Method for internal and external bidirectional carbonization treatment of coarse aggregate on surface layer of steel slag asphalt pavement

Abstract

The invention provides a method for internal and external bidirectional carbonization treatment of coarse aggregate on a surface layer of a steel slag asphalt pavement. The method comprises the following steps: carbonizing the steel slag aggregate from outside to inside by providing a carbonization environment in a normal pressure state; meanwhile, water and CO2 release factors (crystals) are embedded in communicating pores of the steel slag coarse aggregate, water and CO2 are generated through thermal decomposition of the crystals in the carbonization process, the release rate of the water and CO2 is controlled through medium and low temperatures, and f-CaO in the steel slag aggregate is slowly carbonized from inside to outside; and finally, internal and external bidirectional carbonization of the steel slag coarse aggregate is realized on the premise of not pressurizing, so that the volume stability of the steel slag coarse aggregate is improved, and wide application of the steel slag coarse aggregate in asphalt road surface layers is promoted.

Description

Technical Field

[0001] The present invention relates to the field of road engineering, in particular to a method for bidirectional carbonization treatment of coarse aggregate on the surface of a steel slag asphalt pavement. Background Art

[0002] As the world's largest crude steel producer, my country's steel industry produces over 100 million tons of steel slag annually. Currently, the comprehensive utilization rate of steel slag is less than 30%. Traditional landfill disposal not only wastes land resources but also poses the risk of heavy metal leaching pollution. Driven by the "dual carbon" strategy, steel slag, with its ability to sequester 0.3-0.4 tons of CO2 per ton processed, has become a hot topic in carbon-negative material research.

[0003] Currently, the most common method for disposing steel slag is to use it in the production of cement and concrete. However, due to its volume stability, the implementation of the new GB175-2023 regulation has limited its use in cement materials, making its use in road construction another important disposal option. Steel slag is primarily used as an aggregate replacement for asphalt pavements and inorganic binder-stabilized base layers. Due to the large particle size of base layer aggregates and their exposure to water and CO₂ in the environment, the risk of steel slag volume expansion is relatively high. Therefore, in recent years, steel slag aggregate has been increasingly used in asphalt road pavements. Flexible asphalt materials can encapsulate the steel slag aggregate, reducing its exposure to water vapor and CO₂ while also mitigating the resulting crushing damage caused by volume expansion. However, over long-term service, potential water damage to the asphalt pavement and freeze-thaw cycles can accelerate the fracture of the adhesion interface between the steel slag aggregate and the asphalt, exposing the steel slag aggregate and causing carbonization reactions. This leads to volume expansion and damage to the road structure. Therefore, how to effectively and economically carbonize f-CaO and f-MgO in steel slag aggregates and reduce the risk of volume expansion during service is very important for promoting the widespread application of steel slag in asphalt pavements.

[0004] (1) Current engineering practice mainly adopts the natural aging method, which is to place the steel slag in an open space for 1-2 years and use water and CO2 in the air to remove f-CaO and f-MgO. This method is simple and low-cost, but it not only occupies a large amount of land, but also causes certain pollution to the environment. From the perspective of carbonization effect, the natural aging method is also difficult to remove f-CaO and f-MgO inside the steel slag aggregate. On the one hand, the content of water and CO2 in the air is low, and the diffusion rate inside the steel slag is slow; on the other hand, natural aging is a carbonization process from the outside to the inside. The calcium carbonate formed by the preferential carbonization of the steel slag surface will further hinder water and CO2 from entering the interior of the steel slag for in-depth carbonization.

[0005] (2) The carbonization effect of steel slag is related to the particle size. Under the same carbonization method, the larger the steel slag particle size, the weaker the carbonization effect. Therefore, some processing technologies grind steel slag aggregate into fine aggregate or powder to improve the carbonization effect by increasing the specific surface area of ​​steel slag aggregate. However, the grinding process consumes a lot of energy, and fine aggregate steel slag is mainly used to make cement, and its road use demand is relatively small.

[0006] (3) In response to the problems existing in the natural aging method, researchers have developed a variety of methods to accelerate carbonization to improve the carbonization efficiency and degree of steel slag. The new carbonization technology mainly adopts:

[0007] ① Use physical means such as pressurization, heating, and humidification to promote the carbonization reaction of f-CaO and f-MgO;

[0008] ② Calcium and magnesium ions are leached out through chemical means of pickling, and then react with CO2 to form calcium carbonate.

[0009] ③ Using microorganisms (such as urease-producing bacteria and carbonic anhydrase-producing bacteria) to catalyze the carbonation reaction of CO2 and water to accelerate the conversion of f-CaO into CaCO3;

[0010] ④ Combine with other auxiliary technologies such as ultrasound to promote the efficiency of hydration / carbonization.

[0011] Existing carbonization technology can improve the carbonization effect of steel slag to a certain extent, but it still has certain limitations and challenges in engineering applications.

[0012] (1) Conflict between reaction kinetics and engineering economics: Pressurized carbonization can significantly increase the carbonization reaction rate of steel slag aggregates, but the high-temperature and high-pressure carbonization reactor equipment is complex, costly, and energy-intensive, and safety issues need to be considered. The high investment cost of the equipment poses a huge challenge to the application of steel slag in low-value-added road materials;

[0013] (2) The surface carbonization degree of coarse-grained steel slag aggregate is high, while the internal carbonization degree is weak. The uneven carbonization poses a risk of secondary calcification and volume expansion to the long-term service performance of coarse-grained steel slag.

[0014] (3) There is a contradiction between deep carbonization and material properties: although acid leaching (pH < 3) can increase the calcium dissolution rate, it leads to the loss of minerals such as FeO and Fe3O4, significantly reducing the strength and wear resistance of the aggregate;

[0015] (4) The microbial-induced carbonization method is still in the research stage and more work is needed before it can be applied in engineering practice.

[0016] (5) The contradiction between carbonization efficiency and environmental risks: For example, although ultrasonic assistance can improve the CO2 mass transfer efficiency (the diffusion coefficient is increased by times), high-frequency vibration (20-40kHz) can easily cause the secondary release of heavy metal particles (such as Cr and V).

[0017] This technical background reveals the urgent need to develop a steel slag treatment method that is both deeply carbonized and economically feasible, and provides an innovative entry point for the bidirectional carbonization technology of the present invention. Summary of the Invention

[0018] To address the issue of uneven carbonization inside and outside the steel slag when it is used to replace the coarse aggregate in the surface layer of asphalt pavement (particle size 2.36-16mm, divided into four grades: 2.36mm-4.75mm, 4.75mm-9.5mm, 9.5mm-13.2mm, and 13.2mm-16mm), the following technical barriers need to be overcome:

[0019] (1) Limitation of long diffusion paths: Compared with fine aggregate (particle size < 2.36 mm), the effective diffusion path length of CO2 in coarse aggregate increases significantly, resulting in a decrease in carbonization efficiency;

[0020] (2) Surface barrier effect: Natural aging forms a CaCO3 coating on the surface of the steel slag coarse aggregate, which significantly increases the diffusion resistance of water and CO2;

[0021] (3) Gradient carbonization: Under the traditional unidirectional carbonization process, the residual gradient of f-CaO on the surface and inside of the coarse aggregate is large, which leaves the hidden danger of secondary calcification and volume expansion of the internal f-CaO during service after the surface ruptures.

[0022] (4) Dependence on high pressure: Existing technologies mainly rely on pressurization to promote the uniformity of carbonization inside and outside the coarse aggregate of steel slag. However, the application of pressurization in engineering practice still faces economic and safety challenges.

[0023] Therefore, the purpose of the invention is to solve the problem of uneven carbonization inside and outside of steel slag coarse aggregate without applying pressure. To this end, the present invention provides a method for bidirectional carbonization treatment of steel slag asphalt pavement surface coarse aggregate inside and outside. The overall solution is: provide a carbonization environment under normal pressure to carbonize the steel slag aggregate from the outside to the inside; at the same time, bury water and CO2 release factors (crystals) in the connected pores of the steel slag coarse aggregate, and use the crystals to decompose under heat to produce water and CO2 during the carbonization process. By controlling the release rate of water and CO2 at medium and low temperatures, the f-CaO inside the steel slag aggregate is slowly carbonized from the inside to the outside; finally, bidirectional carbonization of steel slag coarse aggregate inside and outside is achieved without applying pressure, thereby improving the volume stability of steel slag coarse aggregate and promoting its wide application in asphalt road surface layers.

[0024] According to the present invention, a method for bidirectional carbonization treatment of coarse aggregate in the surface layer of steel slag asphalt pavement is provided, comprising the following steps:

[0025] Step 1: Pre-treat the steel slag coarse aggregate with weak acid to remove the surface passivation layer;

[0026] Step 2: Lattice channel construction, which involves cooling the supersaturated bicarbonate solution to precipitate crystals, so that the precipitated crystals fill the interconnected pores of the slag aggregate;

[0027] Step 3: Perform the step 2 of vacuum cooling and crystallizing the steel slag aggregate in a saturated bicarbonate solution several times, drain the treated steel slag aggregate, and dry it at a constant temperature to a constant weight for later use;

[0028] Step 4: Internal and external bidirectional carbonization: The steel slag aggregate processed in Step 1-Step 3 is placed in a carbonization box for internal and external bidirectional carbonization under normal pressure.

[0029] Furthermore, the Step 1 adopts a gradient acid etching process to eliminate the surface passivation layer: an acidic solution is prepared, and the steel slag coarse aggregate is immersed in the acidic solution, and the acid solution is uniformly softened and the CaCO3 on the surface passivation layer of the steel slag aggregate is etched by dynamic stirring; then the passivation layer on the surface of the steel slag aggregate is rinsed and brushed with clean water, dried to a constant weight, and cooled to a predetermined temperature for storage.

[0030] Furthermore, the pH value of the acidic solution in Step 1 is 5-6.

[0031] Furthermore, the specific steps of Step 2 include: preparing a supersaturated carbonate solution, immersing the steel slag coarse aggregate pretreated with weak acid in Step 1 into a container containing a supersaturated bicarbonate solution, and then vacuum-controlling the container at a preset temperature; then, exhausting the air in the connected pores of the steel slag aggregate by step-by-step pressure reduction and filling the pores with saturated bicarbonate solution, and then slowly lowering the temperature to a point where the bicarbonate solution can precipitate crystals.

[0032] Furthermore, the specific steps of Step 3 include: cyclically heating and controlling the temperature of a container containing a supersaturated bicarbonate solution, and appropriately adding bicarbonate powder until the bicarbonate solution reaches saturation again and crystallization occurs.

[0033] Furthermore, the specific steps of Step 4 include: external carbonization is achieved through 100% concentration of CO2 circulating gas and a humidity environment of more than 95%; internal carbonization is achieved by slowly decomposing bicarbonate crystals filled in the connected pores at 80-100°C, releasing water and CO2 for internal f-CaO reaction, thereby achieving carbonization in the connected pores inside the steel slag aggregate.

[0034] Furthermore, the acidic solution is one or more of glacial acetic acid, oxalic acid, and citric acid.

[0035] Furthermore, the supersaturation degree of the supersaturated carbonate solution is between 1.05 and 1.35.

[0036] Furthermore, the supersaturated bicarbonate solution can release a supersaturated solution of water and carbon dioxide when heated.

[0037] Furthermore, the supersaturated bicarbonate solution is one or more of sodium bicarbonate (NaHCO3), magnesium bicarbonate (Mg(HCO3)2), and ammonium bicarbonate (NH4HCO3).

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. Compared with the natural aging method, the invention does not require a large amount of land resources to place steel slag, reduces pollution to the environment, has high carbonization efficiency (short treatment cycle), and high carbonization degree.

[0040] 2. Compared with the reactor pressure carbonization method, the invention does not require complex pressurizing equipment, has lower energy consumption, higher safety, simpler process operation, lower cost, and higher economic feasibility for engineering application.

[0041] 3. Compared with the strong acid treatment method, the weak acid acetic acid solution used in the present invention is only used as an auxiliary to remove the surface calcium carbonate, which is less destructive to the steel slag aggregate and will not cause the precipitation of heavy metals to cause environmental pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0043] Figure 1 This is a schematic diagram of the test results of the changes in 13.2-16 mm steel slag CaCO3 after carbonization according to the present invention;

[0044] Figure 2 This is a schematic diagram of the test results of the changes in 9.5mm-13.2mm steel slag CaCO3 after carbonization according to the present invention;

[0045] Figure 3 This is a schematic diagram of the test results of the changes in 4.75mm-9.5mm steel slag CaCO3 after carbonization according to the present invention;

[0046] Figure 4 This is a schematic diagram of the test results of the changes in 2.36mm-4.75mm steel slag CaCO3 after carbonization according to the present invention;

[0047] Figure 5 Schematic diagram of the test results of the change of f-Cao content in coarse aggregates of 13.2-16mm steel slag before and after carbonization;

[0048] Figure 6 This is a schematic diagram of the test results of the changes in f-Cao content in coarse aggregates of 9.5mm-13.2mm steel slag before and after carbonization;

[0049] Figure 7 Schematic diagram of the test results of the change of f-Cao content in coarse aggregates of steel slag of various sizes before and after carbonization;

[0050] Figure 8 Schematic diagram of the test results of the change of f-Cao content in coarse aggregates of 2.36mm-4.75mm steel slag before and after carbonization;

[0051] Figure 9 It is a simplified structural diagram of the carbonization device of the present invention. DETAILED DESCRIPTION

[0052] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0053] In order to use steel slag to replace the surface coarse aggregate of asphalt pavement (particle size 2.36-16 mm, divided into four grades: 2.36 mm-4.75 mm, 4.75 mm-9.5 mm, 9.5 mm-13.2 mm, and 13.2 mm-16 mm), the present invention provides a method for the bidirectional carbonization treatment of the surface coarse aggregate of steel slag asphalt pavement, comprising the following steps:

[0054] Step 1: Pre-treat the steel slag coarse aggregate with weak acid to remove the surface passivation layer. Specifically, a gradient acid etching process is used to eliminate the surface CaCO3 barrier layer: prepare an acidic solution with a pH value of 5-6, immerse the steel slag coarse aggregate in the acidic solution, and use dynamic stirring to achieve uniform softening and etching of the CaCO3 on the surface of the steel slag aggregate by the acid solution. Then rinse with clean water and brush to remove the CaCO3 barrier layer on the surface of the steel slag aggregate, dry it to constant weight, and cool it to a predetermined temperature for storage. In particular, acidic substances such as glacial acetic acid, oxalic acid, citric acid, etc. are used. Generally, the predetermined temperature is set at around 50°C.

[0055] Step 2: Lattice channel construction. Crystals are precipitated by cooling the supersaturated bicarbonate solution, allowing the precipitated crystals to fill the interconnected pores of the steel slag aggregate. Specifically: a supersaturated carbonate solution is prepared, and the steel slag coarse aggregate, which has undergone weak acid pretreatment in Step 1, is immersed in a container containing the supersaturated bicarbonate solution. The container is then vacuum-controlled at a preset temperature. Subsequently, the air in the interconnected pores of the steel slag aggregate is expelled through a stepwise decompression process, and the pores are filled with a saturated bicarbonate solution. The temperature is then slowly lowered to a point where the bicarbonate solution can precipitate crystals. The preset temperature setting varies depending on the bicarbonate solution. The bicarbonate solution can be a supersaturated solution that releases water and carbon dioxide upon heating, such as sodium bicarbonate (NaHCO3), magnesium bicarbonate (Mg(HCO3)2), or ammonium bicarbonate (NH4HCO3).

[0056] Step 3: Repeat the process of Step 2 several times by vacuum cooling and crystallizing the slag aggregate in a saturated bicarbonate solution. Drain the treated slag aggregate and dry it at a constant temperature until it reaches a constant weight. Specifically, a container containing a supersaturated bicarbonate solution is heated in a cycle with controlled temperature, and bicarbonate powder is added as needed until the bicarbonate solution reaches saturation again and crystallizes. Draining the treated slag aggregate can also be accelerated by using an electric fan, for example.

[0057] Step 4: Internal and external bidirectional carbonization: The steel slag aggregate treated in Steps 1-3 is placed in a carbonization chamber for internal and external bidirectional carbonization under normal pressure. External carbonization is achieved through a 100% concentration of CO2 circulating gas and a humidity environment above 95%. Internal carbonization is achieved by slowly decomposing bicarbonate crystals in the interconnected pores at 80-100°C, releasing water and CO2 for the internal f-CaO reaction, thereby achieving carbonization within the interconnected pores of the steel slag aggregate.

[0058] The following uses glacial acetic acid as the acidic substance and NaHCO3 as the bicarbonate as an example to specifically illustrate the implementation method of the present invention. Figure 9 As shown:

[0059] 1. Remove the surface passivation layer (weak acid pretreatment). Use gradient acid etching process to remove the surface CaCO3 barrier layer:

[0060] (1) Prepare the buffered acid system: add glacial acetic acid (CH3COOH) to deionized water at 45±5℃ and adjust the pH to 5-6;

[0061] (2) Acid leaching of steel slag coarse aggregate: Impregnate steel slag coarse aggregate (2.36-16 mm) of the fourth-grade particle size at a solid-liquid ratio of 100-300 g / L for 30-60 minutes. Dynamic stirring (frequency 5-8 times / 10 minutes) is used to achieve uniform softening of the acid solution and erosion of CaCO3 on the surface of the steel slag aggregate.

[0062] (3) Post-treatment process: Rinse the steel slag coarse aggregate after acid leaching with clean water, and use mechanical brushing (such as brushing the surface with a wire brush) to remove the surface CaCO3 barrier layer, drain and dry it to constant weight, and cool it to 50℃ for storage.

[0063] 2. Lattice channel construction (NaHCO3 phase change filling) to achieve crystal growth control.

[0064] (1) Preparation of supersaturated NaHCO3 solution: Prepare a saturated NaHCO3 solution at 50±1℃, and control the supersaturation of the solution between 1.05 and 1.35;

[0065] (2) Vacuum cooling crystallization: The steel slag coarse aggregate after weak acid pretreatment is placed in a mesh basket, and then the mesh basket is hung in a container filled with 50°C saturated NaHCO3 solution until the steel slag aggregate is completely immersed. The entire container is placed in a vacuum temperature-controlled box and the temperature is kept constant at 50°C. Then the pressure is reduced to 40KPa~60KPa in a step-by-step manner to discharge the air in the pores of the steel slag aggregate and fill it with saturated NaHCO3 solution. The rate of pressure reduction is controlled at 5KPa~20KPa / min. When the temperature of the oven slowly drops to 20°C, the steel slag aggregate is taken out and drained.

[0066] (3) Cyclic strengthening mechanism: The NaHCO3 solution from step 1 is reheated to 50°C, and NaHCO3 powder is added until the NaHCO3 solution reaches saturation again and crystallization occurs, with the supersaturation of the solution controlled between 1.05 and 1.35; the drained slag aggregate is placed back into the saturated NaHCO3 solution, and the vacuum cooling and crystallization steps are repeated;

[0067] 3. Carry out the steps of vacuum cooling and crystallization of steel slag aggregate in saturated NaHCO3 solution for 3-5 times, drain the treated steel slag aggregate, dry it at 20℃ to constant weight, or use an electric fan to accelerate drying for later use.

[0068] 4. Internal and external bidirectional carbonization: The steel slag aggregate processed in steps 1-3 is placed in a carbonization box for internal and external bidirectional carbonization. External carbonization is achieved through 100% CO2 circulating gas and a humidity environment above 95%. Internal carbonization is achieved by slowly decomposing NaHCO3 crystals filled in the interconnected pores at 80℃-100℃, releasing water and CO2 for the internal f-CaO reaction. The specific steps are as follows:

[0069] (1) Pour water into the carbonization box, and the water volume shall not exceed one-fifth of the volume of the carbonization box;

[0070] (2) placing the steel slag aggregate in a mesh basket, which is suspended above the carbonization box, with the bottom of the mesh basket at a distance of not less than one-fifth the height of the carbonization box, i.e., above the surface of the bottom water;

[0071] (3) Close the carbonization box and set the temperature to 80-100°C. Introduce CO2 gas at a flow rate of 100-200 ml / min until the box is fully filled with CO2. Then reduce the flow rate to 10-20 ml / min and connect a gas circulation pump to maintain the concentration in the box at 100%. Turn on the micro-mist spray system and continuously spray micron-level water mist to maintain the humidity in the carbonization box at 95-100%. Continue carbonization for 3-7 days.

[0072] (4) The steel slag aggregate after the internal and external bidirectional carbonization treatment is cleaned and rinsed, drained and then dried to a constant weight.

[0073] The following describes in combination with relevant experimental data the beneficial effects of the internal and external bidirectional carbonization treatment of steel slag coarse aggregate with four particle sizes.

[0074] 1. Rinse and dry the coarse aggregate of steel slag with the fourth-grade particle size for later use.

[0075] 2. Titration tests for f-Cao content were conducted on coarse slag aggregates of each particle size (EDTA titration and thermogravimetric analysis, JTG-3432-T0375-2004). A representative piece of slag aggregate from each particle size was subjected to Micro CT scanning (scanning accuracy of 4 microns) and pore and morphology analysis.

[0076] 3. Coarse steel slag aggregate of four particle sizes was acid-leached in a weak acetic acid solution (pH 6) at 50°C for 35 minutes. The aggregate surface was then cleaned with a wire brush and drained for later use. Micro CT scanning (4-micron resolution) was performed on one piece of coarse steel slag aggregate of each particle size after the weak acid wash to analyze its pore structure and morphology.

[0077] 4. Prepare a supersaturated NaHCO3 solution at 50°C with a supersaturation of 1.2. Place coarse steel slag aggregate of four particle sizes into the supersaturated NaHCO3 solution and then place it in a vacuum-controlled oven at 50°C. Evacuate the chamber at a rate of 10 kPa / min until the pressure in the oven drops to 50 kPa. Once the NaHCO3 solution temperature drops to 20°C, remove the steel slag aggregate and drain. Re-heat the solution to 50°C, add NaHCO3 powder to prepare a supersaturated NaHCO3 solution with a supersaturation of 1.2. Place the steel slag aggregate back into the supersaturated NaHCO3 solution and vacuum cool and crystallize. Repeat this step three times. Remove the steel slag aggregate, drain, and dry at 20°C to constant weight. Micro CT scan (4 micron resolution) of each coarse steel slag aggregate after vacuum cooling and crystallization to analyze pore size and morphology.

[0078] 5. The crystallized steel slag coarse aggregate was placed in a carbonization chamber and carbonized using a bidirectional carbonization method. The temperature was set at 90°C, the CO2 flow rate was set at 20 ml / min, the micro-mist nozzle diameter was 0.3 mm, the water mist flow rate was 30 ml / min, and the carbonization time was 48 hours. The carbonized steel slag coarse aggregate was titrated for f-Cao content, and each representative batch of steel slag aggregate was subjected to micro CT scanning (scanning accuracy of 4 microns) to analyze its porosity and morphology.

[0079] Experimental results:

[0080] According to the national standard for testing aggregates for highway engineering (JTG 3432-2024), the f-CaO content of steel slag coarse aggregates of various particle sizes was titrated before and after carbonization for 3 days. The test results are as follows:

[0081] (1) Changes in the quality of steel slag coarse aggregate during processing:

[0082] Table 1. Aggregate quality changes

[0083] original Pickling Mass change rate crystallization Mass change rate carbonization Mass change rate 13.2-16 5.0379 4.9619 -1.53％ 5.1773 4.16％ 4.9655 -4.27％ 9.5-13.2 2.3105 2.2495 -2.71％ 2.3659 4.92％ 2.2481 -5.24％ 4.75-9.5 1.1027 1.0607 -3.96％ 1.0946 3.10％ 1.0433 -4.92％ 2.36-4.75 0.1876 0.1796 -4.45％ 0.1915 6.21％ 0.1733 -10.50％ Total mass 8.6387 8.4517 -2.21％ 8.8293 4.28％ 8.4302 -4.73％

[0084] After weak acid treatment, the quality of the fourth-grade aggregate decreases mainly because the CaCO3 barrier layer formed by natural aging is softened and polished away. The supersaturated NaHCO3 solution is cooled from 50°C to 20°C and crystallized. Under the action of negative pressure (50Kpa), it grows outward along the pores inside the slag aggregate, and undergoes multiple crystallization treatments until the pores are filled. Therefore, the quality of the slag aggregate increases after crystallization. Subsequently, during the carbonization process, the NaHCO3 crystals are decomposed by heat to produce Na2CO3, water and CO2, and water and CO2, which react with the f-CaO inside the slag to produce CaCO3. The Na2CO3 crystals attached to the surface are then washed away, so the quality of the slag decreases after carbonization.

[0085] (2) Changes of CaCO3 in each aggregate during the treatment process Figures 1-4 As shown, the gray lines are CaCO3:

[0086] The results show that the CaCO3 content of the slag aggregate decreased after weak acid pickling treatment, decreased slightly after crystallization, and increased rapidly after three days of carbonization, exceeding the CaCO3 content in the untreated original slag. Visualization results also show a significant increase in CaCO3 content after carbonization (increased density of gray lines). The decrease in CaCO3 content after weak acid pickling is due to the reaction of the weak acid with the CaCO3 coating formed on the surface of the slag aggregate due to natural aging. The slight decrease in CaCO3 content after crystallization is due to the crystallization of NaHCO3 within the pore channels, which is accompanied by volume expansion, causing further flaking of the CaCO3 coating on the pore walls.

[0087] The significant increase in CaCO3 content after carbonization is due to the carbonization of f-CaO to generate a large amount of CaCO3. The changes in the morphology and content of CaCO3 during slag processing fully demonstrate the feasibility and efficiency of the "bidirectional carbonization method for steel slag coarse aggregate" proposed in this invention.

[0088] (3) Changes in f-Cao content in coarse aggregates of steel slag at various levels before and after carbonization, such as Figure 5-Figure 8 As shown:

[0089] The test results show that before treatment, the f-Cao content in steel slag coarse aggregates of various particle sizes was 13.2mm-16mm (23.49%), 9.5mm-13.2mm (20.77%), 4.75mm-9.5mm (19.04%), and 2.36mm-4.75mm (17.26%); after treatment and carbonization for 72 hours, the f-Cao content in steel slag coarse aggregates of various particle sizes was 13.2mm-16mm (6.04%), 9.5mm-13.2mm (4.94%), 4.75mm-9.5mm (5.51%), and 2.36mm-4.75mm (6.04%), and the f-CaO content decreased by 74.28%, 76.21%, 71.08%, and 65.03%, respectively.

[0090] This finding indicates that the carbonization method proposed in this invention is effective in removing f-CaO from steel slag coarse aggregate. However, after three days of carbonization, the f-CaO content still does not meet the industry's general requirements for steel slag used as asphalt mixture aggregate (f-CaO ≤ 3%). Therefore, this invention measured the f-CaO content in steel slag coarse aggregate at various stages of carbonization for five and seven days. The results are shown in the figure below: After five to seven days of carbonization, the f-CaO content in each aggregate will meet the industry's general requirements (i.e., f-CaO ≤ 3%).

[0091] In summary, compared with the natural aging method, which takes up to 1-3 years to carbonize, the present invention does not require a large amount of land resources to store steel slag, reducing environmental pollution, and has high carbonization efficiency, a short processing cycle, and a high degree of carbonization. Compared with the high-pressure reactor carbonization method, the present invention has the following advantages: Compared with the high-pressure reactor carbonization method, the present invention has simple carbonization equipment, does not require pressurization, is safer, and has a very broad prospect for large-scale industrial promotion and application.

[0092] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A method for bidirectional carbonization treatment of the inner and outer surfaces of the coarse aggregate of the surface layer of steel slag asphalt pavement, characterized in that: The following steps are involved: Step 1: Pre-treat the steel slag coarse aggregate with weak acid to remove the surface passivation layer; Step 2: Lattice channel construction, which involves cooling the supersaturated bicarbonate solution to precipitate crystals, so that the precipitated crystals fill the interconnected pores of the slag aggregate; Step 3: Perform the step 2 of vacuum cooling and crystallizing the steel slag aggregate in a saturated bicarbonate solution several times, drain the treated steel slag aggregate, and dry it at a constant temperature to a constant weight for later use; Step 4: Internal and external bidirectional carbonization: The steel slag aggregate processed in Step 1-Step 3 is placed in a carbonization box for internal and external bidirectional carbonization under normal pressure.

2. The method for bidirectional carbonization treatment of surface coarse aggregate of steel slag asphalt pavement according to claim 1, characterized in that: The step 1 adopts a gradient acid etching process to eliminate the surface passivation layer: prepare an acidic solution, immerse the steel slag coarse aggregate in the acidic solution, and achieve uniform softening and etching of CaCO3 on the surface passivation layer of the steel slag aggregate by dynamic stirring; then rinse and brush with clean water to remove the passivation layer on the surface of the steel slag aggregate, dry it to constant weight, and cool it to a predetermined temperature for storage.

3. The method for bidirectional carbonization treatment of surface coarse aggregate of steel slag asphalt pavement according to claim 2, characterized in that: The pH value of the acidic solution in Step 1 is 5-6.

4. The method for bidirectional carbonization treatment of surface coarse aggregate of steel slag asphalt pavement according to claim 1, characterized in that: The specific steps of Step 2 include: preparing a supersaturated carbonate solution, immersing the steel slag coarse aggregate pretreated with weak acid in Step 1 into a container containing the supersaturated bicarbonate solution, and then vacuum-controlling the container at a preset temperature; then, exhausting the air in the interconnected pores of the steel slag aggregate by stepwise pressure reduction until the pores are filled with saturated bicarbonate solution, and then slowly lowering the temperature to a level where the bicarbonate solution can precipitate crystals.

5. The method for bidirectional carbonization treatment of surface coarse aggregate of steel slag asphalt pavement according to claim 1, characterized in that: The specific steps of Step 3 include: cyclically heating and controlling the temperature of a container containing the supersaturated bicarbonate solution, and appropriately adding bicarbonate powder until the bicarbonate solution reaches saturation again and crystallization occurs.

6. The method for treating the inner and outer bidirectional carbonization of the surface coarse aggregate of steel slag asphalt pavement according to claim 1, characterized in that: The specific steps of Step 4 include: external carbonization is achieved through 100% concentration of CO2 circulating gas and a humidity environment of more than 95%; internal carbonization is achieved by slowly decomposing bicarbonate crystals filled in the connected pores at 80-100°C, releasing water and CO2 for internal f-CaO reaction, thereby achieving carbonization in the connected pores inside the steel slag aggregate.

7. The method for bidirectional carbonization treatment of surface coarse aggregate of steel slag asphalt pavement according to claim 2, characterized in that: The acidic solution is one or more of glacial acetic acid, oxalic acid and citric acid.

8. The method for treating the inner and outer bidirectional carbonization of the surface coarse aggregate of steel slag asphalt pavement according to claim 4, characterized in that: The supersaturation degree of the supersaturated carbonate solution is between 1.05 and 1.

35.

9. The method for treating the inner and outer bidirectional carbonization of the surface coarse aggregate of steel slag asphalt pavement according to claim 1, characterized in that: The supersaturated bicarbonate solution can release a supersaturated solution of water and carbon dioxide when heated.

10. The method for treating the inner and outer bidirectional carbonization of the surface coarse aggregate of steel slag asphalt pavement according to claim 1, characterized in that: The supersaturated bicarbonate solution is one or more of sodium bicarbonate (NaHCO3), magnesium bicarbonate (Mg(HCO3)2), and ammonium bicarbonate (NH4HCO3).

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