Concrete slabs under log burner - Tile Advice

[Tilinghelppls]

Hi

We have recently had some old fashioned concrete slabs laid for under a log burner at the time when I asked the builder he advised any natural stone like this would be easy to keep clean and not stain.

I am finding this untrue even without the log burner in and Google seems to suggest they are very porous.

If I sealed the slabs (log burner is not in yet) would this prevent staining pls ? And any recommendations on sealants and how often

Thanks

 

[Tilinghelppls]

Photo of slabs attached that are in fireplace