Rod, freezing during the curing process can ruin the concrete. Ash, calcium, and having the mix done with warm water will accelerate the SETUP time without harming the mix, but it doesn't actually speed the overall CURING time.
Ideally, moderate temperatures (50° to 70°) are best for concrete pours, as concrete will continue to cure & harden for several weeks, and the longer it can stay evenly moist (not wet, just slightly, evenly, damp throughout) the harder it becomes.
If you must pour during colder weather, you'll need to thoroughly cover the pour with thermal blankets, which can be rented, or a generous layer of straw then covered with heavy plastic drape. Lay some 2X lumber around the edges (and across the pour if it's large) to hold down the blankets/drape.
Personally, if it's going to be much below freezing for any length of time, I would put off pouring.