The aim of the present work is to study the effect of limestone and ultra-fine limestone fillers on the physico-mechanical properties of hardened cement pastes. The particle size distribution was determined by laser diffractometer and SEM as well as the mineralogical composition by X-Ray Diffraction (XRD) and Fourier transform Infrared (FT-IR). The OPC was replaced with limestone filler up to 15wt% as well as ultra-fine limestone to form eight mixes. The hydration characteristics of hydrated cement pastes were evaluated by measuring the combined water content, bulk density and compressive strength up to 90 days as well as FTIR and XRD techniques. The results were shown that the coarse limestone filler particles enhance the reaction with C3A forming calcium carboaluminate hydrates which contain higher amount of combined water content than CSH. Also, the ultra-fine limestone filler particles act as a nucleation site for the precipitation of hydration products and activated the hydration of C3S and C2S phases.