It is known that the hydration of cement paste is influenced by a variety of factors, it is also known that some hydration products are gradually dehydrated at elevated temperatures. In doing so, different author studied the dehydration of hydrated cement pastes under different condition. In this work, samples of Hydrated Cement Paste (HCP) were prepared from Portland cement of high initial strength (CP V-ARI) with a water/cement ratio of 0.5. The morphological changes during hydration and dehydration by subsequent heat-treatments were analyzed by X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Thermal Gravimetric Analysis (TGA) was used to study the thermal stability of the HCP. Dehydrated cement powder samples (DCP) were obtained heat treating samples of HCP at 300, 500, 700 and 900°C. After 7 days of curing HCP samples exhibited no significant changes in its structure. HCP dehydrated at 500°C showed the absence of Ca (OH)₂ and calcium silicate hydrate. At 700°C the formation of β-2CaO.SiO₂, 3CaO.SiO₂ and CaO is observed. During heat treatment at 900°C the HCP revealed a significant mass loss of 36%.