What happens when you burn limestone?

It stops being limestone.

Heat the rock to around 900 degrees and it gives up its carbon dioxide. What is left behind is quicklime, a different material that behaves in a different way.

Quicklime is hungry for water. Add some, and the reaction gives off real heat, enough to bring water to the boil.

Add more water and it becomes slaked lime, the soft white material used in mortar and whitewash for thousands of years.

Worth knowing
Self-heating food cans use this reaction. Pull the tab, water reaches the quicklime inside, and the meal warms itself. No flame and no power, just a rock that was burned and is now meeting water again.

Source: European Lime Association.