<p>Offshore deep cement mixing is an essential technique for enhancing marine deposits in undredged land reclamation, tackling the pressing issue of coastal land shortages. This approach relies on a substantial quantity of ordinary Portland cement to produce seawater cement slurry, which stabilizes marine soils in situ at the cost of huge carbon emission. In this study, we propose turning deep cement mixing into a carbon sequestration process, where CO₂ is injected into seawater cement slurry to substantially increase the strength, stiffness and density of the improved marine mud. We also evaluate the in situ efficacy of carbonated deep cement mixing for land reclamation, drawing on detailed material properties and on-site construction data from an operational offshore project. We further examine the microstructure, unconfined and confined compression strengths, and shearing characteristics of the stabilized marine soils. These findings are combined with field observations to construct a numerical model, validated by in situ monitoring records, to assess carbonation’s impact. The results demonstrate that carbonation enhances hydration products and creates a denser pore structure, significantly improving compression and shear properties. Consequently, this leads to better settlement control, increased bearing capacity, and an added advantage of carbon sequestration, supporting sustainable construction practices.</p>