Low additional water full recycled concrete based on recycled fine aggregate wet packing grading optimization and preparation method thereof

Abstract

The application discloses a kind of low additional water full recycled concrete based on recycled fine aggregate wet packing gradation optimization and preparation method thereof, belong to building material technical field.The application is classified into three intervals by natural gradation recycled fine aggregate, after pre-wetting to saturated surface dry state, the wet loose bulk density under each mass proportion combination is determined, and the highest density is selected as the optimal artificial gradation;60% of the saturated surface dry water absorption of all recycled aggregates is determined as additional water;cement, recycled fine aggregate, recycled coarse aggregate, initial water and water reducing agent are used to prepare by using the layered feeding process of "cement pre-mixing→ neat paste preparation→ coarse aggregate→ fine aggregate".The obtained concrete has a slump of 160-190mm, a 28-day compressive strength of 25-40MPa, good fluidity, can meet the construction requirements of conventional concrete structure, and does not need chemical modification, low cost, suitable for building solid waste large-scale utilization.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a low-addition-water fully recycled concrete and its preparation method, which achieves precise control of the true water-cement ratio based on the synergistic regulation of the wet packing gradation optimization of recycled fine aggregate and the layered feeding process. Background Technology

[0002] Existing construction waste recycling mainly focuses on recycled coarse aggregate concrete. Recycled fine aggregate, due to its large specific surface area, abundant adhered old mortar, and high porosity, exhibits strong water absorption, significantly deteriorating the slump of concrete and hindering its widespread application in engineering projects. For recycled concrete, recycled aggregate absorbs water during the fresh mixing stage, and adding additional water can mitigate the deterioration of slump caused by this water absorption. To compensate for this absorption, current technology typically recommends using 80%-100% of the water absorbed by the recycled aggregate from oven-dry to saturated surface-dry as additional water. However, this simple water compensation leads to the recycled aggregate gradually releasing water into the cement paste over time, weakening the strength and interfacial properties of the recycled concrete.

[0003] Current methods for improving the strength of fully recycled concrete mainly focus on the physical reinforcement and chemical modification of recycled aggregates. While these methods can improve aggregate quality, they generally suffer from limitations such as complex processes, high labor and time costs, and poor applicability. Without altering the physicochemical properties of the aggregates, how to precisely control the mix proportions and the preparation process to suppress the early water absorption and later water release effects of recycled aggregates is a critical challenge that urgently needs to be addressed for the large-scale utilization of construction solid waste.

[0004] Current research indicates that the gradation of fine aggregates has a relatively significant impact on bulk density and water absorption. Artificially optimizing the gradation to improve aggregate bulk density and water absorption is a reliable approach to improving the slump and mechanical properties of fully recycled concrete. However, commonly used aggregate gradation optimization models are mainly the Fuller gradation model and the Anderson model, etc. These models primarily recommend artificial gradations based on the proportion of natural aggregates, manufactured sand, or low-absorption recycled aggregates, without fully considering the porous and easily absorbent physical characteristics of recycled aggregates. Therefore, they lack universal applicability to recycled aggregates from different sources, with different saturated surface-dry water absorption rates, and of different qualities.

[0005] In particular, the high proportion of fine aggregate (0-0.30mm) in recycled fine aggregate creates a huge specific surface area, which causes it to absorb water from the cement paste violently in the early stages of mixing, leading to severe slump deterioration and water-reducing agent failure. Conventional techniques compensate for the water absorption of recycled aggregate by adding 80%-100% of the saturated surface-dry water as supplementary water. However, while this high supplementary water approach can temporarily improve workability, it ignores the water release behavior of recycled aggregate during concrete hardening. Because the actual water-cement ratio of the concrete deviates from the design value, as hydration progresses, excess water inside the aggregate is released into the paste, leaving a large number of capillary pores in the interfacial transition zone, which severely weakens the compressive strength and interfacial bond of the concrete.

[0006] Furthermore, existing gradation tests are mostly conducted under completely dry conditions. However, in actual production, recycled aggregates immediately come into contact with moisture upon entering the mixer, altering their packing morphology. The optimal porosity under dry packing conditions is not equivalent to the optimal skeleton morphology encapsulated in cement paste. Therefore, adjusting the mix proportion based on wet loose bulk density, coupled with a scientific feeding sequence, is key to achieving a significant performance leap in fully recycled concrete.

[0007] In summary, there is an urgent need for a low-addition-water fully recycled concrete and its preparation method based on optimized wet packing gradation of recycled fine aggregate, in order to solve the above-mentioned technical problems in this field. Summary of the Invention

[0008] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and solve the problems of insufficient strength in the later stage of existing fully recycled concrete due to high additional water and early slump deterioration due to low additional water. This invention provides a low-addition-water fully recycled concrete based on the optimization of wet packing gradation of recycled fine aggregate and its preparation method.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A low-addition-water fully recycled concrete based on optimized wet packing gradation of recycled fine aggregate, comprising the following components by weight:

[0011] 120 parts of P·O 42.5 cement,

[0012] 150-210 parts of artificially graded recycled fine aggregate,

[0013] 225-315 parts recycled coarse aggregate

[0014] Initial water 40-65 parts

[0015] Add 3-20 parts water.

[0016] 2.16~2.64 parts of polycarboxylate superplasticizer;

[0017] The amount of additional water is determined by 60% of the water absorption mass of all recycled aggregates, which are composed of artificially graded recycled fine aggregates and recycled coarse aggregates, under saturated surface-dry conditions. It is used to compensate for the amount of free water that is rapidly consumed by the recycled aggregates during the fresh mixing stage due to the adhering old mortar layer on the surface and the capillary adsorption of water by the internal high-porosity structure.

[0018] The artificially graded recycled fine aggregate is obtained by grading optimization of naturally graded recycled fine aggregate through a three-zone wet packing method, specifically including the following steps:

[0019] A1: The oven-dried naturally graded recycled fine aggregate was subjected to standard screening to obtain oven-dried fine aggregates in three different particle size ranges, namely high fine aggregate (0~0.3mm), medium fine aggregate (0.3~1.18mm) and low fine aggregate (1.18~4.75mm).

[0020] A2: The high-fineness aggregate, medium-fineness aggregate and low-fineness aggregate obtained in step A1 are pre-wetted with water and then surface dried until all fine aggregates reach a saturated surface-dry state.

[0021] A3: Initially set the upper and lower limits of the mass ratio of three types of recycled fine aggregates, mix the three types of aggregates in saturated surface-dry state according to the preset mass ratios, measure the wet loose bulk density under each mixing ratio, and select the mixing ratio that reaches the peak wet loose bulk density as the optimal artificial gradation.

[0022] A4: Based on the saturated surface-dry water absorption rate data, the mass ratio of each type of saturated surface-dry aggregate in the optimal artificial gradation is converted into its corresponding oven-dry mass ratio. This is used as the optimal ratio of the artificial gradation of recycled fine aggregate. Then, the three oven-dry fine aggregates are weighed and mixed evenly according to this ratio to obtain the artificially graded recycled fine aggregate.

[0023] Furthermore, the mass proportion of the particle size range in the optimal artificial gradation satisfies the following conditions: fine aggregate (0~0.3mm) ≤ 10%, medium fine aggregate (0.3~1.18mm) ≤ 30%, and low fine aggregate (1.18~5mm) ≤ 70%.

[0024] Furthermore, the naturally graded recycled fine aggregate is derived from one or more different construction solid wastes and has different saturated surface-dry water absorption rates.

[0025] Furthermore, the recycled coarse aggregate is a continuous particle size of 5~20mm, wherein the 5~10mm particle size aggregate and the 10~20mm particle size aggregate are mixed and compounded at an oven-dry mass ratio of 3:7.

[0026] Furthermore, the recycled coarse aggregate meets the Class II or higher standards for recycled coarse aggregate.

[0027] Furthermore, the oven-dry mass ratio of the artificially graded recycled fine aggregate to the recycled coarse aggregate is 40:60, i.e., the sand ratio is 40%.

[0028] This invention also provides a method for preparing low-addition-water fully recycled concrete based on wet packing gradation optimization, wherein the cement and recycled aggregate in the concrete material are added in an oven-dry state, comprising the following steps:

[0029] S1: Add P·O 42.5 cement to the mixer for pre-mixing;

[0030] S2: Mix polycarboxylate superplasticizer, initial water and additional water and add to a mixer, then stir until a uniform cement paste is formed;

[0031] S3: Add the recycled coarse aggregate to a mixer and stir to obtain a recycled coarse aggregate mixture;

[0032] S4: Add the artificially graded recycled fine aggregate to the mixer and mix until uniform to obtain the fully recycled concrete mixture.

[0033] Furthermore, the slump of the fully recycled concrete mixture after discharge is 160~190mm. After being poured and molded, the formwork is removed after 24 hours and the concrete is cured according to standard for 28 days. The resulting concrete has a 28-day compressive strength of 25~40MPa.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) This invention differs from the traditional dry packing optimization approach and innovatively proposes an artificial gradation optimization scheme based on "wet loose packing density". Since the wet loose state is closer to the actual distribution of aggregate in cement paste, by precisely controlling the proportion of particle size ranges of 0.0-0.30mm, 0.30-1.18mm and 1.18-4.75mm, the recycled fine aggregate achieves the optimal packing density in the wet state. Thus, while significantly reducing the amount of additional water used (only 60% of the saturated surface dry water absorption), it can still maintain a high slump of 160-190mm, and the synergistic effect of physical filling and gradation is significant.

[0036] (2) By adopting a preparation process that involves pre-mixing the slurry, prioritizing the coating of coarse aggregates with slurry, and adding oven-dried fine aggregates later, the problem of water-reducing agents agglomerating and failing due to the excessive water absorption of porous recycled aggregates is effectively prevented. This ensures the adsorption efficiency of the water-reducing agent on the surface of cement particles and allows the cement paste to preferentially occupy the micro-cracks and pores of the old mortar on the surface of the recycled aggregates. By reducing the amount of additional water used, the increase in internal porosity caused by the subsequent water release behavior of the recycled aggregates is effectively suppressed, and the interfacial bonding strength between the recycled aggregates and the new slurry is significantly improved, thus effectively enhancing the performance of the fully recycled concrete.

[0037] (3) The method of the present invention has strong pertinence and guiding significance for recycled aggregates from different sources and with different original water absorption rates. The full-scale replacement of construction solid waste can be achieved through simple particle size distribution control, which not only greatly reduces the mining pressure of natural sand and gravel, but also has a simple preparation process that does not require complicated chemical pretreatment, and has extremely high economic feasibility, engineering universality and environmental benefits. Attached Figure Description

[0038] Figure 1 This is a schematic flowchart of a method for preparing low-addition-water fully recycled concrete based on optimized wet packing gradation of recycled fine aggregate according to the present invention.

[0039] Figure 2 It is a physical image of naturally graded recycled fine aggregate without artificial optimization.

[0040] Figure 3 This is a wet loose bulk density test chart of recycled fine aggregates conducted in Example 1 of the present invention.

[0041] Figure 4 This is a physical state diagram of the artificially graded recycled fine aggregate used in Embodiment 1 of the present invention.

[0042] Figure 5 This is a diagram showing the compressive failure morphology of the concrete specimen obtained in Example 1 of this invention. Detailed Implementation

[0043] The following content provides a clear and complete description of the technical solutions in the embodiments of the present invention. The described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0044] Example 1

[0045] This embodiment provides a low-addition-water fully recycled concrete material based on optimized wet packing gradation of recycled fine aggregate. The raw material composition, by weight, is as follows:

[0046] 120 parts of P·O 42.5 cement, 150 parts of recycled fine aggregate (using artificial gradation 1), 225 parts of recycled coarse aggregate (the sand content of the fully graded recycled aggregate is 40%), 45.60 parts of initial water, 11.77 parts of additional water, and 2.64 parts of polycarboxylate high-performance water-reducing agent (solid content 30%).

[0047] The method for determining the artificial gradation 1 is as follows:

[0048] like Figure 2This is a physical image of naturally graded recycled fine aggregate without artificial optimization. The particle size distribution is uneven, with a relatively high proportion of fine powder. The original sample of recycled fine aggregate (natural gradation) obtained from crushed construction solid waste was sieved into three particle size ranges: 0-0.30mm, 0.30-1.18mm, and 1.18-4.75mm. The mass percentages of each range in the natural gradation were measured to be 14.69%, 32.89%, and 52.42%, respectively. Preliminary upper and lower limits for the mass percentages of each range were set: 0-0.30mm ≤ 10%, 0.30-1.18mm 20%-30%, and 1.18-4.75mm 60%-70%. Four artificial gradation schemes were designed within these ranges, with the natural gradation serving as a control group. Each grade of aggregate sample was pre-wetted to a saturated surface-dry state (judgment criteria: after soaking in water and drying the surface, the aggregate was placed in a standard mold and lifted vertically; after demolding, the aggregate exhibited a combined shape of an upper truncated cone and a lower cone). The saturated surface-dry samples were then mixed uniformly according to a predetermined mass ratio, and the wet loose bulk density was measured. The results are shown in Table 1. Figure 3 This is a wet bulk density test chart of recycled fine aggregates from artificial gradation 1. The aggregate surface is moist, and the aggregates are piled up in a natural cone shape. Artificial gradation 1, which has the highest wet bulk density, is selected as the optimal artificial gradation, with a wet bulk density of 1506.84 kg / m³. 3 It is 25.6% better than the natural gradation.

[0049] Table 1. Gradation distribution and wet bulk density test results of recycled fine aggregate.

[0050]

[0051] Based on the measured saturated surface-dry water absorption rate data (the saturated surface-dry water absorption rates of high-fineness aggregate, medium-fineness aggregate, and low-fineness aggregate are 11.5%, 7.2%, and 4.1%, respectively), the mass proportions of various saturated surface-dry aggregates in the optimal artificial gradation are converted to oven-dry mass proportions:

[0052] The dry weight percentage of fine aggregate (0-0.30mm) = 5% / (1+11.5%) / 95.04% = 4.71%;

[0053] The dry weight percentage of medium and fine aggregates (0.30-1.18mm) = 25% / (1 + 7.2%) / 95.04% = 24.54%;

[0054] The dry weight percentage of fine aggregate (1.18-4.75mm) = 70% / (1+4.1%) / 95.04% = 70.75%;

[0055] Weigh and mix the three oven-dry fine aggregates according to the above-mentioned oven-dry mass ratio to obtain the artificially graded recycled fine aggregate. Figure 4These are images of the actual state of artificially graded recycled fine aggregate. The particle gradation is relatively regular, and the appearance is better than that of naturally graded aggregate.

[0056] The recycled coarse aggregate is formed by mixing 5-10mm and 10-20mm particle sizes in a mass ratio of 3:7 to form a continuous gradation.

[0057] The method for determining the additional water usage is as follows:

[0058] 150 parts of artificially graded recycled fine aggregate and 225 parts of recycled coarse aggregate were mixed to form 375 parts of fully graded recycled aggregate. A sample in an oven-dry state was soaked in water for 24 hours to reach saturated surface dryness. The water absorption mass was measured to be 19.62 parts. Based on 60%, the additional water was calculated to be 11.77 parts.

[0059] The method for determining the initial water usage is as follows:

[0060] The total design water volume (57.37 parts) is obtained based on the water-cement ratio required for the target strength grade. The remaining water volume (45.60 parts) is obtained after deducting the calculated additional water (11.77 parts).

[0061] like Figure 1 As shown, the preparation method is as follows, wherein cement and recycled aggregate are both added in an oven-dry state:

[0062] S1. Add 120 parts of P·O 42.5 cement to a forced mixer and premix for 20 seconds;

[0063] S2. Mix 2.64 parts of polycarboxylate superplasticizer, 45.60 parts of initial water and 11.77 parts of additional water and add them to the mixer. Mix for 60 seconds until a uniform cement paste is formed.

[0064] S3. Add 225 parts of recycled coarse aggregate to the mixer and mix for 90 seconds to make the aggregate surface evenly coated with slurry.

[0065] S4. Add 150 parts of artificially graded recycled fine aggregate to the mixer and mix for 120 seconds until a uniform fully recycled concrete mixture is formed.

[0066] S5. The material is discharged and a slump test is performed. The slump is measured to be 179.1 mm (in line with the 160-190 mm range). Then it is poured and molded. The mold is removed after 24 hours and the material is cured according to standard for 28 days.

[0067] like Figure 5 This is a diagram showing the compressive failure morphology of the obtained concrete specimens. The failure cracks mainly propagate along the aggregate-paste interface inside the specimens, and the overall failure morphology is relatively good. The 28-day compressive strength was tested to be 34.82 MPa.

[0068] Example 2: Comparison of Sand Ratio Parameter Optimization

[0069] To verify the effect of sand ratio on the performance of low-added-water fully recycled concrete, concrete was prepared with sand ratios of 30%, 35%, 40%, and 45%, respectively. The raw materials and processes were the same as in Example 1, only the ratio of recycled coarse and fine aggregates was adjusted.

[0070] Sand ratio 30% (112.5 parts recycled fine aggregate, 262.5 parts recycled coarse aggregate): slump 91.4 mm, 28-day strength 29.62 MPa, insufficient workability;

[0071] Sand ratio 35% (131.25 parts recycled fine aggregate, 243.75 parts recycled coarse aggregate): slump 120.3 mm, 28-day strength 28.45 MPa;

[0072] Sand ratio 40% (same as Example 1): slump 179mm, 28-day strength 34.82 MPa, no bleeding or segregation;

[0073] With a sand ratio of 45% (168.75 parts recycled fine aggregate and 206.25 parts recycled coarse aggregate), the slump was 239.7 mm, but significant bleeding and segregation occurred, and the strength dropped to 26.15 MPa after 28 days.

[0074] The results show that a sand ratio of 40% achieves the highest strength while ensuring high fluidity, making it the optimal sand ratio.

[0075] Example 3: Verification of the universality of recycled aggregates from different sources

[0076] Recycled aggregates were prepared using construction waste from a different source than that in Example 1:

[0077] Example 3.1 (Low water absorption aggregate): After actual measurement and conversion of coarse and fine aggregates, the saturated surface-dry water absorption rate of the fully graded recycled aggregate was 1.85%. Based on 60% of the total water absorption of the fully graded aggregate, the additional water was determined to be 4.16 parts. Other parameters were the same as in Example 1. The resulting concrete had a slump of 187.2 mm and a 28-day compressive strength of 39.80 MPa.

[0078] Example 3.2 (Medium water absorption aggregate): After actual measurement and conversion of coarse and fine aggregates, the saturated surface-dry water absorption rate of the fully graded recycled aggregate was 3.75%. An additional 8.44 parts of water were determined based on 60% of the total water absorption of the fully graded aggregate. Other parameters were the same as in Example 1. The resulting concrete had a slump of 165.1 mm and a 28-day compressive strength of 35.65 MPa.

[0079] The results show that the low-additional-water process provided by the present invention is applicable to recycled aggregates from different sources and with different original water absorption rates. Moreover, as the water absorption rate of the aggregate itself decreases, the mechanical strength under the same mix proportion is better.

[0080] Comparative Example 1: Comparison of Dry Bulk Density Optimization Grades

[0081] To verify the core technological advantages of the wet loose bulk density proposed in this invention compared to the traditional dry bulk density, three oven-dry fine aggregate ranges were used, the same as in Example 1. The difference was that the concrete was prepared by artificial gradation optimization based on the traditional dry bulk density. The raw material composition and preparation process were the same as in Example 1, the only difference being that in the optimal dry bulk mix obtained based on the oven-dry loose state, the proportion of high fine aggregate (0-0.30mm) was relatively high at 13.5%, medium fine aggregate (0.30-1.18mm) was 21.5%, and low fine aggregate (1.18-4.75mm) was 65.0%.

[0082] The slump measured at discharge was reduced to 138.2 mm, and the 28-day compressive strength was only 30.95 MPa, representing decreases of 22.8% and 11.1% respectively compared to Example 1 (slump 179.1 mm, compressive strength 34.82 MPa). This demonstrates that traditional dry-packing models often incorporate excessive fine powder to fill dry voids, resulting in excessively fluid concrete paste and failing to achieve the preset strength level. This confirms that artificial gradation control based on wet loose bulk density is an effective method to balance strength and high fluidity.

[0083] Comparative Example 2: Natural Grade Pairing

[0084] The raw material composition and preparation process are the same as in Example 1, except that the recycled fine aggregate uses unoptimized artificial gradation (i.e., natural gradation, with a high fine aggregate ratio of 14.69%) and is not screened by wet bulk density test.

[0085] The discharge slump was 145.5 mm, and the 28-day compressive strength was 29.32 MPa, representing decreases of 18.7% and 15.8% respectively compared to Example 1. This demonstrates that optimizing the wet bulk density and limiting the content of fine aggregates in artificial gradation are key to ensuring workability and strength performance in a low-addition-water system.

[0086] Comparative Example 3: Traditional High-Added Water

[0087] The raw material composition and preparation process are the same as in Example 1, except that the additional water is calculated as 80% of the saturated surface dry water absorption mass (i.e., 15.70 parts), and the initial water is adjusted accordingly to 41.67 parts to keep the total water volume unchanged.

[0088] The slump of the discharged material was 195.0 mm, but the 28-day compressive strength was only 30.43 MPa, a decrease of 12.6% compared to Example 1 (34.82 MPa). After demolding, the specimen surface showed obvious pitting, and the internal porosity increased. This demonstrates that while excessive additional water can improve workability, the subsequent water release weakens the interfacial transition zone, resulting in a significant decrease in strength.

[0089] Comparative Example 4: Comparison of Traditional Single-Feed Method

[0090] The raw material composition is the same as in Example 1, but the traditional one-time feeding method is used:

[0091] Add 120 parts of P·O 42.5 cement, 150 parts of artificially graded recycled fine aggregate, and 225 parts of recycled coarse aggregate to the mixer at once and mix for 60 seconds; then add all the water (45.60 parts of initial water + 11.77 parts of additional water) and 2.64 parts of water-reducing agent, and mix for another 150 seconds.

[0092] The slump measured at the discharge was 106.5 mm, a decrease of 40.5% compared to Example 1 (179 mm). Furthermore, the mixture exhibited poor cohesion and slight segregation. This demonstrates that the layered feeding process of this invention can effectively improve the problem of water-reducing agent failure caused by excessive early water absorption in recycled aggregates.

[0093] In summary, this invention successfully solves the technical problem of balancing slump and strength in fully recycled concrete under low-addition-water systems by using an artificial gradation optimization method based on the wet loose bulk density of recycled fine aggregates, supplemented by a layered feeding preparation process. Experimental results show that when the artificial gradation of recycled fine aggregates is controlled within the range recommended by this invention, the wet bulk density between aggregates can be significantly improved, thereby ensuring excellent workability and mechanical properties of the concrete while drastically reducing the amount of additional water used. This invention not only realizes the resource utilization of construction solid waste, but also has a simple and universal preparation process, requiring no high-cost physicochemical modification, and has significant cost advantages, economic benefits, and environmental value. Therefore, the low-addition-water fully recycled concrete and its preparation method based on the optimized wet bulk gradation of recycled fine aggregates provided by this invention have broad market prospects in the fields of solid waste utilization, low-carbon building, and green building materials, and are suitable for promotion in practical engineering applications.

Claims

1. A low water addition full recycled concrete based on recycled fine aggregate wet packing grading optimization, characterized in that, By weight, it includes the following components: 120 parts of P·O 42.5 cement, 150-210 parts of artificially graded recycled fine aggregate, 225-315 parts recycled coarse aggregate Initial water 40-65 parts Add 3-20 parts water. 2.16~2.64 parts of polycarboxylate superplasticizer; The amount of additional water is determined by 60% of the water absorption mass of all recycled aggregates, which are composed of artificially graded recycled fine aggregates and recycled coarse aggregates, under saturated surface-dry conditions. It is used to compensate for the amount of free water that is rapidly consumed by the recycled aggregates during the fresh mixing stage due to the adhering old mortar layer on the surface and the capillary adsorption of water by the internal high-porosity structure. The artificially graded recycled fine aggregate is obtained by grading optimization of naturally graded recycled fine aggregate through a three-zone wet packing method, specifically including the following steps: A1: The oven-dried naturally graded recycled fine aggregate was subjected to standard screening to obtain three different particle size ranges of oven-dried fine aggregate, namely 0~0.3mm high fine aggregate, 0.3~1.18mm medium fine aggregate and 1.18~4.75mm low fine aggregate. A2: The high-fineness aggregate, medium-fineness aggregate and low-fineness aggregate obtained in step A1 are pre-wetted with water and then surface dried until all fine aggregates reach a saturated surface-dry state. A3: The upper and lower limits of the mass ratio of the three types of recycled fine aggregates are initially set as follows: 0-0.30mm≤10%, 0.30-1.18mm20%-30%, and 1.18-4.75mm60%-70%. The three types of aggregates in saturated surface-dry state are mixed according to the preset mass ratio, and the wet loose bulk density at each mixing ratio is measured. The mixing ratio that reaches the peak wet loose bulk density is selected as the optimal artificial gradation. A4: Combining the saturated surface dry water absorption rate data, the mass ratio of each type of saturated surface dry aggregate in the optimal artificial gradation is converted into its corresponding oven-dry mass ratio. This is used as the optimal ratio of the artificial gradation of recycled fine aggregate. Then, the three oven-dry fine aggregates are weighed and mixed evenly according to the oven-dry mass ratio to obtain the artificial gradation of recycled fine aggregate. The method for preparing low-addition-water fully recycled concrete based on optimized wet packing gradation of recycled fine aggregate includes the following steps: S1: Add P·O 42.5 cement to the mixer for pre-mixing; S2: Mix polycarboxylate superplasticizer, initial water and additional water and add to a mixer, then stir until a uniform cement paste is formed; S3: Add the recycled coarse aggregate to a mixer and stir to obtain a recycled coarse aggregate mixture; S4: Add the artificially graded recycled fine aggregate to the mixer and mix until uniform to obtain the fully recycled concrete mixture; The P·O 42.5 cement, recycled coarse aggregate, and artificially graded recycled fine aggregate were all added in an absolutely dry state.

2. A low water addition full recycled concrete based on the optimization of the wet packing grading of recycled fine aggregates according to claim 1, characterized in that, The naturally graded recycled fine aggregate is derived from one or more different construction solid wastes and has different saturated surface-dry water absorption rates.

3. A low water addition full recycled concrete based on the optimization of wet stack grading of recycled fine aggregates according to claim 1, characterized in that, The recycled coarse aggregate is a continuous particle size of 5~20mm, wherein the 5~10mm particle size aggregate and the 10~20mm particle size aggregate are mixed and compounded at an oven-dry mass ratio of 3:

7.

4. The low-water-added full recycled concrete based on the recycled fine aggregate wet packing grading optimization according to claim 1, characterized in that, The recycled coarse aggregate meets the Class II or higher standards for recycled coarse aggregate.

5. The low-water-added full recycled concrete based on the recycled fine aggregate wet packing grading optimization according to claim 1, characterized in that, The oven-dry mass ratio of the artificially graded recycled fine aggregate to the recycled coarse aggregate is 40:60, i.e., the sand ratio is 40%.

6. A low water addition full recycled concrete based on recycled fine aggregate wet packing grading optimization according to claim 1, characterized in that, The slump of the fully recycled concrete mixture after discharge is 160~190mm. After being poured and molded, the formwork is removed after 24 hours and the concrete is cured according to standard for 28 days. The 28-day compressive strength of the resulting concrete is 25~40MPa.

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