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TEOSЦcolloidal silicaЦPDMS-OH hybrid formulation used for stone consolidation.

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Full Paper
Received: 21 August 2009
Revised: 16 February 2010
Accepted: 17 February 2010
Published online in Wiley Interscience: 13 April 2010
(www.interscience.com) DOI 10.1002/aoc.1646
TEOS?colloidal silica?PDMS-OH hybrid
formulation used for stone consolidation
Carmen Salazar-Herna?ndeza, Mar韆 Jesu?s Puy Alquizab , Patricia Salgadoa
and Jorge Cervantesa?
The consolidation of materials concept, which consists of introducing a chemical substance (consolidant) into degraded stone,
has been applied to architectural conservation. Silicon compounds such as tetraethoxysilane (TEOS) are frequently used as a
base for commercial consolidant formulations due to their ability to form a siloxane polymer such as SiO2 . However, the silica
xerogels deposited into the stone show poor performance and the gels obtained are non-porous and tend to crack during the
drying stage. In order to avoid the fractures and to improve gel properties, we propose the synthesis of a hybrid consolidant
based on TEOS and fillers such as colloidal silica (200 nm in diameter) and hydroxy-terminated polydimethylsiloxane (PDMS-OH).
Both additives enhance gel properties such as porosity and elasticity, leading to the formation of non-fractured and permeable
gels. Characterization of the hybrid xerogel was carried out by nitrogen adsorption and 29 Si MAS-NMR. The properties of the
hybrid xerogels were compared with those prepared from a formulation based on TEOS (T-ME) with a composition similar to a
commercial product. In order to evaluate the effectiveness of the hybrid consolidant, it was applied to tuff-stone of historical
monuments in the city of Guanajuato, Mexico. The tuff-stone was also treated with the formulation T-ME. Both treatments were
studied by determining the percentage of consolidant deposited, evaluating changes in porosity and hardness of the treated
c 2010 John Wiley &
stone. The applicability of the hybrid consolidant for the decayed tuff-stone is under study. Copyright Sons, Ltd.
Keywords: hybrid consolidant; TEOS; colloidal silica; PDMS-OH; tuff-stone; consolidation
Introduction
Appl. Organometal. Chem. 2010, 24, 481?488
?
Correspondence to: Jorge Cervantes, Departamento de Qu韒ica, Universidad
de Guanajuato, Guanajuato, 36050, Mexico. E-mail: jauregi@quijote.ugto.mx
a Departamento de Qu韒ica, Universidad de Guanajuato, Guanajuato, Gto.
36050, Mexico
b Departamento de Minas, Metalurgia y Geolog韆, Universidad de Guanajuato,
Guanajuato, Mexico
c 2010 John Wiley & Sons, Ltd.
Copyright 481
A combination of several different degradative factors (natural
weathering, air pollution, etc.) cause the irreparable lost of
historical construction building stones. Monumental stone can be
preserved through consolidation, which consists of introducing a
chemical substance (consolidant) into the decay stone. The ideal
consolidant will restore the mechanical and chemical properties
of the rock.
Commercial consolidants containing alkoxysilanes such as
tetraethylorthosilicate (TEOS) are commonly used to solve the
aforementioned problems. These are low-viscosity fluids which
can penetrate deeply inside the porous network of the stone
and, through a sol?gel process, form a SiO2 gel that works as
a new cement matrix. This is done to re-establish the cohesion
between loosened mineral grains as well as the original mechanical
resistance in the deteriorated material.[1]
However, the effectiveness of these compounds as consolidants
depends on the properties of both the sol and the gel TEOS
phases. The penetration of TEOS into the stone is a function of
the microstructural characteristics of the rock. It also depends on
the viscosity and the surface tension of the TEOS sols. Moreover,
the physical properties of the gel determine the development of
strength and cohesion in the treated stone.[2] TEOS shows excellent
properties in the sol phase, but the gel properties are frequently
not those desired. The tendency of the gel to crack and shrink
during the drying stage is frequently observed.[3 ? 5] The cracking
occurs as a result of a negative stress (capillary pressure).[6,7] Other
important properties such as the thermal expansion coefficient
and porosity of the SiO2 gels deposited tend to be very different
than those corresponding to the stone.[2]
In order to optimize physical properties of TEOS gels, several
approaches have been explored. Scherer et al.[8,9] have proposed
the use of inorganic composites, particle-modified consolidants
(PMCs), obtained by loading TEOS-based consolidant with
different colloidal particles. PMC inclusion in TEOS gel increases
the elastic modulus while decreasing the thermal expansion
coefficient.[10] The gels obtained show a significant reduction
in fracture percentage. Moreover, Mosquera et al.[11,12] have
reported that TEOS?fumed-silica blends render more porous gels
with a reduced degree of shrinkage as compared with the commonly used TEOS-based formulations. More recently, the same
group proposed the use of a surfactant-template to control gel
porosity and diminish the capillary pressure generated during the
drying stage, which leads to gel crack.[13] The surfactant used as
template is a primary amine (n-octyl-amine), where the interaction
between silica and the template is via hydrogen bonding.[14] It is
thus possible to remove the surfactant by evaporation. Although
a small amount of the surfactant is used, the complete elimination
of the surfactant takes a considerable amount of time.
Moreover, synthesis of hybrid materials is very well known.
Hybrid materials are a separate group of materials with properties
of ceramics and organic polymers. The synthesis of hybrid
C. Salazar-Herna?ndez et al.
482
materials is based on the sol?gel process, and it is possible to
introduce organic chains into the inorganic silica skeleton.[15 ? 17]
The addition of organic materials such as organosilanes and
other polymers leads to the formation of an elastic silica network.
Because of these properties, hybrid materials are usually less
sensitive to cracking during the drying stage.
Recently, researchers have suggested the use of hybrid
compounds based on TEOS, polyhedral silsesquioxane (POSS)
and (3-glycidoxypropyl)trimethoxysilane (GPTMS), and also
TEOS, silica nanoparticles and GPTMS as stone consolidant. The
additives of POSS and silica nanoparticles provide a crack-free
gel.[18,19] The use of epoxy-silica polymer has been suggested
as stone consolidant.[20] Another approach to improving the gel
properties of stone consolidant is the incorporation of oligomeric
polydimethylsiloxane (PDMS) to TEOS. The use of ORMOSIL
(TEOS?PDMS) as stone consolidant has been suggested by
Wendler, who refers to it as elastified silicic acid ethyl ester.[21]
The application of this formulation to degrade stone resulted in
improved mechanical properties, and hydrophobic behavior was
also observed.
More recently, Mosquera et al.[22] obtained TEOS?PDMS gels
with controlled porosity using acid catalyst and n-octyl-amine.
PDMS leads to the formation of larger pore size than the gel
containing exclusively silica from TEOS. Then, the surfactant is
easily removed by simple air drying. On the other hand, in a
previous paper, we conducted a study in greater detail on the effect of hydroxyl-terminated polydimethylsiloxane (PDMS-OH) on
TEOS?PDMS-OH formulation using di-n-butyltin dilaurate (DBTL)
as catalyst.[23] DBTL is a well-known neutral polycondensation catalyst used widely in the formation of siloxane bonds (Si?O?Si).[24 ? 26]
We have suggested that the use of PDMS-OH as additive in TEOSDBTL sol?gel systems is a simple and effective way to obtain a more
elastic consolidant phase inside porous stones. Thus, cracking of
alkoxysilane-derived films, one of the most frequently encountered
problems in stone consolidation can be successfully diminished
by the elastification of the rigid tridimensional SiO2 gels normally
obtained. More compliant films were produced in-vitro and in-situ
by adding a relatively small amount of linear segments of PDMSOH (5% w/w) to a typical TEOS-based formulation catalyzed with
DBTL. In addition, viscosity of the sol is practically unvaried after the
addition of PDMS-OH, so the ability to penetrate into the porous
network of the stone is virtually equivalent to commercial products.
In a recent study, we reported the effect of aprotic and protic
solvents on TEOS-xerogel properties when DBTL is used as
polycondensation catalyst,[27] and on the TEOS-DBTL gelation
process.[28] We suggested that TEOS xerogel properties are
improved when a mixture of methylethyl ketone (MEK) and
ethanol are used as solvents. Additionally, Zarraga et al.[29]
reported that the effectiveness of TEOS-based consolidants for
siliceous stone treatments is enhanced when the formulation is
prepared in a MEK?acetone mixture.
In this paper, we propose the use of hybrid materials based on
TEOS and fillers such as colloidal silica (200 nm in diameter) and
PDMS-OH as a new stone consolidant formulation (TEOS?SiO2 ST?PDMS-OH). The hybrid consolidants were characterized using
several different techniques. Textural properties of the hybrid
xerogels were studied through nitrogen adsorption and compared
with TEOS gels (obtained under similar synthesis conditions).
Textural properties for untreated stone samples used in the
formulation evaluation were also obtained. The incorporation
of PDMS-OH in the structure of the hybrid was observed by 29 Si
MAS-NMR analysis and complemented with FT-IR.
www.interscience.wiley.com/journal/aoc
Table 1. TEOS?SiO2 -ST?PDMS-OH hybrid consolidant
TEOS
H1-ME
H2-ME
Additive (% w/w)
Solvent (ml) per
3 g of TEOS
(% w/w)
SiO2 -ST
PDMS-OH
EtOH
MEK
75
50
5
30
20
20
1.5
1.5
1.5
1.5
The effectiveness of the hybrid consolidant proposed was studied by treating tuff-stone of historical monuments in the city of
Guanajuato, Mexico. The stone was characterized mineralogically
and petrographically. Consolidant performance was evaluated by
determining the amount of product absorbed and the changes
in porosity and hardness of the treated stone. To compare
treatment results, a TEOS-based formulation similar to those used
as commercial consolidants was applied to the same stone.
Experimental
Synthesis of TEOS?SiO2 -ST?PDMS-OH Hybrid Materials
The colloidal silica used for the preparation of hybrid materials
was obtained by a variation of the Sto?ber method (SiO2 -ST), as was
previously reported.[27] TEOS (Fluka, 98% purity) was used to prepare TEOS sols with different amounts of colloidal silica. A mixture
of MEK?ethanol (50 : 50% w/v), was used as solvents. All solvents
were analytical grade. In order to start the polycondensation reactions, a 1 : 2:0.03 molar ratio of TEOS?H2 O?DBTL was used. DBTL
(Gelest) was used as polycondensation catalyst. As a pre-hydrolysis
step, the TEOS?SiO2 -ST sols were placed in a round-bottomed flask
with a magnetic stirrer for about two hours at room temperature.
Then, PDMS-OH (Gelest, MW 500?700 Da and OH 4.5?7.5%) was
added, continuing agitation until a homogeneous solution was
obtained. The amounts of TEOS and additives (SiO2 -ST, PDMS-OH)
in the hybrid consolidant were calculated assuming the complete
hydrolysis/polycondensation of TEOS (see Table 1). As can be
seen, two hybrid formulations (H1-ME, H2-ME) with different TEOS
and SiO2 -ST percentages were prepared. The hybrid xerogels were
obtained by pouring the sols into plastic tubes (50 : 10 mm L/D)
closed with a hollowed cover to permit the evaporation of the
solvents at a slow rate. After 4 days, all samples were gelled. The
samples were then allowed to dry under ambient conditions for
2?3 weeks until no changes in weight were detected.
Characterization of TEOS?SiO2 -ST?PDMS-OH Hybrid Consolidant
Textural properties
Textural properties were evaluated via N2 adsorption-desorption
isotherms at 77 K in a Micromeritics ASAP-2010 instrument.
Samples were degassed overnight at 180 ? C and 7 祄Hg prior
to measurements. The surface area was calculated using the BET
method, and the average pore diameter applying the BJH method
to the desorption branch of the isotherm.
29 Si MAS-NMR analysis
The incorporation of PDMS-OH to the inorganic silica fragment
was investigated via 29 Si MAS-NMR using a 300 MHz Varian Unity
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 481?488
TEOS?colloidal silica?PDMS-OH hybrid formulation
Plus spectrometer operating at 59.58 Hz. A 7 mm silicon nitride
rotor with kelF caps was used. The rotor spin rate was 4 kHz, with
delay time of 30 s accumulating 2000 transients.
Table 2. Sol and gel properties of TEOS?SiO2 -ST?PDMS-OH hybrid
Sol properties
Viscosity,
ABET
? (25 ? C, m Gel time
surface
?
Pa s)
(25 C, h) area (m2 g?1 )
Effectiveness of the Consolidation using the Hybrid Formulation
Tuff-stone characterization
XRD characterization ? tuff-stone of a historical monument in the
city of Guanajuato, Mexico was used to observe the effectiveness of
the hybrid consolidant. The monument was dedicated to General
Sostenes Rocha and constructed in 1955. The samples used were
mineralogically characterized by X-ray powder diffraction (XRD)
measured on a Siemens D-500 diffractometer equipped with Cu
tube X-ray and using Cu k? radiation.
Petrographic charaterization ? petrographic analysis on the tuff
stone without and with consolidant formulation H1-ME was carried
out using an Olympus BX41 petrographic microscope. For the
petrography study, the samples were cut into thin sections using
a circular rock cutter and were mounted on glass slides using
epoxy resin. The samples were ground and polished using carbide
grit until a thickness of 0.3 mm was obtained. The sections were
covered with glass slides and the texture and minerals of the stone
were characterized through microscopic analysis. Identification
and characterization of minerals were done using both plane and
cross polarized light.
Tuff-stone consolidation
Samples (2 cm) were treated with two different formulations,
the hybrid consolidant (TEOS?SiO2 -ST?PDMS-OH, H1-ME) and a
TEOS-based formulation (T-ME). The formulations were applied
on stone samples by brushing under laboratory conditions until
the surface remained wet for 1 min. Consolidant performance was
evaluated when no changes in sample weight were detected.
Consolidant evaluation
Absorption of consolidant ? several different tests were conducted
to evaluate the action of both hybrid consolidant (H1-ME) and the
similar commercial consolidant formulation (T-ME). The amount of
consolidant absorbed was determined by weight difference before
and after the stone treatment.[2] Moreover, the consolidation effect
was quantified by the changes in porosity and modification in
hardness of the treated stone. The first test was carried out by
determining water absorption, which was obtained according to
the procedure described in the RILEM II.8 method.[30]
Salts crystallization ? saturated sodium sulfate solution was used
for the salt crystallization test. The samples were exposed to cycles
of salt crystallization that consist of the stone is immersed in
a saturated salt solution for 24 h and then dried to induce the
crystallization of the salt into the porous stone.
Hardness changes ? the hardness of the tuff-stone was measured
using a Cole Parmer Digital Hardness Tester PHT-2500, in a nondestructive test. Using this procedure, it is possible to obtain the
hardness for the self-sample before and after the treatment.
Results and Discussion
Structure and Porosity of TEOS?SiO2 -ST?PDMS-OH Hybrid
Consolidant
Appl. Organometal. Chem. 2010, 24, 481?488
H1-ME
H2-ME
T-ME[26]
1.6
1.8
1.2
96
96
168
436.9
557.4
4.1
Pore
Pore
volume diameter
3
?1
(cm g )
(nm)
0.229
0.4296
0.0033
2.1
3.6
?
Figure 1. N2 adsorption-desorption isotherm (a) T-ME (b) H1-ME and
H2-ME.
PDMS-OH added to the hybrid formulations (H1-ME, H2-ME) was
20% (w/w). With the addition of this polysiloxane concentration, it
is possible to obtain monolithic gels and films without cracks. The
colloidal silica added (SiO2 -ST) was of 5 and 30% (w/w) for H1-ME
and H2-ME, respectively.
According to Table 2, low viscosity (? < 2 mPa s) was observed
for all sols studied, but different gel times were obtained. The
colloidal silica added to the hybrid consolidant slightly increased
sol viscosity but reduced gelation time from 7 to 4 days, as
compared with T-ME. As has been previously reported, the
rheological properties of the consolidant formulation can be
controlled with the amount of solvent added.[10] The viscosities
observed for the hybrid formulation sols were lower than that
observed for the commercial consolidant (2?3.3 mPa s[1] ), but
slightly higher than that observed for T-ME sols. Low viscosity
and gelation time of approximately 2 days are required for
a consolidant, and the hybrid formulations seem to meet
these requirements. It is important to remember that the
aforementioned properties allow deep penetration of the sol
and, therefore, the formation of the gel in the stone.[1] Thus,
the rheological properties of the hybrid formulation allow us to
consider it as a possible stone consolidant.
On the other hand, it is well established that one of the gel
properties for which optimization is suggested with regard to
stone consolidants is porosity.[11 ? 13,22] However, from the porosity
analysis of the xerogel of T-ME, similar to commercial consolidant
(see Table 2), and such materials can be considered non-porous.
The N2 adsorption?desorption isotherm for T-ME is shown in
Fig. 1(a). The addition of colloidal silica particles to the hybrid
xerogels (H1-ME and H2-ME) allows the formation of micro- and
mesoporous materials. Figure 1(b) shows N2 isotherms for these
materials. When 5% (w/w) of SiO2 -ST is used (H1-ME), microporous
materials are formed (isotherm type I). Adding as much as 30%
(w/w) (H2-ME) leads to the formation of mesoporous materials
(isotherm type IV) with an H2 hysteresis loop. According to results
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
483
The sol and gel properties for the hybrid consolidant formulations
(H1-ME, H2-ME) and those of T-ME[27] are shown in Table 2. The
Gel properties
C. Salazar-Herna?ndez et al.
Figure 2. 29 Si MAS NMR (a) T-ME (b) H1-ME.
Table 3. Percentage of Qn obtained for xerogels formulations
Qn (ppm)
T-ME
H1-ME
Q1
Q2
Q3
Q4
?
8.1
8.3
55.3
56.7
36
35
?
Figure 3. Consolidant films (a) T-ME (b) H1-ME (c) H2-ME.
observed, the colloidal silica used as filler in the synthesis of hybrid
consolidant leads to enhanced porosity.
Study of Incorporation of PDMS-OH in the Hybrid Structure
484
The PDMS-OH chemical-bond link to the inorganic silica network
was studied by 29 Si MAS-NMR. T-ME gels contained only SiO2
fragments, as shown in Fig. 2(a). Using the Qn notation,[31,32]
species Q2 (around ?91.2 ppm), Q3 (around ?100.4 ppm) and
Q4 (around ?108.8 ppm) were observed. On the other hand, the
hybrid xerogels (H1-ME, H2-ME) were formed by organic and
inorganic networks. Figure 2(b) illustrates the results observed
for H1-ME. The silica network formed in the new hybrid material
(H1-ME) contains species Q1 (around ?84.1 ppm), Q2 (around
?96.4 ppm) and Q3 (around ?102.1 ppm). As can be observed,
the major Qn intensity signal was Q3 for T-ME, and Q2 for H1-ME. In
Table 3, the percentage of Qn obtained for xerogels formulations
is presented.
On the other hand, the organic network corresponding to
the poly-dimethyl-siloxane fragment in the hybrid consolidant
was observed as D species (-[Si(CH3 )2 O]n -) around ?12 ppm.
The incorporation of PDMS-OH was studied also by FT-IR. The
characteristic signals for C?H and Si?C groups at 2934 and
1261 cm?1 respectively, were observed.
Because of the incorporation of PDMS chains into the silica
skeleton, the elasticity of the gel will be improved, which is
expected to prevent cracking.[21 ? 23] However, using the synthesis
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condition here indicated (percentage of catalyst, molar ratio
of hydrolysis water), a concentration of 20% (w/w) of PDMSOH was required to obtain gels and films free of fracture.
Figure 3 shows the films obtained in Petri dishes for the various
consolidant formulations studied. The films were prepared at
room temperature by pouring 1.5 ml of the formulations into Petri
dishes. A highly fractured film was obtained for T-ME (Fig. 3a), the
formulation similar to a commercial consolidant. Non-fractured
films were observed for the hybrid formulations (H2-ME and
H1-ME) as shown in Fig. (3b, c).
Maravelaki et al.[33,34] reported the use of commercial colloidal
silica (Ludox, SiO2 particles with diameter around 15 nm) as
consolidant, where the particles induced a significant change
in chromatic parameters of the samples treated. According to our
results, coloration and porosity in the hybrid consolidant depend
on the amount of colloidal silica contained into the structure.
H2-ME with a higher content of colloidal silica allows mesoporous
formation with a significant increase in porosity, but forming a
white-colored gel (Fig. 3b). Thus, the application of H2-ME in the
stone may change the visual appearance of the samples treated.
On the other hand, H1-ME with a lower concentration of colloidal
silica leads to formation of microporous gel with a slightly opaque
color (Fig. 3c). These properties suggest the possible use of H1-ME
as a stone consolidant.
From the results obtained related to porosity and PDMS-OH
incorporation, we suggest an idealized structure of the hybrid gel
(Fig. 4). The addition of colloidal silica can improve the porosity
of the gel, while PDMS-OH enables formation of an elastic gel,
preventing the problem of cracking.
Consolidation Effectiveness
Tuff-stone petrography analysis
The mineral crystallographic phases identified for the tuff-stone by
powder XRD were: quartz (SiO2 ), microcline (KAlSi3 O8 ), smectite
(Al,Mg)2 Si4 O10 (OH)2 , mica, talc [Mg3 Si4 O10 (OH)2 ] and hematite
(Fe2 O3 ). Carbonate phases were not observed. Moreover, the
petrographic study of thin sections of the samples indicated that
the rock exhibited characteristics of common phreato-magmatic
eruption products. In this study, samples composed of quartz
(Q) and plagioclases crystals (albite, NaAlSi3 O8 ) (Pg) were observed
(Fig. 5a). Biotite crystals (B) [K(Mg,Fe)3 (Al)Si3 O10 (OH,F)2 ] were also
detected (Fig. 5b). Finally, less than 0.2% of isometric opaque
minerals were observed. In different parts, the samples contained
many irregular shaped spaces or vesicles (v), which indicate
immediate deposition after eruption.[35] The textural and physical
stone properties indicated fragility.
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 481?488
TEOS?colloidal silica?PDMS-OH hybrid formulation
Figure 4. The idealized structure of the hybrid consolidant TEOS/colloidal silica/PDMS-OH.
Figure 5. Tuff-stone petrographic analysis (a) plagioclase and quartz (b) biotite (c) frature quartz (d) lithic tuff.
The tuffs have broken quartz crystals (Fig. 5c), diminishing the
microcrystalline groundmass. The medium-grained matrix was
observed from 0.25 to 2.0 mm. In addition, apparent oxidation of
iron is evidenced by the yellow-red colors along grain boundaries
and space infillings (Fig. 5b).
Microscope analysis confirmed the lithic tuff (L) grained between 2.0 and 16 mm (Fig. 5d). The lithics are generated by
moderate intensity of hydrothermal alteration; commonly, the plagioclase is transformed to albite (NaAlSi3 O8 ), which is replaced by
sericite [KAl5 (OH)2 AlSi3 O10 ] or illite [K0.8 ,Mg0.30 ,Al1.7 (Al0.6 Si13.4 O10 )
(OH)2 ].[36]
According to the XRD and petrographic results, apparent natural
weathering of the stone can be suggested. Two decay factors were
observed: clay minerals such as smectite were identified by XRD,
which could suggest an argillation process on silicate minerals;
in addition, petrographic analysis on tuff showed hydrothermal
activity on biotite crystals.
Consolidantion effectiveness
Appl. Organometal. Chem. 2010, 24, 481?488
Without treatment
T-ME
H-ME
Amount of
product
absorbed
(kg m?2 )
1.47 (0.2)
1.61 (0.3)
Hardness
(HB)
Hardness
increase
(%)
Wateraccessible
porosity
(%)
149 (3)
176 (2)
192 (2)
15
29
20 (2.1)
11.9 (0.54)
3.7 (0.21)
a Standard deviations are in parentheses. The number of samples was
three.
where a SiO2 film (dark film in Fig. 6a) is deposited on the original
cement matrix diminish the vesicles amount (v). Modifications
of biotite crystals (B) were not observed. However, significant
changes could be observed on the quartz crystals (Q) (Fig. 6b).
The quartz crystals (Q) were modified with a spherical structure,
possibly corresponding to the hybrid gel used as consolidant.
Consolidant absorption ? the various properties of the stone
sample before and after treatment with H1-ME and T-ME are shown
in Table 4. The amount of product consumed per surface unit under
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
485
Consolidate Tuff-stone petrography analysis ? Fig. 6 shows the
petrography analysis for the tuff-stone consolidate with H1-ME. An
increase in the cement matrix is suggested according to Fig. 6(a),
Table 4. Properties of stone after consolidationa
C. Salazar-Herna?ndez et al.
Figure 6. Tuff-stone petrographic analysis (a) after treatment with H1-ME (b) changes on quartz crystal using H1-ME as consolidant.
Figure 7. Product applied and absorbed by tuff-stone.
486
treatment (kg m?2 ) was obtained through the mass difference of
the samples before and after the consolidation process. These
values represent the amount of consolidant material polymerized
into the porous structure of the stone and are dependent on
the dimension surface under treatment and on the porosity of
the stone.[2] The tuff-stone porosity (water-accesible porosity)
determined was 20.2 � 2.5%.
According to Fig. 7, the amount of the consolidant applied to
stone was major for samples treated with T-ME. However the
difference between the amounts of consolidant deposited by both
formulations, suggest that tuff-stone samples absorbed similar
quantity of consolidant. An average of 1.5 kg m?2 of consolidant
was deposited for both formulations. Thus, similar consolidant
effects could be expected for both treatments. However, different
consolidant effects (changes in porosity and hardness) were
observed.
Water-accessible porosity after treatment with T-ME was 11%.
Porosity decreases upon treatment because the gel deposited
tends to close the pores of the stone. Moreover, a dramatic
change in stone porosity was observed when the samples were
treated with H1-ME. The water-accessible porosity of the tuff-stone
changed from 20 to 3.7% when H1-ME was applied. In this case,
the porosity decreased by 80%. The changes in porosity observed
in the samples consolidated with H1-ME were equivalent to a
hydrophobic treatment.
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Hardness determination ? the hardness value of the stone before
treatment was 149 HB. After treatment with both T-ME and H1ME formulations, hardness was enhanced (see Table 4). A greater
increase in hardness was observed when H1-ME was applied, from
149 HB to 192 HB (29% hardness increase), while a change from
149 HB to 176 HB (18% hardness increase) was obtained with T-ME
treatment.
Samples treated with H1-ME showed a significant increase in
hardness (29% over the original tuff-stone hardness) and also a
significant reduction in water-accessible porosity. According to
these results, this formulation seems to acts as a consolidant
with significant hydrophobic behavior. A typical hydrophobic
treatment tends to form a protective film on the surface of the
stone that prevents water penetration without increasing the
cohesion of the stone. Thus, no important change in the stone
hardness is expected with this type of treatment.[1]
It is suggested that the ?Si(CH3 )2 - groups of polydimethylsiloxane crosslinked with the inorganic silica skeleton in the hybrid
structure impart the hydrophobic behavior. Additionally, the linear and flexible siloxane chains enhance the elasticity of the gel,
leading to improved hardness of the final materials. Therefore, it is
suggested that the hybrid consolidant reported here can be used
as consolidant and water-repellent in a one-shot treatment.
Salts crystallization ? the results for salt crystallization indicate
that the untreated tuff-stone supported five cycles (Fig. 8a), while
the stone treatment with T-ME increased its resistance to salt
crystallization to seven cycles (Fig. 8b). The major resistance
to this test was observed when the stone was treated with
H1-ME (Fig. 8c). This treatment led the stone to support until
nine cycles. After such a period, changes in the color of the
stone surface were observed and a minimum deterioration in
comparison with the untreated stone or even with respect to the
T-ME sample. After nine cycles of salt crystallization, T-ME was
powdered.
Hybrid formulation allows a major resistance to salt crystallization over the similar commercial formulation (T-ME). A
possible reason is because the H1-ME formulation leads a
porous gel. The porous gel deposited into the stone would
probably conduct a porosity modification of the stone without diminishing drastically the vapor permeability of the rock.
Although it is still necessary to determine the real changes
in the porosity of the samples treated, the present data
suggest that, when H1-ME formulation is applied on the
stone, a permeable consolidation?hydrofugation treatment is
obtained.
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 481?488
TEOS?colloidal silica?PDMS-OH hybrid formulation
Figure 8. Salt crystallization (a) samples (b) after 5 cycles (c) after 6 cycles.
Conclusions
Porosity in the final hybrid xerogels was significantly increased
with the addition of colloidal silica, because the colloidal particles
are caught in the TEOS matrix, forming micro- and mesoporous
structures depending on the percentage of SiO2 -ST in the hybrid
structure. Moreover, the siloxane chains in the hybrid enable
improved elasticity because PDMS-OH is chemically bonded
to the inorganic silica skeleton (TEOS and colloidal silica),
as was confirmed by 29 Si MAS-NMR. The PDMS-OH fragment
crosslinks with the agglomerate of silica particles in the hybrid
microstructure. Thus, the flexibility and mobility of such fragments
lends support to the capillary tension generated during the drying
stage.
The stone treated with the hybrid (TEOS?SiO2 -ST?PDMS-OH)
formulations showed a major increase in hardness, a significant
change in water-accessible porosity and a major salt crystallization
resistance. Although the amounts of consolidant deposited by
H1-ME and T-ME were similar, a greater increase in hardness
was observed for the samples consolidated with the hybrid
formulation.
Furthermore, PDMS-OH significantly modifies the hydrophobic
properties of the stone, and thus its percentage in the formulation
is critical. The applicability of the hybrid consolidant for the
decayed tuff-stone is under study.
Acknowledgments
The authors wish to acknowledge the financial support of the
Consejo Nacional de Ciencia y Tecnolog韆 (CONACyT-Me?xico)
through SEP-CONACYT 25397.
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Copyright www.interscience.wiley.com/journal/aoc
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