*3.1. Mineral Phase of CaO-SiO2-MgO-Al2O3-Cr2O3-FeO System*

The XRD diffraction spectrum of the slag sample at different sampling times is shown in Figure 2. The intensity and position of the diffraction peaks were identical, indicating that the four slag samples contained the same mineral phase, namely, spinel and dicalcium silicate. In other words, the mineral phase structures remained constant as the heating time increased. Thus, prolonging the heating time at 1450 ◦C had no adverse effect on the mineral phase structures of the CaO-SiO2-MgO-Al2O3-Cr2O3-FeO system.

**Figure 2.** Effect of heat treatment time on the XRD diffraction pattern of the slag samples at 1450 ◦C.

The microstructures of the slag samples observed by SEM-EDS are shown in Figure 3. There were three kinds of mineral phases in all samples: (1) the white and regularly shaped phase is the spinel crystal, (2) the black striped phase is the α-C2S phase, and (3) the hoar phase is the glassy matrix. The proportion of spinel phase with respect to the other phases remained nearly stable for all of the executed treatments.

(**a**) 0 min (**b**) 10 min

**Figure 3.** *Cont.*

(**c**) 30 min (**d**) 120 min

**Figure 3.** Effect of heating time on the microstructure of the slag samples at 1450 ◦C.
