DDD - Rehydrate aggregate roots?

All my entities are implementation of interfaces. Most of their properties are read-only.
My repository holds a reference to the library project where i hold all the interfaces, so technically speaking, the repository can save the aggregate root without knowing anything about it's de-facto implementation (something i believe to be a +1).
The problem here is: if most of the properties are read-only, how can I rehydrate a aggregate root without breaking OOP principles? should the repository hold a reference to the domain project and be aware of the concrete implementation of interfaces?