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It was also ground into powder to add to glazes for use in ceramics.
Some of the earliest such uses for the by-products of slag have been found in ancient Egypt.
This rapid cooling, often from a temperature of around 2,600 °F (1,430 °C), is the start of the granulating process.
This process causes several chemical reactions to take place within the slag, and gives the material its cementitious properties.
The smelting of copper, lead and bauxite in non-ferrous smelting, for instance, is designed to remove the iron and silica that often occurs with those ores, and separates them as iron-silicate-based slags.
Slag from steel mills in ferrous smelting, on the other hand, is designed to minimize iron loss and so mainly contains oxides of calcium, silicon, magnesium, and aluminium.
Slag is drawn off the furnace just before the molten steel is poured into ladles for ingotting.
Ferrous and non-ferrous smelting processes produce different slags.
Ground granulated slag reacts with a calcium byproduct created during the reaction of Portland cement to produce cementitious properties.
The major components of these slags therefore include the oxides of calcium, magnesium, silicon, iron, and aluminum, with lesser amounts of manganese, phosphorus, and others depending on the specifics of the raw materials used.
Because of the slowly released phosphate content in phosphorus-containing slag, and because of its liming effect, it is valued as fertilizer in gardens and farms in steel making areas.
However, slags can contain metal sulfides and elemental metals.
While slags are generally used to remove waste in metal smelting, they can also serve other purposes, such as assisting in the temperature control of the smelting, and minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal.
Concrete containing ground granulated slag develops strength over a longer period, leading to reduced permeability and better durability.