Bob, what Carl said is almost correct.
But things are more complicated a lil bit. I had to learn them just about a year back when I poured a slab, or how I should call it if it was a floor of the future 2nd floor of the building and the top of the garage at the same time. The day we poured the weather was fine. About +10 Celsias, well above the freezing. All went smooth, we got 4 mixer trucks off through a truck-mounted pump. I didn't order antyfreeze additive since there was no need. But later in the evening when dark came I found out a small pool on the ground covered with ice. That got me worry and I found one grade below zero Celsias on my window temp. The next day light snow layed on the groung and on the slab also. The trouble was you couldn't cover the concrete with anything because you can't step on it because it was still soft. After 24 hours!! Than there was one interesting view. The plate had two longitutional reinforsments on its inner (lower) side. So it was thicher there. And on the 2nd day in the morning I saw the snow melted by two strips. Almost where the reinforcements were. I poited it was just a little below freezing, about it. And the snow went off completely in a couple of hours. But what I saw was exactly the exhaust warm of concrete setting reaction.
In a couple of days the slab got hard enough to walk on. And I covered it with canvas and poly film. And started reading the theory about the same time. What I was learned is concrete sets only when there's water in it. And it needs good temp for the reaction. If water freezes concrete doesn't set at all. And if it hasn't set enough to get about 30% of its normal strength the water inside it cracks out its structure what follows sufficient drop of strength in the future. So you should protect from freezing. But that's not all. If you keep it unfrozen but still at the temp just a little above the freezing point it sets too slow. And doesn't achieve those 30% of strength to be able to freeze up with no risk. So too important to keep a slab warm for the first 5-7 days.
I got lucky with my structure. Checked it out in the spring and found the concrete steel-solid. But as long as temps kept about freezing for some weeks what followed by real drop of temps I put a wooden stove inside the garage and kept it room temp warm for a month. I should admit it was a kind of nervous month.