Geopolymer-carbonized steel slag composite gelling low-carbon prefabricated sleeper and preparation method thereof

Abstract

The invention relates to the technical field of preparation of modified low-carbon concrete, in particular to a geopolymer-carbonized steel slag composite gelling low-carbon prefabricated sleeper and a preparation method thereof. The invention aims to solve the technical problem that the strength of the sleeper is influenced by microcracks and volume change caused by large self-constriction and delayed expansion of steel slag of the existing geopolymer system. The steel slag is pretreated through physical and chemical combined activation and gradient carbonization processes, and the content of free calcium oxide and magnesium oxide in the steel slag is reduced; a small amount of steel fibers and high-molecular polymer fibers are simultaneously added into the system to enhance the toughness and improve the low-temperature brittleness; furthermore, a microcapsule material with a self-repairing function is introduced into the system, the capsule wall is broken due to stress to release a repairing agent when a microcrack is generated, and the crack is automatically filled and healed. The sleeper is excellent in strength, crack resistance, volume stability and durability, and has remarkable low-carbon environment-friendly benefits and engineering application value.

Description

Technical Field

[0001] The invention relates to the technical field of modified low-carbon concrete preparation, and in particular to a geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper and a preparation method thereof. Background Art

[0002] With the development of high-speed and heavy-haul railways, rail sleepers, as a crucial component of track structures, must possess high strength, durability, and stable dimensional deformation properties. Traditional rail sleepers are mostly made from Portland cement concrete. While its mechanical properties are mature and reliable, its production suffers from high energy consumption and high CO2 emissions. In recent years, geopolymer cementitious materials based on industrial byproducts have attracted considerable attention due to their production process, which does not require high-temperature calcination and emits only about one-tenth the CO2 of equivalent cement. They are now considered a potential low-carbon alternative cementitious material.

[0003] However, geopolymer concrete also faces some technical bottlenecks in practical applications. First, geopolymer systems usually use high-alkali activators, which causes large self-shrinkage and drying shrinkage during the hardening process. Compared with cement-based materials, geopolymer concrete is more prone to microcracks caused by volume shrinkage, which not only weakens its crack resistance, but also reduces its impermeability and durability. Especially in service environments such as freeze-thaw cycles and carbonization, the presence of microcracks will accelerate deterioration, limiting the industrial promotion of geopolymer concrete. Secondly, due to the insufficient toughness of the pure geopolymer system matrix, it exhibits a brittle fracture mode under low temperature or impact load, and needs to be modified to improve toughness and impact resistance.

[0004] Steel slag is an industrial waste product from the steelmaking process, with approximately 0.1 to 0.15 tons of slag produced as a by-product for every ton of steel produced. Steel slag often contains high levels of free calcium oxide and free magnesium oxide. These components slowly hydrate in a humid environment, causing volume expansion. If added to concrete structures, this can lead to the potential for delayed expansion cracking. Therefore, the direct application of untreated steel slag to cement or concrete presents significant stability issues. Traditionally, methods such as aging and rapid cooling have been used to reduce unstable phases in steel slag, but these processes are time-consuming and energy-intensive.

[0005] Combining geopolymer and steel slag to form a cementitious material for preparing concrete is a relatively reliable technical solution. However, due to the properties of the material itself, some problems may arise that affect the ultimate performance of the concrete. Existing technology patent CN112500011A discloses a method for preparing carbonized steel slag lightweight aggregate and concrete containing carbonized steel slag lightweight aggregate. The method mixes steel slag powder, vitrified microspheres, and quicklime, and prepares the lightweight aggregate at a certain water-cement ratio. The aggregate is then carbonized after curing. This method can produce a low-carbon and environmentally friendly cementitious material, but its carbonization process is simple and the material structure design is simple, which cannot guarantee the long-term strength of the concrete after forming.

[0006] In summary, existing carbonized steel slag composite cementitious materials can achieve low-carbon concrete production, but they cannot guarantee the long-term performance and stability of the produced concrete. The urgent technical challenge in this field is to use carbonized steel slag to produce stable, high-performance cementitious materials for concrete applications. Summary of the Invention

[0007] The primary purpose of this invention is to overcome the shortcomings of existing geopolymer cementitious materials used in railroad sleepers, such as large shrinkage, high brittleness, and susceptibility to microcracks, as well as the delayed expansion and volume stability impacted by the direct incorporation of steel slag. The invention provides a low-carbon prefabricated railroad sleeper made of a geopolymer-carbonized steel slag composite cementitious material and a method for its preparation. Through a unique material composition and process design, the sleeper concrete achieves low shrinkage, high toughness, and self-healing properties, while significantly reducing cement usage and carbon emissions.

[0008] The specific technical solutions are as follows: A geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper, wherein the material composition of the prefabricated sleeper comprises a geopolymer-carbonized steel slag composite gelled material, steel fiber, high molecular polymer fiber, self-repairing microcapsule, aggregate and additives.

[0009] Furthermore, the geopolymer-carbonized steel slag composite cementitious material includes slag powder, fly ash and carbonized steel slag.

[0010] Furthermore, the carbonized steel slag is prepared by physical-chemical composite activation and gradient carbonization.

[0011] Furthermore, the physical-chemical composite activation specifically includes: firstly crushing the steel slag to a particle size of less than 5 mm, then grinding for 45 minutes to obtain D 50 The steel slag powder is 30-40 μm, and finally analytical pure gypsum is added and ground evenly.

[0012] Furthermore, the gradient carbonization specifically includes: loading the activated steel slag powder into a closed reaction vessel, introducing a mixed gas of 5% CO2 and 95% air, maintaining a relative humidity of 70%, and pre-carbonizing at 25°C for 30 minutes, then gradually increasing the CO2 concentration to a complete CO2 atmosphere and raising the temperature to 50°C, continuing the reaction for 3 hours, and finally stopping ventilation and naturally curing for 12 hours.

[0013] Furthermore, the basic parameters of the steel fiber include: length 13-15 mm, diameter 0.2-0.3 mm, and tensile strength >1000 MPa; the basic parameters of the polymer fiber include: polypropylene fiber, length 6-9 mm, diameter 20-25 μm; the self-healing microcapsule is an epoxy resin microcapsule, the capsule wall material is phenolic resin, and the particle size is 200-300 μm.

[0014] The present invention also provides a method for preparing a geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper, comprising the following steps: S1: Carbonized slag A is obtained by subjecting steel slag to physical-chemical composite activation and gradient carbonization treatment; S2: Dry mix the geopolymer and carbonized steel slag A for 2 minutes, add steel fiber and high molecular polymer fiber and continue stirring for 1 minute, then add self-repairing microcapsules and slowly stir for 30 seconds, then add additives and continue stirring for 3 minutes, finally add aggregates in sequence and stir until uniform to obtain a uniform mixture B; S3: Pour the uniform mixture B into the standard rail sleeper steel mold, vibrate it into shape, smooth the surface and cover it with a film. Let it stand at room temperature for 2 hours and then steam cure it. After steam curing, demould it and immediately water it for curing. Finally, perform standard curing to complete the prefabricated rail sleeper.

[0015] Furthermore, it is characterized in that the mixing requirement standard of the uniform mixture B described in S2 is: the slump of the uniform mixture B is tested to be 160-180 mm and maintained for 1 hour.

[0016] Furthermore, it is characterized in that the steam curing described in S3 is to move the product into a steam curing tank and perform steam curing at a temperature of 55° C. for 6 hours.

[0017] Furthermore, it is characterized in that the watering maintenance described in S3 is watering and covering with straw mats for 7 days; the standard maintenance described in S3 is maintenance for 28 days in an environment with a temperature of 20±2°C and a relative humidity of ≥95%.

[0018] Compared with the prior art, the present invention has the following beneficial effects: (1) The carbonized steel slag powder in the composite cementitious system used in the present invention greatly reduces the expansion risk, and its small amount of residual active ingredients expands slightly during the hardening process, playing a role in compensating for shrinkage; at the same time, the addition of fly ash reduces water consumption and optimizes the pore structure, which also helps to reduce shrinkage.

[0019] (2) The present invention adopts a physical-chemical composite activation and gradient carbonization process to prepare carbonized steel slag, so that the free oxide content in the steel slag is reduced as much as possible, ensuring the volume stability of the cementitious material in the system and improving the long-term performance of the prefabricated sleepers after preparation.

[0020] (3) The present invention adds composite fibers to the system and coordinates with the cementitious material to improve the basic mechanical strength of the sleeper.

[0021] (4) The microcapsule self-repairing system added in the present invention enables the sleeper to have the ability of "self-repairing" during service. When fine cracks appear, the repair agent is automatically released and heals the cracks, greatly reducing the possibility of harmful media penetrating through the cracks, further ensuring the long-term performance of the sleeper. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a flow chart of the preparation method of the geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper of the present invention; Figure 2 This is a comparison chart of the X-ray diffraction (XRD) test results in Experimental Example 1 of the present invention; Figure 3 This is a comparison chart of the basic mechanical properties test results in Experimental Example 2 of the present invention. DETAILED DESCRIPTION

[0023] The following examples further illustrate and describe the technical solutions of the present invention. It is particularly noted that each specific embodiment is intended to be a concretization and explanation of the technical solutions and should not be construed as limiting the scope of protection of the present invention. Persons of ordinary skill in the art are entitled to modify the technical solutions of these embodiments and to substitute equivalent features for some or all of the technical features. Such modifications or substitutions do not alter the essence of the corresponding technical solutions and do not deviate from the scope of the technical solutions described in the present invention.

[0024] The present invention proposes a geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper and a preparation method thereof. The method adopts a composite modified gelling material system, which is composed of the following key elements: (1) Carbonization stabilized steel slag powder as a gelling component: Industrial waste slag such as converter steel slag is selected and crushed and ground to obtain steel slag powder. Its potential activity is activated by physical-chemical composite method, and gradient carbonization treatment is applied to eliminate the unstable components. Specifically, CO2 is introduced into the steel slag powder in the presence of a small amount of activator to accelerate carbonization, so that the free f -CaO, f -MgO is converted to carbonates as much as possible. This treatment significantly improves the volume stability of the slag, preventing harmful expansion during subsequent concrete curing. Furthermore, a certain amount of fillers, such as calcium carbonate, are produced within the slag, facilitating the densification of the subsequent cementitious system. Furthermore, a small amount of residual uncarbonized calcium oxide slowly hydrates during concrete curing, producing micro-expansion that counteracts geopolymer matrix shrinkage, thereby providing self-stress balancing.

[0025] (2) Slag-fly ash geopolymer matrix: Blast furnace slag powder and fly ash are used as the main gelling precursors and are compounded in proportion to form a geopolymer matrix. Slag is rich in active CaO and SiO2, which can quickly form calcium silicate hydrate (CSH) and aluminate gel when excited by alkali, filling and strengthening the matrix; Al2O3 and SiO2 in fly ash form geopolymer gel under high alkali conditions, and fly ash particles can optimize particle grading and fill pores as microaggregates. The combined effect of the two can take into account both early strength and later performance, and effectively alleviate the high shrinkage and alkali-aggregate reaction risks caused by the rapid reaction of pure slag geopolymer. In the present invention, the mass ratio of slag to fly ash is preferably in the range of about 2:1 to obtain better strength development and shrinkage control effects.

[0026] (3) Fiber toughening and low-temperature embrittlement resistance: A dual fiber reinforcement mechanism is introduced into the cementitious matrix, that is, a certain volume fraction of chopped steel fibers and polymer synthetic fibers are added. The steel fibers are fine in diameter and high in strength, which can bridge micro-cracks in the concrete, prevent cracks from expanding, and significantly improve the tensile and bending toughness of the material. The synthetic fibers are even finer in diameter and have good toughness. They are more evenly distributed in the matrix, mainly inhibiting the formation of plastic shrinkage and early temperature shrinkage cracks, and providing a certain deformation capacity under low temperature conditions to avoid brittle fracture. The synergistic effect of the two fibers makes the sleeper concrete have excellent crack resistance and impact resistance at both room temperature and low temperature, effectively improving the brittle failure tendency of traditional geopolymer concrete at low temperatures.

[0027] (4) Self-repairing microcapsule doping: In order to further improve the durability of the sleeper during its service life, the present invention incorporates a certain proportion of self-repairing microcapsule materials into the concrete. The microcapsules are coated with a thin wall of a polymer that is sensitive to crack stress and filled with a liquid or mortar-like repair agent. When microcracks appear in the sleeper concrete due to load or environmental effects, the stress at the tip of the expanding crack will cause the nearby microcapsule wall to rupture, releasing the internal repair agent into the crack. The repair agent then solidifies and reacts with the matrix, filling the crack and bonding the crack surface, thereby self-healing and sealing the crack at an early stage. This self-repairing function can significantly delay the expansion and connectivity of the crack, maintain the integrity of the sleeper structure, and improve the anti-seepage, anti-freeze and anti-fatigue properties. The amount and particle size of the microcapsules are optimized to ensure sufficient supply of repair agent without adversely affecting the workability and mechanical properties of the concrete. Compared with nano-doping preparations, it has a lower cost and can match the carbonized steel slag-geopolymer system of the present invention.

[0028] (5) Matching the sleeper production process: The sleeper manufacturing process of the present invention fully considers the characteristics of the existing prefabricated sleeper production line, ensuring that the new material system can be directly applied without major process changes. The cementitious material adopts a pre-dry mixing method. When introducing steel fibers and microcapsules, the mixing sequence and time are controlled to avoid fiber agglomeration and capsule damage. The molding process adopts a conventional steel mold casting and vibration molding process, supplemented by a certain amount of pressing to improve the density when necessary. The curing system is similar to that of ordinary concrete sleepers. Indoor natural curing or steam curing can be used to accelerate hardening. Especially under steam curing conditions, this system does not produce harmful expansion. Instead, the moderate temperature increase promotes the reaction of the geopolymer, which rapidly increases the early strength and can reach the required demoulding strength in about 10 hours.

[0029] As attached Figure 1 FIG. 1 is a flow chart of the geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper and its preparation method of the present invention. The detailed preparation steps are as follows: 1. Raw material composition The main raw materials used in the present invention include: blast furnace slag powder, granulated blast furnace slag ground to a specific surface area of ​​400-450m 2 / kg, with overall performance meeting the S95 grade specified in GB / T 18046-2017. Fly ash is Grade 1 fly ash, meeting GB / T 1596-2017. Converter slag is from steel mills, crushed and screened to remove metallic impurities. The activator includes industrial sodium silicate with a modulus of 1.2-1.4 and a density of 1.35 g / mL; industrial sodium hydroxide is prepared from 96% granular caustic soda in a 10 mol / L solution. The reinforcing fibers include chopped steel fibers, 13-15 mm long, 0.2-0.3 mm in diameter, and a tensile strength of >1000 MPa; and polypropylene fibers, 6-9 mm long and 20-25 μm in diameter. The microcapsule repair agent consists of self-prepared epoxy resin microcapsules with a phenolic resin wall material and a particle size of 200-300 μm. The crushed stone aggregate is continuously graded, 5-20 mm in diameter; the fine aggregate is medium sand with a fineness modulus of 2.6.

[0030] 2. Steel slag activation and gradient carbonization treatment The dried converter slag was first crushed to a particle size of less than 5 mm using a jaw crusher, and then ground in a ball mill for 45 minutes to obtain D 50 The steel slag powder is 30-40 μm. According to the mass percentage, take 100 parts of steel slag powder, add 2 parts of analytical pure gypsum and grind evenly to improve the reaction activity of the minerals in the steel slag.

[0031] The mixed powder is then placed into a closed carbonization reactor, and a mixture of 5% CO2 and 95% air is introduced. The relative humidity is maintained at 70% and the temperature is 25°C for pre-carbonization for 30 minutes to allow carbonization reaction to occur first on the surface of the slag particles. Subsequently, the CO2 concentration is gradually increased to a complete CO2 atmosphere and the temperature is raised to 50°C. The reaction is continued for 3 hours, and finally ventilation is stopped and natural curing is carried out for 12 hours.

[0032] After gradient carbonization treatment, the steel slag powder f The -CaO content dropped from the original 4.5% to below 1.0%, which is basically close to a stable state; the carbonized slag powder is slightly moist, and it is dried to a moisture content of <1% for later use.

[0033] 3. Preparation of cementitious materials Slag powder, fly ash and the carbonized steel slag powder are weighed according to the mixing ratio, poured into a blender and dry-mixed for 2 minutes to evenly mix the cementitious material components; then a predetermined amount of steel fiber and polypropylene fiber are added, and stirring is continued for 1 minute to evenly disperse the fibers in the powder; then the microcapsule material is added and stirred at a low speed for 30 seconds to avoid breaking the capsule wall; separately, NaOH solid is dissolved in water according to the mixing ratio and cooled to room temperature, and then mixed with water glass in the required proportion to prepare a uniform alkali activator solution.

[0034] Start the mixer at medium speed and gradually add the alkali activator, the remaining small amount of water, and the admixtures to the mixture. The entire addition process should be completed within 2 minutes. Then, stir at high speed for 3 minutes until the mixture becomes a uniform, viscous slurry with evenly distributed fibers. This is the geopolymer-carbonized steel slag composite cementitious material. If the workability is insufficient, it is recommended to adjust the water volume or add a chloride-free water reducer to improve fluidity.

[0035] 4. Molding and maintenance Mix the composite cementitious material and aggregate according to the proportion and standard, and quickly pour them into the sleeper steel mold that has been cleaned and coated with release agent in advance. The pouring is carried out in two layers, and each layer is vibrated for 20 seconds to eliminate bubbles and compact the concrete. Finally, use a trowel to level the sleeper surface and cover it with plastic film to seal it. Let it stand at room temperature for 2 hours and the ambient temperature should be controlled at 20-25℃.

[0036] After the concrete has initially set, the formwork is moved into the steam curing tank. Steam is supplied as required, and the curing temperature is maintained at 55°C for 6 hours. Heating is then stopped and the test mold is allowed to cool to room temperature with the furnace. The entire curing cycle is controlled within 10 to 12 hours. Immediately after demolding, the surface of the finished sleeper is sprinkled with water and covered with wet straw mats for 7 days of moisturizing curing to ensure later strength development and prevent surface shrinkage cracks. Finally, after 28 days of standard curing, the sleeper product is complete. Standard curing here is carried out in an environment with a temperature of 20±2°C and a relative humidity of ≥95%.

[0037] Example 1 Geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper and its preparation method are as follows: Mix ratio design: The amount of each component is calculated per cubic meter of concrete, as follows: Cementitious materials: 200 kg of slag powder, 100 kg of fly ash, and 60 kg of carbonized steel slag powder; the total amount of the above cementitious materials is 360 kg, which is equivalent to a ratio of slag powder: fly ash: steel slag of approximately 5:2.5:1.5 in the cementitious materials.

[0038] Alkali activator: 8kg of solid NaOH and 80kg of water glass solution are mixed to produce 88kg of alkali solution, which represents 8% of the cementitious material in the form of Na2O and 9% of the cementitious material in the form of SiO2. Clean water is added to bring the mixture to the desired working strength, achieving a water-to-cement ratio (including the water in the alkali solution) of 0.30.

[0039] Aggregate: 1200kg of 5-20mm continuously graded crushed stone, 600kg of sand, and the aggregate volume ratio is 70%.

[0040] Steel fiber: 40kg, equivalent to a volume fraction of 0.5%.

[0041] Polypropylene fiber: 5kg, equivalent to a volume fraction of 0.25%.

[0042] Microcapsule: 50L, equivalent to 45kg, volume dosage 5%, containing 30kg of epoxy resin repair agent.

[0043] Admixture: 5kg of naphthalene-based water-reducing agent, with a water-reducing rate of 15%, and the dosage accounts for 1.4% of the cementitious material.

[0044] S1: Activate and carbonize the steel slag according to the implementation process to obtain 60 kg of carbonized steel slag powder.

[0045] S2: Weigh slag, fly ash and carbonized steel slag powder according to the above ratio and place them in a forced mixer for dry mixing for 2 minutes. Add steel fiber and polypropylene fiber and continue stirring for 1 minute to make the fibers loose and evenly distributed. Then add the pre-prepared epoxy resin microcapsule particles and stir slowly for 30 seconds to avoid damage to the microcapsules. Then start the mixer, add the prepared alkali activator, balance water and admixtures, stir for 3 minutes, and finally add crushed stone and sand in turn and stir until the concrete mixture is uniform. The slump of the uniform mixture should be 160-180 mm and maintained for 1 hour. It can be determined that the mixing requirements have been met.

[0046] S3: Pour the mixture into a standard rail sleeper steel mould, compact it into shape using a flat vibrator, smooth the surface and cover it with a film. After standing at room temperature for 2 hours, move it into a steam curing tank and steam cure it at 55°C for 6 hours. Demould it after a total curing time of 11 hours. Immediately water the demoulded specimen and cover it with straw mat for curing for 7 days to obtain a prefabricated rail sleeper. In actual application, it needs to be cured for another 28 days.

[0047] Example 2 The preparation method of reference example 1 is different in that: Cementitious materials: 205kg slag powder, 105kg fly ash, 50kg carbonized steel slag powder; the total amount of the above cementitious materials is 360kg.

[0048] S3: Demolding after a total curing time of 10 hours. Other steps are the same.

[0049] Example 3 The preparation method of reference example 1 is different in that: Cementitious materials: 195kg slag powder, 95kg fly ash, 70kg carbonized steel slag powder; the total amount of the above cementitious materials is 360kg.

[0050] S3: Demolding after a total curing time of 12 hours. Other steps are the same.

[0051] Example 4 The preparation method of reference example 1 is different in that: Cementitious materials: 230kg slag powder, 70kg fly ash, 60kg carbonized steel slag powder; the total amount of these cementitious materials is 360kg. The other steps are the same.

[0052] Example 5 The preparation method of reference example 1 is different in that: Cementitious materials: 170kg slag powder, 130kg fly ash, 60kg carbonized steel slag powder; the total amount of these cementitious materials is 360kg. The other steps are the same.

[0053] Comparative Example 1 The preparation method of Example 1 is used, except that the slag carbonization process is not performed using a physical-chemical combined with gradient carbonization process. Instead, only a physical activation plus conventional carbonization process is used. Specifically, the dried converter slag is crushed to a particle size of less than 5 mm using a jaw crusher, then ground in a ball mill for 45 minutes. The ball-milled slag powder is then placed in a sealed reaction vessel, introduced with 10% CO2, and carbonized for 24 hours at 50°C and 70% relative humidity, followed by natural curing in air. All other steps are the same.

[0054] Comparative Example 2 The preparation method of Example 1 was used, but no microcapsules were added to the preparation components, and the missing weight of the microcapsules was made up by the aggregate. The other steps were the same.

[0055] Comparative Example 3 The preparation method of Example 1 was used, but steel fiber and polypropylene fiber were not added to the preparation components, and the missing fiber weight was supplemented by aggregate. The other steps were the same.

[0056] Experimental Example 1 The activated carbonized steel slag of S1 in Example 1 and Comparative Example 1 and the unactivated carbonized steel slag used as raw material were subjected to X-ray diffraction (XRD) testing. The testing was performed using an X-ray diffractometer with a scanning diffraction angle 2θ rotation range of 5° to 65° and a scanning speed of 2°∙min. -1 ; Test results are as attached Figure 2 As shown in the figure, it can be seen that the material peaks marked in the uncarbonized activated raw steel slag are significantly different from those in Example 1 and Comparative Example 1. After carbonization treatment, the diffraction peak intensities of silicate phases such as C3S and C2S in the activated carbonized steel slag prepared in Example 1 and Comparative Example 1 are significantly reduced, and the diffraction peak of calcite is significantly enhanced. In addition, compared with Example 1 and Comparative Example 1, Example 1 has a higher degree of carbonization and a higher intensity of related diffraction peaks, which indicates that more f -CaO and f -MgO are transformed into a more stable phase.

[0057] Experimental Example 2 The prefabricated sleepers prepared in Examples 1 to 5 and Comparative Examples 1 to 3 were subjected to basic mechanical property tests, specifically including tests of compressive strength, splitting tensile strength, and flexural strength. The specific test conditions are as follows: Compressive strength: Refer to the standard GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". Pour the mixture from step S3 into a specimen mold to prepare a 150 mm cubic specimen. Follow the subsequent curing steps in the same way. After the final curing for 28 days, use a press to apply uniform load (0.5-0.8 MPa / s) until failure, and record the peak load. Split tensile strength: Refer to ASTM C496 / C496M-17, "Standard Test Method for Split Tensile Strength of Cylindrical Concrete Specimens." Pour the mixture from step S3 into a specimen mold to prepare a cylindrical specimen (φ150×300mm). Follow the same subsequent curing steps. After the final 28-day curing, place the specimen radially and load it with spacers until splitting failure. Calculate the tensile stress. Flexural strength: Referring to the standard JTG 3420-2020 "Test Procedures for Cement and Cement Concrete for Highway Engineering", the mixture in step S3 was poured into the specimen mold to prepare a prismatic specimen (150×150×600mm). The subsequent curing steps were carried out in the same manner. After the final curing for 28 days, the three-point bending method was used to uniformly load the specimen with a span of 450mm until it broke, and the fracture load was recorded. The test results are shown in Table 1 and Appendix. Figure 3 shown.

[0058] Table 1 Comparison of experimental results of Experimental Example 2

[0059] From the above results, it can be seen that the technical solution of the embodiment has better basic mechanical properties due to the process of composite activation and gradient carbonization of steel slag and the addition of composite fiber; the steel slag of comparative example 1 is physically activated and conventionally carbonized, and the mixture of comparative example 1 shows signs of pulping and expansion during the steaming process, and fine network cracks can be observed on the surface of some test pieces. The basic mechanical properties are worse than those of the embodiment. This is because the activation carbonization process of comparative example 1 removes the steel slag in the steel slag. f -CaO and f -MgO conversion is less, the steel slag gradually hydrates in the later stage, causing volume expansion, and the stress accumulation affects the performance of the specimen; Comparative Example 2 is a group example without adding microcapsules, and its basic mechanical properties are basically the same as those in the embodiment, indicating that the addition of microcapsules has little effect on the basic mechanical properties of the sleeper; Comparative Example 3 is a group example without adding composite fibers, and its compressive strength is slightly higher than that of Example 1. The fibers have a slightly weakening effect on the compressive strength, but the flexural strength and splitting strength are relatively low, indicating that the addition of fibers greatly improves the toughness and crack resistance of the material.

[0060] Experimental Example 3 The prefabricated sleepers prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to drying shrinkage testing. Referring to the standard GB / T50082-2009, "Standard for Test Methods for Long-term Performance and Durability of Ordinary Concrete," the mixture from step S3 was poured into test specimens to form 100×100×515 mm prism specimens. Subsequent curing procedures were the same, except that after the specimen molds were removed, the initial lengths were recorded and the specimens were placed in a constant temperature and humidity chamber. Length changes were measured regularly, and shrinkage was calculated after 28 days. The chamber was set at a temperature of 20±2°C and a humidity of 60±5%. A comparometer with an accuracy of 0.001 mm was used as the measuring instrument, and measurements were taken at the 1st, 3rd, 7th, 14th, and 28th day. The test results are shown in Table 2.

[0061] Table 2 Comparison of experimental results of Experimental Example 3

[0062] Note: The unit of drying shrinkage in the table above is “με”, specifically 1με=10 -6 strain.

[0063] From the above results, it can be seen that the technical solution of the embodiment has a lower drying shrinkage rate due to the process of composite activation and gradient carbonization of steel slag and the addition of composite fibers; the drying shrinkage rate of the comparative example 1 steel slag is lower after physical activation and ordinary carbonization treatment, because the steel slag will expand in volume, offsetting part of the shrinkage, but still cannot make up for the decline in its mechanical properties; comparative example 2 is a group example without adding microcapsules, and its drying shrinkage rate is basically the same as that of the embodiment and slightly higher, indicating that the microcapsules themselves have little effect on the structure and will slightly affect the drying shrinkage rate; comparative example 3 is a group example without adding composite fibers, and its drying shrinkage rate is higher. Due to the lack of fiber constraint, shrinkage deformation is easier to release, which proves the effectiveness of fiber in limiting shrinkage cracking of concrete.

[0064] Experimental Example 4 The prefabricated sleepers prepared in Example 1 and Comparative Example 2 were subjected to crack self-repairing performance tests. The test process included: Pre-damage treatment: A servo hydraulic system was used to apply cyclic loading three times; the load value was 50% of the ultimate load, which was determined based on the compressive strength of Experimental Example 2, and the loading rate was 0.2 mm / min. Crack observation: Observe using a digital microscope at a magnification of 200 times, mark five typical cracks, and record the initial width; Self-repair process: Set the room temperature to 25±5℃ and humidity to 50-70% for 14 days. Evaluation of repair effect: Remeasure the crack width.

[0065] The test results are shown in Table 3.

[0066] Table 3 Comparison of experimental results of Experimental Example 4

[0067] From the above results, it can be seen that the addition of microcapsules has a more obvious repair effect. It can better perform self-repair when microcracks appear in the sleeper due to the shrinkage and expansion of its own material or external forces, prevent the expansion of microcracks, and extend the service life of the sleeper.

Claims

1. Geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper, characterized by: The material composition of the prefabricated sleeper includes geopolymer-carbonized steel slag composite gelling material, steel fiber, high molecular polymer fiber, self-repairing microcapsule, aggregate and additives.

2. The geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 1, characterized in that: The geopolymer-carbonized steel slag composite cementitious material comprises slag powder, fly ash and carbonized steel slag.

3. The geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 2, characterized in that: The carbonized steel slag is prepared by physical-chemical composite activation and gradient carbonization.

4. The geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 3, characterized in that: The physical-chemical composite activation specifically includes: firstly crushing the steel slag to a particle size of less than 5 mm, then grinding for 45 minutes to obtain D 50 The steel slag powder is 30-40 μm, and finally analytical pure gypsum is added and ground evenly.

5. The geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 3, characterized in that: The gradient carbonization specifically includes: loading the activated steel slag powder into a closed reaction container, introducing a mixed gas of 5% CO2 and 95% air, maintaining a relative humidity of 70%, and pre-carbonizing at 25°C for 30 minutes, then gradually increasing the CO2 concentration to a complete CO2 atmosphere and raising the temperature to 50°C, continuing the reaction for 3 hours, and finally stopping ventilation and naturally curing for 12 hours.

6. The geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 1, characterized in that: The basic parameters of the steel fiber include: length 13-15 mm, diameter 0.2-0.3 mm, and tensile strength >1000 MPa; the basic parameters of the polymer fiber include: polypropylene fiber, length 6-9 mm, diameter 20-25 μm; the self-repairing microcapsule is an epoxy resin microcapsule, the capsule wall material is phenolic resin, and the particle size is 200-300 μm.

7. The method for preparing the geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to any one of claims 1 to 6, characterized in that: The steps include: S1: Carbonized slag A is obtained by subjecting steel slag to physical-chemical composite activation and gradient carbonization treatment; S2: Dry mix the geopolymer and carbonized steel slag A for 2 minutes, add steel fiber and high molecular polymer fiber and continue stirring for 1 minute, then add self-repairing microcapsules and slowly stir for 30 seconds, then add additives and continue stirring for 3 minutes, finally add aggregates in sequence and stir until uniform to obtain a uniform mixture B; S3: Pour the uniform mixture B into the standard rail sleeper steel mold, vibrate it into shape, smooth the surface and cover it with a film. Let it stand at room temperature for 2 hours and then steam cure it. After steam curing, demould it and immediately water it for curing. Finally, perform standard curing to complete the prefabricated rail sleeper.

8. The method for preparing the geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 7, characterized in that: The mixing requirement standard for the uniform mixture B described in S2 is: the slump of the uniform mixture B is tested to be 160-180 mm and maintained for 1 hour.

9. The method for preparing the geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 7, characterized in that: The steam curing described in S3 is to move the material into a steam curing tank and perform steam curing at 55°C for 6 hours.

10. The method for preparing the geopolymer-carbonized steel slag composite gelled low-carbon prefabricated sleeper according to claim 7, characterized in that: The watering maintenance described in S3 is to water and cover the grass mat for 7 days; the standard maintenance described in S3 is to maintain in an environment with a temperature of 20±2℃ and a relative humidity of ≥95% for 28 days.

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