Poultices for desalination

Author: Hans-Jürgen Schwarz
English version by Sandra Leithäuser
back to Desalination

Permanently wet poultice

The poultice material is kept humid during the application period. There is no, or only a small moisture gradient, resulting in the salt transport being dominated by diffusion. After an adequate duration of the application, a concentration equilibrium between poultice and substrate is effected. Permanently wet poultice material should only be used in cases, where a moisture gradient cannot be set up due to reasons, which lie in the physics of the building. These reasons can include a very damp wall, or when drying is limited. There is also a risk, that the salt accumulation shifts into lower regions of the structure, due to the high moisture penetration.

Drying Poultice

The poultice material dries out more or less completely, depending on the climatic conditions during the application. Capillary transport and diffusion usually effect a flow towards the poultice material, because of the rectified moisture and concentration gradient. If the porosity of poultice and substrate is well matched, the capillary transport is predominant. When gradual drying takes place, salts accumulate in the poultice material. For the treatment of relatively poorly soluble salts (e.g. gypsum) a combination of the two methods can be used. To begin with a permanently wet poultice is applied to dissolve the gypsum, then a drying poultice. Due to the sheer number of influencing factors and their variation during the desalination treatment, no universally applicable procedure can be proposed. Only approximate values can be provided, the exact terms are to be tailored to each individual case. Desalination poultices consist of one or more components (no binder) of the following groups. The components are mixed using deionized water.

• swellable materials from organic origin, no sealing properties, high water absorption and high water retention capacity (e.g. cellulose, paper pulp, aqueous gels)
• mineral components with large specific surface areas and a good ion exchange and adsorption properties (e.g. clay minerals and mixtures such as kaolin, bentonite etc.)
• inerte fillers (in combination with substances of the previous groups) to give the mixture properties such as stability, high porosity or a loose structure, to improve workability or decrease shrinkage.(Quartz sand, quartz flour, silica, good quality sand- rich in quartz, lightweight aggregates or equivalent)

or components are: highly porous, absorbent, flexible and inert mats and fleeces (foams, paper, textile etc.)

The injection poultice procedure

This procedure [Friese.etal:1993]Title: Entsalzung von Ziegelmauerwerk mit dem Injektionskompressenverfahren
Author: Friese, Peter; Hermoneit, Bernd
is carried out by introducing the water for desalination not through the exterior, but through the interior of the wall. At the injection point, the water disperses spherically and when two water- spheres of adjacent injection points overlap, a capillary water flow is generated from the interior of the wall onto the surface of the poultice, where it evaporates. A disadvantage, however, is that holes need to be drilled into the wall, which allow for the injection of water to penetrate behind the contaminated layers.

On brickwork, for example, holes could be drilled into the joints- but this is a decision that needs serious evaluation with regards to ethics and history of the building.

When using this method it is very important to assess the structure of the wall well enough and to know exactly which materials were used for filling the core. The introduction of water into the structure could lead to an increase in weight, leakage of unknown substances, water seepage in other areas of the building, or uncontrollable swelling. Also the amount of water introduced needs to be controlled.

Tabelle 1: Most frequently used poultices (in accordance with [WTA_E 3-13-01/D:2003]The entry doesn't exist yet.)

Poultice composition	Advantages	Disadvantages
Pure cellulose fibres of different length	- pH-neutral - flexible, soft, adherent - high water absorption	- low stability - risk of mould formation
Bentonite, attapulgit, kaolin, pure or mixed with cellulose	- high water retention capacity - kaolin: in solution almost neutral,hardly any alkali can be mobilised	- extremely hard when high clay mineral content is present - sometimes increased pH- values - kaolin: risk of white residue on the surface
Clays (no further specification)	- high water retention capacity	- extremely hard - sometimes increased pH-values - contamination and soluble proportion unpredictable
Newspaper or paper pulp predominantly cellulose/ hemicellulose/ lignin	- high water absorption	- bleaches, additives and colors could be introduced
Cellulose-mixtures with high sand content	- with good quality sands pH-neutral	- properties vary - difficult to process - low stability
Clay mineral-cellulose-sand- mixture (different mixtures, commercially available, predominantly on bentonit basis)	- good stability - machine processable - strength and shrinkage can be controlled through composition	- any mix can become too heavy or too hard - sometimes increased pH-values
Clay minerals-light aggregates-mix, sometimes with cellulose	- good stability - machine processable - low shrinkage, low density	- sometimes increased pH-values - can become too hard, depending on the mix - kaolin: risk of white residue on the surface

Strictly not recommended are components:

• preventing mass transport (risky are film- forming swelling media, or media with hydrophobic additives like Agar- Agar, gelatine, protein- gels, methyl cellulose),

• containing or producing soluble salts

• mineral binders (gypsum, lime, cement) or

• surface- active substances (cleaning agents or washing- active substances)

All material compositions provide some advantages and disadvantages, which are listed in the table above. For this reason the choice of material has to be tailored to each individual operation purpose, with regard to the boundary conditions.

Further quality requirements need to fulfill following demands:

• Poultice components must not contain soluble salts (i.e.< 0,1 M-% total salts)
• The salt content of poultices has to be defined using the usual elution method on a minimum of 3 representative samples. The variation of the results from the arithmetic average has to be stated. The elution method using 20 times the amount of deionized water, over a period of 24h at room temperature and under repeated agitation, has proved its value.
• The ready-for-use mixture should have a pH-value of 6- 10 maximum (tested at 25 °C)
• The poultice mixture should be easy to apply and should adhere well, but not cause any tension to the substrate.
• The poultice mixture should be free from dyeing agents.
• After treatment, it should be possible to remove the poultice without leaving any residue.

The poultice method can be carried out on the object in situ, or in the workshop. The above mentioned parameters can be varied, depending on the kind of poultice used, the existing climate, the surface structure, e.t.c.

When applying the poultice onto vertical surfaces the adhesion is often a problem. The poultice should not slide off the surface through its proper weight and possibly cause damage. Friable surfaces should therefore be consolidated before desalination.

The application of the poultice should be repeated several times. As for the water bath desalination the success of the treatment depends on the pore structure, the kind of salts and salt forming ions present and their distribution, and should be controlled through analysis.

More detailed references can be taken from the code of practice mentioned above.

In chemistry, the term inert is used to describe something that is not chemically reactive. The noble gases are known as inert gases because of their perceived lack of participation in any chemical reactions (e.g. argon).