Page 3 Inside Materials - Newsletter Delft Centre for Materials - November 2013
By Sanne Granneman,
Barbara Lubelli,
and Rob P.J. van Hees
Architectural Engineering and
Technology,
A&BE, TU Delft
The weathering of porous materi-
als due to the presence of salts
has been known since antiquity.
However, serious studies and
investigations were not conducted
until the 19th century. One of the
fi elds of interest includes historic
masonry, where salt damage can
affect natural stone, brick and
mortar (see Figure 1). Mortars
are building materials, which are
for example used in masonry, as
the bonding material in between
bricks or stones or as a plaster or
render. Because of its pore size
distribution (with both coarse
as fi ne pores) and its relatively
low mechanical strength, mortar
is one of the building materials
most affected by salt crystalliza-
tion damage. As a consequence,
replacement of plasters, ren-
ders and pointing mortars often
constitutes a large part of total
restoration costs. Salts present in
masonry can originate, amongst
others, from sea salt spray, rising
damp, road salt, salt storage and
the building materials itself.
The damage caused by salt crystalliza-
tion is closely related to the location
of crystallization. Crystallization at the
ENHANCING SELF-HEALING OF LIME MORTARS
BY BUILT-IN CRYSTALLIZATION INHIBITORS
Figure 1. Salt damage in plaster (Church in de Rijp, the Netherlands) and pointing mortar (building in Oostkapelle, the Netherlands)
be added to a fresh mortar, which can
subsequently be used for restoration or
renovation. Then, when salts penetrate
the materials, they will cause less or
no damage at all.
Amongst the most damaging salts for
building materials are sodium chlo-
ride and sodium sulfate. The initial
phase of the research was directed
towards fi nding suitable modifi ers for
these salts. Sodium chloride is a salt,
which has been already extensively
researched and one of the effective
modifi ers, that has been identifi ed is
sodium ferrocyanide. This molecule
not only inhibits the growth of sodium
chloride but also modifi es the crystal
habit from cubic to dendritic. Previous
laboratory tests show that instead of
forming a layer with cubic shaped crys-
tals, the sodium chloride crystals form
feather-like crystals, which only poorly
adhere to the pore surface (see Figure
2) [2]. In the case of sodium sulfate,
phosphonates are often used as modi-
fi ers. However, these molecules only
work at a specifi c pH range. Due to the
high pH difference between fresh (pH
13) and hardened lime mortar (pH 9)
it would be better to have a modifi er,
which is not pH sensitive. A possible
molecule could be borax (sodium tet-
raborate). This molecule does not have
a pH dependency, but might encoun-
ter other limitations such as complex
formation with other compounds in the
mortar [3].
The research currently carried out >>
surface of a material (effl orescence) is
relatively harmless. However, crystal-
lization inside the material (crypto- or
sub-fl orescence) can lead to severe
damage. The damage is attributed to
crystallization pressure. When a crystal
grows in a confi ned space such as a
pore, it will eventually have no more
room to grow. However, usually there
is still salt in solution present. When
water from this solution evaporates, a
supersaturated solution will form. The
combination of crystal and super-
saturated solution results in a pres-
sure against the pore wall. Eventually
the pressure overcomes the tensile
strength of the material and cracking
and crumbling will occur [1].
Even though much research has been
undertaken, no defi nitive solution to
prevent salt crystallization damage has
been found yet. Recently research has
started to examine the use of crystalli-
zation modifi ers. These molecules alter
the crystallization behaviour. They can
prevent or delay nucleation (inhibi-
tors), promote nucleation (promoter)
and/or adsorb onto specifi c crystal fac-
es, thereby changing the crystal mor-
phology (habit modifi er). The positive
effects of modifying the crystallization
behaviour can be lowering the super-
saturation, or promoting the formation
of effl orescences instead of crypto-
fl orescences [1,2]. One of the aims of
the PhD research of the fi rst author
is to develop a better salt resistant
repair mortar, which already contains
an inhibitor. The molecules will have to
