ShapeCrete (now discontinued) and SculptCrete (which has less videos) are marketed as concrete that works like clay, but sometimes it is used in a wetter gel-like consistency, as seen in the Shapecrete.com videos like this one. Why is this middle-ground gel consistency used instead of a wetter, pourable mix of SculptCrete, a drier clay-like mix of SculptCrete, or another more standard concrete product? By "why", I mean both workability reasons and final result reasons.
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1 Answer
Both ShapeCrete and SculptCrete (and similar products), are cement-based mixtures that are designed to be used in several ways, including molding it like clay. To achieve the clay-like handling and ability to hold fine detail, everything in the mixture has a very small particle size, and they may contain some additives (like clays). If you mix them with the enough water to create a pourable consistency, they can be poured into a mold like regular concrete. If you use less water to create a very firm mix, it handles like clay.
In the video you linked to, the ShapeCrete people wanted to use the material to replicate a house-shaped tissue box. This was a novelty item -- a plastic shell in the shape of a house that fit over a tissue box, and the tissues were dispensed out of the chimney. The goal was a thin-walled replica of that hollow plastic shell.
Pourable
To use ShapeCrete (or almost any cement product), in its pourable form to create a thin-walled replica, they couldn't just plug the chimney and fill that plastic shell as a mold. That would produce a solid block of cement in the shape of the house. They would have had to create an inner mold, nest the two, and pour the material between them. That's a lot of work.
There's a method called rotocasting that creates a thin shell on the inside of a mold. With something like resin, you can pour a little into the mold, and then keep rotating and reorienting the mold to coat the whole inside and keep it a uniform thickness until the resin sets up. With cement-based materials, continually moving and mixing it is the way you keep it liquefied, so it doesn't lend itself to exactly that procedure. With molds of certain shapes, you can spin the form at high speed so centrifugal force holds the cement in a uniform shell until it sets up (a house shape isn't one of those). You can also use a mold material that absorbs water to pull water out of the cement so it doesn't slump, and build up a shell in thin layers. That doesn't work with a plastic mold.
The small particle size and additives of this kind of material give it some strength in a thin shell. The particle size also results in a smoother, more clay-like surface texture than normal concrete, with the ability to hold fine surface detail. Pouring it into a mold is likely to result in some surface bubbles (which, from a previous question, you happen to want). If you didn't want them, they can be reduced with gentle blending to avoid mixing in air, and vibrating the mold to help them rise to the top (which is typically not the exposed face). There are also chemicals that can be added to reduce bubbles.
Clay
The typical way to use ShapeCrete like clay would be to roll it out into a sheet of the desired wall thickness, accurately cut out sections to match the inside surfaces of the house, lay the sections in place, and work the seams to join them. That's a lot of work. Some effort could be saved by using strategically larger pieces, even a single large piece, that get pressed into the mold and formed in place. A thin plastic mold like that tissue box wouldn't hold up to a lot of replications.
Working from a clay consistency will produce even stronger results, the material will be more tightly compacted. The surface texture will be similar to clay, and without the bubbles associated with pourable cement.
Cement/Sand
A cement/sand mix is commonly used for cement sculpting. It's a relatively dry, granular mixture that's a little more moldable than wet beach sand that you might use to make a sand castle. It could be pressed into a thin shell around the inside of the mold, requiring none of the preparation described in the previous alternatives.
If there was any fine, shallow surface detail in the mold (not likely on the inside of a tissue box), it wouldn't pick it up because the material is too coarse. The cement walls would also need a reasonable thickness; something cardboard-thick wouldn't hold up to handling. The surface texture will be grainy, like concrete, but without bubbles.
Gel
That's where the gel consistency comes in, and the use-case they were demonstrating in that video. It provides the benefit of cement/sand in that it saves all of the preparation needed for pourable or clay consistencies for this type of application, and also provides the benefits inherent in the material of strength and the ability to capture detail.
The material can be mixed with an intermediate amount of water to create a consistency half-way between pourable and clay. It's just solid enough for you to be able to handle a blob of it, form a simple shape that you can hold in your hand, and for it to stay where you put it without slumping. It is used like the cement/sand mix described previously -- you put blobs of it into the mold and smear and press it around into a thin shell.
The material is strong enough for a pretty thin shell to hold up to handling. The small particle size will produce a smooth, clay-like surface texture, which will pick up and retain fine detail. As can be seen from the various videos on the ShapeCrete web site, at least one result had some surface bubbles, but most did not. So it appears that the gel consistency is in a gray area on the edge of being pourable, where small differences in water content and/or handling could result in it having some surface bubbles.
