You can use it like giant solid concrete block but not as a filler to reduce the concrete volume. I am not certain how you would tie all of the reinforcement together. If you were planning on stacking it with a really cement rich mortar it would be ok but I agree the biggest problem is cold joints.
"When there is no life in the soil it is just dirt."
If you are using a rototiller, you are doing it wrong. Even on this tiny ad: