Q:Is the CEB clay/cement mixer fully functional?
A: Soil mixer is functional, but it only does about 10 block at a time. So we are reworking that this year to make it do at least 20 per load. We also have not tried it using tractor mounting.
Q: Where could I find cement: clay ratio guideline, to know the % cement to use based on the local clay proprieties?
A: The % of portland cement is by weight - 5%-10% - but depends on your soil - so you have to test by making block - curing for a week - and dropping it in bucket of water over night - to see which mixture is sufficient. If the mixture is sufficient, the block will be just as strong as before (1000+ PSI compressive strength.
Q: Have you tried interlocking blocks such as ones in this YouTube video?
A: No, this is a completely different brick press design and requires significantly increased machine complexity, while not providing definitive overall advantage. See discussion on To Interlock or Not to Interlock.