If calcium carbonate (limestone) is heated above about 800 °C, it decomposes to form calcium oxide (quicklime) and carbon dioxide:

CaCO₃(s) ↓  CaO(s) + CO₂(g)

This is what happens in lime kilns. The reaction is endothermic.

At room temperatrure, however, calcium oxide reacts with carbon dioxide to form calcium carbonate:

CaO(s) + CO₂(g) ↓  CaCO₃(s) 

We make use of this reaction in absorption tubes filled with calcium oxide to protect other chemicals from carbon dioxide in the air. A flask fitted with an absorption tube containing calcium oxide is shown below.

The reaction between calcium oxide and carbon dioxide is exothermic.

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'Transporting' energy

The decomposition of calcium carbonate is reversible and so is the enthalpy change that accompanies it. It's been suggested that calcium oxide might be used to 'transport energy'. The idea is that limestone might be decomposed in parts of the world where solar energy is readily available. The quicklime produced could be transported in huge ships to other parts of the world where there is a great demand for energy. Here the calcium oxide would be combined with carbon dioxide to re-form calcium carbonate, transferring energy to the surroundings in the process.

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Would the decomposition of limestone in this process contribute to the world greenhouse gas effect?

Carbon dioxide is a greenhouse gas but the amount produced by the decomposition of limestone would be offset by an equal amount removed by combination with quicklime.

You can find more information on the enthalpy change of reversible reactions by subscribing to the Energy and change course.