I have bags of various natural stone, ceramic, and porcelain tiles lying around and was hoping to chuck them in my slab base to use them as hardcore. Any reason why I can’t do this?
Obviously I’d break them all up first. I LSO have SOE reinforcing steel mesh going in.

You can but it’s just not good practice.

Do you intend to use a vibrator (poker or beam screed type/air/electric or petrol powered) to vibrate the concrete as you should or are you just going to wet the concrete up (ruining water cement ratio and hence strength ) and use a 4 by 2 screed or whatever for the finish alone.
The latter will give you voids beneath the slabs as you will never get the tiles compacted as you would a sub base type material. Good luck whichever way you chose but I know which one I’d take.

Iolo, not sure but is he not meaning he’s actually gonna use the tiles within the mix and not as the sub base.

Tiles as aggregate in concrete is really not a good idea. The stone size needs to be interlocking (usually fro 20mm down to micromillimeters in size) to ensure strength.
Using tiles in the mix is not such a good idead.
But op, it’s your call and dependant on what use you have for it.

He is chucking it underneath not in the mix. chuck it in, It will probably be better than the ground the concrete will go on anyway. Just bang them about a bit so you don’t have whopping gaps.
Please remember its only bloody concrete . I assume that this isn’t a base for that damn great telescope on top of a mountain.
I would assume its got to be better than a corpse and I am sure that the mafia didn’t ask first!

The op asked for reasons why he shouldnt.
He can do whatever the hell he wants as long as its not something that building control will want to see.

It’s for a timber workshop.
Was thinking, break them up with some old concrete slabs etc, compact them down with a plate compactor, then mesh on top and three inches of concrete on top of that?
Either that, or fill up a skip with all the tiles, buy some crushed hardcore/type 1 and compact that down, then mesh and concrete?
Was hoping to save money on buying hardcore and getting rid of tiles in a skip.
Another good idea bites the dust.

It will be fine as hardcore providing there is a decent amount of it and you are putting a good 6 inch of concrete on it

If you are putting a reinforcing mesh in and your concrete isn’t too wet you should be okay. But I have found that ceramic tiles can float to the surface as they are less dense, ruining any nice smooth finish you’ve spent ages perfecting.

Do not place your mesh unless you space it. Better to place it after pouring and tread in. It does nothing in the bottom of the slab.