Blast furnace slag is used as an admixture for concrete, and its characteristics are excellent in long-term strength and durability. When mixed internally in cement, it is expected to reduce the environmental impact by lowering CO₂ emissions. and it is widely used in the civil engineering and construction industries felid. However, it is difficult to ensure the early strength of concrete using blast furnace slag. Therefore, it is difficult to use concrete for precast products, which requires demolding strength and strength to withstand the early introduction of tension force by prestressing. In this study, focusing on the Blast-furnace slag Portland cement type C, whose internal mixing ratio of blast furnace slag is 70%, to promote the use of blast furnace slag, an experimental study was conducted to improve the early strength of Blast-furnace slag Portland cement type C. The study was undertaken to accelerate the cement hydration reaction using limestone powder, etc., and to determine the suitable curing conditions for early strength improvement, using strength development and analysis of hydration products as indicators. As a result, it was found that the early strength required for precast concrete can be obtained if High-early-strength Portland cement is used as the base cement and limestone powder is used as a substitute for fine aggregate. On the other hand, if the temperature is higher than 40° C, strength development is inferior, and the appropriate curing temperature is in the range of 20° C to 30° C.