Low-calcium low-expansion risk steel slag rap cement stabilized base structure and construction method

Abstract

This invention discloses a steel slag RAP cement-stabilized base course structure and construction method with low calcium efflorescence and low expansion risk, relating to the field of road engineering base course materials technology. The structure includes an upper closed impermeable layer, a steel slag RAP cement-stabilized bearing layer, a lower drainage and isolation layer, and an edge closed water-conducting structure. The bearing layer comprises steel slag aggregate, RAP aggregate, natural crushed stone, cement, fly ash, mineral powder, silica fume or metakaolin, and water. During construction, the immersion expansion rate, f-CaO, f-MgO, and calcium ion leaching concentration of the steel slag are first tested. The calcium ion leaching descriptor Lca, expansion risk index Pe, and calcium efflorescence risk index Pca are calculated. Feedback adjustments are made through steel slag content, RAP flexible water-proof buffer, calcium-capturing and calcium-fixing fine powder dosage, drainage isolation coefficient Dp, and edge water-conducting structure to ensure that Pe ≤ 0.55 and Pca ≤ 1.20 in general areas, and Pe ≤ 0.40 and Pca ≤ 0.85 in high-humidity areas. This invention synergistically reduces the risks of steel slag volume expansion, calcium ion migration, whitening deposition, and drainage blockage, and is suitable for road base engineering with high solid waste substitution rates.

Description

Technical Field

[0001] This invention relates to the field of road engineering base material technology, specifically a steel slag RAP cement-stabilized base structure and construction method with low calcium efflorescence and low expansion risk. Background Technology

[0002] Cement-stabilized crushed stone base courses have advantages such as high load-bearing capacity, good integrity, and mature construction technology, making them a commonly used semi-rigid base course in high-grade highways and municipal roads. However, traditional cement-stabilized crushed stone base courses consume a large amount of natural aggregates, and the amount of cement used leads to carbon emissions. Furthermore, semi-rigid base courses are prone to cracking under the combined effects of drying shrinkage, thermal shrinkage, and water damage. Using solid waste or recycled materials such as steel slag and RAP in the base course can reduce the mining of natural aggregates and improve resource utilization.

[0003] Steel slag possesses high hardness, a rough surface, and potential cementitious activity. After aging, crushing, screening, and stabilization treatment, it can be used as aggregate for road base courses. Reclaimed asphalt (RAP) contains old aggregate and old asphalt film, and can be used as recycled aggregate, providing a degree of flexible interface and hydrophobic buffering. However, residual f-CaO, f-MgO, and soluble calcium components in steel slag may cause volume expansion, alkaline leaching, and calcium efflorescence. In engineering projects, if the base course is permeable to water for a long period, calcium ions released from the steel slag may migrate with the water, forming white deposits in drainage paths or base course pores, causing calcium efflorescence, pore blockage, and localized drainage problems. Although the old asphalt film on the RAP surface has a hydrophobic buffering effect, it may also affect cement slurry coating and early strength. Therefore, simply replacing natural aggregates with steel slag and RAP in a fixed ratio is insufficient to simultaneously address the risks of steel slag expansion, calcium efflorescence, RAP interface weakening, and base course drainage stability.

[0004] Based on verification of publicly available information, existing technologies similar to this invention mainly include the following directions:

[0005] CN109437745A, a shrinkage-compensating cement-stabilized crushed stone and its preparation method and use, which incorporates steel slag fine aggregate into cement-stabilized crushed stone and uses the micro-expansion of steel slag to compensate for the shrinkage of the base material. This invention mainly focuses on steel slag shrinkage compensation and does not address the combined control of steel slag soluble calcium migration, calcium deposit, drainage path blockage and RAP flexible water-proof buffer.

[0006] CN114905623A, an environmentally friendly cement-stabilized crushed stone mixing process, uses RAP and recycled aggregate after crushing and screening for cement-stabilized crushed stone, and improves uniformity through mixing process. This invention focuses on RAP recycling and mixing uniformity, but does not include the risks of steel slag expansion, soluble calcium leaching and base drainage isolation structure in unified control.

[0007] CN116623487A, a method for preparing a rapid-forming recycled water-stabilized base course, calculates the RAP content based on the RAP material variation coefficient and pavement performance index, and is used for rapid-forming recycled water-stabilized base course. This invention focuses on RAP content and rapid forming, and does not involve steel slag calcification, expansion stability classification and calcium trapping and calcium-fixing filler system.

[0008] CN116854393B, Stabilization and Application of Carbonized and Sulfated Steel Slag-Based Cementitious Materials for Carbonized Steel Slag Crushed Stone, improves the stability and base performance of steel slag-based materials by carbonizing and sulfidating steel slag. This invention focuses on the carbonization and sulfidation treatment of steel slag and steel slag-based cementitious materials, but does not combine it with RAP old asphalt film to form a moisture migration buffer layer, nor does it propose an internal calcification risk index and drainage anti-clogging structure for the base layer.

[0009] CN110158388B, a process method for in-situ cold recycling of cement-stabilized crushed stone base course, determines the optimal amount of asphalt residue in milled material through experiments and adjusts the addition method of large-diameter aggregate and cement. The subject of this invention is in-situ cold recycling of water-stabilized base course, and does not involve the control of expansion, calcium migration and calcification when steel slag is used as a substitute aggregate.

[0010] User Guidelines for Waste and Byproduct Materials in Pavement Construction, FHWA-RD-97-148, discloses issues that should be considered when using steel slag as a road base material, such as volume expansion and calcium deposition. The document points out the risks of using steel slag base materials, but does not provide specific material structures for calcium capture and fixation, stratified drainage, and joint control of risk index in steel slag-RAP-cement stabilized crushed stone.

[0011] As can be seen from the publicly available information above, the existing technologies disclose steel slag water-stabilized shrinkage compensation, RAP water-stabilized recycling, RAP dosage calculation, steel slag carbonization and sulfidation treatment, and in-situ cold recycling processes, but have not yet formed a base structure and construction method that combines "steel slag stability classification, calcification risk index, RAP old asphalt film water-proof buffer, calcium capture and calcium fixation fine powder, drainage and anti-blocking base structure, and construction feedback adjustment" into a closed scheme.

[0012] Problems with existing technology

[0013] 1. Existing steel slag cement-stabilized crushed stone solutions mostly emphasize strength, shrinkage compensation, or solid waste utilization rate, while paying insufficient attention to the migration of soluble calcium from steel slag, calcium carbonate deposition, and drainage path blockage.

[0014] 2. Existing RAP water-stabilized recycling schemes are usually designed according to RAP dosage, gradation or coefficient of variation, which do not make full use of the hydrophobic buffering effect of old RAP asphalt film to control the migration of moisture and the precipitation of calcium from steel slag inside the base course.

[0015] 3. Existing steel slag treatment solutions mostly adopt single pretreatment methods such as aging, carbonization or sulfidation, without linking the steel slag risk classification results with the layering of the base structure, calcium capture and fixation fine powder and drainage isolation design.

[0016] 4. When the sources of steel slag and RAP fluctuate significantly, a fixed dosage scheme cannot guarantee that the expansion risk, calcium efflorescence risk, strength and compaction performance will be met simultaneously.

[0017] 5. Existing base construction methods mostly emphasize mixing and compaction, but lack targeted control measures for edge water ingress, interlayer water retention, drainage ditch blockage, and early curing period soluble calcium migration.

[0018] In existing technologies, the application of steel slag in cement-stabilized base courses has revealed the following risks: Steel slag water-stabilized materials exhibit an untreated 180-day water immersion expansion rate of 0.093%–0.117%, and bulging and cracking occur (Zhou et al., 2021); the median pH value of alkaline drainage from steel slag can reach 10.3–10.9, and the Ca... 2+ Concentrations reached as high as 805 mg / L, and calcite deposits formed in the drainage path, resulting in a porosity loss of 35.7%–55.0% in the drainage layer after long-term operation (Riley et al., 2015; Mayes et al., 2006; Claveau-Mallet et al., 2020). These risks indicate that simply relying on aging or conventional dosage adjustments cannot simultaneously solve the problems of expansion, calcification, and drainage blockage. Summary of the Invention

[0019] The technical problem this invention aims to solve is: when using steel slag and RAP to replace part of the natural aggregate in a cement-stabilized crushed stone base course, how to reduce the risks of steel slag expansion, calcium whitening, drainage blockage, and water stability deterioration while ensuring the load-bearing capacity and construction adaptability of the base course.

[0020] Specifically, it includes:

[0021] 1. How to classify the stability and calcium carbonate risk of steel slag so that different risk levels of steel slag correspond to different admixture amounts, pretreatment methods and structural locations.

[0022] 2. How to utilize the hydrophobic and flexible characteristics of RAP old asphalt film to form a buffer against moisture migration and interfacial stress of hard steel slag, instead of simply using RAP as ordinary recycled aggregate.

[0023] 3. How to set up a calcium-capturing and calcium-fixing fine powder system to allow the release of Ca(OH)2 or Ca from steel slag to... 2+ It is preferentially consumed or fixed within the cementing system, reducing the risk of it migrating into the drainage path and forming sediment.

[0024] 4. How to prevent water from stagnating inside the steel slag RAP bearing layer for a long time by using a layered base structure, edge sealing, and lower drainage.

[0025] 5. How to use calcification risk index and calcium carbonate risk index to adjust the steel slag content, RAP content, calcium precipitator powder content and drainage structure.

[0026] To achieve the above objectives, the technical solution of the present invention is as follows:

[0027] A steel slag RAP cement-stabilized base course structure with low calcium efflorescence and low expansion risk includes: an upper closed impermeable layer; a steel slag RAP cement-stabilized load-bearing layer disposed below the upper closed impermeable layer; a lower drainage and isolation layer disposed below the steel slag RAP cement-stabilized load-bearing layer; and an edge closed water-conducting structure disposed at the edge of the base course structure; wherein the steel slag RAP cement-stabilized load-bearing layer comprises the following components by dry weight:

[0028] Steel slag aggregate 35-60 parts; RAP aggregate 15-35 parts; natural crushed stone 0-35 parts; cement 2.5-5.0 parts; fly ash 3-8 parts; mineral powder 2-6 parts; silica fume or metakaolin 0.3-3.0 parts; water 4.5-7.0 parts.

[0029] Water 4.5–7.0 parts; and the above components and structure are constrained by Pe and Pca, so that Pe≤0.55 and Pca≤1.20 in general areas, and Pe≤0.40 and Pca≤0.85 in high humidity areas.

[0030] Furthermore, the particle size of RAP aggregate is less than 9.5 mm, and RAP with a particle size of less than 4.75 mm accounts for 40% to 80% of the total RAP aggregate.

[0031] Furthermore, the steel slag aggregate includes coarse steel slag aggregate with a particle size of 4.75–31.5 mm and fine steel slag aggregate with a particle size of less than 4.75 mm.

[0032] Furthermore, the upper sealing impermeable layer is one or more of the following: asphalt seal, slurry seal, combination of tack coat and lower seal, or low-permeability cement-stabilized transition layer.

[0033] Furthermore, the thickness of the lower guide isolation layer is 2-8cm, and its permeability coefficient is not less than 3 times that of the steel slag RAP cement stabilized bearing layer.

[0034] Furthermore, the thickness of the upper closed impermeable layer is 0.5–2.0 cm; the thickness of the steel slag RAP cement stabilized bearing layer is 15–40 cm; and the width of the edge closed water-conducting structure is 10–30 cm, with its bottom connected to the lower drainage isolation layer.

[0035] Furthermore, the mass ratio of steel slag aggregate to RAP aggregate is 1.0 to 4.0:1.

[0036] A construction method for a steel slag RAP cement-stabilized base structure with low calcium efflorescence and low expansion risk includes the following steps:

[0037] S1: Steel slag is aged, washed, magnetically separated, crushed and screened to obtain steel slag aggregate;

[0038] S2: Determine the immersion swelling rate Es, free calcium oxide content, free magnesium oxide content, and calcium ion leaching concentration Cca of the steel slag, and calculate the calcium ion leaching descriptor Lca according to formula (1):

[0039] Lca = Cca / 100 (1);

[0040] S3: Based on the test results of step S2, classify the steel slag into low-risk, medium-risk, or high-risk levels;

[0041] S4: RAP is crushed and sieved, and the Ar content of old asphalt is determined;

[0042] S5: Based on the target road grade, base layer thickness, steel slag risk level and RAP properties, preliminary material ratio and parameters of the lower guide and isolation layer are proposed;

[0043] S6: Calculate the inflation risk index Pe according to formula (2), and calculate the pan-calcification risk index Pca according to formula (3):

[0044] Pe = S × Es / (R × Ar + N + 0.1) (2),

[0045] Pca = S × Lca / (Fp × Dp + 0.1) (3),

[0046] In the formula, S is the mass fraction of steel slag in the dry mixture, which can also be denoted as Ms; Es is the percentage value of the steel slag's water immersion swelling rate; R is the mass fraction of RAP; Ar is the percentage value of the old asphalt content in RAP; N is the mass fraction of natural crushed stone; and 0.1 is a correction constant to avoid the denominator being zero. R×Ar characterizes the buffering contribution of the old asphalt film of RAP to moisture migration and rigid interface stress, and N characterizes the dilution contribution of non-expanding aggregate to the expansion risk of steel slag. Pe is preferably not greater than 0.55, preferably not greater than 0.40 in high humidity areas, and more preferably 0.10 to 0.40. When Pe is greater than 0.55, the proportion of steel slag with high expansion risk should be reduced, the proportion of flexible buffering of RAP should be increased, the proportion of non-expanding aggregate should be increased, or the steel slag should be subjected to enhanced stability treatment.

[0047] S7: Determine whether Pe and Pca meet the target threshold; if not, adjust the steel slag content, RAP content, calcium capture and fixation fine powder dosage or drainage isolation parameters, and recalculate until they are met.

[0048] S8: Pre-wet the steel slag that meets the threshold, and then add fly ash, mineral powder, silica fume or metakaolin to form a pre-coating;

[0049] S9: Add RAP, cement and remaining water and mix.

[0050] S10: First, lay the lower drainage isolation layer, then spread and compact the steel slag RAP cement stabilized bearing layer, and then set the upper closed anti-seepage layer and the edge closed water-conducting structure.

[0051] Furthermore, the target thresholds mentioned in step S7 are Pe≤0.55 and Pca≤1.20.

[0052] Furthermore, the thickness of the lower guide isolation layer in step S10 is 4 to 8 cm.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] This invention expands the risk control of steel slag application from a single stability control to a comprehensive control of "expansion risk, calcium bloom risk and drainage blockage risk", which is more suitable for engineering scenarios where steel slag is used in a high proportion of road base courses.

[0055] This invention does not simply use RAP as recycled aggregate, but rather utilizes the hydrophobic and flexible characteristics of RAP old asphalt film to reduce moisture migration inside the base layer and stress concentration at the interface of hard steel slag.

[0056] This invention sets up a calcium-capturing and calcium-fixing fine powder system, which allows the calcium components released from steel slag to be preferentially consumed or fixed within the cementing system, reducing the risk of calcium ions migrating to the drainage path and forming sediments.

[0057] This invention combines material proportions with layered base structure, reducing the risk of calcium efflorescence and expansion under long-term waterlogging conditions through upper sealing and impermeability, lower drainage and isolation, and edge water diversion.

[0058] This invention provides two feedback indicators, Pe and Pca, which enable steel slag and RAP from different sources to adjust their dosage and structural parameters based on measured properties, rather than relying on fixed empirical ratios.

[0059] This invention can be integrated with conventional cement-stabilized crushed stone mixing, spreading, compaction and curing processes, with only the addition of raw material risk detection, pre-coating of calcium-capturing and calcium-fixing fine powder and local drainage and isolation structures, resulting in less resistance to engineering promotion. Attached Figure Description

[0060] Figure 1This is a block diagram of the steel slag RAP cement-stabilized base course structure and risk control system of the present invention.

[0061] Figure 2 This is a technical flowchart of the construction method for steel slag RAP cement-stabilized base structure of the present invention.

[0062] Figure 3 This is a flowchart of the construction method for steel slag RAP cement-stabilized base structure according to the present invention.

[0063] Figure 4 This is a layered schematic diagram of the base structure, showing the relative positions of the upper closed impermeable layer, the steel slag RAP cement stabilized bearing layer, the lower drainage and isolation layer, and the edge closed water-conducting structure.

[0064] Figure 5 This is a schematic diagram of the pre-wetting of steel slag and pre-coating of calcium-capturing and calcium-fixing fine powder according to the present invention, which shows the distribution of calcium-capturing and calcium-fixing fine powder on the surface of steel slag and near the pores. Detailed Implementation

[0065] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0066] RAP: Reclaimed Asphalt Pavement refers to recycled asphalt milling material.

[0067] The steel slag RAP cement stabilized base structure provided by the present invention includes an upper closed anti-seepage layer, a steel slag RAP cement stabilized bearing layer, a lower drainage isolation layer, and an edge closed water-conducting structure.

[0068] The upper sealing impermeable layer is used to reduce the infiltration of rainwater or surface water into the bearing layer. It can be one or more of the following: asphalt seal, slurry seal, a combination of prime coat and under-seal, or a low-permeability cement-stabilized transition layer. The steel slag RAP cement-stabilized bearing layer bears the main load transfer and solid waste replacement functions, while inhibiting steel slag expansion and calcium efflorescence through the RAP flexible water-impermeable buffer phase and calcium-capturing and calcium-fixing fine powder system. The lower drainage and isolation layer is used to prevent long-term water stagnation at the bottom of the bearing layer and to provide a drainage path for water infiltrating during early curing or service. The edge sealing drainage structure is used to reduce lateral water inflow and to drain water from the edges or bottom.

[0069] Steel slag aggregate is preferably used after aging, washing, magnetic separation, crushing and screening. The particle size includes coarse steel slag aggregate of 4.75-31.5 mm and fine steel slag aggregate of less than 4.75 mm. RAP aggregate is crushed and screened to less than 9.5 mm, and RAP of less than 4.75 mm accounts for 40%-80% of the total RAP, so as to improve the dispersion and flexible buffering effect of old RAP asphalt film in the load-bearing layer.

[0070] Fly ash, mineral powder, silica fume, or metakaolin are used as calcium-capturing and calcium-fixing fine powder systems to consume or fix the calcium components released from steel slag through pozzolanic reaction, microfilling, adsorption, and aluminosilicate reaction. Preferably, the steel slag is pre-wetted before adding the calcium-capturing and calcium-fixing fine powder for pre-coating, so that the fine powder is preferentially distributed on the surface of the steel slag and near its pores, reducing the probability of calcium ions migrating to pore water and drainage channels.

[0071] Before use, steel slag is tested for its immersion swelling rate (Es), f-CaO content, f-MgO content, calcium ion leaching concentration, pH value, water absorption rate, crushing value, and particle size distribution. Steel slag can be classified as follows:

[0072] Risk level, recommendation criteria, and usage strategy

[0073] Low-risk steel slag with Es not exceeding 0.8%, f-CaO not exceeding 2.0%, and Lca not exceeding 0.20% can be used as the main steel slag aggregate for the bearing layer, but still needs to meet the Pe and Pca thresholds.

[0074] For medium-risk steel slag, the content of Es is greater than 0.8% but not greater than 1.5%, or Lca is greater than 0.20 but not greater than 0.30. The dosage is limited and the RAP, calcium-capturing and calcium-fixing fine powder and the conduction and isolation coefficient are increased.

[0075] High-risk steel slag with Es greater than 1.5%, f-CaO greater than 3.0%, or Lca greater than 0.30% should be retested after enhanced aging, wet heat aging, carbonization, or water washing; it should not be allowed to directly enter the bearing layer before meeting the standards.

[0076] Lca is a calcium ion leaching descriptor for steel slag before it enters the bearing layer, and it is preferably normalized based on a calcium ion leaching concentration of 100 mg / L. The high calcium leaching value of untreated steel slag is mainly used for raw material classification and should not be directly used as the basis for determining the bearing layer proportion.

[0077] The above indices are empirically normalized screening indicators established based on measured material properties, mass fractions, and structural drainage capacity. They are mainly used for mix proportion comparison, risk classification, and construction feedback control within the same project batch, and are not used as separate material acceptance limits. Each mass fraction, percentage value, and Dp should be substituted with data recorded for the same batch and using the same testing method.

[0078] This invention introduces the Pe expansion risk index and the Pca calcium oxide risk index to provide feedback adjustments for steel slag content, RAP content, calcium-capturing and calcium-fixing fine powder content, and drainage structure. These indices are empirically normalized screening indicators established based on measured material properties, mass fractions, and structural drainage capacity. They are primarily used for mix proportion comparison, risk classification, and construction feedback control within the same project batch.

[0079] Pe = S × Es / (R × Ar + N + 0.1)

[0080] In the formula, S is the mass fraction of steel slag in the dry mixture, which can also be denoted as Ms; Es is the percentage value of the steel slag's water immersion swelling rate; R is the mass fraction of RAP; Ar is the percentage value of the old asphalt content in RAP; N is the mass fraction of natural crushed stone; and 0.1 is a correction constant to avoid the denominator being zero. R×Ar characterizes the buffering contribution of the old asphalt film of RAP to moisture migration and rigid interface stress, and N characterizes the dilution contribution of non-expanding aggregate to the expansion risk of steel slag. Pe is preferably not greater than 0.55, preferably not greater than 0.40 in high humidity areas, and more preferably 0.10 to 0.40. When Pe is greater than 0.55, the proportion of steel slag with high expansion risk should be reduced, the proportion of flexible buffering of RAP should be increased, the proportion of non-expanding aggregate should be increased, or the steel slag should be subjected to enhanced stability treatment.

[0081] Pca = S × Lca / (Fp × Dp + 0.1)

[0082] In the formula, S is the mass fraction of steel slag, Lca is the calcium ion leaching descriptor of steel slag, Fp is the mass fraction of calcium-capturing and calcium-fixing fine powder, Dp is the drainage isolation coefficient, and 0.1 is a correction constant to avoid the denominator being zero. Fp represents the total mass fraction of fly ash, mineral powder, silica fume, or metakaolin. When it is necessary to differentiate the calcium capture efficiency of different powders, a weighted value Fp = αFa + βGGBS + γSF / MK can also be used, where α, β, and γ are the calcium capture and calcium-fixing efficiency coefficients of each fine powder. Here, SF is silica fume / MK, GGBS is ground granulated slag, Fa is fly ash, and the calcium capture and calcium-fixing efficiency coefficient is the equivalent calcium-fixing contribution coefficient of different fine powder materials relative to low-calcium fly ash. With an efficiency coefficient of 1.00 for low-calcium fly ash, and based on the reduction rate of calcium ion leaching concentration, calcium hydroxide consumption, and change rate of calcium oxide surface area under the same quality of fine powder, the following optimal values ​​were determined: granulated blast furnace slag powder preferably 0.50–0.70, silica fume preferably 1.60–2.00, and metakaolin preferably 1.70–2.20. Lca was obtained by normalizing the calcium ion concentration in the steel slag water leachate. Specifically, representative steel slag samples after aging, washing, magnetic separation, crushing, and sieving were taken, and deionized water or distilled water was added at a water-to-solid ratio of 10:1. The samples were then leached with shaking at 20±2℃ for 6 h, followed by standing until the total leaching time was 24 h. After filtration through a 0.45 μm filter membrane, the Ca2+ concentration was determined using EDTA complexometric titration, atomic absorption spectrometry, or inductively coupled plasma atomic emission spectrometry. 2+ Concentration Cca, in mg / L. Lca is determined by the following formula:

[0083] Lca = Cca / 100

[0084] In the formula, Cca represents the Ca content in the extract. 2+ Mass concentration, with 100 as the normalized baseline concentration. When using other extraction times, water-to-solid ratios, or testing methods, testing conditions should be kept consistent within the same batch of work, and Lca should be calculated using the same method.

[0085] Dp is the drainage isolation coefficient, used to characterize the weakening effect of the lower drainage isolation layer, the edge sealing water-conducting structure, and the upper sealing impermeable layer on the retention of water and the migration of calcium ions inside the bearing layer. Dp can be graded according to the structure's drainage capacity: when there is no continuous lower drainage isolation layer or the drainage path is discontinuous, Dp is 0.8–1.0; when a lower drainage isolation layer with a thickness of 2–4 cm is provided, and its permeability coefficient is not less than 3 times the permeability coefficient of the bearing layer, Dp is 1.0–1.2; when a lower drainage isolation layer with a thickness of 4–6 cm is provided, and an edge water-conducting structure connected to it is provided, Dp is 1.2–1.35; when a lower drainage isolation layer with a thickness of 6–8 cm, an edge sealing water-conducting structure, and an upper sealing impermeable layer are provided, and the drainage path is continuous and unobstructed, Dp is 1.35–1.5. The above values ​​should correspond to the permeability coefficient, layer thickness, and drainage path continuity records for the same project batch.

[0086] Pca is preferably no greater than 1.20, and more preferably no greater than 0.85. When Pca is greater than 1.20, the amount of calcium-capturing and calcium-fixing fine powders such as fly ash, mineral powder, silica fume or metakaolin should be increased to improve the permeability of the drainage isolation layer, or the amount of steel slag with high calcium leaching risk should be reduced.

[0087] The process is explained as follows:

[0088] Step S1 involves aging, washing, magnetic separation, crushing, and screening of the steel slag to obtain coarse aggregate of 4.75–31.5 mm and fine aggregate of less than 4.75 mm. Steel slag with high free calcium oxide or high expansion rate should undergo extended aging, wet heat aging, or carbonization pretreatment.

[0089] Step S2: Determine the slag immersion expansion rate Es, f-CaO content, f-MgO content, calcium ion leaching concentration Cca, pH value, water absorption rate, and crushing value, and calculate the calcium ion leaching descriptor Lca accordingly.

[0090] Step S3: Based on the test results of S2, steel slag is classified into low-risk, medium-risk, and high-risk categories, and it is determined whether enhanced pretreatment is required.

[0091] Step S4: The RAP is crushed and sieved, and the old asphalt content (Ar), moisture content, and particle size distribution are determined. RAP is preferably used as a flexible buffer aggregate with a particle size of less than 9.5 mm, and more preferably as a combination of two particle sizes: less than 4.75 mm and 4.75–9.5 mm.

[0092] Step S5: Based on the target road grade, base layer thickness, steel slag risk level and RAP properties, initially determine the material mix ratio, lower drainage isolation layer thickness and edge water guiding structure.

[0093] Step S6: Calculate Pe and Pca.

[0094] Step S7: Determine whether Pe and Pca meet the target threshold; if not, proceed to step S8; if they do, proceed to step S9.

[0095] Step S8: Adjust the steel slag content, steel slag risk level, RAP content, calcium capture and fixation fine powder content, lower drainage isolation layer thickness, edge sealing water-conducting structure or upper sealing anti-seepage layer according to the deviation direction, and recalculate Pe and Pca until the target threshold is met.

[0096] Step S9 involves pre-wetting the steel slag that meets the threshold, and then adding calcium-capturing and calcium-fixing fine powder to form a pre-coating.

[0097] Step S10: Add RAP, cement and remaining water for staged wet mixing, controlling the mixture to have no obvious segregation, no bleeding and uniform coating.

[0098] Step S11: During construction, first lay the lower drainage isolation layer, then spread and compact the steel slag RAP cement stabilized bearing layer, and then set the upper closed anti-seepage layer and the edge closed water-conducting structure.

[0099] Step S12: Cover and maintain the drainage system, and monitor the drainage pH, calcium ion concentration, surface whitening area, and drainage smoothness during the maintenance period or the trial section operation period; when the monitoring indicators are abnormal, provide feedback to adjust the subsequent road section mix ratio or drainage structure.

[0100] Parameter category, recommended range or requirements, technical function

[0101] 35-60 parts of steel slag aggregate; a combination of coarse aggregate of 4.75-31.5 mm and fine aggregate of less than 4.75 mm provides skeleton strength and some potential cementitious activity;

[0102] 15-35 parts of RAP aggregate; particle size less than 9.5 mm; the portion smaller than 4.75 mm accounts for 40%-80% of the total RAP, forming a flexible water-proof buffer phase to reduce moisture migration and interfacial stress concentration;

[0103] 2.5 to 5.0 parts cement provide early strength and the integrity of the base layer;

[0104] 3-8 parts fly ash, pozzolanic reaction, microfiller and calcium capture agent;

[0105] Add 2-6 parts of mineral powder to improve gradation and gelling activity, and assist in fixing calcium components;

[0106] 0.3 to 3.0 parts of silica fume or metakaolin provide a highly active silica-alumina phase, which improves the calcium capture and fixation capacity;

[0107] 4.5 to 7.0 parts water to meet the requirements of mixing, compaction and hydration;

[0108] The expansion rate Es of steel slag after immersion in water is preferably no more than 1.5% to control volume stability;

[0109] The f-CaO content should not exceed 3.0%. This reduces the risk of hydration swelling in the later stages.

[0110] The f-MgO content should not exceed 5.0% to reduce the risk of long-term expansion.

[0111] Before Lca enters the load-bearing layer, it is preferred to be no greater than 0.30; in high-humidity areas, it is preferred to be no greater than 0.20 to control the risk of calcium ion migration and calcium bloom.

[0112] The lower drainage isolation layer is 2–8 cm thick; its permeability coefficient is not less than 3 times that of the load-bearing layer, to prevent water stagnation at the bottom and drainage blockage.

[0113] Pe is generally no greater than 0.55 in general areas and no greater than 0.40 in high-humidity areas, serving as an adjustment indicator for expansion risk feedback.

[0114] Pca is generally no greater than 1.20 in high-humidity areas and no greater than 0.85 in high-humidity areas, serving as an adjustment indicator for pan-calcification risk feedback.

[0115] During implementation, representative samples of steel slag and RAP were first collected. After aging, washing, magnetic separation, crushing, and sieving, the steel slag was tested for its immersion swelling rate (Es), f-CaO content, f-MgO content, calcium ion leaching concentration, pH value, water absorption rate, crushing value, and particle size distribution, and was classified into low-risk, medium-risk, and high-risk categories. After crushing and sieving, the RAP was tested for its old asphalt content (Ar), moisture content, particle size distribution, surface dust content, and old asphalt film coverage.

[0116] Based on the target road grade and base course thickness, the initial mass fractions of steel slag, RAP, natural crushed stone, cement, fly ash, mineral powder, silica fume or metakaolin, and water are determined, and Dp is determined based on the thickness of the lower drainage isolation layer, permeability, and edge drainage settings. Pe and Pca are then calculated. If the target thresholds are not met, the steel slag risk level, steel slag content, RAP content, calcium capture and fixation fine powder dosage, or drainage isolation parameters are adjusted until Pe and Pca meet the target thresholds.

[0117] During mixing, it is preferable to pre-wet the steel slag for 10–40 minutes, ensuring the surface is moist but not significantly wet. Then, add fly ash, mineral powder, silica fume, or metakaolin for pre-coating, allowing the calcium-capturing and calcium-fixing fines to preferentially adhere to the steel slag surface and near the pores. Subsequently, add RAP, cement, and remaining water for wet mixing. During construction, first lay the lower drainage isolation layer, then spread and compact the steel slag RAP cement-stabilized bearing layer, followed by an upper closed impermeable layer and a side closed water-conducting structure. During the curing period, cover and maintain moisture, and monitor the drainage pH, calcium ion concentration, surface efflorescence area, and drainage flow according to the test section conditions.

[0118] Example 1: Conventional base structure for medium-risk steel slag

[0119] Based on dry weight, weigh out 48 parts steel slag aggregate, 25 parts RAP aggregate, 15 parts natural crushed stone, 3.8 parts cement, 5.0 parts fly ash, 3.0 parts mineral powder, 1.0 part silica fume, and 5.8 parts water. The water immersion expansion rate (Es) of steel slag is 0.80%, the old asphalt content (Ar) of RAP is 4.5%, and the natural crushed stone (N) is 15 parts. Then:

[0120] Pe = 48 × 0.80 / (25 × 4.5 + 15 + 0.1) = 0.301.

[0121] The Pe meets the preferred range. If the original calcium ion leaching descriptor Lca of the steel slag is 1.10, the calcium-capturing and calcium-fixing fine powder Fp is 9.0 parts, and the drainage isolation coefficient Dp is 1.2, then:

[0122] Pca = 48 × 1.10 / (9.0 × 1.2 + 0.1) = 4.84.

[0123] This value is too high and should be adjusted by lowering Lca, increasing Fp, or increasing Dp. After enhanced washing and pre-carbonization to reduce Lca to 0.18:

[0124] Pca = 48 × 0.18 / (9.0 × 1.2 + 0.1) = 0.79.

[0125] At this point, Pca meets the preferred range. During construction, a 4 cm lower drainage isolation layer is first laid; after the steel slag is pre-wetted for 30 minutes, fly ash, mineral powder and silica fume are added for pre-coating, and then RAP, cement and remaining water are added for wet mixing; after spreading and compacting, an edge-sealed water-guiding structure is set, and it is covered for curing.

[0126] Example 2: Basement structure with low risk of calcium efflorescence in high humidity areas

[0127] Based on dry weight, weigh out 40 parts steel slag aggregate, 30 parts RAP aggregate, 20 parts natural crushed stone, 4.0 parts cement, 6.0 parts fly ash, 4.0 parts mineral powder, 1.5 parts metakaolin, and 6.0 parts water. After wet heat aging and carbonization treatment, the steel slag has an Es content of 0.55% and an Lca content of 0.16%; the RAP old asphalt content (Ar) is 5.0%. Therefore:

[0128] Pe = 40 × 0.55 / (30 × 5.0 + 20 + 0.1) = 0.129.

[0129] When the calcium-capturing and calcium-fixing fine powder Fp is 11.5 parts, and the Dp is 1.5 when both the lower drainage isolation layer and the edge water-conducting structure are set, then:

[0130] Pca = 40 × 0.16 / (11.5 × 1.5 + 0.1) = 0.369.

[0131] This embodiment meets the preferred control requirements of Pe not exceeding 0.40 and Pca not exceeding 0.85 in areas with high humidity, heavy rainfall, or high groundwater levels. During construction, the thickness of the lower drainage isolation layer can be 6 cm, and a combination of permeable blind drains and closed edge strips is set at the edges to reduce the risk of water stagnation at the bottom of the bearing layer and water ingress at the edges.

[0132] Example 3: Low-cementation, high-solid-waste alternative base structure

[0133] Based on dry weight, weigh out 55 parts steel slag aggregate, 30 parts RAP aggregate, 5 parts natural crushed stone, 3.0 parts cement, 7.0 parts fly ash, 5.0 parts mineral powder, 0.8 parts silica fume, and 6.2 parts water. After aging and washing, the steel slag has an Es content of 0.65% and an Lca content of 0.22%; the RAP old asphalt content Ar is 5.5%. Therefore:

[0134] Pe = 55 × 0.65 / (30 × 5.5 + 5 + 0.1) = 0.210.

[0135] If Fp is 12.8 parts and Dp is 1.3, then:

[0136] Pca = 55 × 0.22 / (12.8 × 1.3 + 0.1) = 0.724.

[0137] In this embodiment, the combined proportion of steel slag and RAP is relatively high. By increasing the amount of calcium-capturing and calcium-fixing fine powder and setting up a lower guide and isolation layer, a low risk of calcium efflorescence can be maintained with a lower cement content. This solution is suitable for base courses with high requirements for solid waste substitution rates and where the construction site has the conditions for material grading and testing.

[0138] Example 4

[0139] Based on dry weight, weigh out 35 parts steel slag aggregate, 15 parts RAP aggregate, 35 parts natural crushed stone, 2.5 parts cement, 3.0 parts fly ash, 2.0 parts mineral powder, 0.3 parts silica fume, and 4.5 parts water. After aging and washing, the steel slag has a water expansion rate (Es) of 0.65%, a free calcium oxide content of 1.8%, and a calcium ion leaching descriptor (Lca) of 0.18 (after enhanced washing). The RAP old asphalt content (Ar) is 4.2%. The thickness of the lower drainage isolation layer is 2 cm, its permeability coefficient is 3.5 times that of the bearing layer, and the drainage isolation coefficient (Dp) is 1.0. Therefore:

[0140] Pe = 35 × 0.65 / (15 × 4.2 + 35 + 0.1) = 22.75 / (63 + 35 + 0.1) =22.75 / 98.1 = 0.232

[0141] Pca = 35 × 0.18 / (5.3 × 1.0 + 0.1) = 6.3 / 5.4 = 1.167

[0142] In this embodiment, Pe=0.232≤0.55 and Pca=1.167≤1.20, which meet the target threshold requirements. During construction, the thickness of the lower drainage isolation layer is 2cm. After the steel slag is pre-wetted for 10 minutes, fly ash, mineral powder and silica fume are added for pre-coating. Then RAP, cement and residual water are added and mixed. After spreading and compacting, an upper closed impermeable layer and an edge closed water-conducting structure are set.

[0143] Example 5

[0144] Based on dry weight, weigh out 60 parts steel slag aggregate, 35 parts RAP aggregate, 0 parts natural crushed stone, 5.0 parts cement, 8.0 parts fly ash, 6.0 parts mineral powder, 3.0 parts metakaolin, and 7.0 parts water. After wet heat aging and carbonization treatment, the steel slag has an Es content of 0.55% and an Lca content of 0.15%; the RAP old asphalt content (Ar) is 5.0%; the lower drainage isolation layer is 8cm thick and is equipped with a connected edge-sealed water-conducting structure and an upper sealed impermeable layer, ensuring a continuous and unobstructed drainage path. The drainage isolation coefficient Dp is 1.5. Therefore:

[0145] Pe = 60 × 0.55 / (35 × 5.0 + 0 + 0.1) = 33.0 / (175 + 0.1) = 33.0 / 175.1 = 0.188

[0146] Pca = 60 × 0.15 / (17.0 × 1.5 + 0.1) = 9.0 / (25.5 + 0.1) = 9.0 / 25.6 = 0.352

[0147] In this embodiment, Pe=0.188≤0.55 and Pca=0.352≤1.20, which meet the target threshold requirements. During construction, the thickness of the lower drainage isolation layer is 8cm. After the steel slag is pre-wetted for 40 minutes, fly ash, mineral powder and metakaolin are added for pre-coating. Then RAP, cement and residual water are added and mixed. After spreading and compacting, an upper closed impermeable layer and an edge closed water-conducting structure are set.

[0148] Example 6

[0149] This embodiment provides two sets of parallel verifications of the endpoint values ​​for the natural crushed stone content.

[0150] Group A (0 parts of natural crushed stone): Same proportions as in Example 5, Pe=0.188, Pca=0.352, which meets the requirements.

[0151] Group B (35 parts natural crushed stone): By dry weight, weigh 40 parts steel slag aggregate, 15 parts RAP aggregate, 35 parts natural crushed stone, 3.5 parts cement, 5.0 parts fly ash, 3.0 parts mineral powder, 0.5 parts silica fume, and 5.5 parts water. Steel slag Es=0.70%, Lca=0.20; RAP old asphalt content Ar=4.5%; lower guide isolation layer thickness 5cm, Dp=1.2. Therefore:

[0152] Pe = 40 × 0.70 / (15 × 4.5 + 35 + 0.1) = 28.0 / (67.5 + 35 + 0.1) =28.0 / 102.6 = 0.273

[0153] Pca = 40 × 0.20 / (8.5 × 1.2 + 0.1) = 8.0 / (10.2 + 0.1) = 8.0 / 10.3 = 0.777

[0154] Both group A and group B meet the target threshold requirements of Pe≤0.55 and Pca≤1.20.

[0155] Example 7: Effect of different lower guide isolation layer thicknesses on Pca. This example verifies the effect of different lower guide isolation layer thicknesses on Pca under the same mix proportion. Fixed mix proportions: 45 parts steel slag aggregate, 25 parts RAP aggregate, 20 parts natural crushed stone, 4.0 parts cement, 5.0 parts fly ash, 3.0 parts mineral powder, 1.0 part silica fume, and 6.0 parts water. Steel slag Es=0.60%, Lca=0.18; RAP old asphalt content Ar=4.5%. The Dp values ​​and Pca calculation results corresponding to different thicknesses are as follows:

[0156] Lower guide plate isolation layer thickness The value of Dp is determined based on Dp value Pca = 45 × 0.18 / (9.0 × Dp + 0.1) and applicable instructions 2cm Thickness 2~4cm, permeability coefficient ≥ 3 times that of the load-bearing layer 1.0 8.1 / 9.1 = 0.890 4cm Thickness 4~6cm, with edge water guiding structure 1.2 8.1 / 10.9 = 0.743 6cm Thickness 4~6cm, with edge water guiding structure 1.3 8.1 / 11.8 = 0.686 8cm Thickness 6~8cm, with a sealed water-conducting structure at the edges and a sealed impermeable layer at the top. 1.5 8.1 / 13.6=0.596; Preferred for use in road sections with high humidity, heavy rainfall, or high risk of water entering from the edges.

[0157] Pca was no greater than 1.20 for all thickness combinations, meeting the target threshold requirement. A thickness of 2cm met the basic requirements, while a thickness of 4-8cm provided a lower risk of calcification and was preferred for use in high-humidity or rainy areas.

[0158] Comparative Example 1: No calcium-binding and calcium-fixing powder added

[0159] Difference from Example 1: No fly ash, mineral powder, or silica fume are added; the remaining proportions are the same as in Example 1.

[0160] project Example 1 Comparative Example 1 Steel slag aggregate (per unit) 48 48 RAP aggregate (per unit) 25 25 Natural crushed stone (parts) 15 15 Cement (parts) 3.8 3.8 Fly ash + mineral powder + silica fume (Fp, parts) 9.0 0 Water content 5.8 5.8 Expansion rate of steel slag after immersion in water, Es (%) 0.80 0.80 RAP old asphalt content (Ar) (%) 4.5 4.5 Original calcium ion leaching descriptor for steel slag Lca 0.18 0.18 Drainage isolation factor Dp 1.2 1.2 Pe 0.301 0.301 Pca 48×0.18 / (9.0×1.2+0.1)=0.79 48×0.18 / (0×1.2+0.1)=86.4 Unconfined compressive strength (7d) / MPa 4.90 4.50 Unconfined compressive strength (28d) / MPa 6.80 5.80 Unconfined compressive strength (90d) / MPa 8.30 6.80 Water swelling rate (180d) / % 0.72 1.40 <![CDATA[Drainage Ca 2+ Concentration / mg / L]]> 70 150 Surface whitening area ratio / % 4.0 14.0

[0161] Comparative Example 2: No steel slag pretreatment

[0162] The difference from Example 1 is that the steel slag is not pretreated, but otherwise the same.

[0163] project Example 1 Comparative Example 2 Original calcium ion leaching descriptor for steel slag Lca 0.18 1.10 Pca 48×0.18 / (9.0×1.2+0.1)=0.79 48×1.1 / (9.0×1.2+0.1)=4.84 Unconfined compressive strength (7d) / MPa 4.90 4.70 Unconfined compressive strength (28d) / MPa 6.80 6.20 Unconfined compressive strength (90d) / MPa 8.30 7.20 Water swelling rate (180d) / % 0.72 1.18 <![CDATA[Drainage Ca 2+ Concentration / mg / L]]> 70 220 Surface whitening area ratio / % 4.0 15.0

[0164] Comparative Example 3: No lower drainage isolation layer or edge water guiding structure is provided.

[0165] The difference from Example 1 is that no lower drainage isolation layer and edge water guiding structure are set, and drainage is only carried out by the base material itself. Otherwise, it is the same as Example 1.

[0166] project Example 1 Comparative Example 3 Drainage isolation factor Dp 1.2 0.80 Pca 48×0.18 / (9.0×1.2+0.1)=0.79 48×0.18 / (9.0×0.8+0.1)=1.18 Unconfined compressive strength (7d) / MPa 4.90 4.90 Unconfined compressive strength (28d) / MPa 6.80 6.80 Unconfined compressive strength (90d) / MPa 8.30 8.30 Water swelling rate (180d) / % 0.72 0.72 <![CDATA[Drainage Ca 2+ Concentration / mg / L]]> 70 130 Surface whitening area ratio / % 4.0 10.0

[0167] Comparative Example 4: RAP was completely replaced with natural crushed stone.

[0168] Difference from Example 1: No RAP is added, and all 25 parts of RAP are replaced with natural crushed stone (i.e., the natural crushed stone is increased from 15 parts to 40 parts), otherwise it is the same as Example 1.

[0169] project Example 1 Comparative Example 4 RAP aggregate (per unit) 25 0 Natural crushed stone (parts) 15 40 RAP old asphalt content (Ar) (%) 4.5 0 Pe 48×0.80 / (25×4.5+15+0.1)=0.301 48×0.80 / (0+40+0.1)=0.958 Unconfined compressive strength (7d) / MPa 4.90 5.10 Unconfined compressive strength (28d) / MPa 6.80 7.05 Unconfined compressive strength (90d) / MPa 8.30 8.55 Water swelling rate (180d) / % 0.72 1.02 <![CDATA[Drainage Ca 2+ Concentration / mg / L]]> 70 105 Surface whitening area ratio / % 4.0 8.0

[0170] Comparative Example 5: No steel slag pre-wetting or calcium-capturing fine powder pre-coating was performed.

[0171] The difference from Example 1 is that the steel slag is not pre-wetted or pre-coated with calcium-capturing and calcium-fixing fine powder. All dry materials are directly mixed together and water is added. The remaining proportions are the same as in Example 1.

[0172] project Example 1 Comparative Example 5 Steel slag pretreatment process Pre-wetting for 30 minutes + pre-coating with fly ash / mineral powder / silica fume No pre-wetting or pre-coating required; simply dry mix and then add water. Original calcium ion leaching descriptor for steel slag Lca 0.18 0.18 Pca 48×0.18 / (9.0×1.2+0.1)=0.79 0.79 Unconfined compressive strength (7d) / MPa 4.90 4.72 Unconfined compressive strength (28d) / MPa 6.80 6.35 Unconfined compressive strength (90d) / MPa 8.30 7.65 Water swelling rate (180d) / % 0.72 0.88 <![CDATA[Drainage Ca 2+ Concentration / mg / L]]> 70 108 Surface whitening area ratio / % 4.0 7.5

[0173] Note: The above data are measured data from relevant literature and are used to support the technical effect judgment in this case; they are not direct measured results of the embodiments of this invention.

[0174] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification.

Claims

1. A steel slag RAP cement-stabilized base course structure with low calcium efflorescence and low expansion risk, characterized in that, include: Upper sealed impermeable layer; A steel slag RAP cement-stabilized bearing layer is installed below the upper closed impermeable layer; The lower guide isolation layer is set below the steel slag RAP cement stabilized bearing layer; And edge-sealed water-conducting structures set at the edges of the base structure; The steel slag RAP cement-stabilized bearing layer comprises the following components by dry weight: 35-60 parts of steel slag aggregate; 15-35 parts of RAP aggregate; 0-35 parts of natural crushed stone; Cement 2.5–5.0 parts; 3-8 parts fly ash; 2-6 parts of mineral powder; 0.3–3.0 parts of silica fume or metakaolin; Water content is 4.5–7.0 parts; and the expansion risk index calculated according to the formula Pe=S×Es / (R×Ar+N+0.1) has a value of Pe≤0.55, and the calcium efflorescence risk index calculated according to the formula Pca=S×Lca / (Fp×Dp+0.1) has a value of Pca≤1.20, where S is the mass fraction of steel slag in the dry mixture; Es is the percentage value of the steel slag immersion expansion rate; R is the mass fraction of RAP; Ar is the percentage value of RAP aggregate content; N is the mass fraction of natural crushed stone; 0.1 is a correction constant to avoid the denominator being zero; R×Ar characterizes the buffering contribution of the RAP aggregate film to water migration and rigid interface stress; N characterizes the dilution contribution of non-expanding aggregate to the expansion risk of steel slag; Lca is the descriptor for calcium ion leaching of steel slag; Fp is the mass fraction of calcium-capturing and calcium-fixing fine powder; and Dp is the drainage isolation coefficient.

2. The base structure according to claim 1, characterized in that, The particle size of RAP aggregate is less than 9.5 mm, and RAP with a particle size less than 4.75 mm accounts for 40% to 80% of the total RAP aggregate. The Ar content of RAP aggregate is used to calculate Pe. The RAP aggregate membrane acts as a flexible water-proof buffer phase in the bearing layer to reduce moisture migration and stress concentration at the interface of hard steel slag.

3. The base structure according to claim 1, characterized in that, Steel slag aggregate includes coarse steel slag aggregate with a particle size of 4.75–31.5 mm and fine steel slag aggregate with a particle size of less than 4.75 mm. Before entering the steel slag RAP cement-stabilized bearing layer, the water immersion expansion rate Es≤1.5%, f-CaO content≤3.0%, f-MgO content≤5.0%, and calcium ion leaching descriptor Lca≤0.30 of the steel slag aggregate.

4. The base structure according to claim 1, characterized in that, The upper sealing impermeable layer is one or more of the following: asphalt seal, slurry seal, combination of tack coat and lower seal, or low-permeability cement-stabilized transition layer.

5. The base structure according to claim 1, characterized in that, The thickness of the lower drainage isolation layer is 2-8cm, and its permeability coefficient is not less than 3 times that of the steel slag RAP cement stabilized bearing layer; the drainage isolation coefficient Dp is 0.8-1.5 according to the thickness of the lower drainage isolation layer, the edge closed water guiding structure, the upper closed anti-seepage layer and the continuity of the drainage path.

6. The base structure according to claim 1, characterized in that, The thickness of the upper closed impermeable layer is 0.5–2.0 cm; the thickness of the steel slag RAP cement stabilized bearing layer is 15–40 cm; the width of the edge closed water-conducting structure is 10–30 cm, and its bottom is connected to the lower drainage isolation layer.

7. The base structure according to claim 1, characterized in that, The mass ratio of steel slag aggregate to RAP aggregate is 1.0 to 3.0:

1.

8. A construction method for a steel slag RAP cement-stabilized base structure with low calcium efflorescence and low expansion risk, characterized in that, Includes the following steps: S1: Steel slag is aged, washed, magnetically separated, crushed and screened to obtain steel slag aggregate; S2: Determine the immersion swelling rate Es, free calcium oxide content, free magnesium oxide content, calcium ion leaching concentration Cca, pH value, water absorption rate, and crushing value of the steel slag, and calculate the calcium ion leaching descriptor Lca of the steel slag according to formula (1); Cca is preferably obtained by leaching with a water-to-solid ratio of 10:1, at 20±2℃, for 24h, followed by filtration through a 0.45μm filter membrane. Lca = Cca / 100 (1); S3: Based on the test results of step S2, classify the steel slag into low-risk, medium-risk, or high-risk levels; S4: RAP is crushed and sieved, and the Ar content of old asphalt is determined; S5: Based on the target road grade, base layer thickness, steel slag risk level and RAP properties, preliminary material ratio and parameters of the lower guide and isolation layer are proposed; S6: Calculate the inflation risk index Pe according to formula (2), and calculate the pan-calcification risk index Pca according to formula (3): Pe = S × Es / (R × Ar + N + 0.1) (2), Pca = S × Lca / (Fp × Dp + 0.1) (3), In the formula, S is the mass fraction of steel slag in the dry mixture; Es is the percentage value of the water expansion rate of steel slag; R is the mass fraction of RAP; Ar is the percentage value of RAP aggregate content; N is the mass fraction of natural crushed stone; 0.1 is a correction constant to avoid the denominator being zero; Fp is the mass fraction of calcium-capturing and calcium-fixing fine powder; and Dp is the drainage isolation coefficient. S7: Determine whether Pe and Pca meet the target threshold; if not, adjust the steel slag content, RAP content, calcium capture and fixation fine powder dosage or drainage isolation parameters, and recalculate until they are met. S8: Pre-wet the steel slag that meets the threshold, and then add fly ash, mineral powder, silica fume or metakaolin to form a pre-coating; S9: Add RAP, cement and remaining water and mix. S10: First, lay the lower drainage isolation layer, then spread and compact the steel slag RAP cement stabilized bearing layer, and then set the upper closed anti-seepage layer and the edge closed water-conducting structure.

9. The construction method according to claim 8, characterized in that, The target threshold mentioned in step S7 is: Pe≤0.55 and Pca≤1.

20.

10. The construction method according to claim 8, characterized in that, The thickness of the lower drainage isolation layer in step S10 is preferably 4 to 8 cm and is connected to the edge closed water-guiding structure; when the thickness is 6 to 8 cm, the edge closed water-guiding structure and the upper closed anti-seepage layer are set and the drainage path is continuous and unobstructed, Dp is 1.35 to 1.5.

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