Thermal isolation, smarter than the average bear!
Congratulations on your progress so far. The masons I used to work with, (I was on the carpentry crew), used to add anti freeze to their mortar mix. I've seen this done when the temps would drop below freezing before the mortar would dry completely. Might be worth looking into. On the other hand; I don't think that I would want to be out there handling cold block and brick in those temps if I didn't have to be.