Environmental Engineering Reference
In-Depth Information
very rarely on monuments. If the water content is low, stresses generated by frost
can easily be annihilated because water that is not yet transformed into ice can easily
circulate into the highly permeable porous network and ice can easily expand in the
macropores. Sireuil limestone (
N
t
= 30%,
Ka
= 1,000 mD), for instance, can resist
-8°C frost if not saturated with water.
-
Class 4: microporous stones
. In these stones the average pore radius, access to
which is lower than 1 µm, the ice formation temperature is lower than 0°C.
Letavernier [LET 84] has experimentally shown, for instance, that water contained
in pores with an access radius lower than 0.125 µm freezes at temperatures lower
than -3°C. For instance, the freezing threshold of Comblanchien stone (
r
< 0.02 µm)
is -25°C.
Letavernier established an abacus allowing the determination of conditions in
which limestone becomes sensitive to frost. Four parameters are taken into account:
porosity
N
t
, saturation coefficient
S
, capillary water penetration coefficient
B
and
continuity index
IC
calculated on the basis of ultrasonic speed measurements.
8.3.2
. Salts
Salts are the most important deterioration agents for stones and mortars.
8.3.2.1.
Nature and origin of soluble salts
Among the numerous natural and synthetic salts, only 20 to 30 are frequently
found in buildings [ARN 95, FAS 96]. Their name and chemical formula are given
in Table 8.4.
These salts can be found as ions in water, or in crystalline form in efflorescences
and crusts. The saline systems are constituted of five cations (Ca
2+
, Mg
2+
, Na
+
, K
+
,
NH
4
+
) and five anions (SO
4
=
, Cl
-
, CO
3
=
, NO
3
-
,
C
2
O
5
=
).
The nature of the salts present in a system sometimes gives indications on their
origin (see Table 8.5). For instance, ettringite and thaumasite are known to be
associated with Portland cement and concrete. Sodium carbonate indicates the
presence of an alkaline material (cement, concrete, sodium silicate). Oxalates
(whewellite and weddellite) and nitrates often have a microbiological origin.
Gypsum is often associated with atmospheric pollution, but may also originate from
the mobilization of calcium sulfate-containing building materials (gypsum
containing renders, plasters and joints).
