Accelerated carbonation of basic oxygen furnace slag and the effects on its mechanical properties

• Temperature, CO2 content, and relative humidity all affect carbonation reactions.
• The appropriate carbonation conditions for BOF slag were found out in this study.
• Consumption of free CaO is highly related to decreases in pH value of BOF slag.
• The carbonated shell on BOF slag can be up to about 200 μm in thickness.
• Carbonation is beneficial to the mechanical performance of BOF slag.

Basic oxygen furnace (BOF) slag, a byproduct of steel-making processes, is mainly composed of calcium compounds and thus has high potential for carbonation. The purpose of this study was to use an accelerated carbonation process to treat BOF slag and to examine the effects on the slag properties. Three BOF slags with different particle sizes (3.5–7 mm, 7–15 mm, and 15–25 mm) were tested, and the effects of carbonation temperature, CO2 content, and relative humidity (RH) were investigated. It was found that the pH value of the BOF slag samples decreased after carbonation, a result which was mainly attributed to the transformation from CaO to CaCO3. The appropriate carbonation conditions selected from this study were 200 °C of temperature, 40% of CO2 content, and 60% of RH. The X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy analyses both demonstrated the formation of carbonates in the treated slag. By observing cross-sections of the carbonated slags with an optical microscope, it was revealed that the thickness of a carbonated shell on the BOF slags can achieve about 200 μm. The mechanical properties of BOF slag, including bearing strength and particle cylindrical crushing strength, were improved after carbonation, and this shows that the carbonation of BOF slag can not only capture CO2 but also have benefits to its mechanical performance.