Feasibility of Polyphosphoric Acid in Emulsified Asphalt Modification: Emulsification Characteristics, Rheological Properties, and Modification Mechanism

Abstract

Polyphosphoric acid (PPA), a chemical modifier widely used in petroleum asphalt, results in significant performance improvements. However, its effectiveness for modified emulsified asphalt has not yet been thoroughly verified. This study aims to investigate the emulsification properties, rheological characteristics, compatibility, and modification mechanisms of PPA-modified emulsified asphalt and validate the feasibility of applying PPA for modification. Initially, PPA-modified emulsified asphalt was prepared at different dosages (0%, 0.5%, 1.0%, 1.5%, and 2.0%), and its emulsification characteristics, including evaporation residue properties and storage stability, were evaluated. Subsequently, the rheological performance and compatibility of PPA-modified emulsified asphalt at various temperatures were evaluated using a dynamic shear rheometer. Finally, Fourier transform infrared spectroscopy (FTIR) and fluorescence microscopy (FM) were utilized to investigate the effects of PPA modification on the chemical composition and microscopic characteristics of emulsified asphalt. The results indicated that, with increasing PPA dosage, the softening point of modified emulsified asphalt initially decreased and then increased, while penetration and ductility first increased and then decreased, accompanied by reduced storage stability. Furthermore, PPA modification can enhance the high-temperature stability, fatigue properties, and low-temperature performance of emulsified asphalt, but the effectiveness depended on the dosage of PPA. Specifically, optimal compatibility of modified emulsified asphalt was achieved at a PPA dosage of 1.0%. Notably, PPA underwent hydrolysis within the emulsified asphalt system, leading to modification mechanisms distinct from those observed in base asphalt modification. At a PPA dosage of 1.0%, asphalt particles within the emulsified asphalt exhibited the most uniform distribution. Conversely, excessive PPA dosage (e.g., 2.0%) caused significant particle aggregation, consequently weakening the modification effect.

1. Introduction

Emulsified asphalt is formed by dispersing melted asphalt particles in an aqueous medium containing an emulsifier, resulting in a stable emulsion [1]. Emulsified asphalt is widely employed in pavement construction and maintenance due to its excellent workability and stability. However, ordinary emulsified asphalt suffers from issues such as poor bonding properties, slow strength formation, and susceptibility to aging, which hinder its widespread use in road engineering [2]. Previous research and engineering practices have demonstrated that modified emulsified asphalt can significantly enhance its bonding properties, high-temperature stability, low-temperature crack resistance, water stability, and durability [3,4,5]. Therefore, modifying emulsified asphalt is a crucial research focus for this material and a key factor in ensuring the efficient performance of emulsified asphalt-based road materials.

Currently, the primary method for modifying emulsified asphalt is the addition of polymer modifiers. Commonly used polymer modifiers include styrene–butadiene rubber (SBR), waterborne epoxy resin (WER), and styrene–butadiene block copolymer (SBS). Among these, styrene–butadiene rubber (SBR) is widely used in modified asphalt and emulsified asphalt due to its significant improvements in low-temperature crack resistance, high-temperature stability, adhesion, and other properties. Additionally, SBR can effectively enhance the impermeability and rutting resistance of emulsified asphalt under high-temperature conditions [5,6,7]. However, SBR-modified emulsified asphalt faces challenges, such as insufficient storage stability, limiting its widespread application [8]. Waterborne epoxy resin (WER) is a liquid-phase system in which epoxy resin particles act as the dispersed phase and an aqueous solution serves as the continuous phase. WER can significantly enhance the thermal stability and mechanical properties of emulsified asphalt after curing. However, cured emulsified asphalt exhibits high brittleness, poor ductility, and insufficient adhesion [9,10,11,12]. As a thermoplastic elastomer, styrene–butadiene block copolymer (SBS) enhances the high- and low-temperature performance as well as the water stability of emulsified asphalt. However, SBS-modified asphalt is often difficult to emulsify and lacks long-term storage stability [13,14,15]. Moreover, polymer-modified emulsified asphalt faces challenges, such as high costs and a complex modification process, which require urgent solutions. In contrast, polyphosphoric acid (PPA), as a chemical asphalt modifier, offers several technical advantages, including high modification efficiency, a simple process, good storage stability, and low cost, making it a highly promising option for emulsified asphalt modification [16].

In recent years, PPA, as an economically viable and effective modifier, has attracted significant attention from the pavement engineering applications. Studies indicate that adding PPA increases the content and dispersion of asphaltenes in asphalt, significantly improving its high-temperature stability. However, unlike PPA’s effect on the high-temperature performance of asphalt, its impact on the low-temperature performance of asphalt remains inconclusive due to variations in test temperature, asphalt source, and test methods. Current findings suggest three potential outcomes: PPA may have a positive effect, a negative effect, or no significant effect on low-temperature performance [17,18,19,20,21]. Additionally, PPA-modified asphalt exhibits greater stability during the aging process, with the incorporation of PPA enhancing the asphalt’s aging resistance. By comparing the changes in functional groups before and after asphalt aging, Baumgardner and Dourado observed that PPA exhibits antioxidant properties, which reduce the intensity of oxygen absorption during aging and enhance the asphalt’s anti-aging performance [22,23]. Xu et al. measured the molecular weight and chemical structure of SBR- and PPA/SBR-modified asphalt using Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). They found that PPA- and-SBR modified asphalt undergo complex chemical reactions, leading to changes in its molecular structure, enhancing the stability of composite asphalt, inhibiting polymer macromolecule degradation in SBR, and slowing asphalt aging [24]. Additionally, PPA, a protonic acid with strong polarity, can enhance the compatibility and storage stability of emulsified asphalt when compounded with SBS for modification [25]. Although numerous studies have focused on PPA’s modification of asphalt, most research adds PPA as a polar component to SBS-modified emulsified asphalt to enhance compatibility and storage stability between SBS and asphalt. Therefore, research on using PPA for the sole modification of emulsified asphalt remains relatively limited. Specifically, the impact of PPA on asphalt performance in the water emulsion system requires systematic verification, and the mechanism of PPA’s effect on emulsified asphalt needs further clarification.

This study aims to investigate the modification effects and mechanisms of PPA on emulsified asphalt. Emulsified asphalts with varying PPA contents were prepared, and their storage stability after PPA incorporation was analyzed. Additionally, the basic physical properties, high-temperature stability, low-temperature crack resistance, fatigue performance, and compatibility of emulsified asphalt residue with varying PPA contents were compared and analyzed. Building upon this, the effects of PPA modification on the chemical composition and microstructure of emulsified asphalt residues were investigated using Fourier transform infrared spectroscopy (FTIR) and fluorescence microscopy (FM).

2. Materials and Methods

2.1. Materials

2.1.1. Matrix Asphalt

In this study, Shell 70# petroleum asphalt was used to prepare emulsified asphalt. The technical performance was evaluated based on the standard test methods for bitumen and bituminous mixtures in highway engineering (JTG E20-2011) [26], and the results are presented in

Table 1. Technical indexes of matrix asphalt.

Index	Test Result	Requirement (JTG E20-2011)	Test Method (JTG E20-2011)
Penetration (25 °C), 0.1 mm	77	60~80	T0604
Durability (5 °C, 5 cm·min−1), cm	25.03	≥25	T0605
Softening point, °C	47.7	>46	T0606
Flash point, °C	300	≥260	T0611
Wax content (distillation method), %	1.9	≤2.2	T0615
Density, (g·cm−3)	1.03	-	T0603
Solubility (C2HCl3), %	99.7	≥99.5	T0607

.

2.1.2. PPA

In this study, industrial-grade PPA was used to modify emulsified asphalt, with an H₃PO₄ content of 115.8%, which is provide by JiangSu YunTong New Material Technology Co., Ltd., Nantong, China. The main technical indexes are presented in

Table 2. Technical indexes of polyphosphoric acid.

Index	Requirement	Value
Appearance, %	Colorless or light liquid	Colorless liquid
H3PO4, %	≥115.0	115.8
Fe, %	≤0.002	<0.002
Heavy metal (Pb), %	≤0.005	<0.005
Chloride (Cl), %	≤0.0005	<0.0005

. PPA is an inorganic acid with corrosive properties. PPA reacts with water to form orthophosphoric acid, though its toxicity is relatively low. In this study, all modification procedures adhered strictly to chemical safety protocols.

2.1.3. Emulsifier and Other Additives

In this study, cetyltrimethylammonium bromide (CTAB), a cationic quaternary ammonium salt emulsifier provided by Zhiyuan Chemical Products, Tianjing, China, was used. The hydrophilic–lipophilic balance (HLB) value of this emulsifier is 15.8, close to that of asphalt, and it exhibits good chemical stability and emulsifying properties, making it ideal for asphalt interaction. Its main technical indexes are presented in

Table 3. Technical indexes of hexadecyl trimethyl ammonium bromide (CTAB).

Index		Value	Reference
Content		≥99.0%	/
HLB value		15.8
Melting point		240 °C
Maximum content of impurities	Ethanol dissolution test	Qualified	GB25592 [27]
	Ignition residue (sulfate)	0.05
	Moisture	0.5
	Fe	0.001
	Heavy metal (meter Pb)	0.001

. To facilitate the comparison of the effects of PPA content on emulsified asphalt, the emulsifier content in this study was standardized to 2% of the matrix asphalt’s mass.

Additionally, similar to the preparation of conventionally emulsified asphalt, this study used anhydrous calcium chloride (pH 8–10) as the stabilizer and a hydrochloric acid standard titration solution (1.0009 mol/L) as the pH regulator.

2.2. Preparation of PPA-Modified Emulsified Asphalt

The preparation of modified emulsified asphalt includes three processes: pre-modification emulsification, post-emulsification modification, and simultaneous modification and emulsification [28,29]. Previous studies have shown that emulsified asphalt modified before emulsification exhibits better uniformity and storage stability [30]. Therefore, in this study, the process of modification followed by emulsification was used to prepare PPA-modified emulsified asphalt, as shown in Figure 1. First, the predetermined amount of PPA was added to the matrix asphalt, which was preheated to 155 °C and stirred for 30 min at a mechanical stirring speed of 800 rpm. In addition, the predetermined amount of emulsifier CTAB, stabilizer, and pH regulator were added to the water at 60–65 °C, and the soap lye was formed by uniform mixing and maintained at around 65 °C. Finally, the soap lye and PPA-modified asphalt were sheared for 30 min at a shear speed of 4000 rpm. After stopping the shear, the PPA-modified emulsified asphalt was obtained by cooling it to room temperature. To simplify the expression, PPA-modified emulsified asphalt with PPA contents of 0%, 0.5%, 1.0%, 1.5%, and 2.0% are referred to as 0.0% PPA, 0.5% PPA, 1.0% PPA, 1.5% PPA, and 2.0% PPA, respectively.

Figure 2 illustrates the technical approach used in this study. First, PPA-modified emulsified asphalt with varying PPA contents was prepared using the method outlined in Figure 1, and its storage stability was assessed. Next, the basic physical properties of the PPA-modified emulsified asphalt residues with different PPA contents were compared and analyzed through softening point, penetration, and ductility tests. Additionally, the high, medium, and low-temperature rheological properties of the PPA-modified emulsified asphalt residues were characterized based on the dynamic shear rheological test results. Simultaneously, the compatibility of PPA-modified emulsified asphalt was analyzed using frequency sweeping tests. Finally, the chemical composition and microstructure of the emulsified asphalt residues before and after PPA modification were compared and analyzed using FTIR and FM tests.

2.3. Methods

2.3.1. Emulsification Characteristic Test

The emulsifying properties of PPA-modified emulsified asphalt with varying ratios were evaluated based on 5-day storage stability and basic physical properties (penetration, softening point, and ductility) of the evaporation residues. The tests were conducted according to the standard test methods for bitumen and mixtures in highway engineering (JTG E20-2011) [26].

2.3.2. Dynamic Shear Rheological Test

The high-, medium-, and low-temperature rheological properties of different PPA-modified emulsified asphalt evaporation residues were analyzed using a dynamic shear rheometer, including multistress creep (MSCR) tests, temperature sweeping (TS) tests, linear amplitude sweeping (LAS) tests, and the 4 mm DSR test. Additionally, the compatibility of PPA-modified emulsified asphalt was analyzed using the frequency sweeping (FS) test. The specific parameters for the different DSR tests are provided in

Table 4. Test parameters of dynamic shear rheological tests.

Test	MSCR	TS	LAS	4 mm DSR	FS
Temperature, °C	64	30~80	25	−5, −15	40~80
Temperature increment, °C	/	10	/	/	10
Parallel plate size, mm	25	25	8	4	25
Frequency	/	10 rad/s	0.2~30 Hz, 10 rad/s	0.1~100 rad/s	0.1~100 Hz
Load type	Stress control: 0.1 kPa, 3.2 kPa	Strain control: 1%	Strain control: 0.1% /0.1~30%	Strain control: 0.1%	Strain control: 0.1%
Test method	ASTM D7405 [31]	ASTM D7175 [32]	AASHTO TP 101-14 [33]	Literature [34]	ASTM D7175 [32]

.

2.3.3. Fourier Transform Infrared Spectroscopy Test

Fourier transform infrared spectra (FTIR) of the evaporation residue samples from different PPA-modified emulsified asphalts were obtained in a specific wavenumber range. The specific steps are as follows: The sample is placed on the ATR crystal of the FTIR, ensuring complete coverage of the crystal’s surface. A pressure clamp is then applied to uniformly compress the sample for testing. The test wavelength range is 650–4000 cm⁻¹, with a resolution of 4 cm⁻¹.

2.3.4. Fluorescence Microscope Test

The microstructure of various PPA-modified emulsified asphalts was observed using a fluorescence microscope (FM). The specific steps are as follows: PPA-modified emulsified asphalt is dropped onto a glass slide, covered with a cover glass, and placed in an oven at 163 °C for 1 min to allow even diffusion. The specimen is then removed from the oven and naturally cooled to room temperature. Finally, the microstructure of the PPA-modified emulsified asphalt was observed using a fluorescence microscope, and the particle size of the asphalt was analyzed.

3. Results and Discussion

3.1. Emulsification Characteristic Analysis

3.1.1. Effect of PPA on the Basic Physical Properties

The test results of basic physical properties (softening point, penetration, and ductility) of PPA-modified emulsified asphalt evaporation residue are shown in Figure 3. Compared to the unmodified sample, the addition of 0.5% PPA increases the penetration and ductility of emulsified asphalt, while decreasing the softening point. This trend differs from that observed in PPA-modified matrix asphalt [35]. The test results suggest that PPA interacts differently with matrix asphalt in the emulsified asphalt system, altering its modification effect. As the PPA content increased, the penetration and ductility of emulsified asphalt decreased, while the softening point increased, indicating improved high-temperature performance and reduced low-temperature performance. The possible reason is that PPA dissociates in emulsified asphalt into H₂${\mathrm{P}\mathrm{O}}_4^{-}$ and H⁺. The H⁺ ions then react with asphalt components, breaking asphaltene clusters and dispersing them into smaller asphaltene units [36]. Therefore, the effect of PPA on improving the high-temperature performance of emulsified asphalt becomes more apparent, while its low-temperature performance begins to decline. However, except for the 2.0% content, all the samples meet the specification requirement of being higher than 40 cm [37]. Notably, as the PPA content increases, the softening point of emulsified asphalt evaporation residue initially decreases and then increases, while the penetration and ductility first increase and then decrease. The cause of this nonlinear behavior requires further investigation.

3.1.2. Effect of PPA on the Storage Stability

Following the standardized procedure in Section 2.3.1, we quantitatively assessed the 5-day storage stability of PPA-modified asphalt emulsions. The results, shown in Figure 4, indicate that the storage stability of PPA-modified emulsified asphalt is poor. As the PPA content increases, the storage stability index shows an upward trend, suggesting that the storage stability of emulsified asphalt decreases with higher PPA content. The likely cause is that, during the standing process, the modified emulsified asphalt undergoes demulsification, leading to the aggregation of asphalt particles into a continuous phase. Additionally, the incorporation of PPA increases the viscosity of the continuous phase. Furthermore, the addition of PPA may reduce the adsorption of CTAB emulsifier molecules at the water–asphalt interface, leading to decreased storage stability of the emulsified asphalt [38]. As shown in Figure 4, as the PPA content increases, the color of the upper liquid in the emulsified asphalt after stratification gradually fades, while its pH value remains in the range of 2 to 3. This may be due to the hydration of PPA or its interaction with the emulsifier, though the exact mechanism requires further investigation.

3.2. Rheological Properties of PPA-Modified Emulsified Asphalt

3.2.1. MSCR Analysis

The recovery rate (R_0.1, R_3.2) and nonrecoverable creep compliance (J_nr0.1, J_nr3.2) of PPA-modified emulsified asphalt evaporation residue were determined through the MSCR test at two stress levels: 0.1 kPa and 3.2 kPa. The results are shown in Figure 5. A higher recovery rate and smaller nonrecoverable creep compliance indicate better high-temperature stability of the asphalt. Figure 5 shows that, at stress levels of 0.1 kPa and 3.2 kPa, the nonrecoverable creep compliance of PPA-modified emulsified asphalt follows a similar overall trend. Additionally, it increases nonlinearly with increasing PPA content. Additionally, R_0.1 increases significantly with increasing PPA content, indicating that PPA enhances the deformation recovery ability of emulsified asphalt under low-stress at high temperatures. The key difference is that, at a stress level of 3.2 kPa, R_3.2 is highest and J_nr3.2 is lowest when the PPA content is 1.0%. This result indicates that PPA enhances the high-temperature stability of emulsified asphalt evaporation residue at high-stress levels, but its effectiveness depends on the PPA content. In summary, a higher PPA content generally enhances the deformation resistance of PPA-modified asphalt [39]. However, excessively low- or high-PPA content does not improve the deformation resistance of emulsified asphalt under high stress. Therefore, selecting an appropriate PPA content is crucial for optimizing the performance of PPA-modified emulsified asphalt.

3.2.2. Temperature Sweep Analysis

Asphalt, as a typical viscoelastic material, exhibits significant temperature-dependent mechanical behavior. The complex shear modulus (G*) from the temperature sweeping test effectively reflects the viscoelastic properties of emulsified asphalt. A higher G* indicates a larger elastic component and improved resistance to high-temperature deformation [40]. Figure 6 presents the temperature sweeping test results of emulsified asphalt modified with different PPA contents. Overall, as the temperature increases, the G* of the PPA-modified emulsified asphalt evaporation residue decreases significantly. Notably, under the same temperature conditions, G* exhibits a nonlinear trend as PPA content increases. Compared to 0.0% PPA, the G* of emulsified asphalt decreased after adding 0.5% PPA. Subsequently, G* first increased and then decreased as PPA content increased. When the PPA content is 1.0%, the G* of PPA-modified emulsified asphalt evaporation residue reaches its maximum. The test results show that 1.0% PPA-modified emulsified asphalt consistently exhibits better high-temperature deformation resistance, in line with the conclusion in Section 3.2.1.

To further compare the temperature sensitivity of different PPA-modified emulsified asphalts, the G* of modified emulsified asphalt with varying PPA content was fitted based on the temperature sweeping results using Equation (1). Here, T represents the test temperature, while k and b are the slope and intercept of the linear fit, respectively. The absolute value of the slope |K| obtained from the fitting represents the temperature coefficient of emulsified asphalt, as shown in Figure 7. The smaller the |K| value, the less the influence of temperature change on modified emulsified asphalt [41]. As shown in Figure 7, with increasing PPA content, the |K| value first increases and then decreases, indicating that the temperature sensitivity of emulsified asphalt evaporation residue decreases first and then increases. When the PPA content reaches 1.5%, the temperature sensitivity of PPA-modified emulsified asphalt is lower than that of the 0.0% PPA.

$$lg{G}^{\mathrm{*}}=kT+b$$

(1)

3.2.3. Fatigue Characteristic Analysis

Applying viscoelastic continuum damage (VECD) theory to linear amplitude sweep (LAS) test data [42], the fatigue damage parameters of different PPA-modified emulsified asphalt evaporation residues are obtained, as shown in

Table 5. Fatigue parameters of VECD analysis.

Parameter	0.0% PPA	0.5% PPA	1.0% PPA	1.5% PPA	2.0% PPA
A	50,664.13	39,285.50	77,805.99	56,095.39	90,061.17
B	2.33	2.34	2.56	2.32	2.48
Df	59.67	50.91	61.17	63.21	71.21
Cpeak	0.50	0.54	0.46	0.50	0.46
Stressmax	2,570,712	211,588	254,443	267,318	237,446

. The A value represents the variation in material integrity due to accumulated damage [43], while the B value reflects the sensitivity of fatigue life (N_f) to loading strain [43]. A higher A value indicates that the material exhibits greater fatigue life under low strain, while a higher B value means that the N_f of the material decreases more rapidly with increasing strain. Additionally, D_f represents the damage parameter when the material is approaching fatigue failure, i.e., the fatigue failure point. A larger D_f indicates that the material can withstand more cumulative damage before failure occurs [33]. C_peak represents the material integrity at the peak [33], while Stress_max denotes the peak stress.

Table 5 shows that the A value of 0.5% PPA-modified emulsified asphalt is lower than that of the unmodified asphalt, while the B value remains relatively unchanged. This suggests that the addition of PPA reduces the fatigue resistance of emulsified asphalt but has no significant effect on its stress sensitivity. In contrast, the A value of 1.0%, 1.5%, and 2.0% is higher than that of the unmodified asphalt, with the A values of 1.0% and 2.0% showing more significant increases. However, this is accompanied by higher B values. This indicates that, when the PPA content reaches 1.0%, PPA modification enhances the fatigue resistance of emulsified asphalt but also increases its strain sensitivity. The results of D_f and C_peak also demonstrate that, as PPA content increases, the anti-fatigue performance of emulsified asphalt evaporation residue changes nonlinearly. Based on the parameters in Table 5, the fatigue life (N_f) of different PPA-modified emulsified asphalts was calculated, and the results are shown in Figure 8. In summary, PPA content is a key factor influencing the fatigue life of emulsified asphalt. When the PPA content is low (0.5%), the fatigue performance of PPA-modified emulsified asphalt decreases. When the PPA content reaches 1.0%, its incorporation effectively enhances the fatigue resistance of emulsified asphalt.

3.2.4. 4 mm DSR Analysis

In this study, a 4 mm DSR test was used to evaluate the low-temperature rheological properties of PPA-modified emulsified asphalt evaporation residue.

The master curve of relaxation modulus (Figure A1) is fitted, and G(60s) and m_r(60s) are calculated using Equations (2) and (3), respectively. These values can then be used to quantitatively evaluate the low-temperature performance of PPA-modified emulsified asphalt evaporation residue. The better the low-temperature performance of asphalt, the smaller the absolute value of G(60s) and the larger the absolute value of m_r(60s).

$$G\left(60s\right)=a{x}^2+bx+{\left.c\right|}_{x=1.78}$$

(2)

$$m_r\left(60s\right)=2ax+{\left.b\right|}_{x=1.78}$$

(3)

Figure 9 presents the relaxation modulus G(60s) and relaxation rate index m_r(60s) for evaporation residues across PPA concentrations (0–2.0 wt%). The results shown in Figure 9(b) indicate that the incorporation of PPA increases the absolute value of the relaxation rate index m_r(60s) to varying degrees, allowing PPA-modified emulsified asphalt to more effectively disperse temperature stress. Overall, PPA modification helps improve the low-temperature performance of emulsified asphalt.

3.3. Compatibilization Analysis

Optimum compatibility component is essential to ensuring the effective modification of emulsified asphalt by PPA. The Cole–Cole plots is an effective method for evaluating the compatibility of the modified asphalt system. In the complex viscosity (η*), the real part (η′) is plotted on the x-axis, and the imaginary part (η″) is plotted on the y-axis. A semi-circular or parabolic curve indicates better compatibility of the asphalt [44,45,46]. Additionally, when the curve predominantly lies on the left side of the parabolic peak, the asphalt exhibits more elastic properties. When the curve is predominantly on the right side of the parabolic peak, the asphalt demonstrates more viscous properties [46]. The Cole–Cole diagram of PPA-modified emulsified asphalt under different temperature conditions is shown in Figure 10. At 40 °C, the curves of the five types of emulsified asphalt are semi-arc, indicating good compatibility. The curves for 1.0% and 2.0% PPA-modified emulsified asphalt are primarily located on the left side of the parabola, indicating greater elasticity, while the other three types of emulsified asphalt exhibit symmetrical semi-arcs, suggesting good viscoelasticity. At 50 °C, the curve for 1.0% PPA is semi-circular, indicating the best compatibility. In contrast, 0.0% and 2.0% PPA exhibit a trailing shape, suggesting poor compatibility. For 0.5% and 1.5% PPA, the curves are primarily located on the right side of the parabola, indicating greater viscosity. As the temperature increases further, all samples, except for 1.0% PPA, deviate more from the arc shape, with the data becoming increasingly scattered. In summary, both temperature increase and dosage variation significantly affect the compatibility of emulsified asphalt. Overall, as the PPA content increases, the compatibility of PPA-modified emulsified asphalt first improves and then declines. The compatibility of PPA-modified emulsified asphalt is optimal when the PPA content is 1.0%.

3.4. Analysis of Variance

To further analyze the impact of PPA content on the properties of PPA-modified emulsified asphalt, variance analysis (ANOVA) at a 95% confidence level was performed using IBM SPSS (22.0) software, with PPA content as the variable. The results are presented in

Table 6. The results of variance analysis of PPA content on various indexes.

Index	Sum of Squares	Degree of Freedom	Root Mean Square	F	p-Value
Softening point	13.432	4	3.358	2.231	0.201
Penetration	230.476	4	57.619	0.662	0.645
R3.2	0.000	4	0.000	0.800	0.574
Jnr3.2	9.591	4	2.398	141.040	0.000
Nf2.5	300,079.078	4	75,019.770	0.294	0.871
mr(60s)	0.001	4	0.000	4.484	0.066
G(60s)	0.028	4	0.007	44.675	0.000
Ductility	3294.556	4	823.639	19.233	0.003
Storage stability	680.5434	4	170.136	22.795	0.002

. Table 6 shows that the p-values for J_nr3.2, G(60s), ductility, and 5-day storage stability are all less than 0.05, indicating that PPA content significantly affects the high-temperature, low-temperature, and storage stability properties of emulsified asphalt.

3.5. Micro-Analysis of PPA-Modified Emulsified Asphalt

3.5.1. FTIR Analysis

Figure 11 shows the FTIR test results for different PPA-modified emulsified asphalt evaporation residues. The PPA-modified emulsified asphalt exhibits a distinct absorption peak in the range of 2800~3000 cm⁻¹, likely attributed to the C-H vibrations of cycloalkanes and alkanes. The absorption peaks at 2930 cm⁻¹ and 2860 cm⁻¹ are attributed to the antisymmetric and symmetric stretching vibrations of -CH₂, while the absorption peaks in the range of 1300~1500 cm⁻¹ correspond to the angular vibrations of -CH₃. In addition, the frequency of the asymmetric angular vibration of PPA-modified emulsified asphalt falls within the range of 1454~1465 cm⁻¹, while the frequency of the symmetric angular vibration is around 1380 cm⁻¹. The absorption peak at 1611 cm⁻¹ is primarily attributed to the stretching vibration of the C=C double bond in PPA-modified emulsified asphalt, with its absorption intensity higher than that of the unmodified emulsified asphalt at this peak.

The macroscopic property test results reveal a significant difference in the modification effects of PPA on emulsified asphalt compared to matrix asphalt. Notably, the cationic emulsified asphalt soap has an acidic pH, which facilitates the hydrolysis of PPA into orthophosphate. This reaction process is illustrated in Equation (4) [47].

$$H_{3}P{O}_4\stackrel{p{K}_1=2.1}{\iff }{H}_{2}P{O}_4^{-}+H^{+}\stackrel{p{K}_2=7.2}{\iff }HP{O}_4^{2-}+H^{+}\stackrel{p{K}_3=12.7}{\iff }P{O}_4^{3-}+H^{+}$$

(4)

To further investigate the potential reactions of PPA in the emulsified asphalt system, this study compared the FTIR spectra of 1.0% PPA-modified matrix asphalt and 1.0% PPA-modified emulsified asphalt. The results are presented in Figure 12. Overall, the absorption peaks of the two samples exhibit significant differences in the fingerprint region (400~1330 cm⁻¹). Compared to PPA-modified asphalt, PPA-modified emulsified asphalt exhibits a new absorption peak at 1066 cm⁻¹, likely attributed to the antisymmetric stretching vibration of PO₄ in ${\mathrm{P}\mathrm{O}}_4^{3-}$ following PPA hydrolysis. Therefore, it is hypothesized that PPA undergoes distinct chemical reactions in the emulsified asphalt system compared to the matrix asphalt, leading to significant differences in their modification effects.

3.5.2. Fluorescent Microscope Analysis

Using fluorescence microscopy (FM), images of various PPA-modified emulsified asphalt samples were captured at 400× magnification, as shown in Figure 13. The particle size of asphalt particles was analyzed using ImageJ (Fiji) with JAVA8 software, and the results are presented in Figure 14. Compared to unmodified emulsified asphalt, the incorporation of varying PPA amounts significantly altered the microstructure of the modified emulsified asphalt. As the PPA content increases, the particle size of asphalt particles first decreases and then increases. The emulsified asphalt system exhibited the best compatibility and the smallest particle size at 1.0% PPA. Specifically, compared to 0.0% PPA, the asphalt particle sizes for 1.0% PPA and 1.5% PPA were 7.289 μm and 7.355 μm, respectively. In contrast, the asphalt particle distribution in 1.0% PPA is more uniform. When the PPA content reaches 2.0%, the asphalt particles in the modified emulsified asphalt exhibit significant agglomeration, resulting in larger particle sizes and a more discrete distribution. In conclusion, when the PPA content is no higher than 1.5%, PPA promotes uniform dispersion of asphalt particles, significantly reducing their size. However, excessive PPA content may destabilize the emulsifier, causing uneven dispersion or even aggregation of the asphalt particles during emulsification.

4. Discussion

This study systematically investigated the feasibility of using polyphosphoric acid (PPA) to modify emulsified asphalt, focusing on its emulsification characteristics, rheological behavior, compatibility, and modification mechanism. Based on the experimental results, the main conclusions are as follows:

• As the PPA dosage increased, the softening point of evaporation residues from modified emulsified asphalt initially decreased and subsequently increased, whereas penetration and ductility exhibited an initial increase followed by a decrease. Additionally, incorporating PPA can reduce the storage stability of emulsified asphalt.
• PPA modification can improve the high-temperature stability of evaporation residues from emulsified asphalt, though the extent of improvement depends on the PPA dosage. The optimal high-temperature stability was achieved with a PPA dosage of 1.0%. Additionally, the temperature sensitivity of evaporation residues first decreased and then increased as the PPA content rose. When the dosage is 1.0% or more, PPA-modified emulsified asphalt shows better temperature sensitivity than unmodified emulsified asphalt.
• At a low PPA dosage (0.5%), the fatigue performance of emulsified asphalt decreased after modification. However, when the PPA content reached 1.0%, incorporation of PPA effectively enhanced fatigue resistance. Moreover, PPA enhanced low-temperature performance of evaporation residues, particularly when added at an appropriate dosage (e.g., 0.5%) where its effect is especially pronounced.
• The compatibility of PPA-modified emulsified asphalt initially increased and subsequently decreased as the PPA dosage increased. The optimal compatibility was observed at a PPA dosage of 1.0%.
• Hydrolysis of PPA in the emulsion system altered its chemical structure, resulting in a modification mechanism distinct from that in base asphalt. At a PPA dosage of 1.0%, asphalt particles exhibited a more uniform distribution. However, excessive PPA dosage (e.g., 2.0%) led to significant aggregation of asphalt particles, resulting in larger particle sizes and greater dispersion.

Despite these insights, this study has certain limitations. The long-term aging behavior and field performance of PPA-modified emulsified asphalt require additional validation through future studies. Additionally, further investigation into the interaction mechanisms between PPA and emulsifiers is required to optimize storage stability and elucidate the specific reasons for the nonlinear responses observed with varying PPA dosages.