Rapid and Simple Identification of Free Magnesia in Steelmaking Slag Used for Road Construction Using Cathodoluminescence

Reuse of steelmaking slags is important for the effective use of natural resources. Free magnesia (f-MgO) in steelmaking slag may cause serious problems because of a hydration reaction followed by expansion when it is reused for road construction. We present a promising method to identify f-MgO that causes volume expansion rapidly by investigating cathodoluminescence (CL) images and spectra of a steelmaking slag sample. f-MgO emitted red–orange luminescence from a peak at 755 nm. The mineral phases, 3CaO·SiO₂ and 2CaO·SiO₂, emitted red and yellow luminescence from peaks at 720  and 590 nm, respectively. No luminescence of FeO and 2CaO·Fe₂O₃ was detected. f-MgO changed its composition in the slag sample that was immersed in hot (70 °C) water for a week. f-MgO that was responsible for the volume expansion (combined content of FeO and MnO below 30 mass pct) retained a red–orange luminescence, whereas the other f-MgO lost luminescence. The CL intensity of the f-MgO that retained luminescence was more than 10 times larger than that of 3CaO·SiO₂ and 2CaO·SiO₂. Therefore, we can distinguish f-MgO that causes volume expansion by detecting the intense red–orange luminescence from the peak at 755 nm in the CL image within a few seconds.