Abstract: A new cementing agent, RSGC, designed for filling was prepared with red mud, blast furnace slag, flue gas desulphurization gypsum and a little cement clinker. The adaptability of RSGC used in different mines was studied and the early hydration including microstructure and structural changes of silicates and aluminates were investigated by scanning electron microscopy and nuclear magnetic resonance techniques. It is found that RSGC applies not only to iron ore mines but also to copper mines, gold mines and lead——zinc mines which generally adopt the filling mining method. Filling materials used RSGC exhibit significant strength and water retention advantages over traditional cement filling materials. The RSGC paste hardens at 2.5 h. Ettringite generated before 2.5 h makes important contributions to the setting and hardening. After 4 h, parts of the tetrahedrally-coordinated aluminium turn into an octahedrally-coordinated state, and polymerization of silicates increases. This phenomenon means that ettringite, C——S——H gels and zeolites generate continuously. The combination of these hydration products in various sizes and shapes forms a compact arrangement and a dense structure, which is probably the reason why the RSGC paste obtains strength from 4 h and the strength increases dramatically from that time.