# Potential of Waste Oyster Shells as a Novel Biofiller for Hot-Mix Asphalt
## 3 Results
### Differential Scanning Calorimetry (DSC)  
The differential scanning calorimetry (DSC) was carried out to elucidate the impact oyster shell powder (OSP) on the thermal transitions of asphalt cement. The effect of OSP content (e.g., 0, 5, 10, and 15 wt %) on the glass transition temperature (\(T_{\rm g}\)) of binders obtained from Figure 15 is illustrated in Table 6. Figure 15 demonstrates several thermal effects of the base asphalt (i.e., AP-5 OSP 0 wt %): (a) At very low temperatures (from \(-34\)\({}^{\circ}\)C to \(-15\)\({}^{\circ}\)C), the increase in heat capacity can be attributed to the first glass transition temperature (\(T_{\rm g1}\)) within the hydrocarbon matrix. It arises mainly from the maltene phase (i.e., a mixture of saturates, aromatics, and resins components) and is the most prominent transition. (b) Above \(T_{\rm g1}\), a second glass transition temperature (\(T_{\rm g2}\)) occurs. It comes from a maltene-asphaltene interfacial zone, which is most likely rich in resins. For convenience's sake, it can be defined as \"an interphase\", even though it may not be a strictly separate phase [64; 65]. It becomes clear that the pure AP-5 asphalt is a mixture, and this mixture is not completely homogenous, since it shows more than one \(T_{\rm g}\). Surprisingly, no obvious endothermic or exothermic effects have been shown in the DSC thermograms. This may be because the AP-5 asphalt contains inconsiderable amount of saturates (ca. 4.47 wt %; \(n\)-alkanes (waxes/paraffins), _iso_-alkanes, and cyclo-alkanes), which will bring a possible positive impact on crack susceptibility due to the absence of wax. The DSC curve for OSP, given in Figure 16, exhibits only a sharp endothermic peak located at 820.50 \({}^{\circ}\)C that can be ascribed to the decomposition of CaCO\({}_{3}\) and possible removal of absorbed water molecules that occur according to the following dissociation equation: CaCO\({}_{3}\) (s) + Heat \(\rightarrow\) CaO (s) + CO\({}_{2}\) (g) (1). This reaction is also confirmed by TGA/DTGA analysis. In inert atmosphere, some differences stand out. Figure 16 indicates that there is change in glass transition temperature (\(T_{\rm g}\)) between unfilled and OSP-filled asphalt samples. This fact implies that the presence of OSP affected the \(T_{\rm g}\) of asphalt to some extent. The DSC data of neat asphalt and OSP-asphalt mastics filled with various contents of OSP (e.g., 5, 10, and 15 wt %) are outlined in Table 6. The first glass transition peak temperature (\(T_{\rm g1}\)) of OSP/asphalt composites was smaller than that of neat asphalt. As the filler content increased to 10 and 15 wt %, the \(T_{\rm g1}\) dropped from \(-26.70\) (for neat asphalt: AP-5 OSP 0 wt %) to \(-28.91\)\({}^{\circ}\)C and to \(-28.12\)\({}^{\circ}\)C, respectively. However, the OSP/asphalt composite was also found to have a slightly higher glass transition peak temperature than the base asphalt. The temperature slightly rose from \(-26.70\)\({}^{\circ}\)C to \(-25.61\)\({}^{\circ}\)C with a filler dose of 5 wt %. The \(T_{\rm g}\), though, changed irregularly with the OSP content. This probably happened because when incorporating filler into a viscoelastic matrix, the binder will adsorb on the filler surface, creating a monomolecular layer, which causes an increase in \(T_{\rm g}\), but with higher concentration; the filler will form a superstructure (i.e., chains), which changes the whole inner structure of the binder system, thereby lowering \(T_{\rm g}\). Also, this could be related to the physicochemical interaction effect, which originates from the adsorption of polar groups in asphalt (mainly resins and asphaltenes) onto the surface of OSP particles. This adsorption is partially responsible for the observed changes in thermal properties (e.g., glass transition temperature, \(T_{\rm g}\)) and the rheological properties (e.g., stiffness and viscosity) of OSP/asphalt blends. The amount of polar