Recycled PET aggregate high-ductility cement-based composite material and preparation method thereof

Abstract

The invention discloses a recycled PET aggregate high-ductility cement-based composite material and a preparation method thereof, and belongs to the technical field of concrete. Natural fine aggregate in the cement-based composite material is replaced by recycled PET aggregate with the volume replacement rate of 100%, the water-binder ratio of water to a binding material of the cement-based composite material is 0.21-0.39, and meanwhile, the particle size of the PET aggregate is 72.5-267.0 microns. Experiments prove that more crack initiation points can be formed in an SHCC matrix by adopting PET aggregate to completely replace quartz sand, so that the ultimate tensile strain and crack control capability of the SHCC are remarkably improved, and the performance optimization of the SHCC on the synergistic effect of the matrix strength and the PET particle size is explored. A new thought is provided for performance optimization of SHCC, development of green and environment-friendly concrete materials is facilitated, and reference value is provided for special engineering application.

Description

Technical Field

[0001] The present invention belongs to the technical field of concrete, and particularly relates to a high-ductility cementitious composite material with fully recycled PET aggregates and a preparation method thereof. Background Art

[0002] Concrete is the main material in the construction industry and is facing severe sustainability and performance challenges. First, the traditional concrete production process is accompanied by high resource consumption (80-120 million tons of natural aggregates are consumed globally every year) and high carbon emissions (8% of the global carbon emissions come only from the cement for producing concrete). Second, the inherent brittle characteristics of concrete make it prone to sudden cracking and rapid failure under dynamic loads (such as earthquakes and explosions), threatening the structural safety. In response, Engineered Cementitious Composites (ECC for short) was invented in the early 1990s. ECC exhibits unique multiple-crack cracking and strain-hardening behaviors during the tensile process, and its ultimate tensile strain reaches 300-500 times that of ordinary concrete. Therefore, it is also called Strain-Hardening Cementitious Composites (SHCC). In addition, SHCC has a powerful crack control ability, and its crack width is usually less than 100 μm, which results in its impermeability performance in the cracked state being equivalent to that of sound SHCC. Based on micromechanics and fracture mechanics, the strain-hardening performance and crack control ability of SHCC can be achieved by maintaining a low matrix fracture toughness, and introducing artificial defects (such as recycled fine aggregates and artificial aggregates) has been proven to be an effective method to achieve this purpose, which directly reduces the demand for the exploitation of natural aggregates.

[0003] At present, in response to global environmental protection policies, some studies have used polyethylene terephthalate (PET) aggregates to replace natural aggregates in concrete. PET is the fourth-largest polymer produced in the world, with nearly 70 million tons produced globally each year. As a raw material for plastic bottles, textile fibers, etc., PET waste is difficult to biodegrade, and improper incineration will cause serious environmental pollution. At present, the PET plastic recovery rate in most countries is less than 30%. Some researchers have carried out research on the recycling methods of waste PET plastic products and proposed methods such as enzymatic degradation and chemical recycling to achieve low-carbon recycling of PET. However, the catalytic efficiency of enzymatic degradation is low, and chemical recycling methods require high-temperature and high-pressure conditions, making it difficult to promote these methods on a large scale. In this context, recycling PET waste to prepare artificial aggregates to replace natural aggregates involves relatively low process difficulty, while promoting the recycling of PET waste and reducing the demand for natural aggregates. Research has found that using artificial aggregates such as PET and rubber to replace natural aggregates can form more crack initiation points in the matrix of SHCC, thus significantly improving the ultimate tensile strain and crack control ability of SHCC. Preparing SHCC entirely with PET aggregates may have greater deformation and crack control ability, but there is currently no relevant research report.

[0004] The size and shape of PET play an important role in concrete. Existing studies have shown that differences in the size and apparent shape of PET aggregates can affect the slump of fresh concrete mixtures and ultimately change the mechanical properties. In addition, studies have also shown that when the volume ratio and particle size of PET increase, the compressive strength, splitting tensile strength, and elastic modulus of concrete decrease. Similarly, the mechanical properties of SHCC materials are also affected by the aggregate size. Studies have also shown that by using three different particle sizes of PET (21μm, 107μm, 244μm) in SHCC, at a PET aggregate volume replacement rate of 15%, it is found that appropriately increasing the PET aggregate particle size is beneficial to enhancing the crack control ability of SHCC. However, although introducing 15% PET aggregate can be regarded as artificial defects to improve the deformation ability of SHCC, in the case of high PET replacement of QP, it will lead to an increase in the porosity of the specimen, and due to the low strength and low elastic modulus of PET itself, SHCC with 100% PET aggregate may affect the performance due to stress concentration of PET defects and the increase in porosity. At the same time, when the PET particle size is greater than 244μm, the too large defect size will also cause stress concentration to affect the performance. In addition, the matrix strength also significantly affects the mechanical properties of SHCC. Studies have shown that when the w / b is reduced from 0.32 to 0.14, the compressive strength of SHCC increases from 43MPa to 115MPa, and with the increase in compressive strength, the tensile strength increases from 6.17MPa to 16.46MPa, while the ultimate strain capacity decreases from 10.64% to 8.00%. The higher matrix strength, due to its denser microstructure, improves the matrix strength and the bonding performance between the matrix and the fibers, while the lower matrix strength may lead to smaller matrix toughness, thereby optimizing the multi-crack cracking behavior to enhance the strain. However, for SHCC with all PET aggregates, due to the existence of a large number of matrix defects, it may alleviate the increase in matrix toughness caused by the reduction of the water-binder ratio in a high-strength matrix and may lead to extremely low matrix toughness in a low-strength matrix. Summary of the Invention

[0005] Considering the above technical problems, the present invention proposes a recycled PET aggregate high-ductility cement-based composite material and a preparation method thereof.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] One of the objects of the present invention is to provide a recycled PET aggregate high-ductility cement-based composite material, which replaces the natural fine aggregate in the cement-based composite material with a 100% volume replacement rate of recycled PET aggregate.

[0008] Furthermore, the natural fine aggregate is quartz sand (QP), and the particle size of the quartz sand is 120.0μm.

[0009] Further, the particle size of the recycled PET aggregate is 72.5 - 267.0 μm.

[0010] Further, the water - binder ratio of the recycled PET aggregate high - ductility cement - based composite material is 0.21 - 0.39.

[0011] Further, by unit volume mass, the cement - based composite material is composed of the following raw materials: 937 kg / m 3 of cement, 401 kg / m 3 of fly ash (FA), 281 - 522 kg / m 3 of water, 195 kg / m 3 of PET powder, 0 - 0.8 kg / m 3 of thickening agent (VMA), 0 - 2 kg / m 3 of defoaming agent (DF), 4 - 19.6 kg / m 3 of high - range water - reducing agent (HRWRA), and 19.4 kg / m 3 of 18 / 24 polyethylene (PE) fiber.

[0012] Further, the cement is PII.52.5R ordinary Portland cement.

[0013] Further, the fly ash is F - class fly ash.

[0014] Further, the polyethylene fiber has a length of 18 mm, a diameter of 24 μm, an elastic modulus of 116 GPa, a tensile strength of 3000 MPa, and a density of 0.97 g / cm 3 , and an elongation at break of 3%.

[0015] The second object of the present invention is to provide a preparation method of a recycled PET aggregate high - ductility cement - based composite material, including the following steps: weighing cement, fly ash, PET powder, thickening agent, defoaming agent, high - range water - reducing agent, and polyethylene fiber, adding them into water, and mixing evenly to obtain the recycled PET aggregate high - ductility cement - based composite material.

[0016] Further, the preparation method of the cement - based composite material includes the following steps:

[0017] Mix cement, fly ash, PET powder, thickening agent, and defoaming agent evenly to obtain a mixture;

[0018] Add water and high - range water - reducing agent to the mixture, stir evenly to obtain a slurry;

[0019] Add polyethylene fiber to the slurry and stir to obtain the fresh paste, which is the recycled PET aggregate high - ductility cement - based composite material.

[0020] The third object of the present invention is to provide an application of recycled PET aggregate high-ductility cementitious composites in the field of strain-hardening cementitious composites.

[0021] Compared with the prior art, the present invention has the following advantages and technical effects:

[0022] At an extremely low water-binder ratio, PET replaces quartz sand as a matrix defect to introduce SHCC, thereby alleviating the increase in matrix toughness caused by the decrease in the water-binder ratio. At 0.21 w / b, P-SHCC exhibits the best axial tensile performance. The ultimate tensile strain of the PET-M group with a similar particle size to quartz sand increases by 124.2%, the crack width decreases by 45.0%, the number of cracks increases by 152.2%, while the tensile strength only decreases by 38.9%. In addition, the particle size of PET aggregate has a significant impact on the uniaxial tensile performance of P-SHCC. At all three w / b values, it is shown that the smaller the PET particle size, the higher the ultimate tensile strength and the initial cracking strength. The w / b and PET particle size show a synergistic effect on the tensile ultimate strain. At 0.21 w / b, as the particle size increases, the ultimate tensile strain of P-SHCC becomes larger. The ultimate tensile strain of PET-L is 13.7% higher than that of PET-S. Conversely, it is the opposite at a high w / b. The results of the present invention prove the feasibility of all-PET aggregate SHCC, reveal the influence of PET particle size and matrix strength on the performance of P-SHCC, and provide new ideas for the performance optimization of SHCC. Description of the Drawings

[0023] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0024] Figure 1 is the particle size distribution of different raw materials;

[0025] Figure 2 are the raw materials for preparing P-SHCC in Example 1 and their SEM images;

[0026] Figure 3 is the process flow chart for preparing P-SHCC in Example 1;

[0027] Figure 4 is the test device for uniaxial compression test;

[0028] Figure 5 is the test device for uniaxial tensile test;

[0029] Figure 6 is the fluidity difference diagram for different PET particle size groups and different w / b gradients;

[0030] Figure 7Typical axial compressive failure morphologies of specimens in different PET particle size groups and different w / b gradients;

[0031] Figure 8 Crack widths of PET-M materials with different water-binder ratios;

[0032] Figure 9 Crack widths and spalling behaviors of PET with different particle sizes under a constant water-binder ratio;

[0033] Figure 10 Uniaxial compressive strengths of specimens in different PET particle size groups and different w / b gradients;

[0034] Figure 11 Elastic moduli of specimens in different PET particle size groups and different w / b gradients;

[0035] Figure 12 Microstructural images of different PET particle size groups under 0.30 w / b;

[0036] Figure 13 Failure modes of P-SHCC;

[0037] Figure 14 Stress-strain curves obtained from axial tensile tests;

[0038] Figure 15 SEM images of different PET particle sizes intercepting cracks;

[0039] Figure 16 Influences of water-binder ratio and PET particle size on initial cracking strength, ultimate tensile strength and ultimate tensile strain;

[0040] Figure 17 Energy dispersive spectroscopy (EDS) line scan maps of the interfacial transition zone (ITZ) between aggregate (quartz sand) and matrix in P-SHCC;

[0041] Figure 18 Energy dispersive spectroscopy (EDS) line scan maps of the interfacial transition zone (ITZ) between aggregate (PET) and matrix in P-SHCC. Detailed implementation manners

[0042] Now, various exemplary implementation manners of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and implementation schemes of the present invention.

[0043] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0044] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0045] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the present invention specification are obvious to those skilled in the art. The present invention specification and examples are merely exemplary.

[0046] Regarding the use of "comprising", "including", "having", "containing", etc. in this article, they are all open-ended terms, meaning including but not limited to.

[0047] All raw materials used in the present invention are obtained by purchasing on the market.

[0048] The technical solutions of the present invention are further described below through examples.

[0049] In order to study the influence of PET aggregate particle size on the properties of the composite material, three different sizes of PET particles were selected in the present invention. Through a laser particle size analyzer, PET-S (72.5 μm), PET-M (169.0 μm), and PET-L (267.0 μm) were measured. The PET sizes are shown in Table 1 below, and the particle size distribution is shown in Figure 1 . The elastic modulus of the PET aggregate is 4 GPa. Based on JGJ 52 - 2006 "Standard for Quality and Inspection Methods of Sands and Gravels for Ordinary Concrete", its apparent density is measured as 1.29 g / cm 3 , and the apparent density differences of PET aggregates with different particle sizes are not significant.

[0050] Table 1 PET Particle Size Distribution (unit: μm)

[0051] Particle size Dv(10) Dv(50) Dv(90) PET-S 47.5 72.5 110 PET-M 111 169 250 PET-L 165 267 395

[0052] The raw materials for preparing all-PET aggregate SHCC (P-SHCC) include PII.52.5R ordinary Portland cement, class F fly ash (FA), water, PET plastic powder, quartz sand (QP), thickening agent (VMA), defoaming agent (DF), high-range water reducer (HRWRA), and PE fiber with a length-to-diameter ratio of 750.

[0053] Figure 2 For the raw materials and SEM images of P-SHCC prepared in Example 1, where the average particle size of QP is 120 μm and the density is 2.68 g / cm 3 , and the elastic modulus is 76 GPa.

[0054] Table 2 shows the material parameters of quartz sand and PET particles, and Table 3 shows the performance parameters of PE fiber.

[0055] Table 2 Physical and mechanical properties of quartz sand and PET

[0056]

[0057] Table 3 Physical and mechanical properties of polyethylene fiber

[0058]

[0059] Example 1

[0060] In the present invention, three different w / b mix ratios are set by changing the water-binder ratio. In addition, the polyethylene fiber is 2% of the concrete volume, which is a widely used fiber volume content. The optimal mix ratio obtained through multiple trials is to ensure the uniform dispersion of PE fiber and good workability of the paste, as shown in Table 4.

[0061] Table 4 Mixing ratio of raw materials in P-SHCC (unit: kg / m 3 )

[0062]

[0063] A preparation method for recycled PET aggregate high-ductility cement-based composite material ( Figure 3The process flow chart for preparing P-SHCC in Example 1 is as follows: (1) Add the gelling material, quartz sand or PET powder, thickening agent, and defoaming agent to a blender and stir at a low speed (75 rpm) for 2 - 3 minutes to mix the dry materials evenly. (2) During the 1-minute low-speed (75 rpm) stirring, add the pre-mixed water and water reducer, then stir at a low speed (75 rpm) for 2 minutes, and then stir at a medium speed (135 rpm) for 1 minute to make it into a uniform slurry. (3) Adjust to low-speed (75 rpm) stirring, add the PE fibers in small amounts and multiple times within 2 minutes, then stir at a low speed (75 rpm) for 2 minutes, and then stir at a medium speed (135 rpm) for 2 - 3 minutes and at a high speed (285 rpm) for 1 minute to completely disperse the fibers to obtain the P-SHCC slurry. The entire stirring process takes about 13 - 15 minutes.

[0064] Pour the freshly stirred P-SHCC slurry into a mold and conduct a fluidity test respectively. When pouring into the mold, place the mold on a vibrating table and vibrate it to reduce the air bubbles inside the specimen and ensure the specimen is dense and uniform. Cover the surface of the mold with a polyethylene film to seal and prevent water evaporation. After curing the specimen in a room-temperature environment for 24 hours, demold it, and then soak it in water in a sealed environment for 28 days before conducting the experiment.

[0065] Performance Test

[0066] 1. Fluidity Test: To evaluate the fluidity of all slurries, the present invention conducts a slump test in accordance with ASTM C1437. Use a frustum-shaped mold with an upper diameter of 70 mm, a lower diameter of 100 mm, and a depth of 50 mm, and the mold wall thickness exceeds 5 mm. After completing the experiment, use a caliper to measure the two orthogonal diameters of the bottom surface of the slurry, and take their average value to quantify the flow diameter of P-SHCC, and calculate its fluidity according to the following calculation formula.

[0067]

[0068] Among them, Flow is the fluidity, with the unit of %; D is the flow diameter, with the unit of mm.

[0069] 2. Uniaxial Compression Test: The axial compression test is carried out in accordance with the ASTM C46920 - 20 standard. The specimen is a cylindrical body. An electro-hydraulic servo press is used for the test, and the test device is as shown in Figure 4 . To avoid eccentric loading during the loading process due to the uneven surface of the specimen, apply a layer of high-strength gypsum evenly on the top and bottom of the specimen to level the surface before loading. The specimen is under displacement control mode and is loaded at a loading rate of 0.2 mm / min until failure. During the loading process, two 20-mm strain gauges are respectively arranged in the axial direction of the specimen, and two LVDTs are arranged at axially symmetric positions to monitor the axial deformation.

[0070] 3. Uniaxial Tensile Test: The test specimens were dog-bone shaped with dimensions of 330 mm × 60 mm × 13 mm, as shown in Figure 5 . The axial tensile test was carried out using a microcomputer-controlled electronic universal testing machine for loading. To avoid eccentric loading, universal joints were installed at both ends of the fixture. The axial tensile test loading mode adopted a displacement control mode, and the loading rate was set at 0.5 mm / min. Two displacement gauges with a measuring range of 25 mm were used to measure the axial deformation within the 80 mm region in the middle of the specimen, with a frequency of 1 Hz. The load data was collected by the stress sensor of the universal testing machine, and the deformation data was collected by a high-performance static data acquisition instrument. At the same time, in order to observe the morphology and distribution of cracks during the loading process, white paint was applied to the surface of the dog-bone in advance for observation. During the test, a high-definition camera was used to take pictures at a frequency of 0.1 Hz, and characteristic pictures were selected for binarization processing. The scale was set through software to determine the actual size of each pixel point, thereby obtaining data on the number and width of crack development.

[0071] 4. SEM / EDS Test: A Phenom electron microscope was used to conduct scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) tests on the raw materials and P-SHCC specimens. The samples were cubic samples with a size not greater than 5 mm prepared by cutting. After drying, they were fixed on the sample stage using double-sided conductive adhesive and vacuum sputter-coated with gold to improve conductivity. The energy dispersive spectrometer (EDS) analysis can identify the positions and chemical compositions of different aggregates. Before conducting the EDS measurement, the specimens were polished, the working voltage was set at 15 kV, and a backscattered electron detector (BSD) was used.

[0072] Test Results

[0073] 1. Fluidity

[0074] The tensile strain hardening behavior and the magnitude of the tensile strain capacity of SHCC are closely related to the uniformity of fiber dispersion, and the fiber dispersion is closely related to the fluidity of the fresh SHCC slurry. According to existing research, the flow diameter of SHCC is approximately 100 - 170 mm. Excellent fluidity is the key to ensuring the mechanical properties of SHCC.

[0075] Figure 6 ​Shows the differences in fluidity of P-SHCC under different variable controls. The fluidity is calculated according to the specification (Formula 1) as shown in Table 5. In the PET group, the PET-M group with a similar particle size to the QP group showed the best fluidity at all water-binder ratios. The fluidities of 0.21-P-M, 0.30-P-M, and 0.39-P-M reached 75%, 79%, and 78% respectively. While the fluidity of PET-S was significantly poorer, and the 0.21-P-S, 0.30-P-S, and 0.39-P-S groups only reached 52%, 68%, and 60% respectively. Since PET is a hydrophobic material, the water during the mixing process is not adsorbed by PET but forms a water film on the surface. At the same time, the particle size of PET aggregate is inversely proportional to its specific surface area. The increase in specific surface area (decrease in particle size) will lead to an increase in the water film wrapping the aggregate, which has a negative impact on the fluidity of the paste. In addition, compared with PET-M, 0.21-P-L, 0.30-P-L, and 0.39-P-L also showed poorer fluidity, which were 73%, 65%, and 68% respectively. Introducing aggregates with a particle size larger than the average fiber spacing will cause fiber-aggregate interactions to form aggregates. The increase in the particle size of the aggregates makes it more difficult to achieve uniform dispersion of the fibers. Generally speaking, the larger the size of the aggregate particles, the more fiber caking and interactions there are. For the PET-L group, the PET particle size may exceed the average fiber spacing, reducing the fluidity of P-SHCC. In addition, comparing the PET group with the QP group, it was found that the fluidity of the PET group decreased.

[0076] Table 5 Fluidity of P-SHCC

[0077] Group Flowability (%) 0.21-P-S 52 0.21-P-M 75 0.21-P-L 73 0.21-Q 80 0.30-P-S 68 0.30-P-M 79 0.30-P-L 65 0.30-Q 91 0.39-P-S 60 0.39-P-M 78 0.39-P-L 68 0.39-Q 70

[0078] 2. Influence of PET Particle Size and Water-Binder Ratio on Compressive Behavior

[0079] 1) Failure Mode

[0080] Figure 7 Shows the typical axial compressive failure morphologies of each group of specimens. In each group of cylindrical specimens, it was observed that the main inclined cracks penetrated the cylinder. Compared with the SHCC of the QP group, the introduction of PET aggregate significantly changed the failure mode. The number of cracks in P-SHCC was more, showing a more significant ductile failure characteristic. This is because the elastic modulus of PET is lower, and its introduction enables P-SHCC to generate more cracks and deformations under axial compression. Secondly, as Figure 9, it was observed that an increase in the particle size of PET aggregates would lead to an increase in the crack width of cylindrical specimens. In the groups of coarser aggregate sizes (0.30 - P - M and 0.30 - P - L), spalling even occurred. Conversely, the 0.30 - P - S specimens were relatively intact. This is because the introduction of large - sized aggregates increases some large - sized holes or defects in the concrete structure. When the specimens are subjected to compressive stress, stress concentration occurs at these positions and relatively wide cracks quickly develop. The appearance of wide cracks further intensifies the stress concentration phenomenon, thus further expanding the cracks at these positions.

[0081] In addition, an increase in the water - binder ratio will result in wider cracks and more obvious spalling. Taking PET - M with a particle size similar to that of sand as an example, the cracks are magnified as Figure 8 . This is because a smaller water - binder ratio will lead to an increase in the density of the cementitious matrix and ITZ, thus producing a more compact and uniform microstructure.

[0082] 2) Compressive strength and elastic modulus

[0083] As Figure 10 , Figure 11 and Table 6 show the axial compressive strength and elastic modulus of P - SHCC (calculated according to ASTM - C469, 2020). It can be seen that as the w / b ratio decreases from 0.39 to 0.21, the compressive strength of P - SHCC samples increases under all PET particle - size replacements. This is because the decrease in the w / b ratio increases the density of the interfacial zone, making the microstructure denser and more uniform, thus improving the compressive strength. Replacing QP with PET results in an obvious decrease in compressive strength. For example, compared with 0.21 - Q, 0.30 - Q, 0.39 - Q, the compressive strengths of 0.21 - P - M, 0.30 - P - M, 0.39 - P - M decrease by 36.5%, 26.4%, and 29.6% respectively. This can be attributed to the difference in physical properties between PET materials and QP. The strength and elastic modulus of PET aggregates are much lower than those of QP, becoming a weak link in stress transfer. Secondly, from the morphology of the raw materials, PET tends to be spherical, while QP is multi - angular. Higher roundness and sphericity lead to a weaker interfacial transition zone. In addition, different from QP, PET aggregates have weak bonding with cement paste due to their hydrophobicity. The above factors lead to a larger interfacial transition zone between PET particles and the matrix. Therefore, introducing PET as an artificial defect reduces the compressive strength of the material.

[0084] Meanwhile, it is noted that when the w / b is 0.21, the loss of compressive strength caused by completely replacing QP with PET is significantly greater than that at 0.30 w / b and 0.39 w / b. The w / b is inversely proportional to the mortar viscosity, that is, the mortar viscosity decreases with the increase of w / b. The significant increase in the plastic viscosity of SHCC mortar under low w / b conditions will lead to improved fiber dispersion, but too high plastic viscosity may exacerbate the air entrainment phenomenon, the discharge of internal bubbles in the mortar is blocked, and finally pore defects are formed, reducing the compressive strength of the material. In addition, research shows that the w / b ratio should be limited to about 0.2 to 0.3, so as to achieve a high viscosity with an ideal fiber distribution. Beyond this range, w / b will lead to uneven fiber dispersion. And 0.39 w / b will result in a lower plastic viscosity of the mortar, uneven fiber dispersion, and a decline in mechanical properties due to its too high water-binder ratio.

[0085] In the PET group, there is little difference in the compressive strength between 0.21-P-S, 0.30-P-S, 0.39-P-L and 0.21-P-M, 0.30-P-M, 0.39-P-M. When the PET particle size increases to PET-L, the compressive strength of 0.21-P-L, 0.30-P-L, 0.39-P-L compared with 0.21-P-M, 0.30-P-M, 0.39-P-M decreased by 9.9%, 11.3%, and 13.7% respectively. From Figure 12 It can be seen that the bonding performance between PET aggregates with larger particle sizes and the matrix is poor. At the same time, the use of large particle size aggregates may lead to a higher porosity, and the compressive strength of the sample increases with the decrease of porosity. These factors will all lead to the weakening of the interfacial transition zone between PET aggregates and the matrix or the increase of internal porosity, thereby resulting in a decrease in the compressive strength of the material.

[0086] Figure 11 The elastic modulus of P-SHCC is presented. The change law of the material elastic modulus is similar to that of the compressive strength. The water-binder ratio and the properties of aggregates are two important factors determining the elastic modulus of concrete. The deformation part generated in the concrete is related to the elastic deformation of the aggregates, so the aggregate stiffness determines the elastic modulus of the concrete. Since the stiffness and elastic modulus of PET are much lower than those of natural aggregate QP, the elastic modulus of the PET group will be lower than that of the QP group. The elastic modulus of 0.21-P-M, 0.30-P-M, 0.39-P-M is 41.0%, 34.4%, and 38.6% lower than that of 0.21-Q, 0.30-Q, 0.39-Q respectively. In addition, the elastic modulus decreases with the increase of w / b, which may be related to the porosity. The larger the w / b, the higher the porosity of the concrete, resulting in a lower elastic modulus.

[0087] Table 6 Compressive test results of P-SHCC

[0088] Group Compressive strength (MPa) Elastic modulus (GPa) 0.21-P-S 65.6(5.60) 16.2(2.36) 0.21-P-M 67.9(2.36) 14.8(1.14) 0.21-P-L 61.2(2.64) 14.9(0.48) 0.21-Q 107.0(0.59) 25.1(0.67) 0.30-P-S 57.7(1.13) 13.0(2.53) 0.30-P-M 60.1(1.43) 11.8(1.25) 0.30-P-L 53.2(3.42) 12.1(2.05) 0.30-Q 81.7(4.65) 18.0(1.07) 0.39-P-S 42.0(1.01) 9.9(1.74) 0.39-P-M 40.2(2.21) 7.8(0.16) 0.39-P-L 34.7(1.37) 7.3(0.35) 0.39-Q 57.1(3.61) 12.7(0.65)

[0089] 3. Influence of PET Particle Size and Water-Binder Ratio on Uniaxial Tensile Behavior

[0090] 1) Uniaxial Tensile Failure Mode

[0091] In order to observe the crack development law of P-SHCC under uniaxial tensile load, the crack distribution images of P-SHCC at different strain rates were binarized, and the crack development morphologies of P-SHCC at different strain rates were obtained as Figure 13 shown. It can be seen that under all mix ratios, P-SHCC has the characteristic of typical multi-microcrack development. In order to quantitatively analyze the crack distribution morphologies of P-SHCC under different mix ratios, a 70-mm length region in the center part of the specimen was taken to calculate the crack density and crack width. The statistics of the crack density and crack width in Figure 13 are shown in Table 7.

[0092] Table 7 Tensile Test Results of P-SHCC

[0093]

[0094]

[0095] It can be seen from Table 7 that the crack widths of 0.21-Q, 0.30-Q, and 0.39-Q are 154.63 μm, 129.31 μm, and 102.88 μm respectively. The crack width decreases with the increase of w / b, indicating that the QP group shows stronger crack control ability at higher w / b. Because the low w / b ratio reduces the porosity and densifies the matrix, increasing the bonding force between the fiber and the matrix, resulting in higher cracking stress and enlarged crack width. In addition, compared with the QP group, the PET group shows excellent crack control ability. The crack widths of 0.21-P-M, 0.30-P-M, and 0.39-P-M are reduced by 45.0%, 33.1%, and 7.8% respectively compared with 0.21-Q, 0.30-Q, and 0.39-Q, and the number of cracks increases by 152.1%, 109.1%, and 14.8%, proving that PET improves the crack control ability of SHCC by inducing multi-crack cracking.

[0096] In addition, compared with 0.30-P-S, 0.30-P-M, and 0.30-P-L, the crack widths of 0.21-P-S, 0.21-P-M, and 0.21-P-L decreased by (12.7%, 1.9%, 12.1%) respectively, and the crack numbers increased by (14.8%, 26.1%, 65.5%) respectively. This indicates that P-SHCC with a lower w / b has more obvious and stable multiple crack behavior. As is well known, the tensile properties of SHCC are mainly customized by adjusting micro-mechanical parameters, especially the fiber-matrix interface characteristics and the matrix of SHCC. Moreover, PET replaces QP as a matrix defect introduced into SHCC, thus reasonably weakening the improvement of matrix toughness caused by the decrease in w / b. It can be inferred from this that in the case of a large number of defects in the matrix, the decrease in w / b will cause a reasonable increase in the fiber-matrix friction strength, a decrease in fiber slip under the same cracking stress, resulting in a decrease in crack spacing and an increase in crack number, and ultimately achieving a higher strain capacity while reducing the crack width.

[0097] At the same w / b, the smaller the aggregate particle size, the more the crack width can be reduced. For example, the crack width of 0.21-P-S (61.1 μm) is nearly 50% less than that of 0.21-P-L (122.2 μm). Since the PET material itself has good ductility and can even be used as fibers in SHCC, this phenomenon may be due to the particle bridging effect of PET. Because smaller-sized PET requires more stretching, resulting in a stronger bridging stress, leading to the phenomenon that the crack width of the group with a smaller PET particle size is finer. At the same time, in all w / b of the PET group, it was observed that as the PET particle size increased, the crack number decreased. The crack numbers of 0.21-P-S, 0.30-P-S, and 0.39-P-S decreased by 29.2%, 86.2%, and 28.6% respectively compared with 0.21-P-L, 0.30-P-L, and 0.39-P-L.

[0098] 2) Uniaxial tensile stress-strain curve

[0099] As Figure 14 are the uniaxial tensile stress-strain curves of each mix ratio. All specimens showed typical strain-hardening behavior under the axial tensile load. In the strain-hardening stage, compared with the control group QP group, the fluctuation amplitude of the tensile curve of the PET group is smaller than that of the QP group at all particle sizes, proving that the introduction of PET reduces the fracture toughness of the matrix, which is also consistent with the conclusion that the PET crack width is smaller than that of the QP group shown in the above crack binarization pictures, indicating the improvement of PET in controlling the tensile stress fluctuation and crack width of P-SHCC.

[0100] It should be noted that the strain-hardening curves of 0.30-P-S and 0.39-P-S exhibit a two-stage crack evolution phenomenon, where the slope of the second stage is greater than that of the first stage. This phenomenon is also known as the supersaturated cracking phenomenon. Due to the presence of PET and initial defects, the matrix between two parallel cracks in P-SHCC is actually non-uniform. After the "inflection point", new cracks initiate in adjacent weak areas and connect with the original crack network, forming a secondary stress plateau. According to the theory of Huang et al., the ideal conditions for achieving supersaturated cracking are to meet a lower matrix cracking strength (to minimize the steady-state crack spacing) and a higher fiber bridging force. The PET-S has the smallest size, and smaller aggregates need to be stretched more, which can make the fiber dispersion more uniform and result in a higher fiber bridging force. As can be seen from Table 8, the ultimate tensile stresses of 0.30-P-S and 0.39-P-S are 9.72 MPa and 7.14 MPa respectively, which are 61.2% and 37.3% higher than those of 0.30-P-M and 0.39-P-M, indicating that they have a higher fiber bridging force. In addition, the initial cracking strength of 0.21-P-S (3.2051 MPa) is too high, which inhibits the connection between secondary cracks and the original cracks, while the initial cracking strengths of 0.30-P-S (2.5299 MPa) and 0.39-P-S (2.0171 MPa) with a smaller matrix strength are more suitable for multiple crack cracking. SHCC with the supersaturated cracking phenomenon is more suitable for engineering applications and can provide better safety for buildings.

[0101] 3) Summary of Tensile Mechanical Properties

[0102] Figure 16 Table 8 shows the axial tensile strength, ultimate tensile strain, and cracking strength of P-SHCC under different mix ratios, and the data results are the average values of three specimens.

[0103] Table 8 Summary of Tensile Mechanical Properties

[0104]

[0105]

[0106] By Figure 16As can be seen from (a) in [reference], after replacing the PET aggregate, the initial cracking strength of P-SHCC decreases to varying degrees. Comparing with PET-M with a similar particle size to QP, the initial cracking strength of 0.21-P-M, 0.30-P-M, and 0.39-P-M decreases by (51.1%, 28.7%, 14.6%) respectively compared with 0.21-Q, 0.30-Q, and 0.39-Q. As is well known, the reduction of the initial cracking strength is beneficial to the multiple cracking behavior of SHCC. Therefore, the application of PET aggregate can effectively promote the generation of cracks in P-SHCC, providing a basic condition for the improvement of ductility. At the same time, in the PET group, the smaller the particle size, the higher the initial cracking strength. The initial cracking strength of 0.21-P-S, 0.30-P-S, and 0.39-P-S increases by 47.9%, 19.9%, and 38.4% respectively compared with 0.21-P-L, 0.30-P-L, and 0.39-P-L. According to fracture mechanics, the initial cracking strength is related to the maximum defect size and the matrix fracture toughness. The formula is as follows:

[0107]

[0108] where α is the maximum defect size, with the unit of m; K m is the fracture toughness, with the unit of Y is a dimensionless constant, and larger-sized PET aggregates mean larger defect sizes, resulting in lower initial cracking strength.

[0109] From Figure 16 As can be seen from (b) in [reference], compared with the QP group, the ultimate tensile strength of the PET group shows a downward trend. The ultimate tensile strength of 0.21-P-M, 0.30-P-M, and 0.39-P-M decreases by 21.6%, 38.9%, and 9.7% respectively compared with 0.21-Q, 0.30-Q, and 0.39-Q. The final failure of SHCC and the related ultimate tensile strength are determined by the fiber bridging strength, rather than the maximum defect size. The fiber bridging ability is determined by three aspects: fiber strength, fiber content, and fiber-matrix bonding. PE fibers are hydrophobic, and the fiber-matrix bonding is only maintained by friction. The strength and elastic modulus of PET are very low. After introducing defects, the matrix strength of P-SHCC decreases, and the weaker matrix leads to a decrease in the friction force between the matrix and the fibers. In addition, the low fiber bridging ability is not only attributed to the low interfacial bonding between the fiber and the matrix but also to the uneven distribution of fibers. After fully replacing QP with PET, due to the decrease in fluidity, the fiber distribution will be uneven, reducing the fiber bridging ability.

[0110] In addition, when PET aggregates with different particle sizes are used, as the particle size increases, the ultimate tensile strength of P-SHCC gradually decreases. The ultimate tensile strength of 0.21-P-L, 0.30-P-L, and 0.39-P-L decreases by 9.7%, 42.6%, and 49.3% respectively compared to 0.21-P-S, 0.30-P-S, and 0.39-P-S.

[0111] As can be seen from (c) in Figure 16 , compared with PET-M with similar particle sizes to QP, the ultimate tensile strains of 0.21-P-M, 0.30-P-M, and 0.39-P-M increase by 124.2%, 0.3%, and 33.0% respectively compared to 0.21-Q, 0.30-Q, and 0.39-Q. The increase in the ultimate tensile strain is very obvious at 0.21 w / b. PET replaces QP as the matrix defect introduced into SHCC, thus alleviating the increase in matrix toughness caused by the decrease in the water-binder ratio. However, due to their excessive water-binder ratios at 0.30 w / b and 0.39 w / b, the matrix defects are very large, and the introduction of PET is not obvious in reducing their matrix defects, resulting in an insignificant increase in the ultimate tensile strain compared to the QP group.

[0112] In terms of particle size performance, at 0.21 w / b, the ultimate tensile strain increases with the increase in particle size. The ultimate tensile strain of 0.21-P-L increases by 13.7% compared to 0.21-P-S. At 0.39 w / b, the ultimate tensile strain decreases with the increase in particle size. The ultimate tensile strain of 0.39-P-L decreases by 25.0% compared to 0.39-P-S. The ultimate tensile strain shows different laws under different w / b.

[0113] In the high-strength matrix (0.21 w / b), the matrix toughness is relatively high. Introducing PET with a larger particle size means introducing larger matrix defects. In addition, because PET is more spherical while QP is multi-angular, the bonding performance between PET and the matrix is poorer, and the interfacial transition zone (ITZ) between PET and the matrix is larger, resulting in SHCC being more prone to cracking, which will improve the ductility of SHCC. At 0.39 w / b, according to a large number of research reports, cracks usually propagate along the aggregate / matrix interface in normal-strength concrete. In high-strength matrices, cracks usually propagate through the aggregates in high-strength concrete rather than along the aggregate / matrix interface. Incorporating larger aggregates into low-strength SHCC may therefore increase the tortuosity of the crack path, so more energy is required to drive crack propagation. In low-strength matrices, PET particles will change the crack path. Crack deflection is observed in the area where PET particles intercept the crack plane and is marked with white circles in the electron microscope images. When the crack propagates through PET, the originally stable crack is split and deflected to change the original propagation path ( Figure 15)。Since the matrix strength of 0.39 w / b is relatively low, and due to the smaller particle size of PET-S, it is more densely and uniformly distributed in P-SHCC, and the crack deflection effect is more obvious. The deflection and bifurcation of these cracks contribute to the ductility of SHCC and its crack width control ability. At the same time, the fiber supersaturation phenomenon in the 0.39-P-S group can also confirm this point. Supersaturated SHCC will have many intersecting and bifurcating cracks, and at the same time, it has a stronger crack width control ability, so the smaller the PET particle size in 0.39 w / b, the higher the strain. For the low water-binder ratio of 0.21, due to its high-strength matrix, cracks may develop through the PET aggregates.

[0114] 4. Influence of PET Particle Size and Water-Binder Ratio on the Microstructure of P-SHCC

[0115] 1) Microstructure of P-SHCC with Different Mix Proportions

[0116] Figure 17 、 Figure 18 shows the energy-dispersive spectroscopy line scan (EDS) of the interfacial transition zone (ITZ) between the aggregate and the matrix in P-SHCC under a scanning electron microscope (SEM). Among them, Figure 17 are the data obtained by performing EDS line scanning along the blue arrow direction on cube samples with a size not greater than 5 mm prepared by cutting the non-tensile regions (crack-free regions) of three dog-bone specimens of 0.21-Q, 0.30-Q, and 0.39-Q under the electron microscope. Figure 18 are obtained by performing the same operation on the dog-bone specimens of 0.30-P-S, 0.30-P-M, and 0.30-P-L. As Figure 17 shown, with the increase of w / b, the width of the ITZ slightly increases, indicating that with the increase of w / b, the matrix structure becomes looser, and a low w / b can produce a dense matrix, enhancing the bonding force between the matrix and the aggregate. Figure 18 reflects the ITZ width of P-SHCC under different particle sizes of PET. It can be seen that wider aggregates will result in a larger ITZ width, indicating that it is easier to fill the cementitious material around small-sized PET particles. On the contrary, voids are more likely to appear around large-sized PET particles, thus forming a wider ITZ.

[0117] Comparing Figure 17 in (b) with Figure 18In (b), it can be seen that among QP and PET aggregates with the same water-cement ratio and similar particle size, the ITZ of PET aggregate is wider. This can be attributed to the fact that the shape of PET aggregate is spherical, and the number of edges and the elastic modulus are smaller than those of quartz sand. In addition, unlike QP, PET aggregate cannot interact with the slurry due to its hydrophobicity. These factors together weaken the bite friction between PET aggregate and cement matrix, ultimately leading to a wider interface transition zone. At the same time, it is observed that the surface of the QP group is relatively smooth and dense, without too many pores or cracks, indicating that its structure is relatively uniform and stable. The surface of the PET group appears relatively loose, showing more rough and irregular characteristics.

[0118] The present invention proposes a new type of fiber-reinforced cement-based material - all-PET aggregate fiber-reinforced cement-based material (P-SHCC). At the same time, recycled PET aggregates of different particle sizes are used to completely replace QP and mixed into SHCC. In addition, the influence of different matrices on the performance of P-SHCC is explored by changing the water-cement ratio to change different matrix strengths. The various properties of P-SHCC are studied from the macroscopic level to the microscopic level, including: fluidity, compressive properties, tensile properties and microstructure. The influence law and mechanism of PET aggregate particle size and matrix strength on the multiple cracking behavior, tensile strain capacity and aggregate-cement matrix ITZ of P-SHCC are revealed. Based on the above results, the following conclusions are drawn:

[0119] (1) The use of PET aggregate has a negative effect on the fluidity of P-SHCC fresh slurry. As the particle size of PET aggregate increases, the fluidity first increases and then decreases, that is, the fluidity of the slurry with the addition of PET-M aggregate is the highest.

[0120] (2) The introduction of PET has a significant positive effect on the crack development of P-SHCC. The crack widths of 0.21-PM, 0.30-PM, and 0.39-PM decreased by 44.9%, 33.1%, and 7.8% respectively compared with 0.21-Q, 0.30-Q, and 0.39-Q, and the number of cracks increased by 152.1%, 109.1%, and 14.8%. In addition, the 0.30-PS and 0.39-PS groups showed supersaturated cracking. SHCC with supersaturated cracking is more suitable for engineering applications and can provide better safety for buildings. At the same time, it was observed that under all w / b conditions, the number of cracks decreased with the increase of PET particle size. This is because the cracks in the group with larger particle size are extremely easy to over-expand at the large defect position, which causes the specimen to fail prematurely and reduces the crack density.

[0121] (3) Compared with the QP group, the ultimate tensile strain of the PET group has increased. The ultimate tensile strains of 0.21-P-M, 0.30-P-M, and 0.39-P-M have increased by 124.2%, 0.3%, and 33.0% respectively compared with 0.21-Q, 0.30-Q, and 0.39-Q. The strain increase is more obvious at low water-cement ratios. PET replaces QP as the matrix defect introduced into SHCC, thus alleviating the increase in matrix toughness caused by the decrease in water-cement ratio, resulting in the best performance when PET replaces QP at 0.21 w / b.

[0122] (4) At the microscopic level, the interfacial transition zone (ITZ) between PET aggregates and the cement matrix is wider than that of traditional quartz sand and becomes wider as the particle size of PET aggregates increases. Increasing the water-cement ratio also leads to a wider aggregate-matrix ITZ. Appropriate aggregate size and appropriate w / b value result in the maximum increase in the tensile strain capacity of P-SHCC.

[0123] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A high-ductility cement-based composite material for recycling PET aggregates, characterized in that, Replace the natural fine aggregate in the cement-based composite material with recycled PET aggregate at a volume replacement rate of 100%, wherein the water-binder ratio of the cement-based composite material is 0.21 - 0.39; the particle size of the recycled PET aggregate is 72.5 - 267.0 μm.

2. The recycled PET aggregate high-ductility cementitious composite material according to claim 1, wherein The natural fine aggregate is quartz sand.

3. The recycled PET aggregate high-ductility cementitious composite material according to claim 2, wherein The particle size of the quartz sand is 120.0 μm.

4. The recycled PET aggregate high-ductility cementitious composite material according to any one of claims 1-3, characterized in that, By mass per unit volume, it consists of the following raw materials: 937 kg / m of cement 3 , 401 kg / m of fly ash 3 , 195 kg / m of PET powder 3 , 0 - 0.8 kg / m of thickener 3 , 0 - 2 kg / m of defoamer 3 , 4 - 19.6 kg / m of high-range water reducer 3 and 19.4 kg / m of polyethylene fiber 3 .

5. The recycled PET aggregate high-ductility cementitious composite material according to claim 4, wherein The cement is PII.52.5R ordinary Portland cement.

6. The recycled PET aggregate high-ductility cementitious composite material according to claim 4, wherein The fly ash is class F fly ash.

7. The recycled PET aggregate high-ductility cementitious composite material according to claim 4, wherein The polyethylene fiber has a length of 18 mm, a diameter of 24 μm, an elastic modulus of 116 GPa, a tensile strength of 3000 MPa, and a density of 0.97 g / cm 3 , and an elongation at break of 3%.

8. A method for preparing a recycled PET aggregate high-ductility cement-based composite material according to any one of claims 1-7, comprising the following steps: Weigh Cement, fly ash, PET powder, thickener, defoamer, high-range water reducer, and polyethylene fiber are added to water and mixed evenly to obtain the recycled PET aggregate high-ductility cement-based composite material.

9. The preparation method according to claim 8, wherein, It includes the following steps: Mix cement, fly ash, PET powder, thickener, and defoamer evenly to obtain a mixture. Add water and high-range water reducer to the mixture and stir evenly to obtain a slurry. Add polyethylene fiber to the slurry and stir. The resulting fresh paste is the recycled PET aggregate high-ductility cement-based composite material.

10. Application of the recycled PET aggregate high-ductility cement-based composite material according to any one of claims 1 - 7 in the field of strain-hardening cement-based composites.

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