Monument Future - Siegfried Siegesmund

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calcite between the surface gypsum and the host rock. As 224for the gypsum efflorescences, the most extensive were observed in the WWII tunnels of Yoshimi Hundred Caves, where salt weathering is actually particularly severe, accounting for the crystallization of other sulfates, of Na, Al, Fe, and Mg – jarosite, alunogen, halotrichite, alum-Na, tamarugite, epsomite, and thenardite (Horiguchi et al. 2000; Oguchi et al. 2010). Oya stone also suffers from sulfate-rich efflorescences, constituted of gypsum, mirabilite, and thenardite, each phase preferentially crystallizing in different microenvironments. Low-crystallinity efflorescences were finally found in Taya Caves, composed of chlorides (sylvite, in particular), phosphates, and sulfates.

      Figure 3: Examples of gypsum crusts and efflorescences with the relevant analytical data.

      In addition, we conducted a complementary study of the rock-water interaction in Taya Caves, considering the lack of previous researches and the constant presence of percolating rainfall, rising damp, and extremely high humidity in that environment. We measured an extremely rapid water absorption (~25 %), a more contained yet significant adsorption of hygroscopic water (~5 %), and a very fast decay during the jar slake test (Santi 1998). These findings point out a high susceptibility to clay mineral-promoted swelling and slaking deterioration, which may produce decay patterns like erosion, rounding, scaling, peeling and, in the long term, lead to structural decay and collapses.

       Lithological and environmental constraints

      Gypsum formation in Taya Caves is triggered by the dissolution of the calcareous bioclasts and pyrite crystals in the rock, releasing Ca and S in solution. This is validated by the occurrence of gypsum solely on the fossiliferous rock type, which is also rich in pyrite – diagenetic or secondary, often included inside the shells. Pyrite has been thought to be the precursor also of the sulfate-rich efflorescences in Yoshimi Hundred Caves (Oyama & Chigira 1999), whereas about the gypsum origin 225on Oya stone we need further investigations. The ubiquity of gypsum can be explained in terms of the relevant microenvironmental conditions. The deepest levels of the studied underground sites, the most environmentally isolated, may have a nearly constant RH of about 100 %. This value, under the influence of external airflows, gets significantly lower and more variable closer to the cave entrances, the microclimatic monitoring reveals (Fig. 4). Gypsum has an extremely high deliquescence relative humidity (DRH) – higher than 99 % (Charola et al. 2007) – so that is stable in many environments, even very humid, provided that the substrate is not wet.

      Figure 4: Microclimatic monitoring during summer and early fall (at different distances from the entrances of the underground sites and outdoors).

      With broader fluctuations and lower values of RH, the crystallization of a number of other phases, typically with lower DRH, can occur. Emblematic are the findings on Oya stone, which disclose the exclusive presence of gypsum in the deep, extremely humid quarry levels, whereas the main component of the efflorescences in the middle levels or semi-underground quarries is mirabilite (DRH = 97 % at 15 °C), then replaced by thenardite (DRH = 86 %) in the dryer external environment (Steiger & Asmussen 2008). The efflorescence formation may vary temporally, other than spatially, with the climatic season: like for the sulfates in Yoshimi Hundred Caves, almost all crystallizing during the dry winter season. The conditions of lower and variable RH are associated with more frequent cycles of crystallization/dissolution, hydration/dehydration, hygroscopic adsorption/desorption, and more damaging mechanical stresses generated on the stone. The rate of stone decay related to the simple interaction with liquid water, instead, upsurges in summer and early fall, when most precipitations are concentrated and typhoons may occur.

      The results suggest that the chemistry of salt weathering is often controlled by the contribution of the rock-forming minerals. Another source that is worth mentioning is related to the efflorescences in Taya Caves that, given their composition and the presence of a cultivated land above the cave vault, is possibly represented by the agricultural chemicals used therein, which migrated downwards in solution.

       Summary and conclusions

      The studied sedimentary rocks share the silicate composition, often notable content in clay minerals, low strength, and very high porosity. The microenvironment of the underground sites under investigation is characterized by an extremely high humidity, and salt weathering occurs more easily where/when the RH is lower and inconstant, and the cave walls are not wet. Sulfates are the most common secondary phases, especially gypsum, which has low solubility and the highest DRH, crystallizing even when the RH is close to 100 %. In this regard, the pyrite contained in the rock seems to be the main precursor. On the other hand, water-driven deterioration is greatly affected by the abundance of clay minerals.

      These preliminary results need further investigations, which will concern the continuation of the 226microclimatic monitoring for at least one entire year, the chemical analysis of rainwater and the groundwater collected in the caves, and the finalization of salt sampling and analysis in different seasons.

      The completion of this research will serve as support for the conservation and valorization of the underground cultural heritage of Japan that, because of its own understated form and the low-key advertisement, is often not well known and out of the main tourist circuits.

       Acknowledgements

      This research was funded by JSPS (Japan Society for the Promotion of Science), under a postdoctoral fellowship “standard” granted to L. Germinario (ID no. P18122).

       References

      Charola A. E., Pühringer J., Steiger M. (2007). Gypsum: a review of its role in the deterioration of building materials. Environmental Geology, 52, 339–352

      Horiguchi T., Nakata M., Shikazono N., Honma H. (2000). Occurrence and formation process of alunogen and salt accumulation on the surface of tuff in Yoshimi Hills, Saitama prefecture, Japan. Journal of the Mineralogical Society of Japan, 29 (1), 3–16

      Ikegami S. (2018). The development of rock-cut tombs in the Japanese archipelago. The Rissho International Journal of Academic Research in Culture and Society, 1, 171–199

      Ogata K. (2019). The artistic qualities of religion reliefs in Taya-cave, Japan. In: JpGU Meeting 2019, 26–30 May 2019, Chiba, Japan

      Oguchi C., Takaya Y., Yamazaki M., Ohnishi R., Thidar A., Hatta T. (2010). High acidic sulphate salt production on the cave wall in the Yoshimi Hyaku-Ana historic site, central Japan. In: Proceedings of the XIX CBGA Congress, Thessaloniki, Greece, 100, 413–419

      Oyama T., Chigira M. (1999). Weathering rate of mudstone and tuff on old unlined tunnel walls. Engineering Geology, 55, 15–27

      Santi P. M. (1998). Improving the jar slake, slake index, and slake durability tests for shales. Environmental & Engineering Geoscience, 4 (3), 385–396

      Seiki T., Nishi J., Nishida Y. (2007). Renovation Challenge of Underground Quarries for Oya Tuff. In: 11th ACUUS Conference – Underground space: expanding the frontiers, 10–13 Sept 2007, Athens, Greece

      Steiger M., Asmussen S. (2008). Crystallization of sodium sulfate phases in porous materials: the phase diagram Na2SO 4–H2O and the generation of stress. Geochimica et Cosmochimica Acta, 72, 4291–4306

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       MATERIAL CHARACTERIZATION AND DECAY OF THE LIMESTONES USED IN HISTORICAL STRUCTURES OF MARDİN, TURKEY

      IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT

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