Amateurs, apparently.

I would have thought that actually giving someone 'concrete shoes' would take quite a bit of time and it would almost certainly require either a lot of physical restraint or for the victim to already be dead.
Even with quick drying cement it takes a while for it to set and if it's not quick dry then I think you're looking at hours if not at least a day before it's really solid. The victim's feet would need to be kept relatively immobile while the stuff was setting.
Also, I believe the curing reaction will produce a decent amount of heat and in any case cement is at the very least irritant if not actively corrosive so the effect of prolonged contact with bare skin will be unpleasant anyway (although the victim is presumably doomed anyway if they aren't already dead).

Nasty