A low-carbon cementitious material (LCCM) containing C4A3Š, C4AF, and C2S was designed and synthesized using coal gangue, carbide slag, steel slag as raw materials. The mineral composition of the clinker was optimized, and the calcination system (calcination temperature and holding time) was systematically investigated to determine the optimal parameters. The phase evolution, f-CaO content, and microstructural characteristics of the clinker under different calcination conditions were analyzed. Furthermore, the hydration characteristics of the LCCM was comprehensively characterized through hydration heat analysis, chemically bound water measurement, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The results revealed the hydration product formation, molecular structure evolution, and microstructural development during the hydration process.