Thermal storage stability, rheological properties, and morphological evolution characteristics of micro-foamed asphalt

Abstract The foaming process effectively reduces the viscosity of asphalt, thereby lowering energy consumption during mixture construction. However, conventional foamed asphalt production depends on specialized equipment and strict control of foaming conditions. Incorporating additives that contain water of crystallization to induce micro-foaming in asphalt presents a promising alternative. In this study, three highly crystalline hydrates, namely KAl(SO 4 ) 2 · 12H 2 O, Na 2 CO 3 · 10H 2 O, and Na 2 HPO 4 · 12H 2 O, were introduced into base asphalt to achieve micro-foaming. The effects of the micro-foaming process on the physical properties and chemical structure of asphalt were evaluated through ductility, penetration, softening point, viscosity, and Fourier-transform infrared spectroscopy (FTIR). Additionally, dynamic shear rheometer (DSR) and bending beam rheometer (BBR) were conducted to assess the evolution of viscosity, moisture content, high- and low-temperature performance, and morphological characteristics of micro-foamed asphalt over the foaming period. The results shows that the optimal dosages of KAl(SO 4 ) 2 · 12H 2 O, Na 2 CO 3 · 10H 2 O, and Na 2 HPO 4 · 12H 2 O are 3%, 5%, and 1%, respectively, achieving viscosity reduction rates ranging from 9.92% to 14.62%. Compared to mechanically foamed asphalt, the micro-foamed asphalt exhibited improved physical properties, including an approximately 40% increase in low-temperature ductility and a 60% enhancement in viscosity-reduction stability. Relative to conventional warm-mix processes, the micro-foamed asphalt showed improvements of approximately 141% in fatigue resistance and 215% in cracking resistance. The viscosity-reducing effect remained above 70% for up to 2 h, while moisture content decreased exponentially during micro-foaming, with the final residual water content below 0.05%. Furthermore, a strong linear correlation was observed between apparent and internal bubble sizes, whereas the apparent bubble density exhibited a logarithmic relationship with internal bubble density. These findings demonstrate that the internal foaming state can be effectively characterized by analyzing the apparent bubble parameters of micro-foamed asphalt.