557

SEM BASED STUDIES OF THE COMBINED EFFECTS OF SALT AND BIOLOGICAL WEA THE RING ON CALCAREOUS BUILDING STONES

Viles, H. A.

School of Geography, University of Oxford, Oxford OX1 3TB, UK

Moses, C. A.

Geography Laboratory, University of Sussex, Brighton, BN1 9QN

INTRODUCTION

Scanning electron microscopy (SEM) is a powerful tool in the diagnosis of stone decay and many recent papers include SEM photomicrographs and allied EDAX (Energy Dispersive Analysis of x Rays) information to illustrate and elucidate the active decay processes. For example, field studies of building

stone decay have utilised SEM and EDAX to elucidate decay mechanisms on various calcareous stones (Fassina, 1988; Bello et al., 1991). Short term exposure trials using calcareous substrates have successfully used SEM observations, sometimes combined with weight loss indices and surface roughness profiling, to quantify rates and elucidate mechanisms of decay (Viles, 1990; Moses, 1996). Biological and salt weathering processes have separately been the subject of much study yet, in combination have received little attention. The work reported here aims, through SEM examination of

experimental samples, to elucidate the combined impact of these important processes on calcareous building stone. Two issues remain to be clarified , however, in order to increase the utility and rigour of SEM studies. Firstly, there is an urgent need for standardisation and quantification of SEM observations. Secondly, further work needs to be done to explain the links between morphology and chemical changes at the SEM scale and larger scale symptoms of decay. Recently, there have been calls for the development of 'indicators' of decay, or what could be regarded as relatively simple, quantifiable features which could provide the stone conservator with an indication of the major decay processes occurring (Viles et al, in press) . Standardised SEM observations may well provide suitable indicators of

some decay processes. The present authors have made recent attempts to systematise SEM observations of the morphological characteristics of decay on carbonate stone surfaces, and those of natural rock outcrops (Moses et al, 1995; Moses and Viles, 1996, and table 1). We have proposed the term nanomorphology to describe those morphological styles observable on individual mineral grains (i.e. generally< 100 µmin size) , and micromorphology to describe features influencing the stone fabric (generally < 1 mm in size) in comparison with other relevant scales of stone decay as illustrated in table 2. It should always be remembered that, even at the nano and micromorphological scales, stone decay is a three dimensional process, with a volume of mineral grains and stone being affected rather than a two dimensional surface.

Observations of the morphological expression of biological weathering As shown in table 1, and figures 1 and 2 several seemingly characteristic nanomorphological and

micromorphological features have been ascribed to biological action, especially the chemical activity of microorganisms and biofilms. Many microorganisms are capable, through a range of etching and chelating processes, to bore and burrow their way into mineral surfaces producing distinctive boreholes, pits and channels. Such biochemical impacts are evident not only in subaerial environments (Moses et al., 1995), but also in coastal environments where the combined effects of salt and biological weathering

remain unclear. Several authors have also stressed the potential importance of the physical weathering action of microorganic communities, as wetting and drying cycles lead to swelling and shrinking of cells and mucilage layers, which can prise the grains of stone apart (Moses and Smith, 1993). Such processes seem to leave a micromorphological imprint, altering the near surface fabric of the stone, as well as occasionally a nanomorphological impact as individual grains become cracked and deteriorated

(Cooks and Otto, 1990). In many instances, microorganisms play both chemical and physical roles, creating a nanomorphological network of etch features on mineral grains and crystalline cements within

the stone, as well as the physical damage caused by grains being prised from the surface (Moses and

558

Smith, 1993). Observations of both scales of morphological features would be required in this case to

make a good assessment of the biological contributions to decay.

Table 1 · Carbonate stone weathering nanomorphologies

Morphological feature Origin

Circular etch pits Biological

Circular boreholes Biological

Filament-shaped trenches Biological

Crystal or grain boundary widening Dissolution

Cleavage widening Dissolution

V-in-V etching Dissolution

Blocky etching Dissolution

Stepped etching Dissolution

Rounding Dissolution

Deposition/ crystal growth Weathering-induced mineral transformation

Microfractures Various, e.g. salt action

Table 2: The different scales of building stone weathering

Scale Components affected Nano The minerals Micro The stone fabric Meso The blocks of stone Macro The wall

Figure 1: SEM micrograph showing chasmolithic cyanobacterial colonies etching calcite crystals from a natural limestone outcrop, Japan. Scale bar= 20µm

559

Figure 2: SEM micrograph showing euendolithic cyanobacterial boreholes on terrestrial limestone surfaces from Aldabra Atoll , Indian Ocean. Scale divisions= 10 µm

Observations of the morphological expression of salt weathering

Salts are also effective agents of stone decay, and SEM observations of field samples and experimentally weathered substrates have often been used to help assess their action (Goudie and Viles. 1995; Moses and Smith, 1994). Dated structures provide an opportunity to study rock weathering under effectively controlled conditions. The sea walls of Weston-super-Mare, Avon, UK. for example, have been used for an intensive study of the controlling factors in alveoli development. Alveolar weathering is associated with arid and coastal environments which are characterised by a ready supply of salts and frequent wetting-drying cycles. The presence of salts within alveoli has often been taken as evidence that such features are formed by salt weathering (Mottershead, 1994). Recent work on alveoli at Weston-super-Mare has used petrographic microscopy and SEM to show that salt weathering processes have actually played a minor role in this case. Instead, chemical alteration along inter-grain boundaries and differential swelling and contraction of clays within the rock have resulted in granular disentegration

and micro-lamination of the sandstone (Pye and Mottershead, 1995). Although laboratory studies have shown impressive rock breakdown in a range of salt weathering

simulations it has proved difficult to elucidate the exact causes of breakdown. Goudie and Viles (1995) found, in simulation experiments, that before any visible signs of decay occurred, there were key nanomorphological and micromorphological symptoms, involving predominantly the production of

cracks. It was impossible to link the cracks directly with salt action from SEM observations, and more

detailed studies need to be made to elucidate exactly what is going on. Salts may also have a chemical action on stone which may or may not produce a clear morphological

expression. Gillett (1978), for example, treated a number of stone and aggregate samples with de-icing salts and used SEM to recognise a number of etch features which he ascribed to the chemical action of

the salts. On many carbonate stone surfaces gypsum crust build up is a serious problem, and again there have been many arguments over how the gypsum crust grows at the expense of the underlying stone and what the major symptoms of damage are. SEM and thin section observations of the

560

micromorphology of decay are particularly useful (e.g. Verges-Belmin, 1994) in clarifying the chemical and physical nature of breakdown on encrusted surfaces. Such work also indicates the need to relate micro and nanomorphological observations to internal observations of grain breakdown and alteration

(e.g. pseudomorphism) and the chemical changes associated with such processes.

Combined action of salt and biological weathering

Salt and microorganisms in co-association (as defined by Koestler et al., 1994) may affect building stone surfaces and natural rock outcrops. In coastal environments, for example, the two processes have usually been studied separately and little attempt made to elucidate their combined action. In terms of alveoli, however, Mustoe (1982) suggests that epilithic algae may play a role in protecting the delicate walls which separate them either by retarding evaporation of saline solutions, or by acting as aphysical barrier preventing salts reaching the surface. In polluted, urban atmospheres, salt and biological interactions may also be important. On sooty gypsum crusts, for example, some hardy microorganisms flourish and influence microenvironmental conditions and thus the nature of stone decay. Atmospheric salts may be absorbed by microorganisms and precipitated out on their undersides, forming an altered, often reactive, layer on the stone surface. As the above review shows, studies of a range of natural rock outcrops and building surfaces around the world have not found any clear morphological evidence to show how salt and biological weathering work in co-association, and what factors control the nature of their relationship. Do the two series of processes tend to work synergistically, mutually reinforcing each other?; do they occur in a mutually exclusive fashion, with salt action deteriorating the stone and preventing microorganism colonisation on the one hand, and microorganisms acting as a buffering layer, protecting stone surfaces from salt action on the other hand?; or do they act independently of each other on the same areas of stone? An experimental design to clarify the morphological action of biological and salt weathering In order to try and clarify the co-association of biological and salt processes under a range of conditions, we have designed an experiment as part of a larger project of experiments using SEM techniques to observe and quantify the nano and micromorphological expression of the early stages of decay through a range of processes. These experiments investigate how nano and micromorphological features develop, and we aim to extend the spatial and temporal scale of the experiments in due course to show how these features relate to the visible signs of decay.

We have developed a standardised experimental design for all our initial experiments, building on our first experiment which investigated the impacts of spraying with dilute acids (Viles and Moses, in prep.). Ten c. 5 x 5 x 5 mm cubes of Pentelic Marble and 10 similar sized fresh calcite crystals are subjected to a each treatment (with another group of 10 acting as a control and being treated with deionised water). In the spray experiment, for example, 10 marble and 10 calcite crystals were sprayed daily for five days with 75 ml of 0.02 M solution of HCI, whilst another 10 samples of each were sprayed in a similar fashion with 0.2 M HCI. In terms of the co-association of salt and biological weathering, for example, the experimental design is to treat 10 marble and 10 calcite with salt alone; 10 of each with a culture of microorganisms alone, and 1 O of each with a combination of the two. The salt treatments will involve spraying samples with salt solutions; the microorganism treatments will follow the methods of Koestler et al. , 1985 (using culture conditions of 22° C, 100% RH, day/night 14/10 hours). The combined treatments will involve the microorganism treatment followed by the salt treatment.

After the experiment has been run , samples are taken from each marble and calcite sample and observed with the SEM (as well as identical samples which have received no treatment at all) . In order to standardise and quantify observations, a standard magnification is used (normally x 2000) and 30 random points on the sample observed at this scale. At each points the presence or absence of a checklist (as in table 1) of nanomorphological features (and micromorphological features as well in the

case of the Pentelic marble) is recorded. Standard simple and multivariate statistical tests are used to compare the occurrence of various features under different treatments, to see how diagnostic such features are.

561

Discussion and conclusions

Clearly, investigations of samples from decayed building stone and natural outcrops reveal some suggestive nano- and micromorphological features which may relate to salt and/or biological action.

However, laboratory experiments are needed to clarify the operation of the two, and their co-association. Short-term, small-scale experimental study results should be treated with care, as they may not be representative of larger scale, longer term situations. Thus, for example, as biofilms develop and grow their control over the stone surface microenvironment may change from that of pioneering communities. Experimental results will be reported in the near future in what is very much an on-going experimental programme.

References Bello, M.A. , Martin, L. and Martin, A. 1991 Scanning electron microscopy to establish the marble

weathering mechanism in the Alhambra of Granada (Spain). Scanning Microscopy 5, 645-52. Cooks, J. and Otto, E. 1990 The weathering effects of the lichen Lecidea aft. sarcogynoides (Koerb.) on

Magaliesburg quartzite. Earth Surface Processes and Landforms 15, 491-500. Fassina, V. 1988 Environmental pollution in relation to stone decay. Durability of Building Materials 5, 317-358. Gil Iott, J.E. 1978 Effect of de-icing agents and sulphate solutions on concrete aggregate. Quarterly Journal of

Engineering Geology 11 , 177-192. Goudie, A.S. and Viles, H.A. 1995 The nature and pattern of debris liberation by salt weathering: A laboratory

study. Earth Surface Processes and Landforms 20, 437-449. Koestler, R.J., Charola, A.E. and Wypyski, M. 1985 Microbiologically induced deterioration of dolomitic and

calcitic stone as viewed by scanning electron microscopy. In: Proceedings, 5th lntenrational Congress on Deterioration and Conservation of Stone, Lausanne, 617-626

Koestler, R.J., Brimblecombe, P., Camuffo, D , Ginell, W.S., Graedel, T.E., Leavengood, P., Petushkova, J., Steiger, M., Urzi, C., Verges-Bel min and Warscheid, T. 1994 Group report: How do external environmental factors accelerate change? In: Krumbein, W.E., Brimblecombe, P., Cosgrove, D.E. and Staniforth, S. (eds) Durability and Change. Chichester: John Wiley.

Moses, C.A. 1996 Methods for investigating stone decay mechanisms in polluted and 'clean' environments, Northern Ireland. In: Smith, B.J. and Warke, P.A. (eds) Processes of urban stone decay. London:

Donhead, 212-227. Moses, C.A. and Smith, B.J. 1993 A note on the role of the lichen Collema auriforma in solution basin

development on a Carboniferous limestone substrate. Earth Surface Processes and Landforms 18, 363-

368. Moses, C.A. and Smith, B.J. 1994 Limestone weathering in the supra-tidal zone: an example from Mallorca. In:

Robinson, D.A. and Williams, R.B.G. (eds) Rock weathering and landform evolution Chichester: Wiley,

433-452. Moses, C.A. , Spate, A.P., Smith, D.I. and Greenaway, M.A. 1995 Limestone weathering in eastern Australia. Part

2: Surface micromorphology study. Earth Surface Processes and Landforms 20, 501 -514. Moses, C.A. and Viles, H.A. 1996 Nanoscale morphologies and their role in the development of karren.

In: Fornos, J.-J. and Gines, A. (eds) Karren landforms Palma, Mallorca: Universitat de les Illes Balears,

85-96. Mottershead, D. N. 1994 Spatial variations in intensity of alveolar weathering of a dated sandstone structure in a

coastal environment, Weston-super-Mare, U.K. In: Robinson, D.A. and Williams, R.B.G. (eds) Rock

weathering and landform evolution Chichester: Wiley, 151-174. Mustoe, G.E. 1982 The origin of honeycomb weathering. Bulletin, Geological Society of America 93, 108-115. Pye, K. and Mottershead, D. N. 1995 Honeycomb weathering of Carboniferous sandstone in a sea wall at Weston­

super-Mare, U.K. Quarterly Journal of Engineering Geology 28, 333-347. Verges-Belmin, V. 1994 Pseudomorphism of gypsum after calcite, a new textural feature accounting for the

marble sulphation mechanism. Atmospheric Environment 28, 295-304. Viles, H.A. 1990 The early stages of building stone decay in an urban environment.

Atmospheric Environment 24A, 229-232. Viles, H.A. , Camuffo, D., Fitz, s., Fitzner, B., Lindqvist, 0 , Livingston, R.A. , Maravellaki, PV. , Sabbioni, C. and Warscheid, T. In press. Group report: Whatis the state of our knowledge of the mechanisms of deterioration and

how good are our estimates of rates of deterioration? In: Baer, N.S. and Snethlage, R. (eds) Saving our architectural heritage: The conservation of historic stone structures Dahlem Workshop Report ES20,

Chichester: John Wiley.