High-fire plaster
High -fire gypsum (HBG) is a gypsum-like CaSO 4 modification or so-called anhydrite gypsum that occurs at a comparatively high firing temperature (see also under building plaster ). High-fire gypsum is a historical building material, serves as a binding agent and does not require any inert additives .
High-fire gypsum used to be burned and used in the vicinity of raw gypsum deposits and, after grinding, together with mixing water, results in a high-strength and even largely weather-resistant mortar and exterior plaster.
High-fire plaster differs from double-fire or assembly - or electrician plaster and the like. a. through slow setting and higher water resistance.
Geographical distribution
High-fire gypsum is described in the historical structures of the following countries: Iran , Jordan , Egypt , Libya , Morocco , southern Spain , southern and northern France , Sicily , mainland Italy , Austria , Switzerland , southern and northern Germany . The basis is always natural gypsum deposits, mostly in sufficient storage, occasionally in underground mining , not least in salt domes with a gypsum cap and on diapirs . A prominent occurrence of raw and high-fire plaster of paris is Montmartre - at that time near Paris - where high-fire gypsum was praised for its weather resistance and given its own brand name: montmartrite.
With one exception, high-fire gypsum is widespread in Germany wherever there is or was natural gypsum. This also applies in a positive way to occurrences that are insignificant today, for example in the northern Harz foreland and in the northern German lowlands to Westeregeln near Magdeburg , Lüneburg , Segeberg and Helgoland .
Where there are no local gypsum deposits, burnt and slaked lime served as the appropriate binding agent.
Temporal occurrence
It is not certain whether the plaster of paris described in Çatalhöyük and in the Great Pyramid is really high-fire gypsum. A very old secured high-fire gypsum deposit is the Jordanian desert castle Qasr al-Kharana , built in 710 AD. In Germany, the oldest objects are screeds from Romanesque churches. After that, high-fire gypsum enjoyed a steady boom in Germany until the competing building materials hydraulic lime and Portland cement gained importance from the middle of the 19th century. The production of high-fire plaster did not cease to exist locally until the early 1950s.
In today's monument preservation, high-fire plaster is needed again in France and Germany. In the 1990s, it was often argued that when the high-temperature gypsum ovens went out, practical knowledge of their manufacture and use was also lost. This is only partly true; because in the 1990s and 2000s important relevant literature became known from the year 1600 onwards.
Manufacturing
Raw gypsum must first be crushed to a stone size of 40–160 mm; subsequent drying in air is advisable. As furnaces were manhole, chamber and Grubenöfen, as well as reactors . The simple burning devices were developed empirically or taken over from the lime brandy. Energy-saving and constant quality ovens were designed in France and in Walkenried (Lower Saxony) and continuously improved.
With an amount of 1 tonne, a burning time of about 8 hours may have been sufficient; however, a burning time of around 72 hours is required for 20 tons of material. The firing temperature is 950 ± 50 ° C. The beginning of sintering of the gypsum blocks and a slight deacidification are even mentioned in old literature as desirable criteria for Garbrand. Most of the earlier authors distance themselves from temperatures below 900 ° C and from values higher than 1,000 ° C.
Manual loading of the furnace was characteristic like the manual discharge of the kiln. After firing, the material to be fired is broken to the size of a nut and fed to a suitable grinding unit. As such, the valid edge mill , ordinary mill , the threshing with special flails and Patsch boards. ' Battre comme plâtre ' - beating like plaster of paris (when chopping up and laying screed) is still used in French today in a figurative sense. The maximum grain size is about 6 mm. Thus, the coarse grain also serves as a material-identical aggregate to counter shrinkage cracks during setting and drying out. Vibrating sieves covered with wire have been around for a long time. Before the wire era there were sieves with plant or animal covering. Sieved oversize grain was either re-ground or used as an intermediate layer for screeds .
application
In past epochs, high-fire plaster of paris was used in a very versatile manner; the long-term success in terms of building physics was and is very different.
- Mortar for ridge tiles of gable roofs and mansard roofs , for plain tiles on the inside;
- Exterior and interior plaster over a large area or in compartments , preferably on stone, subordinate to wood and suitable plaster bases; Exterior plaster also colored;
- Inside and outside stucco , window reveals outside;
- Masonry mortar for all kinds of buildings;
- Mortar for church pillars , vaults and vault ribs ;
- Screeds, whether or not colored;
- High-fire gypsum concrete for house walls with and without stone filling;
- High-fire gypsum concrete for compartments in half-timbered buildings, each using plaster casting technology;
- Foundation mortar of houses, castles , even large churches, of cellars with and without buildings above them.
In 1986, the historical structure of Bad Windsheim in West Central Franconia consisted of 64% gypsum stone + high-fire gypsum mortar. Example of the former German school (built in 1569) at Schumberggasse 6: foundations, cellars, vaults, masonry mortar, external plaster, possibly even mortar for ridge tiles - all made of plaster.
Building physics peculiarities
The properly hardened mortar made of high-fire gypsum is said to have high strength and extensive water resistance - not only in historical German and French literature, but above all in the demonstrable willingness of the builders and builders to use this product on the weather side of objects in rainy areas such as Ile -de-France, Hamburg and the Harz Mountains .
Historic dry high-fire gypsum mortar shows the following properties: bulk density 1.6 ± 0.05 g / cm³, compressive strength 30 ± 10 N / mm², water absorption coefficient about 0.2 g / cm². ½ ‑ min ‑ rise coefficient approx. 0.12 cm / ½ min, mean crystallite size 2 × 4 μm², mean pore radius 1 μm.
Thermodynamically , there is no way around the solubility of the high-fire gypsum mortar of 2.6 g calcium sulfate dihydrate (chemical name of gypsum) in one liter of demineralized water . In contrast, going into solution seems to be kinetically slowed down. With the same density, mortar made from high-fire gypsum dissolves more slowly than, for example, mortar made from industrial hard gypsum from the 20th and 21st centuries. The water absorption coefficient of high-fire gypsum is lower than that of hardened hard gypsum of the same bulk density - one can therefore say: Mortar made from high-fire gypsum has the lowest dissolution rate of all gypsum products and therefore the highest resistance to water. The term "water resistance", which is often used, should therefore be understood relatively.
Dissertations from the Crystallographic Institute of the Technical University of Clausthal postulate a certain microstructure of the gypsum dihydrate crystals as a prerequisite for the surprisingly high weather resistance.
In addition to the indisputable limited water solubility, there is also a damage factor in the plaster's tendency to recrystallize , which depends on the wet / dry change and has an unfavorable effect on weather resistance.
It is difficult to make a comparison with lime mortars because they differ greatly in strength and weather resistance in different regions. Soft lime mortars are far superior to high-fire gypsum mortars.
Despite all the disadvantages of high-fire gypsum, there are a multitude of reasons, from the physical to the subjective point of view, which distinguish it positively compared to other binders and which have persuaded the builders and clients of past eras to use high-fire gypsum.
Chemical peculiarities
When gypsum burns, no process CO 2 escapes ; apart from water vapor, only traces of hydrogen sulfide and sulfur trioxide occur, which are more noticeable due to their typical odor than their actual concentration in the exhaust gas. The exhaust gas is dominated by the CO 2 of the firewood and the wood-typical polycyclic hydrocarbons , which can be minimized through optimal wood drying. Coke can also be used as fuel, as can electricity.
A typical property of unmodified gypsum mortars is the chemical increase in volume caused by hydration , which can amount to up to 10 mm / m and impair the plaster adhesion and the screed quality. By adding the smallest amounts of hydrated lime and tartaric acid , the expansion can be reduced to below 1 mm / m; the resulting structural properties in terms of water solubility still have to be checked.
An important aspect when using high-fire gypsum is the interaction with historical or current silica and aluminum compounds on site . In a historical brick- gypsum mortar foundation, for example, driving minerals can form over the course of 250 years - historical high-fire gypsum can also react with modern SiO 2 or Al 2 O 3 -containing building materials and, preferably at low temperatures, cause driving phenomena in the repair mortars .
Conservation importance
In order to prevent such interactions, high-fire plaster of paris from today's production or comparable and tested successor products should be used again when restoring historical objects bound with high-fire plaster. For aesthetic reasons, too, monument preservation prefers to use high-fire plaster of paris for objects bound to high-fire plaster. Technical history museums and certain interest groups are burning high-fire gypsum again today, and special companies mix dry mortar from standardized raw materials that come close to high-fire gypsum - in any case, the quality of a substitute product must be adjusted based on historical specifications. A corresponding aptitude and entrance examination is advisable. Regulations for the technical mortar testing of high-fire plaster are in preparation. In special cases, the authenticity of a historic mortar or a historicizing mortar can be checked with special crystallographic methods - in any case, prior to the current application of high-fire gypsum, monument preservation and chemical advantages with building physics and again chemical disadvantages of high-fire gypsum must be carefully weighed against each other, taking into account the local conditions.
Synonyms
Hochbrntgips, HBG, screed plaster, mason plaster, plâtre surcuit, gesso, yeso, high temperature gypsum plaster
See also
literature
- classical literature by Schickhardt, Schreber, Diderot and d'Alembert, Krünitz, Hertel, Reuleaux, Heusinger von Waldegg, Moye and so on.
- Drawings of industrial high-fire gypsum ovens in the archive of the Walkenried Home History Association .
- Dissertations by Lucas, Rauschenbach, Middendorf, Arens, Bode, Vogel, Weichmann, Jakobsmeier, Follner, Tesch, Haaßengier and others.
- Conference report Quedlinburg . Final report Wigger and Visser.
- Publications by Fischer and Lucas in cement-lime-gypsum.
- Literature Simonin, Bertone, Kulke, Binnewies, Steinbrecher, Lenz and Srocke and others.
- Lecture transcripts of the working group of historicizing gypsum burners.
- Initiatives of the German Gypsum Museum and its friends' association, Walkenried.
- Announcements from the Bad Windsheim , Hundisburg and Westeregeln museums of the history of technology .
- Announcements from the Association pour la valorisation du Gypse et du Plâtre dans les Alpes de Sud .
- Gypsum mortar in historic masonry construction . In: WTA series of scientific and technical working groups for building preservation and monument preservation . Volume 30, Leaflet 2–11.
- Announcements from the Swiss Castle Association .
- GIPS data book . Federal Association of the Gypsum Industry eV, Darmstadt.