Lime mortar

As a building material, the lime mortar is a mixture of slaked lime and sand and is divided into different mortar groups (MG) depending on its composition . This article deals primarily with air lime mortar . Lime mortars with hydraulic components such as pozzolans are also known as water- lime mortars . The setting process of the hydraulic components is more comparable to that of cement .

In 2016 the traditional production of lime mortar was included in the nationwide register of intangible cultural heritage by the German UNESCO Commission .

composition

Processable Luftkalkmörtel (MG 1) contains slaked lime (Ca (OH) ₂ ; also commercially available as a white hydrated lime ) which upon setting in lime (CaCO ₃ corresponds limestone ) is reacted.

If the term lime mortar is not further specified, a plaster or masonry mortar of mortar group MG 2a "lime mortar / hydraulic mortar " ( compressive strength 2.5 MN / m²) according to DIN 1053 (masonry DIN) with hydraulic components is meant.

If cement is also used , the mortar falls into group MG 2b "Lime cement mortar" (up to 5 MN / m²).

Other auxiliary materials such as plaster of paris , wall ties or synthetic resin binders are also possible.

With direct processing of unslaked quicklime , i. H. The extinguishing process takes place when the mortar is mixed , we speak of hot lime mortar - this building material is primarily of historical importance.

To achieve a crack-free surface on uneven or problematic subsoil , hair lime was previously used to which animal hair was added. This mixture was also suitable for covering trapezoidal strips attached to the underside of wooden beam ceilings . The hair pressed through the gaps between the strips served as a plaster base for the plaster that was then applied to the underside of the ceiling.

Masonry mortar is typically made from one part building lime and three parts sand . In order to rule out cracks, up to four parts of sand are added to the plaster mortar . In special cases, such as very fine sand for the production of slurry, welding or filler plaster, 2 parts of sand may be sufficient. When using coarse sand, for example for the production of mortar with open pores, up to five parts of sand can be used.

By increasing the water content, the pore content of the plaster increases. This may be desirable under certain circumstances. At the same time, the risk of drying cracks increases. In order to reduce the risk of scorching , it can therefore be more advantageous to keep the plaster already applied moist by spraying it several times with water instead of increasing the amount of water added. Unless the mortar is exposed to particular impact or pressure loads, it will often be sufficient to keep the mortar moist until it has set on the surface. Due to the hygroscopic nature of the material, the carbonation of the deeper layers nevertheless progresses, so that the strength increases for years.

Acetic clay can be used to improve the adhesion of new lime mortar to old lime plaster substrates with sintered skin or with salt exposure. A 1% solution with a pH of around 4 is typically used.

To increase the porosity , aluminum powder can be added, which leads to the formation of hydrogen .

Setting process

After applying the mushy, water-mixed mortar, it gradually sets. The calcium hydroxide (Ca (OH) ₂ ) is converted into lime (CaCO ₃ ) with carbon dioxide (CO ₂ ) in the air, which reacts with water to form carbonic acid :

${\displaystyle \mathrm {CO_{2}+H_{2}O\longrightarrow H_{2}CO_{3}} }$
${\displaystyle \mathrm {H_{2}CO_{3}+Ca(OH)_{2}\longrightarrow CaCO_{3}+2H_{2}O} }$
(Partial reaction of the technical lime cycle )

Pure air-lime mortar only sets when the carbon dioxide contained in the air comes in. As long as unprocessed lime mortar is covered by a layer of water, it can be stored for a long time without hardening.

On the other hand, the presence of water is also required for carbonation. If the mortar dries out, the setting process slows down, but does not come to a complete standstill, as the humidity in the air generally allows the reaction to proceed.

Lime mortar should not be used at temperatures below 5 ° C.

The fine, needle-like lime crystals that form bind the sand components with which they felt, so that the mortar forms a hard, cohesive solid. An aggregate made of limestone or dolomite is best suited as an aggregate , since a chemical bond then takes place between the calcite crystals of the mortar and the crushed limestone sand.

This process can take many years to complete. It can be accelerated by increasing the concentration of carbon dioxide in the ambient air, for example by burning organic material (such as a coal fire). In some particularly thick walls of old castles, the mortar has not yet set in parts. The setting is delayed in particular by the fact that a sintered layer a few micrometers thick forms on the surface , which consists of highly crystalline, pure calcite (which is highly transparent , this principle enables the fresco technique of wall painting ). However, this layer hinders the diffusion of CO ₂ into the interior, and therefore the carbonation .

Properties and use

Lime mortar is less pressure-resistant than water-based mortar (e.g. cement-bound mortar) and is therefore only suitable for masonry ( masonry mortar ) where no high compressive strength is expected. Lime mortar (without the addition of cement) has a good moisture-regulating effect and can therefore be used for interior plasters .

In addition to the indoor climate advantages, lime plasters and mortars also have ecological advantages over cement plasters, e.g. B. with natural stone masonry for the insects living in it and for the CO ₂ household. The CO ₂ expelled from limestone during the production of quicklime is absorbed again during the setting process. The balance is balanced in contrast to cement mortars, which are also largely made of limestone; no CO ₂ uptake takes place here. But is not included in this balance, the CO ₂ - emissions caused for the recovery of process heat.

Lime mortar has been handcrafted in Rüdersdorf (Märkisch-Oderland district) for more than 200 years.

Norms and standards

• DIN 1053 - masonry
• DIN 18550 - plaster and plaster systems

literature

• H. Künzel, G. Riedl: Factory dry mortar. Lime plasters in monument preservation. In: Bautenschutz u. Building renovation 2/1996

Individual evidence

1. ↑ UNESCO declares Rüdersdorfer Kalkmörtel a cultural heritage ( memento from December 20, 2016 in the Internet Archive ). In: RBB Online from December 9, 2019.
2. ↑ see Meyers Konversations-Lexikon (1885–1890), entry “ceiling”, p. 604, right column
3. ↑ ^{a b} Erwin Emmerling, Stefanie Correll, Andreas Grüner, Ralf Kilian (eds.): Firmitas et Splendor. Vitruvius and the techniques of wall decoration , page 172: "Test series 3 [dry slaked lime]"; Studies from the Chair of Restoration, Art Technology and Conservation Science , Technical University of Munich , Faculty of Architecture
4. ↑ RÖFIX clay solution , technical data sheet, June 10, 2019, Fixit Group
5. ↑ HASIT clay solution, adhesion improver, IN: www.hasit.de, Technical Data Sheet, as of March 30, 2019, Fixit Group
6. ↑ Product data sheet Baumit clay solution , 01.2018; IN: Baumit.com
7. ↑ Konrad Fischer: Moisture and salt in old buildings - collection of facts and tips 2, on wall moisture, salt damage and rising damp, renovation with renovation plaster, monument protection and monument preservation , section "Measures to reduce building-damaging salts: / General procedural information:"; Accessed August 2019
8. ↑ Erwin Emmerling, Stefanie Correll, Andreas Grüner, Ralf Kilian (eds.): Firmitas et Splendor. Vitruvius and the techniques of wall decoration , page 174: "Experiment 5 [porosity, aluminum]"; Studies from the Chair of Restoration, Art Technology and Conservation Science , Technical University of Munich , Faculty of Architecture