OFFICE OF NAVAL RESEARCH
Contract N/N00014-91-J-l 893
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TECHNICAL REPORT NO. 1
Multinuclear NMR Study of Aluminosilicate Sol-Gel
Synthesis Reasons for the Homogeneous Gelation Using
the Prehydrolysis Method
G.A. Pozarnsky and A.V. McCormick
Dept, of Chemical Engineering & Materials Science
University of Minnesota
421 Washington Ave. SE
Minneapolis, MN 55455
Reproduction in whole or in part is permitted for any purpose of the
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MulCinuclear NMR Study of Aluminosilicate Sol-Gel
Synthesis Reasons for Homogeneous Gelation Using the
S. FUNDING NUMBERS
G.A. Pozarnsky and A.V. McCormick
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Dept, of Chemical Engineering & Materials Science
University of Minnesota
421 Washington Ave., SE
Minneapolis, MN 55455
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13. ABSTRACT (Maximum 200 words)
Aluminosilicate gels have been utilized in numerous optical, dielectric, and
catalytic applications. These applications require a high degree of molecular
homogeneity, but reproducible means of achieving this remain unobtainable for
many compositions. In this work, reaction occurring in one of the most
successful sol/gel processes are ecamined by means of both liquid and solid C,
™Si , 7, and 27 Al NMR. The synthesis process studied was the method, optimized
by Krol, of prehydrolyzing tetraethylorthosilicate, TEOS, in ethanol, and
subsequently adding aluminum secbutoxide, and then further water.
After prehydrolysis and addition of aluminum sec-butoxide, an aluminosilicate
precursor is formed in which the aluminum is tetrahedrally coordinated to four
silicate ligands. After adding further water, gelation is accompanied by the
expansion of the tetrahedral aluminum to octahedral coordination; this
apparently occurs by nucleophilic attack of silanol groups on the aluminum in the
aluminosilicate precursor. At sufficiently low water to silicon molar ratios,
where transparent gels and longer gel times result, this coordination
exDansion is preceded by condensation of the silicate ligands to form Si-O-Si ...
14. SUBJECT TERMS 29
aluminosilicate, sol/gel, copolymerization, Si NMR
Z7 A1 NMR, 17 0 NMR
IS. NUMBER OF PAGES
1ft. PRICE COOE
17 security CLASSIFICATION
IB. SECURITY CLASSIFICATION
OF THIS PAGE
19. SECURITY CLASSIFICATION
20. LIMITATION OF ABSTRACT
\S\ 7540-0' -280-5500 Standard form 298 ’iv 2 39)
P»WCr*o^d Oy Std / *9- 5
Multinuclear NMR Study of Aluminosilicate Sol-Gel Synthesis-Reasons
for Homogeneous Gelation Using the Prehydrolysis Method
submitted to Chemistry of Materials, 6/93
G.A. Pozarnsky and A.V. McCormick*
Dept, of Chemical Engineering and Materials Science, University of
Minnesota, Minneapolis, MN 55454
*To whom correspondence should be addressed
Aluminosilicate gels have been utilized in numerous optical, dielectnc, and catalytic
applications. These applications require a high degree of molecular homogeneity, but
reproducible means of achieving this remain unobtainable for many compositions 1 ’ 2 - 3 . In
this work, reactions occurring in one of the most successful sol/gel processes are
examined by means of both liquid and solid 13 C, 29 Si, 17 0, and 27 A1 NMR. The
synthesis process studied was the method, optimized by Krol, of prehydrolyzing
tetraethylorthosilicate, TEOS, in ethanol, and subsequently adding aluminum sec-
butoxide, and then further water 1 .
After prehydrolysis and addition of aluminum sec-butoxide, an aluminosilicate
precursor is formed in which the aluminum is tetrahedraily coordinated to four silicate
ligands. After adding further water, gelation is accompanied by the expansion of the
tetrahedral aluminum to octahedral coordination; this apparently occurs by nucleophilic
attack of silanol groups on the aluminum in the aluminosilicate precursor. At sufficiently
low water to silicon molar ratios, where transparent gels and longer gel times result, this
coordination expansion is preceded by condensation of the silicate ligands to form
Si-O-Si bonds between the aluminosilicate intermediates. At high water levels, though,
coordination expansion proceeds before the formation of Si-O-Si bonds.
Aluminosilicate gels have a number of current and potential applications in optics,
dielectrics, and catalysisIdeally one would want homogeneity on the molecular
scale. The unique processing advantages offered by sol-gel techniques, where dissolv ed
inorganic precursors are mixed in solution, can in principle ensure such homogeneity 5 .
Howev er, these methods have been unable to consistently produce homogeneous gels
over a broad composition range. In fact, few methods provide even transparency, i.e.,
uniformity of composition over a length scale of -0.5 pm 5 . It is worthwhile to
investigate why certain preparation protocols produce more transparent gels.
If one simply mixes tetraethyl orthosilicate, TEOS, aluminum sec-butoxide, and
water, one will in general not obtain a clear gel; this protocol typically leads to the
precipitation of alumina rich phases. The reason is thought to be that the hydrolysis of
the aluminum alkoxide is several orders of magnitude faster than that of the silicon
alkoxide, and this then leads to the early formation of Al-OH-Al bridges and the
subsequent phase separation of bayerite or pseudo-boehmite. To allow the silicate
precursors a headstart in forming Al-O-Si bonds, the silicate alkoxide can be
prehydrolyzed, as suggested by Yoldas 7 . Subsequent addition of the aluminum precursor
and slow addition of the remaining water for hydrolysis and condensation reactions has
been found to produce transparent gels 7,8,9 ’ 10 .
Even with this protocol, a recent study revealed that more homogeneous gels
were synthesized by decreasing the water to silicate molar ratio, R. The gelation rate for
R equal to 4 was 24 minutes, and the resulting gel was inhomogeneous. For R values of
near 2.0, though, gelation was delayed for a week, and the resulting gels were
transparent 6 .
Although NMR studies have been performed on gels synthesized by the
prehydrolysis method 7 - 8 ’ 9 ' 10 ’ 11 , it is not yet clear why only certain prehydrolysis
techniques increase homogeneity and decrease gel time. Pouxviel has used -^Si NMR on
prehydrolysis solutions before and after addition of aluminum sec-butoxide; it was found
that the prehydrolysis of the silicon alkoxide forms various condensed silicates in
addition to the expected hydrolyzed species. However, no other nuclei were observed,
and the value of R was higher than that which would ensure homogeneous gelation 6,7 .
Jonas and Irwin have used 27 A1 MAS NMR on such gels and have found unusually high
amounts of tetrahedral aluminum 9,10 , but Fahrenholtz and co-workers have found both
tetrahedral and octahedral aluminum environments in these sots before gelation 1 *.
In this work, ^Al, 17 0, 29 Si and 15 C NMR are observed at several key steps in
the synthesis to invesUgate the relationship between the hydrolysis ratio (R=H20/Si) and
homogeneity. The gels synthesized were prepared by the prehydrolysis method of Krol,
illustrated in Figure l 4 The preparation method is identical for different R values until
the last step (C), so it may be possible to clearly discern processes responsible for to more
homogeneous gels. A molar ratio of aluminum to silicon of 1/4 and the molar ratio of
HC1 to silicon of 0.006 were used; these are typical values that produce transparent gels at
R = 2 6 . Total water to silicate molar ratios, R, varied from 2.0 to 4.0, but the
prehydrolysis ratio was maintained at 1.2. Gelation rates and homogeneity were similar
to those prev iously reported 6 .
NMR spectra were obtained with a GE 11.77 T spectrometer. MAS NMR
experiments were performed using a Doty 5mm MAS probe using Si 3 N 4 rotors spinning
at 10 kHz. Spectrometer parameters for each nucleus are given in Table 2. A1 NMR
spectra were referenced to an external sample containing Al(H 2 0) 6 3+ . l7 0 NMR spectra
were referenced to water. Both Si and C NMR spectra were referenced to
For 17 0 enrichment, water enriched in 17 0 to 10% atom content, obtained from
Aldrich Chemical, was substituted for normal abundance water (0.037% I7 0). For
prehydrolysis, water at 5% enrichment was used, while additional water at stage C was at
10% enrichment. l7 0 occupies sites of interest as follows 12,13 :
Si-OEt + HjO => Si-OH + EtOH (l)
Si-OH + Al-OBu s =* Al-O-Si + HOBu s
2Si-OH =* Si-O-Si + H 2 Q (iii)
Since alkoxide oxygens are not enriched, they are not observed in the 17 0 NMR spectra.
The processing of all solutions remains identical up to point C in Figure 1. The
amount of added water at C w as varied so that the total w ater to silicate ratio (R) ranged
from 2 to 4. The gelation time is on the order of weeks at R=2, but is only minutes at
R=4, as shown in Table 1 6 .
Prehvdrolvzed silicate solution
The 29 Si NMR spectrum of the prehydrolysis mixture at stage A in the synthesis
is shown in Figure 2. Most of the TEOS has been hydrolyzed and condensed. Q 0 , Qj,
and Q 2 connectivities are shown, and 60 % of the Si present bears silanol groups. The
17 0 NMR spectrum (Figure 3) shows unreacted water (-3 ppm), ethanol (6 ppm), silanol
groups, Si-OH (18 ppm), siloxane bridges Si-O-Si (32 ppm), and siloxane bonds in
cyclic species (Si-O-Si) cyc (44 ppm)[14-19]. These 17 0 environments are consistent
w ith the 29 Si NMR spectrum. The 17 0 shift of unreacted water is 3 ppm lower than for
pure H 2 0 because in the synthesis solution it is participating in hydrogen bonding and
exchange with ethanol.
The 27 A1 spectrum for aluminum sec-butoxide is shown in Figure 4; it is dissolved
in benzene since the pure butoxide liquid is too viscous to produce narrow peaks. The
spectrum displays two broad peaks at 35 and 58 ppm, corresponding to tetrahedral and
penta-coordinated aluminum, and two peaks at 4 and 6 ppm, each corresponding to
octahedrally coordinated aluminum. This spectrum supports the suggestion that the
butoxide is actually a mixture of polynuclear complexes with a variety of aluminum
coordination states 20 . Quadrupolar nutation spectra of the solution showed no additional
Figure 5 shows the 13 C NMR spectrum. Bridging sec-butoxide ligands are
observed at 25,34, and 73 ppm, confirming that aluminum sec-butoxide is oligomeric.
Peaks at 68.7,32, 22.6, and 10 ppm are associated with the terminal sec-butoxide ligands.
The l7 0 NMR spectrum for the aluminum sec-butoxide was not obtained since it was not
possible to enrich the oxygen of the butoxide.
Immediately following addition of aluminum sec-butoxide to the prehvdrolvzed silicate
After addition of aluminum sec-butoxide to the prehydrolyzed silicate solution,
the 29 Si NMR spectrum (Figure 6) shows that all Si-OH groups are consumed. New 29 Si
peaks are observed at -85, -91, and -95 ppm. The peak at -95 ppm has been previously
assigned to an Al-O-Si environment 8 . Those at -85 and -91 ppm are new, and
presumably also represent Al-O-Si environments.
The 27 A1 spectrum (Figure 7) shows two broad peaks at 56 ppm and 4 ppm;
corresponding to tetrahedral and octahedral aluminum 21 . The disappearance of
pentacoordinated aluminum suggests reaction of the aluminum sec-butoxide precursor.
For comparison, the spectrum of a solution with an aluminum to silicon ratio of 3/8 is
shown in Figure 8. The pentacoordinate peak at 38 ppm, similar to that of the original
aluminum sec-butoxide, suggests that with insufficient silicon, butoxide does not
completely react with the prehydrolyzed silicates.
A decoupled 13 C NMR spectrum at stage B is shown in Figure 9. It exhibits
peaks associated with the silicon ethoxide groups(13,59 ppm), ethanol and the aluminum
sec-butoxide groups (68.7,32, 22.6, and 10 ppm). 2-butanol might also result from
alcohol producing condensation of the aluminum sec-butoxide with the hydrolyzed
silicates, but its resonance cannot be distinguished from that of the aluminum sec-
butoxide. While the peak at 59 ppm is the ethoxide ligand of the TEOS, the peak at 58.4
ppm is assigned to a bridging ethoxide group between a silicon and an aluminum. The
peak at 13 ppm is assigned to the methyl group of the ethoxide for both the TEOS and the
ethanol. Peaks at 68.7,32,22.6, and 10 ppm are carbon environments in the sec-butoxide
ligand. The peaks of the bridging sec-butoxide groups in the aluminum precursor have
disappeared, confirming the oligomeric nature of the aluminum precursor has been
disrupted 5 * 20 .
Although 13 C NMR does not help to determine the number of sec-butoxide
ligands still attached to the aluminum, the 17 0 NMR spectrum (Figure 10) indicates a
large amount of enriched 17 0 incorporation into the Al-O-Si site at 29 ppm, so many of
the sec-butoxide ligands on the aluminum must have been removed 14 * 19 . The peak at 44
ppm corresponds to a siloxane environment, but it is not formed in large amounts. The
17 0 NMR spectrum also shows peaks at 7.5, and 80 ppm. The peak at 7.5 ppm falls in
the shift range for neither Si-O-Si, Al-O-Si, nor Si-OH environments 14-19 Since triply
bonded oxygens in A1-OH-A1 env ironments occur at 0 ppm 15 - 18 , though, we tentatively
assign this peak to a Si-OH-AI environment, w hich is the only enriched site possible that
would correspond to this shift. The peak at 80 ppm is associated with neither netw ork
siloxane nor aluminate environments 14 ' 19 , but it does fall into the shift range for
pyroxene siloxane env ironments 19 so it is assigned to a linear Si-O-Si bridge in a new
The chemical shift of the tetrahedral peak in the 27 A1 spectrum of the
prehydrolysis mixture (Figure 5) conforms to a Al(OSi) 4 site 21 • 22 - 23 The large number
of Al-O-Si bridges formed in solution (Figure 10) support this assignment.
Without further addition of water, the solution at stage B will be stable for up to 3
months. Even without addition of the aluminum precursor, the prehydrolyzed TEOS
solution would also gel in approximately 3 months. It is evident, then, that at stage B, the
aluminum sites are "protected"; without further addition of water, gelation seems to be
governed by silicate condensation kinetics.
Following Addition of Final Water
Solution spectra were obtained at stage C only for R=2, since gelation was much
too rapid for R=3 and 4.
The 29 Si NMR spectrum (Figure 11) immediately after final water addition at
p int C for R=2 shows some hydrolyzed Q), Q t and Q 2 silicate sites. No aluminosilicate
peaks remain in solution. After one day of reaction, though, only the Qj and Q 2 species
remain in the solution spectra.
The progression of the 27 A1 NMR spectra for a gelling solution is shown in Figure
12. Addition of water causes a change in the chemical shift of octahedral aluminum (4-6
ppm, in Figure 7) to 0 ppm (Figure 12). The tetrahedral aluminum chemical shift remains
unchanged at 56 ppm, but the intensity is lower. The intensity of both the tetrahedral and
octahedral aluminum peaks decrease with time with no apparent change in proportion,
but 27 A1 MAS NMR of the gelling solution (Figure 13) shows conversion to the
octahedral aluminum environment as gelation proceeds. The signal of the MAS spectrum
is low compared to the liquid NMR spectrum because the sample size in the MAS rotor is
The 13 C NMR spectrum of the solution after water addition (Figure 14) show s
coalescence of the peaks of the bridging ethoxide and TEOS ethoxide groups into a broad
peak at 58 ppm. The 17 0 NMR spectrum (Figure 15) shows an increased number of Si-
OH groups at 18 ppm, Al-OH-Si sites at 7.5 ppm and Al-O-Si bridge sites at 29 ppm.
The Al-O-Si sites are evident in the 17 0 spectra but not the 29 Si spectra due to the poor
sensitivity of the 29 Si nuclei.
The 27 A1 MAS NMR spectra of wet gels at water ratios of R=2 and 4 are shown
in Figure 16. They both show a broad peak corresponding to octahedral aluminum and a
small amount of tetrahedral aluminum; typical of aluminum hydroxides and oxides ’A No
27 A1 chemical shift difference is seen between these gels, although the R=2 gel is much
13 C and I7 0 NMR are more helpful in showing structural differences. 13 C MAS
NMR of the two gels is shown in Figure 17. At R=4.0, only carbon associated with the
sec-butoxide and ethanol are present, bu<. at R=2.0, unhydrolyzed ethoxide groups on the
TEOS at 58 ppm are also present. The 17 0 MAS NMR spectra of the R=2.0 and 4.0 gels
are shown in Figure 18. At both R values, three broad peaks at 24, 7.5, and -5 ppm are
observed. At R=2.0, though, a peak also appears at ~85 ppm. The peak at 7.5 ppm
corresponds to the triply bridging silanol in both gels; it is increased in intensity
compared to the prehydrolysis mixture in Figure 10. The peak at 24 ppm is the Al-O-Si
bridge. The peak at -5 ppm is assigned to a Si-OH environment since no unhydrolyzed
ethoxide groups remain in the gel and since no bridging oxygen environments are present
at R=4. The new peak at 80-90 ppm is assigned to ^ pyroxene-like siloxane bridge.
Thus, w hereas the aluminum coordination expansion occurs for all R values, the more
transparent, more slowly gelling, solutions retain ethoxides on Si sites and produce Si-O-
After Aluminum Sec-Butoxide Addition
The 27 A1 and 17 0 NMR spectra (Figures 7 and 10) of the solution after addition
of the aluminum sec-butoxide suggests sites of tetrahedral aluminum surrounded by four
silicate ligands. The 13 C NMR spectrum (Figure 9) suggests a Si-OEt-Al environment.
This bridging ethoxide is not observed in the 17 0 NMR spectrum only because it is not
an enriched site.
Since the aluminum precursor is only sparingly soluble in ethanol alone, the
presence of prehydrolyzed silicate acts to allow aluminum sec-butoxide to be dissolved in
solution. A deficiency of prehydrolyzed TEOS will not allow complete reaction of the
aluminum sec-butoxide precursor, as shown in Figure 8.
The number of hydrolyzed silicate groups corresponds well with the number of
tetrahedral aluminums surrounded by silicate ligands. There are also competing reactions
to form pure Si-O-Si bridges and Si-OH-Al bonds; both are seen in the spectrum of the
prehydrolysis mixture in Figure 10. There is no distinction, though, between the Al-O-Si
environment bonded to tetrahedral versus octahedral aluminum ,4 .
Thus it is apparent that a new alumino-silicate species is formed at point B in the
synthesis. This species is shown schematically in Figure 19. The 17 Oand ,3 C NMR
spectra suggest this species probably has three Al-O-Si bonds and a fourth bond occupied
by either a bridging silanol or ethoxy ligand.
After Final Water Addition
The idealized precursor shown in Figure 19 can help to explain the homogeneity
and gelation rates as the hydrolysis ratio, R is changed. It is helpful to consider the
amount of w ater added at stage C. Table 3 shows the relation between the overall water
content in the procedure (R) and the amount of water added in stage C (R').
At R=2, the functionality, f, of the new precursor at stage C after hydrolysis with
the remaining water should be less than two. This would suggest that linear
polymerization can occur between the new precursors in solution as show n in Figure 20,
and these Si-O-Si bridges are shown by the 17 0 NMR spectra in Figures 10 and 18. This
Si-O-Si bridges may be the result of polymerization of the aluminosilicate precursors is
shown at site a in Figure 20. This is a site which may be enriched with 17 0 as follows:
2 ?AlOSi(OEt) 2 (OH) =* = AI(0Si(0Et) 2 -0-Si(0Et) 2 0)Air + H ? Q_ (iv)
The 17 0 NMR spectra also suggest coordination expansion of the aluminum by
nucleophilic attack of silanol groups. Figure 18 shows the silanol peak at -5 ppm in the
gel is decreased in intensity, more than can be accounted for by Si-O-Si formation. The
Al-O-Si bridge is shifted upfield by 5 ppm to overlap with the triply bridging silanol
group at 7.5 ppm. The triply bridging silanol group now has twice the intensity of the Al-
In the sequence shown in Figure 20, the hydrolysis of a single ethoxide group of
each silicate ligand can lead to linear polymerization between the precursors. This can be
followed by nucleophilic attack of silanols on the aluminum to expand its coordination
from tetrahedral to octahedral. Gelation may be slow because it is controlled by the
condensation reactions between the alumino-silicate precursors, the kinetics of which are
similar to acid catalyzed TEOS gels.
For R=4, the 17 0 MAS NMR spectrum in Figure 18 shows that silicate
condensation reactions do not occur to any appreciable degree. However, octahedral
aluminum is still formed. This conversion from a tetrahedral to octahedral coordination
coincides with gelation at both high and low R values, but the greater concentration of
the attacking silanol group at higher R apparently causes silanol attack on aluminum to
happen faster than silanols can condense with each other, and this may be associated with
At both high and low water contents, gelation is accompanied by the tetrahedral to
octahedral conversion of aluminum, but it is not yet clear why this expansion of
Although the reason for the relationship between gel homogeneity and aluminum
coordination also is not yet clear, the following is proposed. The coordination shift of
aluminum from tetrahedral to octahedral coordination proceeds by nucleophilic attack of
silanols on the aluminum. The coordination shift is then dependent on the number of
silanol groups present in solution. At low R values, the silanol groups form siloxane
bridges by condensation reaction (iv) before the coordination expansion occurs. The
coordination expansion then is rate limited by the low amount of silanol groups left in
solution. The initial concentration of silanol groups is decreased by reaction (iv) as more
condensed aluminosilicate species are formed. Although water is produced in reaction
(iv) and can form silanol groups by hydrolysis of the ethoxide ligands in solution, there is
only one silanol group formed from the water in reaction (v). The consumption of two
silanol groups in reaction (iv) in a condensation reaction generates only one silanol group
from the water produced, so the concentration of silanol groups is effectively halved,
which slows the rate of coordination expansion. The condensed aluminosilicate species
formed at low R values are also larger and bulkier than the aluminosilicate species
formed at high R values, which introduce steric factors into the reaction.
These silanols which attack the aluminum and expand its coordination become
unavailable for condensation reactions to form siloxane bridges between alumino-silicate
precursors, so the presence of tetrahedral aluminum in a gel is evidence that a high
number of siloxane bridges between precursors have been formed. Aluminum in a
tetrahedral coordination may also fit more easily into the silicate net to form a more
continuous, molecularly homogeneous polymeric network.
The prehydrolysis method of preparation allows the formation of aluminosilicate
precursors in solution in which the aluminum is protected from hydrolysis. At low water
ratios, polymerization between the silicate ligands of these precursors can occur. This
polymerization seems to delay aluminum coordination expansion and gelation.
At high water ratios, all ethoxide groups are hydrolyzed and the resulting silanol
groups attack to expand the aluminum coordination much more rapidly; there is evidently
no time to form siloxane bridges. The lack of homogeneity of these gels is linked to the
fast expansion of coordination of aluminum from tetrahedral to octahedral coordination.
This work was supported by the Office of Naval Research and by a fellowship to GAP
from the University of Minnesota Center for Interfacial Engineering. The authors are
also grateful for helpful discussions w ith Prof. Christopher Macosko (U. of Minnesota)
and Dr. Joseph Bailey (3M).
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R (Molar Ratio of Water/TEOS)
Gel Time (Hours)
2: Spectrometer Parameters
Figure 1: Procedure for Synthesis of Aluminosilicate Gels bv
Krol Method [Ref. 4]
Figure 3: 1 '<) NMR of Prehydrolysis Solution
120.0 100.0 80.0 60.0 40.0 20.0 0.0 - 20.0 - 40.0 - 60 .
Figure 5: ,3 C NMR (Decoupled) of Aluminum Sec-Butox»de in 1'6 H 6
NMR (Decoupled) of Solution at
72.0 64.0 56.0 48.0 40.0 32.0
Figure 10: 17 0 NMR of Solution at
igure II: z *Si NMR of Solution at
- 96 .
Figure 12: ^Al NMR of Solution at C and To delation
Figure 13: Comparison
Hgure 14: ,3 C: NMK (Decoupled) of Solution at
Figure 15: 1 'O NMR of Solution at
Figure 16: Z/ AI MAS NMR of Celled Sols
Figure 17: l3 C MAS NMR of (ielled Sols
Figure 18: 1 '() MAS NMR of (Jelled Sols
"Simplified" Prehydrolyzed Precursor
/° I °
*R 3 H, Et, Si(OEt)3
Low Water to Silicate Ratio (R)
Water in 2nd Step=2-1.2=0.8
<HO)(RO) 2 Si O
(HO)(RO) 2 Si
(RO) 3 Si
O ^i(OR ) 3
<RO) 3 Si
(HO)(RO) 2 Sl /P o
(HO)(RO) 2 Si
'Si(OR) 2 (OH)
O — (RO) 2 Si
(HO)(RO) 2 Si
(HOKRO> 2 Si
Si(OR) 2 _O
Si \ ^
(RO) 2 Si
x Si(OR) 2 (OH)
High Water to Silicate Ratios
Water in 2nd Stepa4.0-1.2*2.8
(RO) 3 SI
3 H 20/Si
(HO) 3 S4
Expansion to Oh Aluminum
—O Si(OH)2 o ^Si(OH) 2
? H H /° H
/* o b 1 .0
^°H | H w _
ei SifQH) 2 Q— -^Al — O — Si(OH) 2 -
. ^-Si(OH ) 2 C Jf\
ll y O d) ' o
▼ / H OH I H
AJ — O — Si(OH) 2 -O / Si (OH) 2 — O
Si (OH) j — o
-► Al O Si (OH) 2 -O
Si (OH) 2 O