Multiscale Mechanisms Governing the Performance Evolution of MgO-GGBS-Stone Powder Lightweight Foamed Soil Under CO2 Curing

To facilitate low-carbon construction technologies in subgrade engineering, this study proposes a carbon-sequestering lightweight foamed soil (C-LFS) using an all-solid-waste binder system in which reactive magnesium oxide (MgO), ground granulated blast-furnace slag (GGBS), and stone powder (SP) fully replace Portland cement. Specimens with target wet densities of 600–800 kg/m 3 were prepared, and macroscopic tests on workability and early setting, dry density, and unconfined compressive strength (UCS) were conducted. SEM and XRD characterization were further performed to reveal carbonation-driven product evolution and microstructural changes. The results show that increasing MgO dosage decreases slurry flowability and shortens setting time. The 28-day UCS exhibits a rise-then-fall trend with both MgO dosage and water-to-binder ratio. At a wet density of 700 kg/m 3 , the optimum MgO dosage and water-to-binder ratio are 35% and 0.65, respectively, yielding a maximum 28-day UCS of 1.132 MPa. Carbonation curing markedly enhances CO 2 uptake and carbonate formation, demonstrating promising CO 2 mineralization potential, with a CO 2 uptake of approximately 67.3 kg CO 2 /t after 5 days of carbonation. However, with prolonged carbonation, the pore-solution pH drops rapidly, suppressing sustained GGBS activation and late-stage MgO hydration; this in turn induces gel-framework weakening (decalcification of C-(A)-S-H), pore-structure coarsening, and microcrack development, causing UCS to decrease continuously with carbonation age, with more pronounced degradation under the combined condition of high MgO dosage and extended carbonation. Despite strength degradation, the 28-day UCS can still be maintained at 0.752 MPa, meeting the strength requirements for subgrade filling. Overall, this study elucidates the trade-off mechanism between CO 2 mineralization and mechanical performance of C-LFS from coupled macro-meso-micro perspectives, providing guidance for the design and application of low-carbon lightweight foamed soil in subgrade backfill and a basis for optimizing the carbonation-curing regime of this material.