The aim of this work is to study the effect of Egyptian limestone microstructure on the activation energy required for the onset of its calcination. Limestone samples were investigated for their bulk density, apparent porosity, crushing strength, microstructure and chemical composition by applying transmitted light microscopy, SEM and X-ray fluorescence methods respectively. The samples were calcined for 0?25-1 h at 800-950uC. The activation energy of calcination was calculated from the Arrhenius equation. Triple junction fractures detected after calcination are mainly attributed to euhedral-subhedral calcite crystals in the sparite matrices of their limestone samples. Samples with higher 'grain' content and lower bulk density show relatively lower activation energy and rate of lime crystallite growth. This is due to the original 'grain' channel pores and the formation of triple junction fractures during calcination, which facilitate transfer of hot gases and diffusivity of the evolved CO 2 . It is finally concluded that calcination of limestone is a temperature, chemical and microstructure dependent process.