Anti-cracking phosphoric acid-based polymer, anti-cracking recycled aggregate and preparation method and application thereof

Abstract

This invention discloses a crack-resistant phosphate-based polymer, crack-resistant recycled aggregate, its preparation method, and its applications. The method uses a 50%–60% (w / w) phosphoric acid solution as the phosphoric acid activation solution. First, metakaolin is added to the phosphoric acid activation solution for mixing, allowing the phosphoric acid to fully decompose the aluminum-silicon structure of the metakaolin. Then, fly ash is added to mix, adjusting the phosphorus-to-aluminum ratio in the reaction system to 1.2–1.4. The mixture is magnetically stirred at 70–80°C until the slurry changes from reddish-brown to gray, forming an inorganic polymer network dominated by P-O-Al and P-O-Si bonds, which is the crack-resistant phosphate-based polymer slurry. Modified recycled aggregate prepared using this phosphate-based polymer slurry, combined with a three-stage curing process, not only exhibits improved crack resistance but also superior mechanical properties. The modified recycled aggregate prepared by this invention has performance close to that of natural aggregates and can be applied to asphalt concrete or asphalt pavements.

Description

Technical Field

[0001] This invention belongs to the field of new building materials technology, and more specifically, relates to a crack-resistant phosphate macropolymer, a crack-resistant recycled aggregate, its preparation method and application. Background Technology

[0002] With the continued deepening of urbanization, the massive accumulation of construction waste has become an urgent problem to be solved. Resource-based disposal of demolition waste not only helps reduce dependence on natural aggregates and alleviate environmental pressure, but also expands the channels for high-value utilization of solid waste, generating positive social benefits. Processing demolition waste through crushing equipment and grading and screening yields recycled aggregates of different particle sizes. However, these recycled aggregates generally suffer from low strength and high water absorption, making it difficult to meet engineering performance requirements when directly used in asphalt concrete or asphalt pavements.

[0003] To improve the performance of recycled coarse aggregates, existing research has explored various strengthening methods. For example, recycled aggregates are ground using a Los Angeles abrasion tester at room temperature, followed by immersion in a low-concentration hydrochloric acid solution to effectively remove aged cement mortar adhering to their surface. Other studies have utilized the binding properties of sodium silicate solution to form a uniform coating layer on the recycled aggregate, filling internal cracks and pores, thereby improving its density and durability. While these methods can improve the mechanical properties of recycled aggregates to some extent, they suffer from drawbacks such as high processing costs and limited strengthening effect.

[0004] Geopolymers, also known as geopolymers, are novel inorganic polymer cementitious materials with a three-dimensional network structure composed of AlO4 and SiO4 tetrahedral structural units. They possess advantages such as rapid early strength development, good heat resistance, and strong bonding ability. However, the performance of geopolymers is affected by various factors, such as the activity and composition of raw materials, the selection of activators, the order of addition, the proportioning design, and curing conditions. This is a multivariate optimization process and the core content of current geopolymer research.

[0005] Chinese patent CN116040993B discloses a method for preparing modified phosphate-based polymers. This method involves mixing an aluminum dihydrogen phosphate solution with a phosphoric acid solution to obtain a mixed solution, then uniformly mixing and stirring this mixed solution with fly ash to ultimately form a phosphate-based polymer with high strength. Furthermore, using phosphate-based polymers to modify recycled aggregates is a feasible technical approach, supported by existing research. For example, Chinese patent CN117700139A discloses a method for modifying recycled aggregates with phosphate-based polymers and its strengthening process. This method involves mixing phosphoric acid, water, and an aluminum source material (such as alumina) to prepare a phosphoric acid activating solution, which is then mixed with a highly active mineral admixture (such as metakaolin) to form a phosphate-based polymer slurry, which is used for surface modification and strength enhancement of recycled aggregates.

[0006] However, while phosphate geopolymer slurry can effectively improve the performance of recycled aggregates by filling the pores and cracks, cracking easily occurs during the preparation of modified recycled aggregates in large-scale practical applications, leading to a significant decline in performance and making it difficult to meet the technical requirements for aggregates used in highway subgrades. Therefore, researching a crack-resistant phosphate-based polymer is beneficial for the large-scale batch modification of recycled aggregates to improve their performance and promote the reuse of recycled aggregates in asphalt concrete. Summary of the Invention

[0007] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a crack-resistant phosphate-based macropolymer, crack-resistant recycled aggregate, its preparation method, and its applications. The aim is to demonstrate that, compared to the metakaolin system, a phosphate-based macropolymer prepared using a metakaolin and fly ash combination system can improve the cracking of modified recycled aggregates. With a fly ash content of 20%–30% and a phosphorus-to-aluminum ratio controlled at 1.2–1.4, the prepared phosphate-based macropolymer significantly reduces cracking in the modified recycled aggregate, thereby solving the technical problem of easy cracking in modified recycled aggregates prepared using existing phosphate-based macropolymers.

[0008] To achieve the above objectives, according to a first aspect of the present invention, a method for preparing a crack-resistant phosphate macropolymer is provided, comprising the following steps:

[0009] A phosphoric acid solution with a mass concentration of 50%~60% was used as the phosphoric acid activation solution, and a combination system of metakaolin and fly ash was used as the reactants;

[0010] First, add metakaolin to the phosphoric acid activation solution and mix to allow the phosphoric acid to fully deconstruct the aluminum-silicon structure of the metakaolin.

[0011] Then add fly ash to mix so that the phosphorus-aluminum ratio in the reaction system is 1.2~1.4. Stir magnetically at 70~80℃ until the slurry changes from reddish-brown to gray, forming an inorganic polymer network mainly composed of PO-Al and PO-Si bonds, which is the crack-resistant phosphate base polymer slurry.

[0012] Preferably, in the preparation method, the solid-liquid ratio of metakaolin to phosphoric acid activation solution is 0.4~0.5:1, and metakaolin is first added to the phosphoric acid activation solution for mixing to obtain reddish-brown slurry A;

[0013] Then, fly ash is added to slurry A and mixed to make the phosphorus-aluminum ratio in the reaction system 1.2~1.4. The mixture is then magnetically stirred at 70~80℃ for 3~4 hours to obtain crack-resistant phosphate macropolymer slurry B. The fly ash content accounts for 20%~30% of the total mass of metakaolin and fly ash.

[0014] Preferably, in the preparation method, fly ash is added to slurry A and mixed to make the phosphorus-aluminum ratio in the reaction system 1.4, wherein the fly ash content accounts for 26.5% of the total mass of metakaolin and fly ash.

[0015] According to a second aspect of the present invention, a crack-resistant phosphate macropolymer is also provided, characterized in that it is prepared by the method described in the present invention.

[0016] Preferably, the crack-resistant phosphate macropolymer is prepared according to the following method:

[0017] A phosphoric acid solution with a mass concentration of 50%~60% was used as the phosphoric acid activation solution, and a combination system of metakaolin and fly ash was used as the reactants;

[0018] With a solid-liquid ratio of metakaolin to phosphoric acid activation solution of 0.4~0.5:1, metakaolin was first added to the phosphoric acid activation solution and mixed to obtain reddish-brown slurry A;

[0019] Then, fly ash is added to slurry A and mixed to make the phosphorus-aluminum ratio in the reaction system 1.4. The mixture is then magnetically stirred at 70-80°C for 3-4 hours to obtain crack-resistant phosphate macropolymer slurry B. The fly ash content accounts for 20%-30% of the total mass of metakaolin and fly ash, preferably 26.5% of the total mass of metakaolin and fly ash.

[0020] According to a third aspect of the invention, the application of the crack-resistant phosphate macropolymer as described herein in the preparation of crack-resistant recycled aggregate is also provided.

[0021] Preferably, in the application, the recycled aggregate is mixed with the crack-resistant phosphate macropolymer slurry and heated in a water bath at 60-80°C until no more continuous small bubbles are generated on the surface of the slurry, thereby obtaining recycled aggregate uniformly coated with phosphate macropolymer slurry;

[0022] Then, a three-stage curing process is adopted: first, curing at room temperature (20℃~30℃) for 10~12 hours, then at a medium temperature (60℃~80℃) for 6~8 hours, and finally at a high temperature (120℃~150℃) for 6~8 hours, followed by natural cooling, which results in crack-resistant recycled aggregate.

[0023] Preferably, in the application, the recycled aggregate is first soaked in a phosphoric acid solution to allow the calcium carbonate in the cement mortar on the surface of the aggregate to react completely with the phosphoric acid to generate carbon dioxide; then it is mixed with crack-resistant phosphoric acid macropolymer slurry and heated in a water bath at 60~80℃ for 3~4 hours.

[0024] Preferably, in the application, the water bath heating temperature is 80°C.

[0025] Preferably, the application involves a three-stage curing process: first, curing at room temperature (25±2℃) for 12 hours; then, curing at a medium temperature (60℃) for 6 hours; and finally, curing at a high temperature (120℃) for 6 hours.

[0026] According to a fourth aspect of the present invention, a crack-resistant recycled aggregate is also provided, which is prepared by modification with phosphate-based polymers as described in the present invention.

[0027] According to a fifth aspect of the invention, an asphalt concrete is also provided, which uses crack-resistant recycled aggregate as described in the invention as aggregate.

[0028] Overall, compared with the prior art, the technical solutions conceived in this invention, based on a combination system of metakaolin and fly ash and a three-stage curing process, can achieve the following beneficial effects:

[0029] 1) The phosphate-based macropolymer provided by this invention uses a 50%~60% (w / w) phosphoric acid solution as the phosphoric acid activating solution. Metakaolin is first added to allow the phosphoric acid to fully decompose the aluminum-silicon structure of the metakaolin. Then, fly ash is added and mixed to achieve a phosphorus-to-aluminum ratio of 1.2~1.4 in the reaction system. This cleverly combines the "ball effect" of fly ash with the "high activity" of metakaolin, while reducing material agglomeration and accumulation. This allows the two to complement each other, generating an inorganic polymer network dominated by PO-Al and PO-Si bonds. The prepared phosphate-based macropolymer slurry exhibits excellent crack resistance.

[0030] 2) Furthermore, the modified recycled aggregate prepared using this phosphate-based polymer not only improves crack resistance but also reduces water absorption by 33.6%–44.8%, crushing index by 16.8%–27.2%, and apparent density to over 2.70 g / cm³, exhibiting superior mechanical properties. The crack-resistant recycled aggregate obtained using this method has performance close to that of natural aggregates and can be applied to asphalt concrete or asphalt pavements. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the entire process of preparing phosphate-based polymer slurry and modifying recycled aggregates.

[0032] Figure 2 This is a response surface plot of variables such as fly ash content, phosphorus-aluminum ratio, and curing temperature, and response quantities such as water absorption rate, apparent density, and crushing value. In the figure, (a), (b), and (c) are the relationships between phosphorus-aluminum ratio and fly ash content, water bath temperature and fly ash content, and water bath temperature and phosphorus-aluminum ratio, respectively, under the condition that the response variable is water absorption rate; (d), (e), and (f) are the relationships between phosphorus-aluminum ratio and fly ash content, water bath temperature and fly ash content, and water bath temperature and phosphorus-aluminum ratio, respectively, under the condition that the response variable is apparent density; (g), (h), and (i) are the relationships between phosphorus-aluminum ratio and fly ash content, water bath temperature and fly ash content, and water bath temperature and phosphorus-aluminum ratio, respectively, under the condition that the response variable is crushing value.

[0033] Figure 3 These are comparison images showing the cracking of modified recycled aggregates in different embodiments and comparative examples. Figure 3 In the figures (a), (b), (c), and (d), respectively, there are physical images of the modified recycled aggregates of Example 2, Example 5, Comparative Example 2, and Comparative Example 3.

[0034] Figure 4 This is a comparison of the surface morphology, interface changes, and SEM images of recycled aggregate before and after modification. The left side of the image shows the recycled aggregate before modification, and the right side shows the recycled aggregate after modification. In the image, (a) shows the surface morphology of the recycled aggregate before and after modification, (b) shows the interface changes of the recycled aggregate before and after modification, RCA represents recycled aggregate, MRCA represents modified recycled aggregate, and (c) shows the microstructure changes of the recycled aggregate before and after modification. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0036] This invention uses phosphoric acid as an activator. By comparing phosphate-based macropolymers prepared from a metakaolin system and a metakaolin-fly ash combination system, it was found that the phosphate-based macropolymer prepared from the metakaolin-fly ash combination system improved the cracking of modified recycled aggregates compared to the metakaolin system. Specifically, adding 20%–30% fly ash (based on the total mass percentage of metakaolin and fly ash) and controlling the phosphorus-aluminum ratio of the reaction system at 1.2–1.4 significantly reduced cracking in the modified recycled aggregates. Applying this phosphate-based macropolymer to the preparation of modified recycled aggregates using a three-stage curing method resulted in modified recycled aggregates that not only reduced cracking but also further improved their mechanical properties. In particular, adding 26.5% fly ash, controlling the phosphorus-aluminum ratio of the reaction system at 1.4, using a water bath temperature of 80°C, and combining three-stage curing resulted in modified recycled aggregates without cracks.

[0037] Based on this discovery, the present invention provides a method for preparing a crack-resistant phosphate macropolymer, comprising the following steps:

[0038] A phosphoric acid solution with a mass concentration of 50%~60% was used as the phosphoric acid activation solution, and a combination of metakaolin and fly ash was used as the reactants. Metakaolin was first added to the phosphoric acid activation solution to allow the phosphoric acid to fully deconstruct the aluminum-silicon structure of the metakaolin. Then, fly ash was added to make the phosphorus-aluminum ratio in the reaction system 1.2~1.4. The mixture was magnetically stirred at 70~80℃ until the slurry changed from reddish-brown to gray, forming an inorganic polymer network mainly composed of PO-Al and PO-Si bonds, which is the crack-resistant phosphoric acid macropolymer slurry.

[0039] This method involves the stepwise addition and mixing of metakaolin and fly ash in a phosphoric acid system. First, metakaolin is added to the phosphoric acid activating solution, allowing the phosphoric acid to fully decompose the aluminosilicate structure of the metakaolin. Then, spherical fly ash is added, reducing the agglomeration of flaky metakaolin and accelerating the reaction. Simultaneously, the fine fly ash particles promptly fill the pores of the geopolymer, increasing its density. This method cleverly combines the "ball effect" of fly ash with the "high activity" of metakaolin, while reducing material agglomeration, allowing the two to complement each other and generate an inorganic polymer network dominated by PO-Al and PO-Si bonds. The structure and properties of this phosphoric acid-based geopolymer differ from traditional alkali-activated geopolymers, exhibiting superior water resistance, early strength, and interfacial adhesion to old cement mortar, as well as excellent crack resistance.

[0040] In some embodiments, the solid-liquid ratio of metakaolin to phosphoric acid activation solution is 0.4~0.5:1. Metakaolin is first added to the phosphoric acid activation solution for mixing, so that the phosphoric acid can fully decompose the aluminum-silicon structure of metakaolin to obtain slurry A.

[0041] Based on the percentage of fly ash content in the total mass of fly ash and metakaolin, fly ash is added to slurry A at a fly ash content of 20% to 30%, and the mixture is reacted at 70 to 80°C for 3 to 4 hours to generate an inorganic polymer network mainly composed of PO-Al and PO-Si bonds, thus obtaining crack-resistant phosphate-based polymer slurry B.

[0042] In some embodiments, the solid-liquid ratio of metakaolin to phosphoric acid activation solution is preferably 0.5:1. After adding metakaolin, stir for 3-5 minutes and let it stand for 15-20 minutes to allow the phosphoric acid to fully decompose the aluminum-silicon structure of metakaolin.

[0043] Furthermore, the preferred fly ash content is 26.5%, the phosphorus-aluminum ratio (P / Al) in the reaction system is 1.4, and the reaction is carried out at 80℃ for 3-4 hours to obtain crack-resistant phosphate macropolymer slurry B.

[0044] The crack-resistant phosphate-based polymer has excellent mechanical properties, with a 3-day compressive strength of 15 MPa and a flexural strength of 2.5 MPa.

[0045] In addition, the present invention also provides the application of the crack-resistant phosphate macropolymer as described in the present invention in the preparation of crack-resistant recycled aggregate.

[0046] The application involves mixing recycled aggregate with the crack-resistant phosphate-based macropolymer slurry B, heating in a water bath at 60-80°C, and continuously stirring to remove air bubbles generated during the reaction until no more small bubbles are produced on the surface of the slurry, ensuring the recycled aggregate is fully and uniformly coated. The treated recycled aggregate is then placed on a sieve to drain excess slurry, followed by a three-stage curing process: first, curing at room temperature (20°C-30°C) for 10-12 hours; then at a medium temperature (60°C-80°C) for 6-8 hours; and finally at a high temperature (120°C-150°C) for 6-8 hours, followed by natural cooling to obtain the modified recycled aggregate. Preferably, the recycled aggregate is first soaked in a low-concentration phosphoric acid solution (1 mol / L) to allow the calcium carbonate in the cement mortar on the aggregate surface to react completely with the phosphoric acid to generate carbon dioxide; then it is mixed with the phosphate-based macropolymer slurry B.

[0047] This method employs a three-stage curing process to further prevent expansion and cracking of the phosphate-based macropolymer shell on the surface of recycled aggregates. The first stage involves room temperature (20℃~30℃) curing for 10~12 hours, allowing the phosphate-based macropolymer slurry to slowly penetrate into the recycled aggregate. Excessive temperature will cause the slurry to evaporate faster than it penetrates, hindering its ability to fill the pores and microcracks within the recycled aggregate. The second stage involves medium temperature (60℃~80℃) curing for 6~8 hours, which allows the slurry to solidify rapidly and form a phosphate-based macropolymer gel layer on the aggregate surface. Medium temperature curing at this stage not only reduces molding time but also effectively avoids expansion stress caused by rapid exothermic reactions, thus reducing the risk of cracking. Finally, the third stage involves high temperature (120℃~150℃) curing for 6~8 hours. At high temperatures, the CaHPO4 formed on the aggregate surface due to the geopolymerization reaction transforms into AlHPO4, which is more stable and less prone to hydrolysis, ensuring the storage stability of the crack-resistant recycled aggregate.

[0048] In some embodiments, the three-stage curing process involves first curing at room temperature (25±2℃) for 12 hours, then curing at medium temperature (60℃) for 6 hours, and finally curing at high temperature (120℃) for 6 hours.

[0049] Recycled aggregates suffer from low strength and poor durability due to their high porosity, high water absorption, and the presence of old cement mortar on their surface, which severely limits their application in asphalt pavements. However, modified recycled aggregates treated using this method can reduce their water absorption by nearly 33.6% to 44.8% and their crushing index by 16.8% to 27.2%, bringing their performance close to that of natural aggregates and enabling their application in asphalt concrete or asphalt pavements.

[0050] The present invention also provides a crack-resistant recycled aggregate, which is prepared by modification with phosphate-based polymers as described in the present invention.

[0051] The present invention also provides an asphalt concrete using crack-resistant recycled aggregate as described in the present invention as aggregate.

[0052] The following are examples.

[0053] The main chemical components and contents of fly ash and metakaolin used in the following embodiments are shown in Table 1.

[0054] Table 1. Main chemical components and contents of fly ash and metakaolin

[0055]

[0056] The recycled aggregates used in the following examples and comparative examples were derived from construction demolition waste. After crushing and sieving, recycled coarse aggregate samples with a particle size of 10-16 mm were obtained, exhibiting a water absorption rate of 4.73%, a crushing value of 25%, and an apparent density of 2.54 g / cm³. The metakaolin had a mesh size of 1250, a pink color, a specific gravity of 2.5 g / cm³, and a specific surface area of ​​20.9 m² / g. The fly ash, after calcination (500℃, 2 h), had a specific gravity of 2.1 g / cm³ and a specific surface area of ​​0.35~0.5 m² / g.

[0057] Example 1: Preparation of Phosphate Base Polymers Based on a Combination System of Metakaolin and Fly Ash

[0058] This embodiment uses phosphoric acid as an activator to compare phosphate-based macropolymers prepared with different ratios of metakaolin and fly ash. The different ratios of metakaolin and fly ash are designed as shown in Table 2. The phosphate-based macropolymers were prepared according to the following steps:

[0059] S1: Mix an 85% phosphoric acid solution with pure water to make the phosphoric acid concentration in the solution 60%, which is used as the phosphoric acid activation solution. First, add metakaolin to the phosphoric acid activation solution for the first stage of mixing. After mixing, stir thoroughly for 3-5 minutes and let stand for 15-20 minutes to allow the phosphoric acid to fully decompose the aluminum-silicon structure of the metakaolin, resulting in a reddish-brown slurry A.

[0060] S2: Different amounts of fly ash (10%, 20%, 30%, and 40% of the total mass of metakaolin and fly ash) are added to slurry A obtained in step S1 for a second stage of mixing. The slurry is then placed on a magnetic stirrer and stirred at a temperature of 70-80°C to accelerate the reaction, allowing PO-Al and PO-Si to combine and form a geopolymer network. In this embodiment, magnetic stirring continues until the slurry color changes from reddish-brown to gray, resulting in a uniform, viscous phosphate-based geopolymer slurry B.

[0061] The obtained phosphate-based polymer slurry B was poured into a 40×40×40mm triple plastic mold and cured in an oven at 60℃. After demolding, it was cured for 7 days, and its mechanical properties were measured. The specific results are shown in Table 2.

[0062] Table 2. 7-day compressive and flexural strength of different phosphate-based polymer blocks.

[0063]

[0064] like Figure 1As shown, recycled aggregate is first soaked in a low-concentration phosphoric acid solution (1 mol / L) to allow the calcium carbonate in the cement mortar on the aggregate surface to react with the phosphoric acid and generate carbon dioxide. Then, it is mixed with a prepared phosphoric acid-based geopolymer slurry B in a third stage. The water bath temperature is set at 60-80℃, and the mixture is continuously stirred to remove air bubbles generated during the reaction, ensuring the recycled aggregate surface is fully coated. Subsequently, a three-stage curing process is performed: first, curing at room temperature (20℃-30℃) for 10-12 hours; then at a medium temperature (60℃-80℃) for 6-8 hours; and finally at a high temperature (120℃-150℃) for 6-8 hours. After slow cooling to room temperature, modified recycled aggregate is obtained. Pre-treatment of the recycled aggregate with phosphoric acid before treatment with the geopolymer slurry reduces the number of air bubbles generated, which is beneficial for the bonding between the geopolymer slurry and the recycled aggregate, reducing the risk of cracking.

[0065] To optimize the preparation process of phosphate-based polymers and modified recycled aggregates, a Box-Behnken design was used for experiments, as detailed below:

[0066] (1) Experimental design using the response surface methodology

[0067] Three key process parameters—fly ash content, phosphorus-aluminum ratio, and water bath temperature—were selected as factors, with water absorption rate (Y1), crushing value (Y2), and apparent density (Y3) as response variables. The levels of each factor are shown in Table 3.

[0068] Table 3 Factors and Levels in Response Surface Method

[0069]

[0070] Note: "Fly ash content" is the percentage of fly ash mass in the total mass of fly ash and metakaolin, %; "phosphorus-aluminum ratio" refers to the molar ratio of phosphorus content in phosphoric acid to the total aluminum content in metakaolin and fly ash; "water bath temperature" refers to the reaction temperature of phosphoric acid macropolymer slurry B with recycled aggregate.

[0071] (2) Experimental design and results

[0072] Seventeen experiments were conducted according to the Box-Behnken design. The specific experimental schemes and results are shown in Table 4.

[0073] Table 4. Experimental Design and Results of Response Surface

[0074]

[0075] (3) Model building and significance analysis

[0076] The experimental data were fitted using multiple regression using Design-Expert software, yielding quadratic polynomial model equations for water absorption rate (Y1), crushing value (Y2), and apparent density (Y3):

[0077] Y1=3.3-0.375A-0.4B-0.1C+0.035A²-0.1C²;

[0078] Y2=2.69+0.0325A+0.0225B+0.0075C-0.0275A²+0.0075C²;

[0079] Y3=20.8-2.09A-1.88B-0.4125C-0.575AB+1.9A².

[0080] Table 5. Significance analysis of each variable.

[0081]

[0082] As shown in Table 5, the p-values ​​of factors A, B and C in the three models are all less than 0.05, and the p-values ​​of A and B are even less than 0.0001. This indicates that these three factors significantly affect the modification effect of phosphate-based polymers on recycled aggregates, especially the fly ash content and the phosphorus-aluminum ratio.

[0083] Use Designexpert software to generate 3D surface plots for each variable, such as... Figure 2 As shown, Figure 2 a, b, and c represent the relationships between the phosphorus-aluminum ratio and fly ash content, water bath temperature and fly ash content, and water bath temperature and phosphorus-aluminum ratio, respectively, under the condition that the response variable is water absorption rate. d, e, and f represent the relationships between the phosphorus-aluminum ratio and fly ash content, water bath temperature and fly ash content, and water bath temperature and phosphorus-aluminum ratio, respectively, under the condition that the response variable is apparent density. g, h, and i represent the relationships between the phosphorus-aluminum ratio and fly ash content, water bath temperature and fly ash content, and water bath temperature and phosphorus-aluminum ratio, respectively, under the condition that the response variable is crushing value. The response variables in the same row are the same (i.e., the vertical axis is the same), and the independent variables in the same column are the same (i.e., the two horizontal axes are the same).

[0084] from Figure 2 The main effects of each variable can be seen, that is, the independent influence of a single variable on the response variable, for example in... Figure 2 In Figure (a), when P / Al is fixed at 1.4, the water absorption rate changes significantly with the amount of fly ash added, and the upward slope of the curve is relatively large, indicating that the amount of fly ash added has a significant impact on the water absorption rate. At the same time, this 3D graph can also highlight the interaction effects between various variables, such as... Figure 2 In the medium (g) range, at lower P / Al ratios, increasing the fly ash content has a relatively low impact on the crushing value, while at higher P / Al ratios, the impact of fly ash content on the crushing value increases significantly, manifested as a steeper slope of the surface.

[0085] (4) Screening the optimal process

[0086] Multi-objective optimization was carried out with the objectives of "minimizing water absorption rate of Y1", "maximizing apparent density of Y2" and "minimizing crushing value of Y3" to obtain the optimal process parameters for phosphate macropolymer: fly ash content of 26.5%, phosphorus-aluminum ratio of 1.4 and water bath temperature of 80℃.

[0087] Verification experiments were conducted under these conditions. The measured values ​​of the modified recycled aggregate were: water absorption 2.61%, crushing value 18.2%, and apparent density 2.72 g / cm³, which were largely consistent with the predicted values. Compared to untreated recycled aggregate, the modified recycled aggregate treated with this phosphate-based polymer showed a 44.8% decrease in water absorption, a 27.2% decrease in crushing value, and a 6.6% increase in apparent density. Lower water absorption indicates reduced porosity and a denser aggregate; a lower crushing value indicates enhanced compressive strength; and increased apparent density indicates tighter particle packing. All three indicators point to improved aggregate density and strength, demonstrating that compared to recycled aggregate, the modified recycled aggregate treated with this phosphate-based polymer is denser, stronger, of higher quality, and exhibits superior crack resistance.

[0088] Example 2

[0089] Phosphate-based polymer slurry was prepared according to the optimal process parameters selected in Example 1: fly ash content 26.5%, phosphorus-aluminum ratio 1.4, water bath temperature 80℃. Modified recycled aggregate was prepared using a three-stage curing system, and the specific steps are as follows:

[0090] S1. Mix 85% phosphoric acid with pure water to make the phosphoric acid solution have a mass concentration of 60%, which is used as the phosphoric acid activation solution; then add metakaolin to the phosphoric acid activation solution and mix to obtain slurry A.

[0091] S2. Add fly ash to the above slurry A to ensure that the solid-liquid ratio of metakaolin, fly ash and phosphoric acid activator is 0.5 (fly ash content is 26.5%), and carry out the second stage of mixing to make the phosphorus-aluminum ratio in the reaction system 1.4. Place the slurry on a magnetic stirrer and set the magnetic stirring temperature to 70~80℃ to accelerate the reaction, so that PO-Al and PO-Si combine to form a geopolymer network, and obtain phosphoric acid geopolymer slurry B for coating recycled aggregate.

[0092] S3. The recycled aggregate is first pretreated by soaking in 1 mol / L low-concentration phosphoric acid. Then, the pretreated recycled aggregate is mixed with phosphoric acid-based polymer slurry B in the third stage. The water bath temperature is set to 80℃ to fully coat the surface of the recycled aggregate. Then, three-stage curing is carried out: first, room temperature (25±2℃) curing for 12 hours, then medium temperature (60℃) curing for 6 hours, and finally high temperature (120℃) curing for 6 hours. After slowly cooling to room temperature, the modified recycled aggregate is obtained.

[0093] Example 3

[0094] The fly ash content was 20%, the phosphorus-aluminum ratio was 1.4, the water bath temperature was 80℃, and other conditions were the same as in Example 2.

[0095] Example 4

[0096] The fly ash content is 30%, the phosphorus-aluminum ratio is 1.4, the water bath temperature is 80℃, and other conditions are the same as in Example 2.

[0097] Example 5

[0098] The fly ash content was 26.5%, the phosphorus-aluminum ratio was 1, the water bath temperature was 80℃, and other conditions were the same as in Example 2.

[0099] Example 6

[0100] The fly ash content is 26.5%, the phosphorus-aluminum ratio is 1.2, the water bath temperature is 80℃, and other conditions are the same as in implementation 2.

[0101] Comparative Example 1

[0102] The same formula as in Example 2 was used, except that conventional high-temperature (120°C) curing was used to prepare modified recycled aggregate.

[0103] Comparative Example 2: Preparation of Phosphate Base Polymers Based on Metakaolin

[0104] The fly ash content was 0, the phosphorus-aluminum ratio was 1.4, the water bath temperature was 80℃, and other conditions were the same as in Example 2.

[0105] Comparative Example 3: Preparation of Phosphate Base Polymers Based on Metakaolin

[0106] The fly ash content is 0, the phosphorus-aluminum ratio is 1.4, the water bath temperature is 80℃, and the curing method is conventional high temperature (120℃) curing.

[0107] Table 6 shows the cracking characteristics of the modified recycled aggregates from Examples 2 to 6 and Comparative Examples 1 to 3; schematic diagrams of cracking of different modified recycled aggregates are shown below. Figure 3 As shown, Figure 3The images show a comparison of surface cracking of the modified recycled aggregates, where (a), (b), (c), and (d) are sample images of Example 2, Example 5, Comparative Example 2, and Comparative Example 3, respectively; the basic physical properties of the different modified recycled aggregates are shown in Table 7.

[0108] Table 6 Record of Macroscopic Cracking

[0109]

[0110] The results from Examples 2 to 4 show that, under the same phosphorus-aluminum ratio, reaction temperature, and curing conditions, phosphate-based macropolymers prepared with different ratios of fly ash and metakaolin have a certain impact on improving the cracking of modified recycled aggregates. Based on the total mass percentage of fly ash and metakaolin, a fly ash content of 20% to 30% is suitable, preferably 20% to 26.5%, and more preferably 26.5%. It is speculated that the appropriate addition of fly ash can provide a micro-aggregate effect, the high phosphorus-aluminum ratio forms a dense structure, and the three-stage stepped curing effectively releases the expansion stress during the curing process. The combined effect of these three factors effectively reduces the cracking of modified recycled aggregates.

[0111] Example 2, employing the optimal formula and process, with a fly ash content of 26.5%, yielded modified recycled aggregate without cracking. Compared to Example 2, Example 3 had a slightly lower fly ash content (20%), resulting in slightly weaker micro-aggregate filling and activity regulation effects, but the overall process still effectively suppressed cracking. Example 4 had a slightly excessive fly ash content (30%), leading to a relatively higher alkalinity and a change in reaction rate, resulting in uneven local shrinkage; however, the three-stage stepped curing still provided remedial protection.

[0112] The results of Examples 2, 5, and 6 show that, under the same fly ash content, reaction temperature, and curing conditions, phosphate-based polymers prepared with different phosphorus-aluminum ratios have a certain impact on the cracking of modified recycled aggregates. In Example 5, the phosphorus-aluminum ratio was too low, resulting in insufficient cross-linking of the [AlO4] tetrahedra, a loose structure, and insufficient strength, making it prone to cracking under shrinkage stress. In Example 6, the phosphorus-aluminum ratio was moderate, and the structure formation was acceptable, but it did not reach the optimal 1.4, indicating room for improvement in density and crack resistance. Therefore, a phosphorus-aluminum ratio of 1.2 to 1.4 is preferred, and a phosphorus-aluminum ratio of 1.4 is more preferred.

[0113] The results of Example 2 and Comparative Example 1 show that, under the same fly ash content, phosphorus-aluminum ratio, and reaction temperature—that is, using the same phosphate-based macropolymer to modify recycled aggregate—different curing conditions have a significant impact on the cracking of the modified recycled aggregate. Comparative Example 1, which directly used high-temperature curing, showed crack widths >0.5 mm in the modified recycled aggregate, indicating significant cracking. This is presumably due to the rapid vaporization of water within the phosphate-based macropolymer slurry, generating enormous vapor pressure, while rapid water loss leads to severe shrinkage, causing the aggregate to crack under thermal shock.

[0114] The results of Example 2 and Comparative Example 2 show that, under the same phosphorus-aluminum ratio, reaction temperature, and curing conditions, the addition of fly ash significantly improves the cracking phenomenon of modified recycled aggregate. Comparative Example 2, using a pure metakaolin system to prepare a phosphate-based macropolymer slurry, resulted in modified recycled aggregate with a large number of cracks distributed throughout, with crack widths of 0.2-0.4 mm. In contrast, the phosphate-based macropolymer slurry prepared using a combination of metakaolin and fly ash in this invention significantly reduces the cracking phenomenon of the modified recycled aggregate, exhibiting crack resistance. The phosphate-based macropolymer slurry prepared with a fly ash content of 26.5% yielded modified recycled aggregate without cracks. However, Comparative Example 3, lacking both fly ash adjustment and the added high-temperature thermal shock, resulted in severe cracking of the modified recycled aggregate.

[0115] Table 7 Comparison of performance of different modified recycled aggregates

[0116]

[0117] As shown in Table 7, Example 2, using optimal process parameters to prepare phosphate-based polymers and combined with three-stage curing, produced modified recycled aggregates with the best water absorption, apparent density, and crushing value, corresponding to optimal surface cracking. This demonstrates that the physical properties of modified recycled aggregates can indirectly reflect the degree of surface cracking. Aggregates with dense structures and excellent mechanical properties are coated with a gel layer formed by phosphate-based polymers, while internal microcracks are also filled. If the inner and outer protective layers are not robust and cracking occurs, the mechanical properties of the obtained modified recycled aggregates will significantly decrease. Therefore, compared to modified recycled aggregates obtained by existing methods, the modified recycled aggregates treated with the phosphate-based polymers prepared based on fly ash and metakaolin of this invention not only effectively reduce cracking but also further improve their mechanical properties.

[0118] To verify the effect of the phosphate-based polymer as a modifier on the improvement of the microstructure of recycled aggregate, scanning electron microscopy was performed on the microstructure of the recycled aggregate before and after modification. The results are as follows: Figure 4 As shown, the comparison of surface morphology, interface changes, and SEM images of recycled aggregate before and after modification is as follows: Figure 4 As shown in the figure, the left side of the figure is the recycled aggregate before modification, and the right side is the recycled aggregate after modification. In the figure, (a) is the surface morphology of the recycled aggregate before and after modification, (b) is the interface change of the recycled aggregate before and after modification, RCA represents recycled aggregate, MRCA represents modified recycled aggregate, and (c) is the micromorphological change of the recycled aggregate before and after modification.

[0119] Figure 4 (a) in the figure is a comparison diagram of the recycled aggregate before and after modification. It can be seen that the surface of the recycled aggregate modified with this phosphate-based geopolymer is coated with a gray geopolymer layer, which has good adhesion to the aggregate and is not easily detached by scraping or soaking. Figure 4 (b) shows the changes in the aggregate interface before and after the modification treatment with this phosphate-based macropolymer. It was found that the original loose and porous old cement mortar layer on the surface of the recycled aggregate after modification with this phosphate-based macropolymer was replaced by a denser macropolymer gel layer. Figure 4 (c) shows the SEM images of the recycled aggregate before and after modification. The surface of the unmodified recycled aggregate is composed of many small particles piled together, with a large number of pores and microcracks. However, after modification with this phosphate-based polymer, the aggregate exhibits a more compact packing, and the number of small particles is significantly reduced.

[0120] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A process for the preparation of a crack resistant phosphosilicate polymer, characterized by, The method comprises the following steps: The phosphoric acid solution with a mass concentration of 50%-60% is used as a phosphoric acid excitation solution, and a combination system of metakaolin and fly ash is used as a reactant; the fly ash accounts for 20%-30% of the total mass of the metakaolin and fly ash; First, the metakaolin is added into the phosphoric acid excitation solution to make the phosphoric acid fully decompose the aluminum-silicon structure of the metakaolin; Then, the fly ash is added to make the phosphorus-aluminum ratio of the reaction system be 1.2-1.4, and the slurry is stirred at 70-80 DEG C by a magnetic force until the slurry changes from red-brown to gray, and an inorganic polymer network mainly formed by P-O-Al and P-O-Si bonds is generated, that is, the anti-cracking phosphoric acid-based polymer slurry.

2. The production method according to claim 1, wherein According to the solid-liquid ratio of the metakaolin to the phosphoric acid excitation solution being 0.4-0.5:1, the metakaolin is first added into the phosphoric acid excitation solution to obtain a red-brown slurry A; Then, the fly ash is added into the slurry A to make the phosphorus-aluminum ratio of the reaction system be 1.2-1.4, and the slurry is stirred at 70-80 DEG C by a magnetic force for 3-4 hours to obtain the anti-cracking phosphoric acid-based polymer slurry B.

3. The production method according to claim 2, wherein The fly ash is added into the slurry A to make the phosphorus-aluminum ratio of the reaction system be 1.4, and the fly ash accounts for 26.5% of the total mass of the metakaolin and fly ash.

4. An anti-cracking phosphoric acid based polymer, characterized in that, The method is prepared by the method as claimed in any one of claims 1 to 3.

5. Use of a phosphonic acid-based polymer as claimed in claim 4 for the preparation of anti-cracking recycled aggregates, characterized by the fact that, First, the recycled aggregate is mixed with the anti-cracking phosphoric acid-based polymer slurry, and heated in a water bath at 60-80 DEG C until no continuous small bubbles are generated on the surface of the slurry to obtain the recycled aggregate uniformly coated with the phosphoric acid-based polymer slurry; Then, the three-stage curing is adopted: first, curing at room temperature of 20-30 DEG C for 10-12 hours, then curing at medium temperature of 60-80 DEG C for 6-8 hours, and finally curing at high temperature of 120-150 DEG C for 6-8 hours, and naturally cooling, that is, the anti-cracking recycled aggregate.

6. Use according to claim 5, wherein First, the recycled aggregate is soaked in the phosphoric acid solution to make the calcium carbonate in the cement mortar on the surface of the aggregate fully react with the phosphoric acid to generate carbon dioxide; then, the aggregate is mixed with the anti-cracking phosphoric acid-based polymer slurry, and heated in a water bath at 60-80 DEG C for 3-4 hours.

7. Use according to claim 6, wherein The three-stage curing is first curing at room temperature of 25±2 DEG C for 12 hours, then curing at medium temperature of 60 DEG C for 6 hours, and finally curing at high temperature of 120 DEG C for 6 hours.

8. An anti-cracking recycled aggregate, characterized in that, The method is prepared by the method as claimed in claim 4.

9. An asphalt concrete, characterized by, The anti-cracking recycled aggregate as claimed in claim 8 is used as the aggregate.

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