Ground granulated blast furnace slag (GGBFS), a by-product of the steel manufacturing industry, is used as an effective partial cement replacement material, and has already been shown to improve several performance characteristics of concrete. The reactivity of GGBFS depends on the properties of slag, which vary with the source of slag, type of raw material used, method and the rate of cooling. A new relationship between the Hydraulic Index (HI) of slag at 7 and 28 days (HI7 and HI28) and the influencing properties of slag, namely, glass content, fineness and chemical composition is given by employing multiple regression analysis on 37 slag samples from various sources. HI7 and HI28 were then mapped onto a Slag Activity Index (SAI) plot, giving an indication of the ranges of strength of slag. 17 refs.