Origin and growth of weathering crusts on ancient marbles in industrial atmosphere

Abstract

The origin and growth of weathering crusts on the ancient marbles of ruins of the Sanctuary of Demeter in the industrial atmosphere of Eleusis in Greece have been investigated. A systematic mineralogical, petrographical and chemical examination of weathered stones and crusts was performed, both in situ and in the lab, on samples taken from different parts of the monument in relation to the surface characteristics as well as to the exposure to rain, sea-salt spray and wet and dry deposition of airborne pollutants and dust. In particular, the various material–environment interactions take place, are characterized by (a) disintegrated “washed-out” surfaces, where products are taken away through dissolution, (b) rusty yellow patinas rich in Fe and Cu, (c) firmly attached black crusts in contact with percolating water, where recrystallized calcite shields amorphous deposits rich in S, Si, Fe and carbonaceous particles, (d) black loose deposits in the water sheltered areas, consisting mainly of gypsum and fly ash particles and (e) cementitious crusts, coating and pitting the horizontal surfaces. Moreover, an interconnected evolution of various physicochemical processes is shown, characteristic of the origin and growth of various crusts, which are formed and classified accordingly.

Introduction

The degree and the distribution of the main types of observable rock deterioration in an urban environment are related to different kinds of wetting and particularly to the exposure of the stone surface to rain (Camuffo et al., 1983). In a polluted environment, two kinds of crusts are usually developed on calcareous rocks; the so-called “white” and “black” crusts.

White crusts are formed through the dissolution of gypsum and the reprecipitation of calcite at the surfaces that are subjected to wash out (Camuffo et al., 1987), which takes away the deposits and the products of dissolution (Camuffo et al., 1982). On the other hand, black crusts are developed by gypsum formation on surfaces sheltered from water and attacked by an SO₂-polluted atmosphere. On those areas protected from intensive wash-out, SO₂ and water vapour or rain water diffuse at a high rate through the pores towards the CaCO₃–gypsum interface. Thus, new porous gypsum film layers must be formed at the CaCO₃–gypsum interface. As the thickness of the gypsum increases, the number and length of the pores decrease due to the larger molecular volume of gypsum compared with that of CaCO₃ and finally, at ca. 30 nm thickness of gypsum, the pores cease to exist. Thereafter, the rate-determining step becomes solid-state diffusion of Ca²⁺ towards the environment (Skoulikidis and Charalambous, 1981). The transformation of the stone into gypsum is not only associated with the presence of gaseous SO₂, but also with S contained in the residual carbonaceous particles of combustion of fossil fuels for domestic heating (Del Monte et al., 1984). A high uptake of atmospheric particulate matter (Fe₂O₃, Al₂O₃, SiO₂, C, etc.) is observed on the weathered surfaces (Moropoulou and Bisbikou, 1995; Rodriguez-Navarro and Sebastian, 1995; Sabbioni, 1995). Silicate particles are emitted by, e.g. the combustion of coal and they are deposited at the exposed surfaces (Camuffo et al., 1987). Various intermediate crust formations are referred to in the literature, according to the wetting type of the surface. However, a number of physicochemical processes determining the origin and growth of deteriorating crusts should be studied with respect to the type of the environmental attack (industrial and marine atmosphere, various types of total suspended particles, etc.) and the type of the respective surface.

The variation of climatic conditions (temperature, air humidity, etc.) contribute substantially to the creation of salts within the pores (Theoulakis and Moropoulou, 1988). NaCl gives rise to the gypsum solubility. The ratio of SO^2-₄/Cl^- in rain water varies according to the amounts of SO₂ in the atmosphere and characterizes the pollution level.

The micro-structural characteristics of the various lithotypes could act as criteria to evaluate their susceptibility to the weathering phenomena caused by the different environmental factors acting. Stones with large pores like marble may contain large amounts of water; stones with very small pores, although they may contain minor amounts of water, experience more frequently condensation and water may remain longer in the pores. The microstructure is also determining the mechanical weathering which accompanies the formation of the crusts on the stone surfaces, due to the polluted atmosphere.

In the present work, the marbles of the ruins of the Demeter Sanctuary in Eleusis, Greece, were subjected to detailed investigation.

The first temple of Demeter and the Eleusinian Sanctuary were founded in the reign of the legendary King Celeus in the Late Helladic period (1500–1425 BC), as a home for Demeter. The present ruins of the Telestirion (Fig. 1) belong to its fourth last building phase design by Ictinus in the Periclean period (Preka-Alexandri, 1991).

The Greater Eleusis area had been a rural area for years, but in the post-war period has become increasingly industrialized, with key manufacturing sectors, basically metallurgical and chemical ones. The Thriasian plain, in particular, has been dramatically “attacked” by several large and small industries causing a decline in environmental standards. In addition, the air pollutants, swept by the prevailing winds to the sea side, remain in Saronikos Bay for a period of time and then recirculate at the coastal areas increasing pollution problems. Air pollution creates aggressive atmospheric conditions which can damage the material cultural heritage of a region. Consequently, the Eleusis area corresponds to a typical urban-centre profile of intense and diversified industrial activity characterized by a highly polluted atmosphere heavily charged by suspended particles (Abazoglou et al., 1990a, Abazoglou et al., 1990b). Hence, the study of neoformations on the monument surfaces in the archaeological site of Eleusis illustrates the variety of stone decay phenomena caused by air pollution and specifically by suspended particle attack, most probably in synergy with marine spray, due to the vicinity to the sea (Zezza, 1994).

Five different carbonate rocks (grey-micritic limestone, white Pentelic marble, white–grey marble, yellow limestone and biomicritic grey limestone) have been identified as building materials in the Sanctuary (Moropoulou et al., 1994a, Moropoulou et al., 1994b); the scope of this work is mainly limited to the weathering of Pentelic marble.

Calcite comprises 96% of Pentelic marble, with about 1.8% dolomite. The remainder of the stone consists of minor quartz and fine silicate minerals (0.79% SiO₂, 0.52% Al₂O₃ and 0.12% FeO) (Dickinson et al., 1988). The petrographic analysis (Moropoulou et al., 1995) shows a medium-grained well recrystallized marble with granoblastic polygonal texture. The grain size indicates increasing intensity of metamorphism that is possibly attributed to high temperature and low deformation rates. The microstructural analysis (Moropoulou et al., 1995) shows a low total porosity (3.64 vol%) of interparticles type, characterized by a network of large fracturing pores between grains. High apparent density (2.71 g cm^-3), low water absorption (0.22%), as well as the wave velocity (5830 m s^-1) depending on the homogeneity of the stone mass, characterize the Pentelic marble.

The main decay patterns in the sequence of decreasing frequency with which they are observed, can be classified as follows:

• — granular disintegration and detachment (Fig. 2a), the most general and important weathering on yellowish limestone and marble, presented at washed-out surface areas, or at surface areas facing the sea,

• — crusts indicating biological attack to the stone,

• — chromatic alteration from the white of the Pentelic marble to the rusty-yellow colour at the washed-out surfaces (Fig. 2b) and mainly where water-rebound phenomena occur (Fig. 2b′),

• — pitting on the marble and limestone surfaces covered by cementitious encrustations, especially on the horizontal surfaces like pavements (Fig. 2c),

• — black crust formations in the form of loose deposits, formed at the surfaces sheltered from rain water and presenting an anomalous relief with high friability (Fig. 2d),

• — black–grey thin crust formations, firmly attached underneath cornices and pillar crowns as well as on the exposed upper perpendicular surfaces, wherever the marble surfaces are in contact with percolating rain water but are not washed out (Fig. 2e).

The real weathering phenomena are usually a combination of the above simplified forms used for the convenience of the analytical study.

In the present work, systematic mineralogical, petrographical and chemical examination is performed on samples from the Eleusis Sanctuary ruins, in an effort to interpret the deterioration phenomena that take place.