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.