Method for preparing concrete by co-doping full-recycled aggregate with large-particle-size broken stones

Abstract

The invention relates to the technical field of construction materials, and particularly discloses a method for preparing concrete by co-doping full-recycled aggregate with large-particle-size broken stones. The method comprises the following steps: mixing recycled fine aggregate and recycled coarse aggregate according to the continuous grading requirement of the coarse aggregate with large-particle-size broken stones to prepare mixed coarse aggregate, replacing cement with 30-50% of fly ash (class-F second grade or above), adding a high-efficiency water reducing agent, and calculating the mixing ratio of the concrete according to an absolute volume method to prepare the concrete. The method for preparing the concrete by co-doping the full-recycled aggregate with the large-particle-size gravel comprises the following steps: S1, pretreating the aggregate; s2, raw material weighing: weighing the following raw materials: cement, a mineral admixture, recycled coarse aggregate, large-particle-size broken stone, recycled fine aggregate, water and a water reducing agent; s3, primary mixing; s4, slurry preparation: mixing and stirring the weighed cement, mineral admixture and water to form uniform cement slurry; and S5, final stirring: adding the cement paste and the water reducing agent into the premixed aggregate together, and stirring. The concrete product prepared by the invention can be widely applied to civil engineering as common concrete; the concrete has the advantages of high mechanical strength, low cost, environmental protection, economy and stable durability.

Description

Technical Field

[0001] This application relates to the field of construction materials technology, and more specifically, it relates to a method for preparing concrete using fully recycled aggregate mixed with large-diameter crushed stone. Background Technology

[0002] Construction materials refer to all kinds of materials used in the construction of buildings, municipal works, transportation, water conservancy and other projects to meet the needs of structural support, decoration and finishing, and functional protection. They are the foundation of building entities and ensure the quality and safety of projects. They are widely used in various infrastructure scenarios such as housing construction, roads and bridges, tunnels and dams, and municipal pipelines. They are rich in variety and can be divided into structural materials, decorative materials and functional materials according to their functions. Some materials have multiple functions. The performance of construction materials must meet national and industry standards. When selecting them, it is necessary to take into account the type of project, the usage environment and design requirements. Concrete is a common construction material that is widely used.

[0003] The relevant concrete uses recycled aggregates to realize the resource utilization of construction waste. However, directly using a single recycled aggregate or a simple mixing method fails to solve the inherent defects caused by the high porosity, high water absorption and old mortar adhering to the surface of the recycled aggregate itself. This results in the rapid loss of workability of the concrete mixture and a weak transition zone at the aggregate-slurry interface, ultimately leading to unstable mechanical properties and durability of the hardened concrete. Summary of the Invention

[0004] To address the issue of unstable mechanical properties and durability of hardened concrete caused by the use of recycled aggregates to recycle construction waste, this application provides a method for preparing concrete using fully recycled aggregates combined with large-diameter crushed stone.

[0005] This application provides a method for preparing concrete using fully recycled aggregate mixed with large-diameter crushed stone, employing the following technical solution: A method for preparing concrete using fully recycled aggregate mixed with large-diameter crushed stone includes the following steps: S1. Aggregate pretreatment: Construction waste is crushed, screened, and dried to obtain recycled coarse aggregate and recycled fine aggregate; natural rock is crushed and screened to obtain large-diameter crushed stone. S2. Raw material weighing: Weigh each raw material according to the following weight ratio: 250-350 parts cement, 800-1000 parts recycled coarse aggregate, 200-400 parts large-diameter crushed stone, 600-800 parts recycled fine aggregate, 150-180 parts water and 5-10 parts water-reducing agent. S3. Initial mixing: Dry mix the weighed recycled coarse aggregate, large-diameter crushed stone and recycled fine aggregate to make them evenly mixed and obtain premixed aggregate; S4. Slurry preparation: Mix the weighed cement, mineral admixtures and water to form a uniform cement slurry; S5. Final mixing: Add the cement paste and water-reducing agent together to the premixed aggregate and mix to obtain concrete.

[0006] By adopting the above technical solution, this method synergistically combines the skeletal support of large-diameter crushed stone with recycled aggregate, thereby improving the particle size distribution and bulk density inside the concrete. Among them, the dry mixing step of aggregate ensures that the unevenly textured recycled aggregate and large-diameter crushed stone are evenly dispersed, laying a structural foundation for the uniform coating of the subsequent cement paste. The two-step mixing process further forms a cement paste with excellent fluidity. Under the action of water-reducing agent, the paste can fill and lubricate the complex voids between premixed aggregates, reduce mixing resistance, and finally obtain a concrete product with workability that meets construction requirements and stable mechanical properties. At the same time, it utilizes construction waste resources to save costs and achieve low carbon emissions.

[0007] Preferably, in step S1, corresponding to the continuously graded 5~20mm, 5~25mm and 5~31.5mm coarse aggregates, the particle size of the recycled coarse aggregates is 5mm~16mm, 5~20mm and 5~25mm respectively, while the particle size of the large-diameter crushed stone is 10mm~20mm, 16~25mm and 20~31.5mm respectively.

[0008] By adopting the above technical solutions, the lower limit of recycled coarse aggregate is controlled at 5mm to avoid excessively fine particles increasing the specific surface area, thereby making full use of mixing water; while the upper limit of 31.5mm meets the maximum particle size requirements of conventional concrete for coarse aggregate; at the same time, the lower limit of large-diameter crushed stone is raised to 10mm to ensure that these crushed stones have sufficient size to play an effective interlocking and supporting role in the aggregate skeleton. An appropriate amount of large-diameter aggregate can improve the skeleton structure of the aggregate system. The two form a gradient complementarity in particle size, with large-diameter crushed stone forming the main skeleton and recycled coarse aggregate filling the gaps between them, together forming a denser aggregate system. This provides a physical basis for reducing the porosity of concrete and improving its final strength.

[0009] Preferably, in step S1, the temperature for drying the recycled coarse aggregate and recycled fine aggregate is 50–70°C, and the time is 60–120 min.

[0010] By adopting the above technical solution, recycled aggregates, due to their adherence to old mortar, have high water absorption characteristics and will rapidly absorb moisture from the cement paste during mixing, leading to a loss of workability. Controlling the drying temperature within the 50-70℃ range is based on a balance between drying efficiency and aggregate performance protection: excessively low temperatures require excessively long drying times, which is not economical, while excessively high temperatures damage the aggregate itself or its old mortar components, causing microcracks. A continuous drying time of 60-120 minutes aims to stably reduce the moisture content of the aggregate to a low and controllable level. This step lays the foundation for subsequent control of the water-cement ratio, ensuring that the mixing water is used for cement hydration rather than being absorbed by the aggregate, thereby guaranteeing the stability of the concrete mixture and the predictability of the hardened concrete strength.

[0011] Preferably, in step S2, the mass ratio of fine aggregate to total aggregate is controlled to be 45-55%; and the mass ratio of recycled coarse aggregate to large-diameter crushed stone is controlled to be 0.6-0.8.

[0012] By adopting the above technical solution, the mass ratio of fine aggregate to total aggregate is maintained at 45% to 55%, ensuring sufficient fine particles to fill the voids between coarse aggregates, thereby achieving high density and pumpability. When the mass ratio of recycled coarse aggregate to large-diameter crushed stone is less than 0.6, there is too much large-diameter crushed stone, the aggregate gaps increase, more paste is needed for filling, the economy decreases, and segregation is easy. When the mass ratio is higher than 0.75, the proportion of recycled aggregate is too high, and its weaknesses of low strength and high water absorption will dominate the concrete performance, and the strengthening effect of large-diameter crushed stone will be weakened. Therefore, the mass ratio of recycled coarse aggregate to large-diameter crushed stone is limited to between 0.6 and 0.8. Through this ratio, a balance is achieved between the economic and environmental advantages of recycled aggregate and the structural strengthening advantages of large-diameter crushed stone.

[0013] Preferably, in step S2, the mineral admixture is fly ash.

[0014] By adopting the above technical solutions, the microsphere effect of fly ash can improve the rheological properties of cement paste and reduce the internal friction of the mixture. This, together with the water-reducing agent, helps the paste to coat the rough interface of the recycled aggregate. At the same time, the pozzolanic activity of fly ash enables it to undergo a secondary reaction with calcium hydroxide, a cement hydration product, in the later stage to generate additional gel. This reaction is beneficial to strengthening the interfacial transition zone between recycled aggregate and new and old cement paste, which is usually the weak link in the performance of all-recycled aggregate concrete. The use of fly ash not only improves the long-term durability of concrete, but also realizes the resource utilization of industrial by-products.

[0015] Preferably, the water-reducing agent in S2 is a polycarboxylate-based water-reducing agent or a naphthalene-based water-reducing agent.

[0016] By adopting the above technical solutions, polycarboxylate superplasticizers can achieve efficient dispersion of cement particles at low dosages through steric hindrance, and naphthalene superplasticizers can achieve efficient dispersion through electrostatic repulsion, thereby reducing the amount of mixing water. At the same time, reducing the water-cement ratio can improve the strength of concrete, thereby compensating for the strength loss caused by the use of recycled aggregates. In addition, the high fluidity provided by the high-efficiency superplasticizer ensures that the cement paste can wet the complex surface and internal pores of the recycled aggregates, forming a dense hydration product structure, thereby improving the microstructure of concrete.

[0017] Preferably, in step S4, the slurry temperature is controlled at 15–30°C, the stirring speed is 300–500 rpm, and the time is 60–120 seconds.

[0018] By adopting the above technical solution, the slurry temperature is maintained at 15 to 30°C, which can avoid excessively high temperature causing cement hydration to be too fast and resulting in loss of workability over time, or excessively low temperature causing slow hydration and affecting early strength development. The relatively high stirring speed of 300 to 500 rpm for 60 to 120 seconds is intended to apply sufficient shear force to fully mix and wet the cement particles, fly ash and mixing water, ensuring that the water-reducing agent is evenly distributed and plays its role, thereby forming a highly fluid slurry with a broken flocculated structure.

[0019] Preferably, in step S3, the dry mixing speed is controlled at 150-250 rpm and the time is 30-60 seconds.

[0020] By adopting the above technical solution, a dry mixing speed of 150 to 250 rpm provides a moderate energy input, which can achieve uniform mixing of different types and particle sizes of aggregates within 30 to 60 seconds, avoiding segregation during subsequent feeding. This step ensures that the subsequently injected cement paste can uniformly contact and coat each type of aggregate, rather than first contacting a certain type of aggregate, thereby ensuring the formation of a uniform interface structure and guaranteeing the overall homogeneity of the concrete.

[0021] Preferably, in step S4, sodium gluconate or sodium citrate is added, and the amount of sodium gluconate or sodium citrate is 0.1% to 0.4% of the total mass of cement and mineral admixtures.

[0022] By adopting the above technical solutions, the mixing process of recycled aggregate concrete is relatively complex. When the amount of aggregate is large, a longer mixing time is required to ensure uniformity. The addition of retarders can delay the initial setting time of cement. These organic retarders inhibit hydration by adsorbing on the surface of cement minerals, thereby providing more time windows for mixing, transportation and construction. Controlling the dosage between 0.1% and 0.4% can delay the time of hydration peak and avoid the equilibrium point of excessive retardation leading to slow strength development. This ensures that the concrete achieves high workability while ensuring that its mechanical properties are not weakened.

[0023] Preferably, in step S5, the stirring temperature is controlled at 10–35°C, the low-speed stirring speed is 100–200 rpm, and the time is 60–90 seconds; the high-speed stirring speed is 300–400 rpm, and the time is 90–120 seconds.

[0024] By adopting the above technical solution, the final mixing stage employs a two-stage mixing process of low speed followed by high speed to ensure that all components are fully and uniformly mixed. In the low-speed mixing stage, the cement paste with a certain fluidity is initially impregnated and covered on the surface of the premixed aggregate under the combined action of water-reducing agent and retarder, achieving preliminary mixing and avoiding slurry splashing. Subsequently, the high-speed mixing stage is entered, where stronger shear force is applied to break up the formed slurry clumps, forcing the cement paste to fill every void between aggregate particles and forcing the paste to penetrate into some open pores on the surface of recycled aggregate, thereby improving the homogeneity and density of concrete. At the same time, the temperature is controlled at 10~35℃ throughout the mixing process to ensure that the cement hydration reaction proceeds normally, ultimately achieving workability, strength, and durability that meet construction requirements.

[0025] In summary, this application has the following beneficial effects: 1. Because this application uses a blend of large-diameter crushed stone and recycled aggregate, and employs a two-step mixing process of first dry-mixing the aggregate and then injecting the slurry, the large-diameter crushed stone provides a solid skeletal support for the concrete, while the pre-dry mixing ensures that the two types of aggregates with different properties are evenly dispersed, laying the foundation for the uniform coating of the subsequent slurry. The two-step mixing method enables the highly fluid cement slurry under the action of the water-reducing agent to efficiently fill the complex voids between the aggregates, thereby reducing the mixing resistance and workability loss caused by the high water absorption rate of the recycled aggregate. Therefore, it achieves the effects of improved workability of the concrete mixture, strengthened interface transition zone, and stable and reliable final mechanical properties.

[0026] 2. In this application, the preferred method is to control the mass ratio range of recycled coarse aggregate to large-diameter crushed stone. This mass ratio range balances the economic and environmental characteristics of recycled aggregate with the structural strengthening effect of large-diameter crushed stone. It avoids the concrete from segregating easily and becoming uneconomical when the mass ratio is too low, and prevents the performance weaknesses of recycled aggregate from dominating the behavior of concrete when the mass ratio is too high. Therefore, it achieves the effect of saving costs by using recycled aggregate while ensuring that the overall skeleton structure of concrete is dense and has high strength, and avoiding segregation or excessive water absorption.

[0027] 3. The method of this application adds a certain amount of sodium gluconate or sodium citrate as a retarder during the slurry preparation stage, and controls the temperature and high / low speed mixing during the final mixing stage. Since the retarder delays the time of cement hydration peak, it provides an operating window for the mixing process. The controlled mixing temperature and segmented mixing process ensure that the slurry wets and mixes the aggregate evenly. Therefore, the concrete mixture has the effect of small slump loss over time, extended construction time, uniform microstructure and high density after hardening, and improved long-term durability. Attached Figure Description

[0028] Figure 1 The flowchart of the method for preparing concrete by combining all recycled aggregate with large-diameter crushed stone according to the embodiment of this application is provided. Detailed Implementation

[0029] The present application will be further described in detail below with reference to the accompanying drawings and embodiments.

[0030] Technical concept: The relevant concrete uses recycled aggregates to realize the resource utilization of construction waste. However, directly using a single recycled aggregate or a simple mixing method fails to solve the inherent defects caused by the high porosity, high water absorption and old mortar adhering to the surface of the recycled aggregate itself. This results in the rapid loss of workability of the concrete mixture and a weak transition zone at the aggregate-slurry interface, ultimately leading to unstable mechanical properties and durability of the hardened concrete.

[0031] See appendix Figure 1 The method for preparing concrete using recycled aggregate mixed with large-diameter crushed stone disclosed in this application includes the following steps: S1, aggregate pretreatment; S2, raw material weighing: weighing the following raw materials: cement, mineral admixtures, recycled coarse aggregate, large-diameter crushed stone, recycled fine aggregate, water, and water-reducing agent; S3, initial mixing; S4, slurry preparation: mixing and stirring the weighed cement, mineral admixtures, and water to form a uniform cement slurry; S5, final mixing: adding the cement slurry and water-reducing agent together to the premixed aggregate and stirring.

[0032] This application employs a blend of large-diameter crushed stone and recycled aggregate, and utilizes a two-step mixing process: first, dry-mix the aggregate, then inject the slurry. The large-diameter crushed stone provides a solid skeletal support for the concrete, while the pre-dry mixing ensures the uniform dispersion of the two types of aggregates with different properties, laying the foundation for the subsequent uniform coating by the slurry. This two-step mixing method allows the highly fluid cement slurry, under the action of the water-reducing agent, to efficiently fill the complex voids between the aggregates, thereby reducing the mixing resistance and workability loss caused by the high water absorption rate of the recycled aggregate. Therefore, it achieves improved workability of the concrete mixture, strengthened interfacial transition zone, and ultimately stable and reliable mechanical properties.

[0033] Example 1 This embodiment provides a method for preparing concrete using fully recycled aggregate mixed with large-diameter crushed stone, comprising the following steps: S1. Aggregate pretreatment: Construction waste is crushed, screened, and dried to obtain recycled coarse aggregate and recycled fine aggregate; natural rock is crushed and screened to obtain large-diameter crushed stone. Among them, the particle size of recycled coarse aggregate is 5~16mm, and the particle size of large-diameter crushed stone is 10~20mm; The temperature for drying recycled coarse aggregate and recycled fine aggregate is 50℃, and the time is 120min. S2. Raw material weighing: Weigh each raw material according to the following weight ratio: 250 parts cement, 150 parts mineral admixture, 800 parts recycled coarse aggregate, 400 parts large-diameter crushed stone, 600 parts recycled fine aggregate, 150 parts water and 5 parts water-reducing agent. Among them, the mineral admixture is fly ash; The water-reducing agent is a polycarboxylate-based water-reducing agent. Among them, the mass ratio of fine aggregate to total aggregate is controlled at 45%; the mass ratio of recycled coarse aggregate to large-diameter crushed stone is controlled at 0.6. S3. Initial mixing: Dry mix the weighed recycled coarse aggregate, large-diameter crushed stone and recycled fine aggregate to make them evenly mixed and obtain premixed aggregate; The dry mixing speed is controlled at 150 rpm for 60 seconds. S4. Slurry preparation: Mix the weighed cement, mineral admixtures and water to form a uniform cement slurry; The slurry temperature was controlled at 15℃, the stirring speed at 300 rpm, and the stirring time at 120 seconds. Sodium gluconate was also added, at a concentration of 0.1% of the total mass of cement and mineral admixtures. S5. Final mixing: Add the cement paste and water-reducing agent together to the premixed aggregate and mix to obtain concrete; The stirring temperature was controlled at 10℃, the low-speed stirring speed was 100 rpm for 90 seconds, and the high-speed stirring speed was 300 rpm for 120 seconds.

[0034] Example 2 This embodiment provides a method for preparing concrete using fully recycled aggregate mixed with large-diameter crushed stone, comprising the following steps: S1. Aggregate pretreatment: Construction waste is crushed, screened, and dried to obtain recycled coarse aggregate and recycled fine aggregate; natural rock is crushed and screened to obtain large-diameter crushed stone. Among them, the particle size of recycled coarse aggregate is 5~20mm, and the particle size of large-diameter crushed stone is 16~25mm; The temperature for drying recycled coarse aggregate and recycled fine aggregate is 60℃, and the time is 90min; S2. Raw material weighing: Weigh each raw material according to the following weight ratio: 300 parts cement, 200 parts mineral admixture, 900 parts recycled coarse aggregate, 300 parts large-diameter crushed stone, 700 parts recycled fine aggregate, 165 parts water and 7.5 parts water-reducing agent. Among them, the mineral admixture is fly ash; Among them, the water-reducing agent is a naphthalene-based water-reducing agent; Among them, the mass ratio of fine aggregate to total aggregate is controlled at 50%; the mass ratio of recycled coarse aggregate to large-diameter crushed stone is controlled at 0.65. S3. Initial mixing: Dry mix the weighed recycled coarse aggregate, large-diameter crushed stone and recycled fine aggregate to make them evenly mixed and obtain premixed aggregate; The dry mixing speed is controlled at 200 rpm for 45 seconds. S4. Slurry preparation: Mix the weighed cement, mineral admixtures and water to form a uniform cement slurry; The slurry temperature was controlled at 22℃, the stirring speed at 400 rpm, and the stirring time at 90 seconds. Sodium citrate was also added, at a rate of 0.25% of the total mass of cement and mineral admixtures. S5. Final mixing: Add the cement paste and water-reducing agent together to the premixed aggregate and mix to obtain concrete; The stirring temperature was controlled at 22℃, the low-speed stirring speed was 150 rpm for 75 seconds, and the high-speed stirring speed was 350 rpm for 105 seconds.

[0035] Example 3 This embodiment provides a method for preparing concrete using fully recycled aggregate mixed with large-diameter crushed stone, comprising the following steps: S1. Aggregate pretreatment: Construction waste is crushed, screened, and dried to obtain recycled coarse aggregate and recycled fine aggregate; natural rock is crushed and screened to obtain large-diameter crushed stone. Among them, the particle size of recycled coarse aggregate is 5~25mm, and the particle size of large-diameter crushed stone is 20~31.5mm; The temperature for drying recycled coarse aggregate and recycled fine aggregate is 70℃, and the time is 60min; S2. Raw material weighing: Weigh each raw material according to the following weight ratio: 350 parts cement, 250 parts mineral admixture, 1000 parts recycled coarse aggregate, 200 parts large-diameter crushed stone, 800 parts recycled fine aggregate, 180 parts water and 10 parts water-reducing agent. Among them, the mineral admixture is fly ash; The water-reducing agent is a polycarboxylate-based water-reducing agent. Among them, the mass ratio of fine aggregate to total aggregate is controlled at 55%; the mass ratio of recycled coarse aggregate to large-diameter crushed stone is controlled at 0.8. S3. Initial mixing: Dry mix the weighed recycled coarse aggregate, large-diameter crushed stone and recycled fine aggregate to make them evenly mixed and obtain premixed aggregate; The dry mixing speed is controlled at 250 rpm for 30 seconds. S4. Slurry preparation: Mix the weighed cement, mineral admixtures and water to form a uniform cement slurry; The slurry temperature was controlled at 30℃, the stirring speed at 500 rpm, and the stirring time at 60 seconds. Sodium gluconate was also added, at a rate of 0.4% of the total mass of cement and mineral admixtures. S5. Final mixing: Add the cement paste and water-reducing agent together to the premixed aggregate and mix to obtain concrete; The stirring temperature was controlled at 35℃, the low-speed stirring speed was 200 rpm for 60 seconds, and the high-speed stirring speed was 400 rpm for 90 seconds.

[0036] Comparative Example 1 The comparative example refers to the content of Example 1, except that in step S1, the particle size of the recycled coarse aggregate is 5~20mm, and the particle size of the large-diameter crushed stone is 16~25mm. The rest is the same as in Example 1.

[0037] Comparative Example 2 The comparative example refers to the content of Example 1, except that in step S1, the temperature for drying the recycled coarse aggregate and recycled fine aggregate is 40°C and the time is 150 min. The rest of the content is the same as in Example 1.

[0038] Comparative Example 3 The comparative example refers to the content of Example 1, except that in step S2, the mass ratio of recycled coarse aggregate to large-diameter crushed stone is controlled to be 0.7, and the rest is the same as in Example 1.

[0039] Comparative Example 4 The comparative example refers to the content of Example 1, except that in step S5, there is no distinction between low-speed and high-speed stirring, and the stirring speed is 200 rpm throughout for 150 seconds. The rest of the content is the same as in Example 1.

[0040] Comparative Example 5 The comparative example refers to the content of Example 1, except that in step S2, large-diameter crushed stone is not used, and its 400 parts are all replaced by recycled coarse aggregate, that is, the total amount of recycled coarse aggregate is 1200 parts. The rest is the same as in Example 1.

[0041] Performance testing Sample preparation: The concrete samples used for performance testing were all prepared according to the proportions and preparation methods described in Examples 1 to 3 and Comparative Examples 1 to 6 of this invention. Sufficient concrete mixtures were prepared for each proportion for subsequent performance tests. After mixing, the performance of fresh concrete was tested immediately. The remaining mixtures were placed in molds of specific sizes and cured in an environment with a temperature of 20±2℃ and a relative humidity of 95% or higher until the specified age before the performance of hardened concrete was tested.

[0042] Slump test: Freshly mixed concrete is poured into a clean slump cone in three layers, each layer is tamped 25 times and the surface is smoothed; the slump cone is lifted vertically and steadily, and the height difference between the height of the cone and the highest point of the concrete specimen after slumping is measured. This difference is the slump value, recorded in millimeters; This test is mainly based on the "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" GB / T50080-2016.

[0043] Compressive strength test: The cubic specimens cured to the specified age are taken out of the standard curing room, wiped with a damp cloth, and placed in the center of the lower platen of the compression testing machine; pressure is continuously and uniformly applied to the specimens at the specified loading rate until the specimens fail, and the failure load is recorded; the compressive strength of the specimens is calculated by dividing the failure load by the bearing area of ​​the specimens; this test is mainly based on the "Standard for Test Methods of Physical and Mechanical Properties of Concrete" GB / T50081-2019.

[0044] Chloride ion migration coefficient test: First, concrete specimens cured to the specified age are cut into slices of a specific thickness and saturated with water under vacuum conditions. Then, the treated specimens are installed in the test apparatus, with sodium chloride solution injected on one side and sodium hydroxide solution injected on the other side, and an external DC electric field is applied. After a certain period of time, the specimens are split open, and silver nitrate solution is sprayed onto the split surface. The chloride ion migration coefficient is calculated by measuring the depth of the colorimetric boundary line. The lower the migration coefficient, the better the resistance to chloride ion penetration. This test is mainly based on the electrical flux method or RCM method in the "Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete" GB / T50082-2009.

[0045] Scanning electron microscopy observation: First, samples were taken from the hardened concrete specimens, and typical areas containing the interface of recycled aggregate, large-diameter crushed stone and cement paste were selected to prepare samples that meet the observation requirements. The samples were placed under a scanning electron microscope, and the structural compactness, morphology of hydration products and microcracks of the interface area were observed under accelerating voltage and magnification.

[0046] Table 1:

[0047] Example Conclusion As can be seen from Examples 1-3 and Comparative Example 1, and Table 1, the configuration of aggregate particle size is the foundation for constructing a robust concrete skeleton. When the aggregate particle size is too small, although high initial fluidity can be obtained, the skeleton interlocking cannot be formed, resulting in poor integrity and increased porosity of the hardened concrete, thereby weakening its compressive strength and impermeability. This invention controls the particle size of recycled coarse aggregate and large-diameter crushed stone within a specific range and forms a gradient to ensure the stability of the aggregate skeleton, thus laying the foundation for obtaining high-performance concrete.

[0048] As can be seen from Examples 1-3 and Comparative Example 2, and Table 1, preheating and drying of recycled aggregates can ensure their high water absorption characteristics and guarantee the workability stability of fresh concrete. Insufficient drying will cause the aggregates to absorb a large amount of mixing water during the mixing process, which will not only reduce fluidity, but also form a weak zone in the aggregate-paste interface area due to local water-cement ratio imbalance, thereby weakening the strength and durability of concrete.

[0049] As can be seen from Examples 1-3 and Comparative Example 3, and Table 1, the mass ratio of recycled coarse aggregate to large-diameter crushed stone can balance economy and structural performance. When the ratio is too low, the large-diameter crushed stone is too concentrated and the recycled aggregate is insufficient, which can easily lead to a large demand for slurry and poor homogeneity of the mixture. However, by controlling the mass ratio within a certain range, the present invention enables the high-strength large-diameter crushed stone to bear the main load, while the recycled aggregate fully fills the gaps. The two work together to form an economical and dense reinforced skeleton system.

[0050] As can be seen from Examples 1-3 and Comparative Example 4, and Table 1, the segmented mixing process of low speed followed by high speed is the guarantee for achieving efficient and uniform mixing between cement paste and aggregate. Simple single-speed mixing cannot overcome the mixing resistance caused by the high water absorption of recycled aggregate, nor can it ensure that the paste penetrates into the pores and interfaces of the aggregate. The segmented mixing process of the present invention first promotes preliminary wetting, and then achieves final homogenization through strong shearing. This is the process for obtaining concrete products with dense microstructure and uniform performance.

[0051] As can be seen from Examples 1-3 and Comparative Example 5, and Table 1, the introduction of large-diameter crushed stone can compensate for the inherent performance deficiencies of all-recycled aggregate concrete. When using recycled aggregates entirely, the overall performance of the concrete has obvious shortcomings due to their low strength and weak interfacial transition zone. However, after adding large-diameter crushed stone, these high-strength natural aggregates significantly improve the load-bearing capacity of the concrete as the main skeleton and improve the internal stress distribution. Thus, while making full use of recycled resources, it ensures that the concrete meets the mechanical and durability performance standards required for structural applications.

[0052] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone, characterized in that, Includes the following steps: S1. Aggregate pretreatment: Construction waste is crushed, screened, and dried to obtain recycled coarse aggregate and recycled fine aggregate; natural rock is crushed and screened to obtain large-diameter crushed stone. S2. Raw material weighing: Weigh each raw material according to the following weight ratio: 250-350 parts cement, 150-250 parts fly ash (Class F, Grade II or above), 800-1000 parts recycled coarse aggregate, 200-400 parts large-diameter crushed stone, 600-800 parts recycled fine aggregate, 150-180 parts water, and 5-10 parts water-reducing agent; S3. Initial mixing: Dry mix the weighed recycled coarse aggregate, large-diameter crushed stone and recycled fine aggregate to make them evenly mixed and obtain premixed aggregate; S4. Slurry preparation: Mix the weighed cement, mineral admixtures and water to form a uniform cement slurry. S5. Final mixing: Add the cement paste and water-reducing agent together to the premixed aggregate and mix to obtain concrete.

2. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, In step S1, corresponding to the continuously graded 5~20mm, 5~25mm and 5~31.5mm coarse aggregates, the particle sizes of the recycled coarse aggregates are 5mm~16mm, 5~20mm and 5~25mm respectively, while the particle sizes of the large-diameter crushed stone are 10mm~20mm, 16~25mm and 20~31.5mm respectively.

3. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, In step S1, the temperature for drying recycled coarse aggregate and recycled fine aggregate is 50–70°C, and the time is 60–120 min.

4. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, In step S2, the mass ratio of fine aggregate to total aggregate is controlled at 45-55%; the mass ratio of recycled coarse aggregate to large-diameter crushed stone is controlled at 0.6-0.

8.

5. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, In step S2, the mineral admixture is fly ash.

6. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, The water-reducing agent in S2 is a polycarboxylate-based water-reducing agent or a naphthalene-based water-reducing agent.

7. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, In step S4, the slurry temperature is controlled at 15–30℃, the stirring speed is 300–500 rpm, and the time is 60–120 seconds.

8. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, In step S3, control the dry mixing speed to be 150-250 rpm and the time to be 30-60 seconds.

9. The method for preparing concrete using all recycled aggregate mixed with large-diameter crushed stone according to claim 1, characterized in that, In step S5, the stirring temperature is controlled at 10–35℃, the low-speed stirring speed is 100–200 rpm, and the time is 60–90 seconds; the high-speed stirring speed is 300–400 rpm, and the time is 90–120 seconds.

Images