# Potential of Waste Oyster Shells as a Novel Biofiller for Hot-Mix Asphalt
## 3 Results
### Conventional Asphalt Binder Tests (Penetration, Softening Point, and Ductility)  
The results of empirical tests such as penetration, softening point, and ductility for unfilled and OSP-filled asphalts are given in Table 4. It is readily seen by reference to this Table that the physical properties of asphalt were greatly affected following the addition of OSP. The penetration and ductility of filled asphalt were decreased, whereas its softening point was increased. The OSP essentially stiffens the binder due to three major reinforcement mechanisms:  
1. Volumetric-filling reinforcement: the stiffening caused by the presence of rigid inclusions (e.g., mineral filler) in a less rigid matrix (e.g., viscoelastic binder). Figure 6: Chemical composition of unfilled and OSP-filled asphalts. Figure 7: Adsorption of aromatic components on oyster shell powder surface at different doses (e.g., 0, 5, 10, 15 wt %) and creation of resin-like structures. 2 Physicochemical reinforcement: the stiffening caused by the interfacial effects between filler particles and asphalt, including adsorption, absorption, and selective sorption. Here, the treated asphalt efficiently creates a rigid layer, which leads to a higher net volume concentration of rigid matter, increasing thereby the mastic stiffness. 3. Particle-interaction reinforcement: the stiffening beyond physicochemical reinforcement and volume filling. This effect increases with increasing filler dosage, as rigid matter accumulates a skeletal framework form [51]. Too much OSP could over-stiffen the road surface, making it more susceptible to cracking or raveling. Therefore, special caution should be taken when using this biofililer. The increase in softening point is desired, as the binder with high softening point is less susceptible to permanent deformation. Therefore, asphalt mixtures prepared with oyster shell powder (OSP) are initially expected to have better rutting performance. Also, the softening point variation is quietly similar to viscosity variation. Actually, these two physical properties are firmly linked, since it is logical to assume that asphalts with high softening point are also more viscous than those that have low softening point. The OSP does not form a homogenous liquid with asphalt, but remains a solid, which has an impact on the increase in viscosity, which is an indicator of fluid resistance against shear [52; 53]. By increasing the addition of a solid that is insoluble in asphalt, one should expect the increase of mixture viscosity, regardless of the asphaltene content in the base asphalt. This prediction was also made based on direct observation taken during the mixing process. Astonishingly, it was noticed that the blends were becoming overly viscous, with further OSP incorporation (particularly with 15 wt %). This increase in viscosity is attributed essentially to the higher asphaltene content of the base AP-5 asphalt (ca. 32.90 wt %). It has been reported that the asphalt with greater asphaltene content, when compounded with basic filler (e.g., CaCO\({}_{3}\)), will result in higher viscosity, while acid filler (e.g., granite) will result in lower viscosity [54]. If a large quantity of biofililer is added, the HMA (i.e., hot-mix asphalt) blend will become difficult to dump, spread, and compact. Accordingly, special attention should be bestowed on the selected OSP dose in order to achieve an adequate consistency. Table 4 also illustrates that, in all cases, as the mineral filler content increased in the asphalt cement, the ductility (at 25 \({}^{\circ}\)C) decreased very rapidly. These results revealed that the mastics tend to crack due to the asphalt elongation, which decreased as the OSP content increased. Low ductility indicates that the bitumen is brittle and will fracture before deforming much under a tensile load. Owing to its high surface area, the OSP formed strong bonds with the binder matrix, promoting the bitumen cohesion.