Multi-scale rheological properties of municipal solid waste fly ash-asphalt mastic materials

2.1. Materials

2.2. Methods

3.1. Brookfield viscosity test

Fig. 1 shows the Brookfield viscosity test results of MSWI fly ash-modified asphalt mastics with different replacement ratios. Under a constant filler-bitumen ratio of 0.8, the Brookfield viscosity of all fly ash-modified asphalt mastics exhibited a general decreasing trend with increasing temperature. Notably, the viscosity differences between different samples became less significant when the temperature reached 175 °C. At any given test temperature, the viscosity of the asphalt mastics increased progressively with higher fly ash content. For instance, at 135 °C, the viscosity of the control sample (with pure mineral filler) was measured at 1937 mPa·s, while the sample containing 100 % fly ash replacement showed a substantially higher viscosity of 4105 mPa·s. This marked viscosity enhancement demonstrates the superior effect of fly ash compared to conventional mineral filler. The observed viscosity improvement may be attributed to the unique physical characteristics of fly ash, including its finer particle size distribution and well-developed porous structure. As the fly ash content increases, its larger specific surface area provides more active sites for interaction with the asphalt binder, thereby significantly enhancing the overall viscosity of the mastic system.

Fig. 1Brookfield viscosity test results for fly ash- asphalt mastics at different temperatures

3.2. Frequency sweep test

The frequency sweep test results of fly ash-asphalt mastics with different replacement ratios are shown in Fig. 2. Fig. 2 displays the curves of complex shear modulus ($G^{*}$*) and phase angle ($\delta$) versus frequency for fly ash-asphalt mastics. Analysis of Fig. 2 reveals that the complex shear modulus ($G^{*}$) of all fly ash-asphalt mastics gradually increases with increasing frequency. Since all curves exhibit similar growth trends under different frequencies, a specific frequency point (10 rad/s) was selected for comparative analysis. The bar chart in Fig. 2 shows that the complex modulus ($G^{*}$) of fly ash-asphalt mastics increases with higher fly ash replacement ratios. Specifically, when the fly ash replacement ratio reaches 100 %, the $G^{*}$value increases by 6 kPa compared to mineral filler-asphalt mastic, demonstrating that replacing mineral filler with fly ash can effectively enhance the complex modulus of asphalt mastics.

3.3. Low-temperature rheological properties of MSWI fly ash-modified asphalt mastics

Fig. 3 shows the creep stiffness (S) and creep rate (m) of two types of MSWI fly ash-modified asphalt mastics with different replacement ratios. As shown in Fig. 3, at the same replacement ratio, the creep stiffness S of all fly ash-modified asphalt mastics increased with decreasing temperature, indicating poorer low-temperature cracking resistance at lower temperatures. Under identical temperature conditions, the S value gradually rose with increasing fly ash content. Both temperature change and fly ash replacement ratios significantly affected the S values of the asphalt mastics. Specifically, when the temperature was –6 °C and the fly ash replacement ratio increased from 25 % to 100 %, the S value increased by 4.7 % to 25.25 % compared to the pure mineral filler asphalt mastic. At –12 °C, the S value increased by 7.1 % to 34.3 %, while at –18 °C, the increase ranged from 0.3 % to 7.3 %.

Fig. 2Complex shear modulus of MSWI fly ash-modified asphalt mastics

Fig. 3Creep stiffness (S) and creep rate (m) of MSWI fly ash-modified asphalt mastics

As evidenced by Fig. 3, all fly ash-modified asphalt mastics exhibited lower m-values compared to pure mineral filler mastics under identical testing conditions. The analysis demonstrates three critical findings: At any given temperature, the m-value showed a progressive decline with higher fly ash replacement ratios. For each fixed replacement ratio, the m-value decreased consistently with reduced temperature. These trends collectively indicate that fly ash incorporation diminishes the low-temperature cracking resistance of asphalt mastics. The underlying mechanism involves the fly ash’s adsorption of certain asphalt components, which produces more pronounced hardening effects than limestone mineral filler. This results in impaired stress relaxation capacity (quantified by reduced m-values) and consequently poorer resistance to thermal cracking at low temperatures.

3.4. Rolling thin film oven (RTFO) aging test

Fig. 4 displays the softening point changes of fly ash-asphalt mastics before and after thin film oven aging. The test results show that after undergoing thin film oven aging, the softening points of all fly ash-asphalt mastics increased, and the magnitude of softening point increment gradually decreased with higher fly ash replacement ratios. The softening point increment of Dongguan fly ash-asphalt mastics was slightly smaller than that of Guangzhou fly ash-asphalt mastics. When the replacement ratio reached 100 %, the softening point increments of Dongguan and Guangzhou fly ash-asphalt mastics decreased to 0.45 °C and 0.5 °C respectively, compared to pure mineral filler-asphalt mastics. This phenomenon may be explained by the following reasons: under high-temperature oxidation, the light components in asphalt partially volatilize while resins transform into asphaltenes, causing the asphalt to exhibit reduced plasticity and consequently an increased softening point. As the fly ash content increases, its microporous structure and larger specific surface area can adsorb more light fractions, increasing the asphaltene content and thereby improving the high-temperature stability of asphalt, which leads to the gradual reduction in softening point increment.

Fig. 4Softening point increment of fly ash- asphalt mastics before and after aging