Siegfried Siegesmund - Monument Future

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) 2to 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.

Table 1:Degree of rehydration of fired gypsum as a function of firing temperature and time.

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 P by SIKA Company was added as a retarder. Tests with ViscoCrete by SIKA Company showed that the addition of a plasticizer to the suspension was not necessary to guarantee sufficient flowability. The gypsum suspension could easily be mixed and processed on-site. Observation of the injection process confirmed the very good flow behaviour of the suspension which ensured the closing of the cracks and cavities in the masonry. The grout remained flowable long enough and the quantity of the water taken from the surroundings were within limits. The masonry showed heavy vertical cracks and numerous holes and cavities in the interior, which had to be grouted from the bottom to the top of the wall with the developed gypsum suspension. First holes were drilled at a spacing of 30 to 50 cm along the course of the crack to grout the masonry. The holes had a diameter of 24 mm and depths of 30 to 80 cm. After completion of drilling, the entire masonry including the cracks was prepared for the subsequent rising grouting. For this purpose the cracks were cleaned of dust and loose objects and the parts of the masonry facade near the masonry facade were secured with loam mortar. This prevented contamination of the masonry during the grouting work and the preceding filling of the cracks. Then grouting was started from bottom to top (Figure 7).

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Figure 7:Gypsum mortar injection into the cracked north wall of the west iwan. Right side: Bottom to top injection in sealed crack.

It was frequently the case that the masonry could be grouted up to 1 m of height from one hole. This demonstrated an effective internal transport of material, with continuous grouting of the entire masonry being guaranteed. At each level, grout escaped at the various holes drilled round the buttress. In total, 7 tons of gypsum suspension were grouted to the east part of the north wall of the west iwan.

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Sobott, R. (2018). Historic and modern gypsum mortar application at the Takht-e Soleyman, Iran. Report about on-site studies and results of sample analyses. unpublished, 25 pp.