salt attack occurs when salts dissolved in water are carried into the stone. the two commonest effects are efflorescence and spalling. salts that expand on crystallization in capillary gaps can cause surface spalling. for example, various magnesium and calcium salts in sea water expand considerably on drying by taking on water of crystallization. however, even sodium chloride, which does not include water of crystallization, can exert considerable expansive forces as its crystals grow.