Type of grout: Unsanded white mixed with fine sand. I used modeling clay to form the tunnels & chambers. I found the clay impossible to remove completely. I had to carve out the colored specks with my dremel tool, then patch up and remodel the chambers with grout. I threw out my modeling clay.
Type of grout: Unsanded grout mixed with coarse dirty sand. I used wood on either side of the tank, covered by plastic cling wrap to facilitate removal after casting the slab. I carved the damp slab with a tile cutting bit and my dremel tool.
Type of grout: Saltillo, sanded grout. It is made for laying floor tile and filling spaces of up to 1.5 inches.
Type of grout: Unsanded grout mixed with perlite. This is the best combination so far. It is half the weight of sanded grout. It is similar in texture to pumice. Carving with a dremel tool is a breeze! Be careful not to add too much perlite as this will cause it to chip and break while carving.
My next experiment will be casting a full sized slab of perlit mixed with saltillo grout and or unsanded grout. My theory is that the perlite adds moisture holding capability to the grout.
I have been trying to think of materials that when put on a carved slab, will be hydrophilic or water-wicking. I have come up with: unsanded grout & fragrance free, pure clay, clumping cat litter, crushed perlite & unsanded grout, unsanded grout & super absorbent polymer granules (crushed).