How long from pour to cold temp?
As long as the ground was not frozen already you may be OK sonce the earth is a pretty good insulator and concert generates heat as it cures.
If you have amy more holes, insulate the top of the concrete right after pouring.
A few inches of dirt, or even some straw and a piece of plastic will hold enough heat to prevent freezing in the top layer.
On big pours calcium chloride is added to release extra heat and prevent freezing. The effect on strength is pretty minimal.