Study on Mix Proportion Optimization and Multi-Scale Mechanism of High-Volume Aeolian Sand Cement-Fly Ash Stabilized Gravel Base

Aeolian sand is abundant in arid deserts, but its high replacement in cement-stabilized bases can reduce strength and raise cracking risk. Strain localization and crack evolution are also poorly quantified. This study aimed to optimize the early age performance of cement-fly ash stabilized aeolian sand gravel (CFSAG) and clarify its failure mechanism. A Box–Behnken response surface methodology varied the cement content, cement-to-fly ash ratio, coarse aggregate gradation, and aeolian sand content. The 7-d unconfined compressive strength (UCS) and splitting tensile strength (STS) were tested. Digital image correlation (DIC) recorded full-field strains and crack metrics in compression and splitting. SEM–EDS was used to interpret microstructural changes. The aeolian sand content dominated UCS, whereas the cement content and cement-to-fly ash ratio mainly controlled STS. Factor interactions were non-negligible and supported the joint optimization of the two strength indices. DIC identified a crack propagation threshold near 0.9 Pmax in splitting. Excess aeolian sand (>50%) caused earlier localization, more cracks, and wider openings. In the appropriate amount of aeolian sand mixtures, hydration products filled voids and improved paste continuity. SEM–EDS indicated that excessive fines increased porosity and weakened the interfacial transition zone. Overall, the combined RSM–DIC–SEM approach links mix design with deformation and microstructure evidence. It provides practical guidance to balance strength and cracking resistance at early ages for cement-stabilized bases in desert highway engineering.