This section will define granulated blast-furnace slag as a byproduct of the steel-making process and detail its chemical properties that make it suitable as a supplementary cementitious material (SCM). It will explore the mechanisms by which GBFS interacts with Portland cement in the hydration process, including the formation of additional cementitious compounds such as calcium silicate hydrate (C-S-H), which contributes to the strength and durability of concrete.