Can't help with the gypsum question. One feature of lime is it has the ability to heal small cracks. I don't know if gypsum does this?

Autogenous Healing - When hairline cracks develop in the mortar, hydrated lime reacts with carbon dioxide in the atmosphere. This reaction produces limestone which helps to seal the crack and fill voids in the mortar. This explains the increased moisture resistance noted after six months of curing in two studies.

Ratios for mixing clay and even lime can vary depending on the clay content of the soil and the type of clay, moisture content of the added binder is also a factor, dry sand, wet sand, dry chips fresh chips. It is advised to do test samples with different known ratios to find the best sand to clay mix, ranging from 1:3 to the other end of the spectrum 3:1. As stated the ratio depends on the clay content of the soil used. So mix according the material at hand. To answer your first question, it will depend on the soil "you" have. My ratio will be different. Test blocks and more test block and it will be revealed.

I find one reason for the lack of use with natural materials is they are not a ready mix, like Betty Crocker cake mix. You have to fool with them, something todays society doesn't have the time for.

Home made cement roof tiles, no nailing required. Slate?

Tim