Anti-cracking phosphoric acid-based geopolymer, anti-cracking recycled aggregate, and preparation method and application of anti-cracking phosphoric acid-based geopolymer and anti-cracking recycled aggregate

Abstract

The invention discloses an anti-cracking phosphoric acid-based geopolymer, an anti-cracking recycled aggregate as well as a preparation method and application of the anti-cracking phosphoric acid-based geopolymer and the anti-cracking recycled aggregate. The method comprises the following steps: by taking a phosphoric acid solution with the mass concentration of 50-60% as a phosphoric acid excitation solution, firstly, adding metakaolin into the phosphoric acid excitation solution, and mixing, so that the aluminum-silicon structure of the metakaolin is fully deconstructed by phosphoric acid; the preparation method comprises the following steps: adding the phosphoric acid into a reaction system, adding fly ash, mixing to enable the phosphorus-aluminum ratio in the reaction system to be 1.2-1.4, magnetically stirring at 70-80 DEG C until the slurry is changed from red brown to gray, and generating an inorganic polymer network mainly comprising P-O-Al and P-O-Si bonds, namely the anti-cracking phosphoric acid-based geopolymer slurry. The modified recycled aggregate prepared by adopting the phosphoric acid-based geopolymer slurry in combination with three-stage maintenance not only has improved cracking resistance, but also has better mechanical properties. The performance of the prepared modified recycled aggregate is close to that of natural aggregate, and the modified recycled aggregate can be applied to asphalt concrete or asphalt pavements.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new building materials, and more particularly relates to an anti-cracking phosphoric acid-based polymer, an anti-cracking recycled aggregate, and a preparation method and application thereof. BACKGROUND

[0002] With the continuous deepening of urbanization, a large amount of construction waste has become a problem to be solved. Resource disposal of construction demolition waste not only helps to reduce the dependence on natural aggregate and alleviate environmental pressure, but also expands the high-value utilization channel of solid waste and produces positive social benefits. The construction demolition waste is processed by a crushing device, and graded and screened to obtain recycled aggregates of different particle sizes. However, such recycled aggregates generally have the disadvantages of low strength and strong water absorption, and it is difficult to meet the engineering performance requirements when directly used in asphalt concrete or asphalt pavement.

[0003] To improve the performance of recycled coarse aggregate, various strengthening methods have been tried in existing research. For example, a Los Angeles abrasion tester is used to polish the recycled aggregate at room temperature, and then a low-concentration hydrochloric acid solution is used for immersion to effectively remove the aged cement mortar attached to the surface. There are also studies that take advantage of the binding properties of sodium silicate solution to form a uniform coating layer on the recycled aggregate, filling the internal cracks and pores, thereby improving its compactness and durability. Although these methods can improve the mechanical properties of recycled aggregate to some extent, they have problems such as high processing cost and limited strengthening range.

[0004] Geopolymer, also known as geopolymer, is a new type of inorganic polymer cementing material composed of AlO4 and SiO4 tetrahedral structural units forming a three-dimensional network structure. It has the advantages of fast early strength development, good heat resistance, and strong bonding ability. However, the performance of geopolymer is influenced by various factors, such as raw material activity and composition, selection of activator, feeding sequence, ratio design, and curing conditions. This is a multi-variable optimization process, which is also the core content of current geopolymer research.

[0005] Chinese patent CN116040993B discloses a preparation method of modified phosphoric acid geopolymer, which is prepared by mixing aluminum dihydrogen phosphate solution and phosphoric acid solution to obtain a mixed solution, then uniformly mixing the mixed solution with fly ash and stirring, and finally forming a phosphoric acid geopolymer with high strength. In addition, using phosphoric acid geopolymer to modify recycled aggregate as a feasible technical path is currently supported by related research. For example, Chinese patent CN117700139A discloses a phosphoric acid geopolymer modified recycled aggregate and a modification and strengthening method thereof. The method is prepared by mixing phosphoric acid, water and aluminum source material (such as aluminum oxide) to prepare a phosphoric acid activator, then mixing it with high-activity mineral admixture (such as metakaolin) to form a phosphoric acid geopolymer slurry, and used for surface modification and strength improvement of recycled aggregate.

[0006] However, although the phosphoric acid geopolymer slurry can effectively improve the performance of recycled aggregate by filling the pores and cracks of the recycled aggregate, cracking phenomenon often occurs during the preparation of modified recycled aggregate in large-scale practical application, which leads to significant decrease in performance and makes it difficult to meet the technical requirements of highway subgrade aggregate. Therefore, it is beneficial to mass modify recycled aggregate to improve its performance and promote the reuse of recycled aggregate in asphalt concrete by studying an anti-cracking phosphoric acid geopolymer. SUMMARY

[0007] In view of the above defects or improvement needs of the prior art, the present application provides an anti-cracking phosphoric acid geopolymer, an anti-cracking recycled aggregate, a preparation method and application thereof, which aims to find that compared with the metakaolin system, the phosphoric acid geopolymer prepared by using the metakaolin and fly ash combined system can improve the cracking condition of the modified recycled aggregate. The fly ash content accounts for 20%~30%, and the phosphorus aluminum ratio of the reaction system is controlled to be 1.2~1.4. The prepared phosphoric acid geopolymer can significantly reduce the cracking of the modified recycled aggregate, thereby solving the technical problem that the modified recycled aggregate prepared by using the existing phosphoric acid geopolymer is prone to cracking.

[0008] To achieve the above-mentioned purpose, according to the first aspect of the present application, a preparation method of an anti-cracking phosphoric acid geopolymer is provided, which comprises the following steps: The phosphoric acid solution with a mass concentration of 50%~60% is used as the phosphoric acid activator, and the metakaolin and fly ash combined system is used as the reactant; First, metakaolin is added to the phosphoric acid activator to make the phosphoric acid fully decompose the aluminum-silicon structure of metakaolin; Then, the fly ash is added to make the phosphorus aluminum ratio in the reaction system be 1.2~1.4, and the slurry is stirred at 70~80℃ until it changes from reddish brown to gray, and an inorganic polymer network mainly composed of P-O-Al and P-O-Si bonds is generated, that is, the anti-cracking phosphoric acid geopolymer slurry.

[0009] Preferably, the preparation method comprises the following steps: adding metakaolin into phosphoric acid activator to obtain a red-brown slurry A according to a solid-liquid ratio of 0.4-0.5:1; Then, fly ash is added into the slurry A to make the phosphorus-aluminum ratio in the reaction system be 1.2-1.4, and the slurry B of the anti-cracking phosphoric acid-based polymer is obtained by magnetic stirring at 70-80℃ for 3-4h; the fly ash content accounts for 20%-30% of the total mass of the metakaolin and fly ash.

[0010] Preferably, the preparation method comprises the following steps: adding fly ash into the slurry A to make the phosphorus-aluminum ratio in the reaction system be 1.4, and the fly ash content accounts for 26.5% of the total mass of the metakaolin and fly ash.

[0011] According to the second aspect of the present application, the anti-cracking phosphoric acid-based polymer is prepared by the method as described in the present application.

[0012] Preferably, the anti-cracking phosphoric acid-based polymer is prepared by the following method: The phosphoric acid solution with a mass concentration of 50%-60% is used as the phosphoric acid activator, and the combination of metakaolin and fly ash is used as the reactant; Preferably, the preparation method comprises the following steps: adding metakaolin into phosphoric acid activator to obtain a red-brown slurry A according to a solid-liquid ratio of 0.4-0.5:1; Then, fly ash is added into the slurry A to make the phosphorus-aluminum ratio in the reaction system be 1.4, and the slurry B of the anti-cracking phosphoric acid-based polymer is obtained by magnetic stirring at 70-80℃ for 3-4h; the fly ash content accounts for 20%-30% of the total mass of the metakaolin and fly ash.

[0013] According to the third aspect of the present application, the anti-cracking phosphoric acid-based polymer as described in the present application is used in the preparation of the anti-cracking recycled aggregate.

[0014] Preferably, the application comprises the following steps: mixing the recycled aggregate with the slurry of the anti-cracking phosphoric acid-based polymer, and heating in a water bath at 60-80℃ until no continuous small bubbles are generated on the surface of the slurry, so as to obtain the recycled aggregate wrapped with the slurry of the anti-cracking phosphoric acid-based polymer; Then, three-stage curing is adopted: curing at room temperature of 20-30℃ for 10-12h, curing at medium temperature of 60-80℃ for 6-8h, and finally curing at high temperature of 120-150℃ for 6-8h, and then naturally cooling, so as to obtain the anti-cracking recycled aggregate.

[0015] Preferably, the application, the recycled aggregate is first soaked with a phosphoric acid solution, so that the calcium carbonate in the cement mortar on the surface of the aggregate reacts completely with the phosphoric acid to generate carbon dioxide; and then mixed with an anti-cracking phosphoric acid-based polymer slurry, and heated in a water bath at 60-80°C for 3-4h.

[0016] Preferably, the application, the water bath heating temperature is 80°C.

[0017] Preferably, the application, the three-stage curing is first cured at room temperature 25±2°C for 12h, then cured at medium temperature 60°C for 6h, and finally cured at high temperature 120°C for 6h.

[0018] According to the fourth aspect of the present application, an anti-cracking recycled aggregate is also provided, which is prepared by modifying the phosphoric acid-based polymer as described in the present application.

[0019] According to the fifth aspect of the present application, an asphalt concrete is also provided, which uses the anti-cracking recycled aggregate as described in the present application as aggregate.

[0020] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects due to the combination of metakaolin and fly ash and three-stage curing: 1) The phosphoric acid-based polymer provided by the present application uses a phosphoric acid solution with a mass concentration of 50%-60% as a phosphoric acid excitation solution, first adds metakaolin to fully decompose the aluminum-silicon structure of metakaolin with phosphoric acid, and then adds fly ash to mix and make the phosphorus-aluminum ratio in the reaction system 1.2-1.4. This not only ingeniously combines the "ball bearing effect" of fly ash and the "high activity" of metakaolin, but also reduces the agglomeration of materials, so that the advantages of the two are complementary, and an inorganic polymer network mainly composed of P-O-Al and P-O-Si bonds is generated, and the prepared phosphoric acid-based polymer slurry has good anti-cracking performance.

[0021] 2) In addition, the modified recycled aggregate prepared by using the present phosphoric acid-based polymer has improved anti-cracking performance, and the water absorption of the modified recycled aggregate is reduced by 33.6-44.8%, the crushing index value is reduced by 16.8%-27.2%, and the apparent density can be increased to more than 2.70g / cm³, having better mechanical properties. The anti-cracking recycled aggregate obtained by the present method has performance close to that of natural aggregate, and can be applied to asphalt concrete or asphalt pavement. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a full process schematic diagram of the preparation of the phosphoric acid-based polymer slurry and the modification treatment of the recycled aggregate.

[0023] Figure 2are response surface plots of the variables of fly ash content, phosphorus aluminum ratio and curing temperature and the response variables of water absorption, apparent density and crushing value, in which (a), (b), (c) are the relationships between the phosphorus aluminum ratio and the fly ash content, the water bath temperature and the fly ash content, and the water bath temperature and the phosphorus aluminum ratio under the condition of the response variable of water absorption; (d), (e), (f) are the relationships between the phosphorus aluminum ratio and the fly ash content, the water bath temperature and the fly ash content, and the water bath temperature and the phosphorus aluminum ratio under the condition of the response variable of apparent density; (g), (h), (i) are the relationships between the phosphorus aluminum ratio and the fly ash content, the water bath temperature and the fly ash content, and the water bath temperature and the phosphorus aluminum ratio under the condition of the response variable of crushing value.

[0024] Figure 3 are comparative diagrams of cracking conditions of modified recycled aggregates of different examples and comparative examples, Figure 3 (a), (b), (c) and (d) in FIG. 8 are actual photos of the modified recycled aggregates of Example 2, Example 5, Comparative Example 2 and Comparative Example 3, respectively.

[0025] Figure 4 are comparative diagrams of surface morphology, interface change and SEM atlas of the recycled aggregate before and after modification, in which the left is the recycled aggregate before modification, and the right is the recycled aggregate after modification; (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 the recycled aggregate, and MRCA represents the modified recycled aggregate, (c) is the micro-morphology change of the recycled aggregate before and after modification. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with the accompanying drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as there is no conflict.

[0027] The application uses phosphoric acid as an activator, and by comparing the phosphoric acid geopolymer prepared by a metakaolin system and a metakaolin and fly ash combined system, it is found that compared with the metakaolin system, the phosphoric acid geopolymer prepared by the metakaolin and fly ash combined system can improve the cracking condition of the modified recycled aggregate. Among them, according to the total mass percentage of metakaolin and fly ash, 20% to 30% of fly ash is added, and the phosphorus aluminum ratio of the reaction system is controlled to be 1.2 to 1.4, so that the phosphoric acid geopolymer prepared can significantly reduce the cracking of the modified recycled aggregate. When the phosphoric acid geopolymer is applied to prepare the modified recycled aggregate, three-stage curing is adopted, and the obtained modified recycled aggregate not only reduces the cracking of the aggregate, but also further improves the mechanical properties. Especially, when 26.5% of fly ash is added, the reaction system phosphorus aluminum ratio is controlled to be 1.4, the water bath temperature is 80℃, and three-stage curing is adopted, the obtained modified recycled aggregate has no cracks.

[0028] Based on this finding, the application provides a preparation method of an anti-cracking phosphoric acid geopolymer, which comprises the following steps: A phosphoric acid solution with a mass concentration of 50% to 60% is used as a phosphoric acid activation solution, and a metakaolin and fly ash combined system is used as a reactant; the metakaolin is first added to the phosphoric acid activation solution to fully decompose the aluminum-silicon structure of the metakaolin, and then the fly ash is added to make the phosphorus aluminum ratio of the reaction system be 1.2 to 1.4, and the slurry is stirred at 70 to 80℃ until the slurry changes from reddish brown to gray, and an inorganic polymer network mainly composed of P-O-Al and P-O-Si bonds is generated, that is, an anti-cracking phosphoric acid geopolymer slurry.

[0029] In the method, the metakaolin and the fly ash are added step by step in the phosphoric acid system, the metakaolin is first added to the phosphoric acid activation solution to fully decompose the aluminum-silicon structure of the metakaolin, the spherical fly ash is then added to reduce the agglomeration of the flaky metakaolin and accelerate the reaction, and the small fly ash particles can timely fill the pores of the geopolymer and increase the compactness of the geopolymer structure. The method ingeniously combines the "ball bearing effect" of the fly ash and the "high activity" of the metakaolin, reduces the agglomeration and accumulation of the materials, and makes the advantages of the two complementary, so as to generate an inorganic polymer network mainly composed of P-O-Al and P-O-Si bonds. The structure and performance of the phosphoric acid geopolymer are different from those of the traditional alkali-activated geopolymer, and the phosphoric acid geopolymer has more advantages in water resistance, early strength and interfacial bonding force with old cement mortar, and has good anti-cracking performance.

[0030] In some embodiments, according to the solid-liquid ratio of metakaolin to phosphoric acid activation solution of 0.4 to 0.5:1, the metakaolin is first added to the phosphoric acid activation solution to fully decompose the aluminum-silicon structure of the metakaolin, and slurry A is obtained; The fly ash is mixed into the slurry A according to the fly ash content of 20%-30% of the total mass percentage of fly ash and metakaolin, and the inorganic polymer network mainly formed by P-O-Al and P-O-Si bonds is generated by reacting at 70-80 DEG C for 3-4 hours to obtain the anti-cracking phosphoric acid-based polymer slurry B.

[0031] In some embodiments, the solid-liquid ratio of metakaolin to phosphoric acid excitation solution is preferably 0.5:1, the metakaolin is stirred for 3-5 minutes after being added, and is aged for 15-20 minutes to make the phosphoric acid fully decompose the aluminum-silicon structure of metakaolin.

[0032] Further, the fly ash content is preferably 26.5%, the phosphorus-aluminum ratio (P / Al) in the reaction system is 1.4, the reaction is carried out at 80 DEG C for 3-4 hours to obtain the anti-cracking phosphoric acid-based polymer slurry B.

[0033] The anti-cracking phosphoric acid-based polymer has excellent mechanical properties, and the 3-day compressive strength can reach 15 MPa, and the flexural strength can reach 2.5 MPa.

[0034] In addition, the application also provides a use of the anti-cracking phosphoric acid-based polymer in preparing anti-cracking recycled aggregate.

[0035] The use is to mix the recycled aggregate with the anti-cracking phosphoric acid-based polymer slurry B, heat in a water bath at 60-80 DEG C, and continuously stir to discharge the bubbles generated in the reaction process until no continuous small bubbles are generated on the surface of the slurry, so that the surface of the recycled aggregate is fully and uniformly coated; the treated recycled aggregate is placed on a sieve to drain the excess slurry, and then three-stage curing is adopted: first, room temperature (20 DEG C-30 DEG C) curing for 10-12 hours, then medium temperature (60 DEG C-80 DEG C) curing for 6-8 hours, and finally high temperature (120 DEG C-150 DEG C) curing for 6-8 hours, and natural cooling, which is the modified recycled aggregate. Preferably, the recycled aggregate is first soaked in a low-concentration phosphoric acid solution (1 mol / L) 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; and then mixed with the phosphoric acid-based polymer slurry B.

[0036] The method can further avoid the expansion and cracking of the phosphoric geopolymer shell on the surface of the recycled aggregate by adopting three-stage curing. The first stage is room temperature (20-30 DEG C) curing for 10-12 hours, mainly to allow the phosphoric geopolymer slurry to slowly penetrate into the recycled aggregate, if a too high temperature is used, the volatilization speed of the slurry will be greater than the penetration speed, which is not conducive to the filling of the slurry in the pores and micro-cracks in the recycled aggregate. Then, the second stage is medium temperature (60-80 DEG C) curing for 6-8 hours, which can make the slurry quickly solidify and form a phosphoric geopolymer gel layer on the surface of the aggregate. At this time, the medium temperature curing can not only reduce the molding time, but also effectively avoid the expansion stress caused by the rapid heat release reaction, thereby reducing the risk of cracking. Finally, the third stage is high temperature (120-150 DEG C) curing for 6-8 hours, and the CaHPO4 on the surface of the aggregate formed by the geopolymerization reaction will be converted into AlHPO4, which has stronger stability and is less likely to hydrolyze, thereby ensuring the storage stability of the anti-cracking recycled aggregate.

[0037] In some embodiments, the three-stage curing is first room temperature (25±2 DEG C) curing for 12 hours, then medium temperature (60 DEG C) curing for 6 hours, and finally high temperature (120 DEG C) curing for 6 hours.

[0038] The recycled aggregate has high porosity, high water absorption rate, and old cement mortar attached to the surface, which leads to low strength and poor durability, so direct use will seriously restrict its application in asphalt pavement. The modified recycled aggregate treated by the method has a water absorption rate reduced by nearly 33.6-44.8%, and a crushing index value reduced by 16.8-27.2%, so that its performance approaches the level of natural aggregate, and can be applied to asphalt concrete or asphalt pavement.

[0039] The application further provides an anti-cracking recycled aggregate prepared by modifying the phosphoric geopolymer as described in the application.

[0040] The application further provides an asphalt concrete using the anti-cracking recycled aggregate as described in the application as aggregate.

[0041] The following is an example The main chemical components and contents of the fly ash and metakaolin used in the following examples are shown in Table 1.

[0042] Table 1 Main chemical components and contents of fly ash and metakaolin

[0043] The recycled aggregate used in the following examples and comparative examples is derived from construction site demolition waste, which is crushed and sieved to obtain recycled coarse aggregate samples with a particle size of 10-16 mm, a water absorption rate of 4.73%, a crushing value of 25%, and an apparent density of 2.54 g / cm3. The metakaolin has a specification of 1250 mesh, a pink color, a specific gravity of 2.5 g / cm3, and a specific surface area of 20.9 m2 / g. The fly ash is calcined (500°C, 2h), has a specific gravity of 2.1 g / cm3, and a specific surface area of 0.35-0.5 m2 / g.

[0044] Example 1 Preparation of phosphoric acid geopolymer based on a combination of metakaolin and fly ash In this example, phosphoric acid is used as an activator to compare the phosphoric acid geopolymer prepared by different proportions of metakaolin and fly ash. The different proportions of metakaolin and fly ash are designed as shown in Table 2. The phosphoric acid geopolymer is prepared according to the following steps: S1: Mix a 85% by mass phosphoric acid solution with pure water to obtain a phosphoric acid solution with a phosphoric acid concentration of 60%, which is used as a 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 min and let stand for 15-20 min to allow the phosphoric acid to fully deconstruct the aluminum-silicon structure of the metakaolin, obtaining a reddish-brown slurry A.

[0045] S2: Add different amounts of fly ash (10%, 20%, 30%, 40% based on the total mass of metakaolin and fly ash) to the slurry A obtained in step S1 for the second stage of mixing. Place the slurry in a magnetic stirrer and set the stirring temperature to 70-80°C to accelerate the reaction, allowing P-O-Al and P-O-Si to combine to form a geopolymer network. In this example, the magnetic stirring continues until the color of the slurry changes from reddish-brown to gray, obtaining a uniform and viscous phosphoric acid geopolymer slurry B.

[0046] Pour the obtained phosphoric acid geopolymer slurry B into a 40x40x40mm three-way plastic mold and cure in an oven at 60°C. After demolding, cure for 7 days and measure the mechanical properties. The specific results are shown in Table 2.

[0047] Table 2 7-day compressive and flexural strength of different phosphoric acid geopolymer test blocks

[0048] As Figure 1As shown, the recycled aggregate is first soaked in a low-concentration phosphoric acid solution (1 mol / L) for pretreatment, so that the calcium carbonate in the cement mortar on the surface of the aggregate reacts with the phosphoric acid to generate carbon dioxide; then the recycled aggregate is mixed with the prepared phosphoric acid-based geopolymer slurry B in the third stage, the water bath temperature is set to 60-80°C, and continuous stirring is performed to discharge the bubbles generated during the reaction, so that the surface of the recycled aggregate is fully coated; then three-stage curing is performed: first, room temperature (20-30°C) curing for 10-12 h, then medium temperature (60-80°C) curing for 6-8 h, and finally high temperature (120-150°C) curing for 6-8 h, and then slowly cooling to room temperature to obtain the modified recycled aggregate. After the recycled aggregate is pretreated with phosphoric acid, the bubbles generated during the treatment with the geopolymer slurry will be relatively reduced, which is beneficial to the combination of the geopolymer slurry and the recycled aggregate and reduces the risk of cracking.

[0049] To optimize the preparation process of the phosphoric acid-based geopolymer and the modified recycled aggregate, Box-Behnken design is used for experiments, as follows: (1) Response surface method experimental design Three key process parameters, i.e., fly ash content, phosphorus-aluminum ratio, and water bath temperature, are selected as factors, and water absorption (Y1), crushing value (Y2), and apparent density (Y3) are selected as response variables. The factor levels are shown in Table 3: Table 3 Response surface method factors and levels

[0050] Note: "Fly ash content" is the mass percentage of fly ash 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 + fly ash; "water bath temperature" refers to the reaction temperature of the phosphoric acid-based geopolymer slurry B and the recycled aggregate.

[0051] (2) Experimental scheme and results According to the Box-Behnken design, 17 groups of experiments are performed, and the experimental scheme and results are shown in Table 4: Table 4 Response surface experimental design and results

[0052] (3) Model establishment and significance analysis The Design-Expert software is used to perform multiple regression fitting on the experimental data, and the quadratic polynomial model equations of water absorption (Y1), crushing value (Y2), and apparent density (Y3) are obtained: Y1=3.3-0.375A-0.4B-0.1C+0.035A²-0.1C²; Y2=2.69+0.0325A+0.0225B+0.0075C-0.0275A²+0.0075C²; Y3=20.8-2.09A-1.88B-0.4125C-0.575AB+1.9A².

[0053] Table 5. Significance analysis of each variable.

[0054] 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.

[0055] 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).

[0056] 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.

[0057] (4) Screening the optimal process 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℃.

[0058] Under this condition, the verification experiment was carried out, and the measured values of the modified recycled aggregate were as follows: water absorption rate 2.61%, crushing value 18.2%, and apparent density 2.72 g / cm3, which were consistent with the predicted values. Compared with the untreated recycled aggregate, the water absorption rate of the modified recycled aggregate treated by the phosphoric acid-based geopolymer decreased by 44.8%, the crushing value decreased by 27.2%, and the apparent density increased by 6.6%. The decrease in water absorption rate means that the porosity is reduced, and the aggregate is more dense; the decrease in crushing value indicates that the compression breaking resistance of the aggregate is enhanced; and the increase in apparent density indicates that the particle accumulation is more compact. The three factors together indicate that the density and strength of the aggregate are improved, and the modified recycled aggregate treated by the phosphoric acid-based geopolymer has better quality and anti-cracking performance than the recycled aggregate.

[0059] Example 2 The phosphoric acid-based geopolymer slurry was prepared according to the optimal process parameters screened in Example 1: fly ash content 26.5%, phosphorus-aluminum ratio 1.4, water bath temperature 80°C, and a three-stage curing system was used to prepare the modified recycled aggregate, and the specific steps were as follows: S1, 85% of phosphoric acid was mixed with pure water to prepare a phosphoric acid solution with a mass concentration of 60%, which was used as a phosphoric acid activator; then metakaolin was added to the phosphoric acid activator and mixed to obtain slurry A; S2, fly ash was added to the above slurry A to ensure that the solid-liquid ratio of metakaolin, fly ash, and phosphoric acid activator was 0.5 (fly ash content was 26.5%), and the mixture was subjected to a second stage of mixing to make the phosphorus-aluminum ratio in the reaction system 1.4. The slurry was placed on a magnetic stirrer and stirred at a temperature of 70-80°C to accelerate the reaction, so that P-O-Al and P-O-Si combined to form a geopolymer network, and a phosphoric acid-based geopolymer slurry B for coating recycled aggregate was obtained.

[0060] S3, the recycled aggregate was first soaked in a low-concentration phosphoric acid solution of 1 mol / L for pretreatment, and then the pretreated recycled aggregate was mixed with the phosphoric acid-based geopolymer slurry B in a third stage, and the water bath temperature was set to 80°C to ensure that the surface of the recycled aggregate was fully coated. Then, a three-stage curing was carried out: first, room temperature (25±2°C) curing for 12 h, then medium temperature (60°C) curing for 6 h, and finally high temperature (120°C) curing for 6 h, and the modified recycled aggregate was obtained after slow cooling to room temperature.

[0061] Example 3 Fly ash content 20%, phosphorus-aluminum ratio 1.4, water bath temperature 80°C, and the others were the same as in Example 2.

[0062] Example 4 Fly ash content 30%, phosphorus-aluminum ratio 1.4, water bath temperature 80°C, and the others were the same as in Example 2.

[0063] Example 5 Fly ash content 26.5%, P / A ratio 1, water bath temperature 80℃, others same as Example 2.

[0064] Example 6 Fly ash content 26.5%, P / A ratio 1.2, water bath temperature 80℃, others same as Example 2.

[0065] Comparative Example 1 The same formula as Example 2 was used, except that conventional high temperature (120℃) curing was used to prepare the modified recycled aggregate.

[0066] Comparative Example 2 Preparation of phosphoric acid-based polymer based on metakaolin Fly ash content 0, P / A ratio 1.4, water bath temperature 80℃, others same as Example 2.

[0067] Comparative Example 3 Preparation of phosphoric acid-based polymer based on metakaolin Fly ash content 0, P / A ratio 1.4, water bath temperature 80℃, curing method using conventional high temperature (120℃) curing.

[0068] The cracking of the modified recycled aggregates of Example 2 to Example 6 and Comparative Example 1 to Comparative Example 3 is shown in Table 6; the schematic diagram of the cracking of different modified recycled aggregates is shown in Figure 3 , Figure 3 is a comparison diagram of the surface cracking of the modified recycled aggregate, wherein (a), (b), (c) and (d) are respectively the sample diagrams of Example 2, Example 5, Comparative Example 2 and Comparative Example 3; the basic physical properties of different modified recycled aggregates are shown in Table 7.

[0069] Table 6 Macro-cracking condition record table

[0070] From the results of Example 2 to Example 4, under the same P / A ratio and reaction temperature and curing conditions, the different proportions of fly ash and metakaolin for preparing the phosphoric acid-based polymer have a certain influence on improving the cracking of the modified recycled aggregate. In terms of the total mass percentage of fly ash and metakaolin, the fly ash content of 20%~30% is more appropriate, preferably the fly ash content of 20%~26.5%, and more preferably the fly ash content of 26.5%. It is speculated that the appropriate addition of fly ash can provide a micro-aggregate effect, a high P / A ratio forms a dense structure, and a three-stage curing effectively releases the expansion stress during the curing process, and the three work together to effectively reduce the cracking of the modified recycled aggregate.

[0071] The fly ash content of Example 2 is 26.5%, and the modified recycled aggregate obtained has no cracks. Compared with Example 2, the fly ash content of Example 3 is slightly lower (20%), the micro aggregate filling and active adjustment is slightly weaker, but the overall process can still effectively inhibit cracking. The fly ash content of Example 4 is slightly excessive (30%), the alkalinity of the system is relatively increased and the reaction rate changes, resulting in local uneven shrinkage, but the three-stage step curing still plays a remedial role.

[0072] From the results of Example 2, Example 5 and Example 6, it can be seen that under the same fly ash content and reaction temperature and curing conditions, the cracking of the modified recycled aggregate prepared by different phosphoalumina ratios has a certain influence. The phosphoalumina ratio of Example 5 is too low, the [AlO4] tetrahedron crosslinking is insufficient, the structure is loose, and the strength is insufficient, which is easy to crack under shrinkage stress. The phosphoalumina ratio of Example 6 is moderate, the structure formation is acceptable, but it fails to reach the optimal 1.4, and the compactness and crack resistance still have room for improvement. Therefore, the phosphoalumina ratio is preferably 1.2-1.4, and more preferably 1.4.

[0073] From the results of Example 2 and Comparative Example 1, it can be seen that under the same fly ash content, phosphoalumina ratio and reaction temperature conditions, i.e. using the same phosphoalumina polymer to modify the recycled aggregate, different curing conditions have a significant influence on the cracking of the modified recycled aggregate. In Comparative Example 1, high-temperature curing is directly used, the crack width of the modified recycled aggregate is >0.5mm, and the cracking phenomenon is obvious. It is speculated that the internal water in the phosphoalumina polymer slurry may have rapidly vaporized, generating a large amount of steam pressure, and the rapid water loss may have caused severe shrinkage, resulting in cracking of the aggregate under thermal shock.

[0074] From the results of Example 2 and Comparative Example 2, it can be seen that under the same phosphoalumina ratio, reaction temperature and curing conditions, it is found that the addition of fly ash can significantly improve the cracking phenomenon of the modified recycled aggregate. In Comparative Example 2, the phosphoalumina polymer slurry prepared by using pure metakaolin system has a large number of cracks distributed on the whole modified recycled aggregate, and the crack width is 0.2-0.4mm. However, the phosphoalumina polymer slurry prepared by using the metakaolin and fly ash combined system of the present application can significantly reduce the cracking phenomenon of the modified recycled aggregate and has the performance of crack resistance. The phosphoalumina polymer slurry prepared by adding fly ash with a content of 26.5% has no cracks. Comparative Example 3 has neither a brittle system adjusted by fly ash nor a high-temperature thermal shock, resulting in severe cracking of the modified recycled aggregate.

[0075] Table 7 Comparison of properties of different modified recycled aggregates

[0076] From Table 7, it can be seen that the modified recycled aggregate prepared by the optimal process parameters of Example 2 and combined with three-stage curing has the best indicators such as water absorption, apparent density and crushing value, and the best surface crack condition. It can be seen that the physical properties of the modified recycled aggregate can also indirectly reflect the good or bad of the surface cracking condition. The aggregate with dense structure and excellent mechanical properties is wrapped by the gel layer formed by the geopolymer on the surface, and the internal micro-cracks are also filled. Once the protection layer inside and outside is not firm, cracking occurs, and the mechanical properties of the obtained modified recycled aggregate will also be significantly reduced. It can be seen that compared with the modified recycled aggregate obtained by the existing method, the modified recycled aggregate obtained by treating the recycled aggregate with the geopolymer prepared based on fly ash-metakaolin in the application can not only effectively reduce the cracking condition, but also further improve the mechanical properties.

[0077] To verify the improvement of the geopolymer in the application as a modifier on the micro-interface of the recycled aggregate, scanning electron microscope analysis is performed on the micro-morphology of the recycled aggregate before and after modification, and the results are shown in Figure 4 , wherein the comparison chart of the surface morphology, interface change and SEM atlas of the recycled aggregate before and after modification is shown in Figure 4 , wherein the left in the figure is before modification of the recycled aggregate, and the right is after modification of the recycled aggregate, (a) in the figure 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, and MRCA represents modified recycled aggregate, and (c) is the micro-morphology change of the recycled aggregate before and after modification.

[0078] Figure 4 (a) in the above is the actual comparison chart of the recycled aggregate before and after modification, and it can be seen that the recycled aggregate after modification with the geopolymer has a layer of gray geopolymer on the surface, and the adhesion with the aggregate is good, and the scraping and water treatment are not easy to fall off. And Figure 4 (b) in the above is the change of the aggregate interface before and after modification with the geopolymer, and it is found that the old cement mortar layer on the surface of the recycled aggregate after modification with the geopolymer is replaced by a more dense geopolymer gel layer. Figure 4 (c) in the above is the SEM atlas of the recycled aggregate before and after modification, and the surface of the unmodified recycled aggregate is accumulated by many small particles, and there are many pores and micro-cracks, and after modification with the geopolymer, the small particles are significantly reduced.

[0079] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the application, and is not intended to limit the application, and any modification, equivalent replacement and improvement made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. A process for the preparation of a crack resistant phosphosilicate polymer, characterized by, The method comprises the following steps: A 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; Metakaolin is first added to the phosphoric acid excitation solution to make the phosphoric acid fully decompose the aluminum-silicon structure of the metakaolin; Then, 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 ℃ 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, an anti-cracking phosphoric acid-based polymer slurry.

2. The production method according to claim 1, wherein According to a solid-liquid ratio of 0.4-0.5:1 of metakaolin to the phosphoric acid excitation solution, metakaolin is first added to the phosphoric acid excitation solution to obtain a red-brown slurry A; Then, fly ash is added to 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 ℃ by a magnetic force for 3-4 h to obtain an anti-cracking phosphoric acid-based polymer slurry B; the fly ash content accounts for 20%-30% of the total mass of metakaolin and fly ash.

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

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

5. Use of the anti-cracking phosphoric acid-based polymer of claim 4 in the preparation of anti-cracking recycled aggregate.

6. The use according to claim 5, wherein the compound is ###0002### First, the recycled aggregate is mixed with the anti-cracking phosphoric acid-based polymer slurry, and heated in a water bath at 60-80 ℃ until no continuous small bubbles are generated on the surface of the slurry to obtain recycled aggregate uniformly coated with the anti-cracking phosphoric acid-based polymer slurry; Then, three-stage curing is adopted: first, curing at room temperature of 20-30 ℃ for 10-12 h, then curing at medium temperature of 60-80 ℃ for 6-8 h, and finally curing at high temperature of 120-150 ℃ for 6-8 h, and natural cooling, to obtain the anti-cracking recycled aggregate.

7. Use according to claim 6, wherein First, the recycled aggregate is soaked in a phosphoric acid solution to make 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 ℃ for 3-4 h.

8. Use according to claim 7, wherein the compound is ###0002### Three-stage curing is adopted: first, curing at room temperature of 25±2 ℃ for 12 h, then curing at medium temperature of 60 ℃ for 6 h, and finally curing at high temperature of 120 ℃ for 6 h.

9. An anti-cracking recycled aggregate, characterized in that, The anti-cracking recycled aggregate is prepared by modifying the phosphoric acid-based polymer of claim 4.

10. An asphalt concrete, characterized by, The anti-cracking recycled aggregate of claim 9 is used as aggregate.

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