Not a chemist, but perusing some websites yielded some information. It seems if you could manufacture sodium silicate and fill a form with it to make a core, then expose it to carbon dioxide (this binds it to a solid) you may have a somewhat solid sculptural product. This is how cannon barrels are made btw, very interesting. If the parameters of your project disclude using armatures, then any wires and supporting rods can be removed (hopefully) after you are done.
Sodium hydroxide heated under pressure with silicon dioxide (sand) will produce sodium silicate.
2NaOH + SiO2 ==> Na2SiO3 + H2O
Sodium hydroxide is made by electrolyzing aqueous solutions of sodium chloride in chlor-alkali membrane cells. Hydrogen and chlorine gas form at the electrodes. Both these gases are dangerous and must be collected or vented away. Here's a website showing how sodium hydroxide (caustic soda) is made.
Chlor-alkali membrane cells As you can see, this is no backyard project, but is (theoretically) possible to do using the materials described. It would be of industrial scale and require expensive equipment and loads of time to account for setbacks, i.e. it may be viable for smaller sculptures (if it works at all), but may not work for larger objects, so much testing would have to be done.
I would not consider it unless the project is part of a R and D team effort with adequate financial underwriting and laboratory resources. Even with access to these things, it would be dangerous and complicated imo.
If the project is a conceptual art project, I would be very interested hearing how you deal with the problem using only sand and sea water and nothing else. I have a friend who is a sculptor, and I will ask him about this. Will let you know what he says. It may be he knows something about doing this an easier way, but I don't think he has done it before.
Les
