BLACK CRUSTS AND THIN BLACK LAYERS IN GRANITIC MONUMENTS: THEIR CHARACTERIZATION AND THE ROLE OF AIR POLLUTION

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1 371 BLACK CRUSTS AND THIN BLACK LAYERS IN GRANITIC MONUMENTS: THEIR CHARACTERIZATION AND THE ROLE OF AIR POLLUTION BEGONHA A. Faculdade de Engenharia, Univ. do Porto, Rua dos Bragas, 4099 Porto Codex, Portugal SEQUEIRA BRAGA M. A. Departamento de Ciencias da Terra, Univ. do Minho, Largo do Par;:o, 4719 Braga Codex, Portugal SUMMARY The dark and dirty appearance of the Oporto.monuments results from the presence of thin black layers and black crusts, covering extensive areas of the fa<;ades. This paper characterizes the mineralogical and chemical composition of these two deteriorations. The decay mechanisms involved are discussed as well as the role of the industrial and urban air pollution in the genesis of the two pathologies. 1. INTRODUCTION Most of the Oporto monuments, namely the Hospital de Santo Antonio, were built with the Oporto granite, a two micas, light gray, medium grained leucogranite. The stones used in the construction of the monuments already had different degrees of weathering inherited from the quarries by hydrolysis of the parent minerals. At the moment, they show severe stone decay. The location and the processes involved in the fonnation of granular disintegration, scales and black crusts in granitic stones were the purpose of earlier studies carried out in the Hospital de Santo Ant6nio by Begonha and Sequeira Braga (1993). Begonha et al. (1994) showed that the weathered granitic stones of the Oporto monuments have a very homogeneous porous network, allowing a fast capillary transfer of the solutions, in spite of the low porosity. This fact favours the concentration of soluble salts and, therefore, stone decay. Black crusts and thin black layers are responsible for the dark and dirty appearance of the Oporto monuments. The purpose of this paper is to characterize these pathologies and the mechanisms involved in their genesis, particularly the role of air pollution. 2. MATERIALS AND METHODS X-ray diffraction (XRD) and chemical analyses by scanning electron microscope (SEM) equipped with a X-ray microprobe (EDS) were carried out in order to characterize thin black layers, their granitic substrates and black crusts of the Hospital de Santo Ant6nio. Concerning the impact of air pollution, chemical analyses of rain water were perfonned in samples collected in the Hospital de Santo Ant6nio by Atomic Absorption Spectroscopy and the methods described in the Standard Methods for the Examination of Water and Wastewater (American Public Health Association et al, 1992). In addition, filters used in monitoring the quality of air in Oporto, the solid material transported by rain water and the material from dry deposition were analysed and observed by SEMEDS. 3. BLACK CRUSTS: RESULTS AND DISCUSSION Black crusts are associated with wet sheltered areas not exposed to runoff, specially in recesses below cornices and ledges and around wall areas wetted by water running through open joints from the above ledges. They have great expression in the granitic monuments in Oporto, contradicting Schiavon et al. (1994). The most remarkable black crusts in the Hospital de Santo Antonio are located on the sheltered surfaces of granitic cantilever slabs existent on the highest structures above the largest roof sector. These samples are almost 1 cm thick and have developed dendritic shapes. In all black crusts areas, soluble salts solutions accumulated by capillary transfer, evaporate on the stone surface. Besides, there is a dry deposition of fly ashes and soot related to air pollution. No biological activity has been observed in the 24 samples studied in the Eastern fa<;ade.

2 372 XRD and SEM results show that black crusts are essentially composed by gypsum crystals, fly ashes and soot, including, at their base, some granite minerals. Gypsum shows several morphologies like perfect crystals (Fig. 1-a, b) and intergrown tabular crystals similar to desert-roses. Under the superficial layer, black crusts are almost exclusively composed by very compact gypsum crystals. Fly ashes show the two typical morphologies and compositions: porous spheres (Fig. 1-c) rich in C, Sand Si with higher or lower amounts of Al, Ca, Fe and K and smooth surface spheres. These particles usually are rich in Si and Al with higher or lower amounts of K, Fe, Ca, Ti and Cl, but there are also smooth surface fly ashes rich in Fe or Ti. The origin of the porous fly ashes and of the Si and Al or Fe rich smooth surface fly ashes is the combustion of coal and fuel in industry. Smooth surface fly ashes rich in Ti can be formed from paintings of iron scrap used in an electric arc furnace located North of Oporto. This type of fly ashes were also observed by Ta basso (1991) in the fa<;ade of the Palazzo dei Celestini in Lecce. Combustion of fuel and natural gas in car engines and house heating originates carbon rich soot about one hundred times smaller than fly ashes. The presence of gypsum on the surfaces of the granitic stones arises the question of the sources of S and Ca. The Oporto granite cannot be the source of calcium since the Cao amount (0.58 %) is quite low in the unweathered rock, strongly decreasing during the weathering phenomenon. Mortars can be a minor source of calcium and sulphur since they contain some gypsum in their composition (identified by XRD) and may react with the percolating water. However, they do not seem to be related to the formation of the majority of the black crusts that have been studied. This is the case of the black crusts located on the sheltered surfaces of the granitic cantilever slabs above mentioned. Mortars are not determinant in black crusts formation in the Oporto monuments. Begonha et al. (1996) showed that rain water can be the most important source of gypsum in black crusts in the granitic monuments of Oporto and that gypsum is the unique sulphate to crystallize in significant amounts by rain water evaporation in Oporto. 4. THIN BLACK LAYERS: RESULTS AND DISCUSSION Thin black layers from Sweden and other European countries have been studied by Nord and Tronner (1992) and by Nord and Ericsson (1993). In Oporto, thin black layers are black or dark brown coloured and usually are homogeneous and lustreless. They are very thin and strongly attached to the granitic substrate. They have a conspicuous distribution on the monuments, appearing in every fa<;ade from the bottom to the top. Normally they are located in rain exposed areas but they are associated neither with areas strongly affected by runoff nor with sheltered areas. Stones damaged by granular disintegration do not display thin black layers. Sometimes, thin black layers do not seem to inflict any damage at all. No significant biological activity has been observed in the 34 studied samples that could be associated with their genesis, contradicting Schiavon (1993). XRD results show that thin black layers are gypsum free or very poor in gypsum. When observed by SEM, the surface looks like a rough fine grained mortar thrown against a wall (Fig. 1-d). It is a quite homogeneous deposit since it is composed by a great number of small particles, rarely surpassing 1 O or 20 micra, and by aggregates of these particles. Amorphous Fe rich particles (Fig. 1-e, f) have been observed as well as solid spherical particles rich in Fe and Si. According to Nord and Ericsson (1993), both types of Fe rich particles are originated from traffic, the wear of engines, brake-shoes or produced by industry. C and 0 rich particles and smooth surface fly ashes rich in Si and Al (Fig. 1-i) or Fe rich (Fig. 1-g, h) were also observed. Occasionally one or two gypsum crystals appear. Porous fly ashes rich in C and S can be rarely observed. SEMEDS chemical analyses were performed in 18 samples of thin black layers (FN), in their granitic substrates (GS) and in eight black crusts samples (CN). The results show that the concentrations of Fe, S, Ca, P, Mg, Ti and Cl are higher in the thin black layers surfaces than in the granitic substrates. On the contrary, Si and K concentrations are lower in the FN samples than in the GS samples. The behaviour of Al, Mn, Na and the trace elements is not clear. All these variations are similar to those referred by Nord and Ericsson (1993). The results of Si0 2, Fe and S0 3 +Ca0 of the FN, GS and CN samples are presented in Fig. 2. It shows an increase in iron in the thin black layers in relation to the granitic substrates and high concentrations of S0 3 +Ca0 in black crusts.

3 373 Fig. 1 - SEM images and spectra: a) b) gypsum in a black crust; c) porous fly ash in a black crust; d) general aspect of a thin black layer; e) f) Fe rich particle in a thin black layer; g) h) Fe rich smooth surface fly ash in a thin black layer; i) Si and Al rich smooth surface fly ashes in a thin black layer. Fe CHEMICAL ANALYSES BY SEMEOS Si02.. \.... S03+CaQ 6. TH~ BLACK LAYERS l!. GRANITIC SUBSTRATE BLACK CRUSTS Fig. 2 - Si0 2, Fe 2 o 3 and S0 3 +Ca0 concentrations of thin black layers, granitic substrates and black crusts.

4 374 ca2+ and soi- values were calculated from the Cao and S0 3 results of the SEMEDS chemical analyses. The Ca2+ and soi- values of the FN and CN samples were plotted in Fig. 3 but separately treated. The equation soi- = ca2+ ( ) was adjusted to the FN values, presenting a low coefficient of correlation r = The adjusted equation is completely different from the SOl = Ca2+ equation of gypsum. Considering the CN values, the linear equation SO- = 6, ,7290 Ca2+ was achieved. The adjustment is significant (r = 0,868) and the adjusted straight line is almost parallel to the straight line representative of the gypsum composition. The excess of S in black crusts relatively to Ca shows the influence of porous fly ashes and other S rich particles in the chemical composition of black crusts surfaces. These statistical results confirm the mineralogical studies obtained by XRD and SEM. They show that thin black layers are basically gypsum free and black crusts are essentially aggregates of gypsum crystals, fly ashes and soot. 80,00 60,00 ' 00 20, Y:'~ ,00 5,00 10,00 ca :i ,00 TH.W Bl.ACK LAYERS BLACK CRUSTS --GYPSUM... ADJUSTMENT (THIN BLACK LAYERS) ADJUSTMENT (BLACK CRUSTS) ADJUSTMENT (THIN BLACK LAYERS) S04 a.0012c... o.1...s r..0,271 ADJUSTMENT (BLACK CRUSTS) S04 8, c. r O.aM Fig. 3 - Statistical correlations between the Ca2+ and so- values of the chemical analyses carried out in thin black layers and black crusts by SEMEDS. Iron concentration in unweathered samples of the Oporto granite is low (Fe = 1.12 %) and it does not increase significantly during the weathering process. Biotite could provide some Fe but it is not much weathered in the stones of the monuments. Therefore, the iron contribution of the Oporto granite to the composition of thin black layers is minimum and it is not the most important source as suggested by Schiavon (1993). Most of the iron is originated from urban and industrial air pollution. Besides, the contact between the granitic substrates and thin black layers is sharp and it is not gradational as referred by Schiavon (1993). S, C and Ca result entirely from air pollution. 5. CONCLUSIONS Thin black layers and black crusts are mineralogically and chemically distinct. Thin black layers are basically gypsum free and iron rich. Black crusts are composed by gypsum crystals, fly ashes and soot. Air pollution plays a decisive role in both stone pathologies. It changes the composition of rain water and is responsible for the dry deposition of damaging compounds. Soluble salts are accumulated in wetted sheltered areas, crystallizing as gypsum when water evaporates. Gypsum cements fly ashes and soot originated from air pollution, forming black crusts. Thin black layers are formed in areas exposed to rain water, not submitted to runoff. In these areas, gypsum crystallizes on the surface but it is removed by rain water. Dry deposition is determinant in these areas, allowing the concentration of iron rich particles and carbon rich particles originated from urban and industrial air pollution in the humid surfaces.

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