So there are several scientific articles on the internet that state that the old 1000+ year construction in Chinese that has plaster on its walls, and mortar used for building, used a secret ingredient that gave it super strength and buildability which was basically sticky rice soup. They would mix this soup into their mix which gave it a inorganic/organic structure many of which are still standing today and gave the plaster awesome water blocking features.
Makes sense. Glutinous rice flour is the principle ingredient of wall paper paste in Asia, which is used for everything paper-related, even making furniture. It's very hard. Not as waterproof as shellac, but a lot better than plaster. They don't mention an antiseptic in the article, though. I've had paste mold because I didn't add enough or the moisture content was too high. Flour, a little sugar, water, and antiseptic. But it needs to be boiled. You'd need a huge pot for an application like that, and probably several batches. If kept in an air-tight container, it will keep for a month or more. It's very cheap and pre-made here, but maybe not available pre-made outside Asia. It is quite thick, so has to be diluted prior to application.
The closest thing to that that i know about is sawdust. We mixed sawdust into our mortar. It slowed the drying time to prevent cracking and may have improved the mortar's R value. I have also seen people making insulated fire bricks by mixing saw dust and clay. Firing the clay burns away the sawdust and leaves a clay foam. Then there is the story from WWII where they mixed sawdust and Ice to create a bullet proof composite that resisted thawing quite nicely. They were going to build a floating airbase with it until they figure out it would take more steel to make the refrigeration units to make the ice than if they just built more steel ships.