Series For Underground Testing. Episode 4: Mineral Substrates

2023 Author: Hannah Pearcy | [email protected]. Last modified: 2023-11-26 11:39

The fourth part of our series on substrate testing deals with mineral substrates.

Which plaster is involved can often be answered by eye. Mineral finishing plaster in the outdoor area can usually be recognized by the typical granularity and structure. In contrast, plaster-bound new plaster in the interior often has a smoothed and denser surface. Numerous defects such as damaged areas, touch-up areas, rust stains, dirt, cracks, moss, algae and fungus infestation or moisture can also be determined visually. To test the surface strength, a scratch test with a solid, angular object or rubbing by hand is required. The plaster's absorbency is checked with a wetting test.

Inspection, scratch and strength tests or wetting tests - in the leaflets of the BFS (Federal Committee for Color and Property Protection) No. 9 "Coatings on mineral exterior plasters" and No. 10 "Coatings, wallpapering and adhesive work on interior plasters" are the test methods and their scope as well as appearances, technical information and measures to be taken.

Determine the cause

Finding a subsurface deficiency is one thing, identifying the cause is another. Only when the cause has been identified and eliminated can the damage be remedied in the long term. Therefore, some appearances and test methods will be discussed in more detail below.

Sintered layers

Shiny spots in the dry state of the surface can indicate sintered layers. These arise when binders accumulate on the surface during processing and drying of the plaster. However, since sintered layers are not always visible as shiny spots, the following check is recommended: First, scratching and the subsequent wetting test ensure clarity. Sintered layers are usually difficult to wet. This can be seen from the fact that the scratch mark shows a significantly better water absorption of the underlying plaster layer, i.e. the scratched depressions are dark in color.

Efflorescence is another subsurface defect that is often encountered with mineral substrates. This term describes the crystallization of salts on the surface. Efflorescence occurs when wall formers such as masonry or concrete are insufficiently protected against rising damp or when plaster systems do not adequately repel water. Ingressing water removes water-soluble salts from the mineral building materials, which migrate capillary through the pores to the surface. The actual salt damage occurs when the dissolved salts dry on the surface. The volume of the salts increases during crystallization, which slowly leads to the destruction of the plaster and existing coatings. An analysis of the individual building salts (chlorides, nitrates, sulfates) is only possible with special indicators in the laboratory. Regardless of which salts are involved in the crystallization, the cause of the exposure to moisture must be identified and remedied to prevent further efflorescence.

Algae and mushrooms

Since the Central European climate offers good living conditions for algae and fungi, this risk for facade surfaces should already be taken into account when planning new buildings or planning renovations. Structural moisture protection must be in the foreground, as algae and fungi cannot grow on dry substrates in the first place. Algae are usually visible on the surface, depending on the type of algae, the areas affected are greenish to bluish black. Mushrooms often appear as a thread-like network. Once the microorganisms have been removed, the surfaces are disinfected.

In order to prevent renewed algae and fungus infestation on vulnerable facade surfaces, it is recommended to equip the facade coatings with film protection. Another option is the use of hydrophilic plasters and facade coatings. Thanks to selected mineral raw materials and their specific product properties, these products create a balanced water balance on the facade surface. The combination of low-nutrient and alkaline coating materials, in conjunction with a largely water-free facade surface, then provides natural protection against microorganisms.

Mineral substrates (4) 1. The rubbing test with the palm of the hand: Is the surface firm or is there abrasion? Mineral substrates (4) 2. Even after wetting the scratch test must be passed dry enough? If necessary, a foil test must be carried out. Mineral sub-founders (4) 4. Sintered layers can be recognized by scratching and moistening. After wetting there is a darkening of the scratch mark. Mineral sub-foundation (4) 5. Rising moisture causes salts to be released from the substrate and thus for flaking plaster layers.

The fifth part of the series will deal with the basic technical standards and requirements that are relevant for the planning of the coating systems to be applied.

Markus Lindenbaum