Abstract
In order to evaluate environmental controls on the soiling formation and decay of building stones a set of mapping and physical and chemical analyses were carried out on granite from a historical church in the polluted centre of Rio de Janeiro. These techniques highlight the increasing of threatening damage on generally perceived as a durable building material, caused by granular disaggregation and contour scaling in areas close to ground level. Mapping also indicated the formation of black crusts over entire building façades, concentrated on areas sheltered from rain-wash. Analyses demonstrated the influence of marine aerosols, rock and mortar composition and mostly of the atmospheric pollutants on the decay and soiling of the granite. Much of the decay is associated specifically with the presence of halite (NaCl) and gypsum ((CaS04)2H2O). The fact that black, gypsum crusts are able to develop over entire façades in a humid subtropical environment is testimony to the high levels of local pollution, especially particulate deposition. Reduced rainwash, in sheltered micro-environments of narrow, canyon-like streets, overcomes the gypsum tendency to be washed away from buildings façades. These observations further highlight that decay processes are primarily controlled by microclimatic conditions.
Highlights
Interactions between the atmosphere and stone used in monuments and buildings invariably lead to the formation of altered surface layers, producing damage to the original stone (Sabbioni 1995)
To highlight the degree of stone weathering in the church façade, those areas most affected by black crusts and weathering were mapped (Figure 2)
A significant correlation between Ca and SO4 is shown in Figure 5 and demonstrates the importance of gypsum formation and how it contributes to weathering on the church façade
Summary
Interactions between the atmosphere and stone used in monuments and buildings invariably lead to the formation of altered surface layers, producing damage to the original stone (Sabbioni 1995). It could be that any gypsum that does form is removed in solution at a rate and a frequency that prevents its accumulation into a coherent and extensive crust The importance of this balance between formation and removal (input Vs output) was recently illustrated in a study of limestones in the polluted environment of central Budapest (Hungary), where it was shown that the combination of low rainfall and high rates of dust deposition enhanced rapid regrowth of gypsum crusts on spalled areas, even where they were exposed to rainwash (Smith et al 2003). In their study, Tang et al (2003) are careful to point out that over the intervening period annual rainfall totals in Pittsburgh had not increased This reflects the understanding that high rainfall, especially associated with low levels of atmospheric pollution, generally negates the formation of gypsum crusts. The examination of superficial crusts formed on a major historic building in the centre of Rio de Janeiro point towards the irrefutable neoformation of gypsum in the crust
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