Q: I want to make some concrete stepping stones with a mosaic surface. What kind of mold, concrete/aggregate mix, and setting technique should I use?
A: The mold depends on the shape you want, the budget, and time constraints. Offhand, I’d say the easiest thing would be to cut a piece of concrete-forming tube (Burketube and Sonotube are common brands) into 6-inch or shorter sections to make round molds. For square or rectangular ones, use wood, nailed together at the corners. Dig these into the ground and fill them with concrete. I’d suggest a mortar mix, if you are trying to set directly into the concrete. This contains mason’s sand (3 parts by volume), cement (1 part), and a little clay (½ part). Mix in only enough water to make it flow—too much water will make a soupy mix and weak concrete.
For the mosaic, I wouldn’t advise using bottle or window glass, although small square glass mosaic tiles (“smalti”) would be fine. They can be further shaped with tile nippers. If you’re going to use scrap ceramic tile, then cutting it into small pieces on a tilesaw would be preferable to trying to break it, which can leave dangerously sharp edges. Natural stone pebbles should work well, especially if you can find them in different colors. Stone tiles also will work. This choice mostly depends what kind of look you are striving for. Be careful not to leave too large a surface covered with a single glossy tile, or it can be a slip-and-fall hazard, especially when wet.
The simplest method would be to set directly into the wet mortar that composes the stepping stones. Remember to sponge off the mosaic surfaces thoroughly before the concrete sets. Alternatively, one can lay the tiles upside down in a bed of sand in the bottom of the mold. After the concrete sets, the whole thing is flipped over, unmolded, and grouted. This permits one to use concrete from a mix truck, which contains gravel instead of sand. One should wet-cure the stepping-stones for at least a week. Another approach is to start with pre-cast stepping stones, prime with admix, then apply thin-set mortar with a notched trowel. Tile can be set either directly or indirectly. After the thin-set mortar is hard, grout with colored tile grout, being sure to leave the tile surfaces as clean as possible. Wait a few days before walking on them.