Research on flame retardant and smoke suppression technology of asphalt and its mechanism

As tunnel traffic infrastructure rapidly expands, the construction of asphalt pavements within tunnels and fire protection measures have become particularly critical. Asphalt, as a commonly used paving material, exhibits combustion characteristics and smoke suppression capabilities that are of paramount importance. This paper first examines the combustion of asphalt, outlining its fire retardant mechanisms. Subsequently, it categorizes and discusses asphalt fire retardant technologies, Explaining the various types of flame retardants and how each works. It further examines how incorporating flame-retardant components and using high-void pavement structures improve fire resistance. The study then addresses research methodologies for evaluating flame retardant performance, listing several testing methods and comparing their ability to assess relevant indicators of asphalt’s fire retardant efficacy. It highlights the limitations of relying on a single indicator in current research and the discrepancies that arise in practical engineering applications. The paper also summarizes the conditions for the currently popular nano-modified asphalts, outlining the parameters for some prevalent nano-modified asphalt formulations. This study highlights the dual role of nanomaterials in enhancing asphalt flame retardancy via char formation and catalytic oxidation, and in concurrently curbing VOC emissions. This work paves the way for developing novel, green, and high-performance asphalt materials. Finally, it concludes by summarizing the research trends and potential future directions for flame retardant asphalt, identifying areas that warrant further investigation.

1 Introduction

With the advancement of road construction, the development of expressways and tunnels in remote mountainous areas has gradually become commonplace (MOT, 2023). In these projects, asphalt materials have increasingly become one of the primary choices due to their superior performance (Tao et al., 2011). However, their potential hazards in tunnel fires also need to be vigilant. Firstly, The burning of asphalt pavement releases significant toxic, carcinogenic black smoke, seriously polluting the environment (Beard, 2016; Tanaka et al., 2016; Yang et al., 2021a). Secondly, as a combustible material, asphalt can act as an accelerant during a fire, leading to a rapid increase in temperature (Moretti et al., 2016; Qiu et al., 2019; Zhao et al., 2015). The enclosed tunnel environment, characterized by poor ventilation, limited heat dissipation, and low oxygen availability, promotes hypoxic combustion. This process releases substantial toxic smoke, severely threatening personnel safety (Tomar and Khurana, 2019; Wan et al., 2023).

Furthermore, it is noteworthy that asphalt materials release significant amounts of volatile organic compounds (VOCs) during high-temperature construction or combustion in fires. These substances not only exacerbate air pollution but also pose potential health risks to construction workers and nearby residents. In recent years, research on the emission characteristics, environmental impacts, and health risks of asphalt-derived VOCs has gained increasing attention. For instance, a recent review systematically summarizes the emission profiles, chemical composition, and potential health hazards of asphalt VOCs, emphasizing the importance of integrating VOC control into flame retardancy research (Li et al., 2024). Therefore, the future development of flame-retardant asphalt technologies should not only focus on suppressing combustion and reducing smoke emissions but also prioritize the control of VOC emissions throughout their entire life cycle, thereby achieving the dual objectives of environmental friendliness and health safety.

For instance, studies have shown that the addition of composite flame retardants can increase the limiting oxygen index (LOI) of asphalt from 19.5% for base asphalt to 25.9%, while smoke suppression efficiency can reach 10.67%, significantly enhancing flame retardancy and smoke suppression. These quantitative metrics underscore the potential of flame-retardant technologies in tunnel asphalt pavements.

Extensive research has been conducted on improving the fire safety of asphalt pavements in tunnels, particularly concerning flame retardant and smoke suppressant materials. Key findings include the identification of two primary flame retardant mechanisms: gas-phase and condensed-phase inhibition, along with appropriate evaluation methods (Tan et al., 2022; Xia et al., 2021c; Xu et al., 2013). Currently, the flame retardant performance of asphalt pavements is commonly enhanced through structural optimization and the careful selection of flame retardant materials. Of the various methods employed, the incorporation of flame retardant and smoke suppressant materials is the predominant approach for improving the flame retardant performance of asphalt pavements. Thus, the quality of the selected flame retardants is directly determinative of the asphalt’s performance. Since the 20th century, interest in various flame retardants for use in road asphalt has grown significantly. While extensive research has been conducted on flame retardant asphalt, a significant knowledge gap persists regarding the combustion performance of both neat and composite asphalt. Additionally, the environmental problems caused by traditional flame retardants also need to be addressed urgently (Li et al., 2023; Xia et al., 2020). Furthermore, scholars have classified and discussed asphalt flame retardant technologies and established flame retardancy evaluation methods such as the oxygen index method, vertical (horizontal) burning tests, and cone calorimetry (Chun et al., 2012; Qun and Wangrui, 2008; Xian-Tao et al., 2013). However, current research still has certain limitations and lacks comprehensive and systematic summaries (Chun et al., 2012; Qun and Wangrui, 2008; Xian-Tao et al., 2013).

In the economic investigation of nano-modified flame retardant asphalt, it is crucial to ensure the uniform dispersion of nanomaterials, with temperature, duration, and shear rate being the key conditions to achieve this goal, as well as ensuring material utilization. Currently, there is a lack of systematic summarization of the conditions required for the use of nanomaterials in flame retardancy, which may lead to material and economic waste. Evaluating the uniform dispersion of nanomaterials in asphalt is also a challenge, and there are no relevant standards for effectively evaluating the uniform dispersion of nanomaterials. Researchers typically use relatively simple methods for evaluation, such as observing the uniform dispersion of nanoparticles using Scanning Electron Microscopy (SEM). To address these issues, more experimental research is needed, followed by the establishment of corresponding standards and evaluation systems. This includes gaining an in-depth understanding of the diffusion behavior of nanomaterials under different temperature, duration, and shear rate conditions, as well as developing more precise and reliable evaluation methods.

Despite considerable progress in flame-retardant asphalt research, several critical research gaps persist, limiting its further development, which constitutes the core issue this paper aims to address. Firstly, existing studies often focus on evaluating the effectiveness of specific flame retardants, lacking a systematic and mechanistic unified understanding of the combustion behavior of both base and composite-modified asphalt within flame-retardant systems. Secondly, while pursuing high flame retardancy, the coordinated control of gaseous toxic by-products such as Volatile Organic Compounds (VOCs) has not received sufficient attention, raising concerns about environmental friendliness. Furthermore, emerging nanomaterial-based flame retardancy faces application bottlenecks including poor dispersion in asphalt, undefined process conditions, and a lack of evaluation standards.

This study aims to transcend the limitations of existing research by bridging these gaps through providing a holistic perspective integrating “mechanism-technology-evaluation-environmental impact.” This paper not only systematically reviews the asphalt combustion process and flame-retardant mechanisms, critiques the advantages and disadvantages of various flame-retardant technologies, and details multi-dimensional performance evaluation methods; more importantly, we explicitly position VOC emission control as a co-equal core dimension alongside flame retardancy and smoke suppression, emphasizing the concept of full life-cycle environmental safety. Concurrently, this paper delves into the synergistic potential of nanomaterials in achieving multiple objectives and identifies future directions for optimizing their dispersion processes. Ultimately, this review provides a clear theoretical framework and development pathway for creating the next-generation of high-performance, green, and safe flame-retardant asphalt materials for tunnels. Figure 1 provides a clear framework for the writing of this paper.

Figure 1

Figure 1. A framework for the writing of this paper.

2 Asphalt combustion process and flame retardant mechanism

2.1 Asphalt combustion process

Asphalt consists of very complex polymers. When exposed to sufficient heat, these polymers establish a combustion cycle (Figure 2) (Kiliaris and Papaspyrides, 2010). The combustion of asphalt is characterized by evaporation burning among liquid combustions; when heated to 300 °C, asphalt ignites (Puente et al., 2016). When the heat released reaches a critical point, it initiates further decomposition reactions in the solid phase. These reactions generate more combustible materials, perpetuating the combustion cycle (Kiliaris and Papaspyrides, 2010; Laoutid et al., 2009; Puente et al., 2016).

Figure 2

Figure 2. Combustion cycle of polymers.

Research has found (Xu and Huang, 2010; Xu and Huang, 2012) that under the condition of a heating rate of 5 °C/min, the combustion of asphalt binder occurs in three successive stages, each characterized by increasingly intense reactions and distinct primary gaseous products (Figure 3).

Figure 3

Figure 3. Rational reaction pathways for the combustion process of asphalt binder.

Different stages can be identified in the combustion process of asphalt (Kai et al., 2020). Based on temperature changes, it can be categorized into an oxygenated pyrolysis stage and a heavy component combustion stage. Moreover, under fire scales ranging from 10 to 50 MW, when the pavement reaches the ignition point of asphalt, the heating rate of the pavement is approximately 5 °C–20 °C/min.

Additionally, based on the differences in combustion products, the asphalt combustion process can also be divided into two stages (Ke et al., 2012; Ke et al., 2009). The first stage involves the combustion of active volatile components, where the concentrations of gases such as CO, NO, and NO₂ rapidly increase and reach their peak. The second stage includes the combustion of secondary volatile components combined with residual char, releasing NO₂ and SO₂. Furthermore, the combustion of asphalt releases substantial amounts of Volatile Organic Compounds, which not only pose threats to human health but also contribute to environmental pollution. Research (Shi et al., 2017) also indicates that compared to other components, the combustion of asphaltene is simpler, involving only the combustion of the carbonized layer.

Table 1 presents the different stages of asphalt combustion as defined by various researchers based on thermogravimetric analysis and combustion product analysis.

Table 1

Table 1. Based on the different stages of thermogravimetric analysis and combustion products.

Additionally, asphalt combustion proceeds through five sequential processes: heating, melting, pyrolysis, ignition, combustion, and expansion. Interrupting any of these processes, as Figure 4 demonstrates, can achieve flame retardancy.

Figure 4

Figure 4. Asphalt combustion process.

2.2 Mechanism of asphalt flame retardation

Currently, it is known that flame retardation can be achieved by limiting a certain stage of asphalt combustion (He et al., 2020), preferably before the actual ignition occurs (Wu et al., 2006). The flame retardant mechanisms of polymeric materials are mainly divided into two categories: gas-phase flame retardant mechanisms and condensed-phase flame retardant mechanisms (Araby et al., 2021; Chen W. et al., 2022). Additionally, asphalt pavements can absorb heat through their own structure, thereby achieving flame retardation. The flame region is typically referred to as the gas phase (Figure 5). In the gas phase, oxygen reacts chemically with volatile pyrolysis products in a high-temperature environment. Therefore, reducing the quantity of reactants and lowering the temperature of the surrounding environment are the most effective methods for inhibiting gas-phase reactions. On the other hand, the region outside the flame zone is commonly known as the condensed phase. Controlling thermally induced degradation in condensed-phase substrates effectively mitigates flammability through suppression of volatilization precursors, thereby diminishing combustion propagation efficiency in organic polymers.

Figure 5

Figure 5. (Araby et al., 2021; Chen W. et al., 2022). (a) Schematic diagram of the entire polymer combustion process. (b) Gas-phase and condensed-phase flame retardant mechanisms.

2.2.1 Gas-phase flame retardant mechanisms

In gas-phase flame retardant mechanisms, flame retardants typically achieve their effect by interrupting the combustion reaction chain and eliminating active free radicals generated during the combustion process. Therefore, the selection and use of flame retardants need to consider their stability and reactivity towards free radicals, which will directly affect the flame retardant effect.

1. Rapid Free Radical Scavenging

Rapid free radical scavenging is a common gas-phase flame retardant mechanism, primarily achieved by introducing flame retardants with high reactivity to scavenge free radicals during combustion. Free radicals are crucial for maintaining the combustion process. By releasing low-energy free radicals that interact with and deactivate high-energy free radicals, flame retardants disrupt the chain reactions vital for combustion. This ultimately reduces the flame spread and provides effective flame retardation (Shen et al., 2022). Under high temperatures, bromine- or chlorine-based flame retardants release halogen free radicals. The released halogen free radicals interact with the reactive free radicals present in the flame, hindering the combustion process and slowing the flame’s spread (Suzhou et al., 1999).

2. Non-radical scavenging

The primary mechanisms of non-radical scavenging include the dilution of inert gases and reducing temperatures both in the flame and on the material’s surface. During combustion, the release of non-combustible gases creates a dilution effect. This effect reduces the flame’s oxygen supply and/or fuel concentration below the flammable limit, leading to a cooling of the gas phase (Zhao et al., 2023). In this mechanism, flame retardants decompose under high temperature conditions to produce non-toxic gases (usually water vapor and carbon dioxide), which can dilute the combustible gases in the combustion zone, reduce the oxygen concentration, and thereby inhibit the spread of the flame. Non-radical scavenging mainly works by absorbing the thermal energy during combustion or reducing the oxygen concentration to prevent the spread of the flame. Some flame retardants with high oxygen indices, such as aluminum hydroxide and calcium hydroxide, can release oxygen and absorb a large amount of heat under high temperature conditions, effectively preventing the spread of the flame (Wang et al., 2023).

2.2.2 Condensed phase flame retardant mechanism

1. Protective Barrier Flame Retardation

This flame retardation method primarily involves the formation of a protective solid barrier through the reaction of flame retardants with the decomposition products of polymers under high temperature conditions. This barrier, typically in the form of a foam or coating, blocks heat and oxygen, delaying or preventing the spread of flames. The barrier may consist of unreacted polymers, carbon, or inorganic salts. Its primary mechanism is physical barrier, mainly used to prevent further contact of heat and oxygen with the material, thereby interrupting the combustion chain reaction. Tan et al. (2020) researched the combined flame retardant effects of kaolin nanotubes (HNTs) and conventional flame retardants (CFR). Their study demonstrated that the combination of these materials forms a dense and thick protective layer at the combustion interface of the modified asphalt, effectively impeding the transfer of heat and flammable gases.

2. Carbonization Flame Retardation

In this flame retardation mechanism, the flame retardant facilitates the conversion of the material into a graphite-like char, providing a barrier against fire. This rigid carbonized layer prevents the further oxidation of internal combustion products. During this process, the flame retardant, to some extent, alters the chemical reaction pathways of the polymer during pyrolysis and combustion, promoting the formation of a stable, continuous, graphite-structured carbonized layer. This carbonized layer or glassy substance acts as an effective barrier between the condensed phase and the gas phase, slowing down the transfer of heat, oxygen, and flammable volatile products within the polymer, thereby effectively preventing the fire from spreading further (Wang et al., 2020; Zongpu et al., 2023). The dense barrier layer formed by slaked lime during asphalt combustion suppresses the fire’s intensity, lowering CO production and smoke emission (Zhu et al., 2018).

Currently, The primary flame retardation mechanisms typically involve simultaneous action in both the condensed phase and the gas phase, and their combined effects have been proven to have a synergistic effect (Camino and Camino, 2019). For example, Zhao et al. (2023) demonstrated that the improved flame retardancy of styrene-butadiene-styrene (SBS) asphalt results from a dense carbonized layer formed through carbonization catalyzed by transition metal compounds. This char layer safeguards the underlying matrix by impeding heat and oxygen transfer in the condensed phase. Furthermore, magnesium hydroxide (MH) releases water, and sodium alginate (SA) emits non-flammable gases, both of which help cool the gas phase combustion zone. Currently, flame retardant development typically utilizes a multi-component compounding approach.

3 Classification of asphalt flame retardant technologies

To ensure road safety and prevent fire hazards, the engineering field widely employs the following three flame retardant technologies: adding flame retardants, incorporating flame retardant components, and adopting high-void ratio pavement structures. The application of these technologies aims to enhance the flame retardant properties of asphalt materials, delay the spread of fire, and effectively reduce the damage caused by fires to the pavement.

3.1 Adding flame retardants

Currently, the most common method for enhancing fire resistance is to add flame retardants to the asphalt mixture, which can significantly improve its fire resistance. These flame retardants typically slow down the spread of fire and reduce the combustion rate, thereby minimizing the damage to the pavement caused by fires. Typically, flame retardants can be categorized into two major groups based on their chemical composition: organic and inorganic. Beyond conventional organic and inorganic flame retardants, bio-based flame retardants developed from renewable resources have emerged as a current research focus (Ge et al., 2025; Ramadoss et al., 2024). Organic flame retardants primarily encompass halogenated, nitrogen-based, and phosphorus-based compounds, whereas inorganic ones mainly contain metal elements like aluminum, magnesium, and calcium. Additionally, they encompass cutting-edge nanoinorganic flame retardants, mainly represented by montmorillonite and expanded graphite. Bio-based flame retardants include herbal residue-derived biochar from recent research (Felix Sahayaraj et al., 2024; Felix Sahayaraj et al., 2022; Iyyadurai et al., 2023). Table 2 provides some representative products along with their respective advantages and disadvantages.

Table 2

Table 2. Mechanisms, advantages, and disadvantages of different flame retardants.

3.1.1 Development of flame retardants

Concerns over the fire safety of asphalt materials in the 1950s spurred the development of flame-retardant asphalt. This led to the invention of flame-retardant asphalt pavement technology in Europe and the United States in the 1980s (Bourbigot et al., 2012). As shown in Figure 6, halogenated flame retardants were once the most commonly used type, favored for their excellent flame-retardant performance. However, some of these flame retardants, such as brominated flame retardants (Author Anonymous, 2011), have been found to have adverse environmental impacts. Nowadays, composite flame retardants and nano-modification are gradually becoming mainstream trends, reflecting our pursuit of more comprehensive and efficient flame retardants. The use of composite flame retardants is no longer limited to single components but involves the ingenious combination of multiple flame-retardant elements or compounds to achieve more comprehensive and synergistic flame-retardant effects (Rubio et al., 2012). In terms of the proportioning of flame retardants, the correct proportional relationship plays a crucial role.

Figure 6

Figure 6. Progress of research on asphalt flame retardant materials.

The components of composite flame retardants work together synergistically, aiming to achieve mutual flame retardation and reduce fire risks. For example, Li et al. (2014) conducted an orthogonal experiment using magnesium hydroxide, ammonium polyphosphate, and melamine, demonstrating enhanced compatibility and a reduced limiting oxygen index for the asphalt binder with their novel composite flame retardant. Xia et al. (2021b) developed a composite flame retardant for asphalt (CFR) composed of CAHC, APP, CRP, and EV, which decomposes sequentially with increasing temperature, effectively interfering with the multi-stage combustion of asphalt. Due to the effects of dilution, cooling, and insulation, it exhibits a comprehensive synergistic flame retardant effect. Aluminium hydroxide (AHO) and organomontmorillonite (OMMT) composites synergistically enhance the flame retardancy of asphalt by promoting charring during combustion (Yang et al., 2020). The carbonaceous char formed during the process acts as a barrier, preventing both oxygen and thermal energy from accessing the unoxidized asphalt beneath. Additionally, this char layer effectively restrains the escape of combustible volatile compounds (Kiliaris and Papaspyrides, 2010). Table 3 summarizes some commonly used flame retardants and their recommended dosages.

Table 3

Table 3. Some recommended dosage levels of currently popular flame retardants.

3.2 Adding flame-retardant components

Incorporating flame retardants is commonly used to prepare flame-retardant asphalt and enhance pavement fire resistance. However, issues such as the compatibility of flame retardants with asphalt and the blending ratio may cause certain damage to the durability of asphalt materials (Wang et al., 2020). As depicted in Figure 7, the impact of various mineral powders on the limiting oxygen index (LOI) of asphalt varies, with the addition of limestone resulting in the least significant increase.

Figure 7

Figure 7. Limiting oxygen index for different mineral powder additions.

In related research, Shuguang et al. (2006), Xian-Tao et al. (2013) observed that, compared to limestone mineral fillers, alkaline fillers have no impact on the high-temperature performance and water stability of asphalt mixtures. This is because alkaline fillers have better adhesion to asphalt than limestone. Although lignin and basalt fibers are widely used, Miber III mineral fiber is a more effective option for improving the flame-retardant performance of asphalt, as shown in Figure 8, due to its higher limiting oxygen index (Xian-Tao et al., 2013). The overall flame retardancy of asphalt mixtures can be potentially enhanced by replacing conventional fillers and fibers with non-combustible mineral powders and fibers. This represents a new method for achieving the goal of flame retardancy in asphalt pavements.

Figure 8

Figure 8. Results of LOI test for three fibre bitumen mastics.

Ren et al. (2023) reported that the synthesis of micro-nano composite fibers (MCE-PAN) from polyacrylonitrile (PAN), multi-walled carbon nanotubes (MWCNTs), and nano-calcium carbonate (nCC), when incorporated into an asphalt binder, yielded a substantial enhancement in fire retardancy. The study observed a 25.24% increment in the limiting oxygen index (LOI), a 10.67% augmentation in smoke suppression efficiency, and a 105 °C elevation in the decomposition temperature, culminating in a marked reduction in smoke emissions.

3.3 Critical assessment of cost, compatibility, and mechanical properties

Nanomaterials (such as layered montmorillonite, nano-SiO₂, and carbon-based nanomaterials) and composite flame retardants can enhance the flame retardancy and smoke suppression properties of asphalt. Nanomaterials have already been investigated for smoke suppression; Chang reported that in asphalt, Nano-CaCO₃ can reduce smoke emission by 4.8%–9.3%, whereas MSHSs can cut it by 38.2% (Chang et al., 2024). However, their engineering application requires balancing cost, compatibility, and mechanical durability: On one hand, the procurement and preparation costs of nano/composite systems are high, requiring high shear, high temperature, and prolonged mixing. This increases material unit costs and construction energy consumption while accelerating asphalt thermal-oxidative aging. Therefore, a cost-benefit analysis should be conducted covering material procurement, processing energy consumption, full-lifecycle maintenance/replacement, and external benefits from VOC reduction. On the other hand, the interface compatibility between flame retardants and the matrix determines dispersion and long-term stability. Agglomeration or sedimentation must be prevented through surface modification/compatibilizers, with systematic reporting of storage stability, phase separation, and results from AFM/SEM/EDS, XRD, FTIR, and rheological testing. Furthermore, flame retardant addition may induce a “stiffening but embrittlement” effect, impacting viscosity, modulus, low-temperature crack resistance, and fatigue life. Therefore, concurrent evaluations should include DSR, BBR/other low-temperature tests, fatigue (four-point bending/complex shear), ITS, Marshall/rut resistance, and AASHTO T283. To balance flame retardancy and engineering properties, adopt synergistic blending to reduce single-component usage, incorporate surface modification/coupling agents, integrate with polymer modification or toughening systems, and optimize process parameters (temperature-time-shear rate). Advance technology adoption through a phased approach: pilot testing (flame retardancy, TG-FTIR) → mechanical/durability characterization → pilot/field validation → life cycle cost and environmental health assessment.

3.4 Flame retardancy of high-air-content pavement structures

High-air-content asphalt pavements achieve flame retardancy primarily through the following three mechanisms: Firstly, in the event of an accident involving the leakage of flammable liquids, the high-porosity structure can quickly absorb the leaked liquid into the pavement interior, preventing it from burning on the surface; Secondly, due to the low oxygen content within the pavement, the absorbed fuel finds it difficult to ignite; Finally, since most of the liquid fuel is absorbed, it cannot spread extensively on the pavement surface, effectively preventing the spread of flames.

Research (Bonati et al., 2013) compared the heat release rate (HRR) curves of two types of mixtures, D-BC series and O-BC series, and found significant differences between their HRR curves. The data indicates that aggregate gradation is a pivotal determinant of the fire-resistant properties of asphalt mixtures. Xiao-Rui et al. (2010), in the study of integrated structure and function of warm-mix multifunctional tunnel pavement, used burning time as an evaluation index for flame retardancy and found that the material exhibits optimal flame retardancy when the void ratio is 20% (Figure 9). Bonati et al. (2015) highlighted that air void content critically influences the ignition behavior of asphalt mixtures. Increased air voids reduce the ignition time of dense-graded mixtures, making them easier to ignite under lower heat fluxes. Hassn et al. (2016) found that compared to dense-graded asphalt mixtures, mixtures with high air void content exhibit higher steady-state temperatures.

Figure 9

Figure 9. Correlation between void ratio and combustion time.

3.5 Summary

Incorporating specific flame retardants into asphalt is a common method to enhance its fire resistance. Common types of flame retardants include bio-based flame retardants, halogen-based, nitrogen-based, phosphorus-based, metal hydroxides, and nanomaterial flame retardants. Adding flame retardant components is another commonly used flame retardant technology, which improves the fire resistance of asphalt by adding flame retardant fibers and minerals to the asphalt. Different types of flame retardant components can adjust the flame retardant effect of asphalt by changing their types, addition methods, and content. The flame retardant technology of high void ratio pavement structure is a technology that realizes asphalt flame retardancy by designing a porous skeleton structure. This technology boosts the flame retardancy of asphalt pavement through increased porosity and surface area. Combining different asphalt flame retardant technologies synergistically can lead to even greater improvements in fire resistance. Examples of multiple technologies synergistic flame retardation: Xia et al. (2024) and Tan et al. (2022) created a flame-retardant warm mix asphalt (FWA) by combining a porous warm mix additive (WMA) with a composite flame retardant (CFR). WMA’s large specific surface area improved CFR particle adsorption, enhancing flame retardancy. Both CFR and FWA significantly reduced smoke release in the initial combustion phase. The sequential decomposition of CFR’s four components at increasing temperatures synergistically prevents asphalt pyrolysis in both the condensed and gas phases.

4 Flame retardant performance research methods

Currently, the main techniques for accurately evaluating the flame retardant properties of asphalt materials fall under two categories: equipment-based and analytical methods. It is crucial to employ appropriate testing methods and instruments. Different testing methods target different performance parameters, such as burning rate, heat release rate, and smoke density. This section will introduce several commonly used flame retardant performance testing methods.

4.1 Limiting oxygen index

The Limiting Oxygen Index (LOI) represents the minimum concentration of oxygen required in an oxygen-nitrogen environment to sustain flaming combustion under specified conditions. However, the limiting oxygen index (LOI) does not provide information on heat release rate (HRR), smoke production, or toxic gas generation. The limiting oxygen index (LOI) is highly dependent on sample thickness and heat release rate, and its use alone can lead to misinterpretation. Therefore, it is recommended to report LOI results in conjunction with methods such as cone calorimetry, vertical or horizontal burning tests, and thermogravimetric analysis (TGA). Additionally, the test conditions for the LOI should be included in the manuscript data to ensure comparability. The calculation formula is as follows:

$LOI=\frac{\left[O_2\right]}{\left[O_2\right]+\left[N_2\right]}\times 100\%$

Where: $\left[O_2\right]$ is the volume flow rate of oxygen in the gas at the limiting oxygen concentration.; $\left[N_2\right]$ is the volume flow rate of nitrogen in the mixed gas flow at the limiting oxygen concentration. Based on the NB/SH/T 0821-2010 standard, flame retardant grades are categorized into five levels, details of which are provided in Table 4:

Table 4

Table 4. Flame retardant grade of NB/SH/T 0821-2010 standard.

4.2 Horizontal burning test

4.2.1 GB/T 2408-2021 rating criteria

Based on the GB/T 2408-2021 rating criteria, the combustion classification of materials is determined by their burning behavior after ignition. Materials can be classified as HB or not reaching HB level, and they can also be classified as HB40 or HB75 level (HB - horizontal burning). For the test, three specimens should be marked with two lines at each specimen, perpendicular to the longitudinal axis of the specimen, at positions 25 ± 1 mm and 100 ± 1 mm from the ignition end. Subsequently, based on their relevant behavior, the linear burning rate vv is calculated to determine their classification, as shown in Table 5.

Table 5

Table 5. GB/T 2408-2021 rating criteria.

4.3 Vertical combustion test

4.3.1 International Electrotechnical Commission standard IEC 60695-11-20

The test procedure of the International Electrotechnical Commission standard IEC 60695-11-20 is as follows: first, a 20 mm high blue flame is generated using an igniter to ignite the sample, and the igniter is removed after 10 s. The time the sample burns with a flame is recorded as t1. If the sample extinguishes within 30 s, the igniter is reignited and the sample is ignited again for 10 s before being removed. The time the sample burns with a flame is recorded as t2, the time it smolders is recorded as t3, and if the sample exhibits dripping that ignites cotton, the test conditions are recorded. The main combustibility classification of materials is divided into three levels, as shown in Table 6.

Table 6

Table 6. IEC 60695-11-20 rating criteria.

4.3.2 GB/T 2408-2021 rating standard

The GB/T 2408-2021 rating standard classifies materials into V-0, V-1, and V-2 grades (vertical burning). If the test results need to meet the grade assessment requirements specified in Table 7. The specific test steps are as follows: Apply the flame to the center below the bottom of the sample at 10 mm and maintain it for 10 s. After 10 s of applying the flame to the sample, immediately move the torch away far enough to avoid affecting the sample, and at the same time, start the timer to measure the afterflame time t1 (unit: seconds), and record t1 as well as whether dripping or burning debris occurs. If so, it is necessary to indicate whether the cotton pad below is ignited. After the second 10-s application of the flame to the sample, immediately extinguish the torch or move it far enough away from the sample so that it no longer affects the sample, and at the same time, use the timer again to measure the sample’s afterflame time t₂ and afterglow time t₃, accurate to the second. Note and record the values of t₂, t₃, and t1+t₃.

Table 7

Table 7. GB/T 2408-2021 rating criteria.

4.4 Thermogravimetric analysis

Thermogravimetric analysis (TGA) characterizes the change in mass of a sample that occurs as its temperature is varied. When asphalt materials are exposed to high temperatures, the presence of flame retardants can be detected by changes in the thermogravimetric curve. Since the 1990s, researchers have utilized thermogravimetric mass spectrometry (TG-MS) to investigate the thermal decomposition and combustion characteristics of polymeric materials (Zhang et al., 2022).

Researchers (Xu et al., 2019) sed TG-MS to study the effects of flame retardants on asphalt’s fire resistance, focusing on mass and energy. They further assessed the flame retardants’ effectiveness using FDS software, examining temperature and smoke distribution. Zhaoyi et al. (2022) concluded that the endothermic flame retardation mechanism, involving targeted decomposition, was active in halloysite nanotube-modified asphalt with conventional flame retardants, based on their thermogravimetric analysis of combustion. Yan et al. (2023) and others compared the thermogravimetric curves of decabromodiphenyl ether flame-retardant asphalt and zinc borate hydrate flame-retardant asphalt, finding that the initial rapid decomposition temperature of decabromodiphenyl ether flame-retardant asphalt was 320 °C, which was 50 °C lower than that of zinc borate hydrate flame-retardant asphalt, indicating that zinc borate hydrate flame-retardant asphalt has better thermal stability.

4.5 Cone calorimeter (CONE)

Compared to traditional asphalt flash point tests, limiting oxygen index measurements, and vertical combustion tests, the cone calorimeter can more realistically simulate fire scenarios and its test results have better correlation with large-scale combustion tests. This test comprehensively evaluates the effectiveness of flame retardants through indicators such as heat release rate (HRR), smoke production rate (SPR), total heat release (THR), total smoke release (TSR), total mass loss (TML), mass loss rate (MLR), carbon monoxide release, time to ignition, and combustion duration.

Li et al. (2023) assessed the flame retardant and smoke suppression effects of asphalt mastic using a cone calorimeter. In another study, Li et al. (2022) investigated the pyrolysis and combustion characteristics of epoxy asphalt, also employing cone calorimeter testing to analyze its heat and smoke release. Their findings indicated that epoxy asphalt exhibited a higher smoke production rate in the initial stage, which subsequently decreased sharply. Wei et al. (2019) found through cone calorimeter testing that phosphorus slag (PS) powder additives can reduce the amount of smoke generated during the mixing and paving process of asphalt mixtures.

4.6 Fourier transform infrared spectrometer (FTIR)

The principle of FTIR technology is based on Fourier transform, which decomposes a beam of infrared light passing through a sample into its component frequencies, generating an infrared spectrum. Through this spectrum, different chemical bonds and functional groups in the sample can be identified and quantified. When used for asphalt materials, FTIR can reveal various organic components in asphalt, including functional groups that affect its combustion characteristics, such as thiols, ethers, and carboxylic acids. GB/T 38309 (2019) specifies the necessary components of the sampling system, the required spectral resolution, the procedures for data collection, and the validation criteria for using FTIR to analyze fire effluent in various fire test scales. Despite this, 3D FTIR spectroscopy offers only qualitative insights into the volatilization of asphalt binders during combustion.

TG-FTIR serves as an instrumental technique for the real-time monitoring of volatile organic compounds (VOCs) speciation during asphalt pyrolysis (Xia et al., 2021a). The mechanisms by which flame retardants suppress VOCs emissions vary significantly with their composition. Inorganic and nano-scale flame retardants, such as aluminum hydroxide (AHO) and organomontmorillonite (OMMT), function primarily through catalytic char formation and physical adsorption, effectively curtailing the release of low-molecular-weight VOC precursors including hydrocarbons and aldehydes. In contrast (Moura et al., 2023), certain halogenated flame retardants, while effective in inhibiting gas-phase combustion, may undergo decomposition that releases halogenated VOCs, thereby contributing to secondary pollution. A semi-quantitative evaluation of VOC suppression efficiency can be achieved by comparing the absorption peak intensities at characteristic wavenumbers across different flame-retardant modified asphalt specimens.

Moura et al. (2023) utilized FTIR and Atomic Force Microscopy to examine the interaction between the warm-mix additive PN2217 and ATH/OMMT flame-retardant asphalt. The study revealed that PN2217 decreases the activation energy of ATH/OMMT asphalt, inhibits the development of a char layer during combustion, and adversely affects its thermal stability. Xia et al. (2021a) used TG-FTIR to analyze SARA fractions with matched flame retardants. This technique simultaneously examines pyrolysis weight loss and identifies gaseous compounds released during pyrolysis in real-time through their characteristic absorption peaks.

4.7 Scanning electron microscopy (SEM)

Scanning Electron Microscopy (SEM) plays a critical role in the study of flame-retardant asphalt. It reveals the structural characteristics of the material’s surface by emitting an electron beam and collecting reflected electrons. SEM provides high-resolution images that detail the microstructure, including surface morphology and particle distribution. Zhaoyi et al. (2022) found that flame retardants form a barrier layer in asphalt residue by enhancing its integrity and density. This was determined through analysis of the residue’s macro and micro morphology using a digital camera and FESEM, thereby achieving flame retardant effects. Wang (Wu et al., 2006) and others’ SEM experiments showed that two novel inorganic flame retardants were uniformly distributed in the asphalt without agglomeration and had good adhesion to the asphalt matrix, exhibiting excellent flame retardant performance. Yang et al. (2021b) used scanning electron microscopy to find that the combustion residue rate of the composite modified asphalt material was significantly reduced, and a significant carbon layer structure was formed on the surface, which played a positive role in inhibiting the combustion process. Figure 10 shows the surface morphology of the char layer after the combustion of the composite-modified asphalt.

Figure 10

Figure 10. Surface morphology of carbon layer after combustion of composite modified bitumen. (a)Base asphalt. (b)ATH (10%)-OMMT (1%). (c)ATH (10%)-OMMT (3%).

4.8 Summary

This section introduces several commonly used flame retardant performance testing methods. Table 8 provides evaluation indicators for different testing methods. Currently, there is no complete set of standards for testing flame retardancy.

Table 8

Table 8. Evaluation criteria for different test methods.

5 Enhancing the flame retardancy of asphalt pavement in highway tunnels with nanotechnology

Unlike traditional materials, nanoparticles possess different physical and chemical properties. This difference is mainly attributed to their elevated surface-area-to-volume ratio and quantum effects resulting from spatial confinement (Yang and Tighe, 2013). Currently, nanoflame retardants are mainly divided into three categories: zero-dimensional nanomaterials, one-dimensional nanomaterials, and two-dimensional nanomaterials. Table 9 summarizes typical nanomaterials used to enhance the flame retardancy of asphalt materials in asphalt pavements and suppress smoke release.

Table 9

Table 9. Typical nanomaterials for flame retardant smoke suppression in asphalt.

Common nano flame retardant/smoke suppressant fillers and their engineering experience can be summarized as follows: Nano silica is typically added at 1.0–5.0 wt%, improving high-temperature strength by increasing binder viscosity, enhancing interfacial bonding, and densifying the carbon layer, while also exhibiting some smoke adsorption capacity. Organomontmorillonite is typically used at 2.0–8.0 wt%, where its layered structure both blocks and catalyzes carbonization; Graphene oxide/graphene oxide requires very low dosages, leveraging high specific surface area and layered barriers to reinforce carbon layers and suppress smoke VOCs, though it demands high dispersion quality and incurs higher costs; Nano-calcium carbonate serves as an economical smoke suppressant/filler, physically trapping smoke particles and synergizing with other nanomaterials; One-dimensional nanomaterials at 0.1–1.0 wt% enhance the mechanical integrity and thermal conductivity of the carbon layer. Engineering note: The above ranges serve as common starting references from literature/engineering practice. Optimal dosages are strongly influenced by the base material, surface modification, preparation temperature, shear rate, and application process. It is recommended to first conduct small-scale screening, then optimize the compounding ratio and dispersion process at a scaled-up level.

Currently, the dispersion conditions in the preparation of nano-modified asphalt are very complex, and increasing the temperature helps nanoparticles to disperse more easily. High temperatures typically reduce asphalt viscosity, but they also accelerate aging, compromising the binder’s durability. High temperatures typically reduce asphalt viscosity, but they also accelerate aging. This compromises the binder’s durability. Consequently, preparation temperatures must be carefully controlled. A significant challenge lies in balancing the need for sufficient heat to achieve uniform dispersion of nano-modified composite materials with the need to minimize asphalt aging. Similarly, while longer mixing times ensure better dispersion, they increase energy consumption, production costs, and waste. Therefore, optimizing both temperature and dispersion time is crucial. Therefore, it is also necessary to select an appropriate mixing time. At the same time, to ensure the full dispersion of nanoparticles, an appropriate mixing speed is also required. However, simply increasing the amount of flame-retardant nanomaterial is not beneficial., a more appropriate ratio of dosage is needed to ensure the economy in the project.

In the field of flame-retardant asphalt pavements, zero-dimensional nanomaterials, with their minute volume and substantial specific surface area, are capable of effectively adsorbing smoke particles and light components, thereby significantly mitigating smoke generation. For instance, Uddin et al. (2024) found that the incorporation of nano-calcium carbonate into asphalt binders can markedly reduce smoke emission. One-dimensional materials, such as carbon nanotubes, also exhibit potential for smoke suppression. Studies by Yang et al. (2013) indicate that while activated carbon demonstrates a pronounced ability to inhibit asphalt smoke, it concurrently exerts an adverse effect on the mechanical properties of the asphalt. Within the domain of two-dimensional materials, Liu et al. (2024) have proposed a scheme based on the combination of nano-calcium carbonate and SBS (styrene-butadiene-styrene block copolymer), effectively diminishing smoke emission from asphalt.

In addition to the aforementioned nanomaterials, Chinese medicine residue biochar (CMRB) has emerged as a promising porous carbon material with significant potential for VOC suppression. The porous structure and abundant oxygen-containing functional groups on CMRB are postulated to facilitate strong interactions with VOC molecules through π-π stacking and physical adsorption, thereby effectively inhibiting their release during asphalt processing (Ge et al., 2025). More importantly, biochar can generate synergistic effects with nanomaterials. For instance, the combination of biochar with organomontmorillonite (OMMT) not only leverages the adsorption capacity of biochar (Wang P. et al., 2022) but also incorporates the catalytic char-forming and barrier properties of OMMT, collectively contributing to the formation of a denser and more stable char layer. This dual mechanism of “adsorption-catalytic char formation” can more efficiently interrupt the combustion cycle and is expected to significantly reduce overall emissions of both VOCs and smoke, thereby achieving a unified approach to flame retardancy and smoke suppression.

Furthermore, the incorporation of layered nanoclays is gaining traction in the field of asphalt materials to bolster their fire-resistant capabilities. These nanoclays are broadly classified into two types: cationic and anionic. Among them, OMMT, or organomontmorillonite, stands out as the prevalent choice of cationic nanoclay owing to its superior compatibility with asphalt binders. On the other hand, anionic nanoclays, exemplified by LDHs (layered double hydroxides), are also emerging as potential candidates for enhancing the fire resistance of asphalt compositions.

Organomontmorillonite (OMMT) offers significant smoke suppression benefits when added to asphalt (Yang and Tighe, 2013). By incorporating OMMT, the release of carbon dioxide and volatile organic compounds (VOCs) from asphalt smoke is effectively diminished. This addition also augments the expansion potential of SBS within the asphalt matrix and improves the interfacial compatibility between SBS and the asphalt. The co-modification of OMMT and SBS results in a robust interlayer structure that effectively prevents the liberation of VOCs during the thermal degradation of asphalt. This leads to a reduction in emissions compared to conventional asphalt, thereby presenting a more eco-friendly alternative. The presence of OMMT in the modified asphalt correlates with an increased concentration of high-molecular-weight compounds, suggesting that OMMT prevents the breakdown of large asphalt molecules into smaller entities, which in turn curtails the release of VOCs. While OMMT has a minimal direct effect on the flame retardancy of asphalt binders (Yang and Tighe, 2013), if discovered that the addition of ATH can markedly enhance the flame-resistant properties of asphalt that has been modified with OMMT.

The efficacy of nanomaterials in mitigating asphalt smoke emissions through mechanisms like spatial confinement, adsorption, and catalytic oxidation is well-documented. Nevertheless, their implementation in asphalt materials is hindered by potential compromises to low-temperature performance and the arduous task of ensuring binder compatibility and nanomaterial dispersion (Caputo et al., 2020).

6 Summary and prospects

Taking a holistic “mechanism–technology–evaluation–environmental impact” perspective, this study systematically reviews advances in flame-retardant and smoke-suppression technologies for asphalt. The central finding is that fire safety can be effectively enhanced through the synergy of gas- and condensed-phase mechanisms, the integration of multiple flame-retardant strategies (additive-, component- and structural-types), and the combined use of multi-dimensional performance assessments. Among these, the application of nanomaterials (e.g., organo-montmorillonite) and composite flame retardants demonstrates dual potential for improving flame resistance while controlling VOC emissions, representing a key direction for future development.

Nevertheless, notable limitations persist. Most current studies focus on single-performance metrics, lacking coordinated optimization and systematic evaluation of flame-retardant efficiency, mechanical properties, long-term durability, and full-life-cycle environmental safety. Dispersion protocols and long-term stability of nanomaterials in field applications, together with standardized testing methods for complex smoke components—especially VOCs—remain to be established.

Future research should aim to develop multifunctional, synergistic flame-retardant systems that integrate flame inhibition, smoke suppression, VOC adsorption, and mechanical reinforcement within a unified design. Simultaneously, comprehensive life-cycle assessment frameworks covering environmental, health, and economic costs are needed to identify truly green and sustainable flame-retardant asphalt materials. Moreover, advancing the standardization of nanocomposite processing and exploring intelligent, responsive flame-retardant materials will be crucial for achieving the next-generation of high-performance, high-safety tunnel asphalt pavements.

Author contributions

YS: Visualization, Resources, Funding acquisition, Conceptualization, Supervision, Data curation, Writing – review and editing, Investigation, Methodology. PL: Supervision, Resources, Software, Methodology, Writing – review and editing, Investigation, Data curation, Visualization, Funding acquisition, Conceptualization. JH: Supervision, Investigation, Software, Conceptualization, Funding acquisition, Writing – review and editing, Visualization, Methodology, Data curation, Resources. BL: Visualization, Investigation, Writing – original draft, Methodology, Data curation, Writing – review and editing. HL: Formal Analysis, Project administration, Data curation, Visualization, Writing – original draft, Methodology, Software, Investigation, Writing – review and editing, Conceptualization.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The research was supported by 2022 Key Science and Technology Projects for the Transportation Industry of the Ministry of Transportation of China (Project No.2022.ZD7-130). The Nanning South Transit Line (from Liujing to Datang Section and fromWuxu Airport to Long’an Extension Section) Highway Scientific Research Topic (Project·No.EHGS-GC-2023-025).

Conflict of interest

Author YS was employed by Nanning Expressway Construction & Development Co., Ltd. Authors PL and JH were employed by Guangxi Nanning Second Ring Expressway Co., Ltd.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The author(s) declared that generative AI was not used in the creation of this manuscript.

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