What are those white patinas that appear on the walls? Why they form and how to remove them

Have you ever noticed those particular white patinas that appear on external walls exposed to the air? These are efflorescences, salt deposits due to a phenomenon well known in the construction world, due to exposure to atmospheric agents, in particular humidity and carbon dioxide. Humidity, or rather water, transports the dissolved salt compounds present in the material which, through chemical reactions, are transformed into insoluble salts e with theevaporation of water they settle in crystals on the surface of the material, forming the characteristic white efflorescences. In other cases, specific chemical reactions can occur that lead to the formation of insoluble salts that settle on the wall. They generally do not represent a structural danger, but if they grow inside the walls they could weaken the structure.

How efflorescence is formed

When a structure, whether an architectural monument or the wall of a house, is exposed to atmospheric agentscomes into contact with humidity, carbon dioxide and other substances present in the environment. This causes the migration towards the outside of the water-soluble salts contained in the material. When the water evaporates, the salts crystallize on the surface, over time forming a clearly visible white accumulation, namely efflorescence.

Among the salts which, following different chemical processes, can give rise to the phenomenon of efflorescence, we find carbonates, sulphates and also some types of nitrates. One of the most common cases is that of deposits calcium carbonateCaCO₃when the building material is the cementused in the production of concrete.

In the process of hardening the material, a number of compounds are formed, including calcium hydroxide, Ca(OH)₂a salt very soluble in water. The humidity in the air, as well as the rain hitting the material, tends to dissolve the calcium hydroxide, transporting it over time through the pores of the concrete, until it reaches the external surface. In contact with the air of the atmosphere, it can react with carbon dioxideCO₂according to the following chemical process:

Ca(OH)₂ + CO₂ → CaCO₃ +H₂OR

From this reaction, calcium hydroxide turns into calcium carbonate, a salt in this case insoluble in water. The subsequent evaporation of H₂Or, it leads to a real “flowering” of the new white salt: efflorescence.

What damage they can cause and how to remove them

Typically, most efflorescence deposits does not harm the building material. It’s mostly about aesthetic alterationssuch as white patinas or actual crusts, certainly not pleasant to look at, but which do not affect the mechanical resistance of the structures.

However, in some cases, efflorescence can have more serious consequences. We then talk about crypto-efflorescencea phenomenon completely similar to the superficial one, with a small but substantial difference: the insoluble salts crystallize oninternal of the material. During the crystallization process, the salts expand, generating pressure that can cause micro-fractures and chipping, which weaken the building structure over time.

Now, if you notice efflorescence deposits on the exterior walls of your home, don’t be alarmed, because, in all likelihood, the structure will still be resistant and solid. In any case, there are methods to remove those aesthetically unpleasant white salts. Initially, the best approach is to go simple mechanical methods: you can use a non-metallic brush to carefully scrape off the white crusts or a strong jet of water, taking care that the salts “slide away” from the wall. Alternatively, it is feasible to operate with chemical methodologiesby gently applying a diluted solution of an acid or simple cooking vinegar. Calcium carbonate itself, among the most common efflorescence deposits, is well dissolved by dilute acid solutions. However, it is preferable to opt for the mechanical method, since the use of acids could damage the surface of the wall itself.