Novel Sustainable Pavement Material: Recycling of Waste Shells as the Filler of Asphalt Mastic

High-quality limestone, as a nonrenewable resource, is decreasing with the maintenance and construction of asphalt pavements. Meanwhile, due to the growth of the shell farming industry, a great amount of waste shells is produced annually, which are mainly disposed in the manner of stacking, landfill, or discarding into the sea directly, leading to serious environmental pollution. This research used mussel, pectinid, oncomelania, and oyster as raw materials to prepare four shell fillers, aiming at substituting nonrenewable mineral resources and recycling the waste shells at the same time. The asphalt mastics were prepared by mixing the base and SBS-modified asphalt with four shell fillers under different filler-asphalt ratios, respectively, and the asphalt mastics with limestone were adopted as control groups. The physical and rheological properties of five asphalt mastics were evaluated, and the chemical composition and the microstructure of five fillers were measured to investigate the acting mechanism between the fillers and asphalt. Results indicate that shells have lower density, hydrophilicity coefficient, and higher specific surface area than limestone, exhibiting a better compatibility with the asphalt. For the properties, in both base and modified asphalt mastics, the mastics with pectinid and oyster shells exhibit the lowest penetration and phase angle, while their complex modulus and softening point are the highest among the five types of mastics, indicating the better antideformation ability. Moreover, with the increasing filler-asphalt ratio, the antideformation ability of the base asphalt mastic increases gradually, while the SBS-modified asphalt mastic exhibits a more complex behavior. In addition, analysis results of fillers demonstrate that the looser structure of shells is attributed to the interlaced layered structure of aragonite and calcite, leading to a larger contact area between the shell and asphalt. Besides, there is some organic matter present in the shell, which plays a role like organic modification, improving the compatibility between the shell and asphalt. The larger contact area and the presence of organic matter in the shell show a synergistic effect in improving the road performance of asphalt mastic. This work verifies the feasibility to replace the conventional mineral fillers with shell fillers, which is beneficial to recycle the waste shell and reduce the consumption of nonrenewable mineral resources.