When pouring cement in cold weather, its a gamble whether the job will come out all-right or will crack etc. I would simply tell the contractor to fix it.
If cement freezes before its dry (initially cured) it will flake and crumble. It is obvious when it freezes.
Yours cracked. This could be from many reasons, however I believe blankets would not have helped.
You are paying for a quality job. The sidewalk cracked. You don't care why. Simply tell the contractor to "Fix It". Its his problem to correct his mistakes.
I wouldn't "Have it out with the guy". You get better results when your polite. Tell him that you want the type of quality job that your paying for.
He may want to wait till spring to repour depending on where you live.
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