Slag is a liquid residue from making iron. It consists of calcium silicate. The silicon dioxide in iron has to be taken away. The calcium oxide reacts with the silicon dioxide solid to make liquid calcium silicate, which is lighter than iron. It is drained out of the blast furnace. Slag can be used to make concrete. Air can be blown through molten slag to make a fluffy insulation made mineral wool.
Concrete made with blast furnace slag is almost pure white compared to ordinary concrete which is greyish.