Slag

Summary: Molten calcium silicate (CaSiO₃) formed in the blast furnace when limestone reacts with sand (SiO₂) impurities. Tags: igcse chemistry definition metals Created: 2026-07-14 Last Updated: 2026-07-14

Slag is the molten calcium silicate (CaSiO₃) produced as a waste product during the extraction of iron in the blast furnace, formed when limestone (calcium carbonate) first thermally decomposes to calcium oxide and then reacts with the sandy silica (SiO₂) impurities present in the iron ore. The overall reaction sequence is CaCO₃ → CaO + CO₂ followed by CaO + SiO₂ → CaSiO₃, effectively converting the unwanted silica gangue into a separable liquid that can be removed from the furnace charge, preventing it from contaminating the iron product. Because slag has a lower density than molten iron, it floats on top of the liquid metal and can be tapped off separately from a higher outlet in the furnace, allowing continuous operation without interrupting iron production. Far from being simply discarded, slag has important commercial uses — it is widely employed in road construction as an aggregate material, in the manufacture of cement and concrete where it improves durability, and as a soil conditioner in agriculture to neutralise acidic soils due to its basic nature. In IGCSE exams, you must be able to write the chemical equation for slag formation and explain its dual role in removing silica impurities and protecting the molten iron from re-oxidation.

Key term — IGCSE Chemistry (0620/0971) glossary.