Treatment of ladle furnace slag by carbonation: Carbon dioxide sequestration, heavy metal immobilization, and strength enhancement

Ladle furnace slag (LFS) is a by-product of the steel industry and is difficult to be reused due to its weak cementitious property, low strength, and potential leaching of heavy metals. The emission of carbon dioxide (CO2) is also a concern for the steel industry. Therefore, the aim of this study was to use CO2 to immobilize heavy metals in LFS and enhance its strength. The LFS specimens were carbonated with different initial water contents, CO2 pressures, and carbonation periods. The carbonated LFS were then studied by leaching test, unconfined compressive strength (UCS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), and field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDX). The results showed that LFS had carbonation reactivity and could sequester CO2 up to 9.6% of its own mass. The carbonation also effectively reduced the leaching of heavy metals from LFS, especially Pb and Zn. The concentrations of leached Pb and Zn of carbonated LFS were significantly reduced from 2760 and 1460 μg/L to 0.11 and 0.56 μg/L, respectively, being one order of magnitude (Pb) or three orders of magnitude (Zn) lower than limits of inert waste and three drinking water regulations. The strength of the carbonated LFS also remarkably increased and was two orders of magnitude higher than that of the uncarbonated LFS. Following the carbonation, calcium carbonate, nesquehonite, and hydromagnesite were produced; these carbonates filled pores and bound LFS particles, which enhanced the strength of LFS.