Natural filler for reconstructing old stone house

Hi all.

I'm restoring an old stone house in the north of Portugal to live in.
the stones are very large and most have huge gaps in them.
Ive seen some kind of foam sprays available in hardware stores but I was wondering if there were any natural options such as maybe clay?
Cheers

Sebastian  

I lived in an old stone building like that.
Mortar was made out of sand, crushed seashells, and I think a little clay.

Unfortunately acid rain does tend to eat away the mortar over time.
Tuckpointing is a real chore too.