P - Technische Universiteit Eindhoven

1. Introduction 15
However, our present knowledge about crystallization pressure does not provide a
complete picture of damage by salt weathering. Materials with different poromechanical
properties react differently on a given crystallization pressure. For one material it can
lead to damage, whereas for another material no damage occurs. Various mechanical
models focus on the effects caused by crystallization pressure, e.g., expansion, viscoelastic
relaxation (damping), cracks, and disintegration (see figure 1.5), related to poromechanical
properties of porous materials [30, 40, 42, 45]. In such models the overall reaction of a
material is called the thermoporomechanical pressure, P m , meaning that it includes both
the thermal and poromechanical reaction of a material. Hence, in general P m can be
written as [42]:
∫
P m ∼ C fill [dε(∆P ) − αdT ] , (1.5)
where C fill is a parameter that depends on the pore filling, ε is the expansion caused by the
crystallization pressure ∆P , α is thermal expansion coefficient and T is the temperature.
The term αdT is responsible for the part ’thermo-’ in the word ’thermoporomechanical’,
whereas the poromechanical properties of a material are hidden in the functional dependence
of the expansion, ε(∆P ), on the crystallization pressure, ∆P . Using this approach,
the actual poromechanical pressure P m occurring in a porous material is a function of
both the internal properties of a material and the effects of crystallization pressure (type
of salt, supersaturation, etc.).
crack
crystal
P cryst
solution
pore wall
viscoelastic damping
P m
Fig. 1.5: Schematic representation of the relation between poromechanical pressure (reaction of
a material) and crystallization pressure.
1.5 Scope of the thesis
The effects of weathering become apparent with time, i.e., with accumulation of weathering
(crystal growth/dissolution) cycles. To understand weathering and damage mechanisms,
the knowledge about crystallization pathways is highly important and required.
The weathering conditions define a crystallization pathway. On the other hand, the
properties of crystalline phase, being formed within these pathways, determine the crystallization
pressure. Finally, damage is a reaction of porous materials on such a pressure.
Therefore, the problem of understanding weathering effects involves three parts: