The effect of elevated temperatures and subsequent rehydration on the microscopic mechanical properties and physicochemical changes of cement pastes was investigated. Cement pastes with different grades (CEM I 42.5, CEM I 52.5) and different water-to-cement ratios (0.3, 0.4) were exposed to target temperatures of 300 °C, 600 °C, and 900 °C, followed by rehydration. Several characterization techniques, including the Vickers microhardness test, X-ray diffraction, thermogravimetry, and