Quantitative characterization of residual water and multi-scale interfacial adhesion analysis for foamed asphalt under different foaming methods

Foamed warm mix asphalt (FWMA) has become an important technological approach for achieving low-carbon road construction because it can reduce mixing and paving temperatures while maintaining favorable workability. However, the water introduced during the foaming process does not completely escape as vapor, and a portion remains in the asphalt system in the form of microbubbles or adsorbed water, potentially affecting interfacial adhesion performance and moisture damage resistance. To address this issue, 70# matrix asphalt (MA) was used in this study to prepare foamed asphalt with three warm mix agents, namely tap water (DW), synthetic zeolite (SZ), and modified activated sintering red mud (MASRM), while limestone and basalt were selected as aggregates. Residual water evolution was characterized through viscosity-time analysis, X-ray microscopic observation, regional image statistical analysis, and mass loss tests. Asphalt-aggregate interfacial performance was evaluated at multiple scales using surface free energy (SFE) theoretical calculations and an improved pull-off test (IPOT). On this basis, the correlation between residual water content and adhesion performance was further investigated. The results show that different foaming methods significantly affect residual water evolution and interfacial adhesion performance. Within the first 3 min, the residual water contents of DW and MASRM decrease by approximately 84% and 90%, respectively, whereas the reduction for SZ is less than 50%, indicating the slower release of bound water from SZ. The water bubble area results show a consistent overall trend, with SZ exhibiting the largest residual water bubble area and MASRM the smallest. The mineral components in the warm mix agents increase the surface polarity of foamed asphalt, thereby contributing to improved interfacial adhesion performance. MASRM exhibits the lowest residual water level and the best overall interfacial performance. At 120 min, the bond strength between MASRM foamed asphalt and limestone reaches 1.88 MPa, which is 12% higher than that of MA. In contrast, the higher residual water content of SZ limits its improvement in interfacial adhesion performance. The correlation analysis generally indicates that interfacial adhesion performance decreases with increasing residual water, while IPOT exhibits a more pronounced response to residual water variation than SFE. In addition, the IPOT results indicate that the limestone exhibits better interfacial adhesion performance and moisture damage resistance than the basalt. Overall, this study establishes a framework for the quantitative characterization of residual water in foamed asphalt and the evaluation of its interfacial effects, providing a basis for the rational selection of foaming warm mix agents and the resource utilization of solid wastes in FWMA applications.