Anti-crack recycled concrete and preparation method thereof

Abstract

The invention relates to the technical field of concrete, in particular to anti-crack recycled concrete and a preparation method thereof. The anti-crack recycled concrete comprises the following raw materials in parts by weight: 30-35 parts of Portland cement, 15-20 parts of recycled aggregate, 12-15 parts of kaolin self-repairing microcapsules, 8-12 parts of basalt fibers, 10-12 parts of fly ash, 3-5 parts of silicon carbide whiskers, 4-6 parts of a polycarboxylate superplasticizer, 3-5 parts of a composite expanding agent and 12-14 parts of water. According to the anti-crack recycled concrete provided by the invention, the strength stability of the recycled concrete is improved, the anti-crack performance of the recycled concrete is improved, and the problem that the strength does not reach the standard due to the fact that the mix proportion of the recycled concrete is difficult to determine is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete, and in particular to a crack-resistant recycled concrete and a preparation method thereof. Background Art

[0002] Recycled concrete (RCA), also known as recycled aggregate concrete, is made by crushing, cleaning, and grading discarded concrete blocks, mixing them in a specific proportion with a grading agent, and then adding cement, water, and other ingredients to partially or completely replace natural aggregates (primarily coarse aggregate) such as sand and gravel. As a green building material, recycled concrete offers significant environmental advantages. With the acceleration of urbanization, the amount of construction waste generated by the demolition and renovation of old buildings continues to grow. As a significant component of construction waste, the recycling of discarded concrete has become an effective way to address environmental issues. Recycled concrete is widely used in road construction, wall materials, and other fields. Compared to ordinary concrete, it offers advantages such as improved crack resistance, cost savings, and environmental friendliness.

[0003] Although recycled concrete has many advantages, there are still some problems in practical applications, especially in terms of crack resistance. The strength of recycled concrete is relatively unstable, and it is easy to have low strength and fail to meet design requirements. This is mainly because the source of recycled aggregate is originally discarded concrete, which itself has certain cracks, and the strength of the aggregate will be seriously affected. In addition, the mix ratio of recycled concrete is also difficult to determine, which can easily lead to the situation where the strength does not meet the standard. The waste materials in the recycled concrete are uneven in size and are mixed with some impurities. These factors will lead to poor crack resistance of recycled concrete, and it is easy to have problems such as cracks during use. Although recycled concrete is less brittle and more tough than ordinary concrete, and its ultimate tensile value is higher than that of ordinary concrete, its crack resistance still needs to be further improved in practical applications. Based on this, the present invention proposes a crack-resistant recycled concrete and a preparation method thereof. Summary of the Invention

[0004] The present invention provides a crack-resistant recycled concrete and a preparation method thereof, which enhances the strength stability of the recycled concrete, improves the crack resistance of the recycled concrete, and solves the problem that the recycled concrete mix ratio is difficult to determine, resulting in substandard strength.

[0005] The technical solutions of the present invention are as follows: In a first aspect, the present invention proposes a crack-resistant recycled concrete comprising the following raw materials in parts by weight: 30-35 parts of Portland cement, 15-20 parts of recycled aggregate, 12-15 parts of kaolin self-repairing microcapsules, 8-12 parts of basalt fiber, 10-12 parts of fly ash, 3-5 parts of silicon carbide whiskers, 4-6 parts of polycarboxylic acid water reducer, 3-5 parts of composite expansion agent, and 12-14 parts of water.

[0006] As a further technical solution, the preparation method of the kaolin self-healing microcapsules includes: adding kaolin to a sodium silicate aqueous solution for ultrasonic dispersion, spray drying to form particle-sized core particles, immersing the core particles in a chitosan-polylactic acid mixture, taking them out and drying them; then spraying glutaraldehyde solution for cross-linking, forming a dense hydrophobic layer after curing, repeating the immersion-spraying steps 2-3 times, and the final wall thickness is 50-80 μm, passing through a 200-mesh sieve, and vacuum drying.

[0007] As a further technical solution, the inlet temperature during the spray drying is 110-120°C, and the outlet temperature is 50-60°C.

[0008] As a further technical solution, the preparation method of the chitosan-polylactic acid mixed solution includes: dissolving chitosan in an acetic acid aqueous solution with a mass concentration of 1-2%, adding polylactic acid particles, stirring at 50-60°C until completely dissolved, adding nano-silica, and ultrasonically dispersing for 20-30 minutes to form a uniform mixed solution.

[0009] As a further technical solution, the weight ratio of the chitosan, acetic acid aqueous solution, polylactic acid particles and nano-silicon dioxide is (3-5): (80-88): (8-12): (1-2).

[0010] As a further technical solution, the drying temperature is 55-65°C; the vacuum drying temperature is 35-45°C, and the time is 22-24 hours.

[0011] As a further technical solution, the glutaraldehyde solution is a glutaraldehyde aqueous solution with a mass concentration of 0.5-1%; the sodium silicate solution is a sodium silicate aqueous solution with a mass concentration of 35-40%.

[0012] As a further technical solution, the weight ratio of the kaolin, sodium silicate aqueous solution, and chitosan-polylactic acid mixed solution is 100:20-30:100-120.

[0013] As a further technical solution, the composite expansion agent includes calcium sulfoaluminate and magnesium oxide in a weight ratio of 1:(2-3).

[0014] In a second aspect, the present invention proposes a method for preparing crack-resistant recycled concrete, the steps comprising: dry mixing silicate cement, fly ash, and recycled aggregate for 5-10 minutes; adding polycarboxylic acid water reducer and water, then adding basalt fiber, silicon carbide whiskers, and composite expansion agent in batches, and stirring at 60-80 rpm for 9-11 minutes; finally, adding self-repairing microcapsules, stirring for 3-5 minutes to avoid breakage, and finally placing into a mold for preparing various concrete prefabricated products, which are obtained after curing.

[0015] The working principle and beneficial effects of the present invention are: The kaolin self-healing microcapsules designed in this invention achieve active repair of concrete cracks through the principles of bionics. The core of this system lies in the construction of a three-level structural system of "core material-wall material-functional layer": first, kaolin is blended with a sodium silicate solution using spray drying technology to form hollow spherical core particles. This structure achieves directional migration of the repair agent through the capillary effect. When the crack expands to the surface of the microcapsule, the spherical structure produces a stress concentration effect, prompting the core material to rupture directionally along the crack direction, ensuring the precise release of the repair agent to the crack tip. In addition, the chitosan-polylactic acid composite wall material is cross-linked with glutaraldehyde to form a semi-permeable membrane structure. The hydrogen bond network between its molecular chains can regulate the release rate of the repair agent. The introduction of nano-silica forms a maze effect in the wall material, extending the diffusion path of the repair agent, so that the release process presents a three-stage characteristic of "early rapid response-mid-term stable release-late-term continuous replenishment", forming a dynamic match with the expansion rate of the concrete crack.

[0016] The calcium sulfoaluminate-magnesium oxide composite expansive agent in this invention achieves precise compensation for concrete shrinkage through a "double-effect reaction" mechanism. Calcium sulfoaluminate reacts with Ca(OH)2 in the initial stages of cement hydration (1-3 days) to form ettringite, effectively offsetting plastic shrinkage stress. This reaction rate synchronizes with the early hydration rate of cement, preventing premature expansion and resulting damage to the interfacial transition zone. Magnesium oxide hydrates to form Mg(OH)2 crystals within 7-28 days, which continuously compensate for drying shrinkage through crystal growth pressure. Its reaction rate is regulated by pH, resulting in a "slow-release" expansion characteristic in the alkaline environment of the cement matrix, reducing the total shrinkage rate over 28 days. Furthermore, this staged expansion transforms the traditional uniform shrinkage stress into a gradient stress field by altering the stress distribution within concrete. The early expansion of calcium sulfoaluminate creates a pre-compressive stress zone on the aggregate surface, while the later expansion of magnesium oxide maintains the stability of this stress field.

[0017] The basalt fibers, silicon carbide whiskers, and kaolin self-healing microcapsules in the present invention constitute a multi-scale reinforcement system that combines both rigidity and flexibility. The basalt fibers dissipate energy at the crack tip through the pull-out effect, and their three-dimensional random distribution forms a spatial network structure, inhibiting the expansion of macroscopic cracks. Silicon carbide whiskers generate stress concentration at the microcrack tips through their high modulus properties, inducing whisker breakage or pull-out. The pull-out process is accompanied by frictional slippage of the SiO2 layer on the whisker surface, consuming additional energy and increasing the impact toughness to 20-21 times. Furthermore, the self-healing microcapsules release a repair agent to fill microcracks in the early stages of crack expansion, reducing the initial damage to the fiber-whisker system. The repaired matrix has a high strength recovery rate, providing continuous load-bearing support for the fibers. At the same time, the damaged interface of the microcapsules provides additional mechanical anchoring points for the fibers, increasing the fiber pull-out work by 20%, forming a cyclic reinforcement mechanism of "repair-reinforcement-repair again."

[0018] The present invention forms a continuous gradation system by combining recycled aggregate with fly ash. The microbead effect of fly ash fills the gaps between aggregates, and the hollow structure of microcapsules further optimizes the pore distribution, thereby improving the density of concrete. The dispersing effect of the water reducer makes the microcapsules, fibers, and whiskers evenly distributed, avoiding stress concentration caused by agglomeration effect. The AFt and Mg(OH)2 generated by the composite expansion agent )2 The crystals form a "whisker-like" reinforcement phase in the ITZ region, and together with the fiber-whisker network, they construct a multi-scale reinforcement system. DETAILED DESCRIPTION

[0019] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0020] The silicate cement model in the present invention is P.O42.5; the recycled aggregate is purchased from Chengtou Construction Waste Disposal (Guangzhou) Co., Ltd.; the basalt fiber is Tai'an Haosong Fiber Co., Ltd., model xwy-16; the fly ash is purchased from Hubei Chengfeng Chemical Co., Ltd.; the silicon carbide whiskers are purchased from Beijing Dekedaojin Technology Co., Ltd., with a length of 3-12 mm and a diameter of 3-6 μm; the polycarboxylate water reducer is a ZJKJ type polycarboxylate water reducer purchased from Jinan Juxin Chemical Co., Ltd.

[0021] Example 1 The present embodiment provides a crack-resistant recycled concrete, which includes the following raw materials in parts by weight: 32 parts of silicate cement, 17 parts of recycled aggregate, 13 parts of kaolin self-repairing microcapsules, 10 parts of basalt fiber, 11 parts of fly ash, 4 parts of silicon carbide whiskers, 5 parts of polycarboxylic acid water reducer, 4 parts of composite expansion agent, and 13 parts of water.

[0022] The preparation method of kaolin self-repairing microcapsules includes: adding kaolin to a sodium silicate aqueous solution with a mass concentration of 37%, ultrasonically dispersing the kaolin, and spray drying the kaolin to form core particles with a particle size of 115°C and an outlet temperature of 55°C during the spray drying process; Chitosan was dissolved in 1.5% acetic acid aqueous solution, polylactic acid particles were added, and the mixture was stirred at 55°C until completely dissolved. Nano-silica was added and ultrasonically dispersed for 25 minutes to form a chitosan-polylactic acid uniform mixture; the weight ratio of chitosan, acetic acid aqueous solution, polylactic acid particles, and nano-silica was 4:84:10:1.5; The core material particles were immersed in a chitosan-polylactic acid mixture, taken out and dried at 60°C; then sprayed with a 0.7% mass concentration of glutaraldehyde aqueous solution for cross-linking, and a dense hydrophobic layer was formed after curing. The dipping-spraying steps were repeated twice, and the final wall thickness was 60 μm. The core material particles were passed through a 200-mesh sieve and vacuum-dried at 40°C for 22 hours.

[0023] The weight ratio of kaolin, sodium silicate aqueous solution and chitosan-polylactic acid mixed solution is 100:25:110.

[0024] The composite expansion agent comprises calcium sulfoaluminate and magnesium oxide in a weight ratio of 1:2.5.

[0025] The method for preparing crack-resistant recycled concrete comprises the following steps: dry-mixing silicate cement, fly ash, and recycled aggregate for 7 minutes; adding a polycarboxylate water reducer and water, then adding basalt fiber, silicon carbide whiskers, and a composite expansion agent in batches, and stirring at 70 rpm for 10 minutes; finally, adding self-repairing microcapsules, stirring for 4 minutes to prevent breakage, and finally placing the microcapsules into a mold for preparing various concrete prefabricated products, and curing them at 25°C and 90% humidity for 28 days to obtain the concrete prefabricated products.

[0026] Example 2 The present embodiment provides a crack-resistant recycled concrete, which includes the following raw materials in parts by weight: 30 parts of silicate cement, 15 parts of recycled aggregate, 12 parts of kaolin self-repairing microcapsules, 8 parts of basalt fiber, 10 parts of fly ash, 3 parts of silicon carbide whiskers, 4 parts of polycarboxylic acid water reducer, 3 parts of composite expansion agent, and 12 parts of water.

[0027] The preparation method of kaolin self-repairing microcapsules includes: adding kaolin to a sodium silicate aqueous solution with a mass concentration of 35% for ultrasonic dispersion, spray drying to form core material particles with a particle size, wherein the inlet temperature during spray drying is 110°C and the outlet temperature is 50°C; Chitosan was dissolved in 1% acetic acid aqueous solution, polylactic acid particles were added, and the mixture was stirred at 50°C until completely dissolved. Nano-silica was added and ultrasonically dispersed for 20 minutes to form a chitosan-polylactic acid uniform mixture; the weight ratio of chitosan, acetic acid aqueous solution, polylactic acid particles, and nano-silica was 3:80:8:1; The core material particles were immersed in a chitosan-polylactic acid mixture, taken out and dried at 55°C; then sprayed with a 0.5% mass concentration of glutaraldehyde aqueous solution for cross-linking, and a dense hydrophobic layer was formed after curing. The dipping-spraying steps were repeated twice, and the final wall thickness was 50 μm. The core material particles were passed through a 200-mesh sieve and vacuum-dried at 35°C for 22 hours.

[0028] The weight ratio of kaolin, sodium silicate aqueous solution and chitosan-polylactic acid mixed solution is 100:20:100.

[0029] The composite expansion agent comprises calcium sulfoaluminate and magnesium oxide in a weight ratio of 1:2.

[0030] The method for preparing crack-resistant recycled concrete comprises the following steps: dry-mixing silicate cement, fly ash, and recycled aggregate for 5 minutes; adding a polycarboxylate water reducer and water, then adding basalt fiber, silicon carbide whiskers, and a composite expansion agent in batches, and stirring at 60 rpm for 9 minutes; finally, adding self-repairing microcapsules, stirring for 3 minutes to prevent breakage, and finally placing the self-repairing microcapsules into a mold for preparing various concrete prefabricated products, and curing them at 25°C and 90% humidity for 28 days to obtain the product.

[0031] Example 3 The present embodiment provides a crack-resistant recycled concrete, which includes the following raw materials in parts by weight: 35 parts of silicate cement, 20 parts of recycled aggregate, 15 parts of kaolin self-repairing microcapsules, 12 parts of basalt fiber, 12 parts of fly ash, 5 parts of silicon carbide whiskers, 6 parts of polycarboxylic acid water reducer, 5 parts of composite expansion agent, and 14 parts of water.

[0032] The preparation method of kaolin self-repairing microcapsules includes: adding kaolin to a sodium silicate aqueous solution with a mass concentration of 40%, ultrasonically dispersing the kaolin, and spray drying the kaolin to form core particles with a particle size, wherein the inlet temperature during spray drying is 120°C and the outlet temperature is 60°C; Chitosan was dissolved in a 2% acetic acid aqueous solution, polylactic acid particles were added, and the mixture was stirred at 60°C until completely dissolved. Nano-silica was added and ultrasonically dispersed for 30 minutes to form a chitosan-polylactic acid uniform mixture. The weight ratio of chitosan, acetic acid aqueous solution, polylactic acid particles, and nano-silica was 5:88:12:2. The core material particles were immersed in a chitosan-polylactic acid mixture, taken out and dried at 65°C; then sprayed with a 1% mass concentration of glutaraldehyde aqueous solution for cross-linking, and a dense hydrophobic layer was formed after curing. The dipping-spraying steps were repeated 3 times, and the final wall thickness was 80 μm. The core material particles were passed through a 200-mesh sieve and vacuum-dried at 45°C for 24 hours.

[0033] The weight ratio of kaolin, sodium silicate aqueous solution and chitosan-polylactic acid mixed solution is 100:30:120.

[0034] The composite expansion agent comprises calcium sulfoaluminate and magnesium oxide in a weight ratio of 1:3.

[0035] The method for preparing crack-resistant recycled concrete comprises the following steps: dry-mixing silicate cement, fly ash, and recycled aggregate for 10 minutes; adding a polycarboxylate water reducer and water, then adding basalt fiber, silicon carbide whiskers, and a composite expansion agent in batches, and stirring at 80 rpm for 11 minutes; finally, adding self-repairing microcapsules, stirring for 5 minutes to prevent breakage, and finally placing the microcapsules into a mold for preparing various concrete prefabricated products, and curing them at 25°C and 90% humidity for 28 days to obtain the product.

[0036] Example 4 The present embodiment provides a crack-resistant recycled concrete, which includes the following raw materials in parts by weight: 30 parts of silicate cement, 20 parts of recycled aggregate, 12 parts of kaolin self-repairing microcapsules, 12 parts of basalt fiber, 10 parts of fly ash, 5 parts of silicon carbide whiskers, 4 parts of polycarboxylic acid water reducer, 5 parts of composite expansion agent, and 12 parts of water.

[0037] The preparation method of kaolin self-repairing microcapsules includes: adding kaolin to a sodium silicate aqueous solution with a mass concentration of 35% for ultrasonic dispersion, spray drying to form core material particles with a particle size, wherein the inlet temperature during spray drying is 120°C and the outlet temperature is 50°C; Chitosan was dissolved in a 2% acetic acid aqueous solution, polylactic acid particles were added, and the mixture was stirred at 50°C until completely dissolved. Nano-silica was added and ultrasonically dispersed for 20 minutes to form a chitosan-polylactic acid uniform mixture. The weight ratio of chitosan, acetic acid aqueous solution, polylactic acid particles, and nano-silica was 5:80:12:1. The core material particles were immersed in a chitosan-polylactic acid mixture, taken out and dried at 65°C; then sprayed with a 0.5% mass concentration of glutaraldehyde aqueous solution for cross-linking, and a dense hydrophobic layer was formed after curing. The dipping-spraying steps were repeated three times, and the final wall thickness was 50 μm. The core material particles were passed through a 200-mesh sieve and vacuum-dried at 45°C for 22 hours.

[0038] The weight ratio of kaolin, sodium silicate aqueous solution and chitosan-polylactic acid mixed solution is 100:30:100.

[0039] The composite expansion agent comprises calcium sulfoaluminate and magnesium oxide in a weight ratio of 1:3.

[0040] The method for preparing crack-resistant recycled concrete comprises the following steps: dry-mixing silicate cement, fly ash, and recycled aggregate for 5 minutes; adding a polycarboxylate water reducer and water, then adding basalt fiber, silicon carbide whiskers, and a composite expansion agent in batches, and stirring at 80 rpm for 10 minutes; finally, adding self-repairing microcapsules, stirring for 3 minutes to prevent breakage, and finally placing the microcapsules into a mold for preparing various concrete prefabricated products, and curing them at 25°C and 90% humidity for 28 days to obtain the concrete prefabricated products.

[0041] Comparative Example 1 Adjustments were made on the basis of Example 1. Unlike Example 1, the preparation method of kaolin self-healing microcapsules in Comparative Example 1 included: dissolving chitosan in an acetic acid aqueous solution with a mass concentration of 1.5%, adding polylactic acid particles, stirring at 55°C until completely dissolved, adding nano-silica, and ultrasonically dispersing for 25 minutes to form a chitosan-polylactic acid uniform mixture; the weight ratio of chitosan, acetic acid aqueous solution, polylactic acid particles and nano-silica was 4:84:10:1.5; immersing kaolin particles in the chitosan-polylactic acid mixture, taking them out and drying them at a temperature of 60°C; then spraying them with a glutaraldehyde aqueous solution with a mass concentration of 0.7% for cross-linking, forming a dense hydrophobic layer after curing, repeating the dipping-spraying steps twice, and the final wall thickness was 60 μm. The particles were passed through a 200-mesh sieve and vacuum-dried at 40°C for 22 hours.

[0042] Comparative Example 2 Adjustments were made on the basis of Example 1. Unlike Example 1, the preparation method of kaolin self-healing microcapsules in Comparative Example 2 included: adding kaolin to a sodium silicate aqueous solution with a mass concentration of 37% for ultrasonic dispersion, spray drying to form particle-sized core particles, the inlet temperature during spray drying was 115°C, and the outlet temperature was 55°C; dissolving chitosan in an acetic acid aqueous solution with a mass concentration of 1.5%, adding polylactic acid particles, stirring at 55°C until completely dissolved, and then ultrasonically dispersing for 25 minutes to form a chitosan-polylactic acid uniform mixture; the weight ratio of chitosan, acetic acid aqueous solution and polylactic acid particles was 4:84:10; immersing the core particles in the chitosan-polylactic acid mixture, taking them out and drying them at a temperature of 60°C; then spraying a glutaraldehyde aqueous solution with a mass concentration of 0.7% for cross-linking, forming a dense hydrophobic layer after curing, repeating the immersion-spraying step twice, and finally having a wall thickness of 60 μm. After passing through a 200 mesh sieve and vacuum drying at 40°C for 22 hours, the obtained microcapsules were Comparative Example 3 Adjustments were made on the basis of Example 1. The difference from Example 1 was that the kaolin self-repairing microcapsules in Comparative Example 3 were replaced with kaolin of equal mass.

[0043] Comparative Example 4 Adjustments were made based on Example 1. The difference from Example 1 was that the kaolin self-healing microcapsules in Comparative Example 4 were replaced with silicon dioxide of equal mass.

[0044] Comparative Example 5 Adjustments were made based on Example 1. The difference from Example 1 was that the composite expansion agent in Comparative Example 5 was replaced by a single expansion agent, calcium sulfoaluminate.

[0045] Comparative Example 6 Adjustments were made based on Example 1. The difference from Example 1 was that the composite expansion agent in Comparative Example 6 was replaced by a single expansion agent, magnesium oxide.

[0046] Test Example 1: The following tests were performed on the crack-resistant recycled concrete prepared in Examples 1-4 and Comparative Examples 1-6: 28d compressive strength: Refer to GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete" for compression test of cubic specimens (150×150×150mm); 28d flexural strength: A three-point bending test was conducted on prismatic specimens (100×100×400mm) in accordance with GB / T 50081-2019; Crack self-repair rate (0.3 mm): After 28 days of curing, the repaired area percentage of pre-cut crack specimens (0.3 mm wide) was observed by SEM. Image analysis software was used to binarize the crack area to distinguish the repaired area (filler) from the unrepaired area (pores). A grayscale threshold segmentation method was set to calibrate the repaired area (white) and the unrepaired area (black). The repaired area percentage (%) = total number of crack pixels / number of repaired area pixels × 100%. High-efficiency repair: 90% or more of the repaired area indicates complete crack closure; effective repair: 70%-89% of the repaired area indicates a majority of the crack is repaired; partial repair: 50%-69% of the repaired area indicates a partial crack is repaired; and low-efficiency repair: <50% of the repaired area indicates an insignificant repair effect. Impact toughness: drop hammer impact test (5kg hammer, 1m height impact test piece until cracking, record the number of impacts); The results are shown in Table 1 below: Table 1

[0047] Combined with the above, Examples 1-4, using kaolin self-healing microcapsules, achieved crack self-healing rates exceeding 85% (maximum 92%), significantly higher than Comparative Example 14. Comparative Example 1 lacked core particles, resulting in a 20% decrease in repair efficiency, demonstrating that the core structure is crucial for sustained release of the repair agent. Comparative Examples 3-4, lacking microcapsules, completely lost their repair function, confirming that microcapsules are a core functional unit of crack-resistant recycled concrete. Examples 1-4 employed a calcium sulfoaluminate-magnesium oxide composite expansive agent, compensating for shrinkage in stages, resulting in superior crack resistance to Comparative Examples 5-6. Comparative Examples 5-6 experienced a 5-10 MPa decrease in compressive strength and reduced impact toughness, demonstrating that the composite expansive agent's staged compensation mechanism can avoid stress concentration. The basalt fiber and silicon carbide whisker combination in Examples 1 and 3 achieved the highest flexural strength and an impact toughness of 20-21 times. Comparative Examples 3-4, lacking microcapsules, weakened the fiber reinforcement effect, demonstrating a synergistic effect between the microcapsules' interfacial reinforcement and the fibers.

[0048] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A crack-resistant recycled concrete, characterized in that: The invention comprises the following raw materials in parts by weight: 30-35 parts of silicate cement, 15-20 parts of recycled aggregate, 12-15 parts of kaolin self-repairing microcapsules, 8-12 parts of basalt fiber, 10-12 parts of fly ash, 3-5 parts of silicon carbide whiskers, 4-6 parts of polycarboxylic acid water reducer, 3-5 parts of composite expansion agent and 12-14 parts of water.

2. The crack-resistant recycled concrete according to claim 1, characterized in that: The preparation method of the kaolin self-repairing microcapsules includes: adding kaolin to a sodium silicate aqueous solution for ultrasonic dispersion, spray drying to form a particle-sized core material particle, immersing the core material particle in a chitosan-polylactic acid mixture, taking out and drying; then spraying glutaraldehyde solution for cross-linking, forming a dense hydrophobic layer after curing, repeating the immersion-spraying steps 2-3 times, and finally obtaining a wall thickness of 50-80 μm, passing through a 200-mesh sieve, and vacuum drying.

3. The crack-resistant recycled concrete according to claim 2, characterized in that: During the spray drying, the inlet temperature is 110-120°C, and the outlet temperature is 50-60°C.

4. The crack-resistant recycled concrete according to claim 2, characterized in that: The preparation method of the chitosan-polylactic acid mixed solution comprises: dissolving chitosan in an acetic acid aqueous solution with a mass concentration of 1-2%, adding polylactic acid particles, stirring at 50-60° C. until completely dissolved, adding nano-silica, and ultrasonically dispersing for 20-30 minutes to form a uniform mixed solution.

5. The crack-resistant recycled concrete according to claim 4, characterized in that: The weight ratio of the chitosan, acetic acid aqueous solution, polylactic acid particles and nano-silicon dioxide is (3-5): (80-88): (8-12): (1-2).

6. The crack-resistant recycled concrete according to claim 2, characterized in that: The drying temperature is 55-65°C; the vacuum drying temperature is 35-45°C, and the time is 22-24 hours.

7. The crack-resistant recycled concrete according to claim 2, characterized in that: The glutaraldehyde solution is a glutaraldehyde aqueous solution with a mass concentration of 0.5-1%; the sodium silicate solution is a sodium silicate aqueous solution with a mass concentration of 35-40%.

8. The crack-resistant recycled concrete according to claim 7, characterized in that: The weight ratio of the kaolin, the sodium silicate aqueous solution, and the chitosan-polylactic acid mixed solution is 100:20-30:100-120.

9. The crack-resistant recycled concrete according to claim 1, characterized in that: The composite expansion agent comprises calcium sulfoaluminate and magnesium oxide in a weight ratio of 1:(2-3).

10. A method for preparing crack-resistant recycled concrete according to any one of claims 1 to 9, characterized in that the steps include: The silicate cement, fly ash and recycled aggregate are dry-mixed for 5-10 minutes; polycarboxylate water reducer and water are added, followed by adding basalt fiber, silicon carbide whiskers and composite expansion agent in batches, and stirring at 60-80 rpm for 9-11 minutes; finally, the self-repairing microcapsules are added and stirred for 3-5 minutes to avoid breakage, and finally, the microcapsules are placed in a mold for preparing various types of concrete prefabricated products, which are then cured.

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