The environmentally friendly magnesium oxychloride cement (denoted as MOC thereinafter) shows promising application prospects. Meanwhile, more emphasis is being paid to the environmental issues created by CO2 emissions. In this paper, carbonation-composite solidification technology is adopted to introduce MOC-lime cementitious material into sludge solidification. The effects of initial water content (H2O/MgCl2 molar ratio), lime content, MgO/MgCl2 molar ratio, and carbonation time on the mechanical properties and micromechanisms of sludge solidification were investigated using unconfined compressive strength (UCS), pH value, carbonation depth, mass loss rate, scanning electron microscopy (SEM), and X-ray diffraction (XRD) tests. The results show that the UCS of sludge solidification decreases with an increase in H2O/MgCl2 molar ratio, and increases first and then decreases with an increase in lime content, reaching a maximum value at H2O/MgCl2 molar ratio of 24.3 and lime content of 4 %. Notably, carbonation significantly improves the UCS of the samples, and with an increase in carbonation time, the mass loss rate and carbonation depth increase, while the pH value decreases. Additionally, uncarbonated samples show an increase in compressive strength with an increase in MgO/MgCl2 molar ratio, as the hydration products gradually transform from amorphous gel to crystalline phases 3, phases 5, and brucite (Mg(OH)2). Finally, XRD and SEM results indicate that the underlying mechanism for the significant improvement in strength and microstructure of the samples after carbonation is the formation of a block-like crystalline network with good binding ability, consisting of chlorartinite, nesquehonite, calcium carbonate, and C-S-H gel. This study promotes the use of MOC-based gel materials as a green stabilizer for sludge solidification, and the carbonation technique employed can improve the mechanical properties of solidified soil and significantly reduce the curing time.

Magnesium oxychloride cement (MOC) is an environment-friendly cement often used for stabilizing soft soils because of its exceptional mechanical properties. In this study, the influence of curing temperature on the strength development of MOC-solidified clay is explored, considering different MgO/MgCl2 molar ratios. Different tests were carried out to study the corresponding effects. The results show that the effect of curing temperature on the strength of MOC-solidified clay differs greatly from that of cement-solidified soil. Increasing the curing temperature leads to strength reduction, whereas decreasing the curing temperature increases the corresponding strength. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses indicate that the variation in type and amount of hydration products of the solidified soil account for the strength development difference between MOC-solidified and cement-solidified soils. A model based on the experimental results is proposed to characterize the relationship between strength development and curing time. The strength influence factor (eta T) and the strength expedite factor (K) were introduced to demonstrate the relationship between strength development at a specific curing temperature as well as at room temperature.