using limestone to fill paver bricks

Sand, hard sharp, is the best. Limestone sand would be OK I would guess.

Concrete is NOT a good idea.

Brick pavers have several very nice advantages over a concrete surface.

1. If something happens, you can remove one or more pavers and reset them. Concrete can't be moved and when something happens to it you just get a crack.
2. Pavers allow drainage. Concrete only has run off.
3. After years when time, weather and roots have made changes; the pavers can be picked up, a little more sand added and rest as smooth and nice as new. You can even move the whole thing. Concrete you brake up and throw away starting over new.
4. Done right with a good foundation, you can drive on pavers the same day you put them in.