Which tooth structure is calcium hydroxide placed?

On dentin: I believe what you're asking is where is it placed. Calcium hydroxide is usually placed in the area where the decay was closest to the nerve (pulp) of the tooth. Calcium hydroxide will help build up a barrier between the nerve and filling. There are some better materials in use now besides calcium hydroxide.

Over the nerve: Calcium hydroxide is used as a base to cover an nor peeve that has been exposed by dental work (in an effort to clean out decay) or it is place in an area that is very close to the nerve without the exposure. It acts like an insulator under a filling or a temporary filling. It is hoped that the tooth will self heal and reduce the possibility of a root canal.