Fillers are specially selected, natural or artificial inorganic materials, which improve the physico-chemical and mechanical properties of the cement such as workability or water retention. They can be inert or have slightly hydraulic, latent hydraulic or pozzolanic properties. They cause no appreciable increase of the water demand of the cement, in addition to not impairing the resistance of the concrete or mortar to deterioration in any way or reducing the corrosion protection of reinforcement. Fillers are normally either limestone or any inert material such as sand. The aim of this investigation is to study the effect of substitution of limestone for Homra in pozzolanic cement. The effect of limestone replacement was studied by the determination of the combined water, free lime contents, bulk density, total porosity and compressive strength. The results show that the addition of limestone reduces the initial and final setting time, as well as total porosity, whereas the free lime and combined water increase with limestone content. It can be concluded that limestone fills the pores between cement particles due to formation of carboaluminate, which may accelerate the setting of cement pastes.