GGBS slag is by product of iron and steel manufacturing process. It contains the entire basic constituent of OPC but not exactly in the same ratio and chemical form. Slag itself has no cementing property but when mixed with OPC, it develops hydraulic properties similar to cement.
Advantages: GGBS replaces a substantial proportion of normal cement OPC in RMC. The mixing varies from 30% to 70% approximately (generally 50%). The higher the proportion, the better is durability for concrete. GGBS is also used in other forms of concrete including site batched and precast.