Modified recycled coarse aggregate, preparation method and self-compacting concrete

Abstract

This invention relates to the technical field of resource recycling of construction solid waste, and discloses a modified recycled coarse aggregate, its preparation method, and self-compacting concrete. The modified recycled coarse aggregate is composed of the following raw materials in parts by weight: 900 parts recycled coarse aggregate, 15-45 parts silica fume, 3-9 parts nano-CaCO3, and 30-90 parts fine steel slag powder. After the modified recycled coarse aggregate is prepared, 300 parts ordinary Portland cement, 750 parts manufactured sand, and 6 parts polycarboxylate superplasticizer are added. Finally, water is added and stirred to obtain self-compacting concrete, with a water-cement ratio of 0.47-0.59. While effectively controlling production costs, the improved RCA (reinforced concrete aggregate) of this invention results in concrete with better mechanical properties and durability (impermeability), significantly improving the resource utilization rate of construction waste and providing a reference for the high-value utilization of construction solid waste.

Description

Technical Field

[0001] This invention belongs to the technical field of resource recycling of construction solid waste, specifically relating to a modified recycled coarse aggregate, its preparation method, and self-compacting concrete. Background Technology

[0002] Recycled concrete coarse aggregate (RCA) originates from concrete building demolition waste, and its utilization can reduce the mining of natural aggregates and lower the environmental burden. However, RCA is covered with old cement mortar, resulting in high porosity, high water absorption, and low strength. Furthermore, compared to natural aggregates, it is more prone to developing more angular features during jaw crushing, leading to a significant decrease in the workability of fresh concrete and its mechanical properties after hardening. Therefore, some researchers have used various modification techniques to optimize RCA, aiming to improve its inherent properties through consolidation strengthening and surface morphology, thereby enhancing the overall performance of recycled concrete.

[0003] Currently, RCA modification methods mainly include: physical modification, chemical modification, polymer modification, mineral admixture encapsulation, and nano-modification. Physical modification primarily involves mechanically grinding recycled aggregates to remove surface mortar, or heating and dehydrating to increase aggregate strength. This method is energy-intensive, has poor controllability, and easily causes secondary damage to the aggregates, negatively impacting later strength and durability. Chemical modification uses high-concentration acid washing or polymer soaking of RCA to remove surface mortar or fill corresponding micro-cracks and pores. This method easily leaves harmful ions and poses a high risk of pollution. While high-concentration acid treatment removes the mortar, it itself damages the aggregates, increasing crushing value and reducing aggregate strength and durability. Polymer soaking is costly and complex. The mineral admixture coating method modifies aggregates using single or composite mineral admixtures such as silica fume and fly ash. While this method can seal surface pores and microcracks, the similar particle size of the admixtures makes it difficult to deeply improve internal aggregate defects, especially microcracks in areas with attached old mortar. Therefore, its effect on optimizing the interfacial transition zone (ITZ) and overall compactness is limited. The nano-modification method treats RCA by adding one or more nanomaterials. However, similarly, the small particle size of nanomaterials limits their defect filling to the nanoscale, offering limited improvement for micron-level or larger defects and minimal strength enhancement. Furthermore, since the modifying materials are all nanoscale, excessive nanoscale materials lead to high costs, difficulty in guaranteeing dispersion effects, low process error tolerance, and large investment in dispersion equipment. In addition, the use of nanoscale materials and silica fume increases water demand, negatively impacting the workability of concrete. Summary of the Invention

[0004] To address the aforementioned technical problems in related technologies, this invention provides a modified recycled coarse aggregate, a preparation method, and self-compacting concrete, which can solve the above problems.

[0005] To achieve the above-mentioned technical objectives, the technical solution of the present invention is implemented as follows:

[0006] A modified recycled coarse aggregate is composed of the following raw materials in parts by weight: 900 parts by weight of recycled coarse aggregate, 15-45 parts by weight of silica fume, 3-9 parts by weight of nano-CaCO3, and 30-90 parts by weight of fine steel slag powder.

[0007] Furthermore, the recycled coarse aggregate is a continuously graded aggregate with a particle size of 5-20 mm obtained by crushing waste concrete.

[0008] Furthermore, the particle size of the silica fume is 0.1~0.3μm.

[0009] Furthermore, the particle size of the nano-CaCO3 is 50~100nm.

[0010] Furthermore, the particle size of the fine steel slag powder is 40~50μm.

[0011] A method for preparing modified recycled coarse aggregate includes the following steps:

[0012] S1. Pretreatment of recycled coarse aggregate: The waste concrete is crushed into continuous graded aggregate with a particle size of 5~20mm. The continuous graded aggregate is washed, dried and then immersed in a 5% mass fraction acetic acid solution for 10 minutes. The immersed continuous graded aggregate is then drained until it is saturated and surface dry.

[0013] S2. Preparation of modifier: 15-45 parts by weight of silica fume, 3-9 parts by weight of nano-CaCO3, and 30-90 parts by weight of fine steel slag powder are dispersed in 75 parts by weight of water and ultrasonically treated for 30 minutes to obtain the modifier.

[0014] S3. Preparation of modified recycled coarse aggregate: 900 parts by weight of the recycled coarse aggregate pretreated in S1 and the modifier prepared in S2 are mixed and stirred for 3 minutes to obtain modified recycled coarse aggregate.

[0015] A self-compacting concrete comprises the modified recycled coarse aggregate prepared above. After the modified recycled coarse aggregate is prepared through the above steps, 300 parts by weight of ordinary Portland cement, 750 parts by weight of manufactured sand, and 6 parts by weight of polycarboxylate superplasticizer are added. Finally, water is added and stirred to obtain the self-compacting concrete, wherein the water-cement ratio is 0.47~0.59.

[0016] Furthermore, the ordinary silicate cement is ordinary silicate cement P·O42.5.

[0017] The beneficial effects of this invention are as follows: This application uses a low-concentration acetic acid solution to treat the aggregates, thus minimizing the risk of pollution and damage to the aggregates. Simultaneously, the use of a small amount of polycarboxylate superplasticizer in the aggregate treatment reduces costs compared to using a larger amount of polymer-modified RCA. Furthermore, the use of mass-produced silica fume in the modifier further reduces costs compared to using only nanoscale materials. While effectively controlling production costs, the improvement effect of this invention on RCA results in concrete with better mechanical properties and durability (impermeability), significantly improving the resource utilization rate of construction waste and providing a reference for the high-value utilization of construction solid waste. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] The present invention will now be described in further detail with reference to the accompanying drawings.

[0020] Figure 1 This is a bar chart showing the compressive strength of the concrete prepared in the embodiments and comparative examples of the present invention at 3 days.

[0021] Figure 2 This is a bar chart showing the compressive strength of the concrete prepared in the embodiments and comparative examples of the present invention at 28 days.

[0022] Figure 3 This is a bar chart showing the compressive strength of the concrete prepared in the embodiments and comparative examples of the present invention at 56 days.

[0023] Figure 4 This is a bar chart showing the slump spread of the concrete prepared in the embodiments and comparative examples of the present invention;

[0024] Figure 5 This is a bar chart showing the unsteady chloride ion migration coefficients of the concrete prepared in the embodiments and comparative examples of the present invention. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art are within the scope of protection of the present invention.

[0026] This application discloses a modified recycled coarse aggregate, which is composed of the following raw materials in parts by weight: 900 parts by weight of recycled coarse aggregate (continuously graded aggregate with a particle size of 5-20 mm obtained by crushing waste concrete), 15-45 parts by weight of silica fume (particle size of 0.1-0.3 μm), 3-9 parts by weight of nano-CaCO3 (particle size of 50-100 nm), and 30-90 parts by weight of fine steel slag powder (particle size of 40-50 μm).

[0027] The preparation method of modified recycled coarse aggregate includes the following steps:

[0028] S1. Pretreatment of recycled coarse aggregate: The waste concrete is crushed into continuously graded aggregate with a particle size of 5~20mm. After the continuously graded aggregate is cleaned and dried, it is immersed in a 5% mass fraction acetic acid solution for 10 minutes (using weak acid for short time is a green process that can not only dissolve residual mortar and activate the surface, but also enhance the permeability of modifier without damaging the aggregate). Then the immersed continuously graded aggregate is drained until it is saturated and surface dry.

[0029] S2. Preparation of the modifier: The silica fume, the nano-CaCO3, and the fine steel slag powder are dispersed in water and ultrasonically treated for 30 minutes to obtain the modifier;

[0030] S3. Preparation of modified recycled coarse aggregate: The pretreated recycled coarse aggregate in S1 and the modifier prepared in S2 are mixed and stirred for 3 minutes to obtain modified recycled coarse aggregate.

[0031] When the modified recycled coarse aggregate is prepared (900 parts by weight of recycled coarse aggregate, 15-45 parts by weight of silica fume, 3-9 parts by weight of nano-CaCO3, 30-90 parts by weight of fine steel slag powder, and 75 parts by weight of water), it is mixed with other dry materials (300 parts by weight of ordinary Portland cement P·O 42.5, 750 parts by weight of manufactured sand, and 6 parts by weight of polycarboxylate superplasticizer), and finally water is added and stirred to obtain self-compacting concrete, wherein the water-cement ratio is 0.47-0.59.

[0032] The water-reducing agent is a polycarboxylate water-reducing agent produced by Nanjing Qiangli Energy Saving Technology Co., Ltd. It is a white powder, and its product parameters are shown in Table 1 below.

[0033] Table 1 Product Parameters of Polycarboxylate Superplasticizer

[0034]

[0035] Continuous gradation refers to a material in which particles of various sizes are evenly distributed, forming a smooth gradation curve, which is suitable for the preparation of engineering materials such as concrete.

[0036] Manufactured sand is rock particles with a diameter of less than 4.75mm produced by sand making machines, jaw crushers, vibrating screens, and other equipment. It does not contain soft weathered rock and belongs to the category of artificial sand.

[0037] Saturated surface dry state refers to a water-containing state in which there is no free water on the surface of a material, but the internal pores are filled with water. It is widely used in civil engineering and materials science. Example 1:

[0038] This embodiment provides a method for synergistic modification of RCA with silica fume, nano-CaCO3, and fine steel slag powder, and the preparation of self-compacting RCA concrete. The specific steps are as follows:

[0039] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0040] (2) Disperse silica fume, nano CaCO3 and fine steel slag powder in water at a weight ratio of 5:1:10, wherein 30 parts of silica fume (0.1~0.3μm), 6 parts of nano CaCO3 (50~100nm), 60 parts of fine steel slag powder (40~50μm), and 75 parts of water are added, and ultrasonic treatment is performed for 30 minutes.

[0041] (3) Mix the modifier in (2) with the pretreated recycled aggregate (900 parts) for 3 minutes, then mix with other dry materials (300 parts of ordinary silicate cement, 750 parts of manufactured sand, and 6 parts by weight of polycarboxylate superplasticizer), and finally add water and mix to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0042] (4) The above mixture was subjected to a slump expansion test for self-compacting concrete. The test method was in accordance with the "Technical Specification for Application of Self-Compacting Concrete" (JGJ / T 283-2012), and the general requirements for slump test in the "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (GB / T 50080-2016). 150 mm cube specimens were prepared for cube compressive strength test. The curing and compressive strength test were in accordance with the "Standard for Test Methods of Mechanical Properties of Ordinary Concrete" (GB / T 50081-2019). The compressive strength age was selected as 3 days, 28 days, and 56 days. 100 mm diameter and 50 mm thick cylindrical specimens were prepared for chloride ion penetration test. The test standard was in accordance with the "Standard for Test Methods of Long-Term Performance and Durability of Concrete" (GB / T 50082-2024). Example 2:

[0043] This embodiment provides a method for synergistic modification of RCA with silica fume, nano-CaCO3, and fine steel slag powder, and the preparation of self-compacting RCA concrete. The specific steps are the same as in Embodiment 1, except that 45 parts of silica fume, 9 parts of nano-CaCO3, and 90 parts of fine steel slag powder are used, and the water-cement ratio is 0.47. Example 3:

[0044] This embodiment provides a method for synergistic modification of RCA with silica fume, nano-CaCO3, and fine steel slag powder, and the preparation of self-compacting RCA concrete. The specific steps are the same as in Embodiment 1, except that 15 parts of silica fume, 3 parts of nano-CaCO3, and 30 parts of fine steel slag powder are used, and the water-cement ratio is 0.59.

[0045] Comparative Example 1

[0046] A method for preparing self-compacting RCA concrete is proposed, but the RCA is not modified. The specific steps are as follows:

[0047] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0048] (2) Add 75 parts of water and treat with ultrasound for 30 minutes.

[0049] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0050] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0051] Comparative Example 2

[0052] A method for preparing self-compacting RCA concrete is proposed, but the RCA undergoes a single silica fume modification treatment. The specific steps are as follows:

[0053] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0054] (2) Add 30 parts of silica fume (particle size 0.1~0.3μm) to 75 parts of water and treat with ultrasound for 30 min.

[0055] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0056] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0057] Comparative Example 3

[0058] A method for preparing self-compacting RCA concrete is proposed, but it involves single modification of RCA with nano-CaCO3. The specific steps are as follows:

[0059] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0060] (2) Add 6 parts of nano CaCO3 (particle size 50~100nm) to 75 parts of water and treat with ultrasound for 30min.

[0061] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0062] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0063] Comparative Example 4

[0064] A method for preparing self-compacting RCA concrete is proposed, but it involves single modification of RCA with fine steel slag powder. The specific steps are as follows:

[0065] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0066] (2) Add 60 parts of fine steel slag powder (particle size 40~50μm) to 75 parts of water and treat with ultrasound for 30 minutes.

[0067] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0068] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0069] Comparative Example 5

[0070] This invention provides a method for synergistic modification of RCA (Reinforced Carbonate) with silica fume, nano-CaCO3, and fine steel slag powder, and the preparation of self-compacting RCA concrete, but without subjecting the RCA to 5% acetic acid treatment. The specific operational steps are as follows:

[0071] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in water for 10 minutes and drain them until they are saturated and surface dry.

[0072] (2) Disperse silica fume, nano CaCO3 and fine steel slag powder in water at a weight ratio of 5:1:10, where silica fume is 30 parts, nano CaCO3 is 6 parts, fine steel slag powder is 60 parts, and water is added to 75 parts. Then, use ultrasonic treatment for 30 minutes.

[0073] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0074] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0075] Comparative Example 6

[0076] A method for preparing self-compacting RCA concrete is proposed, but the RCA undergoes dual modification treatment with silica fume and nano-CaCO3, and the suspension does not contain fine steel slag powder. The specific steps are as follows:

[0077] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0078] (2) Disperse silica fume and nano CaCO3 in water at a weight ratio of 5:1, where silica fume is 30 parts, nano CaCO3 is 6 parts, and water is 75 parts. Then, use ultrasound to treat for 30 minutes.

[0079] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0080] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0081] Comparative Example 7

[0082] A method for preparing self-compacting RCA concrete is proposed, but the RCA undergoes dual modification treatment with silica fume and fine steel slag powder, and the suspension does not contain nano-CaCO3. The specific steps are as follows:

[0083] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0084] (2) Disperse silica fume and fine steel slag powder in water at a weight ratio of 5:10, with 30 parts silica fume, 60 parts fine steel slag powder, and 75 parts water, and treat with ultrasound for 30 minutes.

[0085] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0086] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0087] Comparative Example 8

[0088] A method for preparing self-compacting RCA concrete is proposed, but the RCA undergoes dual modification treatment with nano-CaCO3 and fine steel slag powder, and the suspension does not contain silica fume. The specific steps are as follows:

[0089] (1) Prepare 900 portions of continuous graded aggregate with a particle size of 5~20mm, wash and dry them, then immerse them in a 5% acetic acid solution for 10 minutes, and then drain the soaked continuous graded aggregate until it is saturated and surface dry.

[0090] (2) Disperse nano-CaCO3 and fine steel slag powder in water at a weight ratio of 1:10, wherein 6 parts are nano-CaCO3, 60 parts are fine steel slag powder, and 75 parts are water. Then, use ultrasonic treatment for 30 minutes.

[0091] (3) Stir the solution in (2) with the pretreated recycled aggregate for 3 minutes, then mix with other dry materials (300 parts cement, 750 parts manufactured sand, 6 parts polycarboxylate superplasticizer), and finally add water and stir to obtain self-compacting concrete with a water-cement ratio of 0.525.

[0092] (4) The above mixture was subjected to concrete slump expansion test, compressive strength test and chloride ion penetration test. The test standards were the same as those in Example 1.

[0093] The performance of the RCA concrete prepared in the above examples and comparative examples was tested, and the test results of the examples and comparative examples are shown in Table 2.

[0094] Table 2 Test Results of Examples and Comparative Examples

[0095] project RCA modification methods Collapse spread (mm) Compressive strength in 3 days (MPa) Compressive strength after 28 days (MPa) Compressive strength 56 days (MPa) <![CDATA[Non-steady state chloride migration coefficient (10 -12 m 2 / s)]]> Example 1 <![CDATA[Silica fume (30 parts) + nano-CaCO3 (6 parts) + fine steel slag powder (60 parts)]]> 630 33.2 49.7 63.6 3.4 Example 2 <![CDATA[Silica fume (45 parts) + nano-CaCO3 (9 parts) + fine steel slag powder (90 parts)]]> 610 35.1 51.2 66.7 2.8 Example 3 <![CDATA[Silica fume (15 parts) + nano-CaCO3 (3 parts) + fine steel slag powder (30 parts)]]> 590 27.9 46.3 59.3 4 Comparative Example 1 Unmodified 550 15.3 38.5 47.2 8.8 Comparative Example 2 Silica fume (30 parts) 530 21.8 42.2 49.9 6.3 Comparative Example 3 <![CDATA[Nano CaCO3 (6 parts)]]> 520 22.6 40.3 48.6 7.2 Comparative Example 4 Fine steel slag powder (60 parts) 665 17.6 40.7 50.2 4.9 Comparative Example 5 5% acetic acid soaking was not used 580 23.2 43.6 57.1 4.3 Comparative Example 6 <![CDATA[Silica fume (30 parts) + nano-CaCO3 (6 parts)]]> 490 25.9 42.7 54.1 5.2 Comparative Example 7 Silica fume (30 parts) + fine steel slag powder (60 parts) 650 23.2 41.2 56.3 4.5 Comparative Example 8 <![CDATA[Nano-CaCO3 (6 parts) + fine steel slag powder (60 parts)]]> 645 24.1 40.6 55.8 4.7

[0096] Compared to unmodified RCA concrete (Comparative Example 1), this invention, through the synergistic physical-chemical modification effect of silica fume, nano-CaCO3, and fine steel slag powder, shows that in Example 1, the compressive strength at 3 days, 28 days, and 56 days increased by 117%, 26%, and 35%, respectively, at three curing periods (e.g., ...). Figures 1-3 As shown), the collapse expansion is increased by 18.9% (as shown). Figure 4 As shown), the unsteady-state chloride ion migration coefficient decreased by 61.4% (e.g. Figure 5(As shown). Examples 2 and 3 quantified the effects of the modification amounts of silica fume, nano-CaCO3, and fine steel slag powder on the above indicators. It can be seen that after increasing the content of silica fume, nano-CaCO3, and fine steel slag powder in the modifier (Example 2), compared with Example 1, the compressive strength was improved, the unsteady chloride ion migration coefficient was further reduced, and the slump spread was negatively affected. After decreasing the content of silica fume, nano-CaCO3, and fine steel slag powder in the modifier (Example 3), compared with Example 1, the compressive strength was reduced, the unsteady chloride ion migration coefficient was increased, and the slump spread was also negatively affected. This indicates that the modifier concentration in Example 1 was more reasonable. Comparative Examples 2, 3, and 4 respectively give the values ​​of concrete workability, mechanical properties, and durability indicators under single modification with silica fume, nano-CaCO3, and fine steel slag powder. It can be seen that although the single modification method improved the compressive strength and unsteady chloride ion migration coefficient of the unmodified concrete (Comparative Example 1), the effect was far less than that of Example 1. Compared to Comparative Example 1, Comparative Examples 2 and 3 showed a greater increase in early-age compressive strength, while Comparative Example 4 showed a greater increase in long-age compressive strength. This reflects the role of the three modified materials at each stage of hydration and demonstrates the overall synergistic effect.

[0097] Compared with Comparative Example 5, Example 1 macroscopically characterized the properties of weak acetic acid in dissolving residual mortar, activating the surface, and enhancing the permeability of the modifier. It can be seen that the concrete prepared without acetic acid soaking of RCA showed a decrease in slump spread and compressive strength. The improvement (reduction) effect on the unsteady chloride ion migration coefficient was also less than that of Example 1.

[0098] Comparative Examples 6, 7, and 8 present the values ​​of workability, mechanical properties, and durability of concrete modified with silica fume, nano-CaCO3, and fine steel slag powder, respectively. It can be seen that although the dual-material modification method improves the compressive strength and unsteady chloride ion migration coefficient of the unmodified concrete (Comparative Example 1), the effect is still far inferior to that of Example 1.

[0099] In summary, this invention utilizes the physical synergy of nano-CaCO3 (50~100 nm), silica fume (0.1~0.3 μm), and fine steel slag powder (40~50 μm) to achieve an extremely dense particle packing effect. This results in a continuous gradation from the nanoscale (nano-CaCO3 fills the smallest voids between cement particles and steel slag powder particles) → submicron scale (silica fume fills the slightly larger voids between cement and steel slag powder particles) → micron scale (fine steel slag powder can fill between cement particles and act as an "intermediate" between them), greatly improving the density of the cementitious system. The synergy of these three components forms a tightly packed system of "cement-steel slag powder-silica fume-nano-CaCO3," significantly reducing the original defects and porosity within the paste, laying the most solid physical foundation for concrete to achieve ultra-high strength, low permeability, and high durability.

[0100] This invention utilizes the chemical synergy of three components to optimize the multi-stage, multi-level hydration reaction. The chemical reactions of these three components do not occur in isolation, but rather promote and reinforce each other. The early hydration reaction is dominated by nano-CaCO3, whose enormous specific surface area provides a vast number of nucleation sites. Cement hydration products (CSH gel, CH) preferentially precipitate on the surface of these nanoparticles, rather than spontaneously forming large crystals in solution. This significantly accelerates the early hydration reaction and improves early strength. The Ca on its surface... 2+ The silica fume reacts with the aluminum phase to form aluminocarbonates, further refining the pores. During the mid-stage hydration reaction, silica fume and nano-CaCO3 work synergistically. The secondary hydration reaction of silica fume generates more high-strength CSH gel. The nucleation effect of nano-CaCO3 provides more sites for the pozzolanic reaction of silica fume, promoting a more complete reaction. This further ensures mid-stage strength. It significantly refines capillary pores, causing the pore structure to develop towards a harmless, low-porosity direction. In the later-stage strength growth and stability, the synergistic effect of fine steel slag powder, silica fume, and nano-CaCO3 dominates. The slow release of Ca(OH)2 from the fine steel slag powder provides a stable alkaline environment and reactants for the continuous pozzolanic reaction of silica fume, preventing premature "calcium depletion" and ensuring the continuous progress of the pozzolanic reaction. Simultaneously, the low-alkalinity environment formed by the early reaction of silica fume and nano-CaCO3 weakens the early hydration reaction of steel slag powder. The steel slag powder, "protected" in the early stages, plays a role in the later stages (as cement hydration continuously provides OH, increasing alkalinity), ensuring stable strength growth in the later stages. The hydration reactions of the three components are activated sequentially in the early, middle, and late stages, promoting each other and overcoming the limitations of the single material reaction stage. This results in a smooth transition and continuous growth in strength development, and the formation of a highly dense microstructure.

[0101] In workability tests, the extremely large specific surface area of ​​silica fume and nano-CaCO3 drastically increases water consumption and paste viscosity, making concrete very viscous and reducing its fluidity. However, fine steel slag powder typically has a more regular particle morphology than silica fume and requires relatively less water. Its addition can partially "dilute" the negative effects of the ultra-high specific surface area of ​​silica fume and nano-CaCO3, improving the balance between the cohesiveness and fluidity of the mixture.

[0102] The above analysis shows that the combined use of silica fume, nano-CaCO3 and fine steel slag powder is a high-performance concrete design strategy based on the theory of close packing and multi-scale hydration control. Through the synergistic effect of the three materials, physical filling, chemical reinforcement and workability adjustment are achieved.

[0103] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing modified recycled coarse aggregate, characterized in that, The modified recycled coarse aggregate is composed of the following raw materials in parts by weight: 900 parts by weight of recycled coarse aggregate, 15-45 parts by weight of silica fume, 3-9 parts by weight of nano-CaCO3, and 30-90 parts by weight of fine steel slag powder; it is prepared by the following steps: S1. Pretreatment of recycled coarse aggregate: The waste concrete is crushed into continuous graded aggregate with a particle size of 5~20mm. The continuous graded aggregate is washed, dried and then immersed in a 5% mass fraction acetic acid solution for 10 minutes. The immersed continuous graded aggregate is then drained until it is saturated and surface dry. S2. Preparation of modifier: 15-45 parts by weight of silica fume, 3-9 parts by weight of nano-CaCO3, and 30-90 parts by weight of fine steel slag powder are dispersed in 75 parts by weight of water and ultrasonically treated for 30 minutes to obtain the modifier. S3. Preparation of modified recycled coarse aggregate: 900 parts by weight of the recycled coarse aggregate pretreated in S1 and the modifier prepared in S2 are mixed and stirred for 3 minutes to obtain modified recycled coarse aggregate.

2. The method for preparing modified recycled coarse aggregate according to claim 1, characterized in that, The particle size of the silica fume is 0.1~0.3μm.

3. The method for preparing modified recycled coarse aggregate according to claim 1, characterized in that, The particle size of the nano-CaCO3 is 50~100nm.

4. The method for preparing modified recycled coarse aggregate according to claim 1, characterized in that, The particle size of the fine steel slag powder is 40~50μm.

5. A self-compacting concrete, characterized in that, The mixture includes the modified recycled coarse aggregate prepared as described in claim 1. When the modified recycled coarse aggregate is prepared by the steps in claim 1, 300 parts by weight of ordinary silicate cement, 750 parts by weight of manufactured sand, and 6 parts by weight of polycarboxylate superplasticizer are added. Finally, water is added and stirred to obtain self-compacting concrete, wherein the water-cement ratio is 0.47~0.

59.

6. The self-compacting concrete according to claim 5, characterized in that, The ordinary silicate cement is ordinary silicate cement P·O42.5.

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