Desalination of natural stone and other mineral building materials with the help of compresses is a recognized technique for significantly reducing the amount of structurally damaging salts in buildings.
Compression desalination is a way of reducing salts that can damage the structure of the building close to the surface of porous mineral building materials. One of the objectives here is to reduce damage potential in the long term. This measure can also create the foundation for restoration and preservation measures in the short term, for example the application of stone restoration mortars. Desalting with compresses is usually used for natural stone maintenance and restoration of monuments and occasionally for brick. Desalination may also pose a risk for the object and, for example cause damage to edging and surfaces. Therefore, measures must be taken to check whether the objective can be achieved, whether damaging influences can be reduced sustainably or whether preservation measures can be prepared.
Salts dissolve in water and then crystallize again
Damaging salts have a significant effect on the weathering and damage to building material surfaces. This not only leads to the loss of important cultural monuments, but also to high restoration and maintenance costs. The potential damage caused by salts is largely due to their water solubility and their ability to store moisture from the air. The latter is called hygroscopicity. Under the known salts and those frequently found in the building, nitrate compounds are the most water-soluble followed by chlorides and sulphates. Even with a relative humidity of 50%, nitrates reach their equilibrium moisture percentage. The remaining salts dissolve at humidity levels of around 70 to 80 percent.
When the water partially evaporates again at low humidity, the salts recrystallize. The crystallization process increases the volume of the salts in the pore space and, in turn, subjects the inside of the building material to a mechanical load. This can lead to surface dusting or structural destruction. In the case of cyclic humidity fluctuations, of about 40 to 80 percent, the building materials are exposed to repeated inner mechanical loads. The salts are transported in liquid water and are therefore usually oriented along an evaporation zone. This is usually towards the surface of a building. As the salts are not ‘moved’ when the moisture evaporates, they enrich the evaporation zones near this area.
Salts are transported in two different ways
Salts can only be transported in a soluble form. The transport is distinguished by the movement of the liquids and by its own movement (diffusion). Transport by diffusion is triggered by different salt concentrations within a medium. Thanks to diffusion, concentration differences through to full mixing are equalized (concentration equalization). This physical effect is used when applying the wet compress method. In this method, the compress material is kept wet throughout the desalination period. Smaller, moving parts can also be treated with an immersion desalination agent. Transport through moving fluid (moisture gradient) is driven by different moisture contents or convection as a result of temperature, density or pressure gradients. The salts dissolved in the fluid are also transported by it.
The effectiveness of applying a dry compress is essentially based on moisture transport during drying. This salt transport mechanism can be effectively used for desalination. Both mechanisms can be applied jointly, i.e. in the same compress coat for desalination of building materials.
Compress desalination requires a stable, pre-wet subsurface
An essential requirement for successful compress desalination is an adequately stable subsurface that can bear the compresses and can withstand the subsequent removal without causing any significant loss of substance. As this is not a matter of course on surfaces with serious salt damage, it may need to be consolidated before desalination. Good success has been achieved using silicic acid ester that demonstrates its own ‘secondary porosity’ after the reaction. The salts can also only be successfully transported to the compresses if they are dissolved in water. This means that the building substances almost always needs to be pre-wet before work is commenced. Distilled water is used here (this is also called deionized water or demineralized water). The intensity of pre-wetting (duration and water quantity) depends on the suction behaviour of the subsurface, the type, concentration and depth distribution of the salts, and the moisture distribution in the building materials. It should be noted that there are certain risks associated with pre-wetting. The moisture can penetrate adjacent areas of the construction where it can cause unexpected effects, for instance damage to moisture-sensitive materials. Also, the compresses that usually contain cellulose can be microbially contaminated and even infest the surroundings. It may be necessary to take suitable counter-measures, for instance the fungicide properties of the compresses.
Compresses can be applied manually or mechanically
After pre-wetting, the compress material is mixed with distilled or deionized water and this plastic consistency is applied to the desalinated areas in a in a layer thickness of 15 to 30 millimetre. A thinner layer thickness may be appropriate on delicate surfaces. These compresses can be applied manually or mechanically with a rendering machine in single or multi-level layers. The coat is usually applied manually on smaller areas or sensitive surfaces. Surfaces that require special protection can be covered first with a thin, permeable and load-bearing protective layer that comprises layer cellulose or Japanese paper to relieve subsequent removal. This is dabbed onto the pre-wet stone with a suitable brush without bubbles. When desalinating partial areas, the compresses should protrude at least 20 centimetres above the recognisable salt-loaded area to avoid undesirable edge effects. If desalination is a combination of diffusion transport and moisture transport, the compresses can be kept permanently wet for a period of about a week by covering it with film. The compresses are then left to dry during the further course of the application. This drying process must cover the further action time of the compresses continuously. It is essential here to check the drying progress. If the compress is dried prematurely, or if it has come away from the subsurface, it needs to be removed and after wetting the subsurface again, it needs to be replaced with a fresh compress. The correct application period or number of necessary cycles is difficult to predict. Therefore, if possible, it should be determined by creating sample areas or verified using salt analyses of the compresses after each cycle. A two or three-layer coat that is left to act for about three weeks can be used as a calculation base. The subsurface can be cleaned in a dry manual or a mechanical process depending on the sensitivity of the adhered residues of the compresses. Then the desalination success on the surface material needs to be verified analytically.