The catalyst in this reaction itself is regenerated from the deprotonation of water in its final step, which results in the conversion of its intermediate, an alkoxide.  
The elimination mechanism by which the dehydration of its major product occurs necessarily involves the abstraction of an acidic hydrogen and displacement of the leaving group.  The catalyst in this reaction itself is regenerated from the deprotonation of water in its final step, which results in the conversion of its intermediate, an alkoxide.  An aldehyde or ketone had originally undergone a nucleophilic-addition-type process to produce that alkoxide, which had converted it from a nucleophilic enolate using a hydroxide ion catalyst.  Producing a final product known as a beta-hydroxyaldehyde, FTP, name this condensation reaction whose name combines that of its two final products.
The elimination mechanism by which the dehydration of its major product occurs necessarily involves the abstraction of an acidic hydrogen and displacement of the leaving group.  
aldol condensation