Abstract: Understanding the composition and reactivity of formic acid modified steel slag powder (SS) from micro scale is critical to extending the utilization of steel slag. In this study, four representative sources of SS were selected, which were dry chemically modified by formic acid (FA) with a mass fraction of 4 % on the basis of SS. The backscatter electron microscope, hydration calorimeter, X-ray diffractometer and fourier transform infrared spectrometer were applied to characterize the SS and SS pastes before and after modification. Results show that the 3 d and 7 d compressive strength of the different sources of SS pastes are low, and the 72 h cumulative hydration heat varies between 10~40 J/g due to their different mineral compositions. However, after FC modification, the 3 d compressive strength increases by more than 200 %, and the 72 h cumulative hydration heat rises to above 50 J/g with an increase above 80 %.it is found that the Ca(OH)₂ in SS can react with FA to produce calcium formate, while the calcium silicates in SS are not significantly affected.