The rock material shows a different hydric dilatation. Both varieties show remarkable swelling, while the clastic material (RC) with up to 1.04 mm/m reaches high and critical values (Fig. 4). The fine-grained rock material (RF) only shows half of that dilatation, but a higher anisotropy of more than 50 % (Fig. 4).
Ultrasound velocity decreases in both rock varieties under water-saturated condition (Tab. 2). This particularly affects the RC variety with an average of 38 % and an average of only 7.5 % for the RF variety (Tab. 2).
Table 2: Ultrasonic velocity of the rock samples.
| Sample | X (m/sec) | Y (m/sec) | Z (m/sec) | A (%) |
| RF dry | 1,905 | 2,340 | 1,464 | 37.4 |
| RF wet | 1,791 | 2,110 | 1,366 | 35.2 |
| RF KSE | 2,200 | 2,377 | 1,692 | 23 |
| RF sol | 2,210 | 2,355 | 1,802 | 21.6 |
| RC dry | 1,570 | 1,544 | 1,250 | 20.3 |
| RC wet | 909 | 783 | 930 | 13.8 |
| RC KSE | 1,585 | 1,553 | 1,780 | 12.7 |
| RC sol | 1,593 | 1,610 | 1,795 | 11.3 |
Conclusions
Petrophysical properties
Based on its ultrasonic velocity (Tab. 2) and due to the low values of surface hardness, the natural stone used for the Geghard Monastery can be classified as a low bound stone material with an expected compressive strengh of around 10–20 N/ mm2 (Wedekind et al. 2016). Due to its pore size distribution and the high amount of micropores (Fig. 2 g and h), as well as due to the high S-value, the rock material seems to be sensitive to ice- and salt crystallization (Hirschfeld 1912).
The high hydric dilatation as well as the micoporosity of both rock varieties speak for a certain proportion of swellable clay minerals (Wedekind et al. 2013). Both values correlate with each other (compare Fig. 2 below and Fig. 4). Also, the significant decrease in ultrasound velocity under water-saturated conditions suggests the presence of swellable clay minerals and a softening of the rock structure.
Experimental conservation
Because of the onsite observed weathering forms and the measured values of hydric dilatation, the two rock varieties were treated with a swelling inhibitor and then their hydric dilatation was measured again. After the treatment, the clastic material shows a plain reduction of the hydric dilalation of around 40 %. In the case of the RC variety, a reduction of the anisotropy of the hydric dilatation to nearly zero is remarkable (Fig. 5). Furthermore, the fine-grained material (RF) shows a reduction of more than 50 % (from 0.53 mm/m to 1.8 mm/m) in the Z direction (perpendicular to bedding), whereas the XY-direction shows a lower reduction reaching 0.093 mm/m.
During the consolidation test with the silica sol, gel formation sometimes occurs on the sample surface. This can also be attributed to the pretreatment with the swelling inhibitor. Silica sols are ion-sensitive and can therefore only be used to a very limited extent on saline substrates. And, a swelling inhibitor is actually a saline solution. Consolidation tests with the silica acid esther showed a lower consolidation effect, but also a low darkening of the sample material.
It is striking that there is only a very slight increase in strength, in the case of both consolidants to the two directions parallel to the stratification (XY) (Tab. 2). The increase is around 8 % for the RF by using KSE and 7.5 % using the silica sol. The 262RC variety reaches less than 1 % in the case of KSE and around 3 % for the silica sol. The strengthening effect perpendicular to the bedding (Z) is much higher for both rock types and consolidants (Tab. 2). This attains 16 % for the RF and 30 % for the RC variety by using KSE. By using the silica sol, a consolidation effect can be established in the Z-direction for RF with 19 % and for RC at 30 %.
Figure 4: Hydric dilatation of the two rock varieties.
Figure 5: Reduction of the hydric dilatation after the treatment with the swelling inhibitor.
Conclusions
Treatment with the swelling inhibitor turned out to be a successful conservation strategy, which reduces the hydric dilatation as well as the anisotropic behavior of the rock in the case of hydric swelling. The consolidation was able to stabilize the cohesion of the material and significantly reduce the anisotropic properties in the case of ultrasonic velocity.
Acknowledgements
We would like to thank the Administration of Heritage
