Soluble salt crystallisation causes aesthetical problems and the loss of historic material, degrades health conditions in rooms and often requires high-cost recurrent repairs. In extreme situations, it may endanger the structural safety of old buildings due to a progressive disagregation of the masonry.
To understand better the behaviour of plasters and renders on salt-loaded walls the research programme included:
- Drying experiments monitored by magnetic resonance imaging techniques.
- Salt crystallisation tests on plaster/substrate specimens.
- A study of five salt damaged buildings in Portugal.
- The drying front tends to be located closer to the surface for similar liquid fluxes.
- The drying front is less diffuse, i.e., sharper.
- The height of capillary rise in walls increases.
- The lower the solubility and the more concentrated the solution, the sooner salts will crystallise on their way across the front.
- For salts that may supersaturate or possess several crystal forms that precipitate at different concentrations, such as Na2SO4, a more even distribution of salt tends to arise across the drying front.
Conclusions related to the influence of soluble salts on drying and to the saltaccumulation behaviour of plasters and renders can be made.
Influence of soluble salts on drying
Soluble salts may significantly influence drying of porous building materials. The lower relative equilibrium humidity (RHeq) of salt solutions, in comparison to pure water, induces a lower vapour pressure gradient between the material and the environment. It is also probable that the effective surface of evaporation is reduced, by deposited salt crystals. When salts are present in porous materials, the evaporation rate is lower. The ratio between the liquid flux to the drying front and the evaporation flux from that front is higher and, as a consequence:
Salt-induced dampness arises from the lower drying rate of salt-loaded materials. Even when the air RH is lower than the RHeq of the contaminant solution dampness problems are more likely to occur when salts are present than in the case of pure water, because of the higher height of capillary rise and the drying front located on the outer surface for lower liquid supply fluxes. Hygroscopicity is a particular case of this general feature and occurs when the air RH is higher than the RHeq of the salt solution.
Some salts tend to crystallise at the surface or higher on the walls. Others tend to be deposited within the pores or closer to the ground because of their distinct influence on drying of porous materials. Within evaporative processes, soluble salts crystallise at the drying front. Therefore, the lower the RHeq of the contaminant solution, the higher in walls and the closer to the surface the crystallisation tends to occur.
If the drying front is diffuse rather than sharp, the distribution of salt across that front depends on the solubility properties of the salt solution. Indeed, evaporation occurs all across a diffuse drying front and, hence:
Behaviour of plasters and renders
The behaviour of plasters and renders to salt crystallisation (see Figure) depends on all factors that influence the liquid or vapour fluxes during drying. The type of salt is, as explained above, one of these factors. It was shown by the crystallisation tests on plaster/substrate specimens that a plaster can, for instance, behave as salt-accumulating with sodium sulphate and as salttransporting with sodium chloride.
The substrate material is another important factor because it conditions the liquid flux into the plaster/render. The same plaster can, as observed in the crystallisation tests, behave as salttransporting on one substrate and as salt-accumulating on a different type of material.
Paints too can cause a more superficial deposition of salt and an increase in the moist salt damaged surface area because they can hinder evaporation. The salt crystallisation tests showed that paints can even transform a saltaccumulating into a salt-transporting plaster.
This work was partially carried out within the European research project COMPASS – Compatibility of plasters and renders with salt-loaded substrates in historic buildings. The use of magnetic resonance imaging was carried out at the Centre for Material Research with Magnetic Resonance in Eindhoven University of Technology.