Recycled concrete is used for several different purposes. It can be used "as is" for bulk
fill / drainage / etc.
When cleaned, crushed, and graded for size, it can be used as a base underneath a new concrete pour.
Recycled concrete can also be used as aggregate within the new concrete. Several factors must be taken into account to ensure that it will not weaken the integrity of the new concrete (moisture retention, etc). Typically, up to 30% of the aggregate can be replaced with properly processed recycled concrete.
However, concrete is generally recycled in large scale plants where they can reliably clean, crush, sort, and distribute the incoming material. It is doubtful that you could do this economically in the small scale.
More recycled concrete info
I see deeper problems here. What is the foundation for those walls? Why is all that sand there, and why is it just inches below the door threshold? What is with those gaps in the second picture, that you can see a void behind.
I see the spray foam scraps, which tells me you just closed up those walls, and I see the plastic sheeting that tells me "vapor barrier ends here". Also, the edge of that subfloor is suspiciously clean for the last 6", which tells me a wall was there.
Is this by any chance a porch that you are closing in? Is there a concrete slab under that sand, or soil. If it's just soil, then your are allowing moisture and insects access to your framing.
EDIT - RESPONDING TO COMMENTS
I believe this was a porch that got "annexed" into the interior space of the house.
The right way to do this is to dig all that sand and soil out, and also excavate under the walls (they will need to be temporarily supported). Then prep the excavation (I'm not sure if a vapor barrier is called for, but you have one coming out from your existing foundation, so ask a local builder) and then pour a slab under the whole deal - walls and all. You would probably be replacing the bottom plates of the walls in the process.
A concrete slab is going to be cold in the winter, and I might worry about condensation in that area - plus, the fact that it is sunken, and kind of small - is it really that useful?
If the ceiling height will allow it, wouldn't it be nicer to bring the floor up to match the rest of the house? This will mean re-installing the door, but the you could just frame in a subfloor, put a vapor barrier down, and end up with a MUCH nicer space there.
Best Answer
You do not want to just place the concrete tiles on native ground soil. A proper installation would be to dig out the native soil down to a solid base. Then lay in a layer of crushed rock to aid in water drainage. Directly under the concrete tiles would be some sand that makes it possible to level and even out the tile installation.
With the correct materials that are properly packed in and if the water drainage from the area is implemented correctly an installation should stand up well without adding concrete under the tiles. But keep in mind that they will move around if there is winter frost action, an unstable base soil, very large vehicles driving over them or if there is a regular flow of water over and between the tiles down into the base materials. This movement can be mitigated by placing concrete under the tiles but there is a tendency to make this layer fairly thin and this can still crack and allow tile movements.
My recommendation, if you want a driveway that will stand up to these stresses, would be to not use these tiles at all. Instead put down the crushed rock base and sand as a base. Then place in heavy duty wire mesh AND rebar in both directions in a manner that it ends up in the concrete as opposed to laying on the sand. Pour concrete to a good thickness (using 2x6 forms is far better than using 2x4 forms). Once the concrete is poured and being finished there are any number of techniques that can be used to make it look less like a slab. The simplest would be to surface groove it to look similar to the tiles. Other techniques include embedding brick borders, pebble surface finish, or even brick like patterning rolled into the surface. The latter is often done with the concrete dyed a reddish color.