite microwave absorber, the shell temperature reaches above 100°C within 3 minutes. Unlike traditional oven heating curing, where heat is transferred slowly from the outside to the inside, the outer layer hardens faster, and the reaction bubbles in the inner layer are difficult to expel, leading to cracking; microwave heating curing rapidly conducts heat from the inside to the outside, promoting the expulsion of water vapor from the inside to the outside, achieving rapid reaction hardening and strength improvement of the shell layer. This significantly improves the density at the interface between the modified layer and the recycled aggregate old mortar, providing a guarantee for the anti-stripping performance of the pavement during long-term service, and solving the technical defects of easy shell layer delamination and high internal stress under traditional curing methods. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a comparison image of the modified recycled aggregate (right) and the blank group (left) of Example 6 of the present invention. Figure 2 This is a schematic diagram of the recycled aggregate modification mechanism of the present invention; Figure 3 Infrared thermal images of modified recycled aggregate (left) and blank group 1 (right) after microwave heating (60s) in Example 6 of the present invention; Figure 4 This is a comparison curve of the microwave-heated fracture energy recovery rate of asphalt mixtures prepared from recycled aggregates in Examples 12-15 of this invention and the blank group 2. Detailed Implementation

[0034] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0035] Geopolymers are a novel type of cementitious material with a three-dimensional cross-linked network structure composed of silicon-oxygen tetrahedra and aluminum-oxygen tetrahedra. Compared with traditional cementitious materials such as cement, they have advantages in mechanical properties, durability, and low carbon footprint, energy saving, and environmental friendliness. Using geopolymer slurry to coat and modify recycled aggregates fills the microcracks and pores on the surface of the recycled aggregates, effectively improving problems such as high water absorption and low strength. Furthermore, the raw materials are abundant and inexpensive, the processing is simple, and the use of medium- and low-temperature treatment methods results in low energy consumption, offering significant advantages over existing recycled aggregate modification technologies. Acid-activated geopolymers, compared to alkali-activated geopolymers, exhibit superior early strength, high-temperature stability, corrosion resistance, and bonding properties, making them more suitable for application in the modification of recycled aggregates.

[0036] Regarding microwave self-healing of asphalt pavements, microwave heating self-healing technology, as a non-destructive repair method, offers selective and efficient heating, with its core being the improvement of the microwave absorption efficiency of the mixture. Most existing technologies involve directly incorporating microwave absorbing agents into the asphalt mixture. For example, some researchers have directly incorporated microwave absorbing agents (ferrites, carbon-based materials, etc.) into asphalt through high-temperature shearing. While this can improve the microwave repair efficiency of the mixture, practical applications have revealed that asphalt, being a highly viscoelastic polymer, makes it difficult to uniformly disperse the microwave absorbing agent. This directly leads to uneven heat distribution and localized overheating during microwave heating of the asphalt mixture, easily causing aging in the hottest areas. Furthermore, the addition of the microwave absorbing agent, as a poorly compatible heterogeneous component, directly affects the mechanical properties of the asphalt. It is difficult to achieve higher microwave absorption efficiency through high dosage, and the proportion of asphalt in the mixture is generally less than 10%. Therefore, the dosage of microwave absorber in the mixture is greatly limited, making it difficult to further improve the self-healing efficiency of asphalt mixtures. Other researchers have prepared asphalt mixtures by replacing mineral powder with steel slag powder and microwave absorber powder as fine aggregates. Although this method does not directly incorporate microwave absorber components into asphalt, it will affect the overall gradation of the mixture. Moreover, replacing fine aggregates has a great impact on the asphalt-aggregate ratio, and changes in dosage will have a complex and significant impact on the overall performance of the mixture. Furthermore, existing studies often employ a single absorbing component, such as pure carbon-based materials (graphite, CNTs), which have extremely high dielectric constants and strong loss capabilities. However, due to their permeability being close to 1, the impedance is severely mismatched with the air impedance, causing microwaves to be reflected at the material surface, affecting microwave incidence and significantly reducing microwave absorption efficiency. On the other hand, single ferrite components (such as Fe3O4) have higher complex permeability and a magnetic loss mechanism, which matches the air impedance relatively well, making microwaves easier to incident. However, the magnetic dipole interaction between ferrite powder particles is limited, and its thermal conductivity is poor, with heat concentrated on the particle surface and a slow macroscopic heating rate. At low power, the energy conversion efficiency of magnetic loss is usually lower than that of dielectric loss.

[0037] Recycled aggregate (RCA) suffers from poor performance due to the adhesion of old mortar to its surface, but its porous nature provides room for functional modification. Modifying RCA using a phosphate geopolymer system with added composite microwave absorbers can achieve both physical filling and chemical strengthening effects, while also transforming the aggregate into an "active heating unit" with microwave heating efficiency exceeding that of existing mixtures. This is an effective way to solve the challenges of RCA utilization and asphalt pavement maintenance.

[0038] The mechanism of action of this invention is mainly manifested in two aspects, such as Figure 2 As shown: On the one hand, phosphate geopolymer-modified recycled aggregates utilize the synergistic benefits of chemical reaction and physical filling. The phosphate component in the phosphate geopolymer can react with the mortar (containing alkaline components such as Ca(OH)2) on the surface of the recycled aggregates to generate phosphate minerals, eliminating the root cause of low strength in the recycled aggregates. Then, the phosphate geopolymer slurry physically fills and coats the recycled aggregates, forming a dense inorganic gel shell with a three-dimensional cross-linked network of -O-Al-O-Si-OPO- structure on the surface. This fills the pores and cracks in the recycled aggregates, improving their mechanical properties. On the other hand, to form a high-microwave-response absorbing shell, PAG differs from traditional shell matrices, which merely act as a binder between the absorber and the aggregate. During the research, it was discovered that phosphate geopolymer (PAG) itself possesses certain microwave-response properties. The abundant polar phosphate groups and bound water in the geopolymer gel network exhibit significant dipole-directing polarization loss under microwave fields. Upon microwave irradiation, the PAG matrix responds first, generating heat and lowering the excited energy barrier of the Fe3O4@C magnetic dipole, triggering strong magnetic loss. Simultaneously, the layered graphite structure generates dielectric loss while forming an efficient heat-conducting network within the shell, laterally diffusing heat and achieving uniform heating across the entire depth. Furthermore, the PAG matrix (with a moderate dielectric constant) acts as a transitional matching layer between air and high-loss particles, solving the electromagnetic shielding problem of dielectric loss material graphite powder. The addition of Fe3O4@C modulates the complex permeability of the system, allowing for effective microwave incidence and guiding microwave propagation into the aggregate interior through magnetic loss. Graphite powder, as the primary loss material, converts incident microwaves into heat energy with high dielectric loss. However, increasing the doping amount can cause electromagnetic shielding, leading to impedance mismatch and preventing microwaves from penetrating the interior. Fe3O4@C, by introducing hysteresis loss and natural resonance, allows the magnetic components to compensate for the dielectric mismatch caused by graphite, matching the overall input impedance and spatial impedance of the composite material. This enables a large amount of microwave energy to penetrate the material and be efficiently converted into heat energy.

[0039] This invention addresses the shortcomings of recycled aggregates prepared from existing construction waste concrete in terms of performance, and the problems in traditional microwave self-healing technology for asphalt mixtures, such as uneven heat distribution, electromagnetic wave reflection and shielding effects due to difficulties in dispersing microwave absorbing agents, and the impact on asphalt flowability and road performance. It overcomes the research bottleneck of traditional technology where the addition of microwave absorbing agents affects mixture performance, limiting the dosage of absorbing components and hindering further improvement in microwave efficiency. Furthermore, the research revealed that actual asphalt pavement cracks and defects arise from the large modulus difference between aggregates and asphalt under high-pressure vehicle loads, leading to stress concentration and damage in the aggregate-asphalt interface transition zone. Therefore, almost all cracks and defects occur in this transition zone. Traditional technology, by directly adding microwave absorbing components to asphalt, suffers from uneven dispersion and localized overheating, and also results in energy loss because heat is not immediately transferred to defects. Applying the microwave absorbing shell to the aggregate allows for targeted and rapid heating at cracks and defects, enabling precise and efficient crack repair.

[0040] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0041] In this document, the terms “containing,” “comprising,” or “including” are open-ended expressions, meaning they include the contents specified in this invention but do not exclude other aspects.

[0042] In this document, the terms “optional,” “optionally,” or “optional” generally refer to an event or condition that may, but may not, occur, and the description includes both cases in which the event or condition occurs and cases in which the event or condition does not occur.

[0043] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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 protection scope of the present invention.

[0044] Unless otherwise specified, all reagents used in this invention can be purchased commercially. The aluminosilicate metakaolin (MK) used is from Hebei Jiegui Mineral Products Co., Ltd., and is a reddish-brown powder with a mesh size of 800-1200. Phosphoric acid, lignin sulfonate, and sodium phosphate are from Shanghai Maclean Biochemical Technology Co., Ltd., with phosphoric acid being an 85wt% transparent liquid. Fe3O4 is from Shanghai Maclean Biochemical Technology Co., Ltd., with a particle size of 100nm, and is a black powder. Sodium citrate solution and glucose solution are from Sinopharm Group. Graphite powder is from Shanghai Maclean Biochemical Technology Co., Ltd., and is a silvery-gray flaky powder with a mesh size of 80-120.

[0045] The main chemical components and contents of the recycled aggregate and the silicoaluminous raw material metakaolin used in this invention are shown in Tables 1 and 2.

[0046] Table 1 Main Chemical Components of Recycled Aggregates

[0047] Table 2 Main Chemical Components of Silicon-Aluminum Raw Materials

[0048] Example 1 This embodiment provides a method for preparing microwave-responsive phosphate geopolymer-modified recycled aggregate, including the following steps: S1-1. Add 100g of Fe3O4 powder to 100ml of sodium citrate solution with a concentration of 0.5mol / L, stir magnetically at 60℃ for 1.5h, and centrifuge to obtain pretreated Fe3O4. S1-2. Take 100g of pretreated Fe3O4 and disperse it in 4L of 0.4mol / L glucose solution. After stirring for 15min, transfer it to a hydrothermal reactor and react it in an oven at 160°C for 8h. Then, centrifuge and dry the product to obtain a core-shell structure (Fe3O4@C) with a carbon layer on the surface. S1. Mix 150g of 85wt% phosphoric acid solution, 24.3g of sodium phosphate, and 47.5g of water, and stir until the liquid is clear and transparent to obtain phosphoric acid activator; S2. Mix 32g of composite microwave absorber (Fe3O4@C: graphite powder in a mass ratio of 1:2), 2g of sodium lignosulfonate dispersant, and 144g of metakaolin in a ball mill until homogeneous. Add 221.8g of phosphoric acid activator and stir for 5 minutes to obtain a geopolymer slurry. Pre-cur the slurry in a 40℃ water bath for 1 hour to activate the initial reaction of the raw materials. S3. Pour recycled aggregate with a particle size of 4.75mm-9.5mm into the geopolymer slurry. The mass ratio of aggregate to slurry is 1. Stir and soak at a water bath temperature of 65℃ for 1 hour to allow the geopolymer slurry to fully react and adhere evenly to the surface of the recycled aggregate. S4. Remove the coated recycled aggregate and drain the excess slurry from the surface of the recycled aggregate using a sieve. Then, adopt a staged curing method: first, cure in an oven at 40℃ for 12 hours, and then cure intermittently in a microwave oven at 80W for 1 hour (microwave heating for 1 minute every 10 minutes) to allow the polymer shell layer on the surface of the recycled aggregate to fully react and solidify. After natural cooling, a high-performance modified recycled aggregate with high microwave response capability is obtained.

[0049] Example 2 The difference between this embodiment and embodiment 1 is that the amount of composite microwave absorbing agent in step S3 is 40g, and the metakaolin is changed to 136g, while the rest are the same as in embodiment 1.

[0050] Example 3 The difference between this embodiment and embodiment 1 is that the amount of composite microwave absorbing agent in step S3 is 48g, and the metakaolin is changed to 128g, while the rest are the same as in embodiment 1.

[0051] Example 4 The difference between this embodiment and Embodiment 1 is that the amount of composite microwave absorbing agent in step S3 is 56g, and the metakaolin is changed to 120g, while the rest are the same as in Embodiment 1.

[0052] Example 5 The difference between this embodiment and embodiment 1 is that the amount of composite microwave absorbing agent in step S3 is 64g, and the metakaolin is changed to 112g, while the rest are the same as in embodiment 1.

[0053] Example 6 The difference between this embodiment and Embodiment 1 is that the mass ratio of Fe3O4@C to graphite powder in the composite microwave absorber in step S3 is 1:3, while the rest are the same as in Embodiment 1.

[0054] Example 7 The difference between this embodiment and embodiment 1 is that the mass ratio of Fe3O4@C to graphite powder in the composite microwave absorber in step S3 is 1:4, while the rest are the same as in embodiment 1.

[0055] Example 8 The difference between this embodiment and embodiment 6 is that the particle size of the recycled aggregate in step S4 is 9.5mm-16mm, the mass ratio of aggregate to slurry is 1.2, and it is stirred and soaked for 1 hour at a water bath temperature of 65℃. All other aspects are the same as in embodiment 6.

[0056] Example 9 The difference between this embodiment and embodiment 6 is that the particle size of the recycled aggregate in step S4 is 16mm-20mm, the mass ratio of aggregate to slurry is 1.4, and it is stirred and soaked for 1.2h at a water bath temperature of 65℃. All other aspects are the same as in embodiment 6.

[0057] Example 10 The difference between this embodiment and embodiment 6 is that the particle size of the recycled aggregate in step S4 is 20mm-26.5mm, the mass ratio of aggregate to slurry is 1.4, and it is stirred and soaked for 1.5h at a water bath temperature of 70℃. All other aspects are the same as in embodiment 6.

[0058] Example 11 The difference between this embodiment and embodiment 6 is that the particle size of the recycled aggregate in step S4 is 26.5 mm-31.5 mm, the mass ratio of aggregate to slurry is 1.6, and it is stirred and soaked for 2 hours at a water bath temperature of 70°C. All other aspects are the same as in embodiment 6.

[0059] Example 12 Using the above Examples 6 and 8 as coarse aggregates with a particle size of 4.75mm-16mm, basalt as fine aggregate, and SBS asphalt as binder, SMA-13 ​​Marshall specimens were prepared in accordance with the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG 3410-2025).

[0060] Example 13 Using the above Examples 6, 8 and 9 as coarse aggregates with a particle size of 4.75mm-20mm, basalt as fine aggregate, and SBS asphalt as binder, AC-20 Marshall specimens were prepared in accordance with the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG 3410-2025).

[0061] Example 14 Using the above Examples 6, 8, 9 and 10 as coarse aggregates with a particle size of 4.75mm-26.5mm, basalt as fine aggregate, and SBS asphalt as binder, AC-25 Marshall specimens were prepared in accordance with the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG3410-2025).

[0062] Example 15 Using the above Examples 6, 8, 9, 10 and 11 as coarse aggregates with a particle size of 4.75mm-31.5mm, basalt as fine aggregate, and 70# asphalt as binder, ATB-30 Marshall specimens were prepared in accordance with the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG 3410-2025).

[0063] Comparative Example 1 The difference between this comparative example and Example 1 is that the microwave absorbing agent in step S3 is a single graphite powder, while the rest are the same as in Example 1.

[0064] Comparative Example 2 The difference between this comparative example and Example 1 is that the microwave absorbing agent in step S3 is a single Fe3O4@C, while the rest are consistent with Example 1.

[0065] Comparative Example 3 The difference between this comparative example and Example 1 is that the microwave absorber in step S3 is a single Fe3O4 powder, while the rest are the same as in Example 1.

[0066] Comparative Example 4 The difference between this comparative example and Example 1 is that no microwave absorbing agent is added in step S3, while the rest are the same as in Example 1.

[0067] Comparative Example 5 The difference between this comparative example and Example 1 is that the phased curing method in step S5 is changed to a three-stage oven curing method: first, curing at a low temperature (around 40°C) for 24 hours, then raising the temperature to 60°C for 24 hours, and finally curing at 150°C for 8 hours; the rest is consistent with Example 1.

[0068] Comparative Example 6 The difference between this comparative example and Example 1 is that the concentration of the glucose solution prepared in step S1 is 0.15 mol / L, while the rest are the same as in Example 1.

[0069] Comparative Example 7 The difference between this comparative example and Example 1 is that the concentration of the glucose solution prepared in step S1 is 0.65 mol / L, while the rest are the same as in Example 1.

[0070] Blank Group 1 Untreated recycled aggregate was used as a blank control group 1; Figure 1 This is a comparison image of the modified recycled aggregate from Example 6 (right) and the blank group (left). As can be seen from the images, after modification with phosphate geopolymer, the fragile and porous mortar layer on the surface of the recycled aggregate is transformed into a dense phosphate geopolymer layer, thus repairing the surface defects of the recycled aggregate.

[0071] Blank Group 2 Using the above-mentioned blank group 1 as coarse aggregate, basalt as fine aggregate, and SBS asphalt as binder, SMA-13 ​​Marshall specimens were prepared in accordance with the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG 3410-2025).

[0072] To verify the modification effect of the phosphate geopolymer prepared in this invention on recycled aggregates, the water absorption rate, apparent density, and crushing value of the aggregates in the above examples and comparative examples were tested using the experimental testing methods in "JTG 3432-2024 Highway Engineering Aggregate Test Procedure". The microwave response capability of the aggregates was tested using a laboratory microwave heating device (power 560W) and an infrared camera (the initial temperature of the aggregates was 22℃). The results are shown in Table 3.

[0073] Table 3 Performance Tests of Recycled Aggregates for Each Group

[0074] As shown in the table, the phosphate geopolymer impregnation treatment of recycled aggregates according to the present invention can effectively improve the physical properties of the aggregates, reduce the water absorption rate of the aggregates, increase the surface density of the aggregates, reduce the crushing value, and meet the requirements for aggregates used in road engineering.

[0075] Depend on Figure 3 It can be seen that, when microwave-heated for the same period of time, the temperature rise of the examples was much higher than that of the blank group, and the temperatures of examples 5 and 6 reached over 100℃, demonstrating excellent microwave response capabilities, exceeding the softening point temperature of asphalt (around 80℃). The experiment shows that the geopolymer absorbing shell endows the recycled aggregate with excellent microwave response capabilities, enabling it to play a functional role in efficient microwave heating and self-healing in asphalt mixtures.

[0076] Examples 1-5 investigated the performance of modified recycled aggregates with different amounts of composite microwave absorber. As the amount of microwave absorber increased, the microwave performance of the aggregates also improved significantly, but the road performance of the aggregates also decreased. This is because with high amounts of microwave absorber, magnetic adsorption of the absorber can lead to agglomeration, which destroys the three-dimensional cross-linked inorganic cementitious network of the geopolymer's -O-Al-O-Si-OPO- structure, reduces the strength of the geopolymer shell, and affects the final modification effect.

[0077] Comparative Examples 2 and 3 investigated the microwave absorption properties of Fe3O4 with different structures. They found that directly adding ordinary spherical powdered Fe3O4 to the PAG shell as an absorbing component did not significantly improve microwave performance. However, using core-shell structured Fe3O4@C prepared by a hydrothermal method as the absorbing component effectively improved microwave performance. Further analysis of the material composition revealed that in acidic phosphate geopolymer slurries, Fe3O4 powder was corroded by phosphoric acid and decomposed into Fe... 2+ with Fe 3+ Finally, an iron-containing gel phase component is generated, which improves the performance of the geopolymer, but its microwave absorption performance is greatly affected. However, the core-shell structure Fe3O4@C prepared by hydrothermal method is protected by the outer carbon shell, which prevents the internal Fe3O4 from being eroded by phosphoric acid, and its microwave absorption performance is not affected.

[0078] Comparison of Example 4 with the blank group shows that even without the addition of microwave absorber, the PAG shell itself has certain microwave performance. The abundant polar phosphate groups and bound water in the geopolymer gel network have significant dipole-direction polarization loss under microwave field. Although the microwave performance is not high, as a functional matrix, its dielectric constant is between that of air and high-loss microwave absorbing components, which plays the role of electromagnetic impedance buffer layer. Moreover, it is a relay medium for heat loss, which can rapidly and uniformly diffuse the micro-area heat generated by the microwave absorbing components to the entire aggregate.

[0079] Compared with Example 1, Comparative Example 5, which uses a conventional three-stage oven curing method, showed a decrease in the performance of the modified aggregate. A small number of aggregates developed fine cracks and bulges caused by the inability of air bubbles to escape. This is due to the delayed heat transfer from the outside to the inside in traditional oven curing, which causes the outer layer to harden faster and the inner layer to have difficulty escaping the reaction air bubbles, resulting in cracking. In contrast, Example 1 uses microwave heating curing, which rapidly conducts heat from the inside to the outside, promoting the expulsion of water vapor from the inside to the outside, improving the shell density, reducing surface defects, and greatly shortening the curing cycle.

[0080] Comparative Examples 2, 6, and 7 prepared Fe3O4@C with different shell thicknesses by adjusting the glucose concentration used in the hydration heat method. In Comparative Example 2, the shell thickness of the product was roughly calculated using the BET geometric inverse method based on gas physical adsorption. Through gas physical adsorption, the measured specific surface area S1 of the uncoated nano-Fe3O4 core was found to be 11.6 m². 2 / g, the specific surface area S2 of Fe3O4@C particles in Comparative Example 2 was measured to be 8.4~10.2 m². 2 The change in specific surface area (g / cm³) indirectly indicates the formation of a shell on the particle surface. Using high-purity helium (He) as the reference medium, the particle density was determined to be 4.48 g / cm³ using the gas displacement method. 3 Substituting into the spherical multilayer coating geometric evolution model, the diameter of Fe3O4@C particles is calculated as d2 = 6 / (4.48 × S2 × 10). 6The particle size of Fe3O4@C was calculated to be between 130-160 nm. The particle sizes of Comparative Examples 6 and 7 were measured using the same method. The shell thickness is calculated by subtracting the original Fe3O4 particle size of 100 nm from the Fe3O4@C particle size. The shell thicknesses of Comparative Examples 2, 6, and 7 were measured to be 15-30 nm, 5-15 nm, and 30-50 nm, respectively. Comparison of surface temperatures after microwave heating revealed that Comparative Example 2 exhibited the strongest microwave performance, followed by Comparative Example 7, and the weakest was Comparative Example 6. The thickness of the carbon shell on the Fe3O4 surface has a significant impact on microwave performance. If the shell thickness is too low, it cannot fully coat the Fe3O4, allowing phosphate to still penetrate the shell and react with Fe3O4, thus reducing the material's microwave performance. If the shell thickness is too high, it weakens the intensity of microwave transmission to Fe3O4, preventing Fe3O4 from fully utilizing its microwave performance and slowing down the microwave heating rate. A shell thickness of 15-30 nm can effectively balance the effects of phosphoric acid corrosion and reduced microwave transmission. The shell thickness is sufficient to isolate Fe3O4 from the contact reaction with phosphoric acid without excessively reducing the intensity of microwave transmission, thus optimizing the microwave performance of Fe3O4@C. The Fe3O4@C particle size with a shell thickness of 15-30 nm is 130-160 nm.

[0081] Examples 1, 6, and 7, and Comparative Examples 1 and 2 investigated the effects of different absorber ratios on the microwave performance of the materials. It was found that when the mass ratio of Fe3O4@C to graphite powder was 1:3, the microwave heating rate was significantly higher than the other two groups. Regarding the microwave absorption performance of the materials, impedance matching is a decisive indicator. If the impedance is mismatched, even with strong internal loss capacity, microwaves will be violently reflected at the surface and cannot penetrate the material to be converted into heat energy. Single absorber materials, such as carbon-based absorber graphite powder, are high dielectric loss materials, with a dielectric constant much higher than their magnetic permeability. This leads to strong coherent reflection of electromagnetic waves at the material surface (i.e., electromagnetic shielding), preventing microwaves from penetrating deep into the material and only causing surface heating. Ferrite Fe3O4, on the other hand, is a magnetic loss material with a typically low complex dielectric constant and limited natural resonant frequency. The weak electrical loss capacity of a single magnetic component results in low energy conversion efficiency, making it difficult to efficiently convert incident electromagnetic waves into heat energy. Therefore, this invention combines dielectric loss materials with magnetic loss materials. By changing the material ratio, the impedance of the materials is controlled. Graphite powder provides high electrical loss to ensure energy conversion efficiency, while Fe3O4 provides high magnetic loss to offset the imbalance caused by the excessively high dielectric constant of graphite. The equivalent conductivity of the system is adjusted to a moderate level, guiding microwaves to propagate inward and increasing the penetration depth of microwaves, thus achieving impedance matching. A mass ratio of Fe3O4@C to graphite powder of approximately 1:3 yields the best impedance matching effect. Without changing the total amount of absorbing agent, microwave performance is significantly improved, far exceeding that of a single absorbing component.

[0082] The phosphate geopolymer slurry formulation used in Examples 6 and 8-11 is the same, but the conditions for treating recycled aggregates of different particle sizes are different. The recycled aggregates are divided into five different particle sizes, ranging from 4.75mm to 31.5mm. Different particle sizes result in different specific surface areas of the recycled aggregates, and thus different surface mortar contents for the same mass. Therefore, the specific gravity of the slurry to the aggregate, the treatment time, and the treatment temperature all need to be changed during modification. The smaller the aggregate particle size, the higher the surface mortar content for the same mass, and the higher the required mass ratio of phosphate geopolymer slurry for modification. Furthermore, the reaction between the mortar and excess phosphoric acid in the phosphate geopolymer slurry is exothermic, which promotes the geopolymerization reaction, rapidly increases the slurry viscosity, and shortens the required treatment time. The smaller the particle size and the higher the mortar content, the more vigorous the reaction with phosphoric acid, the higher the exothermic reaction, and the lower the required time and temperature for modification. When modifying large-particle-size coarse aggregates, it is necessary to extend the treatment time and increase the reaction temperature to allow the phosphate geopolymer slurry to initially polymerize on the aggregate surface.

[0083] The prepared Marshall specimens were cut into semi-circular bending (SCB) specimens. The initial fracture energy of the specimens was measured using a universal testing machine (UTM) according to the semi-circular bending test method in the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG 3410-2025). Subsequently, the specimens underwent healing treatment for different durations under microwave irradiation. The recovery rate of fracture energy was used to quantitatively evaluate the microwave self-healing ability of the mixture. Simultaneously, an infrared thermal imager was used to record the highest surface temperature of the specimens. The results are shown in the table below.

[0084] Table 4. Relationship between maximum surface temperature (°C) and time (s) of asphalt mixture under microwave heating

[0085] Note: Since the maximum range of the infrared camera is 160℃, subsequent temperature measurements were not performed.

[0086] In Example 6, microwave irradiation for 60 seconds resulted in the mixture temperature exceeding 90°C. This heat is generated directly in the transition zone between the aggregate and asphalt interface, enabling the asphalt to flow and filling micro-cracks at the interface caused by stress concentration, resulting in high thermal energy utilization.

[0087] Table 5. Initial fracture energy and microwave-heated healing fracture energy of asphalt mixtures

[0088] From the experimental data table above, the initial fracture energy of the asphalt mixture in Example 12 (1324.2 J / m) 2 The concentration of the sample was significantly higher than that of the control group 2 (542.5 J / m³). 2This demonstrates that phosphate geopolymers, through chemical corrosion and physical filling, transform the old mortar layer on the surface of recycled aggregates from a loose state into a dense, high-density shell, significantly enhancing the interfacial adhesion between aggregates and asphalt.

[0089] The ratio of the fracture energy to the initial fracture energy of the self-healing Marshall specimen was calculated to characterize the self-healing rate of the asphalt mixture. The results are as follows: Figure 4 As shown in the figure, the asphalt mixture prepared from modified recycled aggregate in this invention, after being microwaved for 60 seconds, reaches a temperature of over 90°C. This drives the asphalt at the interface to rapidly wet and diffuse into the cracks, resulting in a fracture energy recovery rate of over 80% and a healing rate far exceeding that of the control group 2. This achieves highly efficient microwave-induced self-healing of the asphalt mixture. However, when heated for 120 seconds, the healing rate of each embodiment drops to below 65%. This is because the excessively long heating time leads to excessively low asphalt viscosity at the interface, causing flow and volatilization of lightweight components, which reduces the asphalt's bonding properties.

[0090] The analysis results show that the microwave-responsive phosphate geopolymer-modified recycled aggregate of this invention utilizes chemical reactions to remove surface mortar, physically fills aggregate pores and cracks, and incorporates microwave-absorbing materials, resulting in a multi-faceted synergistic effect. This not only improves the surface porosity and microcracks of the recycled aggregate, reduces water absorption, and enhances mechanical properties, ensuring its road performance meets the requirements for road engineering aggregates, but also, the composite microwave-absorbing agent dispersed in the geopolymer shell endows the recycled aggregate with microwave responsiveness. When applied to asphalt pavement, microwave heating allows the aggregate to rapidly heat up to the asphalt softening point, enabling capillary flow to fill cracks and achieving efficient self-healing of the asphalt mixture. This reduces asphalt pavement maintenance costs, extends the service life of asphalt roads, and achieves a unified low-carbon resource utilization and functionalization of construction solid waste.

[0091] The embodiments described above are some, but not all, of the embodiments of the present invention; the detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention; all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing microwave-responsive phosphate geopolymer-modified recycled aggregate, characterized in that, Includes the following steps: S1. Mix phosphoric acid, phosphate, and water to prepare a phosphoric acid activator; S2. Dry mix the silica-alumina raw material and the composite microwave absorber evenly, add the phosphoric acid activator, stir and mix to obtain a microwave-responsive phosphoric acid geopolymer slurry; S3. Immerse the recycled aggregate in the geopolymer slurry to form a coating layer on the surface of the aggregate; S4. Curing the coated recycled aggregate to solidify the coating layer, resulting in microwave-responsive phosphate geopolymer-modified recycled aggregate. The composite microwave absorbing agent comprises graphite powder and core-shell structured iron oxide; the core of the core-shell structured iron oxide includes iron oxide, the shell is a carbon layer with a thickness of 15-30 nm, and the particle size of the core-shell structured iron oxide is 130 nm-160 nm. The preparation method of the core-shell structured iron(III) oxide includes: S1-1. Add Fe3O4 powder to sodium citrate solution, stir and centrifuge to obtain pretreated Fe3O4; S1-2. The above Fe3O4 is uniformly dispersed in a glucose solution and subjected to a hydrothermal reaction to obtain a core-shell structured iron(III) oxide with a carbon layer protecting its surface; the concentration of the glucose solution is 0.4 mol / L. The curing process in step S4 includes: pre-curing in a 40°C oven for 12 hours, followed by intermittent microwave curing at 80W for 1 hour, with heating for 1 minute every 10 minutes.

2. The preparation method according to claim 1, characterized in that, The composite microwave absorber accounts for 8% to 16% of the total mass of the phosphate geopolymer slurry.

3. The preparation method according to claim 1, characterized in that, In the composite microwave absorbing agent of step S2, the mass ratio of core-shell structured iron oxide to graphite powder is 1:(2-4).

4. The preparation method according to claim 1, characterized in that, The particle size of the silicon-aluminum raw material in step S2 is 800~1200 mesh.

5. The preparation method according to claim 1, characterized in that, The geopolymer slurry in step S2 also includes a dispersant, which includes lignin sulfonate, accounting for 0.3%-0.6% of the total mass of the phosphate geopolymer slurry.

6. The preparation method according to claim 1, characterized in that, The recycled aggregate in step S3 is coarse aggregate with a particle size of 4.75 mm or larger.

7. The preparation method according to claim 1, characterized in that, The phosphate in step S1 includes sodium phosphate, aluminum phosphate, aluminum hydrogen phosphate, or aluminum dihydrogen phosphate; the silicoaluminous raw material in step S2 includes one or more of metakaolin, fly ash, or slag.

8. A microwave-responsive phosphate geopolymer-modified recycled aggregate obtained by the preparation method according to any one of claims 1 to 7.

9. The application of the microwave-responsive phosphate geopolymer-modified recycled aggregate as described in claim 8 in asphalt mixtures, characterized in that, It is used in the upper, middle and lower layers of asphalt mixtures involved in asphalt pavement, as well as flexible base layers.

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