In-situ investigation on the carbonation behaviors of various mineral phases in steel slag: The role of RO phase

Exploring the mineral compositions of steel slag and the microstructure evolutions of its carbonation products is very important for deeply understanding the carbonation mechanism of steel slag, which can promote the application of steel slag in carbon capture and storage. In this paper, the carbonation behaviors of various mineral phases in steel slag were investigated by using an ingenious in-situ observation method through combing laser scanning confocal microscopy (LSCM), backscattered scanning electron microscopy (BSEM) and focused ion beam-transmission electron microscope (FIB-TEM). It is shown that dicalcium silicate (C₂S) and free lime (f-CaO) had high carbonation reactivity, while the calcium aluminoferrite with different Al/Fe ratios and mayenite (C₁₂A₇) exhibited extremely poor carbonation reactivity. The carbonation reactivity of RO phase was positively correlated with its Mg/(Fe + Mn) ratio. RO phase with a Mg/(Fe + Mn) ratio below 0.5 owned almost no carbonation reactivity, while the Mg²⁺ would leach from RO phase with a Mg/(Fe + Mn) ratio higher than 2.0 during the carbonation process, inhibiting the precipitation of CaCO₃, changing the micro morphology of CaCO₃, and generating spheroidal aragonite. The ingenious in-situ observation method developed in this study provides a very visual information, which can be further extended to more fundamental research.