Efflorescence is a chalky white-ish salt deposit left of the exposed face of masonry, concrete, and cementitious building assemblies.
The following picture shows a painted brick screen wall with significant efflorescence.
When a masonry building assembly is subject to or exposed to residual moisture in the form of rainfall, leakage, escape of liquid, or rising damp, the building component will generally have signs of efflorescence on the masonry facings where exposed to air. There are salts that naturally occur and are present in are present in cements, lime motars, and masonry assemblies. When exposed to excessive moisture, especially in a residual naturesidual nature, those those salts are crystalline at a molecular level, similar to sodium chloride (table salt), NaCl, and will dissolve inside of the masonry construction. As the salts become hydrated, the building area will be in a constant cycle of hydration and dehydration. As the water molecules invariably, naturally move towards the source of dry air, they transport the salt molecules molecules. Once the water molecules reach the exterior or exposed face of the masonry, those molecules dry off and go into the atmosphere. Efflorescence, however is left behind in the form of deposits on the face of the masonry.
The picture below shows another closer view of the same brick wall from above. Efflorescence, in this case is trapped behind the paint.
These deposits look like chalky white dust and in a close up resemble small fluffy-like crystalline structures. Calcium carbonate is one of the most common constituent materials in limestone and carbonate is present in almost all types of masonry masonry binders.
Tables salt is sodium chloride, NaCl. By comparison, calcium carbonate is a molecule that is a combination of Calcium, Carbon and three moleles of Oxygen, CaCO3. Calcium carbonate has a relatively low water solubility, limestone for example is largely made of Calcium Carbonate and limestone buildings and statues can stand for centuries. H20, a simple water molecule alone, does not easily solve CaCO3; however with modern pollution rainwater is not pure H2O, today and in recent decades, now rainwater includes high quantities of C02. The presence of CO2 in rainwater, when in contact with calcium carbonate, converts calcium carbonate from CaCO3 to calcium bicarbonate, Ca(HCO3)2. In comparison to calcium carbonate, calcium bicarbonate is highly water soluble. Effectively, when when calcium carbonate is exposed to rainwater it will be highly effective in dissolving calcium carbonate in masonry. By all accounts though, this is an unintended and unwanted consequence and therefore negative to the condition of the building.
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The picture below shows an example of exposed on efflorescence an unpainted brick facade. This efflorescence occurred in a large area of this wall due to concentrated water flows from improperly directed gutter and downspout.
A closer view of the efflorescence shows the crystalline nature of the salt deposits on the face of the brickwork.
The pictures below show a brick masonry wall, built with kiln fired historic common bricks. This building was built in the early 20th century.