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MINERALOGICAL STUDY of STONE DECAY in CHARLES BRIDGE, PRAGUE SUMMARY Charles Bridge Over the Vltava River in Prague

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MINERALOGICAL STUDY of STONE DECAY in CHARLES BRIDGE, PRAGUE SUMMARY Charles Bridge Over the Vltava River in Prague 29 MINERALOGICAL STUDY OF STONE DECAY IN CHARLES BRIDGE, PRAGUE SULOVSKY, PETR; GREGEROVA, MIROSLAVA Dept. of Mineralogy, Petrology and Geochemistry, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic; POSPISIL, PAVEL Technical University of Brno, Dept. of Civil Engineering, Veveri , 600 00 Brno, Czech Republic SUMMARY Charles Bridge over the Vltava river in Prague (Czech Republic) belongs to the most valuable gothic monuments in central Europe. The still increasing degree of deterioration since the general bridge reconstruction in the 1970's made it imperative that a complex re-assessment of its condition was completed prior to the presumed reconstruction of the bridge. An integral part of this project has been petrological, mineralogical and geochemical study of the weathered building stone, reported in this paper. With respect to rock types used for building and repairs as well as to local variability of atmospheric conditions (temperature, wind, shading from sun or rain), the bridge is not a homogeneous structure. As a consequence, an assemblage of neo-formed minerals more variegated than any other earlier described in building object has developed there. The depth distribution of these minerals displays two main types. The more common one consists of a gypsum crust covering the stone, beneath which occurs a zone of highly porous and friable rock. In some places, the zoning is as follows (from the surface inwards): thin cover of iron oxidohydroxides - zone with jarosite I gelous silica cement - zone with gypsum cement - zone with iron oxidohydroxide occurrences. Similar zonation has until now only been reported from concretes. In places shaded from precipitation, efflorescences of many soluble salts have been identified: rock salt, KN03, NaN03, feather alums and many other complex sulphates of K, Na, Al and Fe, some not yet described from nature. The impregnation of the stone surface with insoluble minerals leads to accumulation of high amounts of soluble salts beneath it and to the spallation of the indurated surficial layer, followed by pronounced mechanical erosion of freshly exposed surface. The source of constituents of neogenic minerals are portlandite and calcite (mostly in mortars), minerals containing alkalis (feldspars, mica) and iron (glaukonite, hematite, biotite). The stones and binding materials for repairs should be low in them. 1. INTRODUCTION At first sight, the Charles Bridge appears to be a solid, homogeneous lithic structure. Close observation reveals a variety of materials used therein. During its history, the bridge has been damaged by several floods and wars. Stones and mortars used for repairs and reconstructions differed from the original ones in many respects, and have resulted in a considerable material diversity of the bridge. Owing to changes in their provenance, petrographic character (colour, structure, porosity, mineral composition etc.), composition of binding mortar (lime or cement) , age of embodiment in the bridge, their location in the structure, orientation with respect to wind , sun and other factors, individual ashlars display different stages of degradation. Some of the bridge arches suffered from the formation of fractures, running parallel to the longer axis of the bridge, and in some cases also cutting the ashlars crosswise. The scope of deterioration differs from arch to arch. Generally, 5 to 70% of their surface is to some extent corroded, the depth of total corrosion exceeding s mm in places. The weathering of the building stone of Charles Bridge has been studied by several authors (Konta 1988, Lang 1989, Sramek 1987). Nevertheless. the still increasing degree of deterioration since the general reconstruction in the 1970's made it imperative that a complex re­ assessment of its condition was completed prior to the presumed reconstruction of the bridge. An integral part of this study (Wiczany 1994) has been the mineralogical, petrological, geochemical (Sulovsky, Gregerova; Pospisil, Locker - Wiczany 1994.) and microbiological (Wasserbauer - ibid) study. 30 2. METHODS OF INVESTIGATIONS For the purpose of petrographical and mineralogical study, 21 core samples (diameter of 20 and 35 mm, about 220 mm long) were taken. The selection of sampling points included all main rock types composing the bridge body, ashlars of different age of inclusion in the bridge structure and of different degrees of deterioration. Besides these samples, surface samples of efflorescences and crusts were also taken. The rocks from drill cores were studied in polished thin sections. Rock slices, cut along the core axis, document the surficial, most weathered layer of the stone (40 - 50 mm thick). For comparison, thin sections from deeper levels (from the depth of 100 - 150 mm) of the examined blocks were prepared and observed too. Moreover, a part of the outermost portion of each drillcore (the first 3 mm from the surface) was cut off parallel to the surface, mounted in epoxy resin and carefully lapped until the section's surface revealed grains of secondary minerals covering the rock surface and filling rock pores in the thin surficial layer. The same procedure was applied in case of efflorescences and crusts. The sections were polished and examined with an electron microprobe, together with the polished thin sections. Stone surfaces with the most highly developed efflorescences were also examined, but without mounting in resin, in a scanning electron microscope and identified by EDX spectra; their identification was complemented with XRD and IR spectroscopy. 3. PETROGRAPHIC CHARACTERISTIC OF BUILDING STONES USED Among 21 samples taken from Charles Bridge, four types of sandstone were identified. The most abundant type is si/icarenite, composed of elastic quartz grains (92 - 98 %), smaller amounts of K­ feldspar and rock fragments of quartzite and metaquartzite, the matrix being formed of clay minerals, Fe-oxidohydroxides and silty quartz particles. Gypsum rarely occurs in the pores near the surface. Three samples of eleven silcarenites were strongly porous and friable. Green (glaukonite) sandstone has been used less frequently. It is macroscopically compact, gray-brown and greenish in colour, and has massive structure. In its composition, quartz (90 - 93 %) dominates over K-feldspar, glaukonite, and clay minerals. Binding material is mostly formed of silty quartz, and locally of clay minerals. Clastic psammitic grains are closely packed, empty pores are therefore rare. The subsurface zone is highly friable. The weathering crusts on these stones usually contains not only gypsum, but also jarosite. The least often used rock type is sandstone with Fe-oxidohydroxide cement. It is rusty-brown to dark brown, and very porous. This sandstone is highly friable. The clasts are formed predominantly of quartz (over 95 %), rock fragments of quartzite and metaquartzite. Typical feature is the basal Fe­ oxidohydroxide cement. Close to the ashlar surface, the cement remains preserved only in fragments. All above described sandstone types are Cretaceous, while the arcose sandstone to po/ymict conglomerate (provenance: mostly Kamenne Zehrovice), the second most common rock type used in 1 Prague for building purposes from the 14th till the beginning of the 20 h century (Sramek 1989), is of Carboniferous age. Among the elastic components, quartz grains (85 - 87 %) prevail over K-feldspar (locally up to 15 %), and minor portions of rock fragments (quartz porphyry, sericite schist, metaquartzite and mudstone), plagioclase, altered muscovite and biotite. The matrix consists of silty quartz, clay minerals, calcite, Fe-oxidohydroxides, and in subsurface zones sometimes gypsum. 4. CHARACTERISTIC FEATURES OF ROCK WEATHERING In all the above rock types, the material of the surficial zone differs from that of deeper levels (15-20 cm) of the ashlars in colour, mineral composition, compression strength, in the degree of disintegration, porosity and character of pores. Feldspars in the sub-surface samples are strongly kaolinized and fractured (Sramek and Tolar 1987). The outer zone is usually friable, strongly weathered, with individual elastic grains easily spalling off the surface. Generally, the rock porosity is much higher there than inside 31 the ashlar (see Tab. 1). In some blocks, the depth porosity profile starts with a few millimetres of tightly packed layer encrusted with neogenic minerals, followed by a zone of extremely high porosity 3 - 15 mm thick (see Fig. 1 ). ' Table 1: Physical properties of selected rock samples No Porosity Absorption Volume pH of No of bacte- Rock (mm3.g.1) capacity mass water riae x 103 x) (%) (kg.m-3) extract xx) 6 11, 1 1789 7,5 (0)- 251/0,5/13 sandstone w. 8,0 (I) limonite cement 8 80,8 386/1320/0,9 arcose sandstone to conglomerate 10 13,8(1) 3,5 2191 7,5 (0)- 1000/980/13 quartz sandstone 13,910) 8,5 (I) 11 34,3 (I) 5,0 (0) - 39/15/2 quartz sandstone 27,5 (0) 8,0 (I) 14 41,8 (I) 9,5 1798 5,5 (0)- 1/0,5/2 quartz sandstone 38,0 (0) 4,5 (I) 15 8,8 1838 4,5 (0)- 26/36/52 quartz sandstone 5,0 (I) 16 48 (I) 4,3 2210 glaukonite 34,9 (0) sandstone 17 7,0 (0)- 291/19/1462 sandstone w. 6,0 (I) limonite cement 18 6,9 1982 quartz sandstone 19 11 ,8 (I) 11 ,8 2011 4,5 (0)- 0,710,21- quartz sandstone 2,0 (0) 4,5 (I) 20 31 ,2 (I) 3,4 2235 glaukonite 32,5 (0) sandstone 21 46,9 (I) 6,7 2031 arcose sandstone 31 ,2 (0) to conglomerate x> Porosity measurement performed with mercury porosimeter Carlo Erba AG60 (measurement range of the pore diameter 7500 - 7,5.10-3 mm) xx> Number of bacteriae present in 1 gramm of the rock taken from the depth of 2, 5, and 1O cm - data of VVasserbauer(1994) In the surficial parts, the pore spaces are often fully or partially filled with secondary minerals - gypsum and various other sulphates, gelous silica, chlorides, nitrates. The increase in content of insoluble neo­ formed minerals towards the surface and the opposite behaviour of soluble mineral species has been noticed already by Kaiser (1929) in the case of glaukonite sandstone of the Regensburg Cathedral.
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