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Subgrade treatment has traditionally been achieved using calcium-based cement. However, it does not necessarily enhance sustainable design. Recently, low-carbon alternatives such as portland limestone cement (PLC) have gained attention as substitutes for traditional cement. In addition, recycled concrete aggregate fines (fRCA), a waste product, have shown potential for application in transportation infrastructure because of their enhancements in pavements. This study investigates the effectiveness of PLC and fRCA in improving soil properties under different environmental stressors. Clayey soil was treated with PLC (10% PLC or 10C) and PLC-fRCA mixtures at different ratios (8% PLC/15% fRCA or 8C_15fRCA and 8% PLC/30% fRCA or 8C_30fRCA). Improvements in strength, stiffness, and volumetric changes were evaluated through unconfined compressive strength and repeated load triaxial tests after exposure to various environmental conditioning cycles (0, 6, and 12 cycles of wet–dry or freeze–thaw) in the laboratory. Results indicated that untreated soil collapsed within two cycles of environmental conditioning. In contrast, treated soils exhibited significant improvements in strength and resilience to environmental stressors. Stiffness also improved with treatment, and despite some reduction after exposure to environmental conditioning, treated specimens maintained relatively higher stiffness values. These enhancements are attributed to the formation of strong binding gels from hydration and secondary reactions among PLC, fRCA, and soil, which exhibit strong resistance to moisture intrusion, helping to preserve their engineering properties. Overall, this study provides a comprehensive understanding of the potential of using fRCA as a co-additive to PLC, offering a more sustainable and durable alternative for the long-term performance of transportation infrastructures.