Not an expert on admix, but I've never found that it made any difference in my troughs. The most important thing, for me anyway and I live in Zone 5, is to pack the troughs very tightly while making them so that the water can't seep in, freeze, expand and break the trough apart. Really compress that tufa. I've tried to break my troughs by leaving them outside in a variety of ways: empty, on the ground, raised, full of snow, upside down, full of soil, etc. and they haven't broken. The only ones that did were the first ones I made while learning that weren't packed sufficiently. My troughs aren't particularly porous and won't drain with out drainage holes, but they don't break.

I doubt that any liquid fortifier is going to make a difference with tufa. Concrete, yes, but not tufa.

Tufa contains organic materials, and those materials are going to decompose eventually, creating holes and channels in the pot. Many/most of those holes and channels will trap water, and if that water freezes, it will expand and the pot will either crack or start deteriorating.

You can argue with opinions, but you if you try to argue with the laws of physics, you will lose.

Sue