Efflorescence, in chemistry, is basically the loss of water of crystallization from a hydrated salt to the atmosphere on exposure to air.
and a salt can lose waters of crystallization only if PH2O (calculated frm the K sp for the salt)> Vapour pressure of water in air
here's a small example for u
for the salt, CuSO4 . 5H2O <====> CuSO4 . 3H2O + 2 H2O ................ Ksp
here PH2O = sqrrt (Ksp)
if this value comes to be less than that of Vapour pressure of water in air, sal will not lose its water of crystallization.... as air is already saturated with water vapours