Refractory cement is special. It comes rated in different grades (typically based on max service temperature), and I'd be super cautious about freebies, old mix, or anything not fresh from the maker/vendor. Remember that a wood fire gets hot, like 2000ish F flame temperature.
One excellent page is here:
https://www.sheffield-pottery.com/LOUCAST-3000-CASTABLE-MORTAR-p/lvclc.htm
Read the mixing and working and curing and first-heat instructions to get the full story. It is all about water. Portland explodes not because you degenerate the bonds (that requires the long hot furnace at the plant) but because the free water in the material cannot escape fast enough, and you flash it to steam inside. Portland is a sponge with teeny pores (see also why efflorescence happens on walls exposed to groundwater flows). Water can move through those pores only so fast, and Portland is hygroscopic, meaning there is always water inside. So when you heat it up, water tries to get out, and the more heat, the harder it tries to migrate, until bam, you flash boil and spall.
(I have been near a steam explosion, I worked at a foundry, the melt lead ordered wet scrap dumped in a hot furnace. Bang. Thank God nobody died, but a LOT of glass shattered and the furnace refractory lining needed rebuilding.)
So the moral of the story, in my view, is know where to be obsessive about the right materials. Refractories are one of those. (So, too, is supply side plumbing, I think.)
Happy homesteading!
Mark