P - Technische Universiteit Eindhoven

1. Introduction 13
1 2
initial mass
relative mass
3 4
Disintegration
5 6
cycles
Fig. 1.3: Results of a DIN 52 111 weathering test on a fired-clay brick. In the left graph,
the relative mass of the samples is given as a function of the number of cycles. The
corresponding pictures of one of the samples after each cycle are given at the right.
After the 6th cycle the brick sample of 5x5x5 cm has completely disintegrated.
1.4 Crystallization pressure
Salt weathering is caused by the crystallization of salts within the pores of a material. A
model of crystallization pressure, based on the idea of supersaturation of a salt solution,
was proposed by Correns in 1945 [41]. In this model a salt solution that is supersaturated
with respect to a growing crystal, is proposed as the driving force for crystallization
pressure, as schematically presented in figure 1.4. Hence, the Correns model represents
the crystallization pressure P c as a function of the supersaturation:
P c = NRT
V m
ln
( c
c s
)
, (1.1)
here R is the gas constant, T is the temperature, N is the number of ions in a salt molecule,
V m is the molar volume of the crystalline phase, c/c s is the ratio between the actual and
the saturation concentration of the salt solution. When a hydrated crystalline phase is
formed, i.e., together with salt nuclei also water molecules are incorporated in a crystal,
the effect of hydration has to be taken into account. The incorporation of water molecules
from a solution into a hydrated crystal yields an increase of the actual concentration of
the solution, leading to higher crystallization pressures. Taking into account the effect of
hydration, the extended Correns model can be written in the form [42]:
P c = RT
V m
ln
[ ( c
c s
) N
( 1 − (N − 1)c
1 − (N − 1)c s
(1 + cM H2 O)
(1 + c s M H2 O)
) ν ]
, (1.2)