Tarihi eserlerde kullanılan CaSO4 harç ve sıvalarının incelenmesi

Abstract

Tarihi eserlerin onarımında kullanılan günümüz harç ve sıvalan tarihi eserin dokusuna uyum sağlamadığı zaman eserin bozunmasına neden olur. Tarihi eserlerin gereği gibi korunabilmesi eseri oluşturan elemanların tam olarak tanımlanmasıyla mümkündür. Harç ve sıvalar günümüz yapılarında olduğu gibi tarihi yapılarında en temel elemanlarıdır. Tarihi harç ve sıvaların karakterizasyonu tarihi yapıların restorasyon ve korunması yönünden gittikçe artan bir önemle ele alınmaktadır. Bunun dışında, geçmişten günümüze gelen mimari eserlerin mükemmelliğinin gizemi, geçmişin bilemeyenlerinin araştırılarak günümüz teknolojisine taşınması ile mümkün olacaktır. Bu çalışmada 14. yüzyıl tarihi eserlerinden olan Ahmet Yasevi Türbesinden (Batı Türkistan) alman harç ve sıva örneklerinin kimyasal, mineralojik ve petrogrografik incelenmesi, başlıca yaş ve enstrümental kimyasal yöntemler, x-ışınlar difraksiyonu, Taramalı Elektron mikroskobu ve enerji dispersif analiz yöntemleri kullanılarak yapılmış ve harç ve sıva malzemeleri tanımlanarak restorasyonda kullanılması gereken malzeme önerilmiştir.

The characterization of ancient mortars and plasters has become increasingly important for the restoration and conservation of old monuments particularly after the unsatisfactory results of new materials such as cement, polymer-based mortars and insulating plasters used for restoration purposes. The technology of historical mortar and plaster is rather complicated and the recipe of mortar and plaster differs from place to place depending on the local materials and traditions used in the preparation procedure. The replacement of right materials which have equivalent composition with old mortar and plaster during the restoration of historical monuments can function only when a complete analysis of materials has been carried out. Such investigations need interdisciplinary studies including chemical, physical, petrographical, mineralogical and biological examinations of materials and also numerous of samples should be analysed for the statistical data. During the last decade, extensive studies [1-4] have been done to determine the standardized methods for the characterization of old mortars and plasters. In these studies authors agree that for the ease of application, investigations have to be based on the determination of the most characteristic properties of materials. Besides, non destructive, in situ and possible standardized test procedure should be chosen. It is certain that such an approach would help worldwide classification of old mortars and plasters and thus a technological chronology of old construction materials might be achieved. The present study deals with the identification of CaS04-based mortars and plasters of Ahmet Yasevi's tomb (West Türkistan) from the 14th century. Experimental Part Apparatus Atomic Absorption Spectrometer, (GBC, Model 903, single beam AA), X- Ray Diffractometer (Philips, Model 1040, XRD, Norel Co.), Scanning Electron Microscope, SEM, (JSM T330 Jeol) and Energy Dispersive Analyser, EDA, (Tracor-Noran, Model TN-5500) and Polarizan Microscope (James Swift, double nicol) were used for the measurements. Materials and Methods The mortar (M) and plaster (P) samples were obtained from the different points of the monument. Visual Examination All mortar and plaster samples were examined with respect to the type, size and amount of aggregates, colours and organic matter etc.. Sieve Analysis The weighted amount of samples was reacted 10% of HC1 solution, then filtered, washed with distilled water and dried at 105 °C for 24 hours. The insoluble residue was sieved through 1000 /a, 710 fi, 500 /*, 210 /t, 125 /a, sieve in the given order. The granulometric aggregate size distribution of the insoluble part was determined. Chemical Analysis The elemental analysis of samples was carried out in the solutions obtained after the Na2C03+KN03 and water decomposition of samples. Wet Chemistry methods and Atomic Absorption technique were used for the analysis. Alkali elements were determined after HF -f H2S04 decomposition. Qualitative determination of C032", N03\ N02\ CI" and S042" was carried out and S042" ions were also determined quantitatively. Mineralogical Analysis Mineral composition of mortars and plasters were determined by XRD powder technique using CuKa radiation and Ni filter. Operating conditions were 20 mA and 40 kV. Petrographical Analysis SEM and in situ EDA analysis of the samples give valuable information about the technological characteristics such as texture, particle and aggregate size, shape and orientation as well as the mineral nature and chemical composition of materials. For SEM analysis, the samples were blocked in the epoxy resin (Araldite AY 103) with hardener (HY 956, Ciba-Geigy) under vacuum and were cut with low speed saw (Buhler, Isomet), the sample surfaces then polished with 3 (i, 1 /* and 0.25 fi diamond finisher pastes and coated with carbon, and then examined under scanning electron microscope. Energy dispersive (EDA) analysis of the elements found on more than one spot of the sample surface was carried out. Thin Section Analysis of the samples were also carried out by polarizan microscope. XI Results and Discussion The results of visual examination showed the general appearance of mortars and plasters (Table 1). They were gypsum mortars and plasters supported by broken bricks as pouzzolonic materials. The mortar samples contained aggregates more than plaster samples. Plaster samples appeared containing less additives, but much gypsum as very fine binder. Plaster samples contained much fiber as filler than the mortar samples. Table 1. The results of visual examination. The aggregate size distribution of samples obtained from the sieve analysis were quite analog except the plaster sample of 1. 50% of the sample 1 contained aggregates higher than the diameter of 1 mm. The aggregate sizes of gypsum mortars and plasters differed from the calcium carbonate mortars and plasters with respect to the upper limit [1] which indicates 90% of aggregates pass through the sieve around the aggregate diameter of 1 mm. This was rather reasonable due to the different reactions which will proceed during the preparation of different mortars. The upper limit of gypsum mortar was 80%. The XRD data showed that mortars and plasters consisted mainly of gypsum and quartz. The plaster sample of 1 contained a lesser amount of feldspar (Albite) and clay mineral additionally. The plaster sample of 4 also contain clay mineral more than sample 1. Albite and the clay mineral were considered to be pouzzolanic origin. Wet chemical and AAS analysis confirmed the result of XRD analysis (Table 2 and 3). The main element of the binder was calcium. Plaster samples contained higher amount of calcium than mortar samples indicating the plaster slurry contained lesser amount of additional component. Silicon originated from pouzzolanic xu material. Pouzzolanic materials used were mainly complex aluminium-silicates in nature. The AAS results showed that there was not remarkable amount of the water soluble salts in the samples. This result suggested that the tomb was not seriously affected by the humid conditions. Gypsum mortars and plasters have advantage to the mortars and plasters of calcium carbonate [1] with respect to humidity. Table 2. The results of wet analysis. Sample % SiO, A1,0, Fe203 % TiO, % CaO % MgO % Na,0* % K.O* %LI 1 2 3 4 5 6 6.00 9.00 18.80 13.00 14.00 10.00 5.59 2.49 7.23 3.56 3.12 2.14 3.00 2.30 7.16 5.80 3.28 4.40 27.20 28.36 20.27 30.73 32.86 28.21 Relative precision 1.66-18.5 ppt LI : loss of ignition & r\~*."- : i u.. a a c 3.06 3.87 2.06 2.09 3.95 1.85 0.63 0.29 0.53 0.50 0.25 0.43 0.70 0.83 0.76 1.33 0.73 0.84 10.74 11.73 18.05 11.65 9.68 6.58 Determined by AAS Tablo 3. The results of AAS analysis. Table 4. The AAS results of water soluble salts. xm Table 5. The results of anion analysis. Petrographical analysis carried out by SEM and Polarizan Microscope showed that mortars and plasters were quite homogeneous with respect to opaque and very thin crystals of matrix, but aggregates can not be considered to be spreaded homogeneously. Besides, quite big mass of gypsum crystal was seen in some spot rarely. The aggregates were composed mainly of silicates, but the aggregates from the mixture of silicates, calcite and gypsum were also seen rarely. Frequent calcite mineral was observed. Calcite was considered as secondary occurrence due to the exothermic reaction taking place between gypsum and water. The plaster samples contained the small amount of broken bricks but relatively high amount of pebble like aggregates. The EDA analysis (Table 6) confirmed that SİO2 originated from the pouzzolanic materials, because there was no SİO2 occurence within most of spots of thin sections. Iron(3)oxide also originated from the artificial pouzzolans. Table 6. The results of EDA analysis. a and b indicate different spots on thin section. XIV Conclusions The plaster samples, in general, contained the gypsum of 80%, the organic matter of 10% and the pouzzolanic material of 10%. The mortar samples composed of the gypsum 60-70%, the organic matter of 5-7% and the pouzzolanic material of 20-30%. For the restoration work of Ahmet Yasevi's tomb, however the above given composition can be suggested as the results of the present study, technological parameters such as the particle and aggregate size distribution and the rheological properties of slurries of the mortar and plaster must be taken into consideration.