Seals

There are various ways of sealing this small dome. You can buy a surface-bonding cement, like Sure-Wall Surface Bonding cement, structural grade, and apply a sealing coat. There are various other sealers on the market that can seal your dome. I recommend, also, that you have plenty of heavy-duty plastic (black and clear) on hand for major changes. This material can be draped more-easily over a low-profile dome to seal it in the event of some cracking. (It also seals openings to broken glass windows, etc.)

Plenty of high-grade silicone sealant is good to have on hand. Upon completing the plastering, you will have a very strong structure … strong-enough to allow back-fill of a layer of soil for additional protection. This monolithic structure should have a thick bead of silicone caulk around the outer and inner joint where the dome meets the slab. Caulk also around all pipes, vents, etc.. It goes without saying how strong this integrated structure will be as the dome walls tie directly into the monolithic footing/slab.