I was helping a friend with a reptile backdrop and had some excess grout. We were combining natural red/oxide rich clay from the mountains with unsanded polyblend grout. I decided to pour the grout in some containers lying around, and came back a couple days later when the grout was completely dry. A simple drill was used to carve the chambers and water tower hole. All of the drill bits I used dulled noticeably, so a word of caution to anyone carving with a drill to use an old or cheap drill set when carving grout. I added a perfect cast water tower (see previous posts) in a square container to provide humidity. I used a thin sheet of Plexiglas and a bandsaw to get the round corners.
Overall I really like how the texture of the grout is similar to concrete, it's something I've always wanted to do. It might be hard to notice as well, but some of the clay added to the grout did not break down, leading to a really neat speckled texture. The grout mixture we made was very thick as well, something I'm sure contributed to the strength of this nest, it's basically concrete: no cracks formed when it dried and I'm sure not even Crematogaster could chew through this setup.
Halfway done, you can see the square water reservoir which fits into a rectangular hole.
I'm finishing the other nest from the same grout mixture so stay tuned!