In this study, blended cement pastes are made by different replacements of OPC by alumina (A) using standard water of consistency. The pastes were hydrated for various ages of hydration, up to 28 days. The mechanical and physico-chemical characteristics of the hardened cement pastes were studied and related to the structure of the hardened pastes and the rate of alumina replacement in the blended cement OPC-A pastes. The initial and final setting times of the blended cement pastes were reduced significantly when increasing the amount of partial substitution of alumina in OPC- A cement pastes. It was found that the optimum replacement of OPC by for the improvement of hydraulic properties of blended cement pastes is 5%. The replacement of alumina in mix A5 causes a marked improvement of the microstructure of pastes. The DSC thermograms and XRD analysis for investigated cement pastes indicated the formation of nearly amorphous calcium silicates hydrates, calcium sulphoaluminate hydrates, calcium aluminate hydrates and Portlandite. The SEM micrographs showed that the partial substitution of OPC by 5% alumina leads to more dense structure as compared to the neat OPC paste.