Desalination

Author: Hans-Jürgen Schwarz
back to Measures

Abstract

Increased salt contaminations can be reduced using different methods. These include poultice desalination- also in combination with other methods, the reduction of the salts using a water bath, or methods aided by electric currents. When choosing the method, the protection of the object must always be the first priority. The measures should be accompanied by appropriate investigations to ensure their success.

Introduction

Desalination denotes the removal of salts and salt-forming ions out of the pore structure of porous materials such as natural stone (sandstones, limestones, tuffs, etc.), brick or terracotta and plaster or wall paintings. Treatments can be carried out in situ on the object, or on movable objects in the workshop.

The most commonly encountered salts are sulphates (Gypsum CaSO₄•2H₂O, Mirabilite (Thenardite) Na₂SO4•10H₂O (Na₂SO₄), magnesium sulphate (MgSO4•7H₂O u.a), chlorides (e.g. NaCl) und nitrates (Niter KNO₃ u.a.). In individual cases, different salts can exist beside one another, and a variety of salt-forming ions can be in the pore solution.

Salts can damage the fabric of porous materials and lead to powdering of the surface, sometimes causing substantial loss. The amount of decay and its appearance depend on the kind of crystallizing salts, the concentration of the salt solutions and the 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 making such action impossible. For these reasons, the reduction of the salt content is an indispensable prerequisite for the success and the durability of a conservation measure.

The desalination/ salt reduction can be executed using several different methods [Sawdy.etal:2006]Title: Desalination—rubbing salt into the wound?
Author: Sawdy, Alison; Heritage, Adrian
. The use of plaster/ slurries 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 practicable for objects that can be transported to a workshop, usually sculptures and objects 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 desalinated and circulated, 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 the conductivity of the water.

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 in the pores. Salts concentrated near the surface are removed faster than those from deeper areas. The treatment of life-size figures can take between a few weeks to several months.

On suitable objects, desalination in a water bath has a good chance of success. Specific risk factors are:

• the saturation of the entire pore structure with water: risk for paint layers;
• advanced degree of destruction: flaking of brittle surfaces;
• salts with several hydrate phases: mineral hydration may be triggered, leading to an increase in the volume of the salt, 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. Due to this treatment the desalination can in some cases, be considerably delayed.

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. The motion of a fluid is usually triggered by a moisture gradient (capillary action) or by temperature, density and pressure gradients (convection). In contrast, concentration gradients lead to the diffusion of the salt ions. The transport by capillary action (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
. The transport direction of the ions runs in accordance with the moisture gradient, from the humid to the dryer area. The driving force for ion transport by diffusion is the concentration gradient. The ions diffuse in accordance with the concentration gradient, from the higher to the lower concentration. Diffusion also takes place as surface diffusion on the interface. The convective transport is triggered by pressure, density and temperature differences and can be checked via the water permeability or other tests. This transport process occurs preferentially in larger pores (> 0,1 mm), fissures and voids. The processes described above take place in combination. The scale on which the transport processes contribute to the desalination, depends on the properties of the poultice material, on the environmental and procedural 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 [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.
. The salt reduction using 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 the object. When introducing electric tension to the object the salt ions migrate to the anode or cathode. 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 transition 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 a scale of several ten 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 reaction can cause strong pH fluctuations, leading to a very acidic or a very alcaline environment 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 causes the ion transport to not be set into motion. Hence, brick sized suction cups are placed on the surface of the wall. Under vacuum conditions a liquid is led passed 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
)

Salts are nearly always part of the the reason why historic substance is damaged. When the decision is made, whether a desalination should be carried out, the following considerations are necessary:

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, where the salts are not subject to dissolution and recrystallization cycles, it is a justifiable decision to not carry out a desalination.

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, is a desalination promising. An even spread of the salts at approximately 1 weight/ percent throughout the thickness of the wall, which commonly occurs in the presence of nitrates, cannot be treated successfully. In such cases other solutions must be considered, e. g. a change of use.

Protection of the original substance

During desalination treatment, risks to the object can arise. It has to be taken into consideration, that it may not be possible to remove incorrectly applied poultices from the surface.

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 contaminations 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.

Control measures

For monitoring the success 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 successfully removed up to 90% with a bentonite / sand / cellulose poultice, which was applied twice to the splash zone of the structure. The success of the measure was due to the fact, that the salinization was confined to the uppermost centimeters.

Literature

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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.