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When exposed to elevated temperatures, concrete suffers physicochemical changes, resulting in reduction of mechanical properties, cracking and spalling. These macroscopic changes are related to changes in the microstructure of concrete. In this paper, the microstructure of thermally damaged concrete from real-scale reinforced concrete columns was studied, using scanning electron microscopy (SEM) and x-ray diffraction (XRD). Two real-scale reinforced concrete columns, with different concrete mixtures, were exposed to ISO 834 fire standard curve for 4 hours. After heating, concrete core samples were drilled, sectioned and analyzed in different column section depths. Core samples from reference column not exposed to high temperatures, were also obtained. Macroscopic observations showed a change in the color of concrete exposed to high temperatures. SEM micrographs presented distinct changes in morphology, like coarsening, and the presence of cracks and voids. XRD diagrams showed a reduction of portlandite and presence of larnite as depth increases. [ABSTRACT FROM AUTHOR]