Method for improving high-temperature performance of recycled aggregate slag-fly ash-based geopolymer concrete

Abstract

The invention discloses a method for improving the high temperature performance of recycled aggregate slag-fly ash based geopolymer concrete, and belongs to the technical field of building materials, the recycled aggregate slag-fly ash based geopolymer concrete comprises the following components: fly ash, slag, an activator, fine aggregate, coarse aggregate and water; the coarse aggregate comprises natural aggregate and recycled aggregate; the recycled aggregate accounts for 25-75% of the mass of the coarse aggregate. The ultrasonic pulse speed, the residual compression resistance, the splitting tension, the fracture resistance and the bonding strength of a sample with the coarse aggregate replacement rate of 50% are equivalent to or even better than those of a sample with the coarse aggregate replacement rate of 0%. After a test piece doped with the PVA fiber is exposed at a high temperature, the number of cracks is reduced, the width of the cracks is reduced, the residual mechanical strength and toughness can be improved, and the compression resistance and breaking strength loss rate of the test piece is lower than that of a test piece not doped with the PVA fiber.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials, and in particular relates to a method for improving the high-temperature performance of recycled aggregate slag-fly ash-based polymer concrete. Background Art

[0002] With the acceleration of urbanization and the continuous advancement of infrastructure construction, the demand for concrete, one of the most important building materials, has increased dramatically. However, the production process of traditional cement concrete not only consumes a large amount of natural resources (such as limestone and clay), but also releases large amounts of carbon dioxide (CO2), exacerbating the global greenhouse effect. Therefore, the development of new concrete materials with low carbon emissions and environmental sustainability has become a research hotspot in the field of building materials.

[0003] Geopolymer concrete, a new environmentally friendly building material, has garnered widespread attention in recent years. Its primary raw materials are industrial waste materials like fly ash and slag. These waste materials consume little energy and produce minimal CO2 emissions during production. Geopolymer concrete also boasts excellent mechanical properties, high-temperature resistance, and alkali corrosion resistance, making it considered one of the most promising cement substitutes.

[0004] At the same time, with the rapid development of the construction industry, the amount of construction waste generated is also increasing. Among these, waste concrete, a major component of construction waste, faces urgent challenges in its treatment and reuse. Recycled aggregate technology, which transforms waste concrete into recycled aggregate through crushing and screening processes, not only effectively reduces environmental pollution from construction waste but also enables resource recycling, offering significant economic and environmental benefits.

[0005] Combining geopolymer concrete with recycled aggregate to create recycled aggregate slag-fly ash-based geopolymer concrete (GRAC) not only fully utilizes industrial and construction waste but also creates a new concrete material with excellent performance. However, research on the high-temperature resistance of GRAC is currently insufficient, and concrete's high-temperature resistance is crucial for ensuring the safety of structures under extreme conditions such as fire. High temperatures can alter the chemical and physical structure of concrete, leading to microcracks, spalling, and reduced strength, seriously impacting the safety and durability of structures. Summary of the Invention

[0006] The present invention provides a method for improving the high-temperature performance of recycled aggregate slag-fly ash-based polymer concrete to address the aforementioned problems of the prior art. The present invention aims to develop a method for improving the high-temperature performance of recycled aggregate slag-fly ash-based polymer concrete. By optimizing the concrete mix design, the method improves key performance indicators of GRAC in high-temperature environments, such as residual compressive strength, splitting tensile strength, flexural strength, and bond strength. This provides a scientific basis and technical support for the application of GRAC in high-temperature environments.

[0007] A method for improving the high-temperature performance of recycled aggregate slag-fly ash-based polymer concrete. In the recycled aggregate slag-fly ash-based polymer concrete, coarse aggregate includes natural aggregate and recycled aggregate; the recycled aggregate accounts for 25-75% of the mass of the coarse aggregate.

[0008] Furthermore, the recycled aggregate accounts for 50% of the mass of the coarse aggregate.

[0009] Furthermore, the raw materials of the recycled aggregate slag-fly ash-based polymer concrete include the following components: fly ash, slag, activator, fine aggregate, coarse aggregate and water; wherein the mass ratio of the fly ash, slag, activator, fine aggregate and coarse aggregate is: 152:228:71:679:1108; the amount of water added is based on controlling the water-binder ratio to be 0.34.

[0010] Furthermore, the natural aggregate is natural stone; and the recycled aggregate is recycled stone.

[0011] Furthermore, the natural aggregate has a particle size of 5-20 mm, a bulk density of 1550 kg / m3, a crushing index of 10%, a water absorption rate of 0.8%, and a mud content of 0.7%; and / or the recycled aggregate has a particle size of 5-20 mm, a bulk density of 1380 kg / m 3 , the crushing index is 14%, the water absorption rate is 6.5%, and the mud content is 0.3%.

[0012] Furthermore, the fine aggregate is natural river sand; the fineness modulus of the natural river sand is 2.8 and the apparent density is 2630 kg / m 3 .

[0013] Furthermore, the preparation method of the recycled aggregate slag-fly ash-based polymer concrete includes the following steps: adding fly ash, slag, activator, fine aggregate, coarse aggregate and water into a mixer according to the mass ratio, stirring evenly, pouring the mixture into a mold, vibrating it to make it dense, wrapping it with plastic wrap, and curing it after demolding for 48 hours.

[0014] Furthermore, in the recycled aggregate slag-fly ash-based polymer concrete, the coarse aggregate is recycled aggregate; the raw materials of the recycled aggregate slag-fly ash-based polymer concrete also include PVA fiber, and 6.45 kg of PVA fiber is added to each cubic meter of the recycled aggregate slag-fly ash-based polymer concrete; when the PVA fiber is added, the amount of water added is based on controlling the water-binder ratio to 0.38.

[0015] Furthermore, the PVA fiber has a length of 12 mm, a diameter of 15 μm, a tensile strength of 1650 MPa, an elastic modulus of 40 GPa, and a density of 1.29 g / cm 3 .

[0016] Furthermore, the preparation method of the recycled aggregate slag-fly ash-based polymer concrete includes the following steps: adding fly ash, slag, activator, fine aggregate, coarse aggregate, PVA fiber and water into a mixer according to the mass ratio, stirring evenly, pouring the mixture into a mold, vibrating it to make it dense, wrapping it with plastic wrap, and curing it after demolding for 48 hours.

[0017] Compared with the prior art, the present invention has the following advantages and technical effects:

[0018] The residual compressive strength and flexural strength of the recycled aggregate slag-fly ash-based polymer concrete provided by the present invention both reach their maximum values ​​at 100°C and decrease as the temperature rises. The residual splitting tensile strength decreases at 100°C due to its greater brittleness, and increases slightly at 200°C before beginning to decrease. The ultrasonic pulse velocity, residual compressive strength, splitting tensile strength, flexural strength and bonding strength of the sample with a 50% coarse aggregate replacement rate are equivalent to or even better than those of the sample with 0%. After being exposed to high temperatures, the number of cracks in the specimens mixed with polyvinyl alcohol (PVA) fibers decreases and the crack width becomes smaller, which is beneficial to improving the residual mechanical strength and toughness. The compressive and flexural strength loss rates are lower than those of the specimens without PVA fibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 is the particle gradation curve of aggregate, where the left figure is the cumulative retention percentage of particle size distribution of natural coarse aggregate and recycled coarse aggregate, and the right figure is the cumulative retention percentage of particle size distribution of river sand and medium sand;

[0021] Figure 2 is the heating curve;

[0022] Figure 3 Figure 1 is a diagram of the pull-out test device, where (a) is the pull-out test specimen (mm), (b) the AE sensor, and (c) the pull-out device.

[0023] Figure 4 is the appearance after exposure to different temperatures;

[0024] Figure 5 is the mass loss rate at different temperatures and substitution rates;

[0025] Figure 6 is the ultrasonic pulse velocity at different temperatures and substitution rates;

[0026] Figure 7 This is the failure mode of the compression test block (the red arrow points to the crack location);

[0027] Figure 8 is the residual compressive strength;

[0028] Figure 9 The hydration products after experiencing different high temperatures (magnification 10,000 times);

[0029] Figure 10 is the XRD pattern;

[0030] Figure 11 is the splitting-tension failure mode, where the red color represents aggregate splitting;

[0031] Figure 12 is the residual splitting strength;

[0032] Figure 13 This is the interface transition zone after experiencing different high temperatures (magnification 2000 times);

[0033] Figure 14 is the residual flexural strength;

[0034] Figure 15 Flexural specimen load and AE ringing count;

[0035] Figure 16 The failure mode of the specimen is pull-out;

[0036] Figure 17 is the τ-s curve at different temperatures;

[0037] Figure 18 (a) is the τ-s curve under different substitution rates; (b) is the AE energy diagram of T400-0%; (c) is the AE energy diagram of T400-50%; (d) is the AE energy diagram of T400-50%;

[0038] Figure 19 This is a comparison chart of the calculated strength value and the measured value. DETAILED DESCRIPTION

[0039] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0040] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0041] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice 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 associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0042] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0043] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0044] The present invention tested the mass loss rate, ultrasonic pulse velocity, residual compressive strength, splitting tensile strength, flexural strength, and adhesive strength of GRAC containing 100% RCA after exposure to different temperatures. To compare the effect of RCA content on GRAC performance, GRAC with different substitution ratios was exposed to 400°C for strength testing. Acoustic emission technology was used to monitor internal damage during flexural and pullout tests. To compare the effect of PVA fiber on GRAC performance, PVA fiber was incorporated into GRAC and exposed to temperatures of 100°C, 300°C, 500°C, and 700°C for compressive and flexural strength testing. The microstructure and chemical composition of the GRAC after high temperatures were also analyzed. The results showed that the residual compressive strength, splitting tensile strength, and adhesive strength of GRAC began to decline at 400°C. The flexural strength was more sensitive to temperature, with a significant deterioration beginning at 200°C. Pullout specimens exhibited splitting failure before 300°C, but transitioned to splitting-pullout failure after 400°C. GRAC exhibits optimal mechanical properties at a 50% substitution ratio, with specimen strength comparable to that of natural aggregate concrete. The density of the hydration products decreases with increasing temperature. The width of cracks between aggregate and mortar in the interface transition zone decreases with increasing RCA content. After high-temperature exposure, specimens incorporating PVA fibers exhibit fewer cracks and smaller crack widths, contributing to improved residual mechanical strength and toughness. The loss rates of compressive and flexural strength are lower than those in specimens without PVA fibers.

[0045] In the embodiments of the present invention, S95 grade blast furnace granulated slag and F class II fly ash are used. A mixed solution of water glass with a modulus of 3.09 and NaOH flake crystals with a purity greater than 99% is used as an activator. The RCA (recycled coarse aggregate) used comes from concrete beams stored in an open-air environment for more than two years. After crushing and screening, recycled stone with a particle size of 5-20 mm is obtained. The natural coarse aggregate comes from crushed limestone. The aggregate properties are shown in Table 1. After particle grading testing, they all meet the requirements of the specification "Pebbles, Sand and Gravel for Construction" GB / T14685-2022. The fine aggregate has a fineness modulus of 2.8 and an apparent density of 2630 kg / m 3 The particle size distribution of natural river sand meets the requirements of GB / T14684-2022 standard, such as Figure 1 The particle size distribution curve of the aggregate is shown in Figure 2. The length of the polyvinyl alcohol fiber (PVA) is 12 mm, the diameter is 15 μm, the tensile strength is 1650 MPa, the elastic modulus is 40 GPa, and the density is 1.29 g / cm 3 .

[0046] Table 1 Physical properties of coarse aggregate

[0047]

[0048] Example 1

[0049] The replacement ratio of recycled aggregate to natural stone was set at 0%, 25%, 50%, 75%, and 100%, with a water-binder ratio of 0.34. Fly ash, slag, sodium hydroxide, river sand, natural stone, recycled stone, water glass, and water were weighed according to Table 2. NaOH flake crystals were first dissolved in water to prepare a NaOH solution, which was then mixed with the water glass. The water glass modulus was adjusted to 1 with NaOH to obtain an activator. The activator was cooled and set aside. Fly ash, slag, activator, river sand, natural stone, recycled stone, and water were added to a blender and mixed thoroughly. The mixture was poured into a mold, vibrated to compaction, wrapped in plastic wrap, and cured after demolding for 48 hours to produce the recycled aggregate slag-fly ash-based polymer concrete. The fly ash and slag mixture was mixed at a ratio of 2:3. The RCA water absorption was measured to be 6.9%. The appropriate amount of additional water was added and the concrete was tested under saturated dry conditions.

[0050] The replacement rate of recycled stone for natural stone is set to 100%, and the water-binder ratio is 0.38. PVA fiber is added to the raw materials of the recycled aggregate slag-fly ash-based polymer concrete. 6.45 kg of PVA fiber is added to each cubic meter of the recycled aggregate slag-fly ash-based polymer concrete. When adding the PVA fiber, the amount of water added is based on controlling the water-binder ratio to 0.38. The prepared recycled aggregate slag-fly ash-based polymer concrete is recorded as GPRAC-PVA.

[0051] The mix design is shown in Table 2, where T represents temperature, R represents substitution rate, Tx represents different temperatures (room temperature - 800°C), and PVA represents polyvinyl alcohol fiber. For example, T400-R0 represents a sample with 0% recycled aggregate substitution exposed to 400°C. Specimens measuring 100×100×100mm were cast for compressive and splitting strength testing, 100×100×400mm for flexural strength testing, and 150×150×150mm for tensile strength testing. Three identical samples were prepared for each group. After compacting the mixture on a vibrating table, it was wrapped in plastic wrap to prevent moisture evaporation. Demolding began 48 hours later and curing was carried out at a temperature of 20 ± 1°C and a humidity of 95% for 28 days.

[0052] Table 2 Mix ratio design (kg / m 3 )

[0053]

[0054] Performance test

[0055] High temperature test:

[0056] The GRAC was treated at high temperature using a box-type resistance furnace with dimensions of 400×600×300mm. The heating curve is shown in Figure 2As shown, the heating rate is controlled at 10℃ / min. After reaching the target temperature, the temperature is kept constant for 2 hours and then the power is turned off. The furnace door is opened to allow it to cool naturally. The average cooling rate is calculated by a temperature gun to be about 0.5℃ / min. The test pieces treated at high temperature are placed in a dry place for at least 24 hours before the residual mechanical properties test.

[0057] Physical properties of the sample after high temperature: Observe with naked eyes and analyze the color and cracking of the sample before and after high temperature. Figure 4 According to the mass of the specimen before and after exposure to high temperature, the mass loss rate of the specimens with different temperatures and replacement rates was measured, and the results are shown in Figure 5 The mass loss rate at different temperatures and substitution rates is shown.

[0058] from Figure 4 As can be seen from the figure, after exposure to high temperatures, the color change and cracking of concrete are primarily caused by the gradual dehydration of the cement paste and changes within the aggregate. Specimens exposed to temperatures below 200°C showed little color change and showed no cracks. After exposure to 300°C, the specimens turned yellow. At 400°C, the yellow color deepened, and a small number of fine, short cracks developed at the edges of the specimens. These cracks are caused by thermal incompatibility between the aggregate and the matrix. At 500°C, the yellow color deepened, cracks began to expand, and slight surface flaking occurred. After exposure to 600°C, the yellow color reached its deepest point, and a dense network of short cracks appeared on the surface. At 700°C, the specimens turned white in the center and slightly green at the edges. The cracks widened, and chipping began to occur at the corners. After exposure to 800°C, the specimens' perimeters turned green, with visible burn marks. Cracks covered the entire surface, and chipping became more pronounced. Therefore, only specimens exposed before 700°C were tested for strength. There is basically no significant difference in appearance between the test pieces with different replacement rates after exposure to 400°C. All of them turn yellow and develop fine short cracks.

[0059] Before 100°C, the appearance and color of the specimens incorporating PVA fibers remained largely unchanged. Several short cracks began to appear at 500°C. After reaching 700°C, the cracks in the specimens incorporating PVA fibers connected through the holes, but the crack widths were smaller. Compared to specimens without fibers, specimens incorporating PVA fibers exhibited the fewest cracks at all temperatures. Overall, PVA fibers effectively inhibited the initiation and propagation of cracks on the specimen surfaces, primarily attributable to their improved stress distribution within the matrix.

[0060] from Figure 5The mass loss rate of GRAC increases with increasing exposure temperature. Between 100 and 300°C, the mass loss rate shows a significant upward trend, primarily due to water evaporation within the specimen. Between 400 and 600°C, the mass loss rate increases slowly. The increase in mass loss after 600°C is due to significant chipping at the specimen edges. At 800°C, the specimen exhibits significant chipping and spalling, with the mass loss reaching 10.62%. The mass loss rate increases with increasing recycled aggregate replacement. This is because the water content in GRAC increases with increasing recycled aggregate replacement, leading to greater water loss at the same exposure temperature. The mass loss rate of specimens with PVA addition is essentially the same as that of specimens without fiber addition. However, once the fibers melt, the mass loss rate is lower than that of specimens without fiber addition. This is because fiber melting in GRAC-PVA also causes mass loss.

[0061] Ultrasonic testing method:

[0062] Ultrasonic pulse velocity detection was performed using an ultrasonic non-destructive tester, and the detection method met the requirements of the Technical Specification for Ultrasonic Detection of Concrete Defects (CECS21:2000). For the compressive and splitting tensile test blocks (100×100×100mm), tests were performed separately at the center of the opposite faces of each test block, and three groups of tests were performed on each test block to obtain the average value. The flexural test block (100×100×400mm) was divided into four parts, and each test block was tested 9 times, with the average value taken as the test result. The pull-out specimen (150×150×150mm) was tested on the two opposite faces of the pulse velocity, and the average value was taken as the test result. The propagation speed of the pulse wave in concrete is used to reflect the density of GRAC before and after high temperature, and the rules are as follows: Figure 6 Ultrasonic pulse velocity at different temperatures and substitution rates is shown.

[0063] from Figure 6As can be seen, as the exposure temperature increases, the pulse velocity reaches its highest at 100°C. This is because rising temperature enhances the geopolymerization between the raw materials and the activator, allowing unreacted fly ash and slag particles to continue reacting with the activator to form a geopolymer gel, increasing the density of the GRAC specimen. The decreasing trend in the ultrasonic pulse velocity at 200°C and 300°C is primarily due to the evaporation of water as the temperature increases, which reduces the density of the concrete and causes it to become loose. After 400°C, the ultrasonic pulse velocity shows a significant downward trend due to the appearance of cracks. The gaps within the cracks delay the propagation of sound waves from one surface to the other in the specimen, resulting in a significant decrease in the ultrasonic pulse velocity. With increasing RCA substitution rates, the ultrasonic pulse velocity decreases by -11.16%, -2.39%, -15.54%, and -17.93% at 25%, 50%, 75%, and 100%, respectively. This is due to the lower density of RCA compared to natural aggregate. The smallest decrease in ultrasonic pulse velocity occurs at a 50% substitution rate, owing to the optimal mix ratio, resulting in a denser skeleton structure. Compared with the undoped specimens, the UPV loss rate of the specimens with PVA fibers was consistently the lowest, indicating that the addition of PVA fibers can improve the density of GRAC. The slope of the UPV loss rate of the GRAC-PVA specimen was the largest at 300°C to 500°C, and the smallest at 500°C to 700°C.

[0064] AE test:

[0065] AE testing was performed using a full-information acoustic emission signal analyzer with a signal sampling frequency of 3 Hz and an amplifier gain of 40 dB. The lead was broken at a known location on the specimen, and the velocity of sound was calibrated based on the arrival time at the known sensor. The response amplitude was averaged over three measurements. One of the three test blocks in each group of flexural and bonding specimens was selected for AE testing. High-vacuum silicone grease coupling agent was applied to the acoustic emission sensor, and eight AE sensors were fixed to two opposing surfaces of the specimen using magnetic fixtures. Based on the specimen material properties and test conditions, the threshold value, peak identification time, impact identification time, and impact lockout time were set to 40 dB, 50 μs, 100 μs, and 300 μs, respectively.

[0066] AE ringing count analysis of flexural test piece, by Figure 15It can be seen that some signals are generated at the beginning of loading, which are the initial contact signals between the specimen and the loading device. As the load increases, the ringing count also slowly increases. When the load reaches its peak, concrete cracks are generated and rapidly expand. For specimens with a replacement rate of 0% and 50%, due to their higher strength, a large peak in the ringing count can be found at the moment of failure. After the load reaches its peak, the strength drops rapidly due to cracking of the concrete matrix. The ringing count of the matrix cracking event gradually decreases, but the ringing count is basically stable between 100-200. This is mainly attributed to the gradual expansion of the cracks at this stage. It can also be found that with the increase of RCA content, the number of cracks increases and the damage is more severe.

[0067] Mechanical properties testing

[0068] Mechanical properties tests were carried out in accordance with the "Standard for Test Methods of Physical and Mechanical Properties of Concrete" (GB / T50081-2019), and compressive strength, splitting tensile strength, flexural strength and bond strength tests were performed using a universal testing machine. The loading speed for measuring compressive strength was 0.5 MPa / s, and the loading speed for measuring splitting tensile strength was 0.05 MPa / s. The flexural strength test used displacement loading at a loading speed of 0.2 mm / min. Figure 3 The device shown is used for center pull-out test. The size of bonded specimens is 150×150×150mm. Ribbed steel bars with a diameter of 16mm are used. The length is 570mm and the overlap length is 80mm. In order to prevent the end effect, 316 stainless steel tubes are used in the unbonded area. Displacement loading is controlled with a loading rate of 0.3mm / min until splitting failure occurs. For the split-pulled specimens, in order to ensure that the complete load-slip law is collected, the collection is stopped when the slip exceeds two 16mm. The average value of three samples is calculated each time. If the relative error of the test results of any two specimens in the same group exceeds 20%, they are remade and retested.

[0069] Analysis results of compressive performance after high temperature:

[0070] The specimens heated before 400℃ were subjected to a sharp cracking sound when they were destroyed, with concrete fragments flying everywhere. The cracks on the surface of the specimens expanded from the inside out and continued to spread. The whole process was very fast until a large amount of concrete fell off around them. The specimens heated after 400℃ had micro cracks in the specimens due to the high temperature, which reduced the brittleness of the specimens. A slight crushing sound could be heard during the compression process of the concrete. Figure 7It can be seen that cracks in the concrete under compression primarily occur on the left and right sides of the specimen, with the cracks running diagonally from both sides, dividing the concrete into two conical sections. As the temperature rises, the delamination of aggregate and mortar becomes more pronounced, and the specimen develops more cracks during compression failure. When the temperature reaches 700°C, the overall failure is primarily characterized by disintegration.

[0071] The residual compressive strength of GRAC at different temperatures and different substitution rates is shown in Figure 8 As shown, the compressive strength first increases and then decreases with increasing temperature. Compared to specimens not exposed to high temperatures, the compressive strength at 100°C increases by 14%. This is primarily attributed to the activation of the unhydrated fly ash and slag particles by the elevated temperature, which further hydrates them. The strength decreases by -0.38% and -1.69% at 200°C and 300°C, respectively. This is attributed to the ceramic properties of GRAC and the fact that the mineral admixtures can fill cracks and pores in the residual mortar and react with calcium hydroxide in both the new and old mortars to form silica gel. Therefore, the compressive strength decreases insignificantly before 300°C. Due to the thermal expansion difference between the geopolymer matrix and aggregate, a significant deterioration trend begins after exposure to 400°C. The generation and development of microcracks is another factor contributing to the strength decline of GRAC, consistent with the trend observed in GPC after elevated temperatures. With increasing RCA content, the residual compressive strength of GRAC decreased by -15.25%, -1.39%, -24.34%, and -25.22% at replacement rates of 25%, 50%, 75%, and 100%, respectively. At 50% replacement, due to the optimal mix ratio of recycled and natural aggregates, the compressive strength decreased by only 0.9 MPa compared to the sample with 0% replacement. This is attributed to the improved compatibility between the aggregate and cement paste achieved by GRAC made with 50% RCA and 50% natural aggregate. Furthermore, the roughness of RCA enhances the adhesion between the paste and aggregate. With increasing temperature, the compressive strength loss rate of the GRAC-PVA composite initially decreases and then increases, indicating that the incorporation of PVA fibers enhances compressive strength before 300°C. After exposure to 500°C, the compressive strength loss rate of GRAC incorporating PVA fibers began to increase. This is because the PVA fibers melt and evaporate with the water vapor, creating numerous pores within the concrete. This exacerbates the damage to the GRAC caused by the high temperature, resulting in a decrease in specimen density and a sharp drop in strength. After exposure to 700°C, the strength loss rate reached its highest point, reaching 65.18%.

[0072] Analysis results of splitting tensile properties after high temperature:

[0073] The splitting failure modes of the samples after different high temperatures and 100% replacement rate are shown in Figure 11 shown.

[0074] The residual splitting tensile strength of GRAC at different temperatures and different replacement rates is shown in Figure 12 As the temperature increases, the loss of splitting tensile strength is more serious than that of compressive strength compared with the normal temperature sample. The splitting tensile strength at 100℃, 200℃ and 300℃ decreases by -14.59%, -8.99% and -21.35% respectively. At 100℃, the sample is brittle due to the small tensile-compressive ratio of concrete, so the decreasing trend appears. The small decreasing amplitude at 200℃ is because the pozzolanic reaction makes the unreacted mineral admixture continue to react to fill the cracks and pores. Figure 9 It can also be seen that the density of hydration products does not decrease significantly before 300℃. After 400℃, microcracks are generated due to the temperature gradient and aggregate thermal incompatibility, and the splitting tensile strength is observed to decrease sharply, which is consistent with the reason for the decrease in the compressive strength of GRA. However, the tensile strength is more sensitive to the micro or macro cracks in the sample due to thermal incompatibility than the compressive strength. With the increase of RCA replacement rate, the law of splitting tensile strength is consistent with that of compressive strength, and the strength decreases the least at 50% replacement rate.

[0075] To verify the above-mentioned phenomenon, SEM test is performed on the interface transition zone of samples with different replacement rates. Due to the large difference in elastic modulus between aggregate and mortar, when subjected to temperature, load and other factors, the deformation performance shows difference, making the bonding performance between aggregate and mortar weak, and the fracture is prone to occur at the interface and form through cracks through the relatively low strength mortar, thereby causing the destruction of concrete. Therefore, the interface transition zone between aggregate and mortar is considered to be the weakest part of the structure of concrete. As shown in Figure 13 As shown in the figure, from the interface transition zone of different replacement rates, it can be seen that with the increase of RCA content, the crack width between aggregate and mortar decreases, which may be due to the reaction of SiO2 in the residual mortar attached to RCA with alkali activator to generate silica gel. Although the bonding performance between GRAC mortar and aggregate is good, the mechanical strength decreases with the increase of replacement rate. On the one hand, it may be due to the low strength of recycled aggregate, and on the other hand, it can be found that the mortar density around the interface transition zone decreases, more pores and looser structure appear, and the cracks can be seen to expand from the interface to the mortar at 100% replacement rate, which also proves that RCA can aggravate the damage of GRAC sample.

[0076] Analysis results of bending performance after high temperature:

[0077] The residual bending strength of GRAC at different temperatures and different replacement rates is shown in Figure 14 As shown in the figure, with the increase of exposure temperature, the bending strength first increases and then decreases, which is consistent with the law of compressive strength. At 100℃, the strength is the highest, Figure 10XRD patterns indicate this is due to the high-temperature reaction of calcium oxide and SiO2 to form the hydration product C3S, and the reaction of SiO2 with Al2O3 to form mullite. Unlike the compressive and splitting tensile strength patterns, the flexural strength begins to decline significantly after 200°C. Compared to room temperature, the residual flexural strength after exposure to high temperatures of 200-700°C decreases by -22.23%, -51.29%, -63.61%, -73.68%, -80.53%, and -88.10%, respectively. The decrease at 200°C and 300°C is attributed to the evaporation of bound water, resulting in a decrease in concrete density, which is consistent with the results of ultrasonic pulse velocity measurements. After 400°C, the temperature gradient experienced by the specimens causes cracks in the concrete, reducing the specimen strength. As the RCA substitution rate increases, the residual compressive strength of GRAC decreases by -36.02%, -5.15%, -34.19%, and -37.40% at substitution rates of 25%, 50%, 75%, and 100%, respectively. At a 50% substitution rate, the flexural strength is comparable to that of GRAC made from natural aggregate. This is consistent with the laws governing ultrasonic pulse velocity and compressive and splitting tensile strengths, and is attributed to the optimal mix ratio, resulting in a denser skeleton structure. Other aggregate ratios have little effect on flexural strength.

[0078] from Figure 14 As can be seen in the figure, the flexural strength loss rate of specimens incorporating PVA fibers is also lower than that of specimens without fibers. However, unlike the compressive strength pattern, GRAC-PVA reaches its maximum at 100°C. Before 300°C, the addition of PVA reduces the slope of the strength loss rate, indicating that the PVA fibers act as a bridge between the fiber and the matrix, improving stress transfer efficiency. However, above 300°C, the temperature negatively impacts the bridging strength between the fiber and the matrix, and the slope of the strength loss rate increases.

[0079] Analysis results of bonding performance after high temperature:

[0080] Failure mode: From the bonding failure mechanism, it can be seen that when the area of ​​concrete in front of the steel rib is small and the pressure on the concrete in front of the rib exceeds the compressive strength of the concrete, steel pull-out failure is likely to occur; when the circumferential tensile stress on the concrete around the steel bar exceeds the concrete splitting tensile strength, splitting failure is likely to occur. Since the GRAC compressive strength is relatively high, pull-out failure did not occur in this test. However, since the GRAC splitting tensile strength is relatively low, the tension-compression ratio is relatively small, and the concrete is relatively brittle, two failure modes, splitting failure and splitting pull-out failure, occur in the present invention, see Figure 16As shown in the figure. Before 300°C, due to its high brittleness, the steel bar enters the yield stage after reaching its peak as the load increases. Radial cracks begin to form on the steel bar surface and rapidly expand longitudinally, causing the GRAC to split into 3-4 pieces with a cracking sound even when the steel bar does not slip much. As the exposure temperature increases, the high temperature causes microcracks to develop within the concrete, further degrading the friction and mechanical engagement between the steel bar and the GRAC. The extension of the radial cracks in the concrete occurs simultaneously with the crushing of the concrete in front of the ribs. Therefore, after 400°C, the failure mode of the specimen changes from splitting failure to splitting-pullout failure. Different RCA contents have no significant effect on the failure mode.

[0081] Peak bond stress and slip curves:

[0082] Assuming that the bond stress between the steel bar and concrete is uniformly distributed along the bond length, the average bond strength is defined as the shear force per unit area at the steel bar-concrete interface. The bond strength can be expressed as:

[0083]

[0084] where τ u is the peak bonding stress, F max is the peak tensile load, d is the steel bar diameter 16 mm, and l is the bonding length 5 d.

[0085] The test results of the drawing specimens are summarized in Table 3, where τ r is the residual bond strength. Figure 17 As shown in Figure 3, the bond-slip curves of GRAC at room temperature and after high temperature are also divided into rising section, falling section and residual section like ordinary concrete.

[0086] Table 3 Bond-slip results

[0087]

[0088] Once the ribbed steel bars slip relative to the concrete, the steel bars will form radial compression and circumferential tensile stress on the surrounding concrete. When the circumferential tensile stress exceeds the tensile strength of the concrete, splitting cracks will appear in the concrete around the steel bars. Before exposure to 300°C, the steel bars have reached yield. The circumferential tensile stress generated by the steel bar pulling exceeds the tensile strength of the concrete, resulting in splitting failure. Compared with the test blocks at room temperature, the bond strength after exposure to 100°C, 200°C, and 300°C decreased by 2.8%, 4.7%, and 6.1%, respectively. The strength drop is not obvious because the deformation difference between GRAC and steel bars after high temperature is small, the crack development of GRAC is not significant, and the compressive strength of concrete hardly decreases before 300°C. Therefore, the mechanical bite force and friction between the steel bar and GRAC do not decrease much, so the bond strength changes slightly. As the temperature increases, after 400°C, temperature-induced dehydration of hydration products and the differential thermal expansion between the matrix and aggregate cause a large number of microcracks to develop within the specimens, significantly damaging the GRAC structure and leading to a significant decrease in bond strength. The bond strength decreases by 42.3%, 52.0%, 69.6%, and 83.6% after exposure to 400°C, 500°C, 600°C, and 700°C, respectively. As shown in Figure 17, the τ-s curves before 300°C are essentially identical to those at room temperature. However, as the temperature increases, the difference between the expansion of the concrete surface due to water absorption and the contraction of the rebar during cooling from high temperatures increases, leading to a gradual loss of chemical adhesion and greater susceptibility to rebar pullout. Consequently, with increasing temperature, the slope of the rising segment decreases, while the slip at the peak bond stress increases. Due to the brittle and unpredictable nature of splitting failure, the curves before 300°C show a steeper decline and no slip segment. The decrease is shorter at 400°C and 500°C, and even slower at 600°C and 700°C. This is because high temperatures improve the ductility of concrete. The concrete matrix constrains the steel bars, preventing cracks around them from rapidly expanding, leading to a slow decrease in bond strength. After slip exceeds 8mm, the residual bond strength approaches zero at all temperatures.

[0089] Figure 18 (a) is the τ-s curve under different substitution rates; (b) is the AE energy diagram of T400-0%; (c) is the AE energy diagram of T400-50%; (d) is the AE energy diagram of T400-50%; Figure 18 The results show that the substitution rate has little effect on bond strength, with peak bond stress showing a slight downward trend with the addition of RCA. This is due to the low strength, high water absorption, and high porosity of RCA, which lead to a decrease in bond strength. At a 50% substitution rate, the bond strength is 8.64% higher than that of GRAC prepared with natural coarse aggregate. This is consistent with the conclusions drawn from the compressive, splitting tensile, and flexural strengths, and is attributed to the 50% substitution rate achieving the optimal ratio, which improves the compatibility between aggregate and cement paste.

[0090] AE energy and bond stress τ change with time in the same way, and there are some interference signals in the initial loading stage due to the frictional extrusion process between the specimen and the testing machine and the internal steel bar occlusion effect. In this stage before the AE energy reaches the peak, the mechanical occlusion is small, and the chemical adhesion force is mainly in action, resulting in a slow increase in τ and AE energy. At this time, the generation of AE energy is mainly due to the friction caused by the small displacement between the specimen and the base plate and the small relative movement of the steel bar and the aggregate in the specimen. Since no cracks appear in the specimen at this stage, the AE energy value is generally small. The slope of the rising stage does not differ much because the RCA content has little effect on the chemical adhesion force at the same temperature. When the AE energy reaches the peak, it means that the weak zone of the steel-concrete bond interface begins to crack, and then the AE energy and the bearing capacity of the bond interface decrease continuously. For the specimens with a replacement rate of 0% and 50%, the AE energy can be seen to decrease sharply due to their greater brittleness. With the increase of the replacement rate, the residual strength has little difference, and the AE energy signal in this stage gradually stabilizes in a certain range. The energy generated in this stage is mainly due to the slow expansion of the cracks and the slip of the steel bar.

[0091] Microscopic performance test

[0092] Select the part containing aggregate and mortar in the center of the test block, and prepare samples with a length of 10 mm or less and a thickness of not more than 5 mm. After drying and dust removal of the prepared samples, use the SBC-12 ion sputtering instrument to spray gold plating film. After the sample preparation is completed, use a scanning electron microscope (SEM) to observe the hydration products after different high temperature actions (magnification 10000 times) and the interface transition zone of samples with different replacement rates (magnification 2000 times). Select the part containing only mortar in the center of the test block, and use a mortar and a screen to grind and sieve the mortar to obtain a powder sample of 10 μm or less. Use an x-ray diffractometer to analyze the composition of the cement paste exposed to high temperature. The test scanning rate is 4° / min, the step size is 0.01, the scanning range is 5-80° (2θ), and the anode material used is copper.

[0093] The microstructure differences of GRAC after exposure to high temperature are identified by observing the morphology of the hydration products through SEM. From Figure 9 It can be seen that the density of the hydration products decreases with the increase of temperature. After 400℃, more pores are found, and the size of these pores gradually increases, and the pores are connected to form cracks, and the generation of cracks is one of the main reasons for the decline of the mechanical properties of concrete. Due to the dehydration of the hydration products, the crystals gradually become loose from the close arrangement. After 500℃, due to the obvious dehydration phenomenon, loose network structure can be seen, and the phenomenon is more obvious at 700℃. The dehydration and decomposition of the hydration products in the interior of the concrete is another reason for the decline of the strength of the concrete.

[0094] The XRD patterns of GRAC at different high temperatures are shown in Figure 2. Figure 10 GRAC is mainly composed of quartz (SiO2), anorthite (CaAl2Si2O8), mullite (Al6Si2O 13 ), albite (NaAlSi3O8), dicalcium silicate (C2S), tricalcium silicate (C3S), and calcite (CaCO3). The diffraction peak at 27.5° in the XRD spectrum is attributed to the presence of anorthite, a mineral belonging to the feldspar family. In an alkaline environment, silicon and aluminum elements form mullite. The increase in the Na+ concentration in water glass promotes the synthesis of sodium-containing compounds, thereby facilitating the formation of albite. The diffraction peak near 31° in the XRD spectrum corresponds to dicalcium silicate, which originates from the recycled aggregate (RCA) in GRAC. Under high temperatures, recycled aggregate forms a large number of pores in GRAC, which easily absorb CO2 from the air. Under these conditions, carbonization reactions occur in GRAC, generating a large amount of CaCO3. When the temperature exceeds 400°C, the intensity of the calcite diffraction peak in the XRD spectrum decreases, indicating that CaCO3 begins to gradually decompose.

[0095] Strength calculation

[0096] The residual strength of geopolymer concrete after high temperature exposure is determined by the following formula:

[0097]

[0098] Among them, f T is the residual strength after experiencing high temperature of T degrees Celsius, f0 is the strength at room temperature, k0 and k1 are 1.196 and 0.136 respectively.

[0099] The residual strength in this invention includes cube compressive strength, axial compressive strength, splitting tensile strength, bond strength with steel bars, and flexural strength. The calculation results are shown in Table 4. The measured values ​​are compared with the calculated values. Figure 19 As shown, the average ratio of the calculated value to the measured value is 1.1, where GRAC is the replacement rate of recycled stone for natural stone, which is 100%, and the recycled aggregate slag-fly ash-based polymer concrete without adding PVA fiber;

[0100]

[0101] Table 4 Measured and calculated values ​​of residual strength after high temperature

[0102]

[0103]

[0104] The feasibility and technical effects of the method proposed in the present invention have been verified above. However, this is only a preferred embodiment of the present invention, and the scope of protection of the present invention is not limited to this. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be covered by the scope of protection of the present invention. In addition, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for improving the high temperature performance of recycled aggregate slag-fly ash based polymer concrete, characterized in that: In the recycled aggregate slag-fly ash-based polymer concrete, the coarse aggregate includes natural aggregate and recycled aggregate; the recycled aggregate accounts for 25-75% of the mass of the coarse aggregate.

2. The method for improving the high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to claim 1, characterized in that: The recycled aggregate accounts for 50% of the mass of the coarse aggregate.

3. The method for improving the high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to claim 1, characterized in that: The raw materials of the recycled aggregate slag-fly ash-based polymer concrete include the following components: fly ash, slag, activator, fine aggregate, coarse aggregate and water; wherein the mass ratio of the fly ash, slag, activator, fine aggregate and coarse aggregate is: 152:228:71:679:1108; the amount of water added is based on controlling the water-binder ratio to be 0.

34.

4. The method for improving the high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to claim 3, characterized in that: The natural aggregate is natural stone; the recycled aggregate is recycled stone.

5. The method for improving the high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to claim 4, characterized in that: The particle size of the natural aggregate is 5-20 mm and the bulk density is 1550 kg / m 3 , crushing index is 10%, water absorption rate is 0.8%, mud content is 0.7%; and / or, the particle size of the recycled aggregate is 5-20mm, and the bulk density is 1380kg / m 3 , the crushing index is 14%, the water absorption rate is 6.5%, and the mud content is 0.3%.

6. The method for improving high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to claim 3, characterized in that: The fine aggregate is natural river sand; the fineness modulus of the natural river sand is 2.8, and the apparent density is 2630 kg / m 3 .

7. The method for improving the high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to any one of claims 3 to 6, characterized in that: The preparation method of the recycled aggregate slag-fly ash-based polymer concrete comprises the following steps: adding fly ash, slag, activator, fine aggregate, coarse aggregate and water into a mixer according to mass ratio, stirring evenly, pouring the mixture into a mold, vibrating and compacting it, wrapping it with plastic wrap, and curing it after demolding for 48 hours.

8. The method for improving high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to claim 1, characterized in that: In the recycled aggregate slag-fly ash-based polymer concrete, the coarse aggregate is recycled aggregate; the raw materials of the recycled aggregate slag-fly ash-based polymer concrete also include PVA fiber, and 6.45 kg of PVA fiber is added to each cubic meter of the recycled aggregate slag-fly ash-based polymer concrete; when the PVA fiber is added, the amount of water added is based on controlling the water-binder ratio to 0.

38.

9. The method for improving high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to claim 8, characterized in that: The PVA fiber has a length of 12 mm, a diameter of 15 μm, a tensile strength of 1650 MPa, an elastic modulus of 40 GPa, and a density of 1.29 g / cm 3 .

10. The method for improving the high temperature performance of recycled aggregate slag-fly ash based polymer concrete according to any one of claims 8 to 9, characterized in that: The preparation method of the recycled aggregate slag-fly ash-based polymer concrete comprises the following steps: adding fly ash, slag, an activator, fine aggregate, coarse aggregate, PVA fiber and water into a mixer according to a mass ratio, stirring evenly, pouring the mixture into a mold, vibrating it to make it dense, wrapping it with plastic wrap, and curing it after demolding for 48 hours.

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