If a building site is being demolished and I want to reuse the concrete to make a large sculpture (pour into a mold), must the concrete be fully recycled or is there an easier method to reuse it (such as smashing it down myself and mixing it with cement)?
Concrete is a hard substance created from the chemical reactions of the various components mixed during the build, including water. Once cured, smashing the material into any level of coarseness or fineness will not undo the chemical reactions.
The primary purpose of using recycled concrete debris is usually as filler, to reduce the volume of pour in a construction. I used blocks and chunks on the base of a shed in order to save about ten bags of new mix.
Your suggestion of smashing it down and mixing it with cement would be the only real option available, and you must consider that it's the equivalent of adding any other inert granular substance. It may affect the overall structural strength, if that is a concern.