Granulated blast-furnace slag (GBFS) is obtained by quenching molten iron slag (a by-product of iron and steel-making) into water from a blast furnace in water or steam, to produce a glassy, granular product.

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The chemical composition of a slag varies considerably depending on the composition of the raw materials in the iron production process. Silicate and aluminate from the ore and coke are combined in the blast furnace with limestone or dolomite used as a flux. Slow cooling of slag melts results in an unreactive crystalline material consisting of an assemblage of Ca-Al- Mg silicates. To obtain a good slag reactivity the slag melt needs to be rapidly cooled or quenched. To cool and fragment the slag a granulation process can be applied in which molten slag is subjected to jet streams of water or air under pressure. In order to obtain a suitable reactivity, the obtained fragments are ground to reach the same fineness as Portland cement.