Alkali solution modified recycled concrete and preparation method thereof

Abstract

The invention relates to aqueous alkali modified recycled concrete and a preparation method thereof. The modified recycled concrete is prepared from the following components in parts by mass: 250 to 300 parts of P.II 42.5 R cement, 30 to 50 parts of fly ash, 70 to 90 parts of mineral powder, 0 to 30 parts of silica fume, 160 parts of mixing water, 40 to 50 parts of recycled aggregate pre-wetting water, 420 to 450 parts of river sand, 290 to 300 parts of machine-made sand, 1000 to 1200 parts of recycled coarse aggregate, 3 to 8 parts of water reducing agent and 0 to 3 parts of sodium hydroxide. The preparation method comprises the following steps: firstly, blending recycled coarse aggregate into proper fractal gradation, then mixing the recycled coarse aggregate with an alkali solution for modification, and then mixing with other components to prepare the alkali solution modified recycled concrete. The working performance and the mechanical performance of the recycled concrete can be remarkably improved, the modification method is simple, convenient and rapid, preparation and use are easy to achieve, and great convenience is provided for industrial production.

Description

Technical Field

[0001] The invention relates to the technical field of civil engineering materials, and in particular to an alkali solution modified recycled concrete and a preparation method thereof. Background Art

[0002] Recycled concrete refers to the process of crushing, removing impurities and cleaning waste concrete and waste mortar, mixing particles of various particle sizes in a certain proportion into a recycled aggregate grading system, partially or completely replacing natural aggregates, and then adding cementitious materials, water, admixtures, etc. to prepare new concrete. The application of recycled concrete can help alleviate the contradiction between resources and the environment, namely, the rapid increase of waste concrete and the rapid shortage of natural sand and gravel resources, and is of great significance to promoting sustainable development and resource protection.

[0003] However, recycled aggregate (mainly recycled coarse aggregate) contains a certain amount of mortar, has a rough surface, many edges and corners, and a natural interface transition zone. In addition, a large number of microcracks are generated and developed inside the aggregate during the production process, which makes the recycled aggregate have the characteristics of high water absorption, low density, poor wear resistance and high crushing index. Its properties are generally weaker than natural aggregate, resulting in the working performance, mechanical properties and durability of recycled concrete usually being weaker than ordinary concrete.

[0004] Although existing studies have focused on improving the performance of recycled concrete from four aspects, namely, improving the performance of recycled aggregates, adding admixtures, preparing geopolymer concrete, and incorporating fibers, these methods all have their own limitations. Specifically, the cost of adding admixtures (in the form of additions other than mineral admixtures) and preparing geopolymer concrete is relatively high; the cost of incorporating high elastic modulus fibers is even more significant, while low elastic modulus fibers contribute almost no strength improvement. Existing methods for improving the performance of recycled aggregates mainly use physical, chemical, and biological treatment technologies to remove or strengthen the old mortar on the surface of recycled aggregates, but overall, they are complicated, time-consuming, energy-intensive, waste-intensive, and costly, making them difficult to apply in large-scale industrial applications. Specifically, the heating method and aggregate shaping method are not only energy-intensive and costly, but may also aggravate internal damage to aggregates; acid immersion is likely to leave residual acid ions, which is not conducive to the durability of recycled aggregate concrete and also poses a problem of waste liquid treatment; whether polymer emulsion immersion can improve the performance of recycled concrete is still controversial, and it takes a long time and has low efficiency; volcanic ash immersion is likely to produce new weak interfaces and reduce the workability of recycled concrete; biological calcium fixation and natural carbonization treatment take a long time, making it difficult to meet the needs of large-scale recycled aggregate processing.

[0005] Chinese patent CN202311114204.3 provides a reinforced regenerated bone and its preparation method and regenerated aggregate concrete, which removes the old mortar on the surface by sandblasting, soaks it in polysorbate-80 solution for 4 to 6 hours, removes the remaining old mortar by ultrasound, fishes it out and dries it, and then soaks it in fungal mineralization sedimentation liquid for 20 to 24 hours, calcium lactate solution for 2 to 4 days, and strengthening liquid for 6 to 10 hours, and obtains regenerated interface reinforced regenerated aggregate after drying; however, this process involves multiple soaking and drying, and the operation is complicated and time-consuming. The required solutions (polysorbate-80, fungal mineralization sedimentation liquid, calcium lactate and strengthening liquid) all need to be mixed with multiple components and undergo complex preparation processes such as high-pressure sterilization and cultivation. Therefore, it is not conducive to industrial-scale application. Chinese patent CN202010604227.2 mixes recycled coarse aggregate with microsilica powder and water to form a gel, and after drying, uses a coupling agent to promote anionic polyacrylamide to wrap the aggregate; however, this solution includes two drying steps, resulting in a long processing cycle, and a double layer of interface material is formed on the surface of the aggregate, which increases the interface transition zone, and the internal defect problem is still prominent. Chinese patent CN201710009954.2 uses a sodium hydroxide aqueous solution with a mass concentration of 8-12% to soak the recycled aggregate for 28-32 days, but the patent does not use the soaked recycled aggregate to produce recycled concrete to evaluate its effect. Due to the high mass concentration, this method is not suitable for siliceous pebbles or gravel, and long-term soaking not only hinders the industrialization process, but also poses safety hazards and waste liquid problems. Chinese patent CN202310187218.1 crushes the discarded concrete into 12mm particles, soaks them in a 20% mass concentration hydrochloric acid solution, washes them, mixes them with hydroxyethyl acrylate and diphenyl polydimethylsiloxane, and then crushes them to 5mm. This method causes high energy consumption and severe internal damage due to multiple crushing. The high concentration of hydrochloric acid not only poses a safety hazard, but may also affect the durability of the recycled concrete.

[0006] At present, most methods for optimizing the performance of recycled concrete divide recycled aggregate modification and recycled concrete preparation into two independent parts, which are not closely connected. They have limitations such as high cost, complex process, long time, high energy consumption, and difficulty in industrial application. Therefore, it is still necessary to explore more efficient and economical methods. Summary of the invention

[0007] In view of the above shortcomings of the prior art, the purpose of the present invention is to provide an alkaline solution modified recycled concrete and a preparation method thereof, in order to provide strong support for the industrial production of recycled concrete. The present invention optimizes the recycled concrete, fills the pores and microcracks in the transition zone between the old mortar and the interface, improves its internal microstructure, and achieves the improvement of the performance of the recycled concrete.

[0008] To achieve the above objectives, the present invention adopts the following technical solutions:

[0009] An alkali solution modified recycled concrete is characterized in that it is composed of the following raw materials in parts by mass: 250-300 parts of P·II42.5R cement, 30-50 parts of fly ash, 70-90 parts of mineral powder, 0-30 parts of silica fume, 160 parts of mixing water, 40-50 parts of water for pre-wetting recycled coarse aggregate, 420-450 parts of river sand, 280-300 parts of machine-made sand, 1000-1200 parts of recycled coarse aggregate, 3-8 parts of water reducer, and 0-3 parts of sodium hydroxide.

[0010] Preferably, it is composed of the following raw materials in parts by mass: 265-270 parts of P·II 42.5R cement, 35-40 parts of fly ash, 75-80 parts of mineral powder, 10-20 parts of silica fume, 160 parts of mixing water, 40-50 parts of water for pre-wetting recycled coarse aggregate, 435-440 parts of river sand, 290-300 parts of machine-made sand, 1000-1100 parts of recycled coarse aggregate, 5-7 parts of water reducer, and 0.97-2.44 parts of sodium hydroxide.

[0011] Preferably, the water reducer is a polycarboxylic acid high-efficiency water reducer.

[0012] A method for preparing alkaline solution modified recycled concrete, characterized in that it comprises the following steps:

[0013] (1) Weigh the water reducer and 80% of mixing water in proportion, mix and stir the two to obtain a ready-to-use mixture;

[0014] (2) P·II 42.5R cement, fly ash, mineral powder, silica fume, river sand and machine-made sand are weighed in proportion, poured into a stirring tank for dry mixing, and after mixing evenly, the standby mixture of step (1) is added for wet mixing until a fluid state is reached;

[0015] (3) Add the recycled coarse aggregate into a mixing kettle and mix, adding 10% mixing water during the process until the materials are blended into a uniform paste-like mixture, and then pour and cure the mixture to obtain recycled concrete.

[0016] Preferably, the recycled coarse aggregate is prepared as follows:

[0017] S1. Crushing waste concrete into continuous gradations of 5 to 20 mm, and calculating the mass ratios of three particle size ranges of 5 to 10 mm, 10 to 16 mm, and 16 to 20 mm according to formula (1);

[0018]

[0019] Among them, r g is the aggregate particle size (corresponding to the sieve hole size), r g,min is the minimum particle size in the grading system, r g,max is the maximum particle size in the grading system, F is the fractal dimension, P(rg ) is the passing rate through the sieve hole;

[0020] The fractal dimension F is 2.5 to 2.6;

[0021] S2. Determine the total mass of the required recycled coarse aggregate according to the mix ratio of recycled concrete, and then calculate the mass of each particle size range according to the mass ratio to obtain the prepared recycled coarse aggregate;

[0022] S3. Weigh water for pre-wetting recycled coarse aggregate, sodium hydroxide and 10% mixing water in proportion, mix them evenly to obtain a sodium hydroxide solution; pour the recycled coarse aggregate prepared in S2 into a stirring tank, then pour in the sodium hydroxide solution, and stir to make the recycled coarse aggregate evenly wetted, so as to obtain recycled coarse aggregate modified by the sodium hydroxide solution.

[0023] Due to the porous characteristics of the old mortar on the surface of the recycled aggregate, the sodium hydroxide solution can easily penetrate into the interior of the old mortar, forming an alkaline environment. The alkaline solution can dissolve the silicates and aluminates in the unhydrated cement to form sodium silicate and sodium metaaluminate, which then react with the calcium hydroxide in the old mortar to form hydrated calcium silicate and hydrated calcium aluminate, filling the original micro cracks of the recycled aggregate, thereby achieving the effect of improving the properties of the recycled aggregate. The mechanism is as follows:

[0024] In the Si-O-Si bridging oxygen structure, the 2p electrons of O and the 3p orbital of Si form a π bond. When the hydroxide ion exists, Si 4+ You can pull O around it, breaking the Si—O bond:

[0025]

[0026] Continue with Ca 2+ Reaction to form calcium silicate hydrate:

[0027] -Si-O-+Ca 2+ →-Si-O-Ca-

[0028] -Si-O-Ca-+OH - →-Si-O-Ca-OH (3)

[0029] Alkali dissolution of aluminates:

[0030]

[0031] Continue with Ca 2+ Reaction to form calcium aluminate hydrate.

[0032] Preferably, the fractal dimension F is 2.55.

[0033] Preferably, in step (2), the dry mixing speed is 40 to 50 r / min; in step S3, the stirring speed is 40 to 50 r / min, and the time is 2 to 3 min.

[0034] Preferably, the fly ash is grade II fly ash, and the mineral powder is grade S95.

[0035] Preferably, the stone powder content of the machine-made sand is 10% to 20%.

[0036] Preferably, the water reducing efficiency of the water reducing agent is greater than 12.5%.

[0037] The present invention improves the performance of recycled concrete from three aspects. Compared with the prior art, the present invention is beneficial in that:

[0038] (1) A method for mixing recycled coarse aggregate based on a fractal grading system is proposed. This method makes the aggregate system as close to the theoretical most compact state as possible by accurately adjusting the grading of the aggregate, thereby maximizing the density of the concrete. Given that the content of old mortar attached to the surface of the recycled aggregate is closely related to the fractal dimension, and the content of old mortar directly affects the water absorption of the aggregate, the inventors found that the optimal fractal dimension of the recycled aggregate is different from that of the natural aggregate. The optimal fractal dimension of the recycled aggregate was further determined, providing a new theoretical basis for the efficient use of recycled aggregate - the fractal grading of recycled aggregate can simultaneously characterize the content of old mortar and morphological characteristics.

[0039] (2) The microstructure of the old mortar inside the recycled aggregate is optimized by using an alkaline solution. This process not only improves the interface properties of the aggregate, but also achieves the synergistic effect of alkali and silica fume by adding silica fume, further improving the microstructure of the recycled concrete and thus increasing its strength.

[0040] (3) An innovative four-time feeding / four-stage mixing process was developed. By precisely controlling the feeding sequence and mixing stages of the materials, the uniformity of the concrete mixture was ensured, while the interface bonding between the recycled aggregate and the new cement paste was improved, thereby improving the workability and mechanical properties of the recycled concrete.

[0041] (4) It provides an efficient and convenient recycled aggregate modification technology, greatly simplifies the traditional process, reduces energy consumption and production costs, and achieves rapid preparation and application. It lowers the threshold for industrial production and provides a new efficient and economical way for the large-scale industrial application of recycled aggregates. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The present invention is a production process flow chart of the alkaline solution modified recycled concrete. DETAILED DESCRIPTION

[0043] In order to better understand the present invention, the present invention is further described below in conjunction with specific embodiments and drawings.

[0044] The alkali solution modified recycled concrete of the present invention comprises the following components in parts by weight: 250-300 parts of P·II 42.5R cement, 30-50 parts of fly ash, 70-90 parts of mineral powder, 0-30 parts of silica fume, 160 parts of mixing water, 40-50 parts of water for pre-wetting recycled aggregate, 420-450 parts of river sand, 280-300 parts of machine-made sand, 1000-1200 parts of recycled coarse aggregate, 3-8 parts of water reducer and 0-3 parts of sodium hydroxide.

[0045] Among them, the fly ash and mineral powder are grade II fly ash and grade S95 respectively, the stone powder content of the machine-made sand is 10% to 20%, the water reducer is a polycarboxylic acid high-efficiency water reducer with a water reduction efficiency of more than 12.5%, and the purity of the sodium hydroxide is more than 96%.

[0046] Example 1

[0047] The present embodiment provides an alkali solution modified recycled concrete, comprising the following components and masses (based on 1 cubic meter of recycled concrete): 267 kg of P·II 42.5R cement, 38 kg of fly ash, 76 kg of mineral powder, 15 kg of silica fume, 160 kg of mixing water, 44 kg of water for pre-wetting recycled aggregate, 437 kg of river sand, 291 kg of machine-made sand, 1092 kg of recycled coarse aggregate, 6.2 kg of water reducer, and 1.95 kg of granular sodium hydroxide, wherein the purity of the sodium hydroxide is 96%, and the fractal dimension of the recycled aggregate grading system is 2.55.

[0048] The preparation process of the alkali-modified recycled concrete in this embodiment includes the following key steps:

[0049] (1) Determine the particle size range of the coarse aggregate system as [5, 20] and the fractal dimension as 2.55, and calculate the mass ratios of the three particle size ranges of 5-10 mm, 10-16 mm, and 16-20 mm according to formula (1). According to the mass ratio and the mass of the recycled coarse aggregate required, weigh the recycled aggregate in each particle size range.

[0050]

[0051] Among them, r g is the aggregate particle size (corresponding to the sieve hole size), r g,min is the minimum particle size in the grading system, r g,max is the maximum particle size in the grading system, F is the fractal dimension, P(r g ) is the passing rate through the sieve hole;

[0052] (2) Weigh 10% of mixing water, pre-wetting water for recycled coarse aggregate, and granular sodium hydroxide according to the mix ratio, then gradually pour the sodium hydroxide into the water while constantly stirring to fully dissolve it, thereby obtaining a sodium hydroxide solution.

[0053] (3) Pour the recycled coarse aggregate into a stirrer, add sodium hydroxide solution, and stir at a speed of 45 r / min for 2 to 3 minutes to evenly wet the recycled coarse aggregate to obtain sodium hydroxide solution-modified recycled coarse aggregate.

[0054] (4) Fully blend 80% of the mixing water and water reducer into a uniform mixture for use.

[0055] (5) Pour cement, fly ash, mineral powder, silica ash, river sand and machine-made sand into the mixing kettle one by one, and mix all the materials evenly at a speed of 45r / min. Then, add the reserve mixing agent for wet mixing, and stir until the mixture becomes fluid;

[0056] (6) Pour the sodium hydroxide solution modified recycled coarse aggregate into the mixing tank, and slowly add the remaining 10% mixing water during the mixing process until the materials are blended into a uniform paste mixture. After the mixture is poured out of the mixing tank, it is immediately cast into shape and cured to finally obtain the alkali solution modified recycled concrete.

[0057] Example 2

[0058] The difference between Example 2 and Example 1 is that the mass of sodium hydroxide is 0.97 kg.

[0059] Example 3

[0060] The difference between Example 3 and Example 1 is that the mass of sodium hydroxide is 2.44 kg.

[0061] Example 4

[0062] The difference between Example 4 and Example 1 is that no silica fume is added, no sodium hydroxide is added, and the mass of the water reducing agent is 3.8 kg.

[0063] Example 5

[0064] The difference between Example 5 and Example 1 is that no sodium hydroxide is added and the mass of the water reducing agent is 5.2 kg.

[0065] Example 6

[0066] The difference between Example 6 and Example 1 is that no sodium hydroxide is added, the mass of silica fume is 30 kg, and the mass of water reducing agent is 7.6 kg.

[0067] Comparative Example 1

[0068] The difference between Comparative Example 1 and Example 1 is that no silica fume is added, no sodium hydroxide is added, the fractal dimension of the recycled aggregate is 1.0, and the mass of the water reducing agent is 3.8 kg.

[0069] Comparative Example 2

[0070] The difference between Comparative Example 2 and Example 1 is that no silica fume is added, no sodium hydroxide is added, the fractal dimension of the recycled aggregate is 1.5, and the mass of the water reducer is 3.8 kg.

[0071] Comparative Example 3

[0072] The difference between Comparative Example 3 and Example 1 is that no silica fume is added, no sodium hydroxide is added, the fractal dimension of the recycled aggregate is 2.8, and the mass of the water reducing agent is 3.8 kg.

[0073] The mixing ratio information of Examples 1 to 6 and Comparative Examples 1 to 3 is shown in Table 1.

[0074] Table 1 Mix ratio of recycled concrete (kg / m 3 )

[0075]

[0076] In accordance with the provisions of the "Standard for Test Methods for Performance of Ordinary Concrete Mixtures" (GB / T 50080-2016) and the "Standard for Test Methods for Physical and Mechanical Properties of Ordinary Concrete" (GB / T50081-2019), the working performance and mechanical properties of the recycled concrete of Examples 1 to 6 and Comparative Examples 1 to 3 were tested, and the results are shown in Table 2 and Table 3, respectively.

[0077] Table 2 Working performance indicators of recycled concrete

[0078] project Slump / mm Slump expansion / mm Example 1 200 490 Example 2 210 495 Example 3 190 480 Example 4 160 400 Example 5 140 380 Example 6 120 330 Comparative Example 1 190 425 Comparative Example 2 175 410 Comparative Example 3 150 395

[0079] According to the data in Table 2, it can be found that with the increase of fractal dimension, the slump and slump expansion of recycled concrete show a downward trend, indicating that its working performance deteriorates accordingly. The main reason for this phenomenon is that the recycled aggregate with higher fractal dimension has a higher content of old mortar, which increases the water absorption of the aggregate and reduces the amount of free water available during mixing, thus affecting the fluidity of the mixture. In addition, due to its large specific surface area, the addition of silica fume significantly reduces the fluidity of recycled concrete, and it is necessary to increase the amount of water reducer or use a more efficient water reducer to ensure good fluidity. At the same time, the data in Table 2 also reveal the negative impact of sodium hydroxide addition on the working performance of the mixture, which is manifested in that with the increase of sodium hydroxide addition, the slump and slump expansion of recycled concrete are reduced.

[0080] Although the addition of silica fume and sodium hydroxide has an adverse effect on the working performance of recycled concrete, by appropriately increasing the amount of water reducer, the recycled concrete can still have good working performance. For example, by adjusting the amount of water reducer in Examples 1 to 3, the recycled concrete can maintain excellent working performance even in the presence of silica fume and sodium hydroxide.

[0081] Table 3 Compressive strength of recycled concrete (MPa)

[0082]

[0083] According to the compressive strength of comparative examples 1, 2, 4 and embodiment 4, it can be known that the compressive strength of recycled concrete shows a trend of first increasing and then decreasing with the increase of the gradation fractal dimension. Specifically, when the gradation fractal dimension reaches 2.55, its 7-day, 28-day, 60-day, and 90-day compressive strengths are increased by 15.6%, 17.3%, 14.8%, and 10.1%, respectively, compared with the gradation fractal dimension of 1.0, showing a significant enhancement effect.

[0084] Comparing the compressive strength of Examples 1, 2, and 3, it can be found that with the increase in the amount of sodium hydroxide, the compressive strength of the recycled concrete also shows a pattern of first increasing and then decreasing. The 7-day, 28-day, 60-day, and 90-day compressive strengths of Example 1 were increased by 12.2%, 17.5%, 7.3%, and 7.1%, respectively, compared to Example 5. This result shows that sodium hydroxide has a significant promoting effect on improving the compressive strength of recycled concrete, especially in the early stages.

[0085] The data in Table 3 further reveal that when 15 kg of silica fume and 1.95 kg of sodium hydroxide are added to the recycled concrete (Example 1), its compressive strength is comparable to that of the recycled concrete with only 30 kg of silica fume (Example 6), and its early strength performance is better. This phenomenon shows that there is a synergistic effect between sodium hydroxide and silica fume, which can further optimize the microstructure of recycled concrete, thereby effectively improving its strength.

[0086] It should be emphasized that the above embodiments do not constitute a limitation of the present invention, but are provided as illustrative examples. Although the present invention has been explained in detail through the above embodiments, those skilled in the art can still make adjustments or replace some technical features with equivalents according to the technical solutions disclosed in these embodiments. Any modification, equivalent replacement or improvement within the spirit and principle framework of the present invention shall be regarded as part of the protection scope of the present invention.

Claims

1. An alkaline solution modified recycled concrete, characterized in that: The concrete is composed of the following raw materials in parts by mass: 250-300 parts of P·II 42.5R cement, 30-50 parts of fly ash, 70-90 parts of mineral powder, 0-30 parts of silica fume, 160 parts of mixing water, 40-50 parts of water for pre-wetting recycled coarse aggregate, 420-450 parts of river sand, 280-300 parts of machine-made sand, 1000-1200 parts of recycled coarse aggregate, 3-8 parts of water reducer and 0-3 parts of sodium hydroxide.

2. The alkaline solution modified recycled concrete according to claim 1, characterized in that: The invention is composed of the following raw materials in parts by mass: 265-270 parts of P·II 42.5R cement, 35-40 parts of fly ash, 75-80 parts of mineral powder, 10-20 parts of silica fume, 160 parts of mixing water, 40-50 parts of water for pre-wetting recycled coarse aggregate, 435-440 parts of river sand, 290-300 parts of machine-made sand, 1000-1100 parts of recycled coarse aggregate, 5-7 parts of water reducing agent and 0.97-2.44 parts of sodium hydroxide.

3. The alkaline solution modified recycled concrete according to claim 1 or 2, characterized in that: The water reducer is a polycarboxylic acid high-efficiency water reducer.

4. The method for preparing an alkaline solution modified recycled concrete according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) Weigh the water reducer and 80% of mixing water in proportion, mix and stir the two to obtain a ready-to-use mixture; (2) P·II 42.5R cement, fly ash, mineral powder, silica fume, river sand and machine-made sand are weighed in proportion, poured into a stirring tank for dry mixing, and after mixing evenly, the standby mixture of step (1) is added for wet mixing until a fluid state is reached; (3) Add the recycled coarse aggregate into a mixing kettle and mix, adding 10% mixing water during the process until the materials are blended into a uniform paste-like mixture, and then pour and cure the mixture to obtain recycled concrete.

5. The preparation method according to claim 4, characterized in that: The preparation of the recycled coarse aggregate is as follows: S1. Crushing waste concrete into continuous gradations of 5 to 20 mm, and calculating the mass ratios of three particle size ranges of 5 to 10 mm, 10 to 16 mm, and 16 to 20 mm according to formula (1); Among them, r g is the aggregate particle size, r g,min is the minimum particle size in the grading system, r g,max is the maximum particle size in the grading system, F is the fractal dimension, P(r g ) is the passing rate through the sieve hole; The fractal dimension F is 2.5 to 2.6; S2. Determine the total mass of the required recycled coarse aggregate according to the mix ratio of recycled concrete, and then calculate the mass of each particle size range according to the mass ratio to obtain the prepared recycled coarse aggregate; S3. Weigh water for pre-wetting recycled coarse aggregate, sodium hydroxide and 10% mixing water in proportion, mix them evenly to obtain a sodium hydroxide solution; pour the recycled coarse aggregate prepared in S2 into a stirring tank, then pour in the sodium hydroxide solution, and stir to make the recycled coarse aggregate evenly wetted, so as to obtain recycled coarse aggregate modified by the sodium hydroxide solution.

6. The preparation method according to claim 5, characterized in that: The fractal dimension F is 2.

55.

7. The preparation method according to claim 6, characterized in that: In step (2), the dry mixing speed is 40 to 50 r / min; in step S3, the stirring speed is 40 to 50 r / min, and the time is 2 to 3 min.

8. The preparation method according to any one of claims 4 to 7, characterized in that: The fly ash is grade II fly ash, and the mineral powder is grade S95.

9. The preparation method according to claim 8, characterized in that: The stone powder content of the machine-made sand is 10% to 20%.

10. The preparation method according to claim 9, characterized in that: The water reducing efficiency of the water reducing agent is greater than 12.5%.

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