A method for preparing modified recycled aggregate of retired wind turbine blade based on freeze-thaw interface activation and a method for preparing recycled concrete

Abstract

The application discloses a modified retired wind turbine blade recycled aggregate preparation method based on freeze-thaw interface activation and a recycled concrete preparation method. The modified retired wind turbine blade recycled aggregate preparation method comprises the following steps: obtaining mixed particle size recycled aggregates by mechanically cutting, coarsely crushing and finely crushing the retired wind turbine blade, then pre-drying, soaking in water, taking out, draining and placing in a freeze-thaw box for freeze-thaw circulation to obtain modified recycled aggregates; and replacing part of the natural coarse aggregate with the modified recycled aggregates, mixing and stirring the modified recycled aggregates with natural coarse aggregate, cement, natural sand, water, mineral admixtures and water reducing agent, and then performing gradient carbonization curing to obtain recycled concrete. The method can reduce the problems of poor bonding performance of the retired wind turbine blade aggregate interface, insufficient overall mechanical properties and difficulty in efficient carbon dioxide sealing, and can improve the resource utilization level of the retired wind turbine blade solid waste and the carbon emission reduction benefit.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and low-carbon cement-based material preparation technology. Specifically, it relates to a method for preparing modified recycled aggregates from decommissioned wind turbine blades based on freeze-thaw interface activation and a method for preparing recycled concrete. It is applicable to the high-value resource utilization of decommissioned wind turbine blade composite materials and the preparation of low-carbon cement-based materials. Background Technology

[0002] With the continued rapid growth of global wind power installed capacity, a large number of early-built wind turbine units are gradually entering the end of their service life, resulting in a concentrated and large-scale increase in the number of retired wind turbine blades. Wind turbine blades are typically made of glass fiber reinforced epoxy resin matrix composites, which have advantages such as high strength, high modulus, corrosion resistance, and fatigue resistance. However, precisely because of the stable structure of their thermosetting resins and the difficulty in remelting or degrading them, retired blades face prominent problems such as "difficult degradation, difficult reduction, and difficult high-value utilization" in the disposal and resource utilization process. Traditional landfill and incineration methods not only occupy a large amount of land space and generate potential harmful emissions, but also waste high-performance material resources and are gradually subject to environmental regulations and policies.

[0003] In existing technologies, the main pathways for the resource utilization of retired wind turbine blades include pyrolysis to recover fibers, chemical degradation of resins, and mechanical crushing to prepare fillers or aggregates. Among these, the mechanical crushing-screening-incorporation as recycled aggregate into cement-based materials is simple, low-cost, and easy to promote in engineering, showing promising application prospects. However, because the blade aggregate has a multiphase composite structure of "glass fiber-resin-inorganic filler" and its surface is often covered by a dense resin layer, it has poor hydrophilicity and polarity. When incorporated into a cement-based system, it easily forms a loose, crack-concentrated interfacial transition zone (ITZ), resulting in weak interfacial adhesion and low stress transfer efficiency, making it difficult to fully realize the overall mechanical and durability properties of the material.

[0004] On the other hand, carbonation curing technology has attracted much attention because it can promote the densification and strength increase of cement-based materials, while simultaneously achieving industrial CO2 sequestration. Studies have shown that under suitable temperature, humidity, and CO2 concentration conditions, CO2 can react with alkaline components such as Ca(OH)2 in cement hydration products to generate stable carbonates such as CaCO3, thereby improving pore structure and enhancing material properties. However, most existing carbonation technologies target the "cement paste / mortar / concrete composite," with the carbonation reaction mainly concentrated in the cement matrix. It is difficult to target the fiber-resin interface region within the composite aggregate. Furthermore, due to the density of the resin phase and the lack of effective penetration channels, CO2 has difficulty penetrating deep into the aggregate to achieve targeted modification, resulting in limited strengthening effects on weak interface regions.

[0005] Furthermore, while existing technologies have attempted to improve the surface properties of recycled aggregates through acid washing, alkali washing, and plasma modification, these methods often suffer from limitations such as complex processing, high energy consumption, potential environmental impact, or limitations of only affecting the outer surface of the aggregate, making it difficult to achieve in-depth control over the "internal interface system of the aggregate." In summary, current technologies lack a systematic approach that can construct a microcrack network conducive to CO2 transport within the aggregate through controllable physical means without damaging its overall structure, and combine this with gradient carbonization to achieve synergistic reinforcement of the fiber-resin interface and the aggregate-cement interface. Summary of the Invention

[0006] To address the problems existing in the above-mentioned background technology, the present invention provides a method for preparing recycled aggregates of modified decommissioned wind turbine blades based on freeze-thaw interface activation and a method for preparing recycled concrete. This method reduces problems such as poor interfacial bonding performance, insufficient overall mechanical properties, and difficulty in efficiently sealing carbon dioxide in recycled aggregates of decommissioned wind turbine blades, while improving the resource utilization level and carbon emission reduction benefits of solid waste from decommissioned wind turbine blades.

[0007] The term "subcritical freeze-thaw" in this invention refers to a controlled freeze-thaw process in the process of treating decommissioned wind turbine blade aggregates, where the temperature history, moisture content, and number of freeze-thaw cycles are controlled to induce the formation of microcrack networks only at the internal interface of the aggregates, without causing macroscopic crushing or significant deterioration of the aggregates' strength.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A method for preparing recycled aggregate from modified decommissioned wind turbine blades based on freeze-thaw interface activation includes the following steps: (1) The retired wind turbine blades are mechanically cut, coarsely crushed and finely crushed to obtain recycled aggregate of retired wind turbine blades with mixed particle sizes; the recycled aggregate of retired wind turbine blades is graded and screened by a multi-layer vibrating screen according to particle size ranges of 0~5mm, 5~10mm, 10~16mm, 16~20mm and 20~25mm, and the recycled aggregate of retired wind turbine blades is controlled within the particle size range of 2~25mm, preferably 5~20mm, and fine powder with a particle size of less than 2mm and obvious metal impurities are removed. The recycled aggregate from decommissioned wind turbine blades is used in stages according to different particle size ranges to optimize aggregate gradation and take into account the needs of interface control. By using it in stages, on the one hand, larger particles are used to build a load-bearing skeleton, and smaller particles are used to fill the pores between the skeleton, thereby optimizing aggregate gradation, reducing porosity and improving workability. On the other hand, it increases the overall specific surface area of ​​the aggregate, which is conducive to the full role of freeze-thaw induced crack network and subsequent CO2 targeted carbonization in the aggregate-slurry interface region, thereby improving interfacial bonding performance and overall mechanical properties. (2) Place the recycled aggregate of decommissioned wind turbine blades obtained by screening in step (1) in an oven at (60±5)℃ to pre-dry until the mass is constant. Then, completely immerse the pre-dried recycled aggregate of decommissioned wind turbine blades in water for 4~24h to make the internal pores of the aggregate and the fiber-resin interface area reach a saturated or nearly saturated state. The soaking temperature is preferably 15~30℃. The moisture content at the end of the soaking is basically stable by weighing the change in aggregate mass to ensure that different batches of aggregate have repeatable initial moisture states. (3) Take out the recycled aggregate of decommissioned wind turbine blades after soaking in step (2), drain or wipe off the surface free water so that the interior of the recycled aggregate of decommissioned wind turbine blades is saturated or nearly saturated and there is no obvious water accumulation on the surface. Then place it in a freeze-thaw chamber and perform 5 to 15 freeze-thaw cycles in the temperature range of -20℃ to 25℃. In each freeze-thaw cycle, the low temperature and high temperature platforms are maintained for 3 hours respectively, and the cooling and heating rates are both 5℃ / h. The temperature of the low temperature platform is -20℃ to -10℃ and the temperature of the high temperature platform is 20℃ to 25℃. Modified recycled aggregate of decommissioned wind turbine blades based on freeze-thaw interface activation is obtained. This step precisely controls the temperature range, the internal moisture state of the aggregate, and the number of freeze-thaw cycles, so that the ice expansion stress mainly acts on the fiber-resin interface and the resin matrix, inducing the formation of new nano- to micro-scale cracks inside the aggregate and expanding some of the original micro-cracks, constructing a micro-crack network with a certain degree of connectivity, while not causing macroscopic crushing or significant deterioration of the aggregate strength, thus achieving subcritical activation of the internal interface of the recycled aggregate from decommissioned wind turbine blades.

[0009] Furthermore, in step (1) of the present invention, the length of the decommissioned wind turbine blade after mechanical cutting is 0.5~1.0m, and then it is fed into a jaw crusher for coarse crushing, controlling the output particle size within the range of 50~100mm, and then fed into an impact crusher for fine crushing, controlling the maximum particle size within 25mm.

[0010] Furthermore, the soaking water in step (2) of this invention is tap water or deionized water, the soaking temperature is 15℃~30℃, and the soaking time is preferably 8~16h, so as to ensure that the aggregate reaches a relatively uniform water content state, which is conducive to the formation of a uniform interface crack network during the freeze-thaw process.

[0011] Furthermore, the freeze-thaw cycle number in step (3) of the present invention is preferably 8 to 12 times. When the freeze-thaw cycle number is less than 5 times, the internal crack network of the aggregate is not easy to form. When the freeze-thaw cycle number is greater than 15 times, the aggregate strength decreases significantly, which is not conducive to engineering applications.

[0012] Furthermore, the present invention also provides a method for preparing recycled concrete based on the above-mentioned modified decommissioned wind turbine blade recycled aggregate, comprising the following steps: (1) The modified recycled aggregate of retired wind turbine blades activated by the above-mentioned freeze-thaw interface is used to replace natural coarse aggregate at 5% to 40% of the total mass of coarse aggregate, preferably 10% to 30%, and is mixed with natural coarse aggregate, cement, natural sand, water, mineral admixture and water-reducing agent in a predetermined proportion to prepare fresh recycled concrete; the fresh recycled concrete is poured into shape and vibrated to compact it, cured for 24 hours at 20℃±2℃ and relative humidity not less than 95% before demolding, and continued to be cured according to standard until the predetermined age (e.g. 3d or 7d) to obtain specimens with certain initial strength; (2) The specimens obtained in step (1) are then placed in a CO2-rich sealed carbonization chamber and subjected to gradient CO2 carbonization curing in sequence through the pre-permeation stage, pulse strengthening stage and stabilization post-treatment stage. This allows CO2 to preferentially penetrate into the interior of the recycled aggregate of the decommissioned wind turbine blade and the recycled aggregate-cement interface area along the microcrack network formed by freeze-thaw induction, thereby achieving interface-targeted carbonation modification and obtaining the interface-enhanced recycled concrete based on gradient CO2 targeted carbonation of the present invention.

[0013] Furthermore, the specific methods of step (1) include: (S1) Weigh cement, natural sand, natural coarse aggregate and modified decommissioned wind turbine blade recycled aggregate according to the predetermined ratio, put them into a mixer and dry mix for 1~2 minutes to obtain the first mixture; (S2) Mix 2 / 3 to 3 / 4 of the total water with the water-reducing agent and mineral admixture, and slowly add the mixture to the first mixture. Continue stirring for 2 to 4 minutes to obtain the second mixture. (S3) Add the remaining mixing water to the second mixture to adjust the workability, stir for 1~3 minutes, and obtain fresh recycled concrete with a slump that meets the design requirements; (S4) The freshly mixed material is poured into standard specimens, compacted on a vibrating table, and demolded after standing at room temperature for 24 hours according to the specifications. Then, it is cured under standard curing conditions of 20℃±2℃ and relative humidity≥95% until the predetermined age to obtain specimens with certain initial strength.

[0014] Furthermore, the mixing time for the second mixing in step (S2) is preferably 2 to 3 minutes; the mixing time for the third mixing in step (S3) is preferably 1 to 2 minutes, and the slump of the freshly mixed recycled concrete is controlled at 160 to 200 mm to balance workability and segregation resistance.

[0015] Furthermore, the stirring speed in each step is preferably controlled as follows: The dry mixing speed in step (S1) is 60~100r / min to achieve preliminary uniform dispersion of each solid raw material without excessive crushing of the aggregate; The wet mixing speed in step (S2) is 100~160 r / min to ensure that water, water-reducing agent and mineral admixture are fully dispersed and coated in the slurry; The mixing speed in step (S3) is 80~120r / min. While adjusting the amount of mixing water, the uniformity and stable workability of the freshly mixed recycled concrete are maintained. The vibration time in step (S4) is preferably 30~60s to ensure that the specimen is dense inside and does not segregate excessively.

[0016] Furthermore, the cement mentioned in step (1) is ordinary Portland cement; The mineral admixture is selected from any one or more of slag powder, fly ash, silica fume, and nano silica; The water-to-binder ratio is 0.30~0.45; The fineness modulus of the natural sand is 2.4~2.7; The natural coarse aggregate is continuously graded crushed stone with a particle size range of 5~20mm; The water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent.

[0017] Furthermore, in step (2), the parameters of the carbonation chamber in the pre-permeation stage are set as follows: the volume fraction of CO2 in the carbonation chamber is 60%~80%, the pressure is 0.2~0.5MPa, the temperature is 25℃~40℃, the relative humidity is 60%~80%, and the duration is 12~24h, so that CO2 gradually enters the surface of the cement matrix and the crack channels around the aggregate in a mild diffusion manner, avoiding the formation of a dense carbonate shell on the surface of the specimen and hindering the continued permeation of gas; The parameters of the carbonization chamber during the pulse intensification phase were set as follows: CO2 volume fraction in the carbonization chamber was 60%–90%, temperature was 25℃–40℃, and the pressure in the carbonization chamber was cyclically changed 3–5 times within the range of 0.5–1.2 MPa. Each pressure cycle included: increasing the pressure from a low-pressure plateau (0.3–0.5 MPa) to at least one high-pressure plateau (0.6–1.2 MPa) at a pressurization rate of 0.05–0.10 MPa / min, and holding the pressure at each high-pressure plateau for 2.5–3 hours; then decreasing the pressure back down at a depressurization rate of 0.05–0.10 MPa / min. A low-pressure platform (0.3~0.5MPa) is maintained for 2.5~3 hours; the entire process of "pressure increase-high pressure maintenance-pressure decrease-low pressure maintenance" is recorded as one pressure cycle; through periodic pressure fluctuations, a "pumping effect" is generated, forcibly driving CO2 into the microcrack network inside the recycled aggregate and the aggregate-cement interface transition zone, so that CO2 preferentially accumulates in the weak interface region and undergoes a carbonation reaction; furthermore, the total duration of the pulse strengthening stage is 24~36 hours, and the total duration is determined by the number of pressure cycles, the number of pressure platforms, and the maintenance time of each pressure platform.

[0018] The parameters of the carbonization chamber in the stabilization post-treatment stage were set as follows: after the pulse strengthening stage was completed, the pressure was slowly released to atmospheric pressure at a rate of 0.02~0.05MPa / min, and the specimen was placed in an environment of 60℃~80℃ to dry for 24~48h, so as to remove excess moisture in the interface and pores, promote the stable formation of carbonates and secondary hydration products, reduce residual stress at the interface and improve the stability of the interface structure.

[0019] Furthermore, the present invention provides a recycled concrete prepared by the above method, comprising raw material components in the following weight range: 300-450 parts cement 500-750 parts natural sand 600-900 parts of natural coarse aggregate, 50-300 parts of recycled aggregate from modified decommissioned wind turbine blades. 140-200 parts water 20-80 parts of mineral admixtures 5-15 parts of water-reducing agent.

[0020] After the recycled concrete described in this invention undergoes gradient CO2 targeted carbonization curing, the interface transition zone between the recycled aggregate of the decommissioned wind turbine blades and the cement matrix is ​​filled with a large amount of CaCO3 and hydration products, significantly reducing the number of interface microcracks and transforming the interface zone from a loose, multi-cracked structure into a dense, continuous structure.

[0021] The recycled concrete prepared based on the mix proportion of this invention can meet the mechanical performance requirements of conventional structural applications while taking into account workability, utilization rate of retired wind turbine blade aggregate, and carbon sequestration per unit volume.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) By controlling the freeze-thaw action to a “subcritical” level, the present invention transforms the traditional freeze-thaw process, which aims to completely destroy and separate the material, into a precise activation method for the internal interface of the recycled aggregate of decommissioned wind turbine blades. Under the premise of maintaining the macroscopic integrity of the recycled aggregate of decommissioned wind turbine blades, a controllable microcrack network is induced at the fiber-resin interface and inside the resin, providing a continuous channel for subsequent deep CO2 penetration.

[0023] (2) The present invention avoids the problem of excessive surface density and insufficient internal carbonization caused by a single constant carbonization condition by using a gradient CO2 carbonization curing method that combines pre-permeation, pulse strengthening and stabilization post-treatment. This allows CO2 to not only act on the cement matrix, but also preferentially accumulate in the internal interface region of the aggregate and the aggregate-cement interface transition zone under the drive of pressure fluctuations, thereby achieving targeted carbonation modification and structural optimization of the weak interface region.

[0024] (3) Optionally, the mineral admixture may contain a certain proportion of active silica components (such as silica fume and / or nano silica) to further enhance the secondary hydration and synergistic carbonization of the interface region, but this does not constitute a limitation of the present invention.

[0025] (4) This invention utilizes recycled aggregate from retired wind turbine blades in a large proportion of cement-based systems and seals a large amount of CO2, while taking into account both material performance improvement and environmental benefits. It provides a clear and feasible path for the collaborative carbon reduction and resource utilization of retired wind power equipment and low-carbon building materials, and has good engineering application prospects and environmental benefits. Attached Figure Description

[0026] Figure 1 This is a schematic diagram illustrating the mechanism of action of the recycled aggregate from decommissioned wind turbine blades in this invention through subcritical freeze-thaw interface activation and targeted carbonization.

[0027] Figure 2 This is a microcrack morphology diagram of the recycled aggregate from decommissioned wind turbine blades after freeze-thaw treatment according to the present invention.

[0028] Figure 3 This is a schematic diagram of the process flow for the preparation of recycled aggregate from decommissioned wind turbine blades with a preferred particle size of 5-20 mm, freeze-thaw interface activation treatment, cement-based material molding, and gradient CO2 carbonization curing. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0030] The sources of the raw materials used in the embodiments are shown in Table 1.

[0031] Table 1. Main raw materials and typical performance parameters in the implementation of this invention

[0032] Unless otherwise stated, all raw materials in the following examples meet the requirements of current national or industry standards.

[0033] A method for preparing recycled aggregate from modified decommissioned wind turbine blades based on freeze-thaw interface activation includes the following steps: (1) The retired wind turbine blades are mechanically cut, coarsely crushed and finely crushed to obtain recycled aggregate of retired wind turbine blades with mixed particle sizes; the recycled aggregate of retired wind turbine blades is graded and screened by a multi-layer vibrating screen according to particle size ranges of 0~5mm, 5~10mm, 10~16mm, 16~20mm and 20~25mm, and the recycled aggregate of retired wind turbine blades is controlled within the particle size range of 2~25mm, preferably 5~20mm, and fine powder with a particle size of less than 2mm and obvious metal impurities are removed. The recycled aggregate from decommissioned wind turbine blades is used in stages according to different particle size ranges to optimize aggregate gradation and take into account the needs of interface control. By using it in stages, on the one hand, larger particles are used to build a load-bearing skeleton, and smaller particles are used to fill the pores between the skeleton, thereby optimizing aggregate gradation, reducing porosity and improving workability. On the other hand, it increases the overall specific surface area of ​​the aggregate, which is conducive to the full role of freeze-thaw induced crack network and subsequent CO2 targeted carbonization in the aggregate-slurry interface region, thereby improving interfacial bonding performance and overall mechanical properties. (2) Place the recycled aggregate of decommissioned wind turbine blades obtained by screening in step (1) in an oven at (60±5)℃ to pre-dry until the mass is constant. Then, completely immerse the pre-dried recycled aggregate of decommissioned wind turbine blades in water for 4~24h to make the internal pores of the aggregate and the fiber-resin interface area reach a saturated or nearly saturated state. The soaking temperature is preferably 15~30℃. The moisture content at the end of the soaking is basically stable by weighing the change in aggregate mass to ensure that different batches of aggregate have repeatable initial moisture states. (3) Take out the recycled aggregate of decommissioned wind turbine blades after soaking in step (2), drain or wipe off the surface free water so that the interior of the recycled aggregate of decommissioned wind turbine blades is saturated or nearly saturated and there is no obvious water accumulation on the surface. Then place it in a freeze-thaw chamber and perform 5 to 15 freeze-thaw cycles in the temperature range of -20℃ to 25℃. In each freeze-thaw cycle, the low temperature and high temperature platforms are maintained for 3 hours respectively, and the cooling and heating rates are both 5℃ / h. The temperature of the low temperature platform is -20℃ to -10℃ and the temperature of the high temperature platform is 20℃ to 25℃. Modified recycled aggregate of decommissioned wind turbine blades based on freeze-thaw interface activation is obtained. This step precisely controls the temperature range, the internal moisture state of the aggregate, and the number of freeze-thaw cycles, so that the ice expansion stress mainly acts on the fiber-resin interface and the resin matrix, inducing the formation of new nano- to micro-scale cracks inside the aggregate and expanding some of the original micro-cracks, constructing a micro-crack network with a certain degree of connectivity, while not causing macroscopic crushing or significant deterioration of the aggregate strength, thus achieving subcritical activation of the internal interface of the recycled aggregate from decommissioned wind turbine blades.

[0034] Furthermore, in step (1) of the present invention, the length of the decommissioned wind turbine blade after mechanical cutting is 0.5~1.0m, and then it is fed into a jaw crusher for coarse crushing, controlling the output particle size within the range of 50~100mm, and then fed into an impact crusher for fine crushing, controlling the maximum particle size within 25mm.

[0035] Furthermore, the soaking water in step (2) of this invention is tap water or deionized water, the soaking temperature is 15℃~30℃, and the soaking time is preferably 8~16h, so as to ensure that the aggregate reaches a relatively uniform water content state, which is conducive to the formation of a uniform interface crack network during the freeze-thaw process.

[0036] Furthermore, the freeze-thaw cycle number in step (3) of the present invention is preferably 8 to 12 times. When the freeze-thaw cycle number is less than 5 times, the internal crack network of the aggregate is not easy to form. When the freeze-thaw cycle number is greater than 15 times, the aggregate strength decreases significantly, which is not conducive to engineering applications.

[0037] Furthermore, the present invention also provides a method for preparing recycled concrete based on the above-mentioned modified decommissioned wind turbine blade recycled aggregate, comprising the following steps: (1) The modified recycled aggregate of retired wind turbine blades activated by the above-mentioned freeze-thaw interface is used to replace natural coarse aggregate at 5% to 40% of the total mass of coarse aggregate, preferably 10% to 30%, and is mixed with natural coarse aggregate, cement, natural sand, water, mineral admixture and water-reducing agent in a predetermined proportion to prepare fresh recycled concrete; the fresh recycled concrete is poured into shape and vibrated to compact it, cured for 24 hours at 20℃±2℃ and relative humidity not less than 95% before demolding, and continued to be cured according to standard until the predetermined age (e.g. 3d or 7d) to obtain specimens with certain initial strength; (2) The specimens obtained in step (1) are then placed in a CO2-rich sealed carbonization chamber and subjected to gradient CO2 carbonization curing in sequence through a pre-permeation stage, a pulse strengthening stage, and a stabilization post-treatment stage. This allows CO2 to preferentially penetrate into the interior of the recycled aggregate of the decommissioned wind turbine blade and the recycled aggregate-cement interface region along the microcrack network induced by freeze-thaw cycles, thereby achieving interface-targeted carbonation modification and obtaining the interface-enhanced recycled concrete based on gradient CO2 targeted carbonation as described in this invention.

[0038] Furthermore, the specific methods of step (1) include: (S1) Weigh cement, natural sand, natural coarse aggregate and modified decommissioned wind turbine blade recycled aggregate according to the predetermined ratio, put them into a mixer and dry mix for 1~2 minutes to obtain the first mixture; (S2) Mix 2 / 3 to 3 / 4 of the total water with the water-reducing agent and mineral admixture, and slowly add the mixture to the first mixture. Continue stirring for 2 to 4 minutes to obtain the second mixture. (S3) Add the remaining mixing water to the second mixture to adjust the workability, stir for 1~3 minutes, and obtain fresh recycled concrete with a slump that meets the design requirements; (S4) The freshly mixed material is poured into standard specimens, compacted on a vibrating table, and demolded after standing at room temperature for 24 hours according to the specifications. Then, it is cured under standard curing conditions of 20℃±2℃ and relative humidity≥95% until the predetermined age to obtain specimens with certain initial strength.

[0039] Furthermore, the mixing time for the second mixing in step (S2) is preferably 2 to 3 minutes; the mixing time for the third mixing in step (S3) is preferably 1 to 2 minutes, and the slump of the freshly mixed recycled concrete is controlled at 160 to 200 mm to balance workability and segregation resistance.

[0040] Furthermore, the stirring speed in each step is preferably controlled as follows: The dry mixing speed in step (S1) is 60~100r / min to achieve preliminary uniform dispersion of each solid raw material without excessive crushing of the aggregate; The wet mixing speed in step (S2) is 100~160 r / min to ensure that water, water-reducing agent and mineral admixture are fully dispersed and coated in the slurry; The mixing speed in step (S3) is 80~120r / min. While adjusting the amount of mixing water, the uniformity and stable workability of the freshly mixed recycled concrete are maintained. The vibration time in step (S4) is preferably 30~60s to ensure that the specimen is dense inside and does not segregate excessively.

[0041] Furthermore, the cement mentioned in step (1) is ordinary Portland cement; The mineral admixture is selected from any one or more of slag powder, fly ash, silica fume, and nano silica; The water-to-binder ratio is 0.30~0.45; The fineness modulus of the natural sand is 2.4~2.7; The natural coarse aggregate is continuously graded crushed stone with a particle size range of 5~20mm; The water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent.

[0042] Furthermore, in step (2), the parameters of the carbonation chamber in the pre-permeation stage are set as follows: the volume fraction of CO2 in the carbonation chamber is 60%~80%, the pressure is 0.2~0.5MPa, the temperature is 25℃~40℃, the relative humidity is 60%~80%, and the duration is 12~24h, so that CO2 gradually enters the surface of the cement matrix and the crack channels around the aggregate in a mild diffusion manner, avoiding the formation of a dense carbonate shell on the surface of the specimen and hindering the continued permeation of gas; The parameters of the carbonization chamber during the pulse intensification phase were set as follows: CO2 volume fraction in the carbonization chamber was 60%–90%, temperature was 25℃–40℃, and the pressure in the carbonization chamber was cyclically changed 3–5 times within the range of 0.5–1.2 MPa. Each pressure cycle included: increasing the pressure from a low-pressure plateau (0.3–0.5 MPa) to at least one high-pressure plateau (0.6–1.2 MPa) at a pressurization rate of 0.05–0.10 MPa / min, and holding the pressure at each high-pressure plateau for 2.5–3 hours; then decreasing the pressure back down at a depressurization rate of 0.05–0.10 MPa / min. A low-pressure platform (0.3~0.5MPa) is maintained for 2.5~3 hours; the above process of "pressure increase-high pressure maintenance-pressure decrease-low pressure maintenance" is recorded as one pressure cycle; through periodic pressure fluctuations, a "pumping effect" is generated, which forces CO2 into the microcrack network inside the recycled aggregate and the aggregate-cement interface transition zone, so that CO2 preferentially accumulates in the weak interface region and undergoes a carbonation reaction; furthermore, the total duration of the pulse strengthening stage is 24~36 hours, and the total duration is determined by the number of pressure cycles, the number of pressure platforms, and the maintenance time of each pressure platform.

[0043] The parameters of the carbonization chamber in the stabilization post-treatment stage were set as follows: after the pulse strengthening stage was completed, the pressure was slowly released to atmospheric pressure at a rate of 0.02~0.05MPa / min, and the specimen was placed in an environment of 60℃~80℃ to dry for 24~48h, so as to remove excess moisture in the interface and pores, promote the stable formation of carbonates and secondary hydration products, reduce residual stress at the interface and improve the stability of the interface structure.

[0044] Furthermore, the present invention provides a recycled concrete prepared by the above method, comprising raw material components in the following weight range: 300-450 parts cement 500-750 parts natural sand 600-900 parts of natural coarse aggregate, 50-300 parts of recycled aggregate from modified decommissioned wind turbine blades. 140-200 parts water 20-80 parts of mineral admixtures 5-15 parts of water-reducing agent.

[0045] Depend on Figure 1 This diagram illustrates the mechanism of action of the recycled aggregate from decommissioned wind turbine blades in this invention through subcritical freeze-thaw interface activation and targeted carbonization. Figure 2 It can be seen that the process flow of this invention includes the preparation of recycled aggregate from decommissioned wind turbine blades with a preferred particle size of 5-20 mm, freeze-thaw interface activation treatment, cement-based material molding, and gradient CO2 carbonization curing.

[0046] Example 1 (Typical subcritical freeze-thaw interface activation and three-stage gradient CO2 carbonization process) This embodiment provides a typical process for recycling aggregates from decommissioned wind turbine blades. Using the above-mentioned method for modifying recycled aggregates from decommissioned wind turbine blades, recycled aggregates from decommissioned wind turbine blades are prepared by mechanical cutting and crushing. Subcritical freeze-thaw cycles are used to activate the internal interface of the aggregates. After the cement-based material is formed, a three-stage gradient CO2 carbonization curing process is implemented to obtain recycled concrete with excellent interface properties and carbon fixation function.

[0047] (1) Preparation of recycled aggregate from decommissioned wind turbine blades After the disassembled decommissioned wind turbine blades are transported to the processing site, they are first removed by mechanical cutting to remove metal connectors, bolts, metal sleeves, and other components at the blade root. Obvious surface protective layers, coatings, and attached rubber, plastic, and other impurities are also removed. For areas with deposited dirt or oil, high-pressure water jets or simple cleaning methods can be used to minimize any adverse effects on the interface performance. After initial cleaning, the blades are cut into sections of 0.5 to 1.0 m in length and fed into a jaw crusher for coarse crushing, controlling the output particle size within the range of 50 to 100 mm. Then, they are fed into an impact crusher for secondary crushing, controlling the maximum particle size within 25 mm.

[0048] The crushed product was graded and screened using a multi-layer vibrating screen to obtain materials of 0~5mm, 5~10mm, 10~20mm and >20mm respectively. Large pure resin clumps, slender glass fiber bundles and metal fragments that are easily identifiable during the screening process were manually removed. Particles with a particle size in the range of 5~20mm were used as the recycled aggregate of retired wind turbine blades in this invention.

[0049] The recycled aggregate from the decommissioned wind turbine blades was pre-dried in a 60℃ oven for approximately 24 hours until the difference between two weighings was less than 0.1%. Its apparent density, bulk density, water absorption rate, and crushing value were then measured using current methods. A typical comparison with ordinary crushed stone is shown in Table 2.

[0050] Table 2. Comparison of typical physical properties of recycled aggregate from decommissioned wind turbine blades and ordinary crushed stone.

[0051] As shown in Table 2, the recycled aggregate from decommissioned wind turbine blades has low density, high water absorption and crushing value, complex interface and many internal defects, which are the key targets for control in this invention through "freeze-thaw interface activation + targeted carbonization".

[0052] (2) Subcritical freeze-thaw interface activation The dried 5-20mm retired wind turbine blade recycled aggregate is placed into a plastic container, and tap water is added until the water level is about 5cm above the aggregate. It is then soaked at about 20℃ for 24 hours to allow the internal pores and interface areas of the aggregate to fully absorb water. After soaking, the recycled aggregate is removed, and surface free water is drained or wiped off, ensuring the aggregate is saturated or nearly saturated internally while the surface is free of significant water accumulation. It is then placed in a controlled freeze-thaw test chamber for freeze-thaw cycles. By removing surface free water, agglomeration, surface peeling, and frost heave damage caused by external freezing are avoided, ensuring that the freeze-thaw action primarily occurs in the internal interface area of ​​the aggregate. After draining off the surface free water, the aggregate is laid flat in a stainless steel mesh basket, and the freeze-thaw regime is set as follows: Cooling stage: The temperature is reduced from +5℃ to -20℃ at a rate of about 5℃ / h, and then kept at -20℃ for 3 hours to allow the moisture inside the aggregate to freeze in the confined space, generating a certain amount of ice expansion pressure. Heating stage: The temperature is increased from -20℃ to +25℃ at a rate of about 5℃ / h, and then held at +25℃ for 3 hours to melt the ice crystals, release internal stress, and induce the generation and propagation of microcracks.

[0053] The above-described cooling-heating process is recorded as one freeze-thaw cycle. In this embodiment, the number of cycles is set to 10. After the freeze-thaw cycle, the aggregate is allowed to return to room temperature and its surface moisture is allowed to dry naturally. The mass loss of the aggregate is recorded and its appearance is observed. Under typical conditions, the aggregate mainly exhibits slightly rounded edges and corners, with very little edge peeling. The mass loss rate is controlled within a reasonable range, and the overall block structure is still maintained. Microscopic observation reveals that a crack network mainly at the micron / submicron scale has formed at the fiber-resin interface and inside the resin matrix.

[0054] These types of cracks significantly increase the permeability of CO2 to the interior and interface regions of the aggregate, providing a continuous transport channel. On the other hand, the crack size is controlled by the "subcritical" freeze-thaw intensity, and it does not develop into a macroscopic large crack that destroys the aggregate skeleton, thus taking into account both the "activation" of the interface and the "integrity" of the aggregate.

[0055] (3) Mix proportions and preparation of recycled concrete This embodiment designs C40 grade recycled concrete, using P.O42.5 ordinary Portland cement, medium sand, ordinary crushed stone, and recycled aggregate from decommissioned wind turbine blades pretreated by subcritical freeze-thaw cycles. Based on the total mass of natural coarse aggregate, the proportion of modified recycled aggregate from decommissioned wind turbine blades replacing ordinary crushed stone is 30%. A typical mix design is as follows (in kg / m³): cement 350, slag powder 50, silica fume 15, medium sand 650, ordinary crushed stone 650, modified recycled aggregate from decommissioned wind turbine blades 280, water 180, and water-reducing agent 8. During mixing, medium sand, ordinary crushed stone, modified decommissioned blade recycled aggregate, and cement are first added to a planetary mixer in proportion and dry-mixed for about 2 minutes to ensure that the solid raw materials are evenly mixed. Then, about 2 / 3 of the mixing water is premixed with water-reducing agent, slag powder, and silica fume and added to the mixer for wet mixing for about 2 minutes. The remaining mixing water is then added according to the slump requirements, and mixing continues for 2 minutes. After mixing, the slump is measured and generally controlled within the range of 160~200mm to ensure good workability and pumpability even with the addition of lightweight blade aggregate. The mixed concrete was poured into 40mm×40mm×40mm cube molds, cylindrical molds, and slice molds for interface observation. The concrete was compacted using a vibrating table for 60 seconds, the surface was smoothed, covered with plastic film, and left to stand for 24 hours. After demolding, the specimens were first placed in a standard curing room at 20℃±2℃ and relative humidity≥95% for 3 days to ensure the basic hydration reaction and obtain a certain early strength.

[0056] (4) Three-stage gradient CO2 carbonization curing After pre-curing, the specimens were placed in a sealed carbonization chamber for three-stage gradient CO2 carbonization: Pre-permeation stage: In this stage, the CO2 volume fraction is controlled at 70%, the total pressure is 0.30 MPa, the temperature is controlled at around 30℃, the relative humidity is maintained at 70%, and the carbonization time is about 16 hours. This stage utilizes moderate pressure and humidity conditions to allow CO2 to fully dissolve in the surface layer and pore water of the specimen, and slowly enter the interior of the blade aggregate and the aggregate-slurry interface area through the crack network formed by freeze-thaw induction, avoiding the formation of a dense carbonate shell on the surface that would hinder subsequent gas permeation.

[0057] Pulse-enhanced stage: After pre-permeation, the CO2 volume fraction is maintained at 85%, and the carbonation chamber pressure is periodically varied within the range of 0.5~1.2 MPa. Specifically, the pressure is increased from 0.5 MPa to 0.8 MPa and held for 3 hours, then increased from 0.8 MPa to 1.2 MPa and held for 3 hours, followed by a decrease in pressure at a rate of 0.05~0.10 MPa / min back to 0.5 MPa and held for 3 hours. The pressure sequence of "0.5→0.8→1.2→0.5 MPa" is recorded as one cycle. In this embodiment, the cycle is repeated 3 times, corresponding to a total pulse-enhanced period of approximately 30 hours (excluding the transition time during the pressure increase / decrease process). The frequent pressure increase-decrease process produces a "breathing" or "pumping" effect in the blade aggregate and interface cracks. CO2 that could not be fully penetrated into the depths of the cracks in the previous cycle is further pushed into the depths of the cracks in subsequent cycles, thereby achieving deep carbonation modification of the interface region. Compared with single constant pressure carbonization, pulsed strengthening can significantly increase the propagation depth and number of repetitions of CO2 in the crack channel without accumulating too much carbonate on the surface and causing early blockage.

[0058] Post-stabilization treatment stage: After the pulse intensification stage, the gas supply is stopped and the pressure is slowly released to atmospheric pressure at a rate of about 0.03 MPa / min. After the pressure is released, the sample is left to stand for 2 hours to allow the CO2-water-alkaline substance system inside the sample to gradually reach equilibrium. Then the sample is transferred to a hot air circulating drying oven at 65℃ and dried at 65℃ for 24 hours. The gentle heating promotes the escape of excess moisture and at the same time promotes the rearrangement and recrystallization of newly generated carbonate products and original hydration products, forming a denser and more stable interface structure.

[0059] (5) Typical performance effects Under typical conditions, compared with the control concrete (Comparative Example 1, which uses only recycled aggregate from mechanically crushed decommissioned wind turbine blades and does not undergo freeze-thaw and gradient carbonation treatment), the specimens in this embodiment showed a significant improvement trend in 28-day cubic compressive strength, interfacial microhardness, and 24-hour water absorption. Reasonable exemplary results are shown in Table 3.

[0060] Table 3. Performance trend comparison between Example 1 and the untreated control group

[0061] As shown in Table 3, under the process of the present invention, the 28-day strength of concrete increased from 35.8 MPa to 42.0 MPa, the microhardness of the interface transition zone increased from 2.10 GPa to 2.80 GPa, and the 24-hour water absorption rate decreased from 4.20% to 3.10%, indicating that the internal structure of the recycled aggregate of decommissioned wind turbine blades and the aggregate-slurry interface area achieved significant densification and reinforcement effects.

[0062] Example 2 (The effect of different recycled aggregate replacement rates on performance and carbon sequestration of retired wind turbine blades) Based on Example 1, this embodiment maintains the same preparation process (mechanical crushing + 5~20 mm sieve screening), subcritical freeze-thaw cycles (10 times), and three-stage gradient CO2 carbonization regime for modified decommissioned wind turbine blade recycled aggregate and recycled concrete. Only the substitution rate of modified decommissioned wind turbine blade recycled aggregate in coarse aggregate is adjusted to examine the applicability and performance trend of the process of the present invention under different solid waste utilization levels.

[0063] (1) Concrete mix proportions and mixing Assuming the total coarse aggregate mass remains constant, three substitution rates are set: Example 2-1: The replacement rate of recycled aggregate in modified decommissioned wind turbine blades is 10%; Example 2-2: The replacement rate of recycled aggregate in modified decommissioned wind turbine blades is 20%; Examples 2-3: The recycled aggregate replacement rate of modified decommissioned wind turbine blades is 30%.

[0064] The remaining parts were made up of ordinary crushed stone. The proportions of cementitious materials, water-cement ratio, sand ratio, and water-reducing agent dosage in each group were the same as in Example 1.

[0065] The mixing steps are as follows: During mixing, first add medium sand, ordinary crushed stone, modified decommissioned wind turbine blade recycled aggregate, and cement to the planetary mixer in proportion, and dry mix for about 2 minutes to ensure that the solid raw materials are evenly mixed; then add about 2 / 3 of the mixing water, water-reducing agent, slag powder, and silica fume premixed into the mixer and wet mix for about 2 minutes; then add the remaining mixing water according to the slump adjustment requirements, and continue mixing for 2 minutes until the mixture is uniform and free of dry lumps; after mixing, the slump is measured and generally controlled within the range of 160~200mm, and the difference between the three groups is controlled within ±5mm to reduce the impact of workability differences on performance comparison.

[0066] (2) Specimen molding and curing For each group of concrete, 40mm×40mm×40mm cube compressive strength specimens, interfacial microhardness test specimens, and water absorption test specimens were prepared. The molding method was the same as in Example 1: the mixture was poured into the mold in two layers, each layer was vibrated for 35 seconds, the surface was smoothed, and it was covered with plastic film and left to stand for 24 hours.

[0067] After demolding, all specimens were first pre-cured for 3 days in a standard curing room at 20℃±2℃ and relative humidity ≥95%, and then uniformly transferred to a carbonization box for curing according to the three-stage gradient CO2 carbonization regime described in Example 1. After carbonization was completed, they were transferred back to standard curing conditions for 28 days.

[0068] (3) Performance and carbon fixation effect At 28 days of age, the cubic compressive strength, interfacial transition zone microhardness, and 24-hour water absorption rate of each group of specimens were tested. Simultaneously, the carbon sequestration per unit volume was estimated using gravimetric methods or chemical analysis. Typical trends are shown in Table 4. Table 4 Typical performance trends of the process of the present invention under different blade recycled aggregate replacement rates

[0069] As can be seen from Table 4: When the replacement rate is 20%, the 28-day compressive strength is basically the same as or slightly higher than that of concrete of the same grade in Example 1, the microhardness of the interface remains at a high level, and the structure of the interface transition zone is dense. As the replacement rate increases from 10% to 30%, the carbon sequestration per unit volume increases significantly, indicating that the carbonizable interface and crack channels brought by the recycled aggregate of the blades increase significantly, which is beneficial to CO2 sequestration. When the replacement rate reaches 30%, due to the high proportion of lightweight aggregate and its limited strength, the overall compressive strength decreases slightly, but it is still within the acceptable range for structural applications.

[0070] Taking into account structural load-bearing capacity, safety reserves, solid waste utilization efficiency, and carbon sequestration capacity, this invention preferably adopts a 10% to 20% replacement rate of modified decommissioned wind turbine blade aggregate in ordinary structural components. For non-load-bearing or low-load-bearing components, the replacement rate can be further increased to enhance weight reduction and carbon sequestration effects.

[0071] Example 3 (Optimization of Gradient CO2 Carbonization System Parameters) Based on Example 1, this embodiment adjusts the pressure, time, and temperature parameters for each stage of the three-stage gradient CO2 carbonization process to construct two representative carbonization schemes: Option A: Focuses on increasing carbon sequestration and interface densification; Option B: It takes into account both early-age strength and overall performance.

[0072] The preparation of recycled aggregate from modified decommissioned wind turbine blades, 10 subcritical freeze-thaw pretreatments, concrete mix proportions, and specimen dimensions (40 mm × 40 mm × 40 mm) were all the same as in Example 1.

[0073] (1) Carbonization scheme A (with a focus on enhanced carbonization) Pre-permeation stage: CO2 volume fraction 75%, pressure 0.3MPa, temperature 30℃, relative humidity 60%, duration 20h; Pulse intensification phase: CO2 volume fraction is 85%, and the pressure is cycled 3 times along the path of 0.3→0.6→0.9→0.3 MPa. Each pressure plateau is maintained for 2.5 hours, with a total duration of about 30 hours.

[0074] Post-stabilization treatment stage: After depressurization to atmospheric pressure at a rate of 0.02-0.05 MPa / min, the specimens were dried in hot air at 65℃ for 36 hours.

[0075] This approach extends the pre-penetration and pulse enhancement stages, further increasing the propagation depth and duration of CO2 in the crack channel, making it suitable for scenarios where higher carbon fixation and stronger interface modification are desired.

[0076] (2) Carbonization scheme B (mild carbonization) Pre-permeation stage: CO2 volume fraction 70%, pressure approximately 0.25 MPa, temperature 25℃, relative humidity 70%, duration 16 hours; Pulse intensification phase: CO2 volume fraction 85%, pressure cyclically 3 times along the path 0.3→0.6→0.9→0.3 MPa, each plateau 2.5h, total duration approximately 30h; Post-stabilization treatment stage: After depressurization at a rate of 0.02 to 0.05 MPa / min, dry at 60°C for 24 h.

[0077] The overall carbonization strength of this scheme is slightly lower than that of Example 1 and Scheme A, which helps to control the loss of early-age strength and is suitable for precast components with high requirements for early demolding, tensioning or hoisting.

[0078] (3) Performance comparison Typical trends can be summarized in Table 5: Table 5. Trends in performance and carbon fixation effect under different carbonization regimes

[0079] As can be seen from Table 5: Scheme A is slightly better than Example 1 in terms of carbon fixation and interface hardness, but the 7-day strength is slightly affected; Scheme B has slightly lower carbon fixation and interface modification than Example 1, but it is beneficial to maintain early-age strength. This shows that the gradient CO2 carbonization mode proposed in this invention has a large adjustable space and can be optimized by balancing "strong carbon fixation-strong interface" and "stable early age-stable long-term" according to the design goals of different projects.

[0080] Example 4 (Influence of freeze-thaw interface activation parameters on performance - different soaking times and freeze-thaw cycles) Based on Example 1, this embodiment focuses on examining the effects of soaking time and the number of freeze-thaw cycles on the activation effect of the blade aggregate interface and the concrete performance, in order to support the rationality of the "subcritical freeze-thaw" control range of the present invention.

[0081] The preparation method of the modified decommissioned wind turbine blade recycled aggregate is the same as in Example 1, with a particle size of 5-20 mm; the concrete mix proportion is the same as in Example 1, and the replacement rate of the modified decommissioned wind turbine blade recycled aggregate is 30%. The three sets of parameters are respectively referred to as Example 4-1, Example 4-2 and Example 4-3.

[0082] (1) Combinations of different soaking times and freeze-thaw cycles Example 4-1: Soaking for 8 hours, followed by 6 freeze-thaw cycles; Example 4-2: Soak for 16 hours, freeze-thaw cycle 10 times (similar to Example 1, as recommended operating condition); Example 4-3: Soaking for 24 hours, followed by 14 freeze-thaw cycles (close to the upper limit of the "subcritical" operating condition of this invention).

[0083] The freeze-thaw temperature profiles and single-cycle holding times for the three groups were the same as in Example 1 (from +5°C to...). The aggregate was kept at 20℃ for 3 hours, then heated to +25℃ and kept at +25℃ for 3 hours, with only the soaking time and the number of freeze-thaw cycles adjusted. After soaking, freeze-thaw cycles were performed according to the method in Example 1; after freeze-thaw cycles, the aggregate was visually observed and the mass loss rate was measured.

[0084] (2) Freeze-thaw damage and interfacial activation of aggregates Representative results are shown in Table 6.

[0085] Table 6 Typical Indicators of Leaf Recycled Aggregate under Different Freeze-Thaw Parameters

[0086] As can be seen from Table 6, when the number of freeze-thaw cycles is small and the soaking time is short (Example 4-1), there are insufficient interface cracks and the activation effect is limited; under moderate soaking time and freeze-thaw cycles (Example 4-2), a relatively ideal microcrack network can be formed; when the soaking time and freeze-thaw cycles are further increased (Example 4-3), although the number of interface cracks increases, the aggregate mass loss rate increases, which is close to the "subcritical" upper limit of the present invention.

[0087] (3) Comparison of concrete performance The three aggregate groups were used to prepare concrete. The mixing, molding, pre-curing, and three-stage gradient CO2 carbonation regimes were the same as in Example 1, and 40 mm × 40 mm × 40 mm cubic specimens were prepared. Typical properties at 28 days are shown in Table 7.

[0088] Table 7 Representative values ​​of recycled concrete performance under different freeze-thaw parameters

[0089] The results in Table 7 show that: Example 4-2 (soaking for 16 h and freezing and thawing 10 times) showed the best overall performance in terms of strength, interfacial hardness and water absorption. Considering both interfacial activation and aggregate integrity, it verified the rationality of the "subcritical freeze-thaw" parameter range recommended in this invention. Example 4-1 showed insufficient interface activation, with both interface hardness and compressive strength lower than those of Example 4-2; Although the interfacial microhardness of Example 4-3 is still relatively high, the 28-day strength is slightly lower than that of Example 4-2, indicating that excessive freeze-thaw cycles weaken the aggregate's load-bearing capacity.

[0090] Comparative Example 1 (no freeze-thaw interface activation, only constant CO2 carbonization) This comparative example is a specific explanation of "Comparative Example 1" in Table 3, used to compare the limitations of using only conventional CO2 carbonization when subcritical freeze-thaw interface activation is lacking.

[0091] (1) Aggregate and concrete preparation The aggregate from decommissioned wind turbine blades is only mechanically crushed and screened (5~20 mm), without soaking or freeze-thaw pretreatment. The remaining steps are the same as the aggregate pretreatment in Example 1.

[0092] The concrete mix proportions were the same as in Example 1, with a blade aggregate replacement rate of 30%, and the specimen dimensions were all 40 mm × 40 mm × 40 mm. The mixing, molding, and demolding processes were the same as in Example 1, and the slump of the fresh mix was controlled at 160~200 mm.

[0093] (2) Constant CO2 carbonization After demolding and standard curing for 3 days, the specimens were placed in a carbonization chamber and carbonized at a constant temperature of approximately 20% CO2 volume fraction, 0.15 MPa total pressure, 23℃ temperature and 65% relative humidity for 24 hours. They were then transferred back to the standard curing room for curing for 28 days.

[0094] Under this system, the CO2 pressure is low and there is no significant pressure fluctuation. Furthermore, the blade aggregate has not undergone interface activation treatment and lacks effective crack channels inside.

[0095] (3) Performance and microstructure Test results showed that the 28-day compressive strength of the specimens was approximately 35 MPa, the interfacial microhardness was approximately 2.1 GPa, and the 24-hour water absorption rate was approximately 4.2%. Cross-sectional observation revealed that obvious pore zones and microcracks still existed at the aggregate-slurry interface of the blades. Carbonates were mainly concentrated in the surface slurry and some capillaries, while the formation of carbonates inside the aggregate and at the interface was limited.

[0096] Compared with Example 1, it can be seen that without freeze-thaw interface activation, CO2 is difficult to fully penetrate into the interior and interface area of ​​the blade aggregate, resulting in carbonization mainly remaining on the outer layer, making it difficult to exert the "targeted carbonization" advantage of the present invention.

[0097] Comparative Example 2 (freeze-thaw interface activation, using constant CO2 carbonization) This comparative example is used to highlight that, under the same freeze-thaw interface activation treatment, if the carbonization stage does not use three-stage gradient CO2 curing but instead uses constant pressure conditions for CO2 curing, the penetration depth and number of repeated actions of CO2 in the microcracks inside the modified decommissioned wind turbine blade recycled aggregate and the aggregate-cement interface transition zone are limited, and the interface-targeted modification effect is relatively weakened.

[0098] (1) Aggregate and concrete preparation The preparation, soaking, and subcritical freeze-thaw interface activation pretreatment of the modified decommissioned wind turbine blade recycled aggregate were exactly the same as in Example 1; the concrete mix proportion, mixing steps, specimen size (40 mm × 40 mm × 40 mm), and molding process were also consistent with those in Example 1.

[0099] (2) Constant CO2 curing Unlike Example 1, this comparative example does not use three-stage gradient CO2 carbonation curing after pre-curing, but uses the same constant CO2 carbonation curing as Comparative Example 1. This is used to compare and verify the effect of CO2 on the internal interface region of aggregate and the transition zone of aggregate-cement interface under constant carbonation conditions when freeze-thaw interface activation pretreatment is present.

[0100] Specifically, after the specimens were demolded and cured under standard conditions for 3 days, they were placed in a carbonization chamber and carbonized at a constant temperature of approximately 20% CO2 volume fraction, 0.15 MPa total pressure, 23°C temperature, and 65% relative humidity for 24 hours. They were then transferred back to the standard curing room for curing for 28 days.

[0101] (3) Performance and comparison Test results showed that the 28-day compressive strength of this group of specimens was approximately 38 MPa, the interfacial microhardness was approximately 2.5 GPa, and the 24-hour water absorption rate was approximately 3.6%. Under the same constant CO2 carbonization regime, compared with Comparative Example 1, the specimens in this comparative example showed a more pronounced interfacial densification trend due to the formation of microcrack channels in the aggregate through subcritical freeze-thaw interface activation pretreatment, resulting in a certain decrease in the 24-hour water absorption rate. Compared with Example 1, the interfacial reinforcement and densification effects in this comparative example were relatively limited because it did not employ a three-stage gradient CO2 carbonization process (especially the pulse strengthening stage).

[0102] The above results indicate that freeze-thaw interface activation under constant carbonization conditions can improve the effect of CO2 on the interface region, while gradient CO2 carbonization can further amplify this effect and achieve more complete interface-targeted modification.

[0103] Comparative Example 3 (freeze-thaw interface activation only, no CO2 carbonization) This comparative example is used to highlight that if only physical freeze-thaw interface activation is used without subsequent CO2 carbonization, the "physical-chemical synergistic enhancement" effect emphasized in this invention cannot be achieved.

[0104] (1) Aggregate and concrete preparation The preparation, soaking, and 10 subcritical freeze-thaw pretreatments of the modified decommissioned wind turbine blade recycled aggregate were exactly the same as in Example 1. The concrete mix proportions, mixing steps, specimen dimensions (40 mm × 40 mm × 40 mm), and molding process were also consistent with those in Example 1.

[0105] (2) Maintenance system Unlike Example 1, this comparative example did not undergo any CO2 carbonization curing after demolding, and the specimens were kept in a standard curing environment of 20℃±2℃ and relative humidity ≥95% for 28 days.

[0106] (3) Performance and comparison Tests show that, compared with the "ordinary control group" (which can be regarded as the standard engineering practice) which only mechanically crushes aggregates without freeze-thaw cycles or carbonization, the comparative specimens in this study show a certain degree of improvement in 28-day compressive strength and interfacial microhardness due to increased interface roughness and enhanced mechanical interlocking. However, the interface pores remain relatively interconnected, and the 24-hour water absorption rate only decreases to a limited extent.

[0107] Compared with Example 1, it can be seen that under the same freeze-thaw pretreatment conditions, the lack of the CO2 carbonization stage will result in the crack channels not being fully filled and "repaired", insufficient carbonate formation in the interface area, and the long-term durability, impermeability and carbon fixation ability are significantly inferior to Example 1. This shows that the "freeze-thaw interface activation + gradient CO2 targeted carbonization" emphasized in this invention is a synergistic system, and neither can be omitted.

[0108] Comparative Example 4 (Excessive freeze-thaw, exceeding the subcritical range) This comparative example illustrates that when the number of freeze-thaw cycles is too high and exceeds the "subcritical" interface activation range defined in this invention, although the crack network further expands, the overall strength of the aggregate decreases significantly, and the comprehensive performance deteriorates.

[0109] (1) Transitional freeze-thaw pretreatment Immersion method and freeze-thaw temperature range (+5~) for recycled aggregate from decommissioned wind turbine blades The temperature was 20℃ and the holding time was the same as in Example 1, except that the number of freeze-thaw cycles was increased from 10 to 20.

[0110] After the freeze-thaw cycle, the aggregate was inspected and found to have: the edges and corners became noticeably rounder, some particles showed visible edge peeling, the proportion of fine debris increased significantly during sieving, and the mass loss rate and crushing value were significantly higher than those of the aggregate in Example 1, indicating that the aggregate matrix had deteriorated significantly and was no longer in a "subcritical activation" state.

[0111] (2) Concrete preparation and curing Using the exact same concrete mix proportions and mixing methods as in Example 1, 40 mm × 40 mm × 40 mm cubic concrete specimens were prepared. After demolding and 3 days of pre-curing, the specimens were placed in a carbonation chamber and subjected to the same three-stage gradient CO2 carbonation as in Example 1.

[0112] (3) Performance and failure modes Test results show that the 28-day compressive strength of this group of specimens is significantly lower than that of Example 1, with a decrease of about 10% in some cases. Although the microhardness of the interface is improved to a certain extent, the fracture surface shows that the proportion of aggregate body breakage is increased, the proportion of broken aggregate is large, and the failure mode changes from interface peeling to aggregate failure.

[0113] This indicates that under excessive freeze-thaw conditions, although there are more crack channels and CO2 penetration is more complete, the aggregate itself is "excessively weakened," and the overall load-bearing capacity is impaired, making it difficult to meet engineering strength requirements. This further verifies that the "subcritical freeze-thaw" emphasized in this invention is a reasonable upper limit for interface activation. The number of freeze-thaw cycles and temperature history should be controlled to limit the crack size to the microscopic level, so as to balance CO2 penetration and aggregate integrity.

Claims

1. A method for preparing recycled aggregate from modified decommissioned wind turbine blades based on freeze-thaw interface activation, characterized in that, Includes the following steps: (1) The retired wind turbine blades are mechanically cut, coarsely crushed and finely crushed to obtain recycled aggregate of retired wind turbine blades with mixed particle sizes; the recycled aggregate of retired wind turbine blades is graded and screened by a multi-layer vibrating screen according to particle size ranges of 0~5mm, 5~10mm, 10~16mm, 16~20mm and 20~25mm, and the recycled aggregate of retired wind turbine blades is controlled within the particle size range of 2~25mm by screening, and fine powder with a particle size of less than 2mm and obvious metal impurities are removed. (2) Place the recycled aggregate of decommissioned wind turbine blades obtained by screening in step (1) in an oven at (60±5)℃ to pre-dry until the mass is constant. Then, completely immerse the pre-dried recycled aggregate of decommissioned wind turbine blades in water for 4~24h to make the internal pores of the aggregate and the fiber-resin interface area reach a saturated or nearly saturated state. The moisture content is basically stable at the end of the process by weighing the change in aggregate mass, so as to ensure that different batches of aggregate have repeatable initial moisture states. (3) Take out the recycled aggregate of decommissioned wind turbine blades after soaking in step (2), drain or wipe off the free water on the surface, so that the interior of the recycled aggregate of decommissioned wind turbine blades is saturated or nearly saturated and there is no obvious water accumulation on the surface. Then place it in a freeze-thaw chamber and perform 5 to 15 freeze-thaw cycles in the temperature range of -20℃ to 25℃. In each freeze-thaw cycle, the low temperature and high temperature platforms are maintained for 3 hours respectively, and the cooling and heating rates are both 5℃ / h. The temperature of the low temperature platform is -20℃ to -10℃ and the temperature of the high temperature platform is 20℃ to 25℃, so as to obtain the modified recycled aggregate of decommissioned wind turbine blades based on freeze-thaw interface activation.

2. The preparation method according to claim 1, characterized in that, The retired wind turbine blades described in step (1) are cut to a length of 0.5~1.0m and then fed into a jaw crusher for coarse crushing, controlling the output particle size within the range of 50~100mm. They are then fed into an impact crusher for fine crushing, controlling the maximum particle size within 25mm.

3. The preparation method according to claim 1, characterized in that, The particle size range of the recycled aggregate from the decommissioned wind turbine blades in step (1) is 5~20mm; The soaking water used in step (2) is tap water or deionized water, the soaking temperature is 15℃~30℃, and the soaking time is 8~16h; The freeze-thaw cycle in step (3) is 8 to 12 times.

4. A modified recycled aggregate for decommissioned wind turbine blades prepared by the preparation method according to any one of claims 1 to 3.

5. A method for preparing recycled concrete based on the recycled aggregate from modified decommissioned wind turbine blades as described in claim 4, characterized in that, Includes the following steps: (1) Replace natural coarse aggregate with recycled aggregate from modified retired wind turbine blades at 5% to 40% of the total mass of coarse aggregate, mix it with natural coarse aggregate, cement, natural sand, water, mineral admixtures and water-reducing agent in a predetermined ratio to prepare fresh recycled concrete; pour the fresh recycled concrete into shape and vibrate it to compact it, cure it for 24 hours at 20℃±2℃ and relative humidity not less than 95%, demold it, and continue to cure it according to standard until the predetermined age to obtain specimens with certain initial strength; (2) The specimens obtained in step (1) are then placed in a CO2-rich sealed carbonization chamber and subjected to gradient CO2 carbonation curing in sequence through the pre-permeation stage, pulse strengthening stage and stabilization post-treatment stage to achieve interface-targeted carbonation modification and obtain interface-enhanced recycled concrete based on gradient CO2-targeted carbonation.

6. The preparation method according to claim 5, characterized in that, The modified decommissioned wind turbine blade recycled aggregate mentioned in step (1) replaces natural coarse aggregate by 10% to 30% of the total mass of coarse aggregate; Step (1) includes the following specific methods: (S1) Weigh cement, natural sand, natural coarse aggregate and modified decommissioned wind turbine blade recycled aggregate according to the predetermined ratio, put them into a mixer and dry mix for 1~2 minutes to obtain the first mixture; (S2) Mix 2 / 3 to 3 / 4 of the total water with the water-reducing agent and mineral admixture, and slowly add the mixture to the first mixture. Continue stirring for 2 to 4 minutes to obtain the second mixture. (S3) Add the remaining mixing water to the second mixture to adjust the workability, stir for 1~3 minutes, and obtain fresh recycled concrete with a slump that meets the design requirements; (S4) The freshly mixed material is poured into standard specimens, compacted on a vibrating table, and demolded after standing at room temperature for 24 hours according to the specifications. Then, it is cured under standard curing conditions of 20℃±2℃ and relative humidity≥95% until the predetermined age to obtain specimens with certain initial strength.

7. The preparation method according to claim 6, characterized in that, The mixing time for the second mixing in step (S2) is 2-3 minutes; the mixing time for the third mixing in step (S3) is 1-2 minutes, and the slump of the freshly mixed recycled concrete is controlled at 160-200 mm. The stirring speed in each step is controlled as follows: The dry mixing speed in step (S1) is 60~100 r / min; The wet mixing speed in step (S2) is 100~160 r / min; The mixing speed in step (S3) is 80~120 r / min; The vibration time in step (S4) is preferably 30~60s.

8. The preparation method according to claim 5, characterized in that, The cement mentioned in step (1) is ordinary Portland cement; The mineral admixture is selected from any one or more of slag powder, fly ash, silica fume, and nano silica; The water-to-binder ratio is 0.30~0.45; The fineness modulus of the natural sand is 2.4~2.7; The natural coarse aggregate is continuously graded crushed stone with a particle size range of 5~20mm; The water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent.

9. The preparation method according to claim 5, characterized in that, In step (2), the parameters of the carbonization chamber in the pre-permeation stage are set as follows: the volume fraction of CO2 in the carbonization chamber is 60%~80%, the pressure is 0.2~0.5MPa, the temperature is 25℃~40℃, the relative humidity is 60%~80%, and the duration is 12~24h. The parameters of the carbonization chamber during the pulse-enhanced phase were set as follows: CO2 volume fraction in the carbonization chamber was 60%–90%, temperature was 25℃–40℃, and the pressure in the carbonization chamber was cyclically changed 3–5 times within the range of 0.5–1.2 MPa. Each pressure cycle included: increasing the pressure from a low-pressure platform to at least one high-pressure platform at a pressurization rate of 0.05–0.10 MPa / min, and holding the pressure at each high-pressure platform for 2.5–3 hours; then decreasing the pressure back to the low-pressure platform at a depressurization rate of 0.05–0.10 MPa / min and holding the pressure for 2.5–3 hours. The entire process of "pressurization-high-pressure holding-depressurization-low-pressure holding" was recorded as one pressure cycle. The low-pressure platform pressure was 0.3–0.5 MPa, and the high-pressure platform pressure was 0.6–1.2 MPa. The total duration of the pulse-enhanced phase was 24–36 hours. The parameters of the carbonization chamber during the stabilization post-treatment stage were set as follows: after the pulse strengthening stage was completed, the pressure was slowly released to atmospheric pressure at a rate of 0.02 to 0.05 MPa / min, and the specimen was placed in an environment of 60℃ to 80℃ to dry for 24 to 48 hours.

10. A recycled concrete prepared by the method described in claims 5-9, characterized in that, It includes the following raw material components in the range of parts by weight: 300-450 parts cement 500-750 parts natural sand 600-900 parts of natural coarse aggregate, 50-300 parts of recycled aggregate from modified decommissioned wind turbine blades. 140-200 parts water 20-80 parts of mineral admixtures 5-15 parts of water-reducing agent.

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