Environmental Engineering Reference
In-Depth Information
Butlin
et al.
[BUT 92] even propose a dose-response function for Portland limestone
where NO
2
levels behave as a limiting factor in the annual weight loss of the stone:
( )
[
]
[
]
[ ]
0.0026 H
+
DP %
=+
0.08
0.01 SO
−
0.00012 NO
+
0.00016 rain
+
2
2
where ∆P is the weight loss, [SO
2
] and [NO
2
] the average annual concentrations in
the atmosphere expressed in µg/m
3
, “rain” is the annual rainfall in mm and (H
+
) the
quantity of H
+
ions on the surface considered, expressed in mg/m
2
.
Other more recent studies try to
evaluate the impact of nitrogen compounds
originating from agriculture on monuments, notably in French Brittany, a region
particularly affected by nitrate pollution [CAD 96]. This impact seems to be indirect,
through the change in ecosystems on monument surfaces (lichens, particularly).
Nitrogen compounds are also considered to be responsible for an increase in
colonization of monuments by algae, leading to the development of green veils on
recently constructed or cleaned buildings.
8.3.3.1.2. Impact of sulfur components
The influence of sulfur components has been well studied for calcite-containing
stones, which are particularly sensitive to sulfur dioxide (SO
2
). The influences fall
into two categories: “dry” and “wet” deposits.
Wet deposits form on any surfaces exposed to SO
2
, which reacts with calcite in
presence of water vapor:
SO
+
CaCO
+
2H O
+
0.5O
→
CaSO , 2H O
+
CO
2
3
2
2
4
2
2
In areas exposed to rain, gypsum formed through this reaction enters a solution
during the following rain events. It is then displaced by water run-off, either
eliminated or transferred to other surfaces of the building, where it may accumulate.
In sheltered areas, gypsum is not eliminated and contributes to the formation of
crusts.
Furlan and Girardet [FUR 91] have studied the kinetics of dry sulfur deposit on a
number of lithotypes exposed in many places in the world, on sites sheltered from
rain and water run-off. They show that in 1975, the quantity of dry sulfur deposited
on a porous calcite-rich sandstone per square meter ranged from 0.19 g/m
2
in Saint-
Rémi-lès-Chevreuse close to Paris, France, to 3.14 g/m
2
in Milan, Italy (see Figure
8.20).
