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42 Vth International Brick Masonry Conference
11-2a. Trattamento di Elementi in Laterizio con Resine
Siliconiche Prof. Ing. Paolo Berti
Venice
ABSTRACT
The study of the aggressive salts has been very important with
reference to the walling restoration techniques, it is particularly
important to underline the effects they produce on the brickwork,
on the ways of recognizing them and on the laboratory methods used
to reduce and prevent the damages provoked by them.
Trattamento di elementi componenti le murature (mattoni ecc.) al
fine di aumentare le caratteristiche relative alle resistenze
meccaniche e all'aggressione agli agenti inquinanti. Esperienze su
materiali componenti le murature al fine di prevenire attivazioni
di sali solubili dovute a risalita di umidità caPillare ottenuta
mediante impreg-nazione solto vuoto o non con resine siliconiche in
solventi bassobollenti.
Esperienze in murature vecchie per il fissaggio dei maggiori
gruppi salini, solfati, nitrati, cloruri responsabili di
sfaldamenti; bloccaggio di caPillarità di risalita mediante
impregnazione degli elementi di murature con resine polimerizzanti;
esperienze con resine a catena molecolari variabile.
Building materiais , in particular the burnt day elements
contain different kinds of salts. Although most of them are water
soluble, only some of them, at their natural state, are
hygroscopic. This means that, while ali salts have the property to
absorb different quantities of water, only the hygroscopic ones are
able to absorb the humidity content of the atmosphere. This is very
important with reference to the restoration of stones and
brickworks, because, where hygroscopic salts are not present, it is
possible to create a barrier in order to avoid the water absorption
by the non-hygroscopic salts, allowing them to release water to the
atmosphere. Where hygroscopic salts are present, the above
mentioned damp proof courses are insufficient, as these transmit
the absorbed vapour and converted into water to the non hygroscopic
salts, therefore maintaining them in solution.
Brickworks are subject to two kinds of aggressions: the first,
due to the externai atmospheric condition and there-fore depending
particularly to the pollution; the second, which generally shows
itself after longer periods of time , depends upon the absorption
by water rising due to cap-illarity action, which carries with its
solution salts and actives those already present. Moisture present
in the brickwork is in continuous movement.
In Kieslinger's opinion, there are two different kinds of salts'
movements into the water: a vertical rising movement and an
expansion lllovement towards the externai sur-faces.
Therefore, the salts are carried from the ground into the
brickwork not adequately isolated and finally to the externai
surfaces. The salts which diffuse tend to expend even on very large
surfaces. The diffusion is anyway influ-enced by the capillarity
action, with the result that the upper area of the damaged material
shows more salts con-centration, with consequent humidity. Where
the materiais have a restricted capillarity action , as for
instance with very compact stones, the salts are spreaded out more
evenly. This shows why sometimes, in brickwork at ground levei, the
base moulding is dry, while above there are traces of damp.
Damages due to the aggression of the atmospheric and pollution
agents
I) The carbon dioxide (C02), dissolved in rainwater, react with
the caleite of the building material forming soluble products and
so leacted by the water, the abrasive index shows down and with
time, crushes the building material.
2) Athe AISO sulphate dioxide (S02) which now pres-ent in the
urban atmosphere, transforms even the calei te in hydrate piaste r
(CaS04.ZH20) which is much more soluble in water than the before
men-tioned salt.
3) It has been demonstrated that the nitrogen oxides too have
negative aspects, but their reactions with building materiais are
still unknown. It seems that by the intervent of nitric bacteriums
transforms to nitrate sodium and potassium (NaKn03).
4) Sodium cio ri de (Na CL), particularly in areas near the sea,
coming from the air, forms sodium sulphate (NA2 S04), which is
corrosive and highly hygroscopic.
5) The salts multiply the ionic concentration of the sur-face
and allow the eventual emorescences to be much more soluble than
they are normally.
6) A lot of non-hygroscopic salts become hygroscopic when
combined with other hygroscopic salts; this is the case of
chlorides in presence of sulphates.
7) Water infiltration in the porosity and the subsequent
freezing leads to superficial crushing and a reduction in the
mechan ical characteristics. The result of the combined action of
ali these aggressive agents brings to a washing out of the binder
mineral leaving the underlying stone exposed to more and new
attacks.
Harms to huildings caused hy rising capillary damp
I) Visual damage caused by more or less extended areas of
intense humidity which holds the dirt.
2) Damages due to loss of heat insulation, for which larger
consumes of energy are necessary to keep ini-tial conditions.
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Session 11, Paper 2a, TraUarnento di Elernenti in Laterizio con
Resine Siliconiche 43
3) Damage caused by hygienic nature, for which the intense
humidity helps the growth of moulds with a subsequent diffusion of
spores and* of micotoxines in the environment. In most cases there
can be a slow crushing of the building material.
4) Damage caused by the bursting action of a few hydro-soluble
salts which are present in the building material, are brought to
the surface and destroy the porons structure of the building
material. These are always the more damaging.
In the impossibility to treat a li the salts prescnt in walls,
three important groups shall be considered: sulphates, nitrates and
chlorides.
Sulphates
It's the most common and important group. Sulphates, as we know,
are lhe salts of sulph irid acids (H2 S04)' Since 6% of earth's
crust is composed of sulphates, they are found frequently in their
natural state in ali types of basic building materiais.
These sulphates in their latent state are mostly hygro-scopic
and can absorb large quantities of water. The salts which are more
commonly found in tile walls are the mag-nesium sulphates (which
al-e also the most commonly pres-ent on the surface) followed by
sodium and calcium sul-phates which, having less tendency to move,
remain in the interior of the material. Most or the damages in the
walls are caused by the sulphates of the sulphinic acid plus the
carbonic acid which is a lso found frequently in the atmo-sphere,
are not of damage because its salts, the carbonates, are neither
hygroscopic nor soluble in water. A lot of sul-phates are so
soluble that at normal temperatures they are in a constam state of
solution and successive crysta llization. The subsequent difference
of volume leads to a variation of internai tension of the harmed
material which brings to a material crushing. As an example of the
volume com-binement degree which can occur, we can say that the
sodium can absorb u p to 10 water molecules , when relative
atmospheric humielity is of 75% and so expend its volume by
40%.
We must not forget that sulphates in soluble state, rap-idly
reach the poim of superficia l conelensation, which is not
necessaril y idemical to the one on the externai surface. This can
also be found under the surface. If the point of condensation is in
the externa i surface of the plaster, the crushing of the surface
wi ll occur. Ir the poim is under the surface , the destruction of
the material will occur, par-ticularly in areas subject to
freezing. Taking into account that it is not possible to
immediately carry out laboratory tests, the knowledge of the
sulphates and the texture of the materiais can help to identify the
sulphales . Their con-stam changements of a volume and the varying
in pres-sllre of the damaged materiais brings to a varying of the
internai tensions , as already saiel. The lypical damages of the
sulphates are:
a) Corrosion of lhe surface in the form of "sand" or crushing of
the material , raising to the paiming from lhe surface, eleLachment
of Lhe plaster from the \Vali.
b) Destruction of the harmeel material , always con-nected to a
surface corrosion.
lt's important to remember that the harmed areas do not
necessarily show the signs of humidity, beca use in the case of su
lphates the water is immeeliately given to the atmo-sphere
(evaporation).
Nitrates
Nitl-ates, contraelicting what anyone can think, are the rarest
salts present in walls, unless there are organic eleposits nearby.
A lot of nitrates are hygroscopic, but an important exception is
the potassium nitrate. A few nitrates contains water of
crystallization when precipitated from water solution , in fact,
one or more molecules of water forms part of the crystalline
structure of the soliel material. The nitrates of the nitric aciel
can be found underground, mainly in the form of "Salpetre" K 0 :1
anel
aNoa, in big quamities. They are the most soluble ones of a li
metal salts. Buileling materiais , which are found in areas in
which these sa lts are present, after conta ins latent nitrates.
The use of nitrous (HN02) and nitric acids (HNO:1) in fertilizers
can cause atmosphere pollution. Nitrates or nitric acid (which
become nitrates) containeel in globes of vapour, penetrate in the
externai surfaces of the walls, after the vapour condensation has
taken place. Ir must be pointed out that this kind of contamination
occur particularly in rural areas . The presence of nitrates in
Venice is possibly due to the primitive sanitary solutions of the
ancient anel mielelle ages . The effects of the nitrates,
particularly the calcium ones, are very harmful. The cal-cium
nitrates, being able to absorb large amoums of water and to convert
the vapour imo water and to crystallize from their soluble state at
25°C to a relative atmospheric humidity of about 50%, these
conditions occur often in Ita ly. lt is not only the changing of
volume that causes damage, a lthough is very big, bllt it is a lso
the capacity of storing Lhe water which makes the damaged material
vlll-nerable to freezing. I t must be underlineel that Lhe
exces-sive amollnt of water that can be absorbeel by nitrates
brings to the saturation of other salts and the action of latem
nitrates. The presence of nitrates in the walls can not be
demonstrated without tests in laboratory. Anyway, as for the su
lphates, some damages can be considereel inelicative of its
presence of nitrates. The most importam sign is the concentration
of active nitl-ates in the form of small strips, which vary from
lOto 50 cm of width , which sometÍmes crosses throllghout the
building. Ir can also OCCLll- in single areas which are rarely
large. In any way, the damage caused is serious and the malerial is
destroyeel. The area underneath the one elamageel by the nitrates
is generally elry, anel sol iel if sll lphates anel ch lorides are
not present. It is most importam to be careflll of the area above
the damageel a l'ea, it " 'ill seem shacloweel beca use of
nitl"aLeS anel oLher salts reacting the poim of reactiol1.
Chlorides
These sa lts are mainly founel in areas near the coast and in
lagoon areas. In these areas the chlorides are to be fOllnel in
latem form in raw materiais. It's interesting to
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44
note that the damages, in its natural state, are not
hygro-scopic, but they become so when they combine with other
salts, especially sulphates. For example, sodium chloride. (N ACL)
at its natural state is not hygroscopic and does not absorb vapor.
When it is hygroscopic it can absorb large quantities of water and
vapor. lt crystall izes at 25°C with an atmospheric humidity quite
low (abou t 30%), at this point it rapidly gets r id of humidity.
Since the atmo-spheric humid ity is rarely so low , the damages
caused by cha nges of volume are rarely very big. T he main damage
is caused by large quantities of water in the wa lls which can be
used to cause the reaction of orher sa lts and pro-d uce harmfu l
effects on people li ving in such environ-ments. The absence of
evident damages in materiaIs that have a high percentage of
humidity, but in wh ich ris ing damp does not exist, indicates the
p resence of chlorides. When the chlorides crystallize, crystals
are visible and taste salty. They look like needles and are severa
I centimeters long. After having considered the most important salt
grou p and lhe effects they have on the brickwork, we list the
various experiments and results both in laboratory and in the
field. We can draw the followi ng conelusions on- the way how the
brickwork should be treated so as to resolve the problem. We worked
on buildings with very moist wall s and we saw that the effects of
the salts continued a lso after having stopped the capillary water
with chemical and mechanica l systems. So it was decided to study
the behavior of the hygroscopical salts in re lation to the
con-tempt of h umid ity in the air . The main cause is then water
which can eitheract as a transporteI' or as a salt activator, like
in the case of atmospheric phenomene. lt seems that to stop these
phenomene it would be su fficient to preel ude the passage of the
water into the build ing material by means of su rface treatment
and a horizontal blocking . It is possible to el iminate or reduce
the absorption of water although keeping the stmcture of the walls
intact, by using a silicone tI-eatment. The si licone treated
surfaces gain a strong water-repell ence keeping intact their
original char-acteristics of permeabi lity to a i r, to aqueous
vapor and, to the contrary of resine reinforcements, it does not
modify the specific weight and the coefficient of exp lanation.
Some compounds of silicone and other natured ones have been stud
ied to determine their fitness to be used either for the surface
treatments 01' to impregnate the horizontal blockings. The method
used to test is the "explosion test" (DIN 521 11 ASTM .. ), for th
is ti les, cement and sand were taken. The samples were chosen in
such a way that the various series were the more humugeneous as
possible. Four of these cubes were completely immerged for one minu
te in lhe various tested compounds. T hese samples were left to
rest for three weeks , to allow lhe possibility for lhe products to
complete their hardening. The cubes were finally put for 16 hours
in a saturated solution of sodium sulphate (Na2S04) of which depth
or I cm and temperature of 21°C, then they were dried in an oven
for four hours at IIO°C and subsequently cooled in dessicator for
four hours. The operation called "change" was then repeated on
another face of the cube. The effect of the explosion is based on
the characteristic or sodium sulphate to crysta ll ize up to 32,7°C
with 10 molecules or water, but
Vth International Brick Masonry Conference
at higher temperatures in anhydrous formo lf anydrous salt
becomes in contact with water, it crystall izes and largely
increases its volume form ing the decaidrated sait Na2S04 + H20).
This increase of volume provoques such strain in the inter ior of
the stone to cause the crumbling of the stone itself Similar
crystall izations happen in nature itse lf The little cubes were
treated with the fo llowing ele-ments to impregnate and various
samples were prepared for every treatment because, even if two
stones were taken from the same block , they cou ld give d ifferent
results, an untreated sample was added to ali the series of tests .
Q ual-itative and quantitative valuations can not be in any way
valid, beca use the heterogenety of the samples, the num-ber of
changes necessary to break the sample are repeated periodica ll y
and on ly with marginal differences.
N° I ) With number I it was always marked the intreated cu be.
The untreated cubes were always the first to crack and it takes 3
to 5 cyeles for the ti les and about 10 for the sand, the cement
has in terme-diate values.
N° 2) The samples marked wi th this number were impregnated with
methylsilicate of sodium. The water-repellent effect is created
after a chemical reaction with the carbon dioxide of the ai r,
lead-ing to the formation of a silicone resine accord-ing to the fo
ll owing schema:
OH I
2CH" - Si - ONa + CO2 + H20 --> I
OH
OH I
nxCH" - Si - O H - n H20-----> I
OH
OH I
2CH, - Si - OH + Na2CO" I
OH
OH I
OH I
CH" - Si - O - Si - CH " I I O O I I
HO - Si - O - Si - OH I I
CH" CH"
The groups Si - O - Si are very similar to the qua rtz like
stmcture or·the building materia l and to the methylic radica Is
(CH,,) 'which are respon-sible for the wa ter-repellence and make
for the outside. The presence of sodium makes the resistence to the
crystallization worse and has a tendency to give a whitish color
more the potas-sium silicate. The samples explode afrer about ten
cyeles for the tiles and about twenty cyles for sand, cement
remains an intermediate between the two_
N° :~) Putassium methylsilicate was put on these cubes ,
compared LO sodium salts it has several advan-tages: it has more
resistance to freezing a nd it forms potassiul1l carbonate which is
be tter than sodium during crystallization; it a lsu enables the
samples to resist more cyeles.
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Session lI, Papa 2a, TmUarnento di Elernenti in Lalerizio con
Reúne Siliconiche 45
N° 4) Potassium propylsilicate was applied to these cubes
(-C:sH, instead of CHa). It demonstrated a higher resistance to the
alca lies and a more precious \Vater-repellent effect. It
represents a small improvement with respect to the treatment N° 3,
mainly on the cement sample.
N° 5) "silano metossi funzionale" was applied to these cubes.
Its advamage consists in the possibilities of reacting with
building material, with good infil-tration depth of the molecular
compounds. The cubes of tile explode arter 15- 16 cycles, the cubes
of cement after 20 cycles and the sand ones after 24- 25
cycles.
N° 6) These cubes \Vere treated with a silicone resine which has
a law poim of fusion (P.F.) In practice the silicone resines have
the advantage to not generale reactionate sa lts , and to be able
to be applied several times and resist to atmospheric
precipitations as soon as applied. The resines with a law poim of
fusion P.F. have the incon-venience of becoming soft; and sticky on
the sur-face IVhen it's very hot, and subsequently dirt sticks to
the surface; even the alcal ine stability in this resine results to
be not very good. The resis-tance to the cycles of sand results to
be better than in the test N° 5. On the contrary, the tiles and
cement have reached 20- 21 cycles.
N° 7) These cubes IVere treated with a non-sticky sili-cone
resine which is especially treated to resist strongly alcalies and
has a high capacity of infil-tration. In other laboratory tests
this resine showed a ve l'y good resistance to UV rays. Even though
with the "explosion test" it is not possible to find many
differences from this resine to the others, practical tests of
application make this res-ine preferable to any of the others. With
this res-ine, similarly to the previous one, the results
obtained on the three materiais tend to become uniform and
nearly reach 30 cycles.
N° 8) These cubes IVere treated wirh a stone strength-ening
basely macle from extract of "acido silicico e
metiltrietossisilano" which is distinguished by a high capacity of
penetration thanks to the pres-ence of "metiltI·ietossisilano" this
product solidi-fies and renders water-repellent the treated
material with a reaction which lasts 20 days. This filler underput
to the "explosion test" gives some I'esu lls similar to treatment
N° 7, but we must not forget that the function of lhis produCl is
also to solidify the mineral Slructure, this makes it par-ticu
larly fit to be used on constructions already deteriorated, orher
fillers do not have lhis effecl.
N° 9) These cubes were treated IVith barium hydroxide Ba OH 2
because bariulll has the property of tak-ing lhe place of calcium
in the calcite and chalk or to sodium and potassiulll in their
respective sulphates, forming some carbonates and su l-phates of
barium or even compounds of carbon-ales and calciulll barium
sulphates less soluble. After a few cycles the cubes explode and
appar-ently cio not gain any sensible improvement com-pareci with
lhe test not treatecl.
\Vilh the materiais of the tests 6, 7, 8 \Vere impregnatecl
brickwalls, they presented strong phenolllena of destroy-ment
caused by sa line aggressions of the three groups combinecl, they
also had been treatecl to prevent the rising clamp. The results,
after 3 or 4 months seem to confirm the laboratory tests in the way
rhat the efflorescenzes of slats arter some clays stoppecl and till
today cio not give eviclent signs. This seellls lO confirm the
choices taken, anyway, funher researches and tests are taking place
to g ive a definitive anel sure ans\Ver lO this question which is
so important for the preservation of the artistic and cu ltura l
esta te.
