This work aims to make a comparative study on high-fineness, basic-oxygen-furnace, carbon steel slag (CS) and electric-arc-furnace stainless steel slag (SS) with high fineness to assess the possibility of using them as hydraulic binders. Test results reveal that CS has higher early hydraulic properties than SS, which has a higher rate of activity at later ages than at early ages (but still lower than that of CS), confirmed by compressive strength and mercury intrusion porosimeter (MIP) test results. Calorimetric analysis showed exothermic reactions in both slags. X-ray diffraction (XRD), thermogravimetry/differential scanning calorimetry (TG/DSC), Fourier transform-infrared spectra (FTIR), and scanning electron microscopy (SEM) results indicated that the hydration products of CS were mainly calcium silicate hydrate (C-S-H), calcium aluminate hydrate (C-A-H), calcium aluminate silicate hydrate (C-A-S-H), aluminum hydroxide (Al(OH)3 [AH3]), and calcium hydroxide (Ca(OH)2 [CH]); however, C-S-H was the only hydration product of SS. The microchemical analysis showed that the average silicon/calcium atom ratios for the hydration products in 90-day hydrated CS and SS pastes were 0.31 and 0.52, respectively. Wavelength-dispersive X-ray spectroscopy (WDS) mapping at 90 days for both slags revealed that silicon and calcium are included in the hydrated matrix surrounding slag particles, with the addition of aluminum in CS.