Physical-chemistry of sodium silicate gelation in an alkaline medium

ORGINAL PAPER

Physical-chemistry of sodium silicate gelation in an alkalinemedium

Monique Tohoue Tognonvi • Sylvie Rossignol •

Jean-Pierre Bonnet

Received: 18 November 2010 / Accepted: 1 March 2011 / Published online: 9 March 2011

� Springer Science+Business Media, LLC 2011

Abstract The behavior of sodium silicate solutions in an

alkaline medium has been studied in the 11.56–9 pH range

by adding different amount of hydrochloric acid into

a concentrated commercial solution ([Si] = 7 mol/L,

Si/Na = 1.71, pH = 11.56). The formed products and their

evolution during long ripening (up to 150 days) have been

characterized by cryo-SEM, elementary analysis (ICP-

AES), X-ray diffraction and surface area and relative

density measurements. In the studied narrow ranges of pH

(11.56–9) and silicon concentration (7–0.2 mol/L) four

different situations have been observed: (i) a stable and

clear solution, (ii) a reversible and transparent physical gel;

(iii) a soluble white gel characterized by a significant

contraction during ripening and (iv) an irreversible gel

which presents a slow syneresis leading to a consolidate

solid. The characterizations of the different solids, liquids

and gels have shown that the observed behaviors were the

results of the formation of nanometric soluble

NaSi1.87O4.24 particles and/or insoluble silica-like

(NaSi12.66O25.82) grains and of the contribution of a dis-

solution/precipitation mechanism.

Keywords Sodium silicate solution � Concentrated

solution � Acidification � Gelation in alkaline medium �Syneresis � Dissolution/precipitation

1 Introduction

Binders have been described by Englelleitner [1] as ‘‘an

additive to the material being agglomerated that provides

bonding strength in the final product. A binder can be a

liquid or solid that forms a bridge, film or matrix filler or

that causes a chemical reaction’’. The most commonly used

binder is the cement. It is a mineral powder, which behaves

as a hydraulic binder (consolidation also occurs in water).

The common Portland cement is a mixture of five main

phases: tricalcium silicate (3CaO,SiO2), dicalcium silicate

(2CaO,SiO2), tricalcium aluminate (3CaO,Al2O3), tetra-

calcium ferroaluminate (4CaO,Al2O3,Fe2O3) and calcium

sulphate. The presence of water in the system causes, via a

complex mechanism, the formation of hydrates, which

contribute to the consolidation of the product. It is gener-

ally considered that these silicon-rich hydrates form a gel

that behaves like a binder. The hardening of the cement

paste would correspond to the structuration of this gel that

contains solid particles with a composition of (CaO)x

(SiO2)(H2O)y where the values of x and y depend on the

calcium and silicate content in the aqueous phase [2]. The

attack of amorphous siliceous or silico-aluminate products

by lime saturated aqueous solutions leads also to the for-

mation of CSH gels. This so-called pozzolanic reaction is

at the origin of development of pozzolanic cements where

the Portland clinker is partially substituted by amorphous

silica and/or metakaolin.

During the last few years, a new family of mineral

binders, called geopolymers, has generated a great interest

[3, 4]. These materials are formed by reaction between

siliceous or silico-aluminate powders (metakaolin, fly

ash…) and concentrated alkaline solutions (KOH,

NaOH…). The consolidation of these materials would be

the result of the formation of either a zeolitic-type gel or a

M. T. Tognonvi � S. Rossignol (&) � J.-P. Bonnet

Centre Europeen de la Ceramique, Groupe d’Etude des

Materiaux Heterogenes (GEMH E.A. 3278), ENSCI, 12 Rue

Atlantis, 87068 Limoges Cedex, France

e-mail: [email protected]

123

J Sol-Gel Sci Technol (2011) 58:625–635

DOI 10.1007/s10971-011-2437-4

siliceous or aluminosilicate hydrogel [5]. The formation of

a silicate-based gel seems to be responsible for the binder

behavior of both cement and geoplymer. In such systems

the gelation occurs in situ in a complex and evolving

system. Therefore, the formation mechanism of the binding

minor phase is difficult to analyze.

Alkaline solutions of sodium silicate (water glass) are

often introduced in the geopolymer starting mixture in

order to accelerate the consolidation process. In peculiar

conditions, water glass alone can even behave like a non-

hydraulic binder. In this case the binding gel is formed

directly from the liquid, without any significant reaction

with solid grains, and the progressive evolution from a

solution to a rigid network, via a gel, is characteristic of the

starting solution. In such conditions it is possible to study

the direct influence of the liquid composition on the gela-

tion and strengthening processes.

A combined analysis of 29Si NMR, SAXS and ICP-AES

experiments [6] performed on the concentrated sodium

silicate solution (pH = 11.56; [Si] = 7 mol/L; Si/Na

atomic ratio = 1.71) used in this study has shown that the

observed high siliceous species solubility results from the

formation of a Si7O18H4Na4 neutral complex. The dilution

of such a concentrated solution leads to a progressive

dissociation of these neutral complexes into (Si7O18H4-

Na4-n)n- charged entities and Na? ions (1 B n B 4) for

silicon concentration higher than 4 mol/L. The scattering

entities detected by SAXS being twice bigger in the most

diluted solutions than in the concentrated one, a polycon-

densation reaction between a limited number, between 2

and 8, of (Si7O18H4Na4-n)n- groups should occur. The

acidification of such concentrated or diluted solutions

could affect those neutral or charged species.

A study of the influence of pH and concentration on the

evolution of alkaline commercial solutions of sodium sili-

cate had been realized in order to identify the nature of the

involved gels and to determine the critical parameters for

gelation and consolidation.

2 Experimental part

2.1 The starting solutions

The sodium silicate solution used is a commercial product

from VWR Prolabo. Its molar chemical composition,

determined by emission spectrometry (ICP-AES), corre-

sponds to 3.41 SiO2 and 21 H2O for 1 Na2O). The solution

characteristics are: relative density = 1.32, pH = 11.56,

[Si] = 7.01 mol/L and Si/Na = 1.71.

Dilute hydrochloric acid solutions with concentration

ranging from 0.5 to 2 mol/L were also used in order to

modify both pH and [Si]. These solutions have been

prepared from a 37 HCl wt% commercial solution (Norma

Prolabo) by adding deionized water.

2.2 Products preparation and aging conditions

The studied mixtures have been obtained by progressive

addition of HCl solution into the commercial sodium sili-

cate solution under magnetic stirring at room temperature.

The solutions with [Si] B 2.26 mol/L were firstly dilute

with deionized water before acidification. After acidificat-

ion, the [Si] and pH of the studied solutions were ranged

from 0.2 to 6 mol/L and from 9 to 11.56, respectively. The

pH value was measured all along the process using a

PHM240 METER LAB pH meter (radiometer) operating

with a ‘‘red rod’’-type glass electrode and a T201-type

temperature sensor.

The as-obtained homogeneous solutions (mixtures) were

then transferred into closed plastic vessels for aging at

room temperature.

Depending on pH, Si concentration and behavior during

ripening at room temperature, four different behaviors can

be distinguished:

– (A) a clear and stable solution;

– (B) a stable and transparent gel (Fig. 1a);

(a)

(b)

(c)

4 days 120 days

30 days9 days2 hours40 min

16 min 30 min 5 days 30 days

Fig. 1 Photos showing the evolutions observed during ripening of the

three different gels obtained by acidification of the commercial

sodium silicate solution: a transparent gel (B-type gel) obtained for

[Si] = 5.2 mol/L and pH = 11.10, b white gel (C-type gel) obtained

for [Si] = 3 mol/L, pH = 10.67 and c white gel (D-type gel)

Obtained from [Si] = 2.26 mol/L, pH 10.56

626 J Sol-Gel Sci Technol (2011) 58:625–635

123

– (C) an initial white gel leading during aging to a

progressive separation between a sediment and a

supernatant (Fig. 1b);

– (D) a white gel with a significant syneresis during

ripening (Fig. 1c).

When a gel appeared (B-, C- or D-type gel), a gelation

time (tg) corresponding to the time which elapsed between

the solution preparation and the earliest moment at which

the gel broke away from the wall instead of flowing as a

liquid when the beaker was tilted has been determined [7,

8].

After ripening, when a separation between a clear

supernatant and a white viscous gel, sediment, or brittle

solid was observed (C and D cases), the two phases were

analyzed after a careful separation.

2.3 Characterization techniques

2.3.1 Elementary chemical analysis

The Na and Si concentrations in solutions, gels or solids

were determined using a Thermo Jarrell Ash Corporation

IRIS spectrometer (ICP-AES). Before analysis, solids were

washed and dried (110 �C for 24 h) and then 20 mg were

dissolved in an HF solution using a microwave digestion

technique.

The Cl concentration was determined by potentiometric

titration of the reaction between Cl- and Ag? using a

MeterLab Ion450 Ion analyzer (Radiometer analytical).

2.3.2 Cryogenic scanning electron microscopy

Gels and sediments, previously submitted to a cryogenic

treatment, have been observed by scanning electron

microscopy (SEM). In order to minimize the volume

change of free or weakly bond water, the samples were

cooled under conditions appropriate to the formation of

vitreous ice using a Leica EM PACT high-pressure freezer.

All SEM characterizations were done with a JEOL JSM-

7400F microscope equipped with a field-emission gun and

a cooled stage. High-resolution images were obtained using

low acceleration voltage (1–6 kV).

Each humid sample was introduced into the hollow

(/ = 200 lm) of a copper grid (/ = 1.2 mm) coated with

gold before very ultra-rapid freezing in liquid nitrogen

under a pressure of about 2,000 bars. The frozen sample

was then transferred into a cold chamber (Gatan, Alto

2,500) connected to the microscope, where it was firstly

fractured (freeze-fracture) and secondly warmed from

-150 up to -95 �C in order to remove by sublimation the

ice present on the sample surface. Then, the sample was

cooled at -150 �C for 5 min and coated with gold for 40 s

before its transferred on the cold stage of the SEM using a

protective transfer unit.

2.3.3 X-ray diffraction

X-ray diffraction analysis was performed on powders

obtained after a grinding of samples dried at 25 �C for

72 h. The diffraction diagram was obtained using an INEL

CPS (Curved Position Sensitive Detector) 120 device,

working with CuKa1 radiation (k = 1.540598A) at 40 kV

and 30 mA. The exposure and acquisition times for qual-

itative analyses were both 20 min, the angular range was

10 \ 2h\ 90� and the pace was 0.029.

2.3.4 Specific surface area

The specific surface area of gels and sediments dehydrated

at 200 �C for 2 h was determined by nitrogen adsorption–

desorption isotherms (Micromeritics Flow Sorb II 2,300)

using the BET calculation method. The experiments were

carried out on solid mass of about 11 mg.

3 Results and discussion

3.1 Acidification of the sodium silicate solution

3.1.1 Qualitative description

The addition of hydrochloric acid and water in a com-

mercial sodium silicate solution ([Si] = 7 mol/L and

pH = 11.56) leads before aging for pH [ 9 to the different

situations: All the obtained results are reported in Fig. 2.

It appears that each situation is associated with a specific

pH-[Si] domain:

8

9

10

11

12

0 2 4 6 8

pH

val

ue

[Si] (mol/l)

(a)(b)

(c)(d)

Fig. 2 pH-[Si] domain corresponding at room temperature to

(a) clear solutions, (b) transparent and reversible gels, (c) white and

soluble gels and (d) white and irreversible gels

J Sol-Gel Sci Technol (2011) 58:625–635 627

123

– clear solution (A) for 6 \ [Si] \ 7 mol/L and

11.25 \ pH \ 11.56;

– transparent and reversible gel (B) for 4.14 \ [Si]

\ 6 mol/L and about 10.90 \ pH \ 11.25;

– white and soluble gel (C) for 3 \ [Si] \ 4.14 mol/L

and about 10.56 \ pH \ 10.90;

– white and irreversible gel (D) for 0.2 \ [Si] \ 2.9 mol/L

and about 9 \ pH \ 10.75.

Results reported in Fig. 2 suggest that the nature of the

formed gel would be more dependent on the silicon con-

centration than on the pH.

3.1.2 Influence of chemical parameters on the gelation

time (tg)

3.1.2.1 Influence of pH Figure 3a shows the influence of

pH on the gelation time for three different silicon concen-

trations leading, to the formation, respectively, of B-type

gel ([Si] = 5.2 mol/L), C-type gel ([Si] = 3.6 mol/L) and

D-type gel ([Si] = 2.26 mol/L). In all three cases, the

gelation time increases with pH values. Only C- and D-type

gels can present a very short gelation time.

3.1.2.2 Influence of silicon concentration The influence

of silicon concentration on gelation time (tg) at constant pH

is plotted in Fig. 3b for pH = 11.10 (B-type gel), 10.67

(C-type or D-type gel), 10.56 (C-type or D-type gel) and

10.30 (D-type gel). The gelation time always decreases

when [Si] increases.

The very long ([100 h) and weakly [Si] dependent

gelation times observed when the pH is maintained at

11.10, are characteristic of B-type gel behavior.

At constant pH, a strong tg increase observed for C- and

D-type gels when [Si] decreases (Fig. 3b) could be due to a

dilution effect. The transition between C- and D-type gels

observed at pH 10.67, when the solution is diluted from

[Si] = 3 mol/L down to [Si] = 2.9 mol/L, is not associ-

ated with a strong tg change, suggesting a common origin

for these two type of gels.

3.1.2.3 Chlorine concentration effect The studied solu-

tions are prepared by adding a concentrated or diluted

hydrochloric acid solution to the commercial sodium sili-

cate solution. The added chlorine atoms can act either

directly if Cl- ions are involved in the physico-chemical

process responsible for the gelation or indirectly by

changing the average charge present on the silicate species.

The charge equilibrium in solution leads to Eq. 1:

n Sin�½ � þ OH�½ � þ Cl�½ � ¼ Naþ½ � þ Hþ½ � ð1Þ

Where n- (average negative charge per solvated silicon

atom) is the ratio between the sum of charges associated to

the whole silicate groups and the number of solvated

silicon species. In our concentrated systems, concentra-

tions of OH- (\10-2 mol/L) and H? (\10-9 mol/L) are

negligible compared with [Sin-], [Na?] and [Cl-].

Therefore, charge equilibrium becomes:

(a)

0

1

2

3

4

5

10 10.25 10.5 10.75 11 11.25

log

10tg

(m

in)

pH value

[Si] = 5.2 mol/l

[Si] = 3.6 mol/l

[Si] = 2.26 mol/l

(b)

0

1

2

3

4

5

0 2 4 6

log

10tg

(m

in)

[Si] (mol/l)

pH = 11.10 pH = 10.56

pH = 10.67

pH = 10.30

(c)

0

1

2

3

4

5

0.1 0.2 0.3 0.4 0.5

log

10tg

(min

)

[Cl] (mol/l)

B-type gelC-type gelD-type gel

Fig. 3 Gelation time of different acidified sodium silicate solutions

versus: a pH values for three different [Si]; b silicon concentration for

four different pH values and c chlorine concentration


123

n Sin�½ � ¼ Naþ½ � � Cl�½ � ð2Þ

The gelation times observed for the different studied

solutions are reported in Fig. 3c versus chlorine concen-

tration. For 0.30 B [Cl-] B 0.40 mol/L, the three types of

gels can be obtained. Therefore, it is not possible to asso-

ciate a type of gel with a specific chlorine concentration

domain. These results suggest that Cl- ions are not directly

involved in the gelation process; it acts as a spectator ion.

3.2 Gel ripening

Figure 1 shows the apparent behavior of B-, C- and D-type

gels during aging. B-type gel seems to remain stable with

the time. C- and D-type gels show a significant contraction

during ripening associated with the formation of a super-

natant liquid.

3.2.1 B-type gels

Transparent B-type gels appear at room temperature after a

relatively long maturation time (C4 days). After formation

they do not show any detectable change during aging in

constant conditions (Fig. 1a). However, they are destroyed

by a temperature increase ([50 �C), by mechanical stirring

or by water addition (reversible gels). These behaviors are

characteristic of physical gels constituted of entities

bounded by physical bond (Van der Waals, hydrogen,

Coulombian) [9]. These types of gels are obtained for sil-

icon concentrations ranging from 4 to 6 mol/L; i.e. in a

concentration domain where non-acidified dilute solution

were mainly composed of (Si7O18H4Na4-n)n- entities

(1 B n B 4) [6]. The H? ions added in solution are capable

to combine with these charged species (Si7O18H4Na4-n)n-

leading to the formation of Si7O18H4?nNa4-n neutral

entities likely to aggregate. Therefore, B-type gels could be

made up of aggregated Si7O18H4?nNa4-n particles.

3.2.2 C-type gels

The ripening of white C-type gels leads to the slow for-

mation of a white solid which present a progressive

anisotropic shrinkage. A supernatant liquid is detectable

30 min after gelation. Figure 1b shows a representative

example of an evolution observed during the ripening of a

C-type gel. After a long ripening, a weakly consolidated

solid can be separated from the supernatant. The obtained

solid can be easily dissolved in water. Therefore it appears

that the aging of C-type gels leads to the formation of

soluble precipitate made up of weakly linked grains.

3.2.2.1 Solid phase of C-type gel In order to determine

the apparent volume of the solid phase, the supernatant has

been recovered carefully and filtrated to avoid the presence

of any solid residue in the liquid. Then the supernatant

volume, Vsur, has been measured using a burette. The

difference between the total volume (VT: liquid ? solid)

and the supernatant volume (Vsur) corresponds to the solid

phase apparent volume, consisting of a solid skeleton and

of the interstitial liquid. Some evolutions of the relative

apparent volume of solid phase derived from C-type gels

are shown in Fig. 4. All curves display a similar behavior:

a rapid contraction during the first 9 days, and an asymp-

totic value obtained after about 20 days.

The evolution observed in Fig. 4a is characteristic of the

influence of the starting silicon concentration at constant

pH value: the apparent volume fraction of the solid phase at

the end of the ripening process increases with the silicon

concentration in the solution before gelation and then with

the amount of available solvated silica in the starting

solution.

(a)

(b)

0

0.25

0.5

0.75

1

25 50 75 100

(VT-V

sur)

/VT

Time (days)

[Si] = 4.14 mol/l[Si] = 3.8 mol/l[Si] = 3.6 mol/l

tg

0

0.25

0.5

0.75

1

25 50 75 100

(VT-V

sur)

/VT

Time (days)

pH = 10.90pH = 10.85pH = 10.75pH = 10.67

tg

Fig. 4 Variation, versus ripening time, of the relative apparent

volume of the solid phase derived from different C-type-gels: a for 3

silicon concentrations at pH = 10.90 and b for four pH values and a

[Si] = 3.6 mol/L


123

The evolution observed in Fig. 4b is characteristic of the

influence of pH on the ripening of C-type gels: at constant

initial silicon concentration, higher is the pH value lower is

the relative apparent volume of the final solid (VT-Vsur)/VT.

Such an evolution suggests that the formation of the solid

phase from C-type gels is strongly pH dependant.

3.2.2.2 Morphology and composition of C-type gel Gel

microstructure changes during ripening have been observed

using a scanning electron microscope equipped with a

cryogenic stage. The samples were previously frozen as

described in paragraph 2.3.2. Secondary electrons images

of representative situations observed immediately after

gelation of C-type gels and after a 30 days of ripening are

shown in Fig. 5a. Just after gelation (Fig. 5a1) the gel is

composed of particles homogeneously distributed. The

contraction that occurs during ripening is associated with

pore coalescence (Fig. 5a2). The absence of macroscopic

cohesion of the final solid could be explained by the ten-

dency of the resulting macropores to form macroporosity

lines leading to a microstructure comparable to a stack of

hard aggregates weakly bound each other.

The evolution during ripening of the liquid supernatant

composition has been studied. Each C-type derived gel

products has been let to ripen for a given times and then all

the supernatant formed since gelation has been taken off.

The concentrations of elements present in the supernatant

have been determined by ICP-AES spectrometry for silicon

and sodium elements and by potentiometry for chlorine

ions. The supernatant Si, Na and Cl concentrations deter-

mined for different ripening time are reported in Table 1

for two initial silicon concentrations [Si] (3.6 and

4.14 mol/L) and two pH (10.90 and 10.75). It appears that

silicon and sodium concentrations detected in the super-

natant after a 2 days of ripening are very inferior to those

present in the solution before gelation, indicating that the

white solid contains both silicon and sodium elements. A

significant increase of silicon and sodium concentrations in

the supernatant occurs between the 2nd and the 9th days of

ripening, a period also characterized by a strong contrac-

tion of the solid (Fig. 4), suggesting that this latter phe-

nomenon could be in relation with a dissolution of some

solid species. It can be noticed that the corresponding

increase of silicon and sodium concentrations in the

supernatant occurs without any change of the Si/Na atomic

ratio in the supernatant. After 9 days of ripening, the sili-

con and sodium concentrations in the supernatant appear to

be stabilized (Table 1). The contraction observed for aging

times longer than 9 days (Fig. 4) does not affect signifi-

cantly the silicon and sodium concentrations in the

supernatant.

Results reported in Table 1 show also that the final sil-

icon and sodium concentrations in the supernatant increase

with the pH of the starting solution. This result is in

agreement with the simultaneous decrease of the observed

relative solid volume (Fig. 4b).

Fig. 5 Secondary electron

images of fracture surfaces of

solid formed from a solution of

silicon concentration equal to:

a 4.14 mol/L and pH = 10.85

(C-type gel); b 1.76 mol/L and

pH = 10.75 (D-type gel): (1)

immediately after gelation, (2)

30 days after gelation

(accelerating voltage 3 kV,

frozen samples in a cryogenic

stage)


123

The measured chlorine concentrations in the supernatant

after ripening and that introduced in the initial solution are

always practically identical (Table 1). This result is in

accordance with the hypothesis that chlorine would not be

directly implicated in the gelation process as enunciated in

part 3.1.2.

The white solid formed during ripening of C-type gels is

water-soluble; therefore, the separation of the solid skele-

ton from the interstitial liquid can affect the solid compo-

sition and/or quantity. However an estimation of the solid

skeleton quantity and composition can be performed con-

sidering the conservation of each species mole number and

assuming that the chemical composition of interstitial

liquid and supernatant is the same [10]. The as-determined

volume of the solid skeleton is expressed according to

Eq. 3, 4 in the appendix. According to the Eq. 7 reported in

appendix it is possible to deduce the Si/Na atomic ratio in

the solid from the solid skeleton volume. The results

obtained for three systems ([Si] = 3, 3.6 and 4.14 mol/L)

at pH 10.75 are reported in Table 2. Whatever the initial

composition, the (Si/Na)S atomic ratio tends to increase

slightly during ripening, it is remarkable to note that all the

Si/Na atomic ratios in the solid at the end of ripening are

close to 1.87 [10], in agreement with a solid final compo-

sition (Na2O)0.5(SiO2)1.87 or NaSi1.87O4.24.

According to experimental results, the trend observed

during the ripening of C-type gel can be summarized as

follows. The gel rapidly evolves to form a white solid

which gradually contracts. Wathever the initial conditions,

the solid skeleton always has a chemical composition

characterized by a Si/Na atomic ratio close to 1.87

(NaSi1.87O4.24). During the first part of contraction, sig-

nificant increase of silicon and sodium concentrations is

observed in the supernatant (Table 1). During the shrink-

age, the solid microstructure changes and the formation of

macropores is observed (Fig. 4a). All these evolutions can

result from a continuous process that would begin with the

formation of small NaSi1.87O4.24 solid particles, that grow

in number and size and interact to form a gel that occupies

all the available space [11, 12]. Then, the gel microstruc-

ture evolves via dissolution and precipitation of these

particles leading, simultaneously, to a densification of the

solid (strong contraction) and to the formation of macro-

pore by coalescence of nanopores.

3.2.3 D-type gels

After gelation of D-type gels, a syneresis is always

observed (Fig. 1c). This almost isotropic contraction is

accompanied by a clear liquid discharge. The initial gel and

the consolidated material resulting from syneresis, are not

destroyed by a temperature increase or by a gradual dilu-

tion, therefore, the D-type gels are irreversible. At the end

of the syneresis and after separation from the clear liquid,

the solid obtained is monolithic and strongly consolidated.

The X-ray diffraction diagram of a solid obtained after

5 months of ripening and 72 h of drying at room temper-

ature (silicon concentration = 2.26 mol/L and pH =

10.56) reported in Fig. 6 highlights the amorphous char-

acter of the consolidated product.

3.2.3.1 Behavior of D-type gel during ripening The

evolution during ripening of the solid relative apparent

volume ((Vsur-VT)/VT) for several D-type gels, is shown in

Fig. 7 for various silicon concentration and pH values. The

initial silicon concentration has a marked influence on the

syneresis kinetics at constant pH (Fig. 7a): More concen-

trated is the starting solution shorter is the gelation time and

faster is the syneresis. When the initial silicon concentration

Table 1 Evolution during the ripening of Si, Na and Cl concentra-

tions in the supernatant for different initial solutions: a)

[Si] = 3.6 mol/L and [Na] = 2.11 mol/L; b) [Si] = 4.14 mol/L and

[Na] = 2.42 mol/L, at pH = 10.90 and 10.75

[Si]

(mol/L)

pH tg

(min)

Ripening time

(days)

0 2 9 90

3.6 10.90 342 [Si]sur (mol/L) 2.15 2.48 2.49

[Na]sur (mol/L) 1.31 1.51 1.52

(Si/Na)sur 1.64 1.64 1.63

[Cl]sur (mol/L) 0.31 0.32 0.32 0.32

10.75 120 [Si]sur (mol/L) 1.67 2.30 2.31

[Na]sur (mol/L) 1.04 1.43 1.43

(Si/Na)sur 1.61 1.61 1.62

[Cl]sur (mol/L) 0.34 0.35 0.35 0.35

4.14 10.90 120 [Si]sur (mol/L) – 3.11 3.61 3.61

[Na]sur (mol/L) – 1.86 2.16 2.16

(Si/Na)sur 1.67 1.67 1.67

[Cl]sur (mol/L) 0.38 0.39 0.39 0.39

10.75 30 [Si]sur (mol/L) – 2.37 2.78 2.81

[Na]sur (mol/L) – 1.45 1.71 1.72

(Si/Na)sur 1.634 1.626 1.634

[Cl]sur (mol/L) 0.42 0.43 0.43 0.43

Table 2 Evolution of the estimated solid volume (VS) and compo-

sition ((Si/Na)S) during the ripening of C-type gels obtained from 2

solutions of silicon concentration equal to 3.4, 3.6 and 4.14 mol/L at

pH 10.75 (initial Si/Na atomic ratio = 1.71)

Initial [Si] (mol/L) Ripening time (days) 2 9 90

3.4 VS (cm3) 2.98 2.09 2.08

(Si/Na)S 1.73 1.87 1.87

3.6 VS (cm3) 3.05 2.08 2.07

(Si/Na)S 1.79 1.88 1.87

4.14 VS (cm3) 2.82 2.18 2.14

(Si/Na)S 1.81 1.88 1.87


123

is equal to 1.76 mol/L, the syneresis kinetics is only weakly

pH-dependent (Fig. 7b). When the pH decreases, the solid

apparent relative volume at the end of the ripening process

increases. This observation is in agreement with the

simultaneous increase of the silicon concentration observed

in the supernatant (Table 3). Those results suggest that the

solubility of the solid increases with pH values.

The chemical composition of the liquid phase is reported in

Table 3 for different systems and ripening times. The silicon

and sodium concentrations are always lower in the superna-

tant than in the initial solution. The concentration decrease

affects more silicon than sodium especially for the less con-

centrated solutions. For each given system, it also appears that

the composition of the supernatant does not significantly

change during ripening. This result leads to conclude that the

gel shrinkage takes place without changing the composition of

the supernatant. A decrease of the pH values of solutions of

same initial silicon and sodium concentrations (Table 3) leads

to a decrease of supernatant silicon and sodium concentra-

tions, suggesting an increase of the amount of silicon and

sodium in the solid. It can be also noticed that the Si/Na atomic

ratio in the supernatant decreases with pH: e.g. (Si/Na)sur =

1.29 at pH = 10.75 and 1.08 at pH = 10;56 when the initial

silicon concentration = 1.76 mol/L. This result shows that

the pH value decrease leads to a silicon enrichment of the solid

composition.

3.2.3.2 Morphology and composition of D-type gel The

skeleton volume and the Si/Na atomic ratio, (Si/Na)S, of

the solid formed in these systems had been estimated using

the hypothesis previously considered for the calculation of

the same parameters for C-type gel derived solids. As these

solids are amorphous and silica rich, the relative density of

amorphous silica equal to 2.2 g/cm3 [8, 13] was retained

for calculation. The as-calculated values of solid skeleton

volume (VS) and (Si/Na)S atomic ratio are reported in

Table 4 for different ripening times for three systems

corresponding to initial solutions of pH = 10.56 and sili-

con concentrations = 1.76, 2.26 or 2.81 mol/L respec-

tively. At constant pH, VS increases with the silicon

concentration in the initial solution, i.e. with the amount of

silicon and sodium present in the system. The Si/Na atomic

ratio in the solid is also very sensitive to dilution; when the

initial solution concentration decreases, (Si/Na)S increases.

The microstructure of a D-type gel obtained from a

solution with a starting silicon concentration equal to

1.76 mol/L and a pH = 10.75 was observed by cryogenic

scanning electron microscopy immediately after gelation

and after a 30 days (Fig. 5b). The evolutions of the

microstructure of D-type gels during ripening (Fig. 5a, b)

are rather similar to C-type gels. In both case a pore

enlargement (probably by coalescence) occurs during rip-

ening. However, this pore growth is less pronounced for

D-type gels and the porosity does not seem to move

towards the anisotropic organization responsible for the

Fig. 6 X-ray diffraction diagram of the solid obtained after 5 months

of ripening of a D-type gel and 72 h of drying at room temperature

(initial silicon concentration = 2.26 mol/L and pH = 10.56)

(a)

0

0.25

0.5

0.75

1

40 80 120 160 200

(VT-V

sur)

/VT

Time (days)

[Si] = 0.5 mol/l

[Si] = 1 mol/l

[Si] = 1.76 mol/l

[Si] = 2.81 mol/l

[Si] = 2.26 mol/l

tg

(b)

0

0.25

0.5

0.75

1

40 80 120 160 200

(VT-V

sur)

/VT

Time (days)

pH = 10.75pH = 10.56pH = 10.30pH = 10.10

tg

Fig. 7 Variation of the solid phase relative apparent volume versus

time during ripening of various D-type gels: a different starting

silicon concentrations at pH = 10.56 and b different pH values for an

initial silicon concentration = 1.76 mol/L


123

fragmentation of the C-type gel derived solid. According to

results reported in Table 4, it appears that the volume of

solid skeleton (VS) obtained from a given D-type gel

doesn’t change during syneresis. Therefore, the contraction

observed during ripening (Fig. 7) must correspond to a

gradual decrease of the volume of the interstitial liquid

(VL). The whole of these behaviors (grain growth, pore

growth, consolidation, densification and shrinkage) sug-

gests a microstructural evolution controlled by a dissolu-

tion/precipitation mechanism similar to that which controls

the second step of liquid phase sintering [14].

In order to check the irreversibility of the solids

obtained at the end of the syneresis phenomenon, different

D-type gel derived samples ripened for 150 days have been

washed 70 times with distilled water at room temperature.

A significant amount of silicon and sodium has been

detected by ICP analysis in the first leaches. After 70

washing, silicon and sodium are not yet detected, sug-

gesting that the remaining solid is insoluble in water at

room temperature [10].

These observations suggest that the consolidated solid

formed during ripening of D-type gels is constituted at least

of two phases, one is dissolved in water during the washing

step and the other is insoluble. The continuous evolution of tg

observed at the transition between C- and D-type gel

domains (Fig. 3b, c) and the presence of Si and Na in the

leaches lead to consider the assumption that the soluble

phase present in the D-type gel derived solid and the C-type

derived solid (with a Si/Na atomic ratio = 1.87) are the

same. The insoluble phases obtained after syneresis and

water washing of the solids formed from three solutions of

pH = 10.56 and of initial silicon concentration equal to 1.76,

2.26 or 2.81 mol/L have been analyzed. The results reported

in Table 5 show that the composition of washed solids is very

sodium poor: average molar composition is (0.038 ± 0.003)

Na2O to (0.962 ± 0.003) SiO2; e.i. NaSi12.66O25.82. The

determination by the BET method of the surface area

(Table 5) of these washed solids, with a composition very

close to silica, leads to results very close to the values

(*280 m2/g) observed for silica xerogels [15, 16].

4 Conclusion

The acidification of sodium silicate solution with silicon

and sodium concentrations, respectively, equal to 7 and

4.1 mol/L, leads to several situations in relatively narrow

ranges of pH and concentrations.

i. For the highest pH (11.25 B pH B 11.56) and silicon

concentrations (6 \ [Si] B 7 mol/L), the solution

remains clear and stable with time.

ii. For slightly lower silicon concentrations (4.14 \ [Si]

B 6 mol/L) and pH (10.90 B pH B 11.25), a trans-

parent gel (B-type) is formed. This gel doesn’t show any

change during ripening but it can be destroyed by water

dilution, heating or mechanical stirring. These behav-

iors are characteristic of physical gels constituted of

Table 3 Evolution during ripening of Si, Na and Cl concentrations in

the supernatant for different systems at pH = 10.75 or 10.56 and of

initial concentrations: a) [Si] = 1.76 mol/L and [Na] = 1.03 mol/L;

b) [Si] = 2.26 mol/L and [Na] = 1.32 mol/L

[Si] (mol/

L)

pH tg

(min)

Ripening time

(days)

0 2 9 150

1.76 10.75 1560 [Si]sur (mol/L) – 1.03 1.04 1.07

[Na]sur (mol/L) – 0.80 0.80 0.83

(Si/Na)sur – 1.29 1.29 1.29

[Cl]sur (mol/L) 0.22 0.22 0.22 0.22

10.56 240 [Si]sur (mol/L) – 0.88 0.89 0.90

[Na]sur (mol/L) – 0.82 0.82 0.83

(Si/Na)sur – 1.07 1.08 1.08

[Cl]sur (mol/L) 0.26 0.26 0.26 0.26

2.26 10.75 740 [Si]sur (mol/L) – 1.33 1.35 1.36

[Na]sur (mol/L) – 0.98 1 0.99

(Si/Na)sur – 1.36 1.35 1.37

[Cl]sur (mol/L) 0.28 0.28 0.28 0.28

10.56 16 [Si]sur (mol/L) – 0.95 0.96 0.95

[Na]sur (mol/L) – 0.88 0.87 0.87

(Si/Na)sur – 1.08 1.10 1.09

[Cl]sur (mol/L) 0.34 0.34 0.34 0.34

Table 4 Evolution of solid skeleton volume (VS) and composition

(Si/Na)S during the ripening of D-type gels obtained from three dif-

ferent solutions of pH 10.56 and initial silicon concentration equal to

1.76, 2.26 or 2.81 mol/L

Initial [Si] (mol/L) Ripening time (days) 2 9 150

1.76 Vs (cm3) 1.28 1.27 1.24

(Si/Na)S 3.90 3.87 4.04

2.26 VS (cm3) 1.96 1.96 1.97

(Si/Na)S 2.83 2.75 2.77

2.81 VS (cm3) 2.57 2.56 2.54

(Si/Na)S 2.46 2.47 2.48

Table 5 Chemical composition and specific surface area of the solids

obtained after syneresis (150 days) and water washing (70 times) of

D-type gels derived from three initial solutions of same pH (10.56)

and different silicon concentrations

[Si]initial (mol/L) 1.76 2.26 2.81

Atomic ratio Si/Na of washed solid 13.56 12.46 11.91

Molar composition (%) SiO2 96.40 96.15 95.97

Na2O 3.60 3.85 4.03

SBET (m2/g) 291 282 276


123

entities bounded by physical bond. The constitutives

aggregates would be the Si7O18H4?nNa4-n entities.

iii. In the pH range from 10.56 to 10.90, when the initial

silicon concentration is between 2.9 and 4.14 mol/L, a

soluble white gel (C-type) appears after a short time.

It turns, in few hours, into a white solid with a

chemical composition, NaSi1.87O4.24, independent of

starting conditions. The solid shrinkage observed

during ripening is associated with a silicon and

sodium enrichment of the supernatant, a solid grain

growth and a pore coalescence. This solid can be

easily dissolved by water addition.

iv. When the pH and the initial silicon concentration are

in the range 9–10.75 and 0.2–2.9 mol/L, at least a part

of the formed gel (D-type) is not destroyed by adding

water, raising the temperature or mechanical stirring.

These ‘‘irreversible’’ gels present a syneresis phe-

nomenon. Longer is the gelation time (high pH and

low initial silicon concentration) slower is the syner-

esis kinetics. This phenomenon is associated with a

strong consolidation of the product. The characteriza-

tion of the consolidated product shows the presence of

two solid phases. The first one soluble in pure water,

could be the compound (NaSi1.87O4.24) formed during

ripening of C-type gels. The second corresponds to

nanometrics of an insoluble product whose composi-

tion has been estimated as close to NaSi12.66O25.82.

For a constant pH value (e.g. pH = 10.56), both C- and

D-type gels can be obtained while for a constant [Si], only

one kind of gel exists. Concerning the reaction kinetics, for

a given type of gel (B, C or D), the reaction kinetic depends

on both pH value and [Si], for it increases when the pH

value decreases and [Si] increases.

The microstructual changes of reversible (C-type) and

‘‘irreversible’’ (D-type) gels during ripening are rather

similar. The whole of the observations suggests an evolu-

tion controlled by a dissolution/precipitation process,

leading to grain growth, pore enlargement, consolidation

and densification.

Appendix

1) Volume of the solid skeleton VS

VS ¼MS

qð3Þ

where MS is the mass of the solid skeleton. 2.2 corresponds

to an estimate solid density (g/cm3). [Na�] and [Si�] are

respectively the sodium and silicon initial concentrations in

the solution before gelation. MSiO2= 60 g is the mass of

SiO2 per mol of silicon andMNa2O

2= 31 g the mass of

Na2O = 31 g per mol of sodium. Eq. 3 becomes:

Vs

¼VT 27;27 Si0½ �� SiSur½ �ð Þþ14;09 Na0½ �� NaSur½ �ð Þ½ ��10�3

1� 27;27 SSur½ �þ14;09 NaSur½ �ð Þ�10�3:

ð5Þ

2) Si/Na atomic ratio (Si/Na)S

In the solid skeleton, we have

nSiS ¼ nSi0 � nSisur+ nSiLð Þ

¼ VT Si0½ � � VT � VSð Þ � Sisur½ � ð6Þ

and

nNas¼ nNa0

� nNasur+ nNaL

ð Þ¼ VT Na0½ � � VT � VSð Þ � Nasur½ � ð7Þ

The combination of relations 6 and 7 leads to Eq. 8

Si

Na

� �S

¼ VT Si�½ � � VT � VSð Þ Sisur½ �VT Na�½ � � VT � VSð Þ Nasur½ � ð8Þ

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Vs ¼MSiO2

� VT � Si�½ � � SiSur½ �ð Þ þ Vs � SiSur½ �½ � þ MNa2O

2� VT � Na�½ � � NaSur½ �ð Þ þ Vs � NaSur½ �½ �

2:2ð4Þ


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Physical-chemistry of sodium silicate gelation in an alkaline medium

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