Monument Future - Siegfried Siegesmund

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rel="nofollow" href="#ulink_6078f07a-5dde-5e84-a55c-ea4621f3dd1b">2 Labor für Baudenkmalpflege Naumburg, Rudelsburgpromenade 20C, 06628 Naumburg, Germany

      4 Jäger Ingenieure GmbH, Wichernstraße 12, 01445 Radebeul, Germany

       Abstract

      The preservation of the UNESCO World Heritage Site Takht-e Soleyman is supported by German experts who are working on the 114reinforcement of the badly cracked north wall of the west iwan (large audience hall). The building material gypsum which was used in the past is now also used for up-to-date repair work such as grouting and filling cracks and voids in the historic masonry. Locally quarried gypsum was used to produce a high-fired gypsum mortar in the traditional way. On the basis of laboratory experiments the activation of the hydration of anhydrite and the flow behavior of the mortar suspension was optimized.

      Kewords: Ilkhanid palace, calcium sulfate phases, gypsum calcination, grouting, static safeguarding of masonry

       Introduction

      The UNESCO World Heritage Site Takht-e Soleymann (“Throne of Solomon”) is situated in the West Azerbaijan Province of Iran. The site lies 2200 m above the sea level and has a dry continental climate. In the 6th century AD under Sasanian rule a Zorastrian fire temple and palace complex was built at an artesian lake over the ruins of Achaemenid and Parthian buildings. After the end of the Sasanian dynasty in the 7th century AD the complex fell derelict. In 1221 the Mongols started the invasion of the Islamic world and about 1275 the site was used once again for the construction of an Ilkhanid summer palace. The greatest building within the palace complex was the west iwan, a vaulted audience hall, 11,5 m wide and 27 m long and closed on three sides (Naumann 1977, Huff 2006) which was lavishly decorated inside. With the decline of the Ilkhanid empire the buildings were abandoned and fell into ruin again. Beginning in 1956, the area was investigated by archaeologists and a restoration program was initiated with German support. A huge scaffold was erected on the east side of the west iwan in order to stabilize the ruinous north wall and stop the progressive decay. What was meant as a temporary measure is still in place and actually became an iconic landmark (Figure 1).

      Since 2016 the preservation of the remains of the west iwan is part of a restoration project funded by the Cultural Preservation Program of the Federal Foreign Office. Experts are working on the reinforcement of the badly cracked north wall by strengthening the historic masonry with injection anchors and filling voids and cracks in the wall structure by mortar injections (Fucke, Hansen 2012, Bräunel 2016, Burkert et al. 2019). Gypsum mortar was used for the repair work because plaster is present in Sasanian as well as in Ilkhanid wall structures and its reaction with hydraulic cement mortar may cause the problem of sulphate attack. The project is aiming to strengthen of the wall so that it can withstand earthquakes in a seismically active region and the dismantling of the scaffold which is then no longer required.

       Sasanian and Ilkhanid plaster

      The historic multi-leaf masonry has a filling of stone rubble and mortar. The external leaf was built of travertine worked stones, rubblestones and bricks. Plaster was the binder for the mortar of all building periods and masonry types. Chemical and phase analyses of Sasanian and Ilkhanid plaster samples yielded fairly pure gypsum mortars. Maximum silicon dioxide impurities, mainly as quartz, did not exceed 8.3 mass%.

      The thin section of a Sasanian gypsum mortar revealed a secondray very porous structure (Figure 2). The firing products are more or less dissolved, creating pore space which is surrounded by a gypsum matrix. Brown inclusions in the leached remnants of the fired gypsum must be regarded as impurities in the raw material.

      Figure 1: North wall of the west iwan in the centre, Takht-e Soleyman.

      Figure 2: Sasanian mortar with very porous structure and fragment of gypsum rock (plane polarized light).

      Figure 3: Ilkhanid render with porous structure. The grains in the upper part of photograph exhibit distinct reaction rims (arrows); plain polarized light.

      Embedded in the gypsum matrix are low quantities of gypsum rock fragments and brownish aggregate material with a maximum grain size of about 6 mm. Both may have entered the mortar as contaminants in the furnace or on the construction site. However, an intentional admixture as aggregates cannot be ruled out. Unreacted anhydrite was not detected.As the polarized light microscopy of thin sections showed, the Ilkhanid plaster is quite different from a Sasanian mortar. It contains still a lot of unreacted firing products with a maximum grain size up to 8 mm. Occasionally distinct reaction rims around the grains are visible (Figure 3). Both in the firing products and in the mortar matrix fine clayey (ceramic) particles occur. They most probably derive from impure raw material and not from the intentional admixture of crushed bricks as pozzolanic material. Like the Sasanian plaster the 115Ilkhanid plaster contains gypsum rock fragments with a grain size up to 8 mm. It cannot be decided whether they were added as aggregate or derive from insufficiently fired raw material. Overall, the matrix fabric of the Ilkhanid plaster appears to be denser than that of the Sasanian plaster due to the abundance of unreacted components.

      Figure 4: Furnace for plaster production. Cross section (left) and view into the shaft with installation of temperature measuring device (thermocouple 1).

       Traditional production of gypsum mortar

      For the restoration work locally available material is used. The raw material for the plaster production comes from a quarry some 17 km WNW off the Takht-e Soleyman. Two slightly different qualities of gypsum with respect to the contents of non sulfate accessory minerals are mined. The gypsum rock contains dolomite (ankerite), calcite, quartz, feldspar, clay minerals and celestite as accessory minerals which can sum up to 8–10 mass%.

      The firing of the raw material is done in the traditional way as it was done many centuries ago (Soleymani, Pirak 2012, Sobott 2018). A shaft furnace built of bricks with a diametre of 1.90 m was sunk 2.90 m deep in the ground. The quarried gypsum lumps are piled up in the furnace in such a way that something like a corbeled vault is formed. Large pieces of gypsum rock are on the inside of the construction facing the firing chamber and small pieces are used to fill the space between the furnace wall and the rock pile. The apex of this artful cone-shaped construction surmounts the upper end of the furnace (Figure 4). A mix of combustible material, mostly wood, is piled up inside the vault.

      Once the fire is lit the uncontrolled firing process lasts about 8 hours. Due to the construction of the furnace the temperature distribution in the gypsum filling is extremely variable. Thermocouples installed at different positions in the gypsum pile showed that the temperature difference between the central part and the margin may be as great as 800 °C so that the gypsum lumps are exposed to temperatures ranging from 200 and 1,000 °C (Jäger 2017, Jafarpanah 2017). The cooling period after the extinction of the fire lasts about 24 hours. Then the fired material is removed from the furnace and crushed with large hammers by hand at which high fired gypsum lumps disintegrate easily into powder while low fired lumps break into smaller pieces. The crushed material is sieved and filled into plastic bags. For use at the construction site the material was

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