Abstract: The waste soil generated during shield tunnel construction poses a challenge in terms of stacking, transportation, and treatment. One promising approach is the use of magnesium oxide (MgO) for the solidification treatment of waste soil, as it is abundant in raw materials, has low production energy consumption, and is eco-friendly. The limit moisture content w, unconfined compressive strength qu and modulus of elasticity E₅₀ of MgO solidified and carbonized mucky waste soil were studied by changing the MgO content am, curing age T, and carbonization time H. The solidification and carbonization mechanism of MgO-treated mucky waste soil was discussed in combination with the microstructure and the change rule of mineral composition. The results demonstrate that MgO solidification and carbonization have a significant reinforcing effect on the soil. The reinforcement effect initially increases and then stabilizes with increasing am and T, while it shows an initial increase followed by a decrease with H. Carbonization for 4 hours reduces the soil plasticity index (IP) by more than 57% under different am and T conditions, leading to a significant reduction in soil plasticity. The qu of solidified and carbonized soils reaches a peak value at H of 4 hours, with values up to 1.4 MPa, which is 220%–350% higher than the strength before carbonization. The effect of solidification and carbonization reaction on the increase in E₅₀ of the soil mass is greatly influenced by am. When am exceeds 9%, the E50 increase of the sample carbonized for 4 hours exceeds 500%. During the process of MgO solidification and carbonization, hydration products and carbonization products are continuously generated and developed. They form a network and flower-like microstructure, filling the pores between soil particles, enhancing cementation between soil particles, and densifying the soil structure. The research provides insights into the efficient and eco-friendly treatment and reuse of mucky waste soil produced by earth pressure shield in the Yangtze River Delta.

Key words: waste soil treatment, active magnesium oxide, accelerated carbonization, microstructure, mineral composition, carbonation mechanism