Why does the dissolution of calcium hydroxide have a negative entropy?
When ionic compounds dissolve in water, water molecules form a complex ion around the calcium cation.  The presence of the hydroxide anion has a similar effect, though the polarities of the water ligands is reversed.
Thus, dissolution of Ca(OH)2 in water is actually a chemical reaction.  In some cases, complex ions may be more structured than the crystals of undissolved ions.       In such a case, the entropy change of solution will be negative.