Definition - What does Slag mean?
Slag is the silicon dioxide and metal oxide mixture left over as a by-product of extracting metal from its ore during the smelting process. Once the metal is smelted out of its ore, the ore appears to be glassy. This glassy appearance is caused by the formation of slag on the ore's surface during the smelting process.
Slag consists of the undesired impurities in the metal ore extracted during the smelting process. However, it also protects the liquid metal undergoing the smelting process by providing an oxide layer (crust) on the smelted metal. Once the metal is smelted to a satisfactory level, the slag is skimmed off the top and can be used for secondary purposes such as manufacturing concrete, fertilizers, or road materials.
Corrosionpedia explains Slag
Smelting is the process of extracting large quantities of base metals from their ore. Smelting uses a chemical reducing agent in the presence of heat to decompose the ore (i.e., remove unwanted elements from ore in the form of gas or slag) and subsequently leave behind only the base metal.
Because slag contains unwanted elements from an ore, it is considered a by-product of the smelting process. Although slag is a by-product, it never goes to waste. Slag is used in many industrial processes, such as mixing it with concrete to impart strength, or acting as ballast in road material. It is also used as a component of phosphate fertilizer.