AbstractCement is the strength-forming component of concrete. It has been a major building material for more than a century. However, its production is accountable for a considerable percentage of global CO₂ emissions and is very energy-intensive. The Ordinary Portland Cement (OPC) production is a thermal process at around 1450 °C. This study shows that the reactivation of Hydrated Cement Powder (HCP) can be successful at a much lower temperature. Therefore, the possibility of using HCP to replace parts of OPC in concrete reduces the energy consumption and the CO₂ emissions associated with OPC production. HCP, which may ultimately stem from recycled concrete, needs treatment to produce new concrete of the required mechanical strength. Using reactivated HCP in concrete, an optimum strength is achieved by heating the HCP in the range of 400–800 °C. Among other factors, the type of cement used influences the optimum heating temperature and attainable strength. This paper shows that 600 °C is an optimum heating temperature using the OPC type CEM I 52.5R. The crystalline phase transitions resulting from the thermal treatment were analyzed by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetry (TG). The heat released during hydration was investigated, and scanning electron microscopy (SEM) displays the microstructure evolution. OPC can be partially replaced by thermally treated HCP in mortar, attaining similar mechanical strength values.