Finally managed to complete a new compact Z-axis. It's not very impressive with only 15mm of movement. But it is capable of extremely fine increments. It is essentially a wedge design with a lead screw moving the wedge. It might have been more productive to use multiple vertical lead screws but it was too mechanically challenging to spin all the lead screws at once.

I have also tested multiple variation of the print material. The base is still plaster of paris. The main additive that I found that reduced binder and improved strength was talc powder (baby powder). I also tried to add glycerine to the water binder and no effect was observed on the plaster mixture. The glycerine is now used as a viscosity control for the ink as I have noticed pure water based ink was too runny and had issues with the printhead. 

Hopfully I should be able to do some prints soon.