Flame-retardant asphalt for tunnel pavement and preparation method of flame-retardant asphalt

Abstract

The invention discloses flame-retardant asphalt for a tunnel pavement and a preparation method of the flame-retardant asphalt. The flame-retardant asphalt is based on matrix asphalt, and carbon nanotubes, fly ash and lignin are added as flame-retardant components. The optimal flame-retardant ratio of the flame-retardant asphalt can be further obtained through a central combinatorial design model and a comprehensive evaluation method. The preparation method comprises the working procedures of matrix asphalt heating, flame-retardant component preheating, step-by-step mixing of matrix asphalt and flame-retardant components, high-speed shearing, degassing and the like, can be simultaneously adapted to an indoor test preparation process and an industrial-scale preparation process, and has good adaptability and practicability. According to the invention, the carbon nanotubes, the fly ash and the lignin are introduced to form a composite flame-retardant system, and the three components synergistically act to construct a stable multi-scale flame-retardant structure, so that the flame-retardant property of the asphalt material is remarkably improved, the environmental protection property, the economical efficiency and the engineering adaptability are considered, and the composite flame-retardant asphalt material has remarkable engineering application value and popularization prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of road engineering materials, in particular to flame-retardant asphalt for tunnel pavement and a preparation method thereof, and to the intersection of road paving materials and flame-retardant functional materials. Background Art

[0002] Tunnel engineering, as an essential means of traversing mountainous areas, crossing rivers, and connecting islands, plays a crucial role in my country's integrated transportation system. In recent years, a large number of regional connectivity projects have been implemented, driving rapid development in my country's tunnel construction. This is particularly true in the economically developed eastern coastal regions, where major projects such as undersea and cross-sea tunnels are increasingly being developed to expand urban space and strengthen intercity transportation links. In the past, tunnel pavement designs were designed for fuel-powered vehicles. However, with the rapid development of new energy vehicles in my country in recent years, the types of vehicles used in tunnels are gradually changing. Due to the relatively enclosed space, limited ventilation, and restricted evacuation routes in tunnels, if an electric vehicle is involved in a traffic accident or fire, the fire and toxic smoke can spread rapidly, seriously threatening the lives of passengers, hindering emergency rescue efforts, and potentially causing prolonged traffic disruptions and significant economic losses.

[0003] Existing tunnel pavements in my country are generally paved with traditional asphalt mixtures. While this material has excellent mechanical properties and durability, it is a highly flammable organic material. When exposed to extreme conditions such as open flames, it is prone to melting, carbonization, and even violent combustion. This not only exacerbates the spread of fire but also releases large amounts of toxic and harmful smoke, significantly reducing visibility and oxygen concentration within the tunnel. Therefore, improving the flame retardancy of asphalt pavement materials has become a key step in improving overall tunnel fire safety.

[0004] Currently, asphalt flame-retardant modification technologies primarily include the addition of inorganic flame retardants, organophosphorus flame retardants, and intumescent flame retardant systems. While these technologies have improved asphalt's fire resistance to a certain extent, they also have numerous shortcomings. Inorganic flame retardants require large quantities, which can easily affect the mechanical and construction properties of asphalt. Organic flame retardants can cause environmental pollution and volatility issues. High costs, poor thermal stability, and secondary pollution also limit their large-scale application in engineering projects. Intumescent flame retardant systems generally have poor compatibility with asphalt, which can easily lead to asphalt segregation or stratification, affecting the basic properties of asphalt. They also require a higher dosage to achieve their flame-retardant properties, increasing the material's production cost. Some intumescent flame retardants can produce irritating gases at high temperatures, adversely affecting the tunnel environment. Therefore, there is an urgent need to develop a new type of flame-retardant asphalt material for tunnel pavements that has high flame retardancy, green environmental protection and cost control. It can not only effectively inhibit the spread of fire and prolong the safe evacuation time, but also conform to the concept of sustainable development and have the advantages of being renewable, non-toxic and resource-based, so as to improve the safety level of the tunnel road system from the source and meet the dual needs of tunnel pavements in terms of safety and environmental protection. Summary of the Invention

[0005] In view of the problems that existing asphalt used in tunnel pavements has such as flammability and high smoke density in extreme situations such as fire, the present invention aims to provide a flame-retardant asphalt for tunnel pavements and a preparation method thereof, so as to improve the flame retardant performance and safety of asphalt materials in high-risk environments such as tunnels.

[0006] To achieve the above object, the technical solution of the present invention is as follows: In a first aspect, the present invention provides a flame-retardant asphalt for tunnel pavement, the main material of which is matrix asphalt, which also includes, by mass percentage, 0wt% to 2wt% of carbon nanotubes, 8wt% to 10wt% of fly ash and 5wt% to 7wt% of lignin.

[0007] As a preference, the following method is used to determine the optimal flame retardant ratio of flame retardant asphalt: 1) Using the central composite design model, input the dosage ranges of carbon nanotubes, fly ash, and lignin to generate multiple flame retardant asphalt mix ratios; 2) Using basic asphalt performance indicators and flame retardant performance indicators as evaluation indicators for flame retardant asphalt, the evaluation index values ​​of each ratio scheme were determined through indoor tests. Among them, the basic asphalt performance indicators are ductility, softening point, and needle penetration, and the flame retardant performance indicators are flash point and limiting oxygen index; 3) A comprehensive evaluation method is used to calculate the scores of the basic performance indicators and flame retardant performance indicators of asphalt, obtain the comprehensive score of each ratio scheme, and then determine the optimal flame retardant ratio.

[0008] Preferably, the comprehensive evaluation method adopts the entropy method.

[0009] Preferably, the optimal flame retardant ratio of the flame retardant asphalt is: 1wt% carbon nanotubes, 10wt% fly ash, and 5wt% lignin.

[0010] In a second aspect, the present invention further provides a method for preparing flame-retardant asphalt for tunnel pavement, which is an indoor test preparation method, comprising the following steps: S1: heating the matrix asphalt to a fluid state; S2: preheating carbon nanotubes, fly ash and lignin; S3: Weighing the matrix pitch fluid and the carbon nanotubes, adding the carbon nanotubes to the matrix pitch fluid, heating and stirring, and obtaining a preliminary mixture; S4: Using a high-speed shearing machine to perform high-speed shearing on the preliminary mixture; S5: Weigh fly ash and lignin, add fly ash and lignin to the sheared preliminary mixture in multiple batches, and heat and stir to obtain a secondary mixture; S6: Using a small high-speed shearing machine to perform heat preservation and high-speed shearing on the secondary mixture; S7: Use a small high-speed shearing machine to shear the secondary mixture at a low speed to remove bubbles and obtain flame-retardant asphalt.

[0011] Preferably, the heating temperature, preheating temperature and holding temperature are 155°C-175°C, preferably 165°C.

[0012] Preferably, in S3, the stirring speed is 150-250 r / min, preferably 200 r / min, and the stirring time is 5-15 min, preferably 10 min; in S5, the stirring speed is 150-250 r / min, preferably 200 r / min, and the stirring time is 5-15 min, preferably 10 min.

[0013] Preferably, in S4, the shearing speed is 3000-4000 r / min, preferably 3500 r / min, and the shearing time is 30-50 min, preferably 45 min; in S6, the shearing speed is 3000-4000 r / min, preferably 3500 r / min, and the shearing time is 20-40 min, preferably 30 min; in S7, the shearing speed is 300-700 r / min, preferably 500 r / min, and the shearing time is 5-10 min, preferably 5 min.

[0014] In a third aspect, the present invention also provides another method for preparing flame-retardant asphalt for tunnel pavement, which is an industrial preparation method, comprising the following steps: T1: heating the matrix asphalt to a fluid state; T2: preheating carbon nanotubes, fly ash and lignin; T3: Weighing the matrix asphalt fluid and carbon nanotubes, adding the carbon nanotubes to the matrix asphalt fluid, and dispersing them in a high-speed disperser to obtain a preliminary mixture; T4: Using a large high-speed shearing machine to perform high-speed shearing on the preliminary mixture; T5: Weigh fly ash and lignin, add fly ash and lignin to the preliminary mixture, and disperse them in a high-speed disperser to obtain a secondary mixture; T6: Use a large high-speed shearing machine to perform high-speed shearing on the secondary mixture at a constant temperature; T7: Use a large high-speed shearing machine to shear the secondary mixture at a low speed to remove bubbles and obtain flame-retardant asphalt.

[0015] Preferably, the heating temperature, preheating temperature and holding temperature are all 155°C-175°C, preferably 165°C.

[0016] Preferably, in the T3, the dispersion speed is 150-250 r / min, preferably 200 r / min, and the dispersion time is 5-15 min, preferably 10 min; in the T5, the dispersion speed is 150-250 r / min, preferably 200 r / min, and the dispersion time is 5-15 min, preferably 10 min.

[0017] Preferably, in the T4, the shear speed is 1500-2500 r / min, preferably 2000 r / min, and the shear time is 20-40 min, preferably 30 min; in the T6, the shear speed is 1000-2000 r / min, preferably 1500 r / min, and the shear time is 20-40 min, preferably 30 min; in the T7, the shear speed is 100-300 r / min, preferably 200 r / min, and the shear time is 5-15 min, preferably 10 min.

[0018] Compared with the existing technology, the flame retardant asphalt of the present invention has the following outstanding advantages: The present invention provides a new type of flame-retardant asphalt for tunnel pavement. By introducing carbon nanotubes, fly ash and lignin to form a composite flame-retardant system, it significantly improves the flame-retardant performance of asphalt materials while taking into account environmental protection, economy and engineering adaptability. Carbon nanotubes give the material excellent carbonization and heat insulation capabilities, fly ash provides an inorganic mineral barrier effect, and lignin, as a natural biomass, has good coking and synergistic flame-retardant effects. The three play a synergistic flame-retardant role in the base asphalt matrix through carbonization, coking and ceramicization mechanisms, respectively, to construct a stable multi-scale flame-retardant structure, forming a "low-temperature coking-medium-temperature carbonization-high-temperature ceramicization" three-stage linkage flame-retardant mode; the three cooperate with each other to significantly reduce the heat release rate of flame-retardant asphalt, increase the carbon residue rate, delay the ignition time, and inhibit the release of toxic smoke, thereby comprehensively improving the flame-retardant performance of the material in tunnel fire environments; (1) Significantly enhanced flame retardant performance: The composite additive system can delay flame propagation, improve fire resistance, effectively inhibit dripping and heat release, and meet the tunnel's adaptability to extreme high temperature environments; (2) Green and environmentally friendly: All flame retardant components are derived from renewable resources or industrial solid waste, avoiding the use of toxic chemical flame retardants such as halogens and organophosphorus. There is no secondary pollution during the combustion process, which is in line with the development direction of green building materials. (3) Good cost control and suitable for engineering promotion: the raw materials are widely available and inexpensive, making them suitable for large-scale and engineering applications in tunnel projects; (4) Strong adaptability: The flame retardant system can be modified without significantly changing the conventional asphalt preparation process and construction process, and has good engineering adaptability and replicability. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0020] Figure 1 is a schematic diagram of the asphalt penetration response surface with different flame retardant component dosages; Figure 1 (a) shows the relationship between fly ash and regenerated carbon tubes (carbon nanotubes), Figure 1 (b) shows the relationship between fly ash and lignin, Figure 1 (c) Graph showing the relationship between regenerated carbon tubes and lignin; Figure 2 is a schematic diagram of the asphalt ductility response surface with different flame retardant component dosages; Figure 2 (a) shows the relationship between fly ash and regenerated carbon tubes (carbon nanotubes), Figure 2(b) shows the relationship between fly ash and lignin, Figure 2 (c) Graph showing the relationship between regenerated carbon tubes and lignin; Figure 3 is a schematic diagram of the asphalt flash point response surface with different flame retardant component dosages; Figure 3 (a) shows the relationship between fly ash and regenerated carbon tubes (carbon nanotubes), Figure 3 (b) shows the relationship between fly ash and lignin, Figure 3 (c) Graph showing the relationship between regenerated carbon tubes and lignin; Figure 4 is a schematic diagram of the asphalt oxygen index response surface with different flame retardant component dosages; Figure 4 (a) shows the relationship between fly ash and regenerated carbon tubes (carbon nanotubes), Figure 4 (b) shows the relationship between fly ash and lignin, Figure 4 (c) Graph showing the relationship between regenerated carbon tubes and lignin; Figure 5 is a schematic diagram of the entropy response surface method for different flame retardant component dosages; Figure 5 (a) shows the relationship between fly ash and regenerated carbon tubes (carbon nanotubes), Figure 5 (b) shows the relationship between fly ash and lignin, Figure 5 (c) Graph showing the relationship between regenerated carbon tubes and lignin; Figure 6 Schematic diagram of thermogravimetric analysis of SBS asphalt; Figure 7 This is a schematic diagram of thermogravimetric analysis of flame retardant asphalt; Figure 8 Schematic diagram of heat release comparison between SBS asphalt and flame retardant asphalt in cone calorimetry test; Figure 8 (a) represents the peak heat release rate, Figure 8 (b) represents the total heat release; Figure 9 Schematic diagram of the comparison of mass changes of SBS asphalt and flame retardant asphalt in cone calorimetry test; Figure 10 Schematic diagram comparing the smoke release of SBS asphalt and flame retardant asphalt in cone calorimetry test; Figure 11 Schematic diagram comparing the complex shear modulus and phase angle of SBS asphalt and flame retardant asphalt DSR; Figure 12 Schematic diagram comparing the rutting factors of SBS asphalt and flame retardant asphalt; Figure 13 This is a schematic diagram of the combustion comparison between SBS asphalt and flame retardant asphalt after the cone calorimetry test. Figure 13 (a) represents SBS asphalt, Figure 13 (b) indicates that this product is flame retardant asphalt. DETAILED DESCRIPTION

[0021] In order to more clearly understand the technical solution of the present invention, it is now further described in conjunction with the accompanying drawings and embodiments. It should be understood that, under the premise that no technical conflict occurs, the technical features of the various embodiments in this specification can be combined and replaced with each other. In addition, unless otherwise specified, the technical terms and scientific terms used in the present invention should be based on the conventional understanding of ordinary technicians in the relevant technical field. The terms "including" or "comprising" mentioned in this specification should be understood to mean that the listed contents are not exhaustive, and also include other elements or equivalent alternatives that are not explicitly stated but belong to the same or similar technical fields, and should not be understood as excluding other unlisted technical solutions.

[0022] There are many flame retardant modified materials available. To ensure that asphalt has high flame retardancy, environmental friendliness, and cost controllability, the present invention selects carbon nanotubes, lignin, and fly ash to meet the above three performance requirements.

[0023] Carbon nanotubes, as a nanomaterial with excellent thermal stability and structural rigidity, have been used in some studies to improve the high-temperature stability, anti-aging properties and mechanical modulus of asphalt. However, such studies mostly use them as structural reinforcing agents or functional additives, without systematically designing and verifying their flame retardant effects, let alone considering the renewability of the material source. The carbon nanotubes selected in the present invention are derived from the pyrolysis residue of waste plastics and have typical lamellar and high-carbon structural characteristics. Under the action of heat, they can quickly form an insulating carbon layer, effectively blocking the oxygen and heat transfer required for combustion, significantly inhibiting the spread of fire and the heat release process, reflecting their core functional positioning as a "carbonized skeleton" in the asphalt flame retardant system.

[0024] As a natural biomass polymer material, lignin has been partially used in polymer, concrete and other systems to enhance or assist in flame retardancy. However, there are currently few published documents at home and abroad that use lignin as an independent flame retardant component in road asphalt materials, especially a lack of synergistic analysis and ratio optimization research with other components. In the present invention, lignin is systematically introduced into the asphalt flame retardant system, and its easy coking property during pyrolysis is utilized to increase the carbon residue rate of the material and inhibit smoke release. At the same time, the addition of lignin also improves the flexibility and structural stability of the asphalt, so that the material still has good mechanical properties under high-temperature combustion conditions, demonstrating its application potential under the dual requirements of road safety and green environmental protection performance.

[0025] As an industrial by-product, fly ash is mainly used as a filler or stabilizer in road engineering. In terms of flame retardancy, although fly ash itself has certain thermal stability and ceramic properties, it is often used as an auxiliary material in previous studies, and has limited improvement in flame retardant properties when used alone. The present invention redefines fly ash as an "inorganic thermally inert flame retardant barrier material" and combines the characteristics of the silicon and aluminum components contained in it to form a stable skeleton at high temperatures, so that it plays an important physical insulation and smoke suppression function in asphalt. In particular, it exhibits good synergistic effects after being compounded with carbon nanotubes and lignin, significantly improving the overall thermal stability and flame retardant properties of the material, and the material cost is controllable.

[0026] Implementation method one: The present invention provides a flame retardant asphalt for tunnel pavement. The main material is matrix asphalt. Calculated by mass percentage, the asphalt further comprises 0wt% to 2wt% of carbon nanotubes, 8wt% to 10wt% of fly ash, and 5wt% to 7wt% of lignin.

[0027] The present invention uses three flame retardant components, carbon nanotubes, lignin and fly ash, to construct a composite flame retardant system. The three components exert synergistic flame retardant effects in the base asphalt matrix through carbonization, coking and ceramicization mechanisms, thereby effectively improving the thermal stability and safety performance of the material in high fire risk environments such as tunnels.

[0028] First, carbon nanotubes possess extremely high thermal stability, with a decomposition temperature typically exceeding 600°C. During the asphalt combustion process, carbon nanotubes maintain structural stability in the early stages and gradually carbonize under high temperatures, forming a continuous, dense, and heat-conducting carbon layer. This carbon layer effectively blocks the diffusion of heat and oxygen into the asphalt, significantly slowing the pyrolysis reaction. Furthermore, carbon nanotubes enhance the viscosity and structural integrity of the asphalt system, inhibiting drip melting at high temperatures and helping to slow the spread of flames. Furthermore, carbon nanotubes adsorb free radicals, capturing active groups in the combustion chain reaction and achieving a chemical synergistic effect that inhibits combustion.

[0029] Secondly, lignin is a complex natural aromatic biopolymer with a main decomposition range concentrated between 250 and 400°C. After decomposition, it can produce intermediate products such as phenols and aldehydes, and generate a large amount of carbon residue. Under heating conditions, lignin is easily dehydrated, condensed, and aromatized, and eventually forms a stable carbonaceous structure, thereby constructing a primary coking layer on the asphalt surface. This carbon layer not only enhances the thermal insulation and structural stability of the material, but also works together with carbon nanotubes to construct a more complete carbonization barrier. In addition, the intermediates generated during the pyrolysis of lignin have a certain free radical neutralization ability, which can effectively inhibit the persistence of the combustion reaction. As a flexible biomass, lignin can also improve the ductility of flame-retardant asphalt, allowing the carbon layer to maintain better toughness and integrity at high temperatures.

[0030] Secondly, fly ash is an industrial byproduct produced under high-temperature conditions. It does not burn itself and has extremely high thermal stability, with a melting onset temperature of 800-1300°C. Fly ash is rich in inorganic components such as SiO2 and Al2O3. When heated, it can undergo a glass transition or ceramic reaction, forming a rigid, inert protective layer on the burning surface of asphalt, effectively isolating heat conduction and oxygen diffusion between the external heat source and the internal combustible components. At the same time, the addition of fly ash can reduce the concentration of combustible components in the asphalt system, exerting a dilution effect; its microporous structure can also adsorb flue gas and combustible volatiles, playing a smoke suppression role. Fly ash also has good dispersibility, which helps improve the distribution of lignin and carbon nanotubes in the asphalt matrix, enhancing the synergy and uniformity of the overall flame retardant system.

[0031] The synergistic mechanism of the three flame-retardant components during heating can be summarized as a three-stage linkage model of "low-temperature coking-medium-temperature carbonization-high-temperature ceramicization": Lignin is first pyrolyzed in the 250-400°C range, providing an initial carbon layer and inhibiting free radicals; carbon nanotubes carbonize above 600°C to strengthen the carbon layer structure, forming a highly dense thermal barrier; and fly ash forms an inorganic ceramic skeleton above 800°C, improving the thermal stability of the asphalt's residual structure. These three components work together to significantly reduce the flame-retardant asphalt's heat release rate, increase the carbon residue rate, delay ignition time, and suppress the release of toxic smoke, comprehensively enhancing the material's flame-retardant properties in tunnel fire environments.

[0032] It should be noted that, in addition to the carbon nanotubes obtained by pyrolysis of plastics selected in this embodiment, other carbon nanotubes are also applicable to the present invention.

[0033] The following describes the process for determining the flame-retardant asphalt mix ratio. After deciding on carbon nanotubes, fly ash, and lignin as the flame-retardant components, the authors first investigated existing research literature (including journal articles related to flame-retardant asphalt, tunnel pavement flame-retardant materials, intumescent flame-retardant systems, the flame-retardant effects of carbon nanomaterials, composite flame-retardant systems, and patents related to flame-retardant asphalt, modified asphalt flame-retardant formulations, and the synergistic flame retardancy of nanofillers). The authors determined the maximum individual carbon nanotube, fly ash, and lignin blend ratios. The present invention first constructed an initial mix ratio using the maximum blend ratios of the three components. Single-factor preliminary tests were conducted, including ductility, softening point, penetration, flash point, and limiting oxygen index testing, to preliminarily determine the effective range of each component. It was apparent that the flame-retardant asphalt with this initial mix ratio hardly met the basic performance requirements for asphalt. The authors then determined the next mix ratio using a binary approach or by reducing the blend ratio to observe whether the test results met the requirements. Similarly, once a group meets the basic performance requirements of asphalt, the central composite design model and the flame-retardant asphalt test results determined by it are further used to gradually reduce the proportions that meet the basic performance requirements of asphalt, ultimately determining carbon nanotubes at 0wt% to 2wt%, fly ash at 8wt% to 10wt%, and lignin at 5wt% to 7wt%. This trial-mixing and blending approach can significantly reduce the number of tests required, saving testing time and costs. The flame retardant effect is most coordinated with the material properties, making it feasible for engineering applications. Alternatively, an orthogonal experimental design can be used to test multiple combinations of the three components, comprehensively analyzing the carbon layer formation effect, thermal stability, oxygen index, and changes in the asphalt's road performance, and then determining the dosage of carbon nanotubes, fly ash, and lignin. However, orthogonal experiments will increase the number of experiments.

[0034] It should be noted that the ductility, softening point and needle penetration were tested with reference to the Technical Specifications for Highway Asphalt Pavement Construction JTGF40-2017, and the flash point and limiting oxygen index were tested with reference to the standards "Cleveland Open Cup Method for Determination of Flash Point and Fire Point of Petroleum Products" GB / T3536-2008 and "Oxygen Index Method for Determination of Combustion Performance of Asphalt" NB / SH / T0815-2010. The ductility test of the flame retardant asphalt in the present invention is measured at 5 degrees Celsius. Specific embodiment one: This example, based on relevant research literature, found that the maximum amount of carbon nanotubes (CNTs) is 2wt%, the maximum amount of fly ash is 10wt%, and the maximum amount of lignin is 7wt%. Based on the method described in Implementation Method 1, this example first tested Scheme B2. However, the resulting flame-retardant asphalt did not meet the basic performance requirements. Therefore, further trial preparation and blending processes were carried out. Specifically, the experimental scheme for incorporating different flame-retardant components into the base asphalt is shown in Table 1.

[0036] ; It should be noted that Scheme B1 in the table is only the base asphalt without any flame retardant components added, and serves as the control group for other ratio test schemes that add flame retardant components.

[0037] In each flame retardant asphalt ratio test scheme, the results of the flame retardant asphalt ductility, softening point, penetration, flash point and limiting oxygen index measured by indoor tests are shown in Table 2. For the specific process of the indoor test, please refer to Implementation Method 3 and Specific Example 3.

[0038] ; "Technical Specifications for Highway Asphalt Pavement Construction" JTGF40-2017 stipulates that the minimum ductility requirement for modified asphalt at 5 degrees Celsius (to evaluate the ductility and toughness of modified asphalt at low temperatures) is 20 cm.

[0039] "Technical Specifications for Highway Asphalt Pavement Construction" JTGF40-2017 stipulates that the needle penetration of modified asphalt (25°C, 100g, 5s) in heavy traffic, high temperature or high-performance roads should be 30-60 (0.1mm).

[0040] "Technical Specifications for Highway Asphalt Pavement Construction" JTGF40-2017 stipulates that the softening point of modified asphalt is greater than 65°C and the softening point of base asphalt is greater than 45°C.

[0041] The national standard "Flame-retardant Asphalt Concrete for Roads" GB / T29051-2012 stipulates that the oxygen index (an indicator for evaluating the difficulty of ignition of a material) of asphalt used in tunnel roads should be no less than 23%.

[0042] "Technical Specifications for Highway Asphalt Pavement Construction" JTGF40-2004 stipulates that the flash point of base asphalt is greater than 230°C and the flash point of modified asphalt is greater than 240°C.

[0043] The ductility of mix B2 does not meet the requirements. The oxygen index of mix B1 does not meet the requirements.

[0044] Figure 1-4 The effects of each flame retardant component in the above-mentioned ratio test scheme on the penetration, ductility, flash point and limiting oxygen index of flame retardant asphalt are demonstrated respectively.

[0045] Figure 1The effects of various flame retardant components on the penetration of flame retardant asphalt were demonstrated. With the increase in the amount of recycled carbon nanotubes, the overall penetration showed a trend of first decreasing and then increasing. An appropriate amount of recycled carbon nanotubes can enhance the structural rigidity of the asphalt, making the material more resistant to deformation at high temperatures, resulting in a decrease in penetration. However, when the dosage is too high, the internal structure of the material may become too brittle, causing the penetration to increase. Lignin, as a natural polymer, can moderately increase the consistency and structural stability of asphalt, but its strengthening effect is not as significant as that of recycled carbon nanotubes and fly ash. The fine-grained minerals in fly ash can fill the pores in the asphalt, strengthen the skeleton structure, and significantly improve the density and rigidity of the flame retardant asphalt, thereby significantly reducing the penetration.

[0046] Figure 2 The effect of each flame retardant component on the ductility of flame retardant asphalt is demonstrated. As the content of regenerated carbon nanotubes increases, the ductility shows a trend of first increasing and then decreasing. An appropriate amount of regenerated carbon nanotubes can improve the elasticity and toughness of asphalt, increasing its ductility; however, when the content exceeds a certain range, the rigidity of the system increases, the brittleness increases, and the ductility decreases. The addition of lignin helps to improve the flexibility of the material and form a certain compatible structure with the matrix asphalt, improving the tensile properties. However, if the content is too high, it may lead to a decrease in compatibility, thereby affecting the ductility. The ductility fluctuates with the change of fly ash content, that is, it shows a trend of first increasing, then decreasing, and then increasing again, with a strong nonlinear characteristic. When the content is too high, the filler effect causes the structure to become harder and the ductility to decrease.

[0047] Figure 3 The effects of various flame retardant components on the flash point of flame-retardant asphalt are demonstrated. Regenerated carbon nanotubes and lignin both exhibit a positive correlation with the flash point of asphalt. Their incorporation helps to increase the thermal decomposition temperature of the system, thereby raising the flash point. The effect of increasing fly ash content on the flash point is more complex, showing a trend of initial decline followed by a slight rebound, with significant overall fluctuations. Appropriate amounts of fly ash can indirectly increase the flash point by hindering heat conduction and diluting combustible components. However, excessive amounts can impair thermal stability due to uneven particle distribution or poor interface properties.

[0048] Figure 4 The effects of various flame retardant components on the limiting oxygen index of flame-retardant asphalt are demonstrated. With increasing recycled carbon nanotube content, the limiting oxygen index initially increases and then slightly decreases, with a peak range emerging, indicating a definite optimal content. Lignin significantly improves the limiting oxygen index, while fly ash has a relatively small effect, resulting in an overall smooth curve.

[0049] According to the above results, it is determined that the carbon nanotubes are 0wt% to 2wt%, the fly ash is 8wt% to 10wt%, and the lignin is 5wt% to 7wt%.

[0050] When the carbon nanotube content exceeds 2wt%, the performance of the asphalt material will show significant negative changes. First, carbon nanotubes have a very large specific surface area and high surface energy, making them very prone to agglomeration. Excessive incorporation of carbon nanotubes will lead to uneven dispersion in the asphalt matrix, forming a large number of agglomerates, which in turn will cause local stress concentration within the material, reducing the uniformity and mechanical stability of the overall material. In addition, too many carbon nanotubes significantly increase the rigidity of the asphalt system, resulting in a significant decrease in its high-temperature fluidity and a decrease in its low-temperature ductility, making the material as a whole more fragile and prone to cracking, reducing its service life and reliability.

[0051] Since lignin molecules are highly polar, while asphalt is mainly composed of non-polar components, the addition of excessive lignin can easily cause phase separation, resulting in an uneven mixture structure, which in turn affects the long-term stability and durability of the material. At the same time, excessive lignin addition will significantly increase the rigidity of the asphalt system, reduce the ductility and toughness at low temperatures, make the asphalt material harder and more brittle, and significantly weaken its crack resistance. During the combustion process, lignin produces a large amount of carbonized residue, which may destroy the continuity of the material's carbon layer, reduce the structural integrity of the material, and even have an adverse effect on the microstructure of the asphalt material under non-combustion conditions. When the lignin addition is too low, as a natural carbon source and expandable carbon layer former, it cannot fully participate in the construction of the carbon layer during the combustion process, making it difficult to form a dense and continuous thermal insulation and oxygen barrier, resulting in a significant decrease in the thermal insulation and flame retardancy of the material.

[0052] When the fly ash content exceeds 10wt%, its adverse effects on asphalt performance begin to emerge. Excessive fly ash will dilute the effective binding components in the asphalt, resulting in a decrease in the bonding force between the asphalt and the aggregate, and reducing the overall durability and mechanical properties of the mixture. At the same time, the incorporation of fly ash increases the rigidity of the asphalt system, resulting in a decrease in ductility and flexibility, and a subsequent decrease in needle penetration, which weakens the material's adaptability in low-temperature environments or under dynamic loads. Excessive fly ash may also lead to uneven mixing within the mixture, a loose skeleton structure, and prone to quality problems such as peeling and potholes, seriously affecting the service life and safety performance of the road. When the fly ash content is low, the softening point and flash point of the flame-retardant asphalt are low, and the fly ash cannot build an effective inorganic skeleton structure in the system. Under heating conditions, the asphalt pyrolysis reaction is intensified, and the release of flammable volatiles increases, thereby reducing the overall flame retardant performance.

[0053] Comparing Schemes A1 and A2, with the same fly ash and lignin dosages, increasing the carbon nanotube dosage from 0% (A1) to 2% (A2), the ductility increased from 22.6 cm to 27.0 cm, the softening point rose from 90.4°C to 91.5°C, the flash point increased by 6°C, and the oxygen index significantly increased from 26.20% to 27.90%. The introduction of carbon nanotubes creates a high-thermal conductivity channel structure in the asphalt, facilitating rapid heat conduction and inducing the formation of a char layer, thereby enhancing the material's flame retardancy and thermal stability, validating their synergistic effect with the base flame retardant component.

[0054] Comparing Schemes A2 and A8, with the same carbon nanotube and lignin dosages, but increasing the fly ash dosage from 8% (A2) to 10% (A8), the increased fly ash content raises the flame-retardant asphalt's softening point to 91.8°C and its flash point to 439°C. The oxygen index only slightly increases to 28.10%, but the ductility decreases to 25.0 cm. This comparison reveals that fly ash helps build a porous "skeleton" structure, effectively blocking heat conduction and oxygen diffusion. However, its incremental synergistic effect reaches a saturation point, and excessively high fly ash dosages can reduce the composite's flexibility and ductility, inhibiting ductility gains.

[0055] Comparing Schemes A4 and A5, with the same carbon nanotube and fly ash content, the lignin content was increased from 5% (A4) to 7% (A5). Although A5 has a softening point of 95.5°C, its flash point is 442°C, lower than A4's 452°C. The oxygen index also dropped from 30.20% to 28.70%. This indicates that the lignin content in A4 is more optimal, ensuring the formation of a dense expanded carbon layer without causing a loose carbon layer structure due to overfilling. Scheme A4 performed well in key indicators such as ductility, penetration, flash point, and oxygen index, confirming its status as the most balanced flame-retardant asphalt solution with the most significant synergistic effect.

[0056] Implementation method 2: Although the present invention can determine the ratio range of flame retardant asphalt by the above method, the performance of flame retardant asphalt with different ratios is different. Therefore, the following method can be used to further determine the optimal flame retardant ratio of flame retardant asphalt: 1) Using the central composite design model of Design Expert software, we input the dosage ranges of carbon nanotubes, fly ash, and lignin to generate multiple asphalt flame retardant mix ratios; 2) Using basic asphalt performance indicators and flame retardant performance indicators as evaluation indicators for flame retardant asphalt, the evaluation index values ​​of each ratio scheme were determined through indoor tests. Among them, the basic asphalt performance indicators are ductility, softening point, and needle penetration, and the flame retardant performance indicators are flash point and limiting oxygen index; 3) A comprehensive evaluation method is used to calculate the scores of the basic performance indicators and flame retardant performance indicators of asphalt in each ratio scheme, and the comprehensive score of each ratio scheme is obtained, and then the optimal flame retardant ratio is obtained. Specific embodiment two:

[0058] This example uses the entropy method as a comprehensive evaluation method to evaluate the basic performance indicators and flame retardant performance indicators of asphalt based on the flame retardant asphalt ratio test schemes described in Specific Example 1. The scoring results are shown in Table 3. It should be noted that the ranges for the dosages of the various flame retardant components in this invention apply only to this range; 17 test schemes were generated using Design Expert software. In addition to the entropy method preferred in this example, other evaluation and analysis methods are also applicable to this invention, such as the independent weight coefficient method and the CRITIC method.

[0059] ; Figure 5 A response surface plot is presented using the entropy method to screen the dosage of each flame retardant component. Regenerated carbon nanotubes showed a slight positive correlation with the overall score, demonstrating a moderately positive trend, indicating that within a certain dosage range, they contribute to optimizing the material structure and improving flame retardant efficiency. Lignin exhibited a significant nonlinear effect, with a negative linear coefficient and a positive quadratic coefficient, indicating that its moderate dosage range significantly enhances the overall score (OD value), demonstrating good synergistic flame retardant potential. Regenerated carbon nanotubes and lignin exhibit a positive synergistic effect, which can mitigate the performance fluctuations caused by individual flame retardant components to a certain extent. However, the synergistic incorporation of fly ash and lignin may produce a negative interference effect, hindering the improvement of the overall score (OD value). Response surface analysis revealed that the optimal flame retardant formulation was Scheme A4, with a regenerated carbon nanotube dosage of 1wt%, a fly ash dosage of 10wt%, and a lignin dosage of 5wt%. A more accurate optimal formulation could be obtained by further dividing the dosages of the flame retardant components in the experimental mix (to improve accuracy). However, due to the limited number of experiments, this example is not described in detail.

[0060] The following is an analysis of the differences between the flame retardant asphalt obtained by the flame retardant component of test plan A4 and SBS asphalt in terms of thermogravimetric analysis, total heat release, mass change, smoke release characteristics, dynamic shear rheological properties, and rutting factor.

[0061] The flame retardant asphalt and SBS asphalt of the above-mentioned ratio scheme A4 were tested using a thermogravimetric analyzer. Figure 6 Thermogravimetric analysis results of SBS asphalt are shown below: Figure 7Thermogravimetric analysis results for this flame-retardant asphalt product. In one example, conventional SBS-modified asphalt and composite flame-retardant asphalt samples were subjected to thermal decomposition behavior testing via thermogravimetric analysis to evaluate their thermal stability and pyrolysis characteristics. The test temperature range was from room temperature to 800°C, with the mass change rate and mass derivative changes of the samples recorded during the heating process.

[0062] like Figure 6 As shown, SBS modified asphalt exhibits typical thermogravimetric weight loss characteristics during heating. Its mass change curve indicates that significant pyrolysis reactions begin at approximately 325°C, entering a stage of intense decomposition. Within the temperature range of 325°C to 560°C, its main components rapidly decompose, exhibiting significant mass loss. The pyrolysis rate curve shows that the maximum decomposition rate is reached at approximately 520°C, with the corresponding mass derivative peak approaching -2.0%, reflecting the high reaction intensity during this stage. After reaching 560°C, the mass stabilizes, and the pyrolysis process is essentially complete, with a final residual mass of less than 6%. This indicates that SBS modified asphalt has poor thermal stability in high-temperature environments, a low carbonization residue rate, and limited resistance to thermal decomposition. It is highly susceptible to secondary combustion, making it difficult to meet high-temperature use or fire safety requirements.

[0063] like Figure 7 As shown, in comparison, the flame retardant asphalt provided by the present invention exhibits more excellent thermal stability and flame retardant properties during the entire heating process. Before the temperature reaches 369°C, the sample basically does not suffer from obvious mass loss, the thermogravimetric curve shows a stable trend, and the initial pyrolysis reaction is effectively delayed, indicating that the material has good initial thermal stability. Between 369°C and 640°C, the flame retardant asphalt undergoes a relatively slow multi-stage pyrolysis process, which can be divided into three stages: initial cracking, main pyrolysis and residual stabilization. Among them, the peak pyrolysis rate in the main pyrolysis stage is significantly lower than that of SBS modified asphalt, and does not exceed -1.2%, reflecting that the intensity of the thermal decomposition reaction is weaker and the decomposition process is more gentle and controllable. The final residual mass is about 10%, which is higher than SBS modified asphalt.

[0064] The product of the present invention in the above-mentioned mixing scheme A4 and SBS asphalt were tested using a cone calorimeter. Figure 8 This is a schematic diagram comparing the total heat release of the product of the present invention and SBS asphalt. Figure 9 A schematic diagram comparing mass changes. Figure 10 The figure is a schematic diagram showing the comparison of smoke release. In the cone calorimetry test, the product of the present invention and SBS asphalt showed significant differences in heat release performance.

[0065] Figure 8 In (a), the peak heat release rate of SBS asphalt reaches 892.39kW / m², which is much higher than the 653.6kW / m² of the product of the present invention, indicating that the former releases heat more violently in a fire and the fire develops more rapidly. Figure 8 In (b), the total heat release of SBS asphalt is as high as 217.267MJ / m², while the total heat release of the product of the present invention is 188.972MJ / m², which is reduced by about 13%, effectively suppressing heat release and thus slowing down the development of fire.

[0066] like Figure 9 As shown, during the pyrolysis process, the mass loss rate per unit time of the flame retardant asphalt sample of the present invention and the SBS modified asphalt sample showed a significant difference. The mass change trends of the two materials are consistent with their corresponding heat release rate curves, that is, the mass loss process is accompanied by the release of heat energy, and the two are obviously coupled. After the SBS modified asphalt sample is heated to a certain temperature, the mass loss rate rises rapidly, and the peak rate reaches 0.227g / s, which is much higher than the peak value of 0.064g / s of the product of the present invention. The maximum rate difference between the two is 0.163g / s. This difference quantitatively reflects the difference in pyrolysis intensity between the two materials, indicating that the SBS sample undergoes a more intense and rapid thermal decomposition reaction during the combustion process. The pyrolysis start time of the flame retardant asphalt of the present invention is delayed by 103 seconds compared with the SBS modified asphalt, which significantly improves its stable hysteresis characteristics under high temperature environment.

[0067] like Figure 10 In terms of smoke release characteristics, the product of the present invention also demonstrated superior smoke suppression capabilities. The total smoke volume produced by SBS asphalt during combustion was 50.86 m², significantly higher than the 40.83 m² produced by the product of the present invention, and the smoke release rate increased rapidly in the initial stage. In contrast, the smoke release rate of flame-retardant asphalt was lower overall, with a delayed start time of 150 seconds and a significantly reduced total amount of smoke released.

[0068] The product of the present invention in the above-mentioned mixing scheme A4 and SBS asphalt were tested using a dynamic shear rheometer. Figure 11 This is a schematic diagram comparing the DSR complex shear modulus and phase angle of SBS asphalt and the product of the present invention. In the dynamic shear rheology (DSR) test, the complex shear modulus of the flame retardant asphalt provided by the present invention and the existing SBS modified asphalt under different temperature conditions were compared and analyzed. , phase angle (δ) and rutting factor The law of change with temperature. Figure 11 As shown in Figure 2, with the increase of temperature, the complex modulus of the two asphalts All showed a downward trend, but the flame retardant asphalt was The value is significantly higher than that of SBS modified asphalt, indicating that it has better deformation resistance. Correspondingly, the phase angle of flame retardant asphalt is slightly higher than that of SBS asphalt, indicating that its elasticity is slightly lower, but it still maintains good viscoelastic properties overall.

[0069] The product of the present invention in the above-mentioned mix ratio A4 and SBS asphalt were tested using a rutting tester. Figure 12 The rutting factor comparison diagram of SBS asphalt and flame retardant asphalt is further analyzed by rutting factor. The high temperature rutting resistance of the two asphalts was evaluated. The results showed that the rutting factor of the flame retardant asphalt was always higher than that of the SBS modified asphalt in the range of 50-90°C, and it reached the failure standard. The temperature of the flame retardant asphalt is 94 ° C, while that of the SBS modified asphalt is 90 ° C, which is an increase of about 4 ° C. This shows that the flame retardant asphalt of the present invention has better structural stability and anti-rutting ability under high temperature conditions, and is suitable for tunnel pavements with high use requirements in high temperature environments.

[0070] In order to further evaluate the combustion behavior of the flame retardant asphalt of the present invention, this example also provides a schematic diagram of the combustion comparison of SBS asphalt and the product of the present invention after the cone calorimetry test. Figure 13 Cone calorimetry tests were conducted on the modified asphalt and the conventional SBS modified asphalt, and the residues after combustion were compared and analyzed. Figure 13 As shown, the traditional SBS modified asphalt (left Figure 13 (a)) is heated and carbonized violently during combustion, with a large area of ​​black charred area on the surface and very little residue, indicating that a large amount of thermal decomposition reaction occurs during combustion and a large amount of combustible gas is released, posing a serious fire hazard. In contrast, the flame retardant asphalt provided by the present invention (right) Figure 13 After combustion, (b) forms a foamy carbon layer with a complete structure, full volume, and uniform color. The residue is significantly greater than that of SBS-modified asphalt. This carbon layer has a well-expanded structure and a dense surface of micropores, effectively isolating the further transfer of heat and oxygen, thereby inhibiting the continued combustion of the substrate and providing significant flame retardant protection.

[0071] This invention provides a novel flame-retardant asphalt material suitable for tunnel environments. By incorporating a composite flame-retardant system consisting of carbon nanotubes, fly ash, and lignin, this material significantly enhances its flame retardancy while also balancing environmental friendliness, cost-effectiveness, and engineering adaptability. The carbon nanotubes impart excellent carbonization and thermal insulation capabilities, the fly ash provides an inorganic mineral barrier, and the lignin, a natural biomass with excellent carbonization and synergistic flame-retardant properties, creates a stable, multi-scale flame-retardant structure.

[0072] This material exhibits low smoke density, a long ignition delay, and a low heat release rate during combustion, effectively slowing the spread of fire and suppressing the release of toxic smoke, significantly enhancing fire safety in tunnel pavements. Furthermore, this flame retardant is widely available, renewable, and non-toxic, meeting the requirements of green and low-carbon development. Its simple preparation process makes it suitable for widespread application.

[0073] Implementation method three: The present invention also provides a method for preparing flame-retardant asphalt for tunnel pavement, which is an indoor test preparation method and comprises the following steps: S1: heating the matrix asphalt to a fluid state; S2: preheating carbon nanotubes, fly ash and lignin; S3: Weighing the matrix pitch fluid and the carbon nanotubes, adding the carbon nanotubes to the matrix pitch fluid, heating and stirring, and obtaining a preliminary mixture; S4: Using a high-speed shearing machine to perform high-speed shearing on the preliminary mixture; S5: Weigh fly ash and lignin, add fly ash and lignin to the sheared preliminary mixture in multiple batches, and heat and stir to obtain a secondary mixture; S6: Using a high-speed shearing machine to perform high-speed shearing on the secondary mixture; S7: Use a high-speed shearing machine to perform low-speed shearing on the secondary mixture to remove bubbles and obtain flame-retardant asphalt.

[0074] The main differences between this embodiment and the prior art lie in the order in which the carbon nanotubes, fly ash, and lignin are added. The heating, preheating, and insulation steps are all determined based on the melting characteristics of the base asphalt. The stirring and shearing rates and durations are also determined based on the flow characteristics of the base asphalt fluid itself. Using this preparation method, the flame-retardant asphalt of the present invention can be produced in a laboratory.

[0075] Carbon nanotubes must be added first and treated with high-speed shear at low viscosity to achieve full dispersion and uniform distribution within the matrix asphalt, building a stable carbonized skeleton structure. Fly ash and lignin are added later due to their high dispersibility and thermal stability, and do not significantly affect the initial rheological properties of the system. Specific embodiment three: This embodiment provides a method for preparing flame-retardant asphalt for tunnel pavement, comprising the following steps: S1: Place the matrix asphalt in an oven and heat it for 2 hours until the matrix asphalt is in a fluid state.

[0077] The heating temperature is 165°C.

[0078] S2: Place carbon nanotubes, fly ash and lignin in an oven and preheat for 20 minutes.

[0079] Carbon nanotubes are derived from the pyrolysis residue of waste plastics, fly ash is an inorganic mineral filler produced as an industrial byproduct, and lignin is a byproduct of the papermaking industry. The preheating temperature is 165°C.

[0080] It should be noted that other types of carbon nanotubes, fly ash and lignin are also applicable to the present invention.

[0081] S3: Weigh the matrix asphalt fluid and carbon nanotubes, first add the carbon nanotubes into the matrix asphalt fluid and stir for 5 minutes at a stirring speed of 200 r / min, and heat the container during the stirring process to obtain a preliminary mixture.

[0082] The stirring instrument is a small high-speed disperser, and the container is heated at 165°C.

[0083] It should be noted that the preparation container of the laboratory flame retardant asphalt in this embodiment is 500 ml, and the power of the small high-speed disperser is 500W.

[0084] S4: The preliminary mixture (the matrix asphalt fluid with carbon nanotubes added) is sheared using a small high-speed shearing machine with a shearing time of 45 minutes and a shearing speed of 3500 r / min. The matrix asphalt fluid is kept warm during shearing.

[0085] It should be noted that the power of the laboratory small high-speed shearing machine in the present invention is 1000W, and an electric heating constant temperature jacket is used to keep the asphalt warm at a temperature of 165°C.

[0086] S5: Weigh fly ash and lignin, add fly ash and lignin to the sheared preliminary mixture in multiple times and stir for 10 minutes at a stirring speed of 200 r / min. Heat the container during the stirring process to obtain a secondary mixture.

[0087] The stirring instrument is a small disperser, and the container is heated at 165°C.

[0088] S6: Use a small high-speed shearing machine to shear the secondary mixture (the matrix asphalt fluid with three flame retardants added) with a shearing time of 30 minutes and a shearing speed of 3500r / min. The asphalt is kept warm during shearing.

[0089] It should be noted that the power of the laboratory small high-speed shearing machine in the present invention is 1000W, and an electric heating constant temperature jacket is used to keep the asphalt warm at a temperature of 165°C.

[0090] S7: Adjust the shear rate of the high-speed shear instrument to 500 r / min and the shear time to 5 min. Use the high-speed shear instrument to perform low-speed shearing on the secondary mixture to remove bubbles and obtain flame-retardant asphalt.

[0091] Small high-speed dispersers and small high-speed shears are commonly used in laboratories and correspond to indoor preparation methods. They differ from industrial preparation methods in speed and duration. However, the specific dispersion and shearing parameters are determined based on the characteristics of the base asphalt.

[0092] Implementation method four:

[0093] The present invention also provides another method for preparing flame-retardant asphalt for tunnel pavement, which is an industrial preparation method, comprising the following steps: T1: heating the matrix asphalt to a fluid state; T2: preheating carbon nanotubes, fly ash and lignin; T3: Weighing the matrix asphalt fluid and carbon nanotubes, adding the carbon nanotubes to the matrix asphalt fluid, and dispersing them in a high-speed disperser to obtain a preliminary mixture; T4: Using a large high-speed shearing machine to perform high-speed shearing on the preliminary mixture; T5: Weigh fly ash and lignin, add fly ash and lignin to the preliminary mixture, and disperse them in a high-speed disperser to obtain a secondary mixture; T6: Use a large high-speed shearing machine to perform high-speed shearing on the secondary mixture at a constant temperature; T7: Use a large high-speed shearing machine to shear the secondary mixture at a low speed to remove bubbles and obtain flame-retardant asphalt.

[0094] This embodiment differs primarily from existing techniques in the order and order of addition of carbon nanotubes, fly ash, and lignin. The heating, preheating, and insulation steps are determined based on the melting characteristics of the base asphalt. The stirring and shearing rates and durations are also determined based on the flow characteristics of the base asphalt fluid. This preparation method allows for rapid industrial production of the flame-retardant asphalt of this invention.

[0095] Currently, most research on flame-retardant asphalt, both domestically and internationally, is concentrated in the laboratory stage, using small batches, high-shear mixers, and intermittent heating equipment for batching and shearing. This leads to problems such as uneven component dispersion, unstable thermal control, limited production capacity, and inability to replicate on a large scale, making it difficult to directly translate into engineering applications. However, based on the needs of engineering application scenarios, the present invention has designed a preparation process specifically adapted to industrial conditions, and clearly defines key process parameters, equipment requirements, and operating sequences to ensure that the material maintains stable performance and consistent quality under large-scale production. This embodiment can give the present invention significant engineering application value and promotion prospects. Specific embodiment four:

[0097] This embodiment also provides an industrial preparation method for flame-retardant asphalt for tunnel pavement, comprising the following steps: T1: Place the matrix asphalt in an oven and heat it for 2 hours until the asphalt is in a fluid state.

[0098] The heating temperature is 165°C.

[0099] T2: Place carbon nanotubes, fly ash and lignin in an oven and preheat for 20 minutes.

[0100] Carbon nanotubes are derived from the pyrolysis residue of waste plastics, fly ash is an inorganic mineral filler produced as an industrial byproduct, and lignin is a byproduct of the papermaking industry. The preheating temperature is 165°C.

[0101] T3: Weigh 20 kg of matrix asphalt fluid and pour it into the preparation container; weigh 0.24 kg of carbon nanotubes, add the carbon nanotubes to the matrix asphalt fluid, and disperse them using a large high-speed disperser. The dispersion time is 10 minutes and the disperser speed is 200 r / min. The container is kept warm during the dispersion process; obtain a preliminary mixture.

[0102] The container is made of stainless steel, 30 cm in diameter and 50 cm in height. During industrial production, the volume of asphalt must be greater than two-thirds of the container's volume and less than four-fifths of the container's capacity. An electric thermostat is used to insulate the asphalt. The holding temperature is 165°C.

[0103] It should be noted that the power of the large-scale high-speed disperser used in the industrial preparation of flame-retardant asphalt is 5kW, and large-scale high-speed dispersers of other powers are also applicable to the present invention.

[0104] T4: Use a large high-speed shearing machine to shear the preliminary mixture. The shearing time is 30 minutes and the shearing speed is 2000r / min. The asphalt is kept warm at the same time; the insulation temperature is 165℃.

[0105] In a specific embodiment, an electric heating thermostat is used to keep the asphalt warm.

[0106] It should be noted that the power of the large-scale high-speed shearing instrument used in the industrial preparation of flame-retardant asphalt is 25kW, and large-scale high-speed shearing instruments of other powers are also applicable to the present invention.

[0107] T5: Weigh 2.38 kg of fly ash and 1.19 kg of lignin, add the fly ash and lignin to the sheared preliminary mixture, and disperse using a high-speed disperser for 10 minutes at a stirring speed of 200 r / min. Keep the container warm during stirring to obtain a secondary mixture.

[0108] The asphalt is kept warm using an electric thermostat at a temperature of 165°C.

[0109] It should be noted that the power of the large-scale high-speed disperser used in the industrial preparation of flame-retardant asphalt is 5kW, and large-scale high-speed dispersers of other powers are also applicable to the present invention.

[0110] T6: Use a large high-speed shearing machine to shear the secondary mixture (liquid asphalt with three flame retardant components added). The shearing time is 30 minutes and the shearing speed is 1500r / min. The asphalt is kept warm during shearing.

[0111] The asphalt is kept warm using an electric thermostat at a temperature of 165°C.

[0112] It should be noted that the power of the large-scale high-speed shearing instrument used in the industrial preparation of flame-retardant asphalt is 25kW, and large-scale high-speed shearing instruments of other powers are also applicable to the present invention.

[0113] T7: Adjust the speed of the large high-speed shearing machine to 200r / min and the shearing time to 5min to remove bubbles in the asphalt and obtain flame-retardant asphalt.

[0114] Large-scale high-speed dispersers and large-scale high-speed shears are commonly used in laboratories and correspond to industrial preparation methods. There is no specific distinction between "large" and "small."

[0115] Specifically, the present invention constructs a complete industrial process flow, from heating the base asphalt, preheating the flame retardant component, step-by-step addition of the base asphalt and flame retardant component, high-power shear dispersion, thermal insulation control throughout the entire process, to final degassing and finalizing. The carbon nanotubes are added using a combined high-speed dispersion and high-shear treatment strategy, enabling rapid depolymerization of the nanocomponents under high-temperature, high-shear conditions, forming a preliminary dispersed network. Subsequently, fly ash and lignin are added in stages and sheared continuously, in conjunction with industrial-grade high-power equipment (e.g., a 5kW disperser and a 25kW shearer) to ensure the coordinated distribution of the three components within the asphalt. Furthermore, to meet the requirements of industrial production continuity and batch stability, a medium- and low-speed shear degassing process is specifically incorporated to address issues such as air bubble inclusion and uneven stratification, common in large-scale production.

[0116] In terms of temperature control, this example sets the processing temperature of the asphalt and its components at a uniform 165°C. An electric thermostat jacket and a stainless steel interlayer container are used for temperature control, ensuring stable processing conditions at each stage and adapting to the temperature control requirements of different production scales. This process fully considers the fluidity, safety, and component reaction behavior of the asphalt material, ensuring good scalability and process compatibility.

[0117] This invention not only solves the safety shortcomings of traditional asphalt in high-risk tunnel environments, but also provides a new technical path for the high-value utilization of renewable resources and the development of functional road materials. It has important engineering promotion significance and industrial transformation prospects.

[0118] The above embodiments are only used to illustrate the technical solutions of the present invention and do not constitute a limitation on the scope of protection of the present invention. For those skilled in the art, various equivalent substitutions, modifications, improvements or changes made based on the contents disclosed in the present invention without departing from the technical essence and spirit of the present invention should be deemed to fall within the scope of protection claimed by the present invention.

Claims

1. A flame-retardant asphalt for tunnel pavement, the main material of which is base asphalt, calculated by mass percentage, characterized by: The invention also comprises 0wt% to 2wt% of carbon nanotubes, 8wt% to 10wt% of fly ash and 5wt% to 7wt% of lignin.

2. The flame-retardant asphalt for tunnel pavement according to claim 1, characterized in that: The following method is used to determine the optimal flame retardant ratio of flame retardant asphalt: 1) Using the central composite design model, input the dosage ranges of carbon nanotubes, fly ash, and lignin to generate multiple flame retardant asphalt mix ratios; 2) Using basic asphalt performance indicators and flame retardant performance indicators as evaluation indicators for flame retardant asphalt, the evaluation index values ​​of each ratio scheme were determined through indoor tests. Among them, the basic asphalt performance indicators are ductility, softening point, and needle penetration, and the flame retardant performance indicators are flash point and limiting oxygen index; 3) A comprehensive evaluation method is used to calculate the scores of the basic performance indicators and flame retardant performance indicators of asphalt, obtain the comprehensive score of each ratio scheme, and then determine the optimal flame retardant ratio.

3. The flame-retardant asphalt for tunnel pavement according to claim 2, characterized in that: The comprehensive evaluation method adopts the entropy method.

4. The flame-retardant asphalt for tunnel pavement according to claim 3, characterized in that: The optimal flame retardant ratio of the flame retardant asphalt is: 1 wt% carbon nanotubes, 10 wt% fly ash, and 5 wt% lignin.

5. The method for preparing flame-retardant asphalt for tunnel pavement according to any one of claims 1 to 4 is an indoor test preparation method, characterized in that: The following steps are involved: S1: heating the matrix asphalt to a fluid state; S2: preheating carbon nanotubes, fly ash and lignin; S3: Weighing the matrix pitch fluid and the carbon nanotubes, adding the carbon nanotubes to the matrix pitch fluid, heating and stirring, and obtaining a preliminary mixture; S4: subjecting the preliminary mixture to high-speed shearing at a heat preservation condition; S5: Weigh fly ash and lignin, add the fly ash and lignin to the sheared preliminary mixture, and heat and stir to obtain a secondary mixture; S6: subjecting the secondary mixture to high-speed shearing at a heat preservation condition; S7: The secondary mixture is subjected to low-speed shearing to remove air bubbles therein to obtain flame-retardant asphalt.

6. The method for preparing flame-retardant asphalt for tunnel pavement according to claim 5, characterized in that: The heating temperature, preheating temperature and holding temperature are 155℃-175℃.

7. The method for preparing flame-retardant asphalt for tunnel pavement according to claim 5, characterized in that: In S3, the stirring speed is 150-250 r / min and the stirring time is 5-15 min; In S5, the stirring speed is 150-250 r / min and the stirring time is 5-15 min; In the S4, the shearing speed is 3000-4000 r / min, and the shearing time is 30-50 min; In the S6, the shearing speed is 3000-4000 r / min, and the shearing time is 20-40 min; In the step S7, the shearing speed is 300-700 r / min, and the shearing time is 5-10 min.

8. The method for preparing flame-retardant asphalt for tunnel pavement according to any one of claims 1 to 4 is an industrial preparation method, characterized in that: The following steps are involved: T1: heating the matrix asphalt to a fluid state; T2: preheating carbon nanotubes, fly ash and lignin; T3: Weighing the matrix asphalt fluid and carbon nanotubes, adding the carbon nanotubes to the matrix asphalt fluid, and performing heat preservation and dispersion to obtain a preliminary mixture; T4: subjecting the preliminary mixture to high-speed shearing at a constant temperature; T5: Weigh fly ash and lignin, add fly ash and lignin to the preliminary mixture, and disperse them under heat preservation to obtain a secondary mixture; T6: subjecting the secondary mixture to high-speed shearing at a constant temperature; T7: The secondary mixture is sheared at a low speed to remove bubbles and obtain flame retardant asphalt.

9. The method for preparing flame-retardant asphalt for tunnel pavement according to claim 8, characterized in that: The heating temperature, preheating temperature and holding temperature are 155℃-175℃.

10. The method for preparing flame-retardant asphalt for tunnel pavement according to claim 8, characterized in that: In the T3, the dispersion speed is 150-250 r / min and the dispersion time is 5-15 min; In the T5, the dispersion speed is 150-250 r / min and the dispersion time is 5-15 min; In the T4, the shear speed is 1500-2500 r / min, and the shear time is 20-40 min; In the T6, the shear speed is 1000-2000 r / min, and the shear time is 20-40 min; In the T7, the shearing speed is 100-300 r / min, and the shearing time is 5-15 min.

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