Environmental Engineering Reference
In-Depth Information
Although the global proportion of anthropogenic sources is only 4-5% of global
emissions, their impact, in particular that of ashes, is very important in the formation
of gypsum, which is rich in urban environments [LEF 92].
8.3.3.3
. Degradation patterns linked to atmospheric pollution
SO
2
-rich urban atmospheric pollution is an important source of gypsum
production. This salt, in the same way as other soluble salts, may generate a number
of degradation patterns, either as a primary agent in the form of encrustations or
efflorescence, or as a secondary agent, the action of which is added to that of other
agents, for instance in degradation patterns such as scaling.
In urban environments, particulate and gaseous pollutants interact with the stone
to form deposits on the stone surface, and cause transformations in the stone itself,
just beneath the surface and sometimes deeper. These transformations may occur
even if there is no surface deposit.
8.3.3.3.1. Powdery gray deposits
The composition of these deposits changes a lot from one site to another, but in
most cases they contain silico-aluminates and calcite, because the majority of soils
have this type of composition. Ochre dusts originating from the Sahara have such a
composition; they are transported high in the atmosphere and frequently fall on
southern Europe. Gray deposits found on monuments also contain small quantities
of different particles, such as pollens, ashes linked to fossil fuel burning [LEF 89],
soot, particles originating from building sites, coal micro-fragments [AUS 98] and
hydrocarbons in different states of oxidization. The gray deposits (see Figure 8.22)
are found in areas protected against rain and water run-off [CAM 82].
Figure 8.22.
Powdery gray deposits on a portal sculpture protected
from the rain in Reims cathedral, occidental facade
