Resource-efficient composite foundations using steel slag concrete piles: Field performance and microstructural evidence

Steel slag concrete (SSC) piles represent a resource-efficient option for ground improvement as they valorize steelmaking by-products and reduce dependence on conventional cement and natural aggregates. The major novelty of this study is the field-scale in situ verification of SSC pile composite foundations and their direct comparison with traditional fly ash cement concrete (FCC) piles through a combined macro–micro analysis. This study assessed SSC pile composite foundations using full-scale static load tests coupled with multi-scale material characterization. The results indicated that the SSC and FCC single-pile composite foundations developed comparable ultimate capacities, reaching approximately 2.1–2.4 times that of the natural-site soil. Nevertheless, the SSC foundations demonstrated more favorable pile–soil interactions, which were expressed through a stable and relatively small pile–soil stress ratio and a more effective mobilization of inter-pile soil resistance. Microstructural analyses at 100 and 360 d revealed that SSC formed a denser hydration matrix, a more coherent interfacial transition zone, and a rougher, better interlocked pile–soil interface enriched with hydration products and soil inclusions. These microstructural features underpinned the improved settlement behavior and sustained load-transfer capacity observed during field testing. After one year under dredger-fill conditions, SSC retained low porosity and strong interfacial bonding. This indicates promising durability compared with FCC under the investigated field conditions. Under the adopted A1–A3 calculation boundary, each SSC pile incorporated approximately 0.7 t of steelmaking by-products and showed a preliminary embodied carbon advantage at the material stage compared with FCC. Overall, SSC piles can provide reliable structural performance and a performance-based basis for future optimization- and sustainability-oriented assessments under dredger-fill ground conditions.