Desalination: Difference between revisions
No edit summary |
|||
| Line 40: | Line 40: | ||
== [[Desalination compresses]] == | == [[Desalination compresses]] == | ||
<!-- | <!-- | ||
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 <bib id="Heritage.etal:2008"/>. 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 (<nowiki>></nowiki> 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 <bib id="Pel.etal:2010"/><bib id="Lubelli.etal:2010"/>. | |||
The salt reduction using poultices is the most common method of desalination <bib id="Bourges.etal:2008"/> <bib id="Verges-Belmin.etal:2005"/>. In the last years, important methodical improvements have been achieved, especially due to the EU- project "Desalination" <bib id="Sawdy.etal:2008"/>. | |||
== [[Electrochemical desalination]] == | == [[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- '''A'''ktive '''E'''ntsalzung und '''T'''rocknung- 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 (<bib id="Venzmer:1991" />). 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 <bib id="Snethlage:1994"/>) | |||
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. | |||
--> | --> | ||
Revision as of 08:03, 16 November 2011
<bibimport />
Author: Hans-Jürgen Schwarz
back to Measures
Abstract
If there is an increased salt load, the reduction of the salts is a method to deal with. Salt reduction measures include the possibilities of cpompress desalination, alone or in combination with other methods. Reducing the salt content of an object in a water bath as well as by electric current supported means are other options. When choosing the method the protection of the object has always the first priority. By appropriate investigations the success of a measure has to be ensured.
Introduction
Desalination is the removal of salts and salt-forming ions out of the pore structure of porous materials such as natural rocks (sandstones, limestones, tuffs, etc.), brick or terra cotta, plaster and wall paintings, in situ at the object or on moveable objects in the workshop.
The most commonly encountered salts are sulphates (Gipsum CaSO4•2H2O, Mirabilite (Thenardite) Na2SO4•10H2O (Na2SO4), magnesium sulphate (MgSO4•7H2O u.a), chlorides (NaCl u.a.) und nitrates (Nitrokalit KNO3 u.a.). In individual cases, differnet salts can exist side by side, and a variety of salt-forming ions in the pore solution.
Salts are damaging the structure of porous materials and lead to dusting with sometimes great material loss. The amout of destruction and its appearence depend on the kind of crystallizing salts, the concentration of the salt solutions and the environment conditions. Particularly damaging are climate fluctuations around theDeliquescence Humiditypoint of the salts. In addition, water-soluble salts have an ingfluence on conservation measures such as strengthening, waterproofing, painting or plastering or make even such action impossible. For these reasons looking to the success and the durability of a measure the reduction of the salt content is an indispensable prerequisite.
The desalination/ salt reduction can be done in several ways [Sawdy.etal:2006]Title: Desalination—rubbing salt into the wound?
Author: Sawdy, Alison; Heritage, Adrian
. The use of plaster/ slurries with 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 practicable only for objects that can be transported to a workshop, usually for sculptures and objects that can be removed for a restoration.[Franzen.etal:2008]Title: Water bath desalination of sandstone objects
Author: Franzen, Christoph; Hoferick, Frank; Laue, Steffen; Siedel, Heiner
The object contaminated with salts 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. Easier but not so effective the water is exchanged from time to time. The efficiency of desalination is monitored by measuring the conductivity of the water bath.
Degree and speed of 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. Generally life-size figures need of application from several weeks to several months.
If the material is suitable, desalination in a water bath has a good chance of success. Specific risk factors are:
- the total penetration of the entire pore structure with water, risk for the paint layers;
- advanced degree of destruction: flaking of the brittle surfaces;
- salts with several hydrat phases: hydration may be triggered, leading to a loss of substance.
A pre-consolidation of brittle surfaces with suitable strengthening agents may be possible (for example: silicic acid esters). The desalination can be delayed in some cases considerably.
Desalination compresses
Success control
The salt content in the stone, plaster or brick should be measured before and after the application to check the success of desalination.
On desalination to buildings, there are a few experiences. Basically, one can expect a satisfactory desalination of any kind only if the salts are concentrated near the surface at 1-2 cm depth. Either with compresses or with electrochemical methods the desalination reaches only a few centimeters of the materials.
Example: With a bentonite / sand / cellulose compress applied twice in the splash zone of the "Nürnberger Tor" in Forchheim a NaCl contamination was removed up to 90%. This was due to the fact, that the salinity was confined to the uppermost centimeters.
Literature
[Filter missing]