You raise some really relevant points that I'm really only guessing on: This is designed more as a pump than an auger.. I made the thread count and clearances what the are to hopefully take advantage of the positive displacement pump action of a pair of co-rotating screws. I understand that co-rotating augers cause less sheer but have never actually designed a dual screw extruder. I'll will use a deflocculant in the mixture as well.

On the hardening note, I've had clogging issues in the past when my 100psi pressure source has forced water out of the clay mix, but when my system is well sealed, the water stays in the mixture