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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.