Renders/Mortars

Author: Michael Auras
English translation by Sandra Leithäuser

back to Measures

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.

Abstract

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 the moisture supply has not been blocked from the wall in question, this will most likely result in an increase of moisture underneath renders/plasters 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 .

Restoration plasters / Restoration plaster systems according to WTA

For treatments with a desalination function, restoration plasters according to WTA are only of limited use. In the first instance, they are to produce a damage free wall surface, but at the same time they are to store some portion of the salts in the pores. When the pore structure is saturated with salts, efflorescence appears on the surface and the plaster should be replaced.

At medium to high salt contamination of the substrate, the application of several layers of the restoration plaster system is recommended. Such a system can consist of a porous undercoat for salt storage and a restoration plaster topcoat, provided with hydrophobic elements. If the substrate consists of very dense stone and small joints, it can be an advantage to first punctually apply a slurry undercoat, to improve the adhesion of the following plaster coats.

The salt contamination degrees of nitrates, chlorides and sulphides, defined by the WTA- Code of Practice, are stated in Table 3.

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	Sulphate
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, especially when faced with an evaluation of 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. Sodium or magnesium sulphate, in contrast, have a much higher damage potential, therefore increasing the degree of contamination can be necessary. Additionally, the substrates resilience vis-á-vis the salt crystallization should be included in the evaluation.

Table 2: Requirements to undercoat- and restoration plasters according to WTA-Code of Practice 2-9-04/D

Requirements	Undercoat	Restoration plaster
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 plasters according to 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. Problems with restoration plasters occur mainly when absorbent, unplastered areas adjoin to areas treated with restoration plaster, and moisture replenishment from the substrate could not be fully arrested. In such cases the moisture can be diverted into previously unaffected parts of the wall. Subsequently, damages due to moisture and salts can occur in these areas.

Sacrificial renders

Sacrificial renders are to reduce the contamination of vulnerable surfaces, by transferring the damaging processes, caused for example by salts, into the plaster. Damages to the plaster or its “sacrifice” has to be taken into account, when using this treatment.

The WTA- code of practice WTA-Merkblatt 2-10-06/D defines sacrificial renders as plasters or renders with limited durability and the aim 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 plasters. The WTA has defined requirements for sacrificial plasters and recommends a poultice plaster for salt contaminated brickwork and stonework.

Non- hydraulic lime plasters have been used occasionally as sacrificial plasters, but are only of limited use for salt contaminated walls, because the strengthening of the material can be incomplete or delayed due to a high volume of moisture from the substrate. Lime plasters have a low resistance to damage from crystallizing salts, because of their low strength. This is why damages occur very 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 the 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. The lowering of the effective moisture passage, being triggered by this process, can cause a drying blockade [Kuenzel:1991]Title: Trocknungsblockade durch Mauerversalzung
Author: Künzel, Herbert
, 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
reports salt discharges of between 80 to 800 gr salt per square meter of plaster surface within a few months, when using a natural hydraulic lime plaster.

Poultice plaster

Poultice plasters are aimed to shift salts from the area of damage, i.e. crystallization, and from the substrate that is to be conserved, into the poultice plaster. It should allow high salt storage and thus withdraw some of the salts from the object that is to be protected. The material is of high porosity and uses cement as a binder, but without hydrophobic equipment. The capillary absorbency of the poultice plaster makes the moisture and salt transport through the plaster up to its surface, possible. This increases the drying rate and reduces the danger of moisture accumulation behind the plaster. 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 plaster exhibits a high resistance towards crystallizing salts, due to its hydraulic binder. Long term durability can also be reached at high salt contamination levels of the substrate. The poultice plaster stands out as being of high efficiency regarding the desalination performance. The durability, until first damages appear is usually much better than with lime plaster, even though it is still lower than with the restoration mortar systems devised for desalination. On poultice plaster aesthetic impairments like damp stains or salt efflorescences can soon appear.

Table 3: Requirements for the properties of hardened mortars concerning poultice plasters in accordance with 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

Slurries

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 coat thickness. Porous slurry materials only have a small salt storage capacity. On a slurry treated surface, with a high capillary absorptive capacity, salts appear rapidly, but the use of hydrophobic slurries carries a high risk of damage from salt crystallization, between substrate and slurry. A successful measure has been to apply thin slurry coats on small areas, as a barrier for the protection of salt contaminated original surfaces and new plaster.

Pointing mortar

If the durable or temporary plastering of a salt contaminated wall is not possible, pointing mortars can be adjusted according to the above mentioned plaster concepts. Hereby, the different absorptive capacities of the stonework, 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, that has a low absorptive capacity, the moisture transport to the surface is constricted. This can lead to a high humidity retention in walls made of stonework with a low absorptive capacity. On stonework 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 possibly resulting in its subsequent damage.

Literatur

[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, 197-217.
[Kuenzel:1991]	Künzel, Herbert (1991): Trocknungsblockade durch Mauerversalzung. In: Bautenschutz und Bausanierung, 14 (4), 63-66

The permeability is a measure of the ability of a porous material (often a rock or unconsolidated material) to allow fluids to pass through it. Am material with a high permeability lets the fluid pass through faster than one with a low permeability.

crystallization of a dissolved salt