The formation of these unsightly salt deposits are for the most part, water-soluble salts that come from many possible sources.
For efflorescence to occur there must be water present to dissolve and transport the salts. For water to carry or move salts to the surface there must also be channels through which to move and migrate. The more dense the limestone, the more difficult it will be for water to transport salts to the surface. Conversely, the more porous the material, the greater the ease with which salts are transported and deposited.
Salt-bearing water, on reaching the limestone surface of a structure, evaporates and deposits the salts. When humidity is low, the water may evaporate before reaching the surface of the structure, leaving the salt deposit beneath the surface, and unseen. When the humidity is high, water evaporation is slower allowing more opportunity for visible growth.There must two conditions present to create efflorescence
- A source of water-soluble salts.
- Water moving through the material to carry the salts to the surface. The water evaporates and leaves the white powder (efflorescence) behind.
- They might be more permeable and promote water travel.
- They might tend to have higher water-soluble salts in some batches.