I have had trouble with hardened caustic soda - I put too much in the water and saturated it - had to use a hammer and screwdriver to break it up because it was rock solid.
Your under sink drain trap may also be partially blocked with it too. Best I would guess is to bite the bullet and dismantle the sink drain to clean it out properly. And knock out the solid stuff - once cracked it can usually be easily removed in big chunks. Then hot water for any stubborn bits after the sink drain is back together.
Good luck...
Peccavi, January 2009
