Portland cement (OPC) and steelmaking industries exhibit high production levels and will continue to grow to meet global infrastructure/urbanization requirements. However, their production processes are not environmentally friendly, producing approximately 5-8% of global CO2 emissions. They introduced several improvements in their processes to mitigate their negative externalities towards cleaner production. However, the steelmaking industry generates large volumes of solid waste, such as ladle furnace slag (LFS), which could be used as part of the new sources of supplementary cementitious material (SCM) to reduce OPC consumption once its main technological barriers related to fast setting, variable mechanical performance, volume instability, and environmental impact issues are resolved. This study explores a safe and high-value application that valorizes a local LFS as a new source of SCM. Pastes were prepared to study the effect of medium-to mid-high LFS-OPC content replacement by weight. Fresh/hardened state properties, environmental impact by leaching, and chemical/mineralogical characterization of raw/hydrated samples were tested, and a new methodology was proposed to calculate the degree of hydration of the LFS-OPC system considering the particularities of this type of slag to analyze the long-term performance evolution from 7 to 900 days. Our results suggest that using LFS as part of the era of SCMs is feasible because of its adequate mechanical/environmental performance, reaching values over the compressive strength index performance at 28 days and behaving as an inert waste. It is essential to consider the changes in workability and setting time. In addition, it displayed a higher potential volumetric instability depending on the LFS content. Finally, the microstructural characterization revealed a change in the typical OPC reactions, increasing the AFm/AFt hydration products because of the higher content of Ca/Al reactive phases contributed by the slag.