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The calcium carbonate in the limestone thermally decomposes to form calcium oxide.

calcium carbonate → calcium oxide + carbon dioxide

CaCO₃(s) → CaO(s) + CO₂(g)

The calcium oxide then reacts with silica (sand) impurities in the haematite, to produce slag – which is calcium silicate.

calcium oxide + silica → calcium silicate

CaO(s) + SiO₂(s) → CaSiO₃(l)

This reaction is a neutralisation reaction. Calcium oxide is basic (as it is a metal oxide) and silica is acidic (as it is a non-metal oxide). Molten slag is tapped off at the bottom of the blast furnace above the molten iron as molten slag is less dense than molten iron.