•Hydraulic and pozzolanic behavior of basic oxygen furnace slag (BOFS), ladle furnace slag (LFS), and ground granulated blast furnace slag (GGBFS) are studied.•Multiple tests are performed to determine the feasibility of these slags as SCMs.•GGBFS shows the best performance; BOFS and LFS should not be used as SCMs as-is.•The combined use of test methods is more accurate. Limited applications for the large amounts of steel slags produced globally exist. In this study, the hydraulic and pozzolanic behavior of basic oxygen furnace slag (BOFS) and ladle furnace slag (LFS) are studied and contrasted with the behavior of ground granulated blast furnace slag (GGBFS). Pozzolanic test, isothermal calorimetry, thermogravimetric analysis, and compressive strength test are performed to determine the feasibility of these slags as supplementary cementitious materials (SCMs). The pozzolanic test is performed by measuring the heat release and calcium hydroxide consumption of slag-calcium hydroxide blends at 50 °C and pH 13.5. The other tests are performed on cementitious pastes with 20% slag replacement by mass at 23 °C and 50 °C. GGBFS shows the best performance among all slags in the tests, followed by BOFS, followed by LFS. It is suggested that BOFS and LFS should not be used as SCMs as-is. The poor performance of LFS is due to flash setting, likely caused due to the rapid reaction of C12A7, which causes hydration retardation and poor strength gain. The BOFS shows moderate performance as it has low hydraulic activity, likely explained by its low amorphous content and the presence of low amount of reactive crystalline phases. An important finding from this study is that in order to state whether materials could be used as SCMs or not, the combined use of pozzolanic test, isothermal calorimetry, and compressive strength test is more accurate than the use of only one test method. While the results obtained may here not be broadly generalizable to other materials due to variations in slag chemistry, the approach suggested may be generalizable to obtain insights into whether any powder may be used as SCM.