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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.

The petrographic characteristics of mud supported limestone samples from Egypt were examined before and after calcination at 950uC for 0?25, 0?5, 1 and 2 h. X-ray diffraction (XRD), X-ray fluorescence (XRF), transmitted light microscopy (TLM), scanning electron microscopy (SEM) and X-ray microcomputed tomography (m-CT) were used for technological samples characterisation. Both the free lime content and the hydration behaviour of the resulted quicklime after calcination were measured. The produced lime is unreactive at all applied firing conditions, except the Chalk lime at 1 and 2 h soaking, despite the high free lime content of the samples. The microstructure of the mud supported limestones is of significance on the resulted quicklime reactivity, where microfractures are formed along the cavities inside the microfossils. The mineralogical and the chemical compositions of the studied mud supported limestones are the main controlling variables on the degree of the quicklime crystallinity and consequently the reactivity. Samples enriched with P 2 O 5 show higher crystallinity of the resulted lime crystallites associated with lowest reactivity.

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