The petrographic characteristics of mud supported limestone samples from Egypt were examined before and after calcination at 950°C for 0·25, 0·5, 1 and 2 h. X-ray diffraction (XRD), X-ray fluorescence (XRF), transmitted light microscopy (TLM), scanning electron micro- scopy (SEM) and X-ray microcomputed tomography (μ-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 P2O5 show higher crystallinity of the resulted lime crystallites associated with lowest reactivity.