Monument Future - Siegfried Siegesmund

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than 2 mm. The phase composition of the fired product is variable and depends mainly on the grain size and composition of the raw material, the firing temperature and time, and the resulting partial water vapour pressure on the surface of the lumps. For the assessment of the firing results, 116gypsum pieces adjacent to thermocouples were sampled and studied in situ by colouring tests and in the laboratory by polarized light microscopy of thin sections, X-ray diffraction and thermoanalysis (DTA, DTG). In order to improve the data interpretation of the field samples, experiments with raw material from the gypsum quarry were carried out under controlled temperature and time conditions in the laboratory and the resulting samples analyzed in the same manner.

       Phase composition of the calcined raw material

      Gypsum lumps from the furnace exhibited a temperature dependent zonation with respect to the occurrence of phases and phase morphologies (Lenz, Sobott 2008) . Low fired gypsum lumps typically consist of three zones. A thin surface layer of rehydrated anhydrite III (dehydrated hemihydrate), is followed by a layer of fibrous hemihydrate, and the central part is made up of a mixture of hemihydrate and dihydrate (unconverted gypsum). Thermoanalytic measurements prove anhydrite III to be stable up to 370 °C. It forms fibrous crystals, partly pseudomorph after gypsum. With increasing firing temperature hemihydrate and anhydrite III are converted to anhydrite II (natural anhydrite). Anhydrite II formed in the temperature range between 370 and 800 °C shows elongated fibrous crystals and at temperatures above 800 °C the crystals tend to be short prismatic (Figure 5).

      Figure 5: left: short prismatic anhydrite II crystals in samples fired at 1,100 °C for 5 hours; right: prismatic anhydrite crystals in samples fired at 800 °C for 5 hours; crossed polarized light.

      In a laboratory sample which was fired at 600 °C for 5 hours radial fibrous anhydrite crystals were observed. Obviously the shape of anhydrite II crystals and aggregates in fired samples depends largely on how the heat treatment was executed.

      In thin sections of gypsum samples which were exposed to temperatures above 600 °C isotropic crystals with a hexagon outline were observed.

      They were identified by EDXRF measurements as periclase MgO (Figure 6).

      Müller et al. (2009) described the formation of periclase by a contact metamorphic reaction in dolomite marble at P = 1 kbar and T > 605 °C

      CaMg(CO3)2 → MgO + CaCO3 + CO2 dolomite periclase calcite

      Since idiomorphic to hypidiomorphic dolomite crystals up to 400µm in size were observed in thin sections of the raw material from the gypsum quarry near the Takht-e-Soleyman, it is reasonable to assume that the dolomite disintegrated and was converted to periclase according to the above mentioned chemical reaction. Therefore the appearance of periclase in samples can be regarded as an intrinsic temperature indicator if the pressure dependence of the reaction is known.

      Figure 6: Periclase crystals in anhydrite II matrix; sample from furnace, adjacent to thermocouple that recorded a maximum temperature of 800 °C; plane polarized light.

       117Reactivity of the new gypsum mortar

      Variable amounts of dehydrated or partly dehydrated calcium sulfate phases in the dry mortar and different crystal habits of the phases are the reasons for a variable reactivity of the mixture. For example, the low degree of hydration of the sample fired for 5 hours at 600 °C is possibly due to the radial fibrous structure of anhydrite crystals. The degree of hydration after 3 days and 1 year was determined gravimetrically and by X-Ray diffraction (Rietveld method), respectively, for samples fired at different temperatures and for different times. The results are summarized in Table 1.

      As stated above the fired material is a variable mixture of components with different hydration reaction rates (Glasenapp 1910). Unlike α- and β-hemihydrate, anhydrite III and low fired anhydrite II which hydrate very quickly, high fired anhydrite II reacts very slowly with water. This retarded reaction of anhydrite II can produce cracks in the plaster if the mortar fabric is very dense and cannot compensate the volume increase of the hydrated calcium sulfate phase. Therefore the hydration of high fired anhydrite II has to be activated by a suitable chemical such as citric acid or potassium sulfate. Experiments and analyses proved superfine calcium hydoxide Ca(OH)2 to be a suitable activator to prevent a retarded hydration of high fired anhydrite II. The linear expansion coefficient of the high fired gypsum injection mortar measured after 280 days was reduced to less than 2 mm/m. The compressive strength of the set mortar measured after 28 days was 6.1 MPa and decreased to 2.9 MPa after storage of the samples under water. No cracking was observed in test walls which were built in 2016 and injected with this mortar. Obviously its properties are favourable for the repair and reinforcement of the historic masonry and it is now successfully applied since two years.

Run No. Firing temperature Firing time Reaction time Degree of hydration
1 200 °C – 3 days 85 %
2 200 °C – 365 days ~100 %
3 600 °C 5 hours 3 days 4 %
4 600 °C 24 hours 3 days 15 %
5 600 °C – 365 days 94 %
6 800 °C 5 hours 3 days 13 %
7 ≥ 800 °C > 5 hours 3 days 1 %
8 1,100 °C 5 hours 365 days 72 %

       Static safeguarding of the west iwan

      In a first step of the strengthening of the west iwan the masonry was grouted with a gypsum suspension developed by University of Technology Dresden (TUD) and Jäger Consulting Engineers Ltd. The worm pump SP-20 from Desoi was absolutely necessary for the grouting of large caverns, cavities and crack systems in the north wall of the west iwan. The water-to-gypsum ratio of the aggregate-free suspension was 0.63. The chemical Retardan 200

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