Plaster (building material)

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Plaster as a building material for plaster systems can consist of different materials.

Wet plaster and dry plaster

A fundamental distinction between the building materials for interior plaster is the alternative between wet plaster and dry plaster: The traditional wet plaster (made of binding agent , aggregate and water) is processed when wet and increases the humidity for a longer period of time (see dry residents in the 19th century). Dry plaster , which has become increasingly important in recent decades, consists of building panels that are mounted on stud structures or directly on walls and ceilings to clad them.

Plaster mortar and coating materials

Mineral wet plaster mostly contains hydraulic binders that harden by forming crystals together with water molecules ( hydration ). These binders are called plaster and wall binders . Plaster mortar is generally understood to be building materials that contain such binders (while other building materials applied in thinner layers are called coating materials ). By its crystalline structure of the plaster after the solidifying moisture-proof, but can, depending on the composition and structure of its capillarity absorb a certain amount of water or water vapor. Its alkaline properties (due to the calcium hydroxide it contains ) make it mold-resistant.

On the other hand, there are traditional plasters under the collective name of clay plaster , which do not set, but merely maintain their strength through drying and are again softened by moisture. In the course of the 19th century this led to the extensive replacement of the clay with hydraulic plastering mortar. In interiors with limited humidity, this property is still valued today for regulating the indoor climate .

Other plastering mortars and coating materials have organic binders that harden through polymerization . If they contain aggregate as an aggregate, they are counted among the plaster mortars. Synthetic resin mortar is a fast-curing and waterproof building material for plaster . Silicate plaster hardens through silicification .

Wet plaster can contain mineral and organic additives. The properties of these building materials are influenced by additives such as sealants, setting accelerators, colorants, water repellants or air-entraining agents . Decorative coating materials with organic binders that contain fibers instead of aggregates (such as cotton plaster) are often referred to as plaster. Another aggregate used in the past was asbestos, which was used to improve mechanical properties. In addition to tile adhesives and leveling compounds, plasters are among the so-called new asbestos finds. When renovating, the plaster must therefore be checked for its asbestos content, see VDI guideline 6202, sheet 3.

Plaster mortar according to DIN and EN

Plaster mortar is regulated in the production and processing in DIN V 18550. The mortars are divided into five groups and, depending on the application, are classified as interior or exterior plasters.

The starting materials for the production of plaster mortar are binders, aggregates and additives or agents. As with masonry mortar, these are added to the dry mortar at the factory and according to the manufacturer's instructions. By adding the amount of water specified by the dry mortar manufacturer, the plastering mortar required by the customer is produced in mixing machines on the construction sites that are mounted on the dry mortar silos in a consistently high quality, more effective and also less labor and effort. The dyes are already added to the dry mix for the exterior plaster and generally make an additional painting step superfluous.

The individual groups provide information about the nature of the binders contained and the respective mixing ratio of binder and aggregate .

Plaster mortar groups according to DIN V 18550

Since 2018, DIN 18550 has only differentiated between the mortar groups PI, P II, P III and P IV. A more precise classification takes place in the corresponding European standards.

  • PI is divided into
    • a) Air lime mortar (1 part by volume of hydrated lime / 3 to 4 parts of sand (grain size always less than 4 mm))
    • b) Water-lime mortar consisting of PI a).
    • c) Mortar with hydraulic lime (1 part hydraulic lime / 3 to 4 parts sand).
  • P II lime cement mortar (2 parts hydrated lime / 1 part cement / 9 to 11 parts sand)
  • P III cement mortar (1 part cement / 3 to 4 parts sand)
  • P IV
    • a) gypsum mortar (1 part plaster)
    • b) Gypsum sand mortar (1 part plaster / 3 to 4 parts sand)
    • c) Gypsum lime mortar (1 part hydrated lime / 0.5 to 2 parts plaster)
    • d) Lime gypsum mortar (1 part hydrated lime / 0.1 to 0.5 part plaster of paris)

Plaster mortar groups according to DIN EN 13279

  • Gypsum dry mortar:
    • Gypsum plastering dry mortar B1
    • dry plaster mortar containing gypsum B2
    • Gypsum lime plastering dry mortar B3
    • Light gypsum plastering dry mortar B4
    • Gypsum-based light dry plastering mortar B5
    • Gypsum lime dry plastering mortar B6
    • Gypsum dry mortar for plaster with increased surface hardness B7
  • B1 dry plastering mortar:
Gypsum dry mortar, which consists of at least 50% calcium sulphate as the active main binder and no more than 5% building lime (calcium hydroxide).
  • B2 dry plaster mortar containing gypsum:
Gypsum dry mortar, which consists of less than 50% calcium sulfate as the main active binder and no more than 5% building lime (calcium hydroxide).
  • B3 Gypsum lime plastering dry mortar:
Dry plastering mortar according to 3.3 or dry plastering mortar containing gypsum according to 3.4 with more than 5% building lime (calcium hydroxide). Additives and additives may be added by the manufacturer.
  • B4 Light gypsum plastering dry mortar:
Dry gypsum mortar according to 3.3, 3.4 or 3.5, which contain either inorganic lightweight aggregates, such as expanded perlite or expanded mica, or organic lightweight aggregates.
  • B7 Gypsum plastering dry mortar for plasters with increased surface hardness:
Gypsum dry mortar mixed according to a special recipe for the production of plaster for which increased surface hardness is required

Additives and additives may be added by the manufacturer to all products.

Main areas of application

  • P Ia, b ( interior and exterior plasters with low stress)
  • P Ic ( interior plaster for rooms with normal use, including damp rooms)
  • P II ( interior plaster with increased abrasion resistance, also damp rooms and exterior plaster with water- resistant properties and increased strength)
  • P III ( exterior plaster in the area of ​​the basement exterior wall and exterior plinth area)
  • P IVa, b, c ( interior plaster with increased abrasion resistance and normal use)
  • P IVd ( interior plaster with low stress)
  • PV ( interior plaster with increased abrasion resistance and normal use)

The plaster substrate and its pretreatment play an important role in the selection of the plaster group and are given by the manufacturers of the stone types used with recommendations that must be observed.

Lime plaster

Lime plaster was an important part of construction work up until the early 1980s. This is the result of its excellent hygienic properties. Due to its disadvantages, the comparatively slow setting and the need to apply it in two layers with an intermediate dwell time (setting of the first layer), it has been replaced by the increased use of machine plasters. However, it is becoming increasingly important again.

Modern internal lime plasters (MG P Ic) are already available today, which can be processed mechanically and thus economically, have a setting time that is adapted to the structural requirements and can also be applied in a single layer.

A significant advantage of lime plaster is that significantly less moisture is brought in during processing, which then has to dry out of the masonry. Lime plasters only require around 180 liters of mixing water based on one ton of dry mortar. The significantly faster drying out has a moisture-regulating effect on the entire masonry and, due to its high alkalinity, it also largely prevents the formation of spores (e.g. mold).

Synthetic resin plaster

Synthetic resin plasters are standardized in DIN 18558 Part 1 (as P Org 1 and P Org 2). They are among the most widely used plasters and are also part of plaster systems (such as ETICS). The abbreviation “Org” for “organic” refers to the added binding agent.

Synthetic resin plasters are easy to use as finishing plaster . A polymer dispersion, which is also used in paints, is used as a binder. The aggregates can be mineral or organic. In most cases, the concealed plaster is still of a purely mineral nature, as this ensures better adhesion to the plaster base (masonry).

Thermal insulation plaster

Thermal insulation plaster , more rarely also called thermal plaster, is a plaster mortar with a thermal conductivity equal to / less than 0.2 W / (m · K) as a calculated value. This value is achieved by adding expanded polystyrene (EPS), vermiculite and perlite as additives during industrial production . This building material must also meet at least building material class B I (flame-retardant). If only perlite is added, the thermal insulation plaster meets the highest building material class A I (non-combustible) according to DIN 4102.

Thermal insulation plasters are used as a base plaster on facades and form the basis for the finishing plaster (finishing plaster ) if the thermal insulation properties of the entire wall structure are not achieved (e.g. compliance with the EnEV) or even improved (e.g. due to the wall construction material used) (stone material). B. in the low-energy house ). Due to its low inherent density and strength , thermal insulation plaster has excellent values ​​for vapor permeability (diffusion) and is one of the so-called light plasters (density between 0.6 kg / dm³ and 1.3 kg / dm³) of mortar group PI or P II. However, since the plaster top If it has to be water-repellent and impact-resistant, this material is unsuitable as a finishing coat.

This low bulk density can be achieved by adding mineral and / or organic sands with an open pore structure instead of the usual sands. The properties of the lightweight plaster greatly minimize the later or processing-related crack formation, as this material has a very high elasticity and elasticity with static and thermal stresses and is regularly specially adjusted to the masonry base.

Due to its consistency and adhesiveness, the material can be applied in one operation up to a thickness of more than 5 cm, which is done regularly using a plastering machine and spray nozzle.

More building materials

Depending on its function, the layers of a plaster system can consist of different, coordinated building materials, which are not all plaster mortar in the narrower sense. In addition, plaster bases , reinforcements such as wire mesh for Rabitz or glass fiber mats for alkaline plasters, as well as insulating materials or prefabricated parts made of gypsum, styrofoam, etc. for structural plaster and stucco elements belong to the building materials used.


  • Dietrich Neumann et al. (Ed.): Frick / Knöll Baubauslehre 2, Springer, Wiesbaden 2003, pp. 643–716. ISBN 9783322940858 .
  • Frank Frössel: Lexicon of plastering and stucco technology . Fraunhofer IRB Verlag, Stuttgart 1999, ISBN 978-3-8167-4700-0 .
  • Frank Frössel: Handbook plaster and stucco - production, coating and renovation for new and old buildings . Callwey, Munich 2003, ISBN 978-3-7667-1567-8 .
  • Wilhelm Scholz, Harald Knoblauch, Wolfram Wiese (eds.): Knowledge of building materials . Werner, Düsseldorf 2003, ISBN 978-3-8041-4197-1 .
  • Oskar Emmenegger: Historical cleaning techniques. From architecture to surface design. Zurich 2016, ISBN 978-3-03863-010-4 .

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