GGBS is a fine powder, used in concrete as "cement", major use in ready mixed concrete (RMC) or in the production of Portland slag cement. GGBS replaces a substantial proportion of normal cement (Ordinary Portland Cement) 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 in other forms of concrete including site-batched and pre-cast
Replacement of Portland Cement by adding up to 70% GGBS, reduce cement content in concrete from about 530 Kg/m3 to 160 Kg/m3 and despite of this large reduction of cement in concrete, ultimate strength is not affected, also considerable improvement in durability properties is observed.
GGBS slag is byproduct 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 or activated chemically it develops hydraulic properties similar to cement. The calcium oxide (CaO) found in slag is tied up as calcium silicate, calcium aluminates and calcium aluminosilicate. Although these compounds are not identical to those found in Portland cement (i.e. tricalcium silicate, tricalcium aluminate, etc), they hydrate when activated by calcium hydroxide generated during the hydration of Portland cement. Since GGBS is almost 100% glassy, it is generally more reactive.
This process of quenching is called granulation because it produces glassy granules similar in appearance to coarse sand, having excellent cementitious property when ground to high fineness and mixed with cement.