Evaluation of a novel in situ constructed CO2-carbonated MgO-mixing column: Mechanical performance, carbon sequestration, and microstructural analysis

Cement treatment, such as cement-mixing columns, is commonly used for deep soft soil improvement to increase the bearing capacity and reduce settlement. However, cement production entails high energy consumption and carbon and pollutant emissions. CO 2 capture and mineralization represent promising solutions to these issues. This study proposes a sustainable alternative: a novel CO 2 -carbonated MgO-mixing column that integrates CO 2 mineralization with soil reinforcement. This approach involves in situ mixing of MgO with deep soil to form columns, which are then carbonated and solidified by injecting captured CO 2 through gas-permeable pipe piles, achieving both carbon reduction and soil improvement. In this study, CO 2 -carbonated MgO-mixing columns were comprehensively evaluated to investigate variations in strength, deformation, pH, and CO 2 sequestration with depth. Two rapid and cost-effective methods to assess its mechanical properties, uniformity, and CO 2 sequestration capacity are proposed. The results show that the carbonated MgO-treated soil has good strength along the depth direction, with an average unconfined compressive strength (UCS) of 1.02 MPa and a lower pH than that of cement-mixing columns. It also achieves notable CO 2 sequestration, ranging from 4.88% to 13.10% (average 8.31%), and exhibits good uniformity, as shown by electrical resistivity tests. Needle penetration and electrical resistivity tests could be used to effectively predict the UCS, deformation modulus, and CO 2 sequestration. XRD, FTIR, SEM, and TG-DTG analyses reveal distinct microstructural differences at various depths, with unhydrated MgO, magnesite, and dypingite/hydromagnesite present in shallow columns, and brucite, nesquehonite, and dypingite/hydromagnesite present in deep columns. These products bind soil particles and fill pores, enhancing the strength of the MgO-mixing column.