Limestone
Summary: Limestone is a sedimentary rock composed mainly of calcium carbonate (CaCO₃). In the blast furnace, it thermally decomposes to form calcium oxide and carbon dioxide. The calcium oxide then reacts with sand (SiO₂) to form slag. Tags: igcse chemistry definition metals extraction Created: 2026-07-14 Last Updated: 2026-07-23
Limestone is a naturally occurring sedimentary rock primarily composed of calcium carbonate (CaCO₃). It is one of the three essential raw materials added to the blast furnace alongside hematite (iron ore) and coke (carbon).
Role in the Blast Furnace
Limestone removes the sandy impurity silicon dioxide (SiO₂) from the iron ore:
Step 1: Thermal Decomposition
CaCO₃(s) → CaO(s) + CO₂(g)
At the high furnace temperature (~1500°C), limestone decomposes into calcium oxide (quicklime) and carbon dioxide. This is a thermal decomposition reaction.
Step 2: Slag Formation
CaO(s) + SiO₂(s) → CaSiO₃(l)
The calcium oxide (a basic oxide) reacts with silicon dioxide (an acidic oxide — sand) to form calcium silicate (CaSiO₃), known as slag.
Why Slag Is Important
- Slag is molten and less dense than iron → it floats on top of the molten iron
- This protects the iron from re-oxidation by the hot air blast
- Slag is tapped off separately and used for road construction and cement manufacture
The Limestone Cycle
CaCO₃ →(heat)→ CaO + CO₂ CaO + H₂O → Ca(OH)₂ Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Key Facts
- Limestone = CaCO₃ (calcium carbonate)
- In the blast furnace: CaCO₃ → CaO + CO₂ (thermal decomposition)
- CaO + SiO₂ → CaSiO₃ (slag formation)
- Slag removes sandy impurities from the iron ore
- Slag floats on molten iron → protects from re-oxidation
- Products from limestone are also used in the limestone cycle and to make cement and concrete
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.