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 1H Nuclear Magnetic Resonance spectroscopy, were employed to assess changes in the microscopic mechanical and physicochemical properties of the cement pastes resulting from the heating and rehydration treatments. The results indicate that the cement pastes with higher grades and a higher water-to-cement ratio exhibit better resistance to high temperatures. The heating process alters the water distribution and structure of C-S-H gel, leading to the collapse of its interlayer structure and an increase in gel porosity. Elevated temperatures (300 °C and 600 °C), followed by rehydration, enhance the Vickers microhardness of the cement pastes. However, excessively high temperatures (900 °C) weaken the micro-mechanical properties and may cause damage. Cement pastes heated to 600 °C show a more significant recovery in micro-mechanical properties compared to those heated at 300 °C, which is attributed to the rehydration of a new amorphous nesosilicate phase formed at 600 °C.