Microstructural formation and stiffness evolution of emulsion-stabilized 100% RAP cold recycled mixtures

This study evaluates the coupled effects of asphalt emulsion type, mineral additive, and curing temperature on stiffness development and microstructural formation of 100% reclaimed asphalt pavement (RAP) cold recycled mixtures. Two anionic emulsions and one cationic emulsion were tested with cement contents of 0% and 1.5% at curing temperatures of 25°C and 60°C. Dynamic modulus (|E*|), indirect tensile strength (ITS), tensile strength ratio (TSR), and Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX) analyses were conducted. The results show that the curing temperature is the dominant factor governing microstructural formation and stiffness development of the mixtures. Elevated temperature accelerates emulsion breaking, binder film coalescence, and cement hydration; therefore, specimens cured at 60°C exhibit 35.8% higher |E*| than those cured at 25°C. In addition, emulsion dispersion characteristics also have a significant effect on the mechanical behavior of the mixtures. At 25°C, the Anionic 1 mixture achieved the highest |E*| at low frequencies, and at 60°C, the Anionic 2 mixture developed the highest stiffness, indicating distinct demulsification kinetics and temperature sensitivity. The addition of 1.5% cement had a negligible effect on |E*| at low and intermediate temperatures but significantly increased ITS, TSR, and |E*| under the most critical condition (54°C, 0.1 Hz). Microstructural results showed that elevated curing temperature promotes mineral activation and formation of a hybrid asphalt calcium silicate hydrate (C–S–H) and ettringite (AFt) network. These findings demonstrate that emulsion dispersion characteristics, cement hydration, and curing temperature jointly control microstructure and mechanical performance of 100% RAP cold recycled mixtures.