Sodium silicate is used in the foundry industry as a binder for sand, which hardens almost instantly in the presence of CO2. It's a relatively viscous fluid compared to water. It is lacking in any significant tensile strength, but is hard.
It's relatively cheap, certainly in comparison with silicon rubber or hydrocarbon resins.
It seems to me, that it would make an excellent candidate for a rapid prototyping medium, especially as it can be used as a binder for sand, flour etc. The higher than water viscosity gives the potential to build out at wide angles, with rapid solidification possible.
I don't think there is significant shrinkage on hardening, as with, for instance, wax.
Sand castles anyone ?
