Don't know if this will help, but you might want to try one of the ready mix concrete companies. If you're expecting freezing temps right after you've poured concrete, you would add a bag of calcium chloride to the truck before pouring so the crete 'kicks' before it freezes (it's 'accelerator' or 'kicker' if you call them) - the stuff I've seen them add is ~ 77%.

I realize that you are in a warmer climate, but we used to use it in Richmond Va. , which isn't exactly a cold place Just thought I'd throw out the idea for you. Good luck in finding a reasonably priced source for the calcium!