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BAUMBERG CALCAREOUS SANDSTONE AND OBERNKIRCHENER SANDSTONE, NATURAL SENSOR MATERIALS FOR

ENVIRONMENTAL MONITORING - RESULTS OF A FIELD EXPOSURE STUDY

Mirwald, P.W.*, Bri.iggerhoff, St.** and Fimmel, R.*

* Ins ti tut f. Mineralogie, Univ. Innsbruck ** Zollem Institut beim Deutschen Bergbau-Museum, Bochum

Abstract Monitoring of environmental influences for protection and conservation of -objects at low costs may be achieved by sensor materials. Field exposure experiments (six sites in W-, S-Germany and Tyrol) suggest the possible use of stone materials. Under this aspect field exposure data on Baumberg Calcareous Sandstone (BCS) and on Obernkirchener Sand­stone (OKS) are presented. While BCS is a very sensitive material which reacts, in particular, on S02-immission influences, OKS is an chemically almost inert stone which behaves as a collector, specially for solid components. The exposure data obtained on these two stones allow to distinguish different partial weathering processes out of the bulk degradation which suggests to use these materials as a set of specific sensors.

1. Introduction There is an increasing demand for monitoring of environmental influences in the field of object protection and conservation. In many cases this can be achieved by sensor materials behaving as qualitative indicators of the environement agressivity. So far metals and glasses have been proved to be useful for such purposes (VDI-Richtlinie 3955). Due to the fact that these materials have no relevant porosity, the effects of environmental influences are more or less restricted to the surface of the materials. This has the advantage of a relatively short exposure periods of some months of qualitative sensoring. Porous materials, such as stone, brick or concrete, offer considerably longer exposure durations which range between months to years. In addition, these materials may have the advantage that specific impact sources can be discrimenated. Furthermore, with respect to stone objects these kind of sensors exhibit similar weathering behavior.

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Field exposure experiments allow to select stone materials apt as envi­ronmental sensors for atmospheric immissions. Baumberg Calcareous Sandstone (BCS) which is known to be particularly susceptible for S02 and "dust components" (Luckat, 1981) as well as Obernkirchener Sand­stone (OKS) which is of similar porosity and hygric behavior but chemi­cally an almost inert material, seem be such candidates. This note is concerned with the presentation of an an exposure study under "dry condition" of these two materials which allow to distinguish different partial degradation processes. A practical outcome of this study is that the two stones may be used as sensor materials.

2. Experimental Exposure experiments were conducted on four sites in West- and South­Germany Duisburg (D), (Eifel (Ei), Mtinchen (M), Kempten (Ke)) and two in the Tyrolian Alps (Innsbruck (lbk) and Obergurgl (Og)). All sites are characterized by different climate and anthropogene environment conditions. The duration of the exposures was five years in Germany and two years in Tyrol. Continuously monitored metereological parameters and a selection of important pollution data were available for most of the sites.

Baumberg Calcareous Sandstone (BCS) is a cretaceous biomicritic sedi­ment in Westfalia (Germany) (fossil relics: 50 vol%, clayey calcitic ma­trix: 10 vol.%, quartz: vol.30% , glauconite: vol.10%; porosity: 19 vol%, spec. surface: 10.5 (m2/g), w-value 2.5 (kg/m2h0.5); Obernkirchener Sandstone (OKS) of Lower Saxonia (Germany) is a slightly quarztitic silt-sandstone (quartz: 87 vol.%, clayminerals: vol.13%; porosity: 19 vol%, spec. surface: 2,7 (m2/g), w-value 2.24 (kg/m2h0.5) ) In addition, the starting materials have been carefully characterized with respect to their inherent salt contents (S042-, N03-, Cl-, F-).

Stone slabs (50x50x5 mm) of these materials served as samples. Moun­ted on Mank- carrousels (VDI-Richtlinie 3955), the slabs were exposed under rain protected, "dry conditions" .

3.0 Results and discussion While the data of mass changes will be given as obtained, the gypsum data have been calculated from analysed S-data. The variable parameter used in the correlation diagrams is "S02 presentation" (kg m-2) which

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represents the product of the average of the pollution component, the average wind speed and the duration of exposure (seconds). Compared to previous use of variables such as "immission rate" (mg m-2 d-1) (Luckat, 1981) or "dosis" (mg m-2 month) (Brtiggerhoff & Mirwald, 1991) this parameter configuration yields better linear correlations.

3.1 Data Fig. 1 to 3 give the data of mass changes of the samples from the selected sites under dry weathering conditions and corresponding sulfate contents calculated each plotted versus S02 presentation.

The BCS-samples exposed for five years in Germany exhibit all a typical tendency: the mostly positive mass changes of the samples are smaller than the mass of sulfate contents (fig.1 ). An exception represent the data of the first summer season of exposure, where a considerable mass gain is likely due to biogenis growth. Striking is that after some two years of exposure a significant mass loss is generally noticeable; after three years of exposure again an increase in mass is recorded. The BCS-samples exposed in Tyrol for two years (fig.2) differ significan­tely. Although both parameters of the Innsbruck site, sample mass and mass of sulfate, show also increasing behavior, there is a slight tendency to be observed that the gain in sample mass exceeds that of the sulfate content. A completely different behavior is observed in Obergurgl: very small sulfate contents contrast with dramatic mass losses. These are likely due to extensive frost I thaw events.

Data on OKS (fig.3) are only available from the German sites so far. All samples show again the typical behavior that mass gain is exceeded by the mass of sulfates. However, the mass increase is continuous over the entire exposure duration

3.2 Data discussion Despite the difference in exposure duration between the German and the Tyrolian sites the results allow to draw first principal conclusions.

The inspection of the mass change data on both materials (fig. 1 to 3) shows that BCS is subjected to a higher deposition than OKS. Usually, the amount of analysed sulfate is inferred to result from the reaction of S02-Immision with carbonate components. In contrast to BCS fresh OKS is free of carbonate. However, the analyses of the OKS-samples yielded considerable amounts of sulfate (fig. 3) This leads to the conclu­sion, that this is due to deposition from "exogenic sulfate sources". This

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implies that the bulk amount of sulfate determined in BCS is partially of exogenic and partially of autigenic provenience. From the fact that the sulfate contents exceed the mass changes we conclude that considerable losses in stone material are always given.

3.3 Estimate of partial weathering processes On the basis of the available data it is possible to distinguish a number of various partial processes amounting to the bulk process of stone degradation:

formation of autigenic sulfate (gypsum), - deposition of exogenic sulfate (gypsum?),, - mechanically induced grain loss and - dust deposition (and biogene growth) In a first step of the data analysis the gypsum of the OKS-samples of the German sites has been determined. By subtracting this amount of exo­genic sulfate from the sulfate bulk mass of the BCS-samples one obtains the portion of autigenic gypsum of BKS. By doing so we infer that similar porosity and grain sizes of both materials do not differ much in adsorp­tion behavior and, therefore, no significant error should be introduced in this estimate. In a second step, the difference between mass changes and sulfate bulk mass of BKS has been determined. This gives an estimate for the net losses of substance (stone materials and/or sulfate and/or small amount of "dust"). Based on these two different kinds of determinations the different partial processes for the dry exposed BCS-material have been calculated. (Fig. 4 ). The amount of "dust", which is in terms of mass a relatively unimportant quantity, has been estimated.

- A striking result of this estimate is the considerable amount of exogenic sulfate, in particular for the highly industrialised areas, e.g. around Duisburg, which are already characterized by considerable S02-pollution levels.

- The only site of a really low pollution situation is Obergurgl/Tyrol; the other socalled " clean air areas" around Kempten and Eifel belong reali­stically rather to the medium polluted the areas such as Munich.

- Interestingly, the inner alpine town of Innsbruck is also characterized by a medium pollution situation and does not distinguish from the other middle European areas studied.

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3.4 BCS and OKS as sensor materials. With respect to the applicability of the two stone materials as sensor materials the results may be summarized as follows: BCS shows a very sensitve reactivity on various environmental loads and specific climate impacts. A feature to be taken into account is that the observed mass gain in the first summer season is likely due to bio­genic growth. Similarly, the significant mass losses of BCS after some two years of exposure have to be considered. Less influence due to increa­sing exposure duration is shown by the sulfate bulk mass. After some three years of exposure, there is an indication of saturation noticeable. These observations show that the use of BCS as a sensor material is limited to an exposure duration of about two to three years.

OKS shows a largely unaffected, continuous mass gain over the entire period of exposure. Concerning the sulfate content of OKS which is assumed to be of exogenic origin, there seems a tendency to reach a sort of saturation level after some four years of exposure - at least under the immission conditions of the German sites.

Since the two materials, BKS and OKS, exhibit different degradation pro­cesses they may be used - at simultaneous dry exposure - to discri­menate various environmental sources.

Literature

BRUGGERHOFF, S. und MIRWALD, P.W. (1991): Expositionsversuche mit Naturstein zum Studium von Verwitterungsvorgangen an Denk­malern, 20. Jahrestagung der Ges. f. Umweltsimulation, 15 S., Fraunhofer Inst. Pfinztal, Marz 1991.

LUCKAT, S. (1981): Quantitative Untersuchung des Einflusses von Luft verunreinigungen bei der ZerstOrung von Naturstein. Umweltfor­schungsplan d. Bundesministeriums des Inneren Forschungs­bericht 104 01 018/02.

VDI-Richtlinie 3955, Part 1 and 2: Assessment of effects on materials. Partl (1996): Exposure of steel sheets (Mank's Carroussel); Part 2 (1993): Exposure of glass senors; Kommission Reinhaltung der Luft im VDI und DIN.

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