Early-Age Strength Development and Microstructural Mechanisms of CO ₂ -Cured Recycled Weathered Limestone Concrete

The recycling of weathered limestone from degraded rock subgrades into concrete presents a sustainable strategy for road construction in permafrost regions. This study investigates the early-age mechanical development and microstructural mechanisms of such recycled aggregate concrete enhanced through short-term CO 2 curing. Specimens incorporating weathered limestone coarse aggregate were subjected to standard curing and then controlled carbonization. Macroscopic property evaluation and microscopic characterization via uniaxial compression testing, X-ray diffraction and nuclear magnetic resonance were conducted. The results indicate that early CO 2 curing significantly improves the concrete’s early-age mechanical performance, including compressive strength and stiffness. The 6-hour CO 2 curing significantly improved the early-age compressive strength, increasing from 16.58 MPa to 22.02 MPa at 3 days, whereas its effect on the 28-day strength was limited, with only a marginal rise from 36.30 MPa to 38.38 MPa. Microstructurally, carbonation promotes the conversion of portlandite into calcium carbonate and effectively refines the pore structure, leading to a denser matrix and a reinforced interfacial transition zone. The coupled carbonation-hydration process is identified as the key mechanism driving these enhancements. This work confirms that CO 2 -cured recycled weathered limestone is a viable, sustainable construction material for cold regions, offering the dual environmental benefits of carbon sequestration and valuing waste.