Sustainable valorization of phosphogypsum solid waste as composite filler in asphalt mastic: rheological behavior and construction benefits

Massive phosphogypsum (PG) stockpiles bring persistent environmental risks in China, while heavy-duty channelized traffic increasingly causes premature rutting and fatigue cracking in asphalt pavements. To reconcile engineering requirements with environmental pressures, PG was used as a filler for high-value applications. In this study, asphalt mastic was prepared using Grade A 70 # road asphalt, with mineral powder and PG as a single filler, and a PG–mineral powder composite filler. Temperature–frequency scanning, multiple stress creep recovery (MSCR), and linear amplitude scanning (LAS) tests were subsequently conducted using a dynamic shear rheometer (DSR). The influence of filler type, filler–asphalt ratio, and blend proportions on the mechanism of viscoelastic behavior evolution in mastic was analyzed. It was found that PG filler enhanced the high-temperature deformation resistance of asphalt mastic compared with mineral powder. At high filler–asphalt ratios, the transition of the viscoelastic structure could be delayed, and weak shear thickening characteristics were exhibited under heavy shear loading. A filler–asphalt ratio threshold was observed for the fatigue resistance of PG mastic. At a filler–asphalt ratio of 1.0, its fatigue life was found to surpass that of mastic with mineral powder. For the composite system, a performance-to-blend-ratio threshold was demonstrated. At a 50% PG blend ratio (filler–asphalt ratio was 1.0), optimal fatigue resistance was achieved while maintaining excellent high-temperature stability. Furthermore, a quantitative assessment confirmed that this optimal blend reduces filler processing carbon emissions by 42.9%. These findings provide core theoretical support and key design parameters for the sustainable, large-scale application of PG solid waste in green highway construction.

Due to its excellent ride comfort, ease of construction and maintenance, and low traffic noise, asphalt pavement has become the primary paving type for high-grade highways and urban roads in China (Peng et al., 2026). As China’s transportation network continues to support increasing volumes of heavy-duty traffic, long-term service-related defects in asphalt pavements—such as high-temperature rutting and medium-temperature fatigue cracking—have become increasingly evident. It significantly reduces pavement service levels and increases maintenance costs (Marasteanu and Cannone Falchetto, 2017; Mohammed and Abed, 2024). Driven by both the “Dual Carbon” strategic goals and policies that promote the utilization of bulk industrial solid waste, the development of alternative pavement materials derived from industrial solid byproducts has become a core direction for green, low-carbon development in road engineering (Peng et al., 2023). These materials can simultaneously enhance pavement performance and enable large-scale solid-waste disposal (Ou et al., 2023; Vigneswaran et al., 2024).

Phosphogypsum (PG) is a major industrial byproduct generated during the wet-process phosphoric acid production. Its primary chemical composition is calcium sulfate dihydrate (CaSO 4 ·2H 2 O). However, different pretreatment methods can affect the amount of bound water. The products include CaSO 4 ·0.5H 2 O and CaSO 4 (Ou et al., 2023). In Xiaogan City, Hubei Province, the PG inventory still amounts to 215,800 tonnes. Globally, 100–300 million metric tons of PG are generated annually. However, only about 15% is recycled or put to beneficial use (Ou et al., 2023). Large quantities of PG are stockpiled outdoors. This not only occupies valuable land resources but also poses environmental risks from leaching acidic impurities, soluble phosphorus, and heavy metals contained in PG. These leachates can contaminate soil and groundwater. This severely hampers the sustainable development of the phosphate chemical industry (Wang et al., 2023; Wu et al., 2025). Achieving large-scale, high-value, and environmentally sound utilization of PG is essential to addressing the environmental challenges faced by the phosphate chemical industry while advancing circular economy principles. Road engineering, characterized by high material consumption, strong absorption capacity, and a relatively controllable service environment, is a key application scenario for PG resource utilization. Relevant research has been conducted in areas such as subgrade stabilization and cementitious materials for base courses, yielding phased results (Meskini et al., 2021; Xu et al., 2024; Tomašević Pilipović et al., 2025).

Early research on the application of PG in road construction primarily focused on inorganic binder-stabilized systems for subgrades and base courses. (Dutta and Kumar, 2016) systematically conducted road performance tests on a flyash–lime–PG composite system, confirming that, under optimized mix proportions (fly ash +8% lime +2% PG), the composite system could meet the strength, splitting strength, and durability requirements for road sub-base courses. (Meskini et al., 2021) used fly ash and lime to activate the cementitious properties of PG synergistically. The experimental mix proportions were PG (40%–80%), fly ash (20%–60%), and lime (4%–20%). By generating hydration products such as calcium aluminate, they significantly improved the unconfined compressive strength and wet–dry cycling resistance of the system, thereby demonstrating the feasibility of PG-stabilized road materials. Some studies have further combined solid waste materials such as red mud and blast furnace slag with PG. By synergistically activating these materials, they have produced road subgrade solidification materials with superior performance, thereby expanding the application scenarios for PG (Ma et al., 2022; Zhang et al., 2025). Among them, the optimal mix proportions were as follows: the dosage of cement–PG–modified polymer was 8% of the red mud (Ma et al., 2022), and the PG-to-red mud ratio was 1:4 (Zhang et al., 2025). Furthermore, researchers have optimized the solidification systems and preparation processes for PG-based cementitious materials used in stabilized road layers. The resulting pavement subbase and subgrade materials meet both strength and environmental requirements, validating their potential for engineering applications (Gu et al., 2018; Wang et al., 2023). The aforementioned studies have laid a solid foundation for the application of PG in road engineering. However, its overall applications have primarily been concentrated in the lower and middle structural layers of roads. In contrast, exploration of applications in the asphalt pavement surface course—which presents higher technical barriers and offers greater added value—remains relatively limited.

In recent years, as demand for high-value utilization of solid waste has increased, some researchers have begun exploring PG as a filler in asphalt mastic systems to assess its potential for use in asphalt surface courses. Addressing the widespread issue of low-temperature toughness loss associated with inorganic whisker modification, (Xiao et al., 2025) proposed a composite modification scheme using waste engine oil and anhydrous calcium sulfate whiskers. They demonstrated that 2% waste engine oil can effectively compensate for the decrease in low-temperature flexibility caused by whiskers through lubrication and softening effects, resulting in the low-temperature fracture toughness of the composite-modified mixture exceeding that of the reference group. To improve the interfacial compatibility between PG and asphalt, surface modification techniques have been gradually introduced. Calcium sulfate whiskers surface-modified with stearic acid or silane coupling agents can effectively fill surface defects, enhance interfacial adhesion with asphalt, and significantly improve asphalt’s high-temperature rheological properties and resistance to aging (Meng et al., 2025; Zhang et al., 2025). Building on this, composite modification systems combining PG whiskers with polymers such as SBS and polyurethane have also been preliminarily explored, confirming that the synergistic effect of rigid reinforcement and flexible toughening can simultaneously improve the high- and low-temperature performance of asphalt (Huang et al., 2025; Meng et al., 2025). Furthermore, hybrid reinforcement systems combining PG whiskers with polyester fibers and basalt fibers have been applied to asphalt mixtures, further enhancing the mixtures’ overall road performance through synergistic toughening effects at the micro- and nano-scales (Wang et al., 2020; He et al., 2024; Yang et al., 2024).

From the perspective of evaluation methods, asphalt mastic—as the core component of the continuous phase in asphalt mixtures—consists of asphalt and dispersed mineral filler particles. Its viscoelastic rheological behavior directly determines the mixture’s resistance to high-temperature shear deformation and the rate of fatigue damage evolution, making it a key factor in regulating the overall service performance of asphalt pavements (Wu et al., 2021; Zhou et al., 2023). Recently, researchers have used PG as a substitute for mineral powder, thereby expanding the range of applications for PG. Ou et al. (2023) systematically compared the performance of asphalt mastic (filler-asphalt ratio was 1.0) in which mineral powder was replaced with PG. They confirmed that PG fillers primarily enhance the high-temperature modulus and elastic recovery ratio of the binder through physical hardening effects, while reducing irreversible creep modulus; however, they also slightly reduce the fatigue life. Yin et al. (2025) used PG whiskers as an alternative filler to prepare asphalt mixtures. The filler-asphalt ratio was fixed at 1.0, and the ratios of PG to mineral powder were 40%–60% (by volume). They found that they improved high-temperature stability and water stability more effectively than ordinary Portland cement fillers, and could enhance the fatigue performance of the mixture through whisker bridging. The findings regarding the factors affecting the fatigue properties of asphalt mixtures here show a pattern opposite to that reported in the study by Ou et al.

In summary, the existing research on PG-asphalt mastic still has the following gaps: The application of PG in road construction has primarily focused on inorganic structural layers such as subgrade and base layer. Research on its use as a filler in asphalt surface layers lacks depth and a systematic approach, with no comprehensive comparisons with conventional mineral fillers. Different studies have even reached conflicting conclusions.

Research on PG-mineral powder composite filler systems remains unexplored. The synergistic effects of blending and the optimal dosage range remain unclear. At the same time, there is a lack of in-depth analysis of the intrinsic mechanisms underlying the viscoelastic structural transitions in PG mastic, which prevents mechanistic design of supporting materials for engineering applications.

Existing studies lack a quantitative assessment of the filler-phase environmental benefits and direct construction-period economic costs associated with PG-modified asphalt, particularly regarding solid waste consumption potential, carbon emission reductions during filler production, and direct material cost savings.

Based on this, 70 # base asphalt was used to prepare asphalt mastic containing mineral powder, PG as a single filler, or a PG–mineral powder composite filler. The dynamic shear rheometer (DSR) was used to conduct three tests: temperature–frequency scanning, MSCR, and LAS. A systematic comparative analysis was performed to examine the influence of filler type, filler-asphalt ratio, and composite blend content on the high-temperature rheological properties, creep recovery performance, and fatigue resistance of the asphalt mastic. By combining this analysis with the Black Curve, the evolution mechanism of the viscoelastic behavior of the mastic was interpreted, and the performance advantages of the PG filler, along with the optimal blending scheme, were identified. In addition, a quantitative assessment was conducted to evaluate the solid waste absorption capacity, carbon emission reductions, and economic feasibility of the composite system. A theoretical basis and key technical parameters are thus provided to support the large-scale, high-value application of PG solid waste in asphalt pavement layers.

This section provides a systematic overview of the basic properties of the materials used in the test. The procedure for preparing mastic samples, test conditions, and evaluation criteria for three types of rheological tests: temperature-frequency scanning, MSCR, and LAS.

Asphalt

Grade A 70 # base asphalt was used in the experiment, and its basic technical properties are presented in Table 1. All properties meet the requirements of the Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) (Ministry of Communications of the People’s Republic of China, 2004). The asphalt is black and solid at room temperature and exhibits typical viscoelastic characteristics at high temperatures. In this study, the base asphalt was used as the reference sample, and the evolution of the rheological properties of the asphalt mastic system after filler incorporation was compared.

Filler

The mineral powder used in the test was an alkaline limestone-based road construction powder. It has a uniform particle size distribution, an apparent density of approximately 2.5 g/cm 3, and a hydrophilicity coefficient of less than 1. It is the most commonly used conventional filler in asphalt mixtures. In this study, it was used as the control filler to compare the effects of PG modification.

The PG used in the tests was obtained as an industrial byproduct from a phosphate chemical plant. It was dried at 105 °C to remove free water before use. The image comparing PG and mineral powder is shown in Figure 1.

For ease of presentation, in the subsequent data figures, mineral powder, PG, and the composite filler are denoted by the subscripts mp, pg, and mix, respectively.

To assist in the mechanistic analysis of the subsequent experimental results, the two fillers were characterized using the following methods.

pH

The pH values of PG and mineral powder were measured and compared using a pH meter. The test procedure was as follows: 25 g of the sample was placed in a beaker, 100 mL of distilled water was added, and the mixture was stirred with a magnetic stirrer for 10 min to ensure thorough mixing. After standing for 5 min, the pH of the supernatant was measured with a pH meter. The pH of fillers directly affects the performance of asphalt mixtures; excessively high or low pH values can cause problems in asphalt mixture performance. The test results showed that the pH of the mineral powder was 8.67, indicating weak alkalinity, while the pH of PG was 6.36, indicating slight acidity, which poses no notable risk of acid corrosion to the mastic system.

Particle size distribution

The particle size distributions of mineral powder and PG were measured using a laser particle size analyzer, with water and anhydrous ethanol used as the dispersing media, respectively. Each sample was tested three times, and the average values were used for analysis. The results are shown in Figure 2. It can be seen that the particle size of mineral powder is slightly smaller than that of PG, which is consistent with the results reported by (Ou et al., 2023).

XRD

Before leaving the factory, the PG was washed and dried to remove hazardous substances, which may affect the amount of bound water. To investigate the specific morphology of the filler, XRD analysis was performed. XRD analysis was performed on the two fillers with a step size of 0.02° and a scanning range of 5°–90°. The results are shown in Figure 3.

The XRD results show that the mineral powder is dominated by calcite (CaCO 3), with characteristic diffraction peaks at 2θ = 23.1°, 29.4°, 35.9°, 39.4°, 43.1°, 47.5°, and 48.5°, corresponding to the (012), (104), (110), (113), (202), (018), and (116) crystal planes, respectively. The PG is dominated by calcium sulfate hemihydrate (CaSO 4 ·0.5H 2 O, bassanite), with principal peaks at 2θ = 14.7°, 25.6°, 29.7°, 31.9°, and 33.2°, corresponding to the (020), (021), (200), (041), and (−221) crystal planes, respectively. The absence of the characteristic dihydrate peak at approximately 11.6° (020).

Experimental design

Preparation of asphalt mastic

Asphalt mastic samples were prepared using a high-speed mixing method. The base asphalt was heated to 135 °C, and fillers were added in batches. The mastic was stirred uniformly at 1,000 r/min for 10 min until the system was homogeneous, free of agglomerates, and free of visible air bubbles. It was then allowed to stand to remove air bubbles before use.

Two types of mastic systems were tested, with the specific protocols as follows: Single-filler asphalt mastic: Using mineral powder and PG as single fillers, respectively. The filler-asphalt ratio was set at 0.6, 0.8, 1.0, and 1.2. This was used to analyze how filler type and dosage affect the rheological properties of the mastic.

PG-mineral powder composite asphalt mastic: With the filler-asphalt ratio fixed at 1.0, the proportion of PG by mass relative to the total filler mass was varied to establish a gradation of 0% (pure mineral powder mastic), 25%, 50%, 75%, and 100% (pure PG mastic). This was done to investigate the synergistic effects of the composite system and identify the optimal dosage range.

Rheological properties testing methods

Rheological tests were conducted using a dynamic shear rheometer (DSR, Figure 4) to characterize the viscoelastic behavior and in-service performance of the mastic by precisely controlling temperature, loading frequency, and strain level. All rheological tests were conducted twice, and the average value was adopted for result analysis. Based on the raw data, the CV values for G* and G * /sin δ were below 4%, those for R and J nr were below 5%, and the CV for fatigue life (N f) was below 10%, indicating good repeatability of the test results. Specific tests included the following: Temperature-frequency sweep

A 25 mm-diameter parallel-plate was used, with a test gap set at 1 mm. Preliminary tests confirmed that a 1% strain level fell within the sample’s linear viscoelastic range and was used as the control strain. Four temperature gradients (40, 50, 60, and 70 °C) were used, with a loading frequency range of 0.1–100 rad/s. The complex shear modulus (G *) and phase angle (δ)were measured under different temperature-frequency conditions, and the rutting factor (G * /sin δ) was calculated to characterize the resistance to rutting deformation at high temperatures. 2. MSCR Test

A 25 mm-diameter parallel-plate with a test gap of 1 mm was used. The test temperatures were set to 58, 64, and 70 °C, corresponding to the typical summer service temperature range for road pavements in China. Two levels of shear stress—0.1 kPa and 3.2 kPa—were applied. For each stress level, a cycling mode of 1 s creep loading and 9 s unloading and recovery was employed, with 10 loading cycles repeated.

Evaluation parameters were calculated: non-recoverable creep compliance (J nr) and creep recovery rate (R). Lower J nr and higher R indicate greater resistance to permanent deformation in mastic. 3. LAS Test

An 8-mm-diameter parallel-plate with a 2-mm test gap was used. The test temperature was set to 25 °C, and the loading frequency was 10 Hz. A strain-controlled loading mode was employed, with the strain amplitude linearly increasing from 0.1% to 30%. Continuous amplitude loading was used to simulate the accumulation of fatigue damage under alternating pavement loads. Based on the Viscoelastic Continuous Medium Damage (VECD) theory, the fatigue life N f was calculated at three strain levels (2.5%, 5%, and 10%) to evaluate resistance to fatigue cracking quantitatively.

Temperature-frequency sweep test analysis

Rheological properties of base asphalt

To determine the rheological properties of the reference mastic, temperature-frequency sweep tests were conducted. The test results for the G *, δ, and G * /sin δ are shown in Figure 5. Within the test temperature range, the viscoelastic structure of the base asphalt remained unchanged. A comparison of Figures 5a,c reveals that the frequency response curves of G * and G * /sin δ remain parallel at different temperatures, with only an overall shift in the modulus values. This indicates that the flow characteristics of the asphalt did not change with temperature, and that no abrupt changes in structural type or viscoelastic ratio mechanisms occurred, consistent with the time-temperature equivalence (TTS) principle. Furthermore, the curves are parallel, without any intersections or distortions, confirming that the selected strain and frequency ranges were appropriate. The base asphalt remained within the linear viscoelastic range throughout the testing, indicating that the data are reliable and free from structural damage or nonlinear rheological interference.

The high-temperature deformation resistance of the base asphalt decreased at temperatures above 60 °C. As the temperature rises to 60 °C, the G * and G * /sin δ curves shift downward, indicating that the base asphalt stiffness rapidly decreases. At high temperatures, it was highly susceptible to shear deformation and exhibited poor rutting resistance, indicating a decline in rutting resistance. Combined with the phase angle variation curve in Figure 5b, it can also be observed that between 40°C and 50 °C, the phase angle curve generally exhibited a linear trend on a semi-logarithmic scale. However, after reaching 60 °C, the curve’s frequency response changed. In the low-frequency range below 1 rad/s, the phase angle reached 90°, indicating that the asphalt exhibits almost entirely viscous behavior at this frequency. Since low-frequency loading corresponds to slow vehicle speeds, it can be predicted that the base asphalt’s high-temperature rutting resistance will decrease under high-temperature conditions or during slow vehicle loading.

Effect of single filler on rheological properties

Based on the existing evaluation system, the G * /sin δ can comprehensively reflect the resistance of asphalt and mastic to high-temperature deformation. In the following analysis, G*/sinδ was used as the primary evaluation indicator to examine the effects of mineral powder and PG fillers comprehensively—both individually and in combination—on the rheological properties of mastic. The results of temperature-frequency sweep tests for mastic with single fillers are shown in Figures 6, 7. Both mineral powder and PG can enhance the high-temperature deformation resistance of asphalt mastic while maintaining good linear viscoelastic response. As shown in Figures 6, 7, the G * /sin δ values of the mastic containing these two types of fillers were higher than those of the base asphalt. Under a loading frequency of 1 rad/s at 40 °C, the G * /sin δ for mineral powder and PG mastic (filler-asphalt ratio of 1.0) had rutting factors of 18,798.3 Pa and 28,022.6 Pa, respectively, at 40 °C, which are 3.10 and 4.62 times that of the base asphalt (6,063.0 Pa); at 50 °C, they were 3,225.9 Pa and 4,298.5 Pa, respectively, which were 3.17 times and 4.23 times that of the base asphalt (1,016.8 Pa); at 70 °C, they were 0.1979 Pa and 0.2939 Pa, respectively, which were 3.39 times and 5.03 times that of the base asphalt (0.0584 Pa). Furthermore, on a double-logarithmic coordinate system, the rutting factors of both types of asphalt-filler mastic exhibited linear growth with increasing frequency. The frequency response curves at the same temperature were parallel to each other. They showed no distortion, indicating that the addition of filler did not disrupt the asphalt’s original linear viscoelastic nature. This behavior may be attributed to the following proposed mechanism: filler particles fill the interstitial spaces between asphalt molecules and may adsorb light components, potentially forming a “filler–structurally bound bitumen” framework. Furthermore, the filler-asphalt ratio range (0.6–1.2) used in this experiment likely fell within the optimal range, which may allow the asphalt to sufficiently encapsulate the particles and reduce agglomeration. It should be noted that these mechanistic interpretations are inferred from rheological evidence and would benefit from direct microstructural validation (e.g., SEM or AFM) in future studies. This conclusion validated the core reinforcing role of filler in asphalt mixtures and provided a basis for determining filler dosage in mixture design.

Under the same temperature, frequency, and filler-asphalt ratio conditions, PG enhanced the high-temperature performance of asphalt mastic more effectively than mineral powder. Taking a filler-asphalt ratio of 1.0 and a loading frequency of 1 rad/s as an example, the rutting factor ratio between PG and mineral powder mastics was 1.49 at 40 °C, 1.33 at 50 °C, and 1.48 at 70 °C. Similar proportional relationships were observed for other filler-asphalt ratios. This is consistent with the patterns and orders of magnitude of the effects of PG on rheological properties reported in the literature (Ou et al., 2023).

PG with a high filler-asphalt ratio could enhance the high-temperature deformation resistance of asphalt mastic. Notably, as temperature increased from 50 °C to 60 °C, the G * /sin δ values of all mineral powder asphalt mastic decreased by 2 to 3 orders of magnitude. However, when the filler-asphalt ratio was 1.0 or higher, PG asphalt mastic still maintained relatively high G * /sin δ values (as shown in the marked section of Figure 7c). When the temperature reached 70 °C, even asphalt with a high PG content showed a marked decrease in G * /sin δ values, indicating that the reinforcing effect of PG diminishes as the temperature continues to rise. This may be because the 50 °C–60 °C range is the critical transition zone where asphalt shifts from a viscoelastic to a visco-fluid state. It is also possible that PG particles at a high filler-asphalt ratio may form a continuous spatial skeleton; their surface-active sites could strongly adsorb the asphalt’s acidic components, potentially forming a thick, stable structurally bound bitumen film that may suppress viscous flow through a proposed dual mechanism of “skeletal support and interfacial reinforcement” (Ma et al., 2022). However, when the temperature rises to 70 °C, the thermal motion of asphalt molecules may exceed the interfacial binding energy and the shear strength of the proposed skeleton, potentially leading to the dissociation of the structurally bound bitumen film and the slip of the particle skeleton. Thereby diminishing the reinforcing effect of PG. These analytical results provide a theoretical basis for determining the appropriate temperature range for PG-asphalt mastic.

Effect of composite fillers on rheological properties

To investigate the effect of blending PG with mineral powder on the rheological properties of asphalt mastic, temperature-frequency sweep tests were conducted on asphalt mastic with different PG contents at a filler-asphalt ratio of 1.0. The test results are shown in Figure 8. The use of composite fillers to regulate the high-temperature deformation resistance of asphalt mastic exhibited a threshold effect with respect to the blending ratio. At low blending ratios (25%), performance decreased slightly in the medium-to high-temperature range. However, at a blending ratio ≥50%, performance increased. An analysis of the rutting factor under 1 rad/s showed that the increase in the composite mastic compared to the single-mineral powder mastic is illustrated in Figure 9 (analysis at 60 °C and above is conducted separately). It can be observed that the rutting factor increases gradually as the PG content increases. When the content reached 50% or higher, the reinforcing effect of the PG particles gradually increased. Therefore, considering high-temperature deformation resistance alone, low-content formulations below 25% should be avoided in engineering applications. A reasonable content range of 50%–75% should be selected based on actual requirements.

The high-temperature performance of the composite filler mastic within the critical temperature range of 60 °C was improved compared to that of the single-mineral powder mastic. However, it remained lower than that of the single-PG-asphalt mastic. As indicated by the markings in Figure 8c, the rutting factor of the mastic gradually increased with the addition of PG. However, it did not exhibit the order-of-magnitude jump observed in the single-PG mastic. This may be explained as follows: as the temperature rises, the PG content in the composite system may not reach the critical concentration required to trigger strong interfacial adsorption, which could prevent the formation of a continuous three-dimensional network skeleton. Consequently, performance may only be marginally improved through localized skeletal reinforcement. In engineering applications requiring good rutting resistance at the critical temperature of 60 °C, a single-component PG filler system should be used.

Analysis of the viscoelastic behavior of asphalt mastic based on the Black Curve

The above analysis showed that the addition of filler did not alter the viscoelasticity of asphalt. However, a comparison of the G * and δ curves revealed that the viscoelastic behavior of different samples at different temperatures still varies notably, with a major impact on their in-service performance. To further analyze the mechanism by which the filler affects the viscoelastic behavior of the asphalt binder, the Black Curve is employed in this section. The Black Curve plots lg| G * | on the y-axis and δ on the x-axis, thereby eliminating the influence of loading frequency.

The base asphalt curve shown in Figure 10 was first analyzed. The curve displayed a smooth, continuous distribution of data points, and its shape remained essentially consistent across different temperatures. In the 40 °C–50 °C range, the curve shifted to the right with increasing temperature, indicating a decrease in elastic properties and an increase in viscous properties. As the temperature continued to rise above 60 °C, the curve shifted downward overall. At this point, the test temperature exceeded both the asphalt softening point and the asphalt glass transition temperature. The viscous components of the base asphalt became dominant, and its elastic recovery capacity at high temperatures is weak. It is the fundamental reason for its poor high-temperature deformation resistance.

The Black Curve comparisons across different combinations of filler types and content are shown in Figures 11–13 (the x- and y-axis scales are consistent across all three figures). The mineral powder-asphalt mastic enhanced overall stiffness solely through physical filling, without altering the intrinsic viscoelastic transition behavior. As shown in Figure 11, as the filler-asphalt ratio increased from 0.6 to 1.2, the Black Curve gradually shifted toward lower phase angles and higher modulus, indicating that the elastic proportion of the mastic and its overall stiffness increased in tandem with the filler content. However, across all samples, the data points in the 40°C–70 °C temperature range were continuously distributed along a single smooth curve, with no obvious segments or inflection points. This indicated that the addition of mineral powder did not disrupt the time-temperature equivalence of the asphalt system, and the mastic consistently maintained a uniform viscoelastic structure. It should be noted that, for all filler-asphalt ratios, the data points at 60 °C highly overlapped with those at 70 °C, both falling within the high-phase-angle viscous region. This indicated that mineral powder cannot increase the viscoelastic transition temperature of the asphalt, and that viscous flow remained the dominant behavior of the binder at 60 °C. This finding is corroborated at the intrinsic structural level by the experimental results in Section 3.1.2, which showed an order-of-magnitude decrease in the rutting factor of the mineral powder-modified asphalt at 60 °C.

PG with a high filler-asphalt ratio could induce a viscoelastic structural transition in the asphalt mastic, increasing the elastic component at critical high temperatures. In PG asphalt mastic, high-temperature sensitivity increased markedly once the filler-asphalt ratio reached a certain level. This is manifested by a change in the Black Curve at 60 °C when the ratio reached 1.0. The distribution position on the coordinate axes shifted from the previous range corresponding to 70 °C to a range consistent with 40 °C–50 °C. It indicated that incorporating PG altered the compositional characteristics of the asphalt mastic, thereby improving its high-temperature stability.

The viscoelastic behavior of the PG–mineral powder composite mastic evolved gradually and did not trigger a critical structural transition. As shown in Figure 13, as the PG content increased from 25% to 75%, the Black Curve gradually shifted toward lower phase angles and higher modulus. The elastic fraction of the mastic gradually increased with increasing content, but the overall curve remained continuous and smooth, with no obvious separation at any temperature measurement points. It indicated that the composite system did not alter the homogeneous rheological structure. Specifically, at a PG content of 25%, the shift in the curve was minimal, and its shape closely resembled that of a mastic with mineral powder alone. This corresponded to the threshold effect described earlier, where performance improvements at medium and high temperatures were not notable at low content levels. At 50% and 75% content, the elastic proportion of the mastic gradually increased with increasing content. However, it remained in the viscous region at 60 °C with a high phase angle and did not shift toward the medium-temperature region. This may indicate that the blended system did not reach the critical concentration required to trigger strong interfacial adsorption. Performance improvements may be attributed primarily to physical filling and localized skeletal reinforcement. It is consistent with the conclusion in Section 3.1.3 that the composite mastic did not exhibit an order-of-magnitude performance leap at 60 °C.

MSCR test analysis

Temperature-frequency scanning tests can characterize the high-temperature rheological parameters of materials within the linear viscoelastic range. In comparison, MSCR tests can simulate the cyclic heavy-load shear effects experienced by actual road pavements, more closely reflecting the evolution of permanent deformation under service conditions. In this section, tests were conducted at two stress levels (0.1 kPa and 3.2 kPa) and at three typical temperatures (58, 64, and 70 °C). The R and J nr were used to systematically reveal the mechanisms by which filler type and content regulate the high-temperature resistance to permanent deformation and stress stability of asphalt mastic. The test results are shown in Figures 14–16. 1. The deformation resistance of mineral powder asphalt mastic was superior to that of base asphalt. However, the performance degraded markedly under the combined effects of high temperature and high stress. Specifically:

As shown in Figure 14, the J nr of the mineral powder mastic decreased monotonically as the filler-asphalt ratio increased under two stresses, while R increased simultaneously. Moreover, their overall performance was far superior to that of base asphalt. At 58 °C, the J nr0.1 values of mineral powder mastic with filler-asphalt ratios ranging from 0.6 to 1.2 were 64.9%, 72.0%, 78.3%, and 84.5% lower, respectively, than that of the base asphalt (3.40 kPa -1). Corresponding to increases in R 0.1 of 46.2%, 149.2%, 203.1%, and 300.3%, respectively. This indicated that the filling and adsorption effects of mineral powder particles can effectively reduce the proportion of viscous components and enhance the elastic recovery capacity of the mastic. Under a heavy load stress of 3.2 kPa, the J nr3.2 values of samples with the same filler-asphalt ratio decreased by 61.2%, 68.4%, 78.3%, and 83.5%, respectively, compared to the base asphalt. The R 3.2 values increased by 122.0%, 187.8%, 287.8%, and 329.3%, respectively. Compared with samples under 0.1 kPa stress, the R 3.2 values of samples with various filler-asphalt ratios decreased by 66.5%, 74.4%, 71.7%, and 76.3%, respectively, indicating that under high stress, the proportion of elastic deformation in the mastic decreased. In contrast, the proportion of permanent deformation increased markedly.

An increase in temperature degraded the high-temperature performance of the mineral powder asphalt mastic. Taking the sample with a filler-asphalt ratio of 1.0 as an example, when the temperature rose from 58 °C to 64 °C, J nr0.1 and J nr3.2 increased to 1.69 and 1.82 kPa -1, respectively, representing increases of 129.4% and 138.3%. R 0.1 and R 3.2 decreased to 3.63% and 0.52%, respectively, representing a decline of 35.4% and 67.3%. This pattern aligned with the abrupt drop in the rutting factor observed near 60 °C during the temperature-frequency sweep in Section 3.1, confirming the intrinsic characteristic of the mineral powder mastic to rapidly transition to a visco-fluid state within the critical high-temperature range. As the temperature rose to 70 °C, J nr3.2 reached 4.09 kPa -1; only the sample with a filler-asphalt ratio of 1.2 retained a slight elastic recovery capacity of 0.07%. R 3.2 for all other samples dropped to 0, indicating that the mastic had essentially entered a purely viscous flow state.

The creep recovery performance of PG asphalt mastic was superior to that of mineral powder mastic. It exhibited shear-thickening characteristics at high filler-asphalt ratios, and this performance advantage was amplified in the critical high-temperature range. Specifically:

As shown in Figure 15, under identical test parameters, the J nr values of PG mastic were lower than those of mineral powder mastic, while the R values were higher. The modification effect on the base asphalt was far stronger than that of conventional mineral powder fillers. Specifically, at 58 °C and 0.1 kPa stress, the J nr0.1 values of PG mastic with filler-asphalt ratios of 0.6–1.2 were 1.3%, 30.3%, 57.6%, and 68.1% lower, respectively, than those of mineral powder mastic with the same filler-asphalt ratios. A comparison of R 0.1 values showed that the 0.6 filler-asphalt ratio resulted in a 56.3% increase; the 0.8 ratio remained essentially unchanged. The 1.0 and 1.2 ratios showed marked increases of 110.0% and 128.3%, respectively.

The elastic recovery rate of samples with a filler-asphalt ratio of 1.0 or higher doubled, suggesting that high-content PG may form a stable, elastic-reinforcing structure within the mastic. Under a heavy-load stress of 3.2 kPa at 58 °C, the J nr 3.2 values of PG mastic with filler-asphalt ratios of 1.0 and 1.2 were 59.4% and 74.0% lower, respectively, than those of mineral powder mastic. It is noteworthy that the J nr 3.2 and J nr 0.1 values of the high filler-asphalt ratios were essentially equal or even slightly lower, exhibiting a weak shear-thickening characteristic. This may be attributed to a spatial skeleton potentially formed by the interlocking of PG particles. Under moderately high stress, particle rearrangement and structural compaction may occur, thereby potentially enhancing the overall resistance to shear deformation. This advantageous characteristic under heavy-load pavement conditions is not present in mineral powder mastic. ② When the temperature rose to 64 °C, the performance advantages of the PG mastic became even more pronounced. The J nr 3.2 value for the sample with a filler-asphalt ratio of 1.0 was 0.75 kPa -1, a 58.8% reduction compared to the mineral powder mastic. The R 3.2 value was 1.09%, which is 2.1 times that of the mineral powder mastic. The performance gap between the two types of fillers widened compared to 58 °C. This conclusion is consistent with the literature (Ou et al., 2023). This confirms, from the perspective of permanent deformation, that high PG content delayed the viscoelastic transition of the asphalt mastic. At the extremely high temperature of 70 °C, the J nr 3.2 of the PG mastic with a filler-asphalt ratio of 1.0 was still only 1.75 kPa -1, a 57.2% reduction compared to the mineral powder mastic. Moreover, R 3.2 maintained an effective elastic recovery capacity of 0.44%, whereas the mineral powder mastic had essentially lost it. This indicated that PG can enhance the high-temperature stability of the mastic through its intrinsic structure, enabling it to retain some deformation recovery even at extremely high temperatures and heavy loads.

The performance of the composite mastic improved gradually as the PG content increased. A content threshold effect was observed across all cases at both stress levels, accompanied by a simultaneous improvement in stress stability. Specifically:

As shown in Figure 16, under a filler-asphalt ratio of 1.0, the results of both stress tests indicated that the composite mastic’s resistance to permanent deformation at high temperatures gradually increased with rising PG content, and a clear content threshold existed. At 58 °C and 0.1 kPa stress, the J nr 0.1 values of composite mastic with 25%, 50%, and 75% PG content were 18.4%, 18.3%, and 29.9% lower, respectively, than those of pure mineral powder mastic. Correspondingly, for R 0.1 at low PG content levels (25%–50%), the elastic recovery rate was similar to that of the pure mineral powder mastic. However, the rate of improvement accelerated at content levels of 75%–100%, exhibiting a clear threshold characteristic. Under a heavy load stress of 3.2 kPa at 58 °C, the J nr 3.2 values of the composite mastic with 25%, 50%, and 75% PG content were reduced by 14.5%, 17.6%, and 27.7%, respectively, compared to the pure mineral powder mastic. R 3.2 values were 1.27%, 1.20%, and 1.26%, respectively. At low blending ratios, the values were even slightly lower than those of the pure mineral powder mastic, which may indicate that the addition of a small amount of PG interfered with the mineral powder’s original dense packing structure. The elastic recovery capacity under high stress was not effectively enhanced. Only when the blending ratio reached 75% did the skeletal reinforcement effect of PG begin to become apparent.

At 64 °C, the J nr 3.2 value of the composite mastic with a 75% blend ratio was 27.6% lower than that of the pure mineral powder mastic, further widening the performance advantage observed at 58 °C. However, it remained 76.3% higher than that of pure PG mastic (0.75 kPa -1), and no performance leap was observed. This indicated that the composite system did not reach the critical concentration required to trigger continuous spatial scaffolding and strong interfacial adsorption, a finding consistent with the conclusions of the Black Curve analysis. Under the coupled effects of high temperature (70 °C) and high stress (3.2 kPa), the R 3.2 values for the composite mastic with 25% and 50% PG content turned negative, indicating that the mastic underwent delayed plastic deformation. Deformation continued to develop even after unloading, and elastic recovery capacity was completely lost. Only the 75% blend sample retained a slight positive recovery. In comparison, the pure PG mastic maintained a recovery ratio of 0.44%, further confirming the positive correlation between PG content and the mastic’s high-temperature structural stability.

LAS test analysis

LAS tests can simulate the evolution of fatigue cracking under alternating pavement loads by continuously increasing shear amplitudes. They can effectively characterize the fatigue damage resistance of asphalt mastic at different strain levels. In this section, three strain levels—2.5%, 5%, and 10%—were selected. Fatigue life (N f) was adopted as the evaluation criterion. The influence of filler type, filler content, and the blending method of PG and mineral powder on the fatigue performance of asphalt binders was analyzed, and the fatigue damage mechanisms of different binder systems were revealed. The test results are shown in Figures 17–19. The fatigue performance of mineral powder asphalt mastic decreased as the filler-asphalt ratio increased. As shown in Figure 17, the fatigue life of the mineral powder asphalt mastic decreased monotonically with increasing filler-asphalt ratio. It was lower than that of the base asphalt at all strain levels. This indicated that mineral powder fillers enhanced high-temperature stiffness but reduced fatigue-crack resistance. Taking the 10% high-strain level as an example, the N f values of mineral powder asphalt mastics with filler-asphalt ratios ranging from 0.6 to 1.2 were reduced by 90.1%, 93.2%, 96.6%, and 98.3%, respectively, compared to the base asphalt. At the low strain level of 2.5%, the corresponding N f values decreased by 90.0%, 92.6%, 96.4%, and 98.0%, respectively. The extent of deterioration was essentially consistent across different strain levels, indicating that the negative impact on fatigue performance does not vary fundamentally with strain level. This was because mineral powder particles primarily enhance the modulus of the mastic through physical filling. As the filler-asphalt ratio increased, the proportion of free asphalt continued to decrease, resulting in reduced deformability and increased brittleness. Under cyclic shear loading, stress concentrations may be more likely to occur at the particle–bitumen interfaces, potentially accelerating the initiation and propagation of microcracks, which may ultimately lead to the observed decline in fatigue life as filler content increases (Al-Mosawe et al., 2026).

The fatigue performance of PG asphalt mastic followed a trend of first increasing and then decreasing with the filler-asphalt ratio, exhibiting fatigue resistance advantage at the critical filler-asphalt ratio. As shown in Figure 18, as the filler-asphalt ratio increased, the PG mastic reached its peak at a filler-asphalt ratio of 1.0, which differed markedly from the monotonically deteriorating behavior of mineral powder mastic. Under low filler-asphalt ratios (≤0.8), the N f of PG mastic is lower than that of mineral powder mastic at the same ratio. For example, at a 5% strain level, the N f values of PG mastic with filler-asphalt ratios of 0.6 and 0.8 were 76.5% and 55.2% lower, respectively. This may be explained as follows: at low blending ratios, PG particles may be dispersed and unable to form a continuous matrix. Moreover, local stress concentrations potentially induced by the particles’ sharp edges may accelerate the initiation of microcracks. At this stage, the enhanced interfacial adsorption effect has not yet been fully realized, resulting in overall fatigue resistance that is weaker than that of mineral powder mastic with a more uniform particle size distribution. When the filler-asphalt ratio increased to 1.0, the fatigue performance reached an inflection point and surpassed that of mineral powder mastic. For example, at 10% strain, the N f value was 36.7% higher than that of mineral powder mastic; at 2.5% strain, it was 92.2% higher. However, when the filler-asphalt ratio was increased to 1.2, the asphalt was unable to encapsulate the excess filler particles fully. This led to an increase in interfacial defects and a thinning of the asphalt film, causing the mastic’s brittleness to rise again and resulting in a further decrease in fatigue life at 10% strain.

The fatigue performance of the PG-mineral powder composite mastic exhibited a dosage threshold. A 50% dosage achieved the optimal balance between high-temperature and fatigue performance. As shown in Figure 19, under a filler-asphalt ratio of 1.0, the N f of the composite mastic first increased and then gradually declined as the PG content rose, reaching a peak fatigue life at a 50% PG content, with fatigue resistance far superior to that of mastic containing a single filler. Specifically, at a low strain level of 2.5%, the N f values of the composite mastic with 25%, 50%, and 75% PG content increased by 302.3%, 1772.9%, and 1,009.1%, respectively, compared to the pure mineral powder mastic (6,140 cycles). At a high strain level of 10%, they increased by 252.4%, 1,389.8%, and 702.7%, respectively. Among these, the composite mastic with a 50% PG content exhibited the best fatigue performance, achieving a fatigue life of 67.3% of that of the base asphalt at 2.5% strain—far exceeding that of single-filler systems—and successfully balancing high-temperature stiffness with fatigue toughness. When the PG content exceeded 50%, the particle size distribution balance was disrupted, the modulus of the mastic continued to rise, brittleness gradually became apparent, and fatigue performance consequently declined. Combined with the high-temperature rheological properties discussed in Section 3.2, it was evident that the composite mastic with a 50% PG content not only exhibited superior resistance to permanent deformation at high temperatures compared to pure mineral powder mastic but also achieved an order-of-magnitude improvement in fatigue resistance. This effectively resolved the technical contradiction in conventional filler systems where it is difficult to simultaneously enhance high-temperature stability and fatigue resistance, making it the composite formulation with the optimal overall performance and engineering application value.

It should be noted that, when considering the fatigue performance of PG as a complete replacement, this shows a trend opposite to that reported in the literature (Ou et al., 2023), but consistent with the trend of improved fatigue performance in PG asphalt mixtures (Yin et al., 2025). More importantly, this study clearly demonstrates the nonlinear evolution of fatigue properties in composite fillers as a function of PG content. This serves as a valuable supplement to existing research and will help advance the practical application of PG fillers.

Environmental and economic analysis

In addition to optimizing mechanical and rheological performance, evaluating the potential environmental benefits and construction-period economic feasibility of industrial solid waste valorization is essential for assessing its practical engineering applicability.

Quantitative evaluation of environmental impact and solid waste consumption

To clarify the environmental contributions of the composite mastic system, a functional unit model of a “1-km standard two-way, four-lane highway asphalt surface course” was established based on general highway engineering technical specifications to quantify raw material consumption, solid waste absorption, and carbon emissions.

The basic boundary conditions of the model are defined as follows: paving length 1,000 m, width 20 m, surface course thickness 5 cm (total surface volume 1,000 m 3). The typical compacted density of the asphalt mixture is 2.4 t/m 3 (total mixture mass 2,400 t/km). Based on the optimal filler–asphalt ratio of 1.0 determined in this study, the filler mass proportion in the mixture is set at 5.0%, corresponding to a total filler demand of 120 t per km of highway surface course. The comparison scenarios are: the control group (0% PG, i.e., 100% limestone mineral powder, amounting to 120 t) and the optimal recommended group (50% PG composite system, i.e., 60 t PG and 60 t mineral powder).

Solid Waste Consumption Potential

Under the optimal recommended mix proportion, 1 km of highway surface course directly consumes 60 t of PG solid waste. Based on the historical inventory of 215,800 t in Xiaogan City, applying this composite filler system to regional highway network construction would require paving only about 3,596 km of equivalent lanes to completely clear the local historical stockpile. Considering that the total highway mileage in the Xiaogan region reached 19,256 km, this demand accounts for only 18.6% of the local network. Furthermore, since many major highways in this region exceed four lanes and the filler can also be utilized in underlying asphalt binder and base layers, this required percentage would be even lower in practice. This provides a high-capacity, low-risk, and rapid absorption pathway for regional bulk chemical solid waste.

Quantitative Reduction of Carbon Emissions During Filler Production

Conventional limestone mineral powder production involves energy-intensive processes such as quarrying, coarse crushing, grinding, and screening. According to regional baseline data for road construction materials, its greenhouse gas emission factor for production is approximately 35 kg CO 2 e/t (Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2019). Conversely, PG is a byproduct of wet-process phosphoric acid production, meaning its upstream carbon emissions are allocated to the chemical production phase. Utilizing it as an alternative road filler requires only simple low-temperature drying and short-distance transport, with a conservative direct carbon emission factor for processing and transport estimated at <10 kg CO 2 e/t (Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2019).

Consequently, the direct raw material carbon emission reduction (△ E CO2) per km of composite mastic asphalt surface course is calculated as Equation 1:

This calculation indicates that replacing 50% of conventional mineral powder with PG reduces the raw material carbon footprint of the filler phase by 42.9%, making a positive contribution to low-carbon roadway construction.

Quantitative assessment of economic benefits during construction period

Currently, the comprehensive delivered price of high-quality limestone mineral powder for road construction in the regional market is approximately 180 RMB/t. In contrast, as an industrial waste urgently awaiting disposal by chemical producers, PG has a near-zero ex-factory raw material cost (excluding local municipal treatment subsidies in Xiaogan for a conservative estimate). Accounting for basic loading, unloading, storage, and short-distance handling expenses, its comprehensive utilization cost is only about 40 RMB/t.

Therefore, the direct material cost savings (△ C mat) for filler per kilometer of highway surface course during initial construction as Equation 2:

This demonstrates that under the premise of maintaining existing construction equipment and procedures, procurement costs for the filler phase can be directly reduced by 38.9%.

PG is an industrial solid waste generated in enormous quantities. A new pathway for its effective disposal and resource recovery can be provided by partially replacing mineral powder with PG as a filler. In this study, temperature–frequency scanning, MSCR, and LAS tests were conducted to compare the effects of mineral powder systematically, PG as a single filler, and a PG–mineral powder composite filler on the temperature–frequency response characteristics, high-temperature creep recovery capacity, and fatigue resistance of asphalt mastic. The main conclusions are as follows: There are differences in the rheological properties between asphalt mastic and base asphalt. Therefore, it is necessary to conduct a comprehensive evaluation of rheological properties of mastic that better reflects its actual in-service performance. Within a filler-asphalt ratio range of 0.6–1.2, the rutting factor of filler-modified asphalt paste can reach several times that of base asphalt. The complex shear modulus and the proportion of viscoelastic components changes. The reinforcement mechanisms of different fillers differ markedly. Relying solely on base asphalt indicators cannot accurately reflect the actual in-service behavior of the mastic in the mixture. Traditional evaluations of asphalt rheological properties should shift toward assessments of asphalt mastic.

The high-temperature rheological and creep recovery properties of PG asphalt mastic are superior to those of mineral powder mastic. However, its fatigue resistance exhibited a distinct threshold effect with respect to filler-asphalt ratio. Temperature-frequency scanning and MSCR tests indicated that the high-temperature performance of PG-asphalt mastic was better than that of mineral powder mastic, and it also exhibited slight shear thickening characteristics at high filler-asphalt ratios. LAS tests indicated that at low filler-asphalt ratios (≤0.8), the fatigue life of PG mastic was lower than that of mineral powder mastic with the same content. However, once the filler-asphalt ratio reached 1.0, its performance outperformed that of traditional mineral powder mastic, with the improvement particularly pronounced at a low strain level of 2.5%.

Both the high-temperature and fatigue properties of PG-mineral powder composite mastic exhibited threshold effects with respect to the content. At a filler-asphalt ratio of 1.0 adopted in this work, a 25% blending ratio resulted in only limited performance improvement. Once≥50% was reached, high-temperature performance gradually improved with increasing proportion. Fatigue life followed an initial increase, then a decrease, with blending ratio, peaking at 50%, and overall performance was superior to that of pure mineral powder mastic systems.

Comprehensive analysis indicated that the composite system with a 50% PG content exhibited the best overall performance and was recommended as the preferred blending scheme for engineering applications. At a 50% blending ratio, the fatigue life of composite mastic at 2.5% strain was 17.7 times higher than that of pure mineral powder mastic, while its high-temperature deformation resistance was also superior. This approach balances mastic-scale rheological performance with solid waste consumption potential and demonstrates a favorable pathway for simultaneously improving high-temperature stability and fatigue resistance at the mastic scale.

The 50% PG composite system shows favorable potential for solid waste valorization at the filler phase. Under the simplified estimation framework adopted in this study, replacing 50% of limestone mineral powder with PG can consume 60 t of PG solid waste per kilometer of highway surface course, reduce filler-phase production carbon emissions by 42.9%, and lower direct filler material costs by 38.9% during the construction period.

This study clarified the rheological reinforcement mechanism and performance advantages of PG filler at the mastic scale. However, the research system can be further refined. In future research, we should focus on the pre-treatment Process, micro-scale modification mechanisms, the evolution of other properties of asphalt mastic, and the evaluation of the corresponding asphalt mixture properties and their engineering applications. This would reveal the intrinsic mechanisms of performance regulation at the microscopic level, providing more systematic theoretical support and technical parameters for the large-scale application of PG solid waste in road engineering.