Polymer modified alkali-activated recycled concrete and preparation method thereof

Abstract

The invention discloses polymer modified alkali-activated recycled concrete and a preparation method thereof, and belongs to the technical field of concrete preparation, the polymer modified alkali-activated recycled concrete comprises the following raw materials: fly ash, slag powder, fine aggregate, coarse aggregate, an alkali activator, water, a polymer modifier and a defoaming agent; the polymer modifier is obtained by mixing epoxy resin and a curing agent, and the coarse aggregate comprises natural coarse aggregate and recycled coarse aggregate. According to the invention, fly ash and slag powder are used as precursor materials, a mixed solution of water glass (Na2SiO3 solution) and NaOH is used as an alkali activator, and fine aggregate, natural coarse aggregate and recycled coarse aggregate are matched with epoxy resin, a curing agent and a defoaming agent to prepare the polymer modified alkali-activated recycled concrete (PARAC). The influence of the epoxy resin and the recycled aggregate on the durability of the PARAC is researched by adjusting the mixing amount of the epoxy resin and the replacement rate of the recycled aggregate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of concrete preparation, and particularly relates to a polymer-modified alkali-activated recycled concrete and a preparation method thereof. BACKGROUND

[0002] Alkali-activated cementitious material is an inorganic cementitious material generated by the reaction of alkali activator and silicate raw material (fly ash, slag, metakaolin, limestone, slag, etc.). Its preparation process does not depend on limestone calcination, thus having significant low-carbon environmental protection advantages, and can effectively utilize industrial waste slag, in line with the concept of sustainable development. Compared with ordinary cement concrete, alkali-activated concrete exhibits many advantages, such as excellent mechanical properties, good stability and strength under the action of external forces such as pressure and tension, excellent durability, excellent resistance to complex environments such as dry-wet cycle, chemical corrosion, and temperature change, and less performance degradation. In addition, after a series of processing and treatment of waste concrete, masonry and other building materials, recycled aggregate can be obtained for preparing concrete. By mixing recycled aggregate into alkali-activated concrete, alkali-activated recycled concrete can be prepared. By converting construction waste into recycled aggregate and applying it to alkali-activated concrete, the consumption of natural resources is reduced, the negative impact of construction waste on the environment is reduced, and the recycling of resources and the protection of the environment are realized.

[0003] However, the performance of alkali-activated recycled concrete is generally lower than that of ordinary alkali-activated concrete, especially in terms of compressive strength, crack resistance and durability. Therefore, how to improve the performance of alkali-activated recycled concrete has become one of the current research hotspots. Polymer modification technology as an effective means of enhancing concrete has been widely used in the field of building materials in recent years. By mixing polymers into concrete, the mechanical properties, durability and crack resistance of concrete can be significantly improved. The network structure formed by the polymer in the concrete can effectively fill the pores inside the concrete, improve its compactness, and enhance the durability of the concrete. Among them, epoxy resin as a high-performance polymer has excellent adhesion, chemical corrosion resistance and mechanical strength, and has received widespread attention in the field of concrete modification in recent years. Epoxy resin can significantly improve the mechanical properties and durability of concrete, especially in terms of impermeability, freeze-thaw resistance and chemical corrosion resistance. However, there are still deficiencies in the research on epoxy resin modified alkali-activated recycled concrete at home and abroad, especially in the optimization of the amount of epoxy resin, which lacks systematic experimental data support.

[0004] In addition, in the field of building materials, domestic and foreign scholars have carried out extensive research on the durability of alkali-activated concrete, the performance of polymer modified alkali-activated concrete and the performance of recycled aggregate concrete, and have achieved fruitful results. However, the exploration of the synergistic effect of polymers and recycled aggregates on the durability of alkali-activated concrete is still lacking.

[0005] (1) Existing researches are mostly focused on polymer modified concrete, while the research on polymer modified alkali-activated concrete is relatively scarce. This limitation of research seriously hinders the optimization of alkali-activated concrete performance and its popularization and application in practical engineering.

[0006] (2) Although a lot of research has been done on alkali-activated recycled concrete, the research on the synergistic effect of polymers and recycled aggregates on alkali-activated concrete is still lacking, and the mechanism of the effect of the addition of polymers and recycled aggregates on the performance of alkali-activated concrete is still unclear.

[0007] (3) With the increasing standards of infrastructure construction such as water conservancy projects, alkali-activated recycled concrete, which has the advantages of environmental friendliness and sustainable development, faces the challenge of insufficient strength performance in practical engineering applications. In order to meet the diversified needs of modern engineering structures for durability, it is urgent to build a performance optimization system based on the synergistic effect of epoxy resin and recycled aggregates. SUMMARY

[0008] To solve the above technical problems, the present application provides a polymer modified alkali-activated recycled concrete and a preparation method thereof. Fly ash and slag powder are used as precursor materials, a mixed solution of water glass (Na2SiO3 solution) and NaOH is used as an alkali activator, and fine aggregate, natural coarse aggregate and recycled coarse aggregate are used together with epoxy resin, curing agent and defoaming agent to prepare polymer modified alkali-activated recycled concrete (PARAC). That is, by adjusting the dosage of epoxy resin and the replacement rate of recycled aggregate, the influence of the two on the durability of PARAC is explored.

[0009] To achieve the above purpose, the present application provides the following technical solutions:

[0010] One of the technical solutions of the present application is:

[0011] A polymer modified alkali-activated recycled concrete, in addition to water, comprises the following raw materials:

[0012] Fly ash, slag powder, fine aggregate, coarse aggregate, alkali activator, water, polymer modifier and defoaming agent;

[0013] The polymer modifier is obtained by mixing epoxy resin and curing agent;

[0014] The coarse aggregate comprises natural coarse aggregate and recycled coarse aggregate.

[0015] Optionally, the mass ratio of the epoxy resin and the curing agent is 2:1, wherein the epoxy resin is preferably bisphenol A type epoxy resin.

[0016] Optionally, in the polymer modified alkali-activated recycled concrete, the dry weight of the epoxy resin is 2-8wt% of the total weight of the fly ash and the slag powder (precursor); wherein the epoxy resin is an emulsion with a solid content of 50%, and the dry weight = the mass of the epoxy resin emulsion x the solid content.

[0017] Further, the dry weight of the epoxy resin is 6wt% of the total weight of the fly ash and the slag powder. Hereinafter, 6% of the amount of the epoxy resin corresponds to this, and the others are the same.

[0018] Further, the modulus of the alkali activator is 1.3.

[0019] Optionally, the mass ratio of the recycled coarse aggregate and the natural coarse aggregate is 1:3-3:1, i.e. the mass percentage of the recycled coarse aggregate in the coarse aggregate is 25%-75%.

[0020] Further, the mass ratio of the recycled coarse aggregate and the natural coarse aggregate is 1:1.

[0021] Optionally, the mass ratio of the fly ash, the slag powder, the fine aggregate, the coarse aggregate, the alkali activator, the polymer modifier and the defoaming agent is: 300:200:645:967:183.5:30-90:0-1.

[0022] The second technical solution of the present application is:

[0023] A preparation method of a polymer modified alkali-activated recycled concrete, comprising the following steps:

[0024] The recycled coarse aggregate is stirred with additional water until uniform, and then the fine aggregate (natural river sand), the natural coarse aggregate, the fly ash and the slag powder are sequentially added to the mixture, so that the dry materials are fully dispersed and mixed to obtain material 1;

[0025] The polymer modifier and the defoaming agent are mechanically stirred until uniform, and then they are mixed with the alkali activator, the mixing water and the material 1, and then sequentially subjected to molding, vibration forming, demolding and curing to prepare the polymer modified alkali-activated recycled concrete.

[0026] Optionally, the curing condition is that the curing is carried out in a standard curing room for 28d.

[0027] Compared with the prior art, the present application has the following advantages and technical effects:

[0028] The application provides a polymer modified alkali-activated recycled aggregate concrete (PARAC) and a preparation method thereof.

[0029] The application uses fly ash and slag powder as cementitious materials, uses a mixed solution of water glass and NaOH as an alkali activator, introduces bisphenol A type epoxy resin and a curing agent in a proportion to form a polymer modifier, and adds a defoaming agent to optimize construction performance.

[0030] The inventors find that the addition of an appropriate amount of epoxy resin can improve the compactness of the microstructure of the alkali-activated system, enhance the bonding capacity of the interface transition zone, and thus improve the impermeability, frost resistance and carbonation resistance of the concrete; and the adverse effects of the recycled coarse aggregate are effectively alleviated under the synergistic action of the polymer, realizing the unification of resource recycling and performance optimization. Especially when the epoxy resin content is 6wt%, the concrete exhibits optimal comprehensive performance.

[0031] The application not only expands the functional modification path of the alkali-activated concrete, but also provides a new idea for the high-value utilization of the recycled aggregate, has important engineering application prospects for promoting the development of green and low-carbon building materials, and meets the needs of high-standard infrastructure construction such as water conservancy projects. BRIEF DESCRIPTION OF DRAWINGS

[0032] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application, illustrate the preferred embodiments of the application, and are incorporated in and constitute a part of this application. In the drawings:

[0033] Figure 1 A process flow chart for preparing the polymer modified alkali-activated recycled aggregate concrete according to the embodiments of the application;

[0034] Figure 2 Influence of epoxy resin content on the water penetration height of the PARAC test piece;

[0035] Figure 3 Water penetration height morphology of the PARAC test piece prepared by different recycled aggregate replacement rates;

[0036] Figure 4 Influence of recycled aggregate replacement rate on the water penetration height of the PARAC test piece, wherein (a) is the epoxy resin content of 0%, and (b) is the epoxy resin content of 6%;

[0037] Figure 5Effect of epoxy resin content on unsteady chloride ion migration coefficient of PARAC specimen

[0038] Figure 6 Effect of recycled aggregate replacement rate on unsteady chloride ion migration coefficient of PARAC specimen, wherein (a) is epoxy resin content of 0%, (b) is epoxy resin content of 6%;

[0039] Figure 7 Effect of epoxy resin content on carbonation depth of PARAC specimen

[0040] Figure 8 Effect of epoxy resin content on compressive strength of PARAC specimen before and after carbonation for 28 days

[0041] Figure 9 Effect of recycled aggregate content on carbonation depth of PARAC specimen, wherein (a) is epoxy resin content of 0%, (b) is epoxy resin content of 6%;

[0042] Figure 10 Effect of recycled aggregate replacement rate on compressive strength of PARAC specimen before and after carbonation for 28 days, wherein (a) is epoxy resin content of 0%, (b) is epoxy resin content of 6%;

[0043] Figure 11 Effect of epoxy resin content on mass loss rate of PARAC specimen

[0044] Figure 12 Effect of epoxy resin content on relative dynamic elastic modulus of PARAC specimen

[0045] Figure 13 Effect of recycled aggregate replacement rate on mass loss rate of PARAC specimen, wherein (a) is epoxy resin content of 0%, (b) is epoxy resin content of 6%;

[0046] Figure 14 Effect of recycled aggregate replacement rate on relative dynamic elastic modulus of PARAC specimen, wherein (a) is epoxy resin content of 0%, (b) is epoxy resin content of 6%;

[0047] Figure 15 SEM image of interface transition zone of PARAC specimen

[0048] Figure 16 Microstructure changes of matrix before and after freeze-thaw of E0R50 group, E2R50 group, E6R50 group and E8R50 group

[0049] Figure 17The XRD patterns of the test pieces before and after carbonization of the E0R50 group, the E2R50 group, the E6R50 group, and the E8R50 group, wherein (a) is the effect of the carbonization age on the PARAC test piece, and (b) is the effect of the epoxy resin content on the PARAC test piece;

[0050] Figure 18 The FTIR patterns of the test pieces of the E0R0 group, the E2R0 group, the E4R0 group, the E6R0 group, and the E8R0 group;

[0051] Figure 19 The change graph of the porosity of the four groups of test pieces of the E0R50 group, the E2R50 group, the E6R50 group, and the E8R50 group before and after freezing and thawing;

[0052] Figure 20 The pore size distribution graph of the PARAC test piece before and after the freeze resistance performance test. DETAILED DESCRIPTION

[0053] The various illustrative embodiments of the present application will now be described in detail below. This detailed description is merely intended to teach a person skilled in the art further details about the various aspects and features of the present application and is not intended to limit the scope of the application. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and for teaching one skilled in the art to various embodiments of the present application.

[0054] It should be understood that the terms used in the present application merely describe particular embodiments and are not intended to limit the present application. In addition, for numerical ranges in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is specifically disclosed. Each smaller range within any stated range or within any stated intermediate value is also encompassed within the scope of the present application. The upper limit and the lower limit of these smaller ranges can be included or excluded independently.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All documents mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict, the content of the present specification will control.

[0056] Various modifications and changes can be made to the specific embodiments of the present application described herein without departing from the scope or spirit of the application. Other embodiments of the application will be apparent to those of ordinary skill in the art from the description and examples presented herein. The description and examples are illustrative of the application and are not intended to limit the scope of the application.

[0057] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having" or the like are open-ended terms that are intended to mean including but not limited to.

[0058] As used herein, the term "room temperature" refers to 20-30℃, unless otherwise specified.

[0059] As used herein, the term "parts" refers to mass parts, unless otherwise specified.

[0060] The raw materials used in the present application are commercially available.

[0061] The fly ash and slag powder are used as precursor materials, and the physical parameters of the two are shown in Table 1, and the chemical components are shown in Table 2.

[0062] Table 1 Physical parameters of precursor materials

[0063]

[0064] Table 2 Chemical components of precursors

[0065] Chemical composition SiO2 CaO Al2O3 MgO SO3 Fe2O3 Na2O K2O TiO2 Other Fly ash 52.78 3.35 34.18 0.31 0.49 5.64 0.15 1.65 1.13 0.32 Slag powder 34.21 36.71 19.48 6.12 1.05 0.38 0.18 0.38 1.09 0.40

[0066] A mixed solution of water glass (Na2SiO3 solution) and NaOH is used as an alkali activator, wherein the water glass is produced by Zhengzhou Xinghe Dry Materials Co., Ltd., and the NaOH is purchased from Inner Mongolia Junzheng Chemical Industry Co., Ltd., with a purity of 99.5%; the modulus (M s = SiO2 / Na2O) of the overall alkali activator is 1.3.

[0067] The natural coarse aggregate is natural crushed stone, and the physical properties of the recycled crushed stone (recycled coarse aggregate) are shown in Table 3.

[0068] Table 3 Physical properties of coarse aggregate

[0069]

[0070]

[0071] The mixing water is tap water.

[0072] The polymer modifier is prepared by mixing an aqueous epoxy resin emulsion and a curing agent, both of which are produced by Shenzhen Jitian Chemical Co., Ltd., and the physical properties of the epoxy resin are shown in Table 4.

[0073] Table 4 Physical properties of epoxy resin and curing agent

[0074]

[0075] The defoaming agent is DWQ-110 type organic silicon defoaming agent.

[0076] The technical solutions of the present application are further illustrated by examples below.

[0077] Example 1

[0078] A polymer modified alkali-activated recycled concrete, comprising the following raw materials: fly ash, slag powder, fine aggregate (natural river sand), natural coarse aggregate, recycled coarse aggregate, alkali activator, water, polymer modifier and defoaming agent;

[0079] As Figure 1 shown, the preparation method of the above polymer modified alkali-activated recycled concrete comprises the following steps:

[0080] The recycled coarse aggregate and the corresponding proportion of additional water are sequentially added into the mixer and stirred for 5 min to make the recycled coarse aggregate fully absorb water. Then, the natural river sand and the natural coarse aggregate are added into the mixer and stirred for 3 min. After the aggregate mixing is completed, the precursors (fly ash and slag powder) are added into the mixer and stirred for 2 min to make the dry materials fully dispersed and mixed uniformly, avoiding a large amount of agglomeration of the raw materials. At the same time, the epoxy resin emulsion, the curing agent and the defoaming agent are mechanically stirred for 2 min to ensure that the epoxy resin and the curing agent are fully mixed. Then, the pre-stirred mixture of the epoxy resin, the curing agent and the defoaming agent is added into the mixer together with the alkali activator and the additional water (mixing water) and stirred for 2 min to complete the mixing. After that, the freshly mixed concrete is loaded into a test mold and placed on a vibrating table for vibration. After the test piece is formed, the surface of the test piece is covered with plastic wrap to maintain the surface humidity of the test piece, and after being placed at room temperature for 24 h, the test piece is demolded, i.e. the epoxy resin modified alkali-activated recycled concrete is obtained, and then the test piece is placed in a standard curing room for curing for 28 d for testing.

[0081] The specific mixing proportion in Example 1 is shown in Table 5. Among them, E represents epoxy resin, R represents recycled aggregate, and the group without adding epoxy resin and recycled aggregate is the control group, corresponding to the number E0R0; for example, the number E6R50 indicates that the corresponding test group has an epoxy resin content of 6% and a recycled aggregate replacement rate of 50%. Based on the mechanical performance requirements of concrete, while maximizing the use of recycled aggregate, according to the “Technical Specification for Application of Recycled Aggregate” (JGJ / T 240-2011), the recycled coarse aggregate replacement rate is fixed at 50% in the group only changing the epoxy resin content.

[0082] Among them, the polymer content is determined as 0%, 2%, 4%, 6% and 8%; the recycled aggregate replacement rate is determined as 0%, 25%, 50%, 75% and 100%; a total of 13 groups of mixing proportions.

[0083] Table 5 Mixing proportion of PARAC

[0084]

[0085] Effect verification

[0086] I. Water penetration resistance test

[0087] According to the relevant specifications, the water penetration height method is selected in the present application. The instrument used for the penetration resistance test is NELD-HP1006 type automatic pressure regulating concrete penetration resistance instrument produced by Beijing Nai'erde Intelligent Technology Co., Ltd. The prepared test piece is a circular truncated cone with an upper diameter of 175 mm, a lower diameter of 185 mm and a height of 150 mm. For 13 kinds of mix proportions, 6 test pieces are prepared for each group of mix proportions, totaling 78 test pieces. The specific test steps are as follows:

[0088] (1) Test piece curing and sealing: the test piece is taken out after 27 days of standard curing in the curing room, wiped clean and naturally dried. The next day, the test piece is sealed using a full-circle rubber ring.

[0089] (2) Test operation and result measurement: the sealed test piece is pressed into a special steel mold using a press, then it is securely installed on the penetration resistance instrument and the screws are tightened in place. After accurately adjusting the water pressure parameter of the penetration resistance instrument to 1.2 MPa, the penetration resistance test is officially started. After 24 hours of standard test period, the steel mold is removed from the instrument and the test piece is taken out. The test piece is split using a press, then a waterproof pen is used to mark the water mark boundary on the split surface. To ensure data reliability, 10 measurement points are evenly selected on the split surface of each test piece, and each test group contains 6 parallel test pieces. Finally, the average water penetration height of the test piece is calculated according to formula (1) to evaluate the penetration resistance of the material.

[0090]

[0091] In the formula: - average water penetration depth of the test piece (mm);

[0092] h i - water penetration depth of the jth measurement point of the test piece (mm).

[0093] II. Chloride ion penetration resistance test

[0094] The RCM method is used for the chloride ion penetration resistance test of the present application, and the corresponding test piece size is φ100mmx100mm. For each group of mix proportions, 3 test pieces are prepared, and for 18 mix proportions, a total of 54 test pieces are prepared. The instrument used for the chloride ion penetration resistance test is CABR-RCM6 type concrete chloride ion diffusion coefficient tester produced by Jianyanhua Measuring Instrument Equipment Co., Ltd. The specific test steps are as follows:

[0095] (1) Pretreatment of specimens: The concrete specimens were cylinders with a height of 50 mm and a diameter of 100 mm. Before the formal test, the cut specimens were vacuum saturated using a CABR-BSY type intelligent vacuum saturation instrument for concrete. At the same time, the 0.3 mol / L NaOH solution and the 10% NaCl solution required for the test were prepared the day before the test.

[0096] (2) Install the specimen: Install the saturated specimen in the RCM test tank, ensuring that both ends of the specimen are in close contact with the anode plate and cathode plate respectively, to ensure good sealing and prevent solution leakage. Pour 3.0% sodium chloride solution into the anode tank, so that the solution level is about 20 mm above the top surface of the specimen; pour 0.3 mol / L NaOH solution into the cathode tank.

[0097] (3) Test Procedure: Connect the DC regulated power supply, connect the anode plate to the positive terminal of the chloride ion diffusion coefficient measuring instrument, and connect the cathode plate to the negative terminal. Before starting the test, calibrate the instrument's initial current and other key parameters. During the test, record the test data in real time. The test duration is 24 hours.

[0098] (4) Post-test treatment: Disconnect the power supply, remove the specimen, and rinse the surface with clean water to remove the solution. Split the specimen along the diameter direction using a press, and spray 0.1 mol / L AgNO3 solution evenly on the split surface. Wait about 15 minutes and observe the color change inside the concrete. Take about 10 points evenly, measure them with vernier calipers, and take the average value as the penetration depth of the specimen. Calculate the unsteady chloride ion migration coefficient of the specimen according to formula (2).

[0099]

[0100] In the formula: D RCM —Unstable chloride ion migration coefficient of the specimen (m) 2 / s);

[0101] X d —Average chloride ion penetration depth (mm);

[0102] U—The absolute value of the voltage during the test (V);

[0103] T—The average initial and final temperatures of the anolyte solution (°C);

[0104] L—Specimen thickness (mm);

[0105] t — duration of the experiment (h).

[0106] III. Carbonization Resistance Test

[0107] In the carbonation test of PARAC, 18 test pieces are prepared for each group of mix proportion. Considering that the present application has 13 different groups of mix proportion, a total of 234 test pieces are prepared. The instrument used in the carbonation test is CABR-HTX12 type concrete carbonation test box produced by Jianyanhua Measurement Technology Co., Ltd. The specific test steps are as follows:

[0108] (1) Test piece pretreatment: After curing, the test piece is taken out from the standard curing room, dried in an oven at 60±5℃ for 48h to make the test piece reach a constant weight, and then taken out and cooled to room temperature. The test piece is sealed with melted paraffin on all surfaces except the surface that needs to be exposed to the carbonation environment to ensure that carbon dioxide enters the test piece only from the specified surface.

[0109] (2) Test process: The treated test piece is placed in the carbonation test box, and the test pieces should be kept at a certain distance and not contact each other, and the distance from the carbonation box wall should not be less than 50mm. The temperature in the carbonation test box is adjusted to 20±5℃, the relative humidity is 70±5%, and carbon dioxide gas is introduced to make the carbon dioxide concentration in the box stable at 20±3%.

[0110] (3) Post-test processing: After reaching the specified carbonation time (3d, 7d, 14d, 28d), 3 test pieces are taken out and split with a press. Phenolphthalein alcohol solution is uniformly sprayed on the cut surface or inner wall of the drilled hole. Since phenolphthalein turns red in alkaline environment and does not change color in neutral or acidic environment, the uncarbonated part of the concrete will turn red, while the carbonated part will not change color. After the color is stable, the vernier caliper is used to measure the carbonation depth, and the average carbonation depth is calculated according to formula (3).

[0111]

[0112] In the formula: —The average carbonation depth of the test piece after carbonation for t days (mm);

[0113] d i —The carbonation depth of the i-th measuring point of the test piece (mm).

[0114] After completing the concrete carbonation test, the remaining 3 test pieces from the 28-day carbonation test group and 3 uncarbonated test pieces are taken out for compression test. The test uses a WHY-2000 type microcomputer controlled pressure testing machine. In order to ensure the accuracy and reliability of the test data, the loading rate of the testing machine needs to be accurately set, which is adjusted to 0.5MPa / s.

[0115] Four, frost resistance test

[0116] The anti-freezing test of PARAC adopts the quick freezing method. The test specimens are strictly prepared according to the standard size of 100 mm x 100 mm x 400 mm. For 13 different mix proportions, 3 test specimens are prepared for each mix proportion, and a total of 39 test specimens are prepared. The equipment used in the test is the HC-HDK type concrete quick freezing and thawing test machine produced by Jianyanhua Instrument Equipment Co., Ltd. During the operation of the freezing and thawing machine, the minimum anti-freezing temperature of the supplemented anti-freezing liquid can reach -35℃, which can meet the requirements of the test for low temperature environment. The specific test steps are as follows:

[0117] (1) Test specimen pretreatment: After completing the standard curing procedure, the anti-freezing performance test specimen is taken out of the curing room, and then it is completely immersed in clean water for saturation treatment. The soaking time is 4d to ensure that the test specimen reaches a fully saturated state. After completing the saturation treatment, the test specimen surface is wiped with a dry cloth to remove excess water. Then the basic parameter measurement is carried out, including recording the initial mass, original size and other basic physical parameters of the test specimen. Among them, the determination of the transverse fundamental frequency uses the NEL-DTA type dynamic modulus tester manufactured by Beijing Nai'er Instrument Equipment Co., Ltd.

[0118] (2) Test process: After the measurement is completed, the test specimen is placed in the test specimen box, and appropriate clean water is filled between the test specimen and the test specimen box to ensure that the test specimen can be fully soaked in water. The test specimen box with the test specimen is placed in the freezing and thawing test machine, the lower limit of the temperature is set to -18℃, and the upper limit of the temperature is set to 5℃. Start the equipment and begin the freezing and thawing cycle test. One freezing and thawing cycle includes freezing and thawing processes, the freezing time is controlled within 2-4 hours, the thawing time is controlled within 2-3 hours, and the total time of each cycle is controlled within 3-4 hours. During the test process, the operation of the equipment is checked regularly to ensure that the temperature, time and other parameters meet the requirements.

[0119] (3) Test treatment: every 25 freezing and thawing cycles, the test specimen is taken out of the freezing and thawing test machine, the surface water is wiped off, the mass of the test specimen is weighed with an electronic balance, and the mass loss is recorded; the dynamic modulus of the test specimen is measured with the dynamic modulus tester, and the dynamic modulus loss rate is calculated. When the relative dynamic modulus of the test specimen decreases to below 60% or the mass loss rate reaches 5%, the test is stopped. The mass loss rate and the relative dynamic modulus of the test specimen are calculated according to formula (4) and formula (5), and the measured values of each group of test specimens are taken according to the value taking method of the anti-freezing test in the standard "Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete" (GB / T 50082-2009).

[0120]

[0121] In the formula: ΔW N The mass loss rate of the test specimen after N freezing and thawing cycles (%);

[0122] W N—The mass (g) of the specimen after N freeze-thaw cycles;

[0123] W0—Initial mass (g) of the specimen before the freeze-thaw test.

[0124]

[0125] In the formula: P N —Relative dynamic elastic modulus of the specimen after N freeze-thaw cycles (%);

[0126] f N —The transverse fundamental frequency (Hz) of the specimen after N freeze-thaw cycles;

[0127] f0—The transverse fundamental frequency (Hz) of the specimen before the freeze-thaw test.

[0128] Figure 2 The effect of epoxy resin content on the water penetration height of PARAC specimens was investigated. With a fixed recycled aggregate replacement rate of 50%, epoxy resin contents were set at 0%, 2%, 4%, 6%, and 8%. The variation of water penetration height in PARAC with epoxy resin content is shown below. Figure 2 As shown in the figure. The experimental results show that the water permeability characteristics of PARAC material exhibit a non-linear relationship with the epoxy resin content. As the epoxy resin content gradually increases, the water permeability height first decreases and then increases, correspondingly, its impermeability performance undergoes a transformation process of first strengthening and then weakening. By comparing the test data of samples with different mix proportions, the water permeability heights of the E2R50, E4R50, E6R50, and E8R50 test groups were measured to be 69.4 mm, 63.6 mm, 56.4 mm, and 59.7 mm, respectively. Among them, the E6R50 group showed the best impermeability characteristics, with its water permeability height reaching the lowest value. Compared with the control group E0R50, the water permeability heights of each test group decreased by 7.99%, 15.68%, 25.23%, and 20.85%, respectively, fully demonstrating the significant improvement effect of epoxy resin modification on the impermeability of alkali-activated concrete. The improvement of PARAC's impermeability performance by epoxy resin is mainly attributed to its filling effect. During the curing process, the epoxy resin and the curing agent undergo a cross-linking reaction to form a three-dimensional network polymer structure. This structure effectively fills the pores in the concrete matrix. Simultaneously, this filling effect significantly reduces internal water seepage channels, thereby substantially improving impermeability. In other words, the incorporation of epoxy resin can improve the durability of concrete by reducing its permeability.

[0129] Figure 3 The water penetration height morphology of PARAC specimens prepared with different recycled aggregate replacement rates was shown. By fixing the epoxy resin dosage at 0% and 6%, and adding recycled aggregate at replacement rates of 0%, 50%, and 100%, the influence of the single variable of recycled aggregate replacement rate on the water penetration resistance of PARAC was investigated.

[0130] Figure 4 Figure 6 is a graph showing the influence of recycled aggregate replacement rate on the water penetration height of the PARAC specimens; wherein (a) is the epoxy resin content of 0%, (b) is the epoxy resin content of 6%. It can be seen from the figure that in the control group without adding epoxy resin, the average water penetration height of the NC specimen is 63.4 mm. As the recycled aggregate replacement rate increases from 25% to 100%, the water penetration height of the PARAC specimen increases by 7.73%, 18.97%, 27.92% and 38.64% respectively, showing an obvious increasing trend. In the test group with a fixed epoxy resin content of 6%, the baseline water penetration height of the E0R50 control group is 75.4 mm, and when the recycled aggregate replacement rate changes in the range of 25% to 100%, the water penetration height of the specimen increases by 6.76%, 17.99%, 24.03% and 33.4% respectively. The test results of the two groups show that there is a significant positive correlation between the water penetration height of the PARAC material and the recycled aggregate replacement rate, regardless of whether the epoxy resin is added. This regular change confirms that the introduction of recycled aggregate will reduce the impermeability of the material. Although the overall water penetration height of the test group with 6% epoxy resin is higher than that of the group without adding, the increase is relatively small, indicating that the epoxy resin can alleviate the negative impact of recycled aggregate on impermeability to a certain extent.

[0131] By comparing the test results of the two groups of PARAC specimens, the change trend of the average water penetration height is analyzed, and the incorporation of recycled aggregate has a negative impact on the water permeability of PARAC. The reasons include the following aspects: first, compared with natural aggregate, the defects of recycled aggregate provide a convenient channel for water penetration. Inside the concrete, water can spread through these pores and cracks, thereby weakening the impermeability of PARAC. Second, the old mortar layer remaining on the surface of recycled aggregate forms a unique "aggregate-old mortar-new mortar" triple interface transition zone in the concrete matrix. Compared with the traditional interface transition zone formed by natural aggregate, it has more complex microstructure characteristics: its porosity is significantly improved, the structural density is reduced, and the interface bonding strength is decreased. When the PARAC material shrinks in volume, stress concentration mainly occurs in these weak interface areas, making it easier for micro-cracks to initiate and propagate. In addition, under the same recycled aggregate replacement rate, it is found that the weakening degree of recycled aggregate on the water permeability of PARAC with epoxy resin is relatively weak, which is due to the addition of epoxy resin to compensate for some of the defects introduced by recycled aggregate. Epoxy resin can fill pores and enhance interface bonding, thereby inhibiting the negative impact of recycled aggregate on the water permeability of PARAC to a certain extent, reducing its weakening effect.

[0132] Figure 5The influence of epoxy resin content on the unsteady-state chloride ion migration coefficient of the PARAC specimen; To explore the influence of epoxy resin on the chloride ion penetration resistance of alkali-activated recycled concrete, the recycled aggregate replacement rate was fixed at 50%, and the epoxy resin content was 0%, 2%, 4%, 6%, and 8%. The chloride ion penetration resistance test was carried out. As can be seen from the figure, the unsteady-state chloride ion migration coefficient of PARAC decreases with the increase of epoxy resin content. That is, the unsteady-state chloride ion migration coefficient of E6R50 group is the smallest, and its chloride ion resistance is the best. Compared with E0R50 group, the unsteady-state chloride ion migration coefficients of E2R50 group, E4R50 group, E6R50 group and E8R50 group are reduced by 14.01%, 22.69%, 38.28% and 29.73% respectively.

[0133] The mechanism of epoxy resin enhancing the chloride ion penetration resistance of alkali-activated recycled concrete includes the following two aspects: epoxy resin itself has hydrophobicity, when it is uniformly distributed in alkali-activated concrete, it can change the properties of the internal pore wall and surface of the concrete, making it change from hydrophilic to hydrophobic. Since chloride ions need to be in aqueous solution to diffuse inside the concrete, the decrease in the hydrophilicity of the concrete hinders the migration of chloride ions. The contact angle between water and hydrophobic surface increases, making it difficult to spread and penetrate inside the concrete, thereby reducing the possibility of chloride ion migration with water, and thus improving the chloride ion penetration resistance of alkali-activated concrete. Secondly, after the epoxy resin is cured, it acts like a "filler" that divides the originally connected large pores into many small and isolated pores. Chloride ion diffusion inside the concrete mainly occurs through pores, and the refinement of the pore structure significantly prolongs the diffusion path of chloride ions, increasing the difficulty of chloride ion penetration, thereby improving the chloride ion penetration resistance of alkali-activated concrete.

[0134] Figure 6 The influence of recycled aggregate replacement rate on the unsteady-state chloride ion migration coefficient of the PARAC specimen; (a) is the epoxy resin content of 0%, (b) is the epoxy resin content of 6%. Through analysis and observation, it can be found that whether the epoxy resin is added in the PARAC, with the gradual increase of the recycled aggregate replacement rate, the unsteady-state chloride ion migration coefficient shows a continuous upward trend. This phenomenon clearly shows that the addition of recycled aggregate will weaken the chloride ion penetration resistance of PARAC. Among them, the unsteady-state chloride ion migration coefficient of E6R100 group is lower than that of the control group NC specimen. It can be inferred that in PARAC, the enhancement effect of epoxy resin on the chloride ion penetration resistance is greater than the weakening effect of recycled aggregate on the performance.

[0135] In the PARAC test group without adding epoxy resin, the measured value of the unsteady-state chloride ion migration coefficient of the control group NC specimen is 12.76 x 10 -12 m 2 / s. With the replacement rate of recycled aggregate increasing from 25% to 100%, the chloride migration coefficient of the samples increased by 5.79%, 14.67%, 24.09% and 30.18%, respectively, showing an obvious increasing trend. In the test group with the addition of epoxy resin, the baseline migration coefficient of the control group E6R0 sample was 7.82 x 10 -12 m 2 / s. It was significantly lower than the non-added group. When the replacement rate of recycled aggregate changed in the same range, the increase in the migration coefficient of the samples was 5.20%, 15.52%, 26.26% and 32.22%, respectively. This series of data further quantified the degree of influence of recycled aggregate on the chloride ion penetration resistance of PARAC.

[0136] Figure 7 To explore the influence of epoxy resin content on the carbonation depth of PARAC specimens, the replacement rate of recycled aggregate was fixed at 50%, and the epoxy resin was mixed at 0%, 2%, 4%, 6% and 8%, respectively, to explore the influence of a single variable of epoxy resin on the carbonation resistance of PARAC. As can be seen from the figure, the addition of epoxy resin significantly improves the carbonation resistance of the material. At 7 days of carbonation age, the carbonation depth of the control group E0R50 was 19.6 mm, while the carbonation depths of the samples with 2%, 4%, 6% and 8% epoxy resin content were reduced by 20.95%, 35.14%, 29.05% and 23.65%, respectively. After extending the carbonation time to 28 days, the carbonation depth of the control group increased to 31.7 mm, and the carbonation depth of each test group decreased by 6.62%, 15.14%, 26.5% and 18.93%, respectively. Among them, the material showed the best carbonation resistance at 6% epoxy resin content. The reason why epoxy resin enhances the carbonation resistance of PARAC can be attributed to two points: first, the epoxy resin and epoxy resin curing agent undergo a curing reaction and cross-link into a network structure through gradual polymerization, filling the pores in alkali-activated recycled concrete, making the specimen more dense, and thus reducing the carbonation depth. Second, the molecular structure of epoxy resin contains several hydrophilic functional groups. When epoxy resin is mixed into alkali-activated concrete, its hydrophilic functional groups combine with the hydrophilic functional groups on the surface of the alkali-activated concrete matrix through chemical bonding or physical adsorption mechanisms, which builds a cross-linking network at the microscopic level, thus cross-linking the entire system. By observing the carbonation rate of the specimen at different time periods, it can be found that the carbonation depth does not increase linearly with time. With the passage of time, the carbonation rate of the concrete in the later stage gradually decreases, and this rate decreases significantly in the modified concrete with the addition of epoxy resin. During the carbonation process of the surface layer of concrete, calcium carbonate is generated to fill the pores, and epoxy resin also fills the pores. The two filling effects work together to significantly reduce the diffusion coefficient of carbon dioxide in concrete, effectively slowing down the subsequent carbonation process of the concrete.

[0137] Figure 8 The influence of the epoxy resin content on the compressive strength of the PARAC specimens after carbonization for 28 days can be seen from the figure. After carbonization for 28 days, the compressive strength of the PARAC specimens decreased to varying degrees. Specifically, the compressive strength of the E0R50 group, the E2R50 group, the E4R50 group, the E6R50 group, and the E8R50 group decreased by 26.95%, 22.12%, 21.49%, 19.03%, and 27.36%, respectively, compared with the pre-carbonization compressive strength. It can be seen that, similar to the change rule of the carbonation depth of the PARAC specimens, there is a significant nonlinear relationship between the strength loss rate of the PARAC specimens after carbonization treatment and the epoxy resin content. When the epoxy resin content increases from 0% to 6%, the strength loss rate continuously decreases, reaching a minimum value at a content of 6%. However, when the content exceeds 6%, the strength loss rate shows an upward trend. From a microscopic perspective, the decrease in the compressive strength of the specimens after carbonization is due to the decalcification reaction between the C-(A)-S-H gel in the material matrix and carbon dioxide after carbon dioxide diffuses into the PARAC. This chemical reaction destroys the original stable gel structure, causing the overall structure of the material matrix to become loose, thereby leading to a decrease in the compressive strength. With the addition of epoxy resin, the degree of reduction in compressive strength decreases, which may be due to the filling of the pores inside the structure after the epoxy resin is cured.

[0138] Through the analysis of the carbonation depth and the compressive strength of the PARAC specimens, it can be clearly found that the addition of epoxy resin can effectively improve the carbonation resistance of PARAC. Specifically, with the change of the epoxy resin content, the improvement effect on the carbonation resistance of PARAC shows a trend of first increasing and then weakening. When the epoxy resin content reaches 6%, the improvement effect on the carbonation resistance of PARAC reaches a maximum. Epoxy resin mainly plays two important roles in filling pores and cross-linking inside the structure of PARAC.

[0139] Figure 9The influence of recycled aggregate content on carbonation depth of PARAC specimens, wherein (a) is the epoxy resin content of 0%; (b) is the epoxy resin content of 6%. By fixing the epoxy resin content at 0% and 6%, the recycled aggregate is mixed in at 0%, 25%, 50%, 75%, and 100% replacement rates, respectively, to explore the influence of a single variable of recycled aggregate on the carbonation resistance of PARAC. As can be seen from the figure, in the test group without adding epoxy resin, the carbonation depth of the control group NC specimen is 17.4 mm at 7 days of carbonation age. As the replacement rate of recycled aggregate increases from 25% to 100%, the carbonation depth of the specimen increases by 7.14%, 14.29%, 19.48%, and 25.32%, respectively. After extending the carbonation time to 28 days, the carbonation depth of the control group increases to 27.8 mm, and the carbonation depth of each test group increases by 9.71%, 14.03%, 20.14%, and 29.83%, respectively. In the test group with 6% epoxy resin, the carbonation depth of the control group E6R0 is 11.3 mm at 7 days of carbonation age, which is significantly lower than that of the group without adding. As the replacement rate of recycled aggregate changes in the same range, the carbonation depth of the specimen increases by 6.64%, 13.52%, 17.81%, and 24.69%, respectively. At 28 days of carbonation age, the carbonation depth of the control group E6R0 is 19.2 mm, and the increase of each test group is 6.68%, 12.79%, 18.98%, and 27.49%, respectively.

[0140] Figure 10 The influence of recycled aggregate replacement rate on the compressive strength of PARAC specimens after carbonation for 28 days, wherein (a) is the epoxy resin content of 0%, (b) is the epoxy resin content of 6%. As can be seen from the figure, in the PARAC test group without adding epoxy resin, the compressive strength of the E0R0 group, the E0R25 group, the E0R50 group, the E0R75 group, and the E0R100 group before carbonation decreases by 23.22%, 26.93%, 26.95%, 30.55%, and 33.89%, respectively, compared with the compressive strength after 28 days of carbonization. In the PARAC test group with epoxy resin, the compressive strength of the E6R0 group, the E6R25 group, the E6R50 group, the E6R75 group, and the E6R100 group before carbonation decreases by 16.54%, 18.73%, 20.29%, 24.78%, and 27.31%, respectively, compared with the compressive strength after 28 days of carbonization. As can be clearly seen from the data in the figure, the compressive strength of the material after carbonation decreases, which may be due to the carbonation reaction in the pore solution, which causes Na + The Na+ in the alkaline silicate matrix is abstracted and combined with other substances to form bicarbonate and carbonate, which changes the original microstructure of the material, increases the porosity of the structure, and thus reduces the strength.

[0141] Through the in-depth analysis of the two core evaluation indexes of carbonation depth and 28d carbonation compressive strength of different carbonation age specimens of PARAC specimens, it can be concluded that the incorporation of recycled aggregates will cause the decline of PARAC carbonation resistance. Specifically, with the gradual increase of the replacement rate of recycled aggregates, the decline of PARAC carbonation resistance is more and more obvious, and they show a significant positive correlation. When the replacement rate of recycled aggregates reaches 100%, the negative impact on the carbonation resistance of PARAC reaches the maximum. There are great differences between recycled aggregates and natural aggregates in performance, and these differences affect the carbonation resistance of concrete from the following two main aspects. First, the porosity of recycled aggregates is significantly higher than that of natural aggregates. Under the same mix proportion conditions, the porosity of concrete prepared with recycled aggregates also increases accordingly. This high porosity provides more channels for the penetration of carbon dioxide and other gases, thus promoting the progress of carbonation reaction and significantly reducing the carbonation resistance of recycled concrete. Second, recycled aggregates have high water absorption capacity. In order to ensure the workability of concrete, additional water is usually added during the preparation of concrete. This increase in water-cement ratio will make the internal structure of concrete become more loose, not only providing more reaction sites for carbonation reaction, but also accelerating the diffusion speed of carbon dioxide in the internal structure of concrete, thus further increasing the carbonation rate.

[0142] Figure 11 For the influence of epoxy resin content on the mass loss rate of PARAC specimens, when the replacement rate of recycled aggregates is fixed at 50%, the relationship between the mass loss rate of PARAC specimens and the number of freeze-thaw cycles under different epoxy resin contents is shown in the figure. The test data show that with the accumulation of freeze-thaw cycles, the mass loss of each group of specimens shows an obvious increasing trend. Specifically, when the freeze-thaw cycle reaches 50 times, the mass loss rate increases significantly, showing that the slope of the curve increases significantly. Alkali-activated concrete reaction depends on a large amount of alkali, but after the reaction is completed, the excess alkali solution cannot be completely consumed, resulting in a large amount of residual alkali in the pore solution of alkali-activated concrete. During the freezing stage, due to the presence of a large amount of residual alkali solution in the pore solution, the pore solution of alkali-activated concrete always maintains a concave liquid surface state. This makes it produce a large deformation. With the increase of freeze-thaw cycle number, the fatigue stress of the matrix gradually accumulates, and the surface of alkali-activated concrete begins to fall off. In addition, there is a clear correlation between the number of freeze-thaw cycles and the surface damage phenomenon. With the continuous increase of the number of freeze-thaw cycles, the phenomenon of concrete surface spalling and falling off becomes more and more serious. Due to the loss of concrete material caused by spalling and falling off, the mass loss of concrete gradually increases.

[0143] With the increase of epoxy resin content, the mass loss rate of the specimen presents a trend of first decreasing and then increasing under the same number of freeze-thaw cycles. When the epoxy resin content is 6%, the mass loss rate is the minimum. The mass loss rate of the E0R50 group specimen without epoxy resin is 5.35% after 75 freeze-thaw cycles, which has reached the failure standard. The mass loss rates of the other groups with different amounts of epoxy resin (E2R50 group, E4R50 group, E6R50 group and E8R50 group) are 5.98%, 5.78%, 4.56% and 5.53% respectively after 100 freeze-thaw cycles. Among them, the mass loss rate of the E6R50 group specimen is the lowest, indicating that it has better frost resistance.

[0144] Under freeze-thaw cycle conditions, uneven stress distribution occurs in the concrete due to temperature changes, while the polymer network formed by the epoxy resin has excellent stress dispersion ability, which can effectively alleviate these internal stresses. Specifically, the epoxy resin network absorbs energy through its elastic deformation ability, preventing the expansion of micro-cracks, and ultimately achieving the purpose of protecting the integrity of the concrete structure. In addition, the epoxy resin not only fills the pores, but also forms a continuous hydrophobic network structure inside the alkali-activated concrete, preventing the intrusion of external water. Water is a key factor that causes freeze-thaw damage, and reducing water entering the concrete interior can effectively reduce the damage caused by freeze-thaw cycles. At the same time, a strong bond is formed between the epoxy resin cured product and the aggregate, further stabilizing the internal structure of the concrete. The combined action of pore filling and strong bonding significantly reduces the mass loss rate of the concrete during freeze-thaw cycles, thereby improving the frost resistance and durability of the concrete.

[0145] Figure 12 For the effect of epoxy resin content on the relative dynamic elastic modulus of the PARAC specimen, it can be seen from the figure that there is a significant negative correlation between the relative dynamic elastic modulus of the PARAC specimen and the number of freeze-thaw cycles. Specifically, when the number of freeze-thaw cycles reaches 50 times, the rate of decline in the relative dynamic elastic modulus accelerates significantly. The relative dynamic elastic modulus of the E0R50 group specimen without epoxy resin is 53.3% after 75 freeze-thaw cycles, which has reached the failure standard. The mass loss rates of the other groups with different amounts of epoxy resin (E2R50 group, E4R50 group, E6R50 group and E8R50 group) are 38.5%, 43.9%, 65.2% and 53.5% respectively after 100 freeze-thaw cycles. Among them, the relative dynamic elastic modulus of the E6R50 group is the highest, indicating that it has the best frost resistance. The continuous decrease in the relative dynamic elastic modulus is because the high concentration of unreacted alkali solution in the pores of the alkali-activated concrete can accelerate the polymerization rate and deepen the reaction degree, but the rapid reaction rate will quickly release a large amount of heat, and the resulting thermal stress will cause the matrix to generate a large number of micro-cracks.

[0146] By analyzing the two key evaluation indicators of the quality loss rate and the relative dynamic elastic modulus of the PARAC test piece, it can be clearly found that the incorporation of epoxy resin can effectively improve the frost resistance of PARAC. Specifically, with the increase of the content of epoxy resin, the improvement effect on the frost resistance of PARAC presents a trend of first increasing and then weakening. When the content of epoxy resin reaches 6%, the improvement effect on the frost resistance of PARAC reaches the maximum. Epoxy resin in PARAC mainly plays three main roles of filling pores, blocking moisture and enhancing adhesion.

[0147] Figure 13 The influence of the replacement rate of recycled aggregate on the quality loss rate of the PARAC test piece, where (a) is the content of epoxy resin is 0%, (b) is the content of epoxy resin is 6%. As can be seen from the figure, whether or not there is epoxy resin incorporated, the incorporation of recycled aggregate will cause the quality loss rate of PARAC to increase under the same number of freeze-thaw cycles. For the PARAC test group without incorporating epoxy resin, when the number of freeze-thaw cycles reaches 75 times, the quality loss rates of the NC group, the E0R25 group, the E0R50 group, the E0R75 group and the E0R100 group are 4.52%, 5.02%, 5.35%, 5.65% and 6.13% respectively, among which, except for the NC group, the rest of the groups all reach the failure standard, while the ultimate freeze-thaw cycle number of the NC group is 100 times. For the PARAC test group with epoxy resin, when the number of freeze-thaw cycles reaches 125 times, the quality loss rates of the E6R0 group, the E6R25 group, the E6R50 group, the E6R75 group and the E6R100 group are 4.62%, 5.12%, 5.56%, 5.87% and 6.34% respectively, among which, the ultimate freeze-thaw cycle number of the E6R0 group is 150 times. That is, the internal defects of recycled aggregate will cause micro-cracks and pores during the pouring and molding of alkali-activated recycled concrete, and in the freeze-thaw cycle, the ice expansion stress repeatedly acts on these pores and micro-cracks, causing them to continuously expand and connect with each other, eventually forming frost heaving cracks in the concrete and causing damage to the concrete surface, which seriously affects its structural performance. In addition, the proportion of recycled aggregate in concrete is positively correlated with the number of pores and micro-cracks, that is, the higher the content of recycled aggregate, the more pores and micro-cracks in the concrete, resulting in a decrease in the frost resistance of alkali-activated recycled concrete. In the test group with 6% of epoxy resin incorporated, the epoxy resin particles will gather to form a continuous film structure, promoting the slurry to uniformly adhere and wrap around the surface of the aggregate, not only enhancing the mutual connection strength between aggregates, but also significantly improving the adhesion between the aggregate network, and effectively filling the micro-cracks existing in the recycled aggregate and at the interface between new and old mortar. Thus, the quality loss rate of the concrete is reduced and the frost resistance is improved.

[0148] Figure 14The influence of recycled aggregate replacement ratio on the relative dynamic modulus of PARAC specimens, wherein (a) is the epoxy resin content of 0%, (b) is the epoxy resin content of 6%. As can be seen from the figure, whether or not there is epoxy resin mixed in, the incorporation of recycled aggregate will result in a decrease in the relative dynamic modulus of PARAC under the same number of freeze-thaw cycles. For the PARAC test group without mixing epoxy resin, when the number of freeze-thaw cycles reaches 75 times, all groups except the NC group reach the failure standard, and the ultimate freeze-thaw cycle number of the NC group is 100. For the PARAC test group mixed with epoxy resin, when the number of freeze-thaw cycles reaches 125 times, all groups except the E6R0 group reach the failure standard, and the ultimate freeze-thaw cycle number of the E6R0 group is 150. Under the same number of freeze-thaw cycles, as the recycled aggregate replacement ratio increases, the relative dynamic modulus of PARAC decreases, reflecting the increase and expansion of internal micro-cracks and pores in concrete, as well as the increase of pores near the bonding interface between cementitious materials and aggregate.

[0149] Through in-depth analysis of the test results, it can be concluded that the incorporation of recycled aggregate will cause the frost resistance of PARAC to decrease. Specifically, as the recycled aggregate replacement ratio and the frost resistance of PARAC show a significant negative correlation. When the recycled aggregate replacement ratio reaches 100%, the negative impact on the frost resistance of PARAC reaches the maximum. This is because there are significant differences in performance between recycled aggregate and natural aggregate, which directly affect the frost resistance of concrete. Compared with natural aggregate, recycled aggregate has experienced various loads and environmental effects, and there are a large number of cracks and pores inside. In addition, the water absorption of recycled aggregate is higher than that of natural aggregate. During the freeze-thaw process, the ice formed by the absorbed water will expand in volume, which will generate a large expansion pressure on the internal structure of the concrete, thereby reducing the frost resistance of the concrete. At the same time, the bonding force of the interface transition zone between recycled aggregate and new cementitious materials is poor, and crack propagation is more likely to occur.

[0150] Six, Micro-morphology

[0151] Figure 15 The SEM image of the interface transition zone of the PARAC specimen (E6R50 specimen). As can be seen from the figure, the interface zone structure is loose, and there are a large number of cracks and pores. This is mainly due to the damage to the cement mortar during the crushing and recycling of aggregate, resulting in a decrease in its bonding performance. The formation of a new interface zone structure between recycled aggregate and new alkali-activated cementitious materials is relatively complex, and the reaction between the alkali activator and the active ingredients on the surface of the recycled aggregate generates hydration products, which helps to enhance the bonding strength of the new interface zone. In addition, due to the high water absorption of recycled aggregate, local pores and weak areas can be formed. Therefore, as the recycled aggregate content increases, the durability of PARAC decreases continuously.

[0152] Figure 16 The microstructure of the matrix before and after freeze-thaw of the E0R50 group, the E2R50 group, the E6R50 group, and the E8R50 group changed. As can be seen from the figure, with the increase and decrease of the epoxy resin, the matrix of the PARAC test piece gradually becomes more dense, and the holes and micro-cracks are reduced. Under the condition of low dosage, the epoxy resin begins to play a role in improving the interface transition zone between the recycled aggregate and the alkali-activated cementitious material. A small amount of epoxy resin (E2R50 (before freezing)) can penetrate into the small pores and cracks of the matrix, enhancing the bonding force between the two, making the structure of the matrix relatively tight. However, due to the limited dosage, this improvement is not comprehensive enough, and there are still some unfilled pores. As can be seen from the microstructure of E6R50 (before freezing), the matrix becomes more dense, and there are almost no obvious pores and cracks. The epoxy resin forms a continuous film at the interface, firmly bonding the recycled aggregate and the cementitious material together, greatly improving the bonding strength of the interface. As can be seen from the microstructure of E6R50 (after freezing), due to the high viscosity of the epoxy resin, it is easy to aggregate under high dosage, forming larger clumps. These clumps not only cannot be uniformly dispersed in the concrete, but also can form defects in the local area, affecting the uniformity of the microstructure of the concrete.

[0153] By comparing the SEM images of the PARAC test piece before and after freeze-thaw cycles, it can be clearly observed that the cracks in the matrix after freeze-thaw increase. The effect of ice expansion pressure causes the original pores to be further expanded, and a large number of new cracks and pores are generated. However, with the increase of the dosage of epoxy resin, the cracks generated in the PARAC test piece are reduced. After the epoxy resin solidifies in the concrete, it will form a three-dimensional network structure, tightly connecting the various components of the concrete together, thereby inhibiting the expansion of cracks, reducing the connectivity of cracks, and reducing the possibility of damaging the internal structure of the concrete through cracks, thereby improving the frost resistance of the concrete. As can be seen from the SEM images, the matrix of the E6R50 group test piece is the most dense, with few and narrow cracks, and thus the E6R50 group test piece has the best frost resistance among the four groups.

[0154] Figure 17The XRD patterns of the test pieces before and after carbonation of the E0R50 group, the E2R50 group, the E6R50 group, and the E8R50 group, wherein (a) is the influence of the carbonation age on the PARAC test piece, and (b) is the influence of the epoxy resin content on the PARAC test piece. From the figure, the crystal characteristics of the PARAC test piece can be clearly observed. At 2θ = 26.7°, a strong quartz crystal main diffraction peak appears; at 2θ = 21°, 2θ = 39.5°, and 2θ = 51°, there are also quartz crystal diffraction peaks. These diffraction peaks correspond to SiO2 in the unreacted or incompletely reacted fly ash. In an alkaline environment, SiO2 and silicate aluminate are necessary components for the formation of silicate aluminate gel, which helps to make the microstructure of the material more uniform and improves the density of the obtained alkali-activated concrete. In the XRD pattern, the diffraction peaks of potassium feldspar and sodium feldspar are detected at 2θ = 27.5° and 2θ = 28°, which is due to the fact that during the recycling process of recycled aggregates, old building slurry is inevitably carried, and these slurry is mainly composed of cement hydration products. In the alkali-activated concrete system, the presence of alkali solution changes the chemical reaction path, and the mineral components in the cement undergo a series of reactions under the action of the alkali solution, finally forming calcite and dolomite.

[0155] As can be seen from figure (a), the intensity of the calcite characteristic peak in the diffraction pattern of the test piece after carbonation treatment increases significantly with the deepening of the carbonation degree. This phenomenon confirms that calcite is one of the main products of the carbonation reaction of PARAC. By comparing the changes in the diffraction peaks of the E6R50 group before and after carbonation, the diffraction peaks of quartz crystals are enhanced, which is mainly due to the influence of carbonation on its hydration products. During the carbonation process, CO2 will react with calcium ions in the C-A-S-H gel in the matrix, and the pH value of the alkaline solution affects the carbonation products of PARAC. If the alkalinity of the alkaline solution is strong, the main product after carbonation is calcium carbonate; if the alkalinity is relatively weak, calcium bicarbonate becomes the main carbonation product. In addition, the N-A-S-H gel in the matrix may also decompose to form SiO2 and Na2CO3, etc. C(N)-A-S-H gel, as the main hydration product of alkali-activated concrete, originally has high structural stability. However, the carbonation process will cause the decalcification reaction of the gel, leading to the depolymerization of the gel structure. These structural deteriorations ultimately lead to the loosening of the concrete matrix, forming a connected pore network and accelerating the degradation of the material performance. This microstructure deterioration is directly reflected in the macroscopic mechanical properties, ultimately leading to the reduction of the compressive strength of the PARAC test piece after carbonation.

[0156] As can be seen from Figure (b), the changes of diffraction peaks of the specimens in E0R50 group, E2R50 group, E6R50 group and E8R50 group after carbonization can be seen. Compared with the specimens in E0R50 group without epoxy resin, the characteristic diffraction peak intensity of calcite in the specimens with epoxy resin is lower. This is because the epoxy resin forms a film-like structure inside the PARAC, which prevents the full contact of CO2 with the internal basic substances, thereby inhibiting the generation of calcium carbonate. At the same time, the presence of epoxy resin slows down the decalcification process of C(N)-A-S-H gel. With the increase of the amount of epoxy resin, the amount of carbonization product decreases, and the degree of damage to the structure of the hydration product gradually decreases. When the amount of epoxy resin reaches a certain value, the carbonation resistance of concrete is significantly improved. However, too high a content of epoxy resin may reduce its carbonation resistance due to air entraining, affecting its overall performance.

[0157] Figure 18 The FTIR spectra of the specimens in E0R0 group, E2R0 group, E4R0 group, E6R0 group and E8R0 group, i.e. the FTIR test of alkali-activated concrete with only epoxy resin, can be seen from the figure. The E0R0 group, E2R0 group, E4R0 group, E6R0 group and E8R0 group all have a peak at 3400 cm -1 , which corresponds to the stretching vibration peak of -OH. The absorption peaks appearing in the range of 900-1200 cm -1 correspond to the vibration peaks of silicon-oxygen tetrahedron, reflecting the stretching vibration of silicon-oxygen tetrahedron in C-S-H gel and C-A-S-H gel. The specimen in E0R0 group has a clear absorption peak in this region, while when the content of epoxy resin is 2%, the peak position moves slightly to high wave number; when the content reaches 4% and 6%, the peak position moves more obviously, and the peak intensity increases. The movement of the peak position to high wave number indicates that the connection degree of silicon-oxygen tetrahedron increases, and the polymerization degree improves. Epoxy resin may promote the further connection and polymerization of silicon-oxygen tetrahedron. The absorption peaks appearing in the range of 400-700 cm -1 correspond to the bending vibration of Si-O-Si, Si-O-Al and Al-O. The increase of peak intensity with the addition of epoxy resin indicates that the addition of epoxy resin affects the structure and vibration mode of aluminosilicate minerals. Epoxy resin fills in the pores of aluminosilicate minerals, making the structure more stable. Through the analysis of the FTIR test results of epoxy resin modified alkali-activated concrete, it can be seen that the addition of epoxy resin has an impact on the chemical structure inside the concrete. It not only promotes the geological polymerization reaction, but also improves the structural stability of aluminosilicate minerals. These microscopic changes help to improve the macroscopic performance of concrete.

[0158] Figure 19The pore size distribution graph of the PARAC test piece before and after the frost resistance test is shown in the figure. The pores of the concrete can be divided according to the pore size, the pores with a pore size less than 20nm are classified as harmless pores, the pores with a pore size between 20nm and 50nm are less harmful pores, the pores with a pore size in the range of 50nm to 200nm are harmful pores, and the pores with a pore size more than 200nm are defined as harmful pores. It can be seen from the pore size distribution graph that the incorporation of epoxy resin significantly optimizes the pore size of the PARAC test piece. With the incorporation of epoxy resin, the harmful pores and harmful pores in the test piece are reduced, and the harmless pores are increased, which is mainly due to the filling effect and film forming effect of the epoxy resin. Although the incorporation of epoxy resin cannot significantly reduce the porosity, it refines the internal pore size, so that the internal structure of the PARAC test piece is more dense, thereby reducing the permeability of the test piece, and it is difficult for water, chloride ions and CO2 to enter the internal structure, thereby improving the impermeability, carbonation resistance and frost resistance of the PARAC test piece. By comparing the pore size distribution of the PARAC test piece before and after the freeze-thaw cycle, it can be found that the proportion of pores with a pore size of more than 50nm increases after the freeze-thaw cycle. This is because under the action of freeze-thaw cycle, the migration of water in the concrete and the ice expansion of pore water cause part of the harmless pores or less harmful pores to be gradually expanded and become harmful pores. After multiple freeze-thaw cycles, the pore size distribution of the E6R50 group test piece is optimal, and the proportion of harmful pores is the smallest, indicating that in this test, the frost resistance of the PARAC is best when the epoxy resin content is 6%.

[0159] Figure 20 The pore size distribution graph of the PARAC test piece before and after the frost resistance test is shown in the figure. The pores of the concrete can be divided according to the pore size, the pores with a pore size less than 20nm are classified as harmless pores, the pores with a pore size between 20nm and 50nm are less harmful pores, the pores with a pore size in the range of 50nm to 200nm are harmful pores, and the pores with a pore size more than 200nm are defined as harmful pores. It can be seen from the pore size distribution graph that the incorporation of epoxy resin significantly optimizes the pore size of the PARAC test piece. With the incorporation of epoxy resin, the harmful pores and harmful pores in the test piece are reduced, and the harmless pores are increased, which is mainly due to the filling effect and film forming effect of the epoxy resin. Although the incorporation of epoxy resin cannot significantly reduce the porosity, it refines the internal pore size, so that the internal structure of the PARAC test piece is more dense, thereby reducing the permeability of the test piece, and it is difficult for water, chloride ions and CO2 to enter the internal structure, thereby improving the impermeability, carbonation resistance and frost resistance of the PARAC test piece. By comparing the pore size distribution of the PARAC test piece before and after the freeze-thaw cycle, it can be found that the proportion of pores with a pore size of more than 50nm increases after the freeze-thaw cycle. This is because under the action of freeze-thaw cycle, the migration of water in the concrete and the ice expansion of pore water cause part of the harmless pores or less harmful pores to be gradually expanded and become harmful pores. After multiple freeze-thaw cycles, the pore size distribution of the E6R50 group test piece is optimal, and the proportion of harmful pores is the smallest, indicating that in this test, the frost resistance of the PARAC is best when the epoxy resin content is 6%.

[0160] In summary, the following conclusions can be drawn from the above-mentioned tests of the present application:

[0161] (1) When the epoxy resin content gradually increases from 0% to 8%, the average water penetration height and the unsteady-state chloride ion migration coefficient of the PARAC show a trend of first decreasing and then increasing. When the epoxy resin content reaches 6%, the impermeability and the resistance to chloride ion penetration of the PARAC reach the best state. With the increase of the recycled aggregate replacement rate from 0% to 100%, the average water penetration height and the unsteady-state chloride ion migration coefficient of the PARAC show a trend of continuously rising, and the impermeability and the resistance to chloride ion penetration of the PARAC continuously decrease. This phenomenon shows that the incorporation of recycled aggregate has a significant negative impact on the impermeability and the resistance to chloride ion penetration of the PARAC, and the appropriate incorporation of epoxy resin can effectively compensate for the performance defects caused by recycled aggregate.

[0162] (2) Under the same carbonation time conditions, with the increase of the epoxy resin content, the carbonation depth of the PARAC shows a trend of first decreasing and then increasing, and with the increase of the recycled aggregate replacement rate, the carbonation depth of the PARAC continuously rises. After the carbonation resistance test, the compressive strength of the PARAC decreases to different degrees.

[0163] (3) Under the same number of freeze-thaw cycles, when the epoxy resin content increases from 0% to 8%, the mass loss rate of the PARAC first decreases and then increases, and the relative dynamic elastic modulus gradually first increases and then decreases.

[0164] (4) The epoxy resin and the curing agent form a three-dimensional polymer network structure through cross-linking and curing reaction, which can fill the pores in the concrete matrix, improve the interface transition zone, optimize the pore structure, and enhance the bonding force inside the matrix. This structure not only improves the compactness of the concrete, but also enhances its durability.

[0165] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A polymer-modified alkali-activated recycled concrete, characterized by, In addition to water, the following raw materials are also included: fly ash, slag powder, fine aggregate, coarse aggregate, alkali activator, polymer modifier and defoaming agent; the polymer modifier is obtained by mixing epoxy resin and curing agent; the coarse aggregate includes natural coarse aggregate and recycled coarse aggregate.

2. The polymer-modified alkali-activated recycled concrete according to claim 1, characterized in that, the mass ratio of the epoxy resin to the curing agent is 2:

1.

3. The polymer-modified alkali-activated recycled concrete according to claim 1, characterized in that, In the polymer modified alkali activated recycled concrete, the dry weight of the epoxy resin is 2-8wt% of the total weight of the fly ash and the slag powder.

4. The polymer-modified alkali-activated recycled concrete according to claim 3, characterized in that, the dry weight of the epoxy resin is 6wt% of the total weight of the fly ash and the slag powder.

5. The polymer-modified alkali-activated recycled concrete according to claim 5, characterized in that, the modulus of the alkali activator is 1.

3.

6. The polymer-modified alkali-activated recycled concrete according to claim 1, characterized in that, the mass ratio of the recycled coarse aggregate to the natural coarse aggregate is 1:3-3:

1.

7. The polymer-modified alkali-activated recycled concrete according to claim 6, characterized in that, the mass ratio of the recycled coarse aggregate to the natural coarse aggregate is 1:

1.

8. The polymer-modified alkali-activated recycled concrete according to claim 1, characterized in that, the mass ratio of the fly ash, slag powder, fine aggregate, coarse aggregate, alkali activator, polymer modifier and defoaming agent is 300:200:645:967:183.5:30-90:0-1.

9. A method for producing a polymer-modified alkali-activated recycled concrete, characterized by, the method comprises the following steps: the recycled coarse aggregate is stirred with additional water until uniform, then the fine aggregate, the natural coarse aggregate, the fly ash and the slag powder are sequentially added thereto and stirred until uniform, to obtain material 1; the polymer modifier and the defoaming agent are mechanically stirred until uniform, then they are mixed with the alkali activator, the mixing water and the material 1, and sequentially subjected to molding, demolding and curing, to obtain the polymer modified alkali activated recycled concrete according to any one of claims 1-8.

10. A method of producing a polymer-modified alkali-activated recycled concrete according to claim 9, characterized in that, the curing condition is that the product is cured in a standard curing room for 28 days.

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