Desalination

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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

Desalination with compresses relies on the principle of solving salts in water and transport it from the salt-loaded, porous, mineral building materials into the compress. The transport of the salts in solution can take place both both by diffusion and by movement of the fluid. The motion of a fluid is usually troggered by a moisture gradient (capillary) or by temperature, density and pressure gradients (convection).

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

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