Hot lime mortar

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The building material hot lime mortar (or hot lime mortar ) is a mixture of quick lime , silicate sands or brick dust and water.

Manufacturing

Alternately, a layer of quicklime and (as required) four to seven layers of moist, silicate-rich sand are poured over with just enough water so that the lime is slaked at the same time . The mortar created in this way with the development of heat can be stored for days with the sparing addition of water, but should only be tapped cold after the end of the reaction, mixed with water until the mortar is consistent, mixed well and processed. When tapping, make sure that the mixing ratio (sand to lime) is retained. The mortar hardens with high strength development. Hand-warm processing only makes sense if tension is to be created in the masonry, for example with arches and vaults or during repair work, because warm, freshly slaked mortar expands a little while it hardens. Normal masonry would be blown up when it hardens if the lime in the masonry is still slaked, especially if it has not been mixed thoroughly. Such damage can occur years after application.

With this special extinguishing technique (at temperatures up to 300 ° C) it is possible to influence air lime in such a way that mortar can be produced with it, which behaves like (weak) hydraulic mortar. In contrast to the sump lime technique, it is not extinguished with excess water, but with moist, silicate-containing sand or brick dust , whereby hot, highly alkaline steam on the surfaces of these silicate aggregates leads to the breakdown of silicon dioxide and the release of silicic acids , which in conjunction with calcium hydroxide leads to the formation of Calcium hydrosilicates is possible, which leads to hydraulic hardening reactions. Alternatively, ground quicklime can also be added to conventional slaked lime or ready-made lime mortar in order to change its properties. This also makes it easier to control the typical lime-scale shrinkage phenomena.

Hydraulic binders , on the other hand, have to be extinguished dry with a precisely calculated amount of water vapor , since the hydraulic components bind when water is added, i.e. they cannot be sumped.

properties

Hot lime mortars have the same physical properties as hydraulic lime mortars and have the advantage over cement-bound mortars of absorbing and releasing moisture contained in the masonry. They harden largely without cracks and have a high level of pressure and abrasion resistance. In addition, hot lime mortars can be removed more easily despite their high strength. Hot lime mortars only acquire their high strength after several weeks and may need to be re-moistened during this time and not be exposed to direct sunlight. For example, you can wrap the masonry with damp cloths.

Animal blood, urine or wine are also mentioned as additives in the literature, which further break down the silicate additives and influence the hydraulic properties of the mortar. This means that waterproof lime mortars are also possible.

application

The hot lime technique is a traditional but out-of-the-way process that is now only used in monument preservation. This technique is also known as quick erase or direct erase . Because of their ease of manufacture and strength, these mortars were used as casting mortars for shell masonry or as a binding agent for bricks and stones, but not for plasters, where higher quality slaked limes were preferred. If the mortar is not mixed well before it is applied, noticeable lime sparrows can form.

See also

literature

  • Günter Binding: The medieval construction company in contemporary images. Stuttgart 2001, ISBN 3-8062-1634-7 .
  • Antje Sander-Berke: Supplying building materials to late medieval cities in Northern Germany. Cologne 1995, ISBN 3-412-03895-4 .
  • Günter Binding: Construction in the Middle Ages. Cologne 1993, ISBN 3-534-10908-2 .

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