Ron, My limited experience as been one wants the mix to be more "packable" than pourable. Like adobe in feel. I would keep the portland cement ratio under 10% of the total. Too much water, same as any cement mixture, makes for a weak product, however, if used as mortar between bricks naturally one would mix with more water with no perlite. Many years ago I built a little Gingerly furnace and not finding a source of fire clay I busted up old red bricks, whew, now that was a messy hard job
I should mention these furnaces have a metal casing of some sort. I would think that using chicken wire embedded in the inside, pack a layer of mix over the sand, then trimmed, overlapped chicken wire(triangle shaped) pressed into it, followed by the rest of cement would be a good idea. Reverse that if inside a kettle form.
Hey Don, based on your experience, how good is that refractory mixture at holding its shape without a restricting mold? Do you think forming a dome style oven over top of a sand mound base would work with the refractory cement mixture? Like they do with forming a temporary clay oven? ,