Abstract: Based on the slag extruded precast pile technology and active MgO-CO₂ carbonation curing method, the feasibility of using MgO instead of cement as curing agent was studied. Effects of the pre-carbonation duration on the particle gradation, mass and moisture content, and the unconfined compressive strength of MgO-cured compacted soil were investigated by combining the X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) tests. The products of carbonization process and microstructure evolution characteristics were analyzed, and the mechanism of the influence of pre-carbonation duration on the strength of MgO-cured compacted soil was revealed. In pre-carbonation duration of 1−5 h, the mass of compacted soil increases rapidly and then tends to slow down with the carbonization time, but the water content shows in the opposite trend. The coefficient of uniformity C_u and the compactability increase with the rising number of particles in size of 1−10 m. The unconfined compressive strength increases rapidly then slows down to be stabilized. During the carbonization process, the carbonization products generated by the carbonation process are wrapped around the soil particle surface to improve the hardness. Morever, the carbonization products fill the large to super larger pores (4−40 m) in soil during the compaction, and MgO continues to hydrate and carbonize during the maintenance stage to further fill soil pores, leading to improve the degree of densification. The three mechanisms described above act synergistically to enhance the macroscopic strength of the cured compacted soil. However, with longer pre-carbonation durations, the formation of a passivating layer inhibits further carbonation. In addition, the initial low water content limits MgO hydration, reducing carbonation during the carbonation stage. These factors lead to a transition from rapid to slower strength development in the cured compacted soil.

Key words: MgO, carbonation, compressive strength, soil consolidation, microscopic mechanisms