Since 1918, The NSA has promoted the use of Blast Furnace and Steel Furnace Slag. Blast Furnace slag has been called the “All-Purpose Aggregate” as it can be used in all construction applications as either a normal weight or lightweight (expanded or pelletized) aggregate depending on how it was formed and processed. Blast Furnace Slag is also quickly quenched by water or air to produce Granulated Blast Furnace Slag.