Reuse slab mortar for Foundation

Hello.

I'm looking to re-pave the patio. I've removed the slabs, and I will get onto removing the old mortar.

Could I use the old mortar as a basis for the foundation for a small shed as part of a future project?

I was thinking I could use it along with some aggregate, compact it with a wacker plate before topping it off with concrete.

Appreciate any help on this.

I would do and have done. I knocked down an old 70s concrete block wall last year and used the many broken blocks and dry mortar to form part of the hardcore base for my new patio. I then used a dry sand/cement mix which was all compacted down and finally topped with a wet mortar with slabs on top. As long as the hardcore is a good 4" deep and the mortar bed is around 3" you'll be fine.