## 3 Results and Discussion
### The Contact Angle Test Results
Before and after short-term thermal oxidation, the adhesion work between COBA and steel slag was 7% and 9% higher than that between COBA and limestone, respectively. The adhesion work between 15% COBA and steel slag was up to 55.16 mJ/m\({}^{2}\), which was 6.9% higher than that between COBA and limestone at the same bio-oil dosage. This was because the content of SiO\({}_{2}\) in steel slag is higher than that in limestone, and the acid molecules in COBA are more likely to react with a high-valence metal salt to form alkyl acids, which accumulate and crystallize on the surface of aggregates to form adhesion [24]. Moreover, based on the physical adsorption theory, the greater the roughness of the aggregate surface, the more effective the contact area, and subsequently the more favorable to asphalt adsorption. Steel slag is also more conducive to COBA adsorption, forming adhesion because of its porous surface and higher roughness than limestone. (3) Work of exfoliation of COBA-aggregate  
The work of exfoliation refers to the energy released by asphalt during the displacement of water from the surface of aggregates. The smaller the absolute value of exfoliation work, the greater the bonding strength of asphalt and aggregates under water conditions, and the stronger the ability of the mixture to resist water damage. According to Figure 5c,d, for the virgin samples, the exfoliation work between COBA and aggregates decreases with the increase in bio-oil content. This indicates that bio-oil exhibits the effect of improving the anti-stripping at the interface of asphalt-aggregate. Under the action of thermal oxidation, the work of the exfoliation of COBA increased, probably due to the decrease in internal resin content and increase in asphaltene content in COBA after aging. This accordingly reduces the adhesion of COBA-aggregate, thus weakening its resistance to water damage. In terms of aggregate type, the exfoliation work of COBA-limestone was much smaller than that of COBA-steel slag, which was only half of it. According to Table 4, the SiO\({}_{2}\) dosage in steel slag was 8.4 times that in limestone, so COBA interacted with steel slag to form more acidic compounds, which could provide better adhesion. However, carboxylic acid compounds can ionize readily underwater and increase the moisture susceptibility of asphalt concrete, which results in a rapid decrease in the interfacial adhesion [25]. Also, the silicate in the aggregates and the H in the water produce a coulomb interaction and combine in the form of hydrogen bonds, causing steel slag to demonstrate a stronger hygroscopicity than limestone. In addition, the steel slag itself will have a hydration reaction with water, which can replace water during the hydration process, so the exfoliation work increases and the resistance to water damage stripping decreases. (4) Compatibility ratio of COBA  
The compatibility ratio is used to characterize the compatibility of asphalt and aggregates. The greater the compatibility ratio, the better the adhesion stability of the asphalt mixture. From Figure 5e,f, the compatibility ratio of COBA and limestone was twice higher than that of COBA and steel slag. The steel slag had a higher hygroscopicity and limestone was less affected by water, with a much lower exfoliation work than that of steel slag. Furthermore, with CaCO\({}_{3}\) the as main component, limestone is more alkaline than steel slag, so it has a stronger acid-base reaction with COBA and better adhesion stability. Of course, the large adhesion work between steel slag and COBA is accompanied by large exfoliation work. This indicated that its adhesion with asphalt cannot work stably in the presence of water, which needs to be further explored.