Limestone is mainly calcium carbonate, CaCO3 which breaks down when heated to form calcium oxide and carbon dioxide (thermal decomposition)
Calcium oxide + water forms calcium hydroxide
Limestone and its products have many uses including cement, mortar and concrete.
Calcium carbonate --- (heat) ----> calcium oxide + carbon dioxide
Other metal carbonates decompose in the same way, including sodium carbonate, magnesium carbonate and copper carbonate.
Metals high up in the reactivity series need a lot of energy to decompose them.
Metals low down in the reactivity series have carbonates that are easily decomposed.