This study compares the influence of two limestone fineness on strength development and cement hydration. The replacement of clinker by 20% of fine limestone (dv,50 = 2 μm and SSA = 4,21 m2/g) gives similar strength development to PC up to 7 days. Fine limestone enhances cement hydration and offsets the dilution effect caused by the decrease of clinker amount. The accelerating effect of the limestone was explained by a higher degree of undersaturation with respect to C3S. The influence of coarse limestone (dv,50 = 130 μm and SSA = 0,46 m2/g) on cement hydration is limited to dilution effect. The combination of fine and coarse limestone was also studied. A linear correlation was obtained between strength and gel space ratio for all investigated systems. Furthermore, the results showed that despite the similarity of specific surface area, limestone is more effective than quartz for the enhancement of cement hydration.