Well ionic gelation is one type of reaction which gives instantaneous gels - The reaction involves mixing of a polymeric solution with a crosslinking solution which leads to instantaneous formation of gels. The most commonly used combinations are Alginate-CaCl2 and Chitosan-Tripolyphosphate systems where Alginate and Chitosan are the polymers and CaCl2 and Tripolyphosphate are the crosslinkers respectively.

The mechanical strength of these gels depends on the polymer : crosslinker ratio, however I am not sure if the strength meets your requirement. Hope that helps.

Srujan

Gelation of polyvinylalcohol solutions can be suddenly triggered by addition of crosslinkers such as organic titanates (Tyzors) or boric acid. The system requires mixing, though. The final viscosity can be extremely high, up to a solid-like material.

If you want a transparent, hard solid, epoxy resins could do.

Another option would be heating/firing a sodium silicate solution, which will turn into a glassy solid, if your system can take the heat.