# Potential of Waste Oyster Shells as a Novel Biofiller for Hot-Mix Asphalt
## 3 Results
### Scanning Electron Microscope (SEM)  
The possible interaction between OSP particles and asphalt binder was further assessed by SEM imaging analysis for a better understanding of the rheological findings. This approach permitted the identification of inorganic filler distribution inside the bituminous matrix, thus helping to explain  
Figure 10: Effect of temperature on the fatigue cracking parameter of unfilled and OSP-filled asphalts under long-term aging condition (PAV: pressure aging vessel). the differences detected in fatigue and rutting behaviors between unfilled and OSP-filled asphalts. Figures 11 and 12(A1,2-D1,2) display the SEM patterns of OSP, base asphalt (i.e., AP-5 OSP 0 wt %), and asphalt treated with different concentrations of OSP (e.g., 5, 10, 15 wt %). The SEM micrographs were taken at different levels of magnification (e.g., \(\times\)100, \(\times\)500, and \(\times\)3000). In the low magnification SEM images (Figure 11, \(\times\)100), it can clearly be seen that the OSP comprises some large particles and high fraction of fine particles, which resulted in a more heterogeneous distribution. The comminuted powder exhibits an irregular crystal structure of densely attached fine grains (e.g., width 1-130 \(\upmu\)m, length 1-246 \(\upmu\)m) caused probably by the mechanical effect during dry grinding. These results are also being well observed in particle size distribution curves in Figure 3. The filler is detectable as angular and rough particles with schist structure and low propensity to agglomerate. By examining the SEM images of AP-5 asphalt treated with 5 and 10 wt % OSP, the presence of filler in the asphalttic matrix can be easily detected in the form of uneven distributed aggregates. On the contrary, it is worth mentioning that in the case of neat asphalt (i.e., AP-5 OSP 0 wt %), the SEM image reveals the existence of a continuous network that is characterized by a uniform, smooth, and homogeneous surface. As seen in Figure 12 (B1,2,C1,2), the OSP in different shapes (i.e., mainly in flat shapes) could be obviously identified in the asphalt bulk. Generally, it can be found that the oyster shell powder (i.e., dispersed phase) and asphalttic material (i.e., viscoelastic matrix) exist independently, establishing that there is perhaps no chemical reaction between them. Hence, as one mineral admixture, OSP mainly plays its role through \"micro-aggregate filling effect\" and \"morphological effect\" in the bituminous material. The micro-aggregate filling effect means that the micro-particles of mineral admixture spread evenly across the basic phase of asphalt with the consolidation and filling effects. On the other side, the morphological effect arises from the effect caused by grain composition, surface property, internal structure, external morphology, and other physical features of the particles. These two effects supplement each other and are interrelated. The oyster shell powder obtained with dry ball grinding is finer and barely homogeneous, and thus its addition made the particle stacking in the asphalt much closer. It can be assumed that there is fairly better interface binding among particles, so the asphalt strength is improved, especially for early strength (generated particularly with 5 and 10 wt % of filler). Therefore, when the content of added OSP is increased to 15 wt %, the asphalt strength (i.e., stiffness and hardness) shows an increased trend. This tendency to rise is largely dictated by the physical properties of oyster shell powder such as gradation, specific surface area, shape, and porosity. Oyster shell powder is an amorphous material that agglomerates at higher concentration and can significantly affect the mechanical properties of resultant composite. Figure 11: Scanning electron microscope (SEM) micrographs of oyster shell powder (OSP) obtained with dry ball grinding at different magnifications \(\times\)100 (**A**) and \(\times\) 500 (**B**).