masonry sealant on cob

Hey folks,

Anyone have any experience using a masonry sealant on cob? Specifically, I'm interested in cob tub/hot tub applications. I've seen the old video of the cob hot tub attached to the wood fired oven....but this seems to be the only recorded application. I'm sure there are more. I'm curious about long term water exposure and using possibly some portland in the final layer then sealing as you would a concrete hot tub or pool. Benefit would be a "more" natural process and the ability to sculpt the product more than straight concrete would allow.

Thoughts? Experience? Photos?

I am confident any cob hot tub would have a waterproof liner.
Either a plastic tub or a concrete one.
In my experience failure would occur with out either.

Cob expert Miguel Elliott (way above my Uncle Mud pay grade) responded to this exact question when I asked:
"Ceramicrete [but it is] hard to find though. Try Grancrete. Have to special order it. Hydraulic lime plaster first, then trowel on the grancrete. That should work. It has worked for him several times.

Regards,
--Mud

Great natural builder Massey Burke has also done this with a polished lime plaster called tadelakt which is beautiful and a huge amount of work.

If the sealant is a lithium silicate or sodium silicate based one, I don't think it would work. They seal by a chemical reaction to the byproducts of the hydration process of concrete. Cob would likely not have calcium hydroxide with which to react. Though I wonder what if one was to add calcium hydroxide to cob while mixing and then try it? I don't think I would test it out on a hot tub though!

Chris McClellan wrote:Cob expert Miguel Elliott (way above my Uncle Mud pay grade) responded to this exact question when I asked:
"Ceramicrete [but it is] hard to find though. Try Grancrete. Have to special order it. Hydraulic lime plaster first, then trowel on the grancrete. That should work. It has worked for him several times.

Regards,
--Mud

Thanks, Mud. Did you end up trying it? I definitely thought about the idea of a lime plaster topcoat for the finish as a better option so this is good to know.

Chris McClellan wrote:Great natural builder Massey Burke has also done this with a polished lime plaster called tadelakt which is beautiful and a huge amount of work.

I've seen some good examples of a hybrid Tadelakt that was a bit less work....essentially a series of finishing trowels with black soap  vs the heavy workload of actually stone polishing....it makes me wonder if the troweled finish + those sealers you mentioned instead of black soap would be a good combination of both.