Performance evolution from macro to micro scales of cold-bonded silty soil environmentally friendly lightweight aggregates and their subgrade application

Waste soil generated during construction is often underutilized, while the continued consumption of natural aggregates increases resource demand and environmental burdens. In this study, waste silty soil was converted into cold-bonded lightweight aggregates (CBLAs), which were incorporated into silty soil as a cleaner alternative to natural aggregates for silty subgrade improvement. Microstructural characterization and performance evaluation were conducted to investigate the internal structure and engineering performance of the CBLAs, using X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), water absorption and single-particle compressive strength tests, dry–wet cycling tests, and field validation. Results indicate that an 8% cement dosage represents a critical threshold for producing high-strength CBLAs, corresponding to the formation of a continuous load-bearing cementitious network. Statistical analyses indicate that the single-particle compressive strength of the CBLAs generally follows a lognormal distribution. Under dry–wet cycling, strength evolution exhibits a three-stage pattern with rapid decline at low dosages, partial compensation at medium dosages, and stabilization at high dosages. These changes are governed by the evolution of hydration products and variations in pore connectivity. The incorporation of 30% CBLAs doubled the CBR relative to untreated soil and yielded resilient modulus values comparable to those of gravel–soil mixtures but with lower variability. These results confirm the engineering feasibility of using CBLAs produced from waste silty soil as a cleaner alternative to natural aggregates for silty subgrade improvement.