Here's my first! and only, so far. My teen-aged daughter's bathroom. Tadelakt over gypsum plaster on drywall. The second photo is a crack developing already on the wall that has been covered in cement board and taped with OPC thinset. We were going to tile the wall, but the tadelakt is so beautiful, we decided to cover this wall with it instead of tile. This is the back side of the shower wall, so I don't think it will fail, but I'll never do this again. Tadelakt over gypsum seems to be working great though.
siu-yu man wrote:Bill, curious, in retrospect, how would you do this differently? what did not work?
In retrospect, I would not use any OPC based products with any kind of lime plaster. They behave too differently and this resulted in cracks appearing at the seams between the sheets of cement board.
Gypsum and lime get along famously. The predominant plastering system in homes 100 to 150 years old is lime render, gypsum plaster and then colored lime top coat, so this is what I mostly use in my restoration work. A modern interpretation of that system is what you see in the first photo. I installed drywall on the stud wall, then plastered that with a greenguard certified all purpose joint compound then top coated with type s hydrated lime aged 1 month mixed with brick slurry from a brick saw, application by 6 inch brush and immediately trowel burnished, dried leather hard and then local goat soap dissolved into warm water is painted on with the same 6" natural bristle paint brush and burnished with a cut piece of local limestone.
I am a restoration contractor, so I always have a bottom line. I experiment with combining ancestral techniques with modern industrial products on my own home in order to work out the bugs before doing any job professionally. This really isn't natural building, so I have been reluctant to share some of my techniques here on Permies, but I have decided that less than perfect is sometimes good enough.
Bill can expand this perhaps if he can, but you don't (at least in most...not all) formula recipes, add soap to the mix. It is only employed as a "burnishing paste" to render a more water resistant surface for lime plasters in damp and wet areas such as bathrooms, kitchens and patios. This is a detail process and I hope to publish more when I can...
First start with a really good lime plaster that is heavily burnished so that it is starting to polish. Then shave a bit of olive oil soap into the water, not so much that it gets all soap bubbly, but enough to see that there is soap in it. Then take a good polishing trowel, one that is highly polished and flexible and polish the soap by painting on a bit and burnishing while wet. The soap and the free lime on the surface react chemically to create calcium stearate, aka soap scum. The scum waterproofs the plaster by filling in the pores as you burnish.
I don't use a stone, just a good stainless steel trowel, but I'm not going for waterproof, just stain and water resistance. The polish makes for great lighting and gives the room a glow.