Abstract: As the largest iron and steel producing country, China creates a large amount of steel and blast furnace slag each year. If these metallurgical slags are not properly handled, not only do they occupy land and damage the environment, but they also result in a waste of resources. Using steel slag as a building material usually leads to poor stability due to the high free lime (f-CaO) content. To improve its stability and gelation performance, steel slag usually needs to be modified. In this study, based on the chemical and mineral compositions of steel and blast furnace slags, a thermodynamic calculation of the modification process was performed. The results show that SiO₂ in blast furnace slag reacts with f-CaO in steel slag to generate a gelatinous phase and f-CaO in the modified steelmaking slag decreases simultaneously. Here, the properties of steelmaking slags modified with hot blast furnace slag, including the mineral composition, f-CaO content, and viscosity, was studied using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and energy spectrum analysis. The results show that viscosity increases with increasing blast furnace slag content and the f-CaO and RO contents decrease. The gelation performance of the modified steelmaking slags is improved, especially in steelmaking slags modified with 10% blast furnace slag at 1550℃. When the content of 2CaO·SiO₂ and 3CaO·SiO₂ increase significantly and the f-CaO content decreases to 1.64%, the modified slags meet the requirements for building materials. Additionally, the study also attempted to use coke to reduce the iron in the modified slag. As a result, the facile separation of slag and iron is realized, and the grindability of the modified slag has been improved.