Correlation Between Packing Voids and Fatigue Performance in Sludge Gasification Slag-Cement-Stabilized Macadam

The fatigue resistance of cement-stabilized macadam (CSM) plays a vital role in ensuring the long-term durability of pavement structures. However, limited cementitious material (CM) content often leads to high packing voids, which significantly compromise fatigue performance. Existing studies have rarely explored the coupled mechanism between pore structure and fatigue behavior, especially in the context of solid-waste-based CMs. In this study, a cost-effective alkali-activated sludge gasification slag (ASS) was proposed as a sustainable CM substitute for ordinary Portland cement (OPC) in CSM. A dual evaluation approach combining cross-sectional image analysis and fatigue loading tests was employed to reveal the effect pathway of void structure optimization on fatigue resistance. The results showed that ASS exhibited excellent cementitious reactivity, forming highly polymerized C-A-S-H/C-S-H gels that contributed to a denser microstructure and superior mechanical performance. At a 6% binder dosage, the void ratio of ASS–CSM was reduced to 30%, 3% lower than that of OPC–CSM. The 28-day unconfined compressive strength and compressive resilient modulus reached 5.7 MPa and 1183 MPa, representing improvements of 35.7% and 4.1% compared to those of OPC. Under cyclic loading, the ASS system achieved higher energy absorption and more uniform stress distribution, effectively suppressing fatigue crack initiation and propagation. Moreover, the production cost and carbon emissions of ASS were 249.52 CNY/t and 174.51 kg CO2e/t—reductions of 10.9% and 76.2% relative to those of OPC, respectively. These findings demonstrate that ASS not only improves fatigue performance through pore structure refinement but also offers significant economic and environmental advantages, providing a theoretical foundation for the large-scale application of solid-waste-based binders in pavement engineering.