Poultices for desalination

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

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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 kkaolin 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.

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).