Abstract: Environmental problems associated with the large volumes of waste soil generated by large-diameter slurry shield tunneling have become increasingly severe. MgO-based solidification-carbonation is an efficient, environmentally friendly, and sustainable method for modifying waste soil. However, the governing patterns and mechanisms of mass transfer during carbonation remain unclear. To address this issue, this study quantitatively determined the pore water and CO₂ contents in carbonated soils with different MgO contents and carbonation times based on the water transformation mechanism. It then investigated the mass-change behavior of each soil component, proposed a mass transfer mechanism and a mass transfer model, and analyzed the relationships between the model and the mechanical properties of the soil. The results show that soil mass changed during carbonation consist of CO₂ absorption and pore water evaporation. The variations in the contents of each soil component conforms to the water transformation mechanism and the synergistic carbon–water interaction. The proportion of soil mass increase caused by CO₂ absorption increased with the prolongation of carbonation time, with a maximum absorption capacity of 197 kg/m3. Meanwhile, water transformation gradually shifted toward the formation of mineral water, with up to 48% of pore water converted into carbonation-derived mineral water. Fitting results indicated that, as the MgO content increased, the ratio of pore water consumption to CO₂ absorption decreased, suggesting improved CO₂ utilization efficiency during carbonation. By establishing a mass transfer model based on the water transformation principle, the changes in unconfined compressive strength (UCS) and elastic modulus could be indirectly inferred from variations in the individual components of the model. Among these variables, pore water variation exhibited an exponential relationship with UCS and a linear relationship with elastic modulus. The calculation method for analyzing mechanical properties through the mass transfer model provided a novel approach for predicting the properties of MgO-based solidified/carbonated high-plasticity silt generated by slurry shield tunneling.

Key words: slurry shield, MgO, carbonation, water transfer, mass transfer