AbstractSteel slag needs aging treatment before being used for the coarse aggregate of asphalt mixtures because it has volumetric instability due to the hydration of calcium oxide. This paper investigates the effect of hydration products on the bonding properties between asphalt binder and steel slag coarse aggregates subjected to natural aging treatment. A scanning electron microscope, a fluorescence microscope, and X-ray diffraction testing were used to study the surface morphology, microstructure, and mineral phase of the hydration product layer generated on the aggregate surface of steel slag. A modified water boiling test was conducted to evaluate the water stripping resistance of asphalt film with different boiling times. Pull-off testing on real steel slag aggregates was performed to investigate the effect of the hydration product layer and surface treatment on the bonding strength and interface failure mode. Test results indicated that the hydration products exhibited a loose and porous structure and formed an interlayer to prevent the asphalt film from directly adhering to the aggregate surface of steel slag. The hydration product layer could improve the initial stripping resistance but impair the long-term stripping resistance with increased boiling time. The interlayer of hydration products tended to be the weakest link between asphalt film and steel slag aggregate when subjected to pull-off strength damage. The bonding strength showed a negative linear relation with the stripping rate of asphalt film. After steel slag surface treatment with water-based capillary inorganic waterproofer, the pull-off failure of hydration product layer was avoided and thus improved the bonding strength between asphalt film and steel slag aggregate. The bonding strength and failure mode between asphalt-slag adhesion could be well examined by using pull-off tests. Better insights into asphalt–slag adhesion are useful to design durable asphalt mixtures containing steel slag aggregate.