From Saltwiki

Author: Michael Auras
English translation by Sandra Leithäuser

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N. B.

This is a translation from the German SalzWiki . The German language does not necessarily distinguish between plaster and render. In this text the term plaster refers to application to indoor walls, while render refers to external ones. The term plaster is usually associated with either gypsum (from which it originated) or lime plaster. Most restoration and desalination mortar or plaster systems can be used on external and internal walls and do not contain gypsum.


Desalination render systems are often used when poultices or water bath treatments are not an option, e.g., not logistically or economically viable. The conservation measures to be considered should be based on diagnosis drawn from examination and analysis. Renders, plaster and mortar systems are selected with respect to the applicable requirements:

  • Salts and humidity are to be blocked from entering the render/plaster,
  • The effects of salt or humidity exposure is to be concealed,
  • Salts are to be retained inside the render/plaster or mortar,
  • Salts and humidity are to permeate to the plaster surface

Through the choice of binder, aggregate, additives and admixtures the properties of modern renders, plasters and mortars can be adjusted to fulfill the tasks mentioned above. Dense cement mortars or lime-cement mortars, which have been adjusted to be water repellent, have an inhibiting effect (hydrophobic renders/plasters) on salt migrating into them. If rising moisture has not been blocked in the wall, this will most likely result in an increase of moisture content underneath the render/plaster with low capillary absorption capacity. This may lead to the blistering of the render/plaster or the diversion of moisture and salts into other, previously unaffected areas of the wall, causing further damage. If the plaster has a high capillary absorption capacity (lime plaster, gypsum plaster or lime-cement plaster without water repellent or inhibiting admixtures), the moisture and salts will migrate into the render/plaster and accumulate within the pores as well as at the wall-render interface. Eventual salt crystallization will lead to the destruction of the render/plaster. However, this migration of salt solutions into the render/plaster can be exploited to safeguard precious surfaces (sacrificial plasters) or to remove salts from the wall (poultice plasters). The first step is to make a basic decision:

  • Is the purpose of the render to show a damage free wall surface for a given amount of time? Or
  • Is the purpose of the render to reduce the salt contamination?

In this case it may have an negative aesthetic effect and a diminished durability.

The following sections describe the various alternatives.

Restoration plasters for desalination

Dehumidifying/ desalination restoration plasters are ready- mixed mortars with a high porosity, salt absorption capability, water vapor permeability and heat insulation power. Effect:

  • Shifting evaporation levels of moisture in the wall, from the surface into the plaster coat.
  • Gradual absorption of the dissolved salts from the wall into the plaster fabric without the salts causing damage through crystallization.

The International Association for Science and Technology of Building Maintenance and Preservation of Monuments (WTA) has issued standards for restoration mortars, plasters and renders (in German). WTA-Merkblatt 2-9-04/D[1] [2].

Restoration renders (according to the WTA)

For restoration renders with a desalination function, these systems are only of limited use according to the WTA. Their main aim is to produce a damage free wall surface, which at the same time cam "store" some salts within the pores. When the pore structure is saturated with salts, efflorescence appears on the surface and the render/plaster needs to be replaced.

At medium to high salt contamination of the substrate, the application of several layers of the restoration render system is recommended. This system can consist in a porous undercoat for salt storage and a restoration render finishing layer having hydrophobic characteristics. If the substrate consists of very dense stone and small joints, it can be an advantage to first locally apply a slurry undercoat, to improve the adhesion of the subsequent render coats.

The salt contamination degree for chloride, nitrates, and sulfates, defined by the WTA- Code of Practice, are listed in Table 1.

Table 1: Evaluation of the salt contamination according to WTA-Code of Practice 2-9-04/D. Data in M.-%.
Degree of contamination Chloride Nitrate Sulfate
Low contamination < 0.2 < 0.1 < 0.5
Medium contamination 0.2 to 0.5 0.1 to 0.3 0.5 - 1.5
High contamination > 0.5 > 0.3 > 1.5

In addition to the WTA- Code of Practice, it is recommended to differentiate when faced with a sulphate contamination, which cations are present. If gypsum- a salt with relatively low solubility- is present, the degree of contamination can possibly be lowered. Contrariwise, sodium or magnesium sulfate have a much higher damage potential, therefore increasing the degree of contamination comes necessary. Additionally, the substrate resilience vis-á-vis the salt crystallization should be included in the evaluation (see Table 2).

Table 2: Requirements to undercoat- and restoration renders according to WTA-Code of Practice 2-9-04/D
Requirements Undercoat Restoration render
Consistency of fresh mortar (Slump) 170 ± 5 mm 170 ± 5 mm
Air- void content of fresh mortar > 20 Vol.-% > 25 Vol.-%
Water vapour diffusion resistance factor µ < 18 µ < 12
Water penetration after 24 hours > 5 mm < 5 mm
Capillary absorption W24 > 1.0 kg/m² W24 > 0.3 kg/m²
Porosity of hardened mortars > 45 Vol.-% > 40 Vol.-%
Density of hardened mortars < 1.400 kg/m³
Compressive strength after 28 days > ßD Restoration mortar < 1.5 to 5.0 N/mm²
Flexural strength after 28 days < declared array
Compressive-/flexural strength after 28 days < 3

Restoration renders according to the WTA have been successfully used in the past. However, they restrict the drying of masonry, therefore it is imperative to undertake measures for the reduction of moisture replenishment, such as rising damp. Problems with restoration renders occur mainly when absorbent, unrendered areas are next to rendered ones and rising damp in the wall was not completely blocked. In such cases the moisture can be diverted into previously unaffected parts of the wall and damages can occur there subsequently.

Sacrificial renders

Sacrificial renders aim to reduce the deterioration of vulnerable surfaces by transferring the salt contamination into them. Damages to the render causes its “sacrifice”. This has to be taken into account when considering their application.

The WTA- code of practice WTA-Merkblatt 2-10-06/D [1][3] defines sacrificial renders as having limited durability and their aim is to provide a remedial and protective function. The code of practice distinguishes between various kinds of damages and their sources and between the properties of the different sacrificial renders. The WTA has defined the requirements for sacrificial renders and recommends a poultice plaster for salt contaminated brickwork and stonework.

Non-hydraulic lime plasters have been used occasionally as sacrificial renders, but they are only of limited use for salt contaminated walls because the setting of the render (and its strengthening) can be incomplete or delayed due to a high volume of moisture from the substrate. Because of the low mechanical strength, lime renders have a low resistance to deterioration from salt crystallization. And is the reason for damages to be seen soon after their application. The high capillary absorptive capacity in lime plasters causes the salts to be carried to the surface, where the humidity evaporates and salts crystallize. In the presence of high humidity and high salt contamination levels, the rapid salt accumulation in combination with moderate porosity (approximately 30 Vol.-%) can lead to premature closure of the pores on the plaster surface. This process lowers the effective moisture passage causing a drying blockade [Kuenzel:1991]Title: Trocknungsblockade durch Mauerversalzung
Author: Künzel, Herbert
Link to Google Scholar
, with subsequent damage due to the rise of the humidity level in the structure. If only moderate moisture replenishment is taking place, lime mortars can contribute with considerable efficiency to the extraction of salts from brickwork or stonework. Auras [Auras:2008]Title: Poultices and mortars for salt contaminated masonry and stone objects
Author: Auras, Michael
Link to Google Scholar
reports salt discharges of between 80 to 800 grams of salt per square meter of plaster surface within a few months, when using a natural hydraulic lime plaster.

Poultice renders

Poultice renders are aimed to shift salts from the area of damage in the substrate to be conserved into them. They should allow high salt storage and thus withdraw some of the salts from the substrate that is to be protected. In general they need to have high porosity and use cement as a binder to provide mechanical resistance to salt crystallization. Obviously, they should not be hydrophobic. Their capillary absorption allows moisture and salt transport through them right to their surface increasing their drying rate and reducing the danger of moisture accumulation in the substrate. The true porosity of up to 60 Vol.-% produces a low water vapor diffusion resistance, hereby additionally benefiting the drying process. Despite being adjusted to fit masonry of low cohesiveness, the poultice render exhibits a high resistance towards crystallizing salts, due to its hydraulic binder. Long term durability can be reached even at high salt contamination levels of the substrate. The poultice render stands out as being of high efficiency for its desalination performance (Table 3). The durability, until first damages appear, is in general far better than for lime renders, though it may be lower than for the restoration renders discussed above. It is to be noted that on poultice renders aesthetic issues such as moisture stains or salt efflorescences can soon appear.

Table 3: Requirements for the properties of hardened mortars concerning poultice renders according to the WTA-Code of Practise 2-10-06/D
Porosity > 60 Vol.-%
Bulk Density > 1.0 kg/cm³
Water vapor diffusion resistance factor µ < 10
Capillary absorption w > 1.0 kg/m²h½
Compressive strength ßD < 5.0 N/mm²
Bond strength ßHZ < Substrate
Elastic modulus or Young´s modulus E < Substrate
Drying < 1 kg/m²d


Slurry sample on the exterior wall of the protestant reformed church in Eilsum, Germany

Thinly applied slurries are generally not very appropriate as coatings on salt contaminated masonry, because they can only develop a low protective effect, due to their low coat thickness. Porous slurry materials only have a small salt storage capacity. On a slurry treated surface, with a high capillary absorptive capacity, salts will fast appear. The application of a thin slurry coat (locally on salt contaminated surfaces) prior to that of a new render has proved successful by serving as a barrier between the substrate and the render thus improving the overall performance. Using a hydrophobic slurry carries the risk of damage from salt crystallization at the slurry-substrate interface, inducing its detachment.

Pointing mortar

If rendering of a salt contaminated wall is not possible, either in the short or long term, pointing mortars can be adjusted according to the above discussed render possibilities. For this purpose, the different absorptive capacities of stone or brick-work, bedding mortars and pointing mortars, are to be considered. For example, a historic lime based bedding mortar with a high absorptive capacity, would presumably carry out moisture and salts to a great extent. If the masonry joint is now pointed up with a mortar having a low absorptive capacity, the moisture transport to the surface is reduced. This can lead to high humidity retention in masonry with a low absorptive capacity. On masonry with a high absorptive capacity, however, the humidity and salt transport is diverted into the stone, shifting the salt contamination from the mortar into the stonework and very likely resulting in its subsequent damage.



[Auras:2008]Auras, Michael (2008): Poultices and mortars for salt contaminated masonry and stone objects. In: Ottosen, Lisbeth M.; Rörig-Dalgaard, Inge; Larsen, Poul Klenz; Brajer, Isabelle; Bøllingtoft, Peder; Marciniak, Mette; Svane, Maja (eds.): Salt Weathering on Buildings and Stone Sculptures, Technical University of Denmark, Lyngby, Denmark, 197-217Link to Google ScholarFulltext link
[Kuenzel:1991]Künzel, Herbert (1991): Trocknungsblockade durch Mauerversalzung. Bautenschutz und Bausanierung, 14 (4), 63-66Link to Google Scholar