alt=""/>
Figure 2: a) The remains of medieval architectural structures in Sanahin, Armenia shows the huge amounts of mortar within the building structure. b) Cross section of a wall at the Avan Cathedral. c) The critical static situation at the top of the walls at Avan Cathdral. Restoration mortar from the 1941 campaign is exposed in the lower parts of inner masonry. d) Widespread salt contamination and weathering at the Zvartnots Cathedral. e) Huge remnants of the inner masonry and preserved plaster on some walls at the Zwartnots Cathedral shown in the museum of the site (early 20th century). f) Cement mortar injections done by restoration in the 1960 s also documented in the museum.
Methods of investigations
Onsite investigations were done by comparable observations, damage mapping, electrical capacity and conductivity measurements on single building stones.
The electrical conductivity and capacity is basically dependent on the material and is also influenced by the moisture content of the material and the presence of ionic compounds. The used device (surveymaster/protimeter) measures moisture in buildings and other related structures both on (conductivity) and below (capacity) the surface.
Sampling of the drilling powder was also done. Field studies were done on a southwestern wall of the Zwartnotz ruins and the westen front facade of the Avan Cathedral.
This study concentrates on the historical mortars of the Zvartnots Cathedral (ZM), the Avan Cathedral (AM) and the restoration mortar (RM) from 1941 used for Avan (Fig. 4). From these three mortars an analysis of their aggregate/binder ratio and the grading curve of its aggregate were done in a mobile laboratory. Petrographical and fabric analyses of the building materials were performed on thin sections under a polarization microscope and cathodoluminescence (CL) microscopy. Hydrostatic weighing on the samples was carried out to acquire the particle and bulk density as well as the porosity (DIN 52102). Mercury intrusion porosimetry was used to acquire the pore radii distribution (Fig. 3d, h, l).
Results
Methods of construction
Double shell masonry structures are characteristic of many ruins of the rich cultural heritage in Armenia. The building stones that form the exterior and interior masonry are held together between the two masonry shells by lime mortar and quarry stone masonry. The percentage of both structural elements in rising walls is about 50 % (Fig. 2a, b and e).
The inner shell of the masonry has a very solid and durable mortar. This mortar seems to literally glue the two wall shells together and seems to be the reason why many church buildings, as ruins, have still been partly preserved (Fig. 2a).
The mortars
The mortars of both historical buildings are lime mortars with volcanic aggregates with a high amount of glass-rich, amorphous material. These are characterized by a predominantly dense and binder-rich, light and dense matrix surrounded by aggregates of different sizes and color (Fig. 3a–3h). In contrast, the restoration mortar of the Avan Cathedral shows single quartz grains, feldspar crystals and sub-rounded inclusions clearly identified as lime accumulations (Fig. 3i). The historical Avan mortar (AM) mostly shows reddish, large pumice-rich grains (Fig. 3e). They reach a size up to 5 mm. The Zvartnots mortar shows more basaltic and andesitic fragments of sand particle size (0.063–2 mm). The matrix of both historical 140mortars is extremely dense and shows a firm connection to the grains (Fig. 3b, c and f and g). The Zvartnots mortar shows larger accumulations (–1 mm) of crystallized lime, which show multicolored effects in polarized light (Fig. 3b). These accumulations show a distinct red color under cathodoluminescence (CL) and could be a dolomitic lime. The matrix of the historical mortar shows a slight dark reddish structure, that can be identified as calcite-silica compounds (3c). A similar slight dark red color of the matrix could be observed for the mortar of the Avan Cathedral, while only light reddish lime accumulations are visible in CL-light (3g). The lime inclusions within the restoration mortar (RM) shows a similar intense light reddish color but no crystallization (Fig. 3k). The binding cement of the mortar is made from fine crystals, showing a multicolored birefringence in polarized light (Fig. 3j). These crystals are identified as calcium hydroxide or portlandite, which only forms single grain contacts. The blue glowing areas are probably claystone fragments (Fig. 3k) or alteration products (Fig. 3c).
Figure 3: The three investigated mortars. a–d) The mortar of the Zvartnots Cathedral (ZM), a) hand specimen, b) thin section under polarized light and c) under cathodoluminescence. d) The pore size distribution. e–h) The mortar of the Avan Church (AM) and i–l) the restoration mortar of Avan (RM) in the same order.
The pore size distribution of the three mortars is different. In the Zvartnots mortar smaller capillary active pores dominate the pore structure attaining 73 %. The microporosity (0.001–0.1 µm) reaches 20 % for the ZM and 28 % for the AM. Both mortars are characterized by a bimodal pore size structure, while the finer pores dominate (Fig. 3d, 3 h). The restoration mortar of the Avan Cathedral attains an amount of micropores of 64 %. A bimodal pore size structure is also formed in this case, dominated by small pores as well (Fig. 3l).
The historical mortars are lime mortars and show different binder contents. In the case of the original Avan Cathedral mortar, the lime content is 60 %, and only 25 % for the Zvartnots Cathedral. However, it must be taken in mind that the percentages are by weight and not by parts by volume. The Zwartnots mortar has a comparatively heavy aggregate mainly from basalt, which is why it also has the highest density of all mortars (Tab. 1). Therefore, the binder content appears relatively low. The binder-aggregate ratio of the restoration mortar (RM) shows a lime content of 34 %. In terms 141of the grading curve, there are huge differences between the historical mortars and the restoration mortar of the 1941 work. The historical mortars show a falling aggregate content from 0.5 mm to 63 µm and only a small amount of fine sands (Fig. 4). In contrast, the restoration mortar (RM) is dominated by fine sands with a clear domination of the grain size > 5 mm.
Figure 4: The grading curve of the three mortars.
Volcanic building stones
The tuffs, which were used in the Yerevan region in historical times, belong to the varieties of the Hoktemberyan Brown (HB) and the Hoktemberyan Red (HR) or the Hoktemberyan Black (HBl). Basalt is partly found at the Zvartnots Cathedral and can be compared with the Armenia Black (AB) variety. A detailed description of all building stones is given in Pötzl et al. 2018.
The porosity of the building stones ranges between 14.1–35.6 %, the particle density between 2.46 and 2.93 g/cm3 (Tab. 1). The porosity of the different mortars differs between 31.3 and 47.6 %, with densities between 1.2 and 1.7 g/cm3. Their particle densities range between 2.2 and 2.6 g/cm3 (Tab. 1).
Table 1: Porosity and density of the investigated stone and mortar samples. * Pötzl et. al 2018
| Sample |
|
