The effect of limestone on the compressive strength, hydration and microstructural development of Ultra High Performance Concrete (UHPC) was investigated.This conference paper summarizes the results of a larger study submitted for publication.The mix with 54% replacement of cement by limestone showed better workability and higher compressive strength (170MPa at 56 days) compared to a classical mix (155MPa) with no limestone replacement.The hydration kinetics were compared for different replacement levels using isothermal calorimetry.The results show that the released heat evolution normalized per unit volume of binder corresponds well to the compressive strength development.The UHPC with 54% limestone has similar hydration kinetics to the classical UHPC.The comparison of phase development between classical UHPC and the one with 54% limestone was carried out using X-ray diffraction with Rietveld method.The results show that the microstructure was developed rapidly in the first 7 days for both mixes, but due to the dilution effect of limestone addition the low clinker UHPC has more Portlandite formed, and kinetics of pozzolanic reaction is higher than the classical one.Compared to the classical UHPC, the hydration degree of cement in low clinker UHPC was increased from 39% to 66%.The image analysis of UHPC matrices were determined on scanning electron microscopy.The images show that the low clinker UHPC has the same dense microstructure as the classical UHPC.