What to add to a broken conctrete sub base

[master1234]

I have dug up an old tarmac drive which had approximately 4-5" of concrete underneath it.
I am breaking the concrete down into lumps approx 6" square and adding other harcore (from previous building projects I have an abundance of bricks, paving slabs etc) to it to give a depth of 6".
My question is whether I would be better adding a thin layer of concrete (again from previous projects I have surplus ballast available) to the top of the rubble to bind/seal it or should I add compavted sand/ballast or other materials instead to complete my sub base?

[lutonlagerlout]

make sure you wack the hardcore with a wacker plate first ,to make sure theres no voids or soft spots,then lean mix the top with the ballast if you like,this is like a 1:10 mix with very little water,should look crunchy
make sure you get the levels right or it will all have to come back out
regards tony