Effects of Geopolymers Synthesized with Red Mud and Fly Ash as Warm-Mix Asphalt Additives on the Rheological Properties of Asphalt Mastics and Asphalt–Filler Interaction Ability

This study synthesizes a new geopolymer warm-mix asphalt (WMA) additive using red mud (RM) and fly ash (FA) as precursor materials. Orthogonal tests and thermogravimetric tests were carried out to determine the optimum preparation process for RM–FA geopolymer WMA additive. X-ray diffractometer, scanning electron microscope, and fourier transform infrared spectrometer were used to compare and analyze the microstructure and mineral composition of RM-FA geopolymer WMA additive and limestone powder. Viscosity tests, dynamic shear rheometer tests, and bending beam rheometer tests were employed to study the effect of RM–FA geopolymer WMA additive on the rheological properties of base asphalt mastic, SBS-modified asphalt mastic, and high ductility modified asphalt mastic. The K-B-G* and Ziegel-A-δ parameters were used to evaluate the asphalt–filler interaction ability. Toxicity Characteristic Leaching Procedure testing was utilized to evaluate the leaching characteristics of heavy metal ions from RM–FA geopolymers. The results indicate that RM–FA geopolymers exhibit exceptional dehydration performance, achieving a weight loss rate of 10.8% at 135°C. During the geopolymerization reaction, RM–FA geopolymer WMA additives form a complex mineral composition and pore structure. RM–FA geopolymers effectively reduce the viscosity of asphalt mastics, lowering the mixing and compaction temperatures of asphalt mixtures by about 15°C. Simultaneously, RM–FA geopolymers enhance asphalt–filler interaction ability, and significantly improve the high-temperature rheological properties of asphalt mastic, but slightly reduce its low-temperature rheological properties. The leaching concentrations of heavy metals in RM–FA geopolymer are below standard limits. Heavy metals are effectively immobilized within the geopolymer. Therefore, RM–FA geopolymers can be used as WMA additives to reduce the viscosity of asphalt mastics, improve their high-temperature rheological properties, and enhance asphalt–filler interaction.