A method for promoting the interface fusion of new and old asphalts in emulsified asphalt cold recycled mixture

Abstract

The present application relates to the technical field of road engineering materials, and particularly relates to a method for promoting interface fusion of new and old asphalts in emulsified asphalt cold recycling mixture, wherein an isocyanate component and a hydroxyl-containing component are pre-reacted to prepare an interface coupling pre-reaction agent containing a urethane structure and having a residual free NCO mass fraction of 0.5% to 5%; then the pre-reaction agent is added into the mixture of RAP and emulsified asphalt for mixing in the initial demulsification stage of the emulsified asphalt, and interface fusion is completed through molding and curing. The present application effectively inhibits the water phase side reaction of isocyanate through pre-reaction, realizes chemical crosslinking of the interface of new and old asphalts, and significantly improves the interface fusion degree; the prepared mixture has no obvious foaming, the strength and water stability are greatly improved, and the existing construction equipment does not need to be changed, and the present application is suitable for high-proportion RAP cold recycling engineering.

Description

Technical Field

[0001] This invention relates to the field of road engineering materials technology, specifically to a method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures. Background Technology

[0002] Emulsified asphalt cold recycling technology is a core green technology in the field of road maintenance. It involves mixing old asphalt pavement material (RAP) with emulsified asphalt, new aggregates, and water at room temperature to create recycled mixtures, achieving resource utilization of waste pavement materials and offering significant advantages in energy saving, environmental protection, and economy. However, the engineering application of this technology is still constrained by the key bottleneck of insufficient fusion between the old and new asphalt interfaces: After long-term road use, the aged asphalt in RAP accumulates a large number of highly polar oxygen-containing functional groups such as carbonyl and sulfoxide groups in its molecular structure. This results in poor thermodynamic compatibility and weak molecular interdiffusion ability with the new asphalt formed after the emulsification of the emulsified asphalt, easily forming a weak interfacial transition zone with obvious gaps. This weak interfacial transition zone becomes the main channel for stress concentration and moisture intrusion within the mixture, directly leading to a significant deterioration in the mechanical strength, water stability, and fatigue durability of the cold recycled mixture.

[0003] To address the aforementioned issues, existing technologies typically employ regenerators, reducing agents, or interface modifiers to adjust aged asphalt, thereby improving its rheological properties or compatibility. For example, by partially reducing the oxidized groups in aged asphalt using reducing regenerators, the degree of aging can be reduced to some extent, and polarity can be adjusted. Furthermore, isocyanate components possess high chemical reactivity and, theoretically, can undergo addition reactions with hydroxyl-containing structures, thus forming a certain degree of chemical coupling.

[0004] However, existing technologies still have significant limitations in the special system of cold recycling of emulsified asphalt, which involves high water content and dynamic demulsification. On the one hand, while adjusting the polarity of old asphalt through reduction can improve some compatibility, it is often difficult to achieve effective chemical coupling at the interface between new and old asphalt. On the other hand, if isocyanate materials are directly introduced into the cold recycling system of emulsified asphalt, their active -NCO groups readily react with water in the system to generate urea structures and release carbon dioxide, resulting in foaming of the mixture, increased porosity, loose structure, and performance dispersion.

[0005] Furthermore, the following mutually restrictive technical challenges exist in this type of high water-content system:

[0006] Firstly, isocyanates need to retain a certain level of reactivity in order to continue chemical coupling at the interface between new and old asphalt; however, if the retained reactivity is too high, it is prone to side reactions with water.

[0007] Secondly, the modified product after pre-reaction not only needs to reduce the isocyanate's sensitivity to water, but also needs to maintain its affinity and dispersibility to both new and old asphalt phases so that it can effectively reach and act on the interface.

[0008] Third, the timing of adding the interfacial coupling agent is not arbitrary. If added too early, there is more free water in the system, and residual -NCO can still easily participate in side reactions; if added too late, the interface between the old and new asphalt has basically formed, and the coupling agent cannot fully participate in the interface reconstruction.

[0009] Fourth, the above factors are not independent of each other, but are coupled with the degree of pre-reaction, residual free NCO content, dosage, RAP moisture content and conditioning conditions, making it difficult to obtain stable results through optimization of a single parameter.

[0010] Therefore, how to suppress the side reaction between isocyanate and water in the cold recycling system of emulsified asphalt with high water content, while preserving its effective chemical coupling ability at the interface between new and old asphalt, and further determine key conditions such as the degree of pre-reaction, residual free NCO content and timing of addition, has become a technical problem that urgently needs to be solved in this field.

[0011] In view of the above-mentioned defects, the inventors of this invention have finally obtained this invention after a long period of research and practice. Summary of the Invention

[0012] The purpose of this invention is to solve the problems of suppressing the side reactions of isocyanate water reaction, achieving directional chemical coupling between isocyanate and aged asphalt hydroxyl groups, and strengthening the molecular fusion of new and old asphalt interfaces, and to provide a method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures.

[0013] To achieve the above objectives, this invention discloses a method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures, comprising the following steps:

[0014] S1, prepare old asphalt pavement materials, new aggregates, emulsified asphalt, water, isocyanate components and hydroxyl-containing components that have been crushed and screened;

[0015] S2, the isocyanate component is pre-reacted with the hydroxyl-containing component to generate an interfacial coupling pre-reactant with a carbamate structure, wherein the mass fraction of residual free NCO in the interfacial coupling pre-reactant is 0.5%-5%;

[0016] S3 involves mixing old asphalt pavement materials, new aggregates, and emulsified asphalt. When the emulsified asphalt enters the initial demulsification stage, an interfacial coupling pre-reaction agent is added to continue mixing to obtain a mixture.

[0017] S4 involves compacting the mixed material into shape, curing it, and completing the fusion process at the interface between the old and new asphalt.

[0018] In step S1, the isocyanate component is any one or a combination of several of diphenylmethane diisocyanate (MDI), polymeric MDI, or their prepolymers.

[0019] In step S1, the hydroxyl-containing component is any one or a combination of several of the following: castor oil-based regenerator, polyether alcohol regenerator, asphalt-based hydroxyl regenerator, or regenerated aged asphalt containing active hydroxyl groups; the mass ratio of the isocyanate component to the hydroxyl-containing component is 1:2-5.

[0020] The preparation process of the reduced aged asphalt with active hydroxyl groups is as follows: the aged asphalt in the old asphalt pavement material is subjected to a reduction reaction, so that some of the oxide groups in the aged asphalt are converted into hydroxyl groups; the hydrogenation reducing agent used in the reduction reaction is any one or a combination of sodium borohydride, diimide, and transition metal complex hydrogenation catalyst; the amount of hydrogenation reducing agent added is 0.2%-1.0% of the mass of the aged asphalt; the reduction reaction is carried out at 25±3℃ and 300-500rpm for 10-20 minutes.

[0021] In step S2, the stirring rate of the pre-reaction is 600-800 rpm, the reaction temperature is 50-90℃, and the reaction time is 20-40 minutes. During the pre-reaction, the residual free NCO mass fraction needs to be controlled at 0.5%-5%, more preferably 1.0%-4.8%. If the residual free NCO mass fraction exceeds the range of 0.5%-5%, it can be adjusted by adding isocyanate components or hydroxyl-containing components.

[0022] In step S3, the initial demulsification stage can be determined by a combination of appearance, free water state, and coating state. Preferably, it is the stage where there is no continuous free water at the bottom of the mixture, more than 70% asphalt coating forms on the aggregate surface, and the mixture changes from a fluid slurry to a loosely aggregated state. This timing is beneficial for balancing the control of residual -NCO side reactions with the effectiveness of interfacial coupling.

[0023] In step S3, the amount of the interface coupling pre-reactant is 0.3%-1.5% of the total mass of the mixture.

[0024] In step S3, the old asphalt pavement material and new aggregate are first dry-mixed for 45-60 seconds, then emulsified asphalt is added and wet-mixed for 60-90 seconds to achieve a preliminary coating state and initiate the initial demulsification stage of the emulsified asphalt. After adding the pre-reactive agent, wet mixing continues for 60-90 seconds to ensure that the interface coupling pre-reactive agent is uniformly dispersed in the system and acts on the interface between the old and new asphalt. If necessary, additional water can be added to fine-tune the mixing state, and wet mixing continues until all aggregate particles are uniformly coated with asphalt slurry. The total mixing time preferably does not exceed 240 seconds.

[0025] In step S4, the compaction process is as follows: the mixture is compacted on both sides or rolled by roller at room temperature or 40-60℃; the Marshall test mold is compacted on both sides 75 times each, and the roller rolling is rolled at a rate of 42 times / minute, with 12-15 rolling passes.

[0026] In step S4, the curing conditions are: curing at 40-60℃ and relative humidity >85% for no less than 3 days, or curing at 25±5℃ under sealed conditions for no less than 7 days.

[0027] This invention modifies isocyanate through a pre-reaction method, enabling it to preferentially react with hydroxyl-containing components, consuming some of the active -NCO groups, and forming a prepolymer with asphalt affinity segments. This reduces the direct side reaction between isocyanate and water in the emulsified asphalt system from the source, avoiding foaming and increased porosity of the mixture.

[0028] This invention controls the residual free NCO mass fraction in the pre-reaction system to be between 0.5% and 5%, so that limited secondary chemical reactions occur at the interface: the residual -NCO groups can continue to undergo addition reactions with the hydroxyl groups at the interface of new and old asphalt to form chemical cross-linking bonds, while avoiding the embrittlement of the mixture caused by excessive reaction.

[0029] This invention utilizes the exothermic addition reaction of isocyanate and hydroxyl groups. The released heat can increase the local temperature at the interface between new and old asphalt, reduce the viscosity of asphalt, promote the interdiffusion of molecules between new and old asphalt, and further enhance the interface fusion effect.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1. This invention does not directly introduce isocyanate into the cold recycling system of high water content emulsified asphalt. Instead, it first consumes some of the highly active -NCO groups through the pre-reaction of isocyanate with hydroxyl-containing components, and constructs an interfacial coupling pre-reactant that has both asphalt affinity and limited residual reactivity. This reduces the probability of direct side reactions between isocyanate and water from the source and reduces the risk of foaming and increased porosity.

[0032] 2. The interface coupling pre-reactant constructed by the present invention can form urethane chemical cross-linking bonds at the interface between new and old asphalt, realizing directional chemical coupling between new and old asphalt, rather than traditional physical mixing; at the same time, the asphalt affinity segments of the pre-reactant can be embedded in the new and old asphalt phases to form stable molecular bridges, significantly improving the fusion bandwidth of the new and old asphalt interface, realizing deep molecular fusion between the two at the microscopic level, and reducing the weak interface transition zone.

[0033] 3. This invention reveals that the residual free NCO mass fraction is not necessarily better the higher or lower it is, but rather needs to be controlled within the range of 0.5%-5% to simultaneously balance side reaction control and interfacial secondary coupling activity. When the residual free NCO is too low, the interfacial secondary reaction is insufficient; when it is too high, it easily leads to enhanced side reactions and causes localized foaming. This window exhibits significant nonlinear characteristics.

[0034] 4. This invention further discovers that the timing of adding the interface coupling pre-reactant has a significant impact on the final effect. Only when added during the initial demulsification stage of emulsified asphalt can the pre-reactant more effectively participate in the interface formation and reconstruction process, thereby improving the degree of interface fusion between the new and old asphalts, under the condition that the free water in the system is relatively limited and the interface between the new and old asphalts is still reconstructable.

[0035] 5. The interface-induced melting method of the present invention has excellent adaptability to high proportions of RAP. Even when the RAP content reaches 80% or more, after adding the interface coupling pre-reactant, the freeze-thaw splitting strength ratio of the mixture can still be stabilized at over 85%, which greatly expands the applicable RAP content range of emulsified asphalt cold recycling technology and further improves the resource utilization rate of waste pavement materials.

[0036] 6. The heat released by the addition reaction of isocyanate and hydroxyl groups can increase the interface temperature between new and old asphalt, promote intermolecular diffusion, and further enhance the interface fusion effect. At the same time, it eliminates the need for additional heating steps, which is in line with the concept of cold recycling technology, saving energy and reducing resource waste. Detailed Implementation

[0037] The above-mentioned and other technical features and advantages of the present invention will be described in more detail below with reference to the embodiments.

[0038] In the following examples and comparative examples, the indirect tensile strength (ITS), freeze-thaw splitting strength ratio (TSR), void ratio, etc., were all tested in accordance with the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTGE20-2011).

[0039] Example 1

[0040] (1) Select milled old asphalt mixture (RAP), after crushing and screening, its asphalt content is measured to be 5.0%, the gradation meets the AC-20 median requirement, and the moisture content is controlled to be 2.5%; the RAP content is designed to be 80% (mass fraction), and 20% of 5-10mm new limestone aggregate is added;

[0041] (2) Slow-cracking and fast-setting cationic emulsified asphalt was used, with an evaporation residue content of 62% and an Engler viscosity of 11 at 25°C. Sodium borohydride was selected as the hydrogenation reducing agent, with an addition amount of 0.5% of the mass of aged asphalt. It was reacted with the aged asphalt in RAP at room temperature of 25°C and a stirring rate of 400 rpm for 15 minutes to obtain reduced aged asphalt containing active hydroxyl groups.

[0042] (3) Select polymeric MDI as isocyanate component and castor oil-based regenerator as hydroxyl-containing regenerator, weigh them at a mass ratio of 1:3, pre-react at a constant temperature of 70℃ and a stirring rate of 700rpm for 30min, and detect that the residual free NCO mass fraction is 2.1% to obtain the interface coupling pre-reactant;

[0043] (4) The amount of emulsified asphalt is 3.5% of the dry aggregate mass of the mixture, and the amount of interface coupling pre-reaction agent is 0.6% of the total mass of the mixture; dry mix RAP with new aggregate for 60 seconds, add emulsified asphalt and wet mix for 80 seconds until the emulsified asphalt enters the initial demulsification stage; add interface coupling pre-reaction agent and continue wet mixing for 80 seconds, add external water to fine-tune the state and wet mix for 30 seconds, for a total mixing time of 210 seconds;

[0044] (5) The mixture is loaded into a standard Marshall mold, compacted 75 times on each side, and cured for 3 days at 60°C and 90% relative humidity.

[0045] Test results show that, compared with conventional emulsified asphalt cold recycled mixture without the addition of interfacial coupling pre-reactant, the mixture obtained in this embodiment has an ITS of 22%, a TSR of 18%, a porosity of 4.2%, no obvious air bubbles were observed inside the specimen, the mixing uniformity is good, and the cross-sectional interface area is relatively continuous and dense.

[0046] Example 2

[0047] To investigate the effect of the timing of adding the interfacial coupling pre-reactant on the performance of the mixture, four groups were set up under the same conditions as in Example 1:

[0048] Group A: Add the interface coupling pre-reaction agent before adding emulsified asphalt, i.e. during the dry mixing stage of RAP and new aggregates;

[0049] Group B: Add the interfacial coupling pre-reaction agent after the emulsified asphalt is added and when the mixture reaches the initial demulsification stage;

[0050] Group C: Add an interfacial coupling pre-reaction agent after the emulsified asphalt has basically broken down;

[0051] Group D: Add an interfacial coupling pre-reactant to the end before molding.

[0052] The test results are shown in Table 1.

[0053] Table 1 Comparison of mixture performance at different addition times

[0054]

[0055] Microscopic observation and testing of the specimen cross-sections revealed that in Group A, the pre-reactant was added too early, resulting in a scattered distribution of its effective action sites after the subsequent introduction of emulsified asphalt and water, and traces of local side reactions were also observed. In Groups C and D, the pre-reactant was added too late, making it difficult for it to fully participate in the formation process of the interface between the new and old asphalt, and the interface transition zone remained relatively clear. Only in Group B, when the pre-reactant was added at the initial demulsification stage, was the interface zone most continuous, with the fewest air bubbles, and the best mechanical strength and water stability.

[0056] This demonstrates that adding the mixture during the initial demulsification stage of the emulsified asphalt is necessary to simultaneously address the requirements of restricted free water, suppressed side reactions, and reconfigurable interfaces, thereby maximizing the interfacial coupling effect of this invention.

[0057] Example 3

[0058] The difference between this embodiment and Embodiment 1 is that the dosage of the interface coupling pre-reactant is fixed at 0.6%, and the residual free NCO mass fraction is controlled to be 0.3%, 1.0%, 2.1%, 4.8%, and 5.5% by adjusting the degree of pre-reaction. All other conditions are the same as in Embodiment 1. The test results are shown in Table 2.

[0059] Table 2 Performance indicators of mixtures with different residual free NCO contents

[0060]

[0061] As shown in Table 2, when the residual free NCO mass fraction is less than 0.5%, there are insufficient activation sites in the pre-reactant that can further participate in the secondary interfacial reaction, resulting in limited interfacial enhancement. When it is in the range of 0.5%-5%, there are no obvious bubbles in the mixture, and the strength and water stability are significantly improved. When it exceeds 5%, there is too much residual active NCO in the system, which is prone to side reactions with water, leading to local bubbles and performance degradation.

[0062] Therefore, it is evident that a higher or lower residual free NCO content is not necessarily better; rather, there exists a window range that balances side reaction control and interfacial activity. Below this range, secondary interfacial reactions are insufficient, while above it, water-related side reactions intensify. Thus, a residual free NCO mass fraction of 0.5%–5% is preferred.

[0063] Example 4

[0064] The difference between this embodiment and Embodiment 1 is that the residual free NCO mass fraction is fixed at 2.1%, and the amount of interfacial coupling pre-reactant (total mass percentage of the mixture) is adjusted to 0.2%, 0.3%, 0.6%, 1.2%, 1.5%, and 2.0%, respectively. All other conditions are the same as in Embodiment 1. The test results are shown in Table 3.

[0065] Table 3 Performance indicators of the mixture under different pre-reactant dosages

[0066]

[0067] Table 3 shows that when the dosage is below 0.3%, the interfacial coupling agent is insufficient and the effect is limited. When the dosage of the pre-reactant is in the range of 0.3%-1.5%, the strength and water stability are significantly improved, and the porosity is basically stable. When the dosage is above 1.5%, it may cause local oil bleeding, decrease the stability of the system, and worsen the economy. Therefore, the preferred dosage of the pre-reactant is 0.3%-1.5% of the total mass of the mixture.

[0068] Example 5

[0069] The difference between this embodiment and Example 1 is that the RAP dosage was adjusted to 50%, 60%, 70%, 80%, and 85%, respectively, while the dosage of new aggregate was adjusted to maintain the gradation in accordance with the AC-20 median requirement. The pre-reactant dosage was fixed at 0.6%, and the residual free NCO mass fraction was 2.1%. All other conditions were the same as in Example 1. The TSR of the mixture before and after adding the pre-reactant was tested, and the results are shown in Table 4.

[0070] Table 4. TSR of mixtures with different RAP dosages

[0071]

[0072] As shown in Table 4, with the increase of RAP content, the TSR of emulsified asphalt cold recycling mixtures without the method of this invention decreased significantly, indicating that the interface fusion problem between new and old asphalt is more prominent under high RAP content conditions. However, after adopting the method of this invention, the TSR of the mixture under all RAP content conditions remained at a high level, especially under high RAP content conditions, the improvement was more significant. This shows that the method of this invention can effectively address the interface fusion problem caused by high RAP content and significantly broaden the applicable RAP content range of emulsified asphalt cold recycling technology.

[0073] Example 6

[0074] The conditions in this embodiment are the same as in Example 1. To examine the difference between adding isocyanate directly and adding it after pre-reaction, the following three groups were set up:

[0075] Group A: Add polymeric MDI directly;

[0076] Group B: MDI prepolymer was added directly, but its residual free NCO mass fraction was not controlled;

[0077] Group C: The interfacial coupling pre-reactant described in this invention is added, and the residual free NCO mass fraction is controlled at 2.1%.

[0078] The test results are shown in Table 5.

[0079] Table 5 Comparison of mixture performance under different isocyanate introduction methods

[0080]

[0081] Among them, obvious pores and bubbles appeared inside the specimens of Group A, indicating that the free NCO had a strong side reaction with the water in the system; although Group B was better than Group A, the performance was still unstable due to local side reactions or insufficient interfacial activity because the residual free NCO was not controlled; Group C balanced the lower risk of side reactions and better interfacial coupling ability, and had the best performance and stable results.

[0082] This indicates that the present invention requires pre-reaction to convert isocyanate into a carbamate-containing structure and control residual free NCO within a reasonable range in order to balance side reaction control and interfacial coupling.

[0083] Example 7

[0084] To verify the rationality of the pre-reaction temperature range, with the reaction time fixed at 30 min and other conditions the same as in Example 1, the pre-reaction temperature was set to 40℃, 50℃, 70℃, 90℃ and 100℃ respectively. The mass fraction of residual free NCO in the pre-reactant and the performance of the mixture were tested, and the results are shown in Table 6.

[0085] Table 6 Comparison of mixture properties at different pre-reaction temperatures

[0086]

[0087] Table 6 shows that when the pre-reaction temperature is too low, the isocyanate reacts insufficiently with the hydroxyl-containing components, resulting in a high residual free NCO content, which makes it prone to side reactions in the subsequent cold recycling high-moisture system. Conversely, when the temperature is too high, the pre-reaction is excessive, resulting in a low residual active NCO content. Simultaneously, the viscosity of the pre-reactant increases, and its construction dispersibility deteriorates, hindering its effective action at the interface between new and old asphalt. Considering the factors of side reaction suppression, residual interfacial activity, and construction dispersibility, the optimal pre-reaction temperature is 50-90℃.

[0088] Example 8

[0089] With the pre-reaction temperature fixed at 70℃ and other conditions the same as in Example 1, the pre-reaction time was set to 10 min, 20 min, 30 min, 40 min and 60 min respectively, and the test results are shown in Table 7.

[0090] Table 7 Comparison of mixture performance under different pre-reaction times

[0091]

[0092] Table 7 shows that when the pre-reaction time is too short, the isocyanate is not fully pre-reacted, resulting in high levels of free NCO and significant side reactions. When the pre-reaction time is too long, the pre-reaction process is excessive, leading to insufficient residual active NCO, weakened interfacial coupling, and increased system viscosity, which is detrimental to dispersion during construction. Therefore, the optimal pre-reaction time is 20-40 min.

[0093] The above description is merely a preferred embodiment of the present invention and is illustrative rather than restrictive. Those skilled in the art will understand that many changes, modifications, and even equivalents can be made within the spirit and scope defined by the claims of the present invention, all of which will fall within the protection scope of the present invention.

Claims

1. A method of promoting interfacial fusion of old and new asphalts in an emulsified asphalt cold recycled mixture, characterized in that, Includes the following steps: S1, prepare old asphalt pavement materials, new aggregates, emulsified asphalt, water, isocyanate components and hydroxyl-containing components that have been crushed and screened; S2, the isocyanate component is pre-reacted with the hydroxyl-containing component to generate an interfacial coupling pre-reactant with a carbamate structure, wherein the mass fraction of residual free NCO in the interfacial coupling pre-reactant is 0.5%-5%; S3 involves mixing old asphalt pavement materials, new aggregates, and emulsified asphalt. When the emulsified asphalt enters the initial demulsification stage, an interfacial coupling pre-reaction agent is added to continue mixing to obtain a mixture. S4 involves compacting the mixed material into shape, curing it, and completing the fusion process at the interface between the old and new asphalt.

2. The method of claim 1, wherein the method is characterized by, In step S1, the isocyanate component is any one or a combination of several of diphenylmethane diisocyanate, polymeric diphenylmethane diisocyanate, or their prepolymers.

3. The method of claim 1, wherein the method is characterized by, In step S1, the hydroxyl-containing component is any one or a combination of several of the following: castor oil-based regenerator, polyether alcohol regenerator, asphalt-based hydroxyl regenerator, or regenerated aged asphalt containing active hydroxyl groups; the mass ratio of the isocyanate component to the hydroxyl-containing component is 1:2-5.

4. The method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures as described in claim 3, characterized in that, The preparation process of the reduced aged asphalt with active hydroxyl groups is as follows: the aged asphalt in the old asphalt pavement material is subjected to a reduction reaction, so that some of the oxide groups in the aged asphalt are converted into hydroxyl groups; the hydrogenation reducing agent used in the reduction reaction is any one or a combination of sodium borohydride, diimide, and transition metal complex hydrogenation catalyst; the amount of hydrogenation reducing agent added is 0.2%-1.0% of the mass of the aged asphalt; the reduction reaction is carried out at 25±3℃ and 300-500rpm for 10-20 minutes.

5. The method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures as described in claim 1, characterized in that, In step S2, the stirring rate of the pre-reaction is 600-800 rpm, the reaction temperature is 50-90℃, and the reaction time is 20-40 minutes. During the pre-reaction, the residual free NCO mass fraction needs to be controlled at 0.5%-5%. If the residual free NCO mass fraction exceeds the range of 0.5%-5%, it can be adjusted by adding isocyanate components or hydroxyl-containing components.

6. The method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures as described in claim 1, characterized in that, In step S3, the initial demulsification stage can be judged comprehensively by appearance, free water state and coating state. It is the stage where there is no continuous free water at the bottom of the mixture, more than 70% asphalt coating is formed on the surface of the aggregate, and the mixture changes from a fluid slurry to a loosely clump-like state.

7. The method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures as described in claim 1, characterized in that, In step S3, the amount of the interface coupling pre-reactant is 0.3%-1.5% of the total mass of the mixture.

8. The method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures as described in claim 1, characterized in that, In step S3, the old asphalt pavement material and the new aggregate are first dry-mixed for 45-60 seconds, then emulsified asphalt is added and wet-mixed for 60-90 seconds until initial demulsification is achieved. After adding the pre-reaction agent, wet mixing continues for 60-90 seconds, with the total mixing time not exceeding 240 seconds.

9. The method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures as described in claim 1, characterized in that, In step S4, the compaction process is as follows: the mixture is compacted on both sides or rolled by roller at room temperature or 40-60℃; the Marshall test mold is compacted 75 times on each side, and the roller rolling rate is 42 times / minute, with 12-15 rolling passes.

10. The method for promoting the fusion of new and old asphalt interfaces in emulsified asphalt cold recycled mixtures as described in claim 1, characterized in that, In step S4, the curing conditions are: curing for no less than 3 days at 40-60℃ and relative humidity >85%, or curing for no less than 7 days at room temperature 25±5℃ under sealed conditions.

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