The dry mortar you referred to was clay. We had expected to be building it with "normal" clay that had been just dug out of the ground.
There was plenty of "normal" clay available on site. However when the friends we were building the oven for tried to dig it out they found it was full of large stones, which wouldn't work well for oven building.
They had trouble tracking down alternative supplies of clay locally. In the end they stumbled upon some unusual clay at a local quarry. What they found was ball clay.
https://en.wikipedia.org/wiki/Ball_clayWhat they ended up with was 500kg of a mixture of fine powdered clay and up to fist sized lumps of dry compacted clay. It was not at all what we expected to be working with !
We tried stomping on it, crushing under paving stones, rolling things over it. After several hours work (not shown in that video) we still had a mixture of powder and solid lumps.
We were about to give up, we couldn't see a way of breaking the lumps up in a sensible time. Our last idea was just to add water, stomp on it and hope.
Thankfully that worked, though the resulting clay was still a bit lumpy in places.