Design and performance evaluation of steel slag–based geopolymer grouted semi-flexible asphalt concrete

To promote the value-added recycling of industrial solid wastes and enhance the overall performance of semi-flexible pavements, a novel steel slag–based geopolymer (SSBG) grouted asphalt concrete was developed in this study. By optimizing the mix proportions of the large-void asphalt concrete skeleton and the SSBG grout, the evolution of material properties was systematically investigated. For the geopolymer grout, the effects of different water-solid ratios (W/S: 0.40–0.60) on workability, mechanical properties, and microstructural characteristics were comprehensively evaluated. Meanwhile, the influence of grout W/S on the road performance of semi-flexible asphalt concrete was also examined. Finally, a multi-objective decision-making approach was employed to determine the optimal preparation parameters. The results indicate that the asphalt–aggregate ratio of the asphalt skeleton is 3.7%, corresponding to air void content of 24.6% and a connected void content of 23.7%, which provides a favorable balance between structural stability and grout infiltration capacity. As the W/S increases, the fluidity and setting time of the geopolymer grout increase, whereas its mechanical strength decreases. XRD, FTIR, SEM–EDS, and TG–DTG analyses reveal that a W/S of 0.50 facilitates the formation of a dense and highly polymerized C(N)-A-S-H gel network. An excessively low W/S impairs grout fluidity and infiltration, while an overly high W/S dilutes the alkaline environment and suppresses geopolymerization. The road performance of SSBG grouted asphalt concrete initially improves and then deteriorates with increasing W/S. At the optimal W/S ratio of 0.50, the geopolymer grout achieved a 28d compressive strength of 53.4 MPa and a groutability of 95.8%. The resulting semi-flexible asphalt concrete exhibited superior rutting resistance with a dynamic stability of 25800 cycles/mm. This optimized system facilitates the value-added recycling of steelmaking and metallurgical residues, offering a practical solution to advance sustainable semi-flexible pavement materials with enhanced performance