I think the mechanic I suggested in this thread is better.
That does make sense.
You don't; the idea is to make PSTE be able to turn into CNCT, not SAND. The SAND has to be mixed in to make PSTE(CNCT) (or PSTE(SAND), depending on which direction we're going); for PSTE to turn into SAND, the SAND has to already be in there, and PSTE is a mixture of CLST (aka cement) and WATR, there's no SAND already mixed in, so it doesn't make sense. For a liquid that turns into it's ctype, use RSST.