Salt efflorescence

Salt efflorescence is an expression for the deposits of salts on the surface of masonry. Salt efflorescence is formation of powdery, usually white, crystals or coating on masonry surfaces. The deposition occurs when soluble salt is present, and dissolved in moisture within the wall. The moisture and diluted salts move towards the surface for evaporation, causing the salts to form superficial crystal formations. Salt efflorescence can appear both internally and externally, and examples of salt efflorescence in both cases are shown in Figure 4–17. As the salt migrates with moisture, the presence of salt efflorescence can be an indication of moisture problems in general, and often rising damp can be the driving factor for the salt depositions.

Salts in masonry can originate from the original building materials themselves, or from atmospheric pollutants in the surrounding air. The buildings may be constructed on saline soil, or the soil may have become saline from agricultural fertilizer. The history of a historic building may also yield information on salt origins, if it has been used for e.g. manure, salt or gunpowder storage. Finally, nowadays a major contributor to salty masonry is road salt for removal of ice and snow on roads and pavements.

Figure 4–17: Examples of salt efflorescence in masonry walls; top row: externally, middle row: internally, bottom row: façade with salt attack.

The location of a building furthermore influences the salinity of soil, air and masonry, and thus there are salt exposure classes based on location. Buildings located in coastal areas, or adjacent to roads that are salted in winter, are therefore classified as being exposed. Red bricks usually have a higher porosity when compared to yellow bricks, and thereby attract more moisture and salt. For this reason, red bricks are more prone to salt damage compared to yellow bricks.

The previous use of the building can also have an impact of the quantity of salt inside the masonry, e.g. if the building has been used to store gun powder (RIBuild Deliverable D1.1, 2015).

Table 4‑9 sums up what to look for with regard to salt efflorescence, and where on the building to pay special attention.

Table 4‑9: What to look for, and where with regard to salt efflorescence.

As removal of the cause for salt efflorescence is difficult if not impossible, it is recommended not to proceed with internal insulation if salt efflorescence is detected.

Salt migrates with moisture, and the efflorescence occurs at the surface. Often the salts will migrate with rising damp, so basement walls, or the façade near the plinth are often good indicators for presence of salt. Visible salt deposits in masonry often appears as white, crystal-like, powdery substance, or merely as a white coating. Note that loose external salt efflorescence may easily be weathered away by the climate, and therefore may not be visible at the time of inspection.

Sub-florescence is the accumulation of salt crystals beneath the surface of the masonry. Due to expansion of the salt, and crystallization pressure, the salt can cause serious damage to masonry. Similar to frost damage, sub-florescence can cause scaling of the surface. The presence of salt becomes apparent if the outer layer detaches. Salt efflorescence is an indication of the presence of moisture. Therefore, the moisture source should be located and remedied prior to installation of internal insulation.

Moisture damage to facades can be often be associated with presence of salt. Moisture damage is often caused by a lack of horizontal and vertical sealing, insufficient protection in the splash water area, incorrect material selection, damage to the roof drainage system, leaking roofs, window connections or damage to building services. The water dissolves the harmful salts from the wall and transports them to zones with lower humidity (surface), where favorable crystallization conditions are present. Walls containing salts may even be more susceptible to moisture, as the salts can absorb moisture from the surrounding air (osmosis).

For a complete diagnosis of salt content in masonry, the salinity can be measured in drill dust samples, and should not exceed 0.5 % by weight. As salt has hygroscopic properties, it will in itself attract moisture from the air in high relative humidity conditions, and bind this moisture. Thus, the moisture level is kept rather constant with the presence of salt, which will crystallize where the moisture level is not constant, i.e. at the surface.

Frequent damage patterns of salt efflorescence are e.g.

• Efflorescence of salts, crust formation on surfaces

• Damage due to salt crystallization (spalling, shell formation and attrition), also within the stone structure due to volume increase and high crystallization pressure with insufficiently resistant facade material.

• Hygroscopic water damage, visible by damp stains, often already with attacked surface; moisture from the environment is bound by salt-loaded areas in the material in this case.

• For plastered surfaces: Flaking on paint with efflorescence (often in the base area, at the eaves, under window parapets)

• Damage in pedestal areas caused by frost-thaw salts

As part of a renovation measure of a salt-loaded facade, the causes of the moisture and salt load must first be checked and measures taken to eliminate them (e.g. blocking, drying, etc.). The degree of salinity should be determined to derive the necessary steps.

Salt complicates DELPHIN simulations by affecting the moisture content, and the characterization model does not include a parameter to describe salt storage capacity. Furthermore, it was not possible to get a clear picture of the number of damages related to presence of salt or locate it to specific types of buildings (RIBuild Deliverable D2.2, 2019). Therefore, to be on the safe side, it was decided within RIBuild not to include salt in the simulations and for the WP6 web tool not to recommend the use of internal insulation if visual inspection could detect (unwanted) presence of salt.

Remedial actions if salt efflorescence is identified

As removal of the cause for salt efflorescence is difficult if not impossible, it is recommended not to proceed with internal insulation if salt efflorescence is detected.

There are different methods for salt remediation.

• With a planned drying, the salts will also reach the facade surface with the moisture transport. These must always be removed dry on the surface.

• In the case of brick facades, the joints must be completely renovated up to at least 50 cm beyond the affected areas.

• In the case of partial high salt contents, the salt-loaded areas are removed, i.e. contaminated plaster or stones are removed and new plaster or stone replacement is applied, resulting in a natural reduction of the salt content.

• Chemical processes, in which easily soluble salts are converted into poorly soluble or insoluble salts.

• Physical processes in which salts are dissolved and transported to a defined location where they crystallize. This is achieved, for example, with compresses that are applied to the wall surface and removed again after the salt has been added.

• Coatings and plaster systems are frequently used. However, this depends on the surface design of the façade (plaster façade). The best known of these are pore-hydrophobic restoration plasters with high water vapour permeability and reduced capillary conductivity. After the plaster has been saturated with salts, the restoration plaster has fulfilled its function and must be removed.

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