THE EFFECT of replacing ordinary Portland cement (OPC) and granulated blast furnace slag (GBFS) with by- pass cement dust (CKD) at 0%, 2.5%, 5%, and10 mass % after 3, 7, 28 and 90 days of curing period was studded. One blend of slag rich cement was prepared, namely30/70 mass % Portland cement and granulated slag, respectively. Two different mixes were made from this blend, the first contains (70% GBFS (with variable amounts of OPC and CKD, the second contains (30% OPC)with variable amounts of GBFS and CKD. The hydration behavior was followed by estimation of combined water, bulk density and gel/space ratio. The required water for standard consistency, setting times and compressive strength were also determined. The required water for standard consistency increases with CKD content. The compressive strength and bulk density decrease with cement kiln dust. Combined  water content increases with CKD. Initial sitting time for blended cement pastes with (30% OPC) is elongated up to 5 mass % then accelerated. The final sitting time of cement pasts (30 % OPC)) is elongated with CKD. The results obtained were confirmed by XRD.