Blast-furnace slag (BFS) is formed from a liquid at 1350-1550 °C during the manufacture of iron. When reaching the bottom of the furnace, the liquid slag forms a layer on top of the molten iron due to its lower density. Being separated from the molten iron, the liquid slag is cooled in the air or with water. Depending on the way of cool process, three main categories of BFS could be produced, which are namely ground granulated BFS, pelletized slag and air-cooled slag.