GROUNDED blast furnace slag GBFS is one of common supplementary cementitious materials in Portland cement concrete, and is capable of improving the performance of concrete and reducing the costs. There are strong environmental and energy reasons for developing a wide range of pozzolanic cements. The aim of this work is to study the effect of substitution of cement with GBFS by the determination of water of consistency, initial and final setting times, combined water and free lime contents, bulk density, total porosity and compressive strength. The results show that the substitution of GBFS decreases the water of consistency, initial and final setting times, 50 % GBFS increases compressive strength as well as total porosity, whereas the free lime and combined water decrease with the granulated slag content. The hydration products of some GBFS cement pastes were identified by the aid of XRD, DTA and IR techniques.