Desalination

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser
back to Measures

Abstract

Increased salt contamination can be reduced using different methods. These include poultice desalination, by themselves or in conjunction with other methods; the use of a water baths, or methods aided by electric currents. When choosing a method, the first priority should be towards not damaging the object. And they should be accompanied by appropriate investigations to ensure their applicability.

Introduction

Desalination refers to the removal of salts, i.e., their ions, from the pore system of porous materials such as natural stone (sandstones, limestones, tuffs, etc.), brick or terracotta, renders/plasters, or wall paintings. Treatments can be carried out in situ, or in a workshop for movable objects.

The most commonly encountered salts are sulfates (Gypsum CaSO₄•2H₂O, Mirabilite (Thenardite) Na₂SO4•10H₂O (Na₂SO₄), magnesium sulfate (MgSO4•7H₂O and other hydrates), chlorides (e.g., NaCl) and nitrates (Niter KNO₃ u.a.). In general, different salts can co-exist and a variety of salt-forming ions can be found in pore solutions.

Salts can damage the fabric of porous materials and lead to powdering of the surface, sometimes causing substantial loss. The amount of deterioration and its appearance depends on the type of salt(s) crystallizing, the amount of salt(s) present, and environmental conditions. Particularly damaging are climate fluctuations around the Deliquescence point of the salts. In addition, water-soluble salts have an influence on conservation measures such as consolidation, treatment with hydrophobic materials and painting or plastering, often hindering them. For these reasons, reduction of the salt content is an indispensable prerequisite for the success and durability of a conservation measure as well as for reducing the deterioration rate of the object in question.

The desalination/ salt reduction can be carried out by several different methods [Sawdy.etal:2006]Title: Desalination—rubbing salt into the wound?
Author: Sawdy, Alison; Heritage, Adrian
. The use of renders/ mortars on salt-contaminated objects [Auras:2008]Title: Poultices and mortars for salt contaminated masonry and stone objects
Author: Auras, Michael
is described elsewhere.

Water bath desalination

This method is only applicable for objects that can be transported to a workshop, usually sculptures and items that can be removed from their permanent location.[Franzen.etal:2008]Title: Water bath desalination of sandstone objects
Author: Franzen, Christoph; Hoferick, Frank; Laue, Steffen; Siedel, Heiner

The salt contaminated object is placed in a bath of cold or slightly warm water. In doing so, the water can be recirculated through a deionizing system to enhance the desalination process. An easier, but less effective method is to change the water from time to time. The efficiency of the desalination is monitored by measuring water conductivity.

Degree and speed of the desalination depends on the size of the object, the properties of the material (e.g., fine pores or coarsely porous stone), the type and amount of salts and salt-forming ions and their distribution within the pores. Salts concentrated near the surface are removed faster than those found in-depth. The treatment of life-size figures can take between a few weeks to several months.

On suitable objects, desalination in a water bath can be successful. Specific risk factors are:

• Saturation of the entire pore structure with water: Risk for paint layers;
• Increased deterioration: Flaking of brittle surfaces;
• Presence of salts with several hydrate phases: Phase changes may induce mechanical stresses, which can cause a loss of substance to the object.

A pre-consolidation of brittle surfaces with a suitable strengthening agent, such as silicic acid esters may be possible. However, this treatment may considerably delay the desalination in some cases.

Poultices for desalination

Desalination using poultices relies on the principle that salts dissolved in water are transported from the salt-contaminated, porous, mineral building materials into the poultice. The transport of salt solutions can take place both by diffusion and by movement of the fluid. This is triggered by a moisture gradient, i.e., from the humid to the dryer area (capillary action, i.e., advection) or by temperature, density and pressure gradients (convection). In contrast, concentration gradients lead to the diffusion of the salt ions. Advection is determined by the pore structure of the building material and is characterized by the water absorption coefficient [Heritage.etal:2008]Title: How do conservators tackle desalination? An international survey of current poulticing methods
Author: Heritage, Adrian; Sawdy, Alison; Funke, Fredericke; Vergès-Belmin, Veronique; Bourges, Anne
. Diffusion is triggered by concentration gradients, i.e., from higher to lower concentration. It can also takes place as surface diffusion at interfaces. The convective transport is triggered by pressure, density and temperature differences and can be checked via water permeability tests. This transport process occurs preferentially in larger pores (> 0,1 mm), fissures and voids. The three transport processes described above take place simultaneously. The scale on which the transport processes contribute to the desalination, depends on the properties of the poultice material, on the environmental and implementation conditions. Essentially, the moisture and salt currents are influenced by a complex interplay between moisture condition, salt distribution, the properties and particularly the porosity of the substrate and that of the poultice [Pel.etal:2010]Title: Physical principles and efficiency of salt extraction by poulticing
Author: Pel, Leo; Sawdy, Alison; Voronina, V.
[Lubelli.etal:2010]Title: Desalination of masonry structures: fine tuning of pore-size distribution of poultices to substrate properties
Author: Lubelli, Barbara; van Hees, Rob P. J.
. Salt reduction through the use of poultices is the most common method of desalination [Bourges.etal:2008]Title: Comparison and optimization of five desalination systems on inner walls of Saint Philibert church in Dijon, France
Author: Bourgès, Anne; Vergès-Belmin, Veronique
[Verges-Belmin.etal:2005]Title: Desalination of masonries and monumental sculptures by poulticing: a review
Author: Vergès-Belmin, Veronique; Siedel, Heiner
. In the last years, important methodical improvements have been achieved, especially due to the EU- project "Desalination" [Sawdy.etal:2008]Title: A review of salt transport in porous media, assessment methods and salt reduction treatments
Author: Sawdy, Alison; Heritage, Adrian; Pel, Leo
.

Electrochemical desalination

Electrochemical desalination can be conducted in the workshop or on site, on a building. When introducing electric tension to the object the salt ions migrate to the anode or cathode depending on their charges. The electrodes have to be laid into a poultice or mortared into a gap in the masonry. Basic principles of this method are the processes known as electrokinetics and electroosmosis, respectively. If an electric field is installed with the help of electrodes, the ions migrate to the oppositely charged poles. The electrochemical desalination according to the principle of electroosmosis (including the AET- Aktive Entsalzung und Trocknung- active desalination and drying method) has been a matter of controversial discussion in the literature. For an efficient desalination a number of rod-shaped electrodes are needed, that can be mortared into crevices or drill holes. The distance between electrodes should not be more than 30 cm. A better solution appears to be the use of net shaped electrodes, that are placed in a poultice onto the surface. A major problem consists in producing a similarly good electrical transmission on every electrode. Otherwise the current only flows to one electrode, which cannot be verified without special circuits. The applied tension has to be on the order of some 20-50 volts. This can lead to health and safety issues when used outdoors. The method of desalination only works if sufficient moisture is present and for this reason the objects have to be kept humid.

Due to the very complicated and not well-established discharge reaction of the ions on the electrodes, a return migration of ion complexes can take place. This particularly applies to ions with amphoteric properties (e.g., magnesium ions). Furthermore, the discharge reactions can cause strong pH fluctuations, leading to a very acidic or a very alkaline environments and to damages in the vicinity of the electrodes.

In the event of a high salt contamination, a new method, developed by Friese can be applied ([Venzmer:1991]Title: Sanierung feuchter und versalzener Wände
Author: Venzmer, Helmuth
). In such cases the influence of electric fields serves to stop the ion transport in the material. Hence, brick sized suction cups are placed on the surface of the wall. Under vacuum conditions a liquid is led past the sample surface. The liquid moistens the surface and sets the salt ion transport into motion. The salts are virtually washed off the surface and transported away.

Evaluation criteria

(according to [Snethlage:1994]Title: Entsalzung
Author: Snethlage, Rolf
)

The presence of soluble salts is nearly always part of the reason why historic building materials have deteriorated. To determine whether a desalination should be carried out, the following points need to be considered:

Evaluation (Risk assessment) of hazards to the substance of objects affected by salt contamination

It is to be balanced, whether the desalination measures may cause greater loss to the substance, than leaving the object in its present condition. If the climate can be stabilized to a degree, so that the salts are not subject to dissolution and recrystallization cycles, then not carring out a desalination is justified.

Usefulness of the desalination

It is necessary to assess whether it is at all possible to carry out a successful desalination. Only if the salts are situated near the surface, can a desalination process be relatively successful. A uniform distribution of salts at approximately 1% by weight throughout the wall thickness, which commonly occurs in the presence of nitrates, cannot be treated successfully. In such cases other solutions must be considered, e.g., giving the building a different use.

Protection of the original substance

During desalination treatments, risks to the object can arise. For example, incorrectly applied poultices may not be easily removed from the surface and their removal may cause more damage.

Adverse effects on other conservation measures

Low salt concentrations do not compromise the success of other conservation measures. Desalination in such cases may therefore be omitted. However, only a material-specific threshold value for salt contamination can be specified, because the porosity, pore radius distribution and climate plays an important role. In individual cases decisions have to be made with reference to a survey or condition report. It is known that high concentrations of salts can have adverse effects on consolidation with silicic acid esters or on hydrophobic treatments. Also the durability of the measure will be affected and in the case of water repellent treatments may result in increased deterioration.

Control measures

To evaluate the effectiveness of the desalination, the salt content in stone, plaster or brick should be measured before and after the treatment.

There are only a few experiences on the desalination of entire buildings. Basically, a satisfactory desalination of any kind can only be expected, if the salts are concentrated near the surface at 1-2 cm depth. With both, poultices and electrochemical methods, the desalination will only reach a few centimeters into the materials.

Example: At "Nürnberger Tor" in Forchheim a NaCl contamination was successful in removing nearly 90% of the salt using a bentonite / sand / cellulose poultice, which was applied twice to the splash zone of the structure. The success of the measure was can be attributed to the fact that the salt was confined to the few outermost centimeters.

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

The permeability is a measure of the ability of a porous material (often a rock or unconsolidated material) to allow fluids to pass through it. Am material with a high permeability lets the fluid pass through faster than one with a low permeability.