Clay Minerals (1996) 31,225-232
STABILITY
OF S E P I O L I T E IN N E U T R A L A N D
MEDIA AT ROOM TEMPERATURE
S. M A R T I N E Z - R A M I R E Z ,
F. P U E R T A S
ALKALINE
AND M . T . B L A N C O - V A R E L A
Instituto de Ciencias de la Construcci6n "Eduardo Torroja " ( C.S.L C. ), C/ Serrano Galvache s/n, Apartado de Correos
19002, 28033 Madrid, Spain
(Received 27 June 1994; revised 11 September 1995)
A B S T R A C T: The chemical and structural stability of a Spanish sepiolite from Vallecas, Spain, was
studied in 0.04 N NaOH (pH = 12.6), a saturated solution of Ca(OH)2 (pH = 12.6) and deionized/
distilled water (pH = 7). The chemical stability was evaluated by determining the concentrations of
Mg, Si and Ca in the solid residues during the kinetic test. The structural stability was studied by
determining the changes in specific surface area and 'crystallite' size. The specific surface was
determined by the BET method and the 'crystallite' size by the profile of the X-ray diffraction line.
The main conclusion was that sepiolite is a material with high chemical stability in neutral and
alkaline media, with no significant structural changes. However, in alkaline media, greater
dissolution of silica was observed than in neutral media, the silica being derived from the most
external tetrahedra attached to the OH- groups.
Sepiolite is a hydrated Mg silicate, with the
formula, according to Brauner & Preisinger
(1956): Si12MgsO3o(OH)a(H20)4.SH20. Its structure
is composed of sheets of silica tetrahedra joined by
O ions to a central octahedral plane of Mg ions.
The silica tetrahedra are inverted every six units,
forming longitudinal channels of dimensions 0.36
x 1.06 nm. On the surface of the mineral, cracks in
the most external S i - O - S i groups and formation
of silanol groups are produced which confer
adsorbent properties on the sepiolite.
Adequate thermal and acid treatments can modify
the channels (Serna et al., 1975; Fern~indezAlvarez, 1978), the surface silanol groups, the
Lewis and Br6nsted centres (Jim6nez-L6pez et al.,
1978) etc., modifying in turn the structure, porosity
and superficial activity of the sepiolite (RodriguezReinoso et al., 1981; L6pez-Gonz~ilez et al., 198l).
Abdul-Latif & Weaver (1969) studied the kinetics
of acid dissolution of sepiolite and Corma et al.
(1986) proved that at pH<3, the octahedral sheet
was attacked, and on increasing the temperature and
reaction time, there was also an attack on the
tetrahedral silicate sheet.
The silanol groups are capable of reacting with
certain organic substances by forming covalent
links and by modifying the structure and surface
of the sepiolite (Serratosa, 1978; Hermosfn &
Cornejo, 1986).
As can be deduced from the above, there is a
very broad and exhaustive knowledge of the
behaviour and stability of sepiolite in acid media
and under different thermal treatments. However,
no data have been published on the behaviour of the
sepiolite in an alkaline medium, such as in a system
based on Portland cement or lime.
The objective of this study was to understand the
behaviour and stability of the Vallecas sepiolite in
aqueous solutions of neutral and alkaline pH at
room temperature.
EXPERIMENTAL
A sepiolite from Vallecas, Spain, supplied by Tolsa,
was used. The material was mineralogically
characterized by X-ray diffraction (XRD), X-ray
fluorescence (XRF), infrared (IR) spectroscopy, and
differential thermal/thermogravimetric analyses
(DTA/TGA). Contents of Fe203, A1203, SIO2,
CaO and MgO were determined by conventional
chemical analyses, weight loss at 1000~
and
insoluble residue in hot HC1 (1:5).
The fibre length of the initial sepiolite and that of
the sepiolite treated with water and basic media for
9 1996 The Mineralogical Society
S. Marffnez-Ramfrez et al.
226
28 days was determined by transmission electron
microscopy (TEM).
The aggressive solutions used in this study were
a saturated solution of Ca(OH)2 (pH = 12.6) and a
solution of 0.04 N NaOH (pH = 12.6). Deionized/
distilled water was used to carry out a blank
treatment.
The methodology used was as follows: to 15 g of
sepiolite were added 100 ml of the corresponding
solution, being shaken and maintained in an inert
atmosphere of N2 for 1, 3, 7 and 28 days. At the
end of this time the mixtures were filtered under
N2, and the solid residues and the filtrates were
collected. This process was carried out under N2 to
avoid carbonation of the samples, which occurs in
Ca(OH)2 media.
The Mg and Ca contents of the filtrate were
analysed quantitatively by selective electrodes, and
Si by inductively coupled plasma (ICP). The Ca,
Mg and Si were analysed quantitatively by XRF in
the solid residue. The solid samples were also
studied by IR, the spectra being recorded on a
polycarbonate support filter by preparing films.
The N2 adsorption isotherms were obtained and
the BET method applied for both the original
sepiolite and for samples exposed to the different
neutral and alkaline treatments in order to
determine the values of the specific surface area
for each case.
The 'crystallite' size in the different samples was
determined by XRD through line profile analysis
(LPA) (Delhez et al., 1982; Klug, 1974; Warren
1969).
~ 4 % . The remaining 18% corresponds to the
mixture of quartz, feldspar and mica.
The TEM image of the untreated sepiolite fibres
indicates that the average length is 0.88 ~tm.
Stability of sepiolite in water
When sepiolite is placed in water, a hydrolytic
phenomenon is produced which liberates some O H ions from the silanol groups ( S i - O H ) present in its
structure. As a consequence of this hydrolysis, the
pH of the solution reaches a value close to eight.
After the treatments, the solid residue was
examined by XRD and IR spectroscopy and in all
cases, quartz, calcite and sepiolite were recognized,
showing that minor components of these minerals
are not dissolved.
In Fig. la, the evolution with time of the Ca, Mg
and Si contents in the filtrates can be observed. It
can be seen that on the first day of treatment, the
three elements can be detected in minimal
quantities, of the order of 0.011% Ca, 0.006% Mg
and 0.001% Si. These values remain practically
constant in the following stages of the study. Using
these results, it can be shown that the maximum
quantities of Ca, Mg and Si released from the
sepiolite in distilled water are of the order of 2 rag/l,
1 rag/l, and 0.24 mg/1, respectively.
%
4O
b)
35
Si
30
25
20
Mg
15
RESULTS
AND
DISCUSSION
10
Ca
5
Characterization of the sepiolite
The chemical analysis of the sepiolite yielded
SiO2 55.10%; MgO 19.43%; A1203 5.91%; CaO
5.13%; Fe203 1.96%; SO3 0%; weight loss
(1000~
12.02% and solid residue insoluble in
1:5 HC1 0.1%.
Mineralogical characterization of the sample
indicated that the major component was sepiolite,
although calcium carbonate (as calcite) and quartz
were present as minor components. Assuming that all
of the Mg is due to sepiolite, the sample is 78% pure.
The weight loss is caused by the decarbonation
of the sample indicating that the percentage of
calcite, which occurs as a minor component, is
0.02
a)
Ca
0.015
~
o.oo5
Mg
O.Ol
Si
time (days)
FIG. 1. (a) Dissolved Ca, Mg and Si in filtrated liquid
in H20 medium (in percentages). (b) Amounts of Ca,
Mg and Si in solid residue in H20 medium (in
percentages).
Stability of sepiolite
The contents of Ca, Mg and Si in the solid
residues during the same treatment times are shown
in Fig. lb. These levels scarcely change over the
time scales used in this study. However, at one day,
a slight decrease in Ca, Mg and Si can be observed.
This coincides with the small increases detected for
these elements in the filtrates, implying that
solubility of the sepiolite in water is very low.
Figure 2 shows the IR absorption spectra of the
initial sepiolite and samples treated in water for 1,
Z
O
o~
-5
Z
I.-
227
3, 7 and 28 days, within a range' of 4000 to 3000
cm -1. In this region, a sharp band appears, located
at 3720 cm -1 due to the vibration stresses in the
O H - ions of the S i - O H groups. The spectrum also
shows another narrow band at 3680 cm -1 attributed
to the vibration of the O H - groups coordinated with
three Mg 2+ ions, [(Mg)3OH)]; finally, the two wide
and very intense bands located at 3620-3540 cm -1
were ascribed to the molecular vibrations in zeolitic
water weakly bonded to the sepiolite structure by
hydrogen bridges. All of these assignments have
been made following work by Serna et al. (1975)
and Corma et al. (1985).
Analysis of the IR spectra shown in Fig. 2
reveals that the frequencies of the characteristic
absorptions of the sepiolite, in all of the samples
studied, are located at the same values. However,
the bands located in the 3620-3540 cm -1 region
undergo modifications in their intensity. It is
believed that these changes are due to the same
process used to obtain the film and to their being
recorded in vacuum conditions in the spectrophotometric chamber. These bands are ascribed to
zeolitic water, weakly bonded to the sepiolite
structure by hydrogen bridges, thus facilitating its
partial elimination during recording of the IR
spectrum.
The intensity of the vibration bands of the O H groups, corresponding to coordination water molecules that vibrate at 3627 cm -~, decreases with
treatment time. These molecules are placed in the
most external part of the sepiolite and joined to the
changes in sepiolite surface indicated that small
changes are being produced in the external clay
structure.
The average length of the sepiolite fibres
maintained in water for 28 days was 0.58 /~m.
Stability o f sepiolite in Ca(OH) 2 medium
4000
36oo
3600
cm-1
F1G. 2. Infrared absorption spectra of the films in the
region 4000-3000 cm - l for: (a) untreated sepiolite;
(b) sepiolite treated in H20 for one day; (c) three days;
(d) seven days; and (e) 28 days.
The Ca(OH)2 solution initially had a pH of 12.6,
which decreases when sepiolite was added, reaching
a value of ~ 11. A representation of the Mg and Si
contents at the various ages studied in the solid
residues is shown in Fig. 3a. In this case, only Mg
and Si were analysed, as the treating medium was a
saturated solution of Ca(OH)2. In the solid residue
(Fig. 3a), the Mg and Si contents decrease in the
first day and thereafter remain rather unchanged.
The variation in the Si and Mg contents with
respect to the initial sepiolite is 3.9 wt% weight for
the Si and 1.2 wt% weight for Mg. By comparing
S. Martlnez-Ramfrez et al.
228
%
ao
b)
25
si
20
Mg
15
10
Ca
5
30"
25"
a)
S|
k.
20"
15
Mg
10
Z
O
03
o3
5
o
; ; ; ; ,'o,~ ,'~,'61'8='o ='= =',=='6 ='a 3o
time (days)
CO
Z
n."
I--
FIG. 3. (a) Amounts of Mg and Si in solid residue in
Ca(OH)2 medium. (b) Amounts of Ca, Mg and Si in
solid residue in the NaOH medium.
these results with those described in distilled water
treatment the Si and Mg contents of the sepiolite
were found to be more soluble in Ca(OH)2 solution.
Figure 4 shows the IR spectra of the films in the
region of 4 0 0 0 - 3 0 0 0 cm - j , for the samples before
and after digestion in a Ca(OH)2 solution for 1, 3, 7
and 28 days. The frequencies of the characteristic
bands do not show any changes for the samples
treated with a Ca(OH)z solution. It is worth noting
slight modifications such as in a widening of the
band located at 3720 cm -1 and assigned to the
S i - O H bonds (silanols), and in the intensity of the
vibration bands of coordination water molecules
O H - groups. These changes became more apparent
in the sample maintained for 28 days.
In strong alkaline media, it is known that the
macroanionic structures of Si and A1 tetrahedra in
clays and amorphous compounds are unstable, as
they undergo partial or total dissolution. From the
results obtained in this study, it can be observed
that the sepiolite maintained in a Ca(OH)a saturated
solution undergoes very little structural alteration in
a medium with such a high pH. In the analysis of
the solid residues, a decrease in the Si content can
be observed, indicating that a very small amount of
dissolution of Si tetrahedra has taken place in the
4000
3500
30'00
cm-1
FIG. 4. Infrared absorption spectra of the films in the
region of 4000-3000cm -~ for: (a) untreated sepiolite;
(b) sepiolite treated in Ca(OH)2 for one day; (c) three
days; (d) seven days; and (e) 28 days.
structure of the sepiolite in an alkaline medium.
This phenomenon was not observed in water.
The rupture mechanisms in the silicate structures
in an alkaline medium can take place in the
following ways (Puertas, 1993):
-Si-O-Si+ - O H ---+ S i - O - + - S i - O H (a)
- S i - O - S i + HOH ---+ 2( - S i - O H )
(b)
Stability of sepiolite
In both cases the attack is hydroxylic-hydrolytic,
the difference lying in the type of dissolution, being
congruent in (a) and incongruent in (b). In both
mechanisms, a fixation of the OH- groups is
produced by the broken Si-O-Si bonds. The
decrease in the pH value (from 12.6 to approximately 11) of the solutions as a consequence of
placing sepiolite in a Ca(OH)2 medium may
indicate that a partial dissolution of some Si
tetrahedra has taken place with fixation of OHgroups, with the subsequent decrease in the pH.
This is supported by IR spectroscopy, as it is only
the absorption located at 3720 cm -1, due to the
vibrations of the Si-OH groups, which undergoes
widening of the spectral band of the samples
maintained in the basic solution. The existence of
new S i - O H bonds would justify the widening
observed in this absorption.
These dissolutions only take place in very small
quantities.
The average length of the sepiolite fibres
maintained in calcium hydroxide saturated solution
for 28 days, determined by TEM, is 0.38 ~tm. This
value is lower than both the initial sepiolite and that
of the sample maintained in water. These data
confirm the fact that small changes occurred in the
sepiolite fibre surface.
229
:E
<[
r,.f-
Stability of sepiolite in a NaOH medium
A study was conducted of the stability of
sepiolite in a NaOH medium, similar to that
carded out above. The initial NaOH solution had
a pH of 12.4, but the addition of sepiolite reduced
the pH to 8.6.
Figure 3b shows the concentrations of Ca, Mg
and Si in the solid residues plotted against the time
of treatment. Whilst the Ca does not change, the
percentages of Mg and Si in the solid residue
decrease, reaching a maximum after 7 days in both
cases. The amounts of released Mg and Si were
found to be 1% and 1.6%, respectively. The
Ca(OH)2 medium appears, therefore, to be more
aggressive towards sepiolite than 0.04 N NaOH.
The IR spectra in this medium are plotted in
Fig. 5. Note that modifications also take place in
the intensity of the bands due to the zeolitic water,
due to the process of obtaining and recording the
films, as previously discussed. In this case, a
widening of the band assigned to the Si-OH
vibrations is also produced, although it is more
apparent than in the case of Ca(OH)z. The vibration
400o
3, oo
o'oo
CITt- I
FIG. 5. Infrared absorption spectra of the films in the
region 4000-3000cm-1 for (a) untreated sepiolite;
(b) sepiolite treated in NaOH for one day; (c) three
days; (d) seven days; and (e) 28 days.
frequency values, however, show no significant
changes. A decrease in vibration band intensity of
the coordination water OH- groups that vibrate at
3627 cm-1 is produced.
As previously stated, in strongly alkaline media,
the Si tetrahedra of clay materials undergo partial
or total dissolution processes, through the mechanisms that have been previously proposed. Under
230
S. Marffnez-Ramfrez et al.
each attack, the S i - O - S i groups break and fixation
of O H - takes place simultaneously; this lowers the
pH of the solution. This is also confirmed by the IR
spectra in which a widening of the band assigned to
the S i - O H vibrations can be observed9 These
modifications, with those produced in the coordination water molecules noted before (3627cm-1),
indicate that small structural changes are produced
in the surface silanols.
The median length of sepiolite fibre maintained
in NaOH solution for 28 days is 0.30 Ixm.
Comparing this value to that of initial sepiolite
sample, it is clear that fibre ruptures are produced9
Analysis of the solid residues revealed a slight
solubility of the Mg and Si, which may confirm the
breakdown of the Si-O-Si groups as mentioned
above9 However, it should be noted that the
modifications produced are slight and occur
during the first stages, after one or even three
days. Later, however, the sepiolite did not undergo
alteration9
240
b)
22O
200
i~
NaOH
,..
,
. .....
3
9 100
.--..--.-....--::-.:
9 160
v.z.
"
H=O
9 140
1804
a)
160
0
M-
14o
NaOH
m
120-
10(~
Ca(OH)~
5
10
15
20
25
30
35
time (days)
FIG. 6. (a) 'Crystallite' size of treated sepiolite in
different dispersant media. (b) Specific surface (BET)
of treated sepiolite in different dispersant media.
Analysis of the ' crystallite' size by XRD and
of the specific surface
Figure 6a shows the 'crystallite' size, as
determined by XRD, for the sepiolite exposed to
the different treatments in the aggressive media
studied.
These analyses have been carried out on the
11.9 A (7.38~
diffraction line. This line can be
found at low values of 20, is one of the most
intense and is free from interference from any other
sepiolite line. All of these characteristics make it
ideal for profile analysis.
In Fig. 6a, it can be noted that in all of the media
studied a similar increase took place in the size of
the 'crystallite' at one day. In a water medium, this
size is maintained at an almost constant level up to
28 days of treatment9 In a NaOH medium, the value
reached after one day decreases slightly at three
days, and is maintained constant for 28 days.
Finally, in a Ca(OH)2 medium, alternate increases
and decreases in the values occurred at the different
ages, reaching a value close to the initial one at 28
days.
In the sample maintained in water for 28 days,
the 'crystallite' size is 40 times smaller than that of
the fibre length9 For the samples treated in Ca(OH)z
and NaOH, this factor is 30.
Whilst after one day all the sizes of the
'crystallite' are practically the same, at 28 days
they are different, the highest value being reached
in water and the lowest being that which
corresponds to the aggressive Ca(OH)z medium9
In Fig. 6b, the specific surface of the sepiolite
exposed to treatments in alkaline and neutral media
is shown. In a water medium a decrease of the
specific surface of sepiolite was produced, being at
a maximum at three days. At seven days, it
increases, and then decreases again at 28 days,
reaching a value close to that obtained after three
days of treatment.
In the NaOH medium, the specific surface of
sepiolite behaves similarly as in water9 After one
and three days treatment, the specific surface
decreased. A small increase can be observed at
longer treatment times.
In a Ca(OH)2 medium, the specific surface of
sepiolite undergoes a series of fluctuations, with
alternating maxima and minima, reaching a
maximum surface value at 28 days. In this
Ca(OH)z medium, a small carbonation of the
solution is produced, and small CaCO3 crystals
are formed (Fig. 7c). The high values of specific
surface in this case are justified by the specific
surface determination of the formed carbonate.
Therefore, the sample with the greatest specific
surface corresponds to that maintained in Ca(OH)z;
Stability of sepiolite
a
b
c
d
231
FIG. 7. Sepiolite fibre length determined by TEM. (a) original sample; (b) in water for 28 days; (c) in Ca(OH)2
solution for 28 days; (d) in NaOH solution for 28 days.
the sample treated with water shows the minimum
specific surface. The surface changes undergone by
sepiolite fibres justified the small changes produced
in the 'crystallite' size and those produced in the
specific surface.
Finally, it is worth mentioning, that in the last
treatment age (28 days), the sepiolite maintained in
Ca(OH)2 had the greatest specific surface and the
smallest microcrystallinity, whereas the sample
treated with water showed the opposite behaviour.
This could be due to the aggressive action of
Ca(OH)2 which provoked the dissolution of a small
quantity of Si (as discussed above), probably
resulting from the external silanol groups.
CONCLUSIONS
Sepiolite is a material with low solubility in a
neutral medium and in saturated Ca(OH)2 and
NaOH solutions (pH = 12.6). Treatment for 28
days in these media produced small surface
structural changes, mainly in external S i - O H
groups and coordination water molecules. These
changes also produced a decrease in the length of
the sepiolite fibre and changes of the sepiolite
surface.
ACKNOWLEDGMENTS
The authors wish to thank the EU (through its STEP
programme) and the C.I.C.Y.T. for funding both
research projects (STEP-CT90-0107 and PAT911056) without which these research projects could
not have been conducted. The authors also wish to
thank Dr J. Cornejo, Instituto de Recursos Naturales y
Agrobiologfa, Seville, for plotting the N2 adsorption
isotherms using the BET method. The authors also
wish to thank to Dr Jestis Rinc6n (Instituto de Ciencias
de la Construcci6n "Eduardo Torroja") for his help in
preparation of TEM samples and image interpretation.
S. Martfnez-Ram&ez et al.
232
REFERENCES
ABDUL-LATIF N. & WEAVER C.E. (1969) Kinetics of
acid-dissolution of palygorskite (attapulgite) and
sepiolite. Clays Clay Miner., 17, 169-178.
BRAUNER K. & PRESINGER A. (1956) Struktur und
Entstchen des Sepioliths. Mineral. Petrogr. Mitt. 6,
120.
CORMA A., MIFSUD A. & PEREZJ. (1986) Etude cinetique
de l'attaque acide de la sepiolite: modifications des
proprietes texturales. Clay Miner. 21, 69-84.
CORMA A., PEREZ-PARIENTE J. & SORIA J. (1985)
Physico-chemical characterization of Cu 2+ exchanged sepiolite. Clay Miner. 20, 467-475.
DELHEZ R., H. DE KEIJSERTh. & MITTEMEIJERE.J. (1982)
Determination of crystallite size and lattice distortions through X-ray diffraction line profile analysis.
Recipes, methods and comments. Fresenius Zeitschr.
Anal. Chem. 312, 1-16.
FERNANDEZ-ALVAREZT. (1978) Efecto de la deshidrataci6n sobre las propiedades adsorbentes de la
palygorskita y sepiolita. Clay Miner. 13, 325-335.
HERMOSIN M.C. & CORNEJO J. (1986) Methylation of
sepiolite and palygorskite with diazomethane. Clays
Clay Miner. 34, 591-596.
J1MENEZ-LOPEZ A., LOPEz-GoNZALEZ J. DE O., RAMIREZSAENZ A., RODRIGUEZ REINOSO F., VALENZUELACALAHORRO C. & ZURITA-HERRERA L. (1978)
Evolution of surface area in a sepiolite as a function
of acid and heat treatments. Clay Miner. 13,
375--386.
KLUG H.P. (1974) X-ray Dif~action Procedures .for
PolycrystaUine and Amorphous Materials. 2 na edition,
John Wiley & Sons, New York. Chapters 5 and 9.
LOPEZ-GONZALEZ J. DE D., RAMIREZ-SAENZ A.,
RODRIGUEZ-REINOSO F., VALENZUELA-CALAHORROC.
8L ZURITA-HERRERA L. (1981) Activaci6n de una
sepiolita con disoluciones diluidas de HNO 3 y
posteriores tratamientos t6rmicos: I. Estudio de la
superficie especifica. Clay Miner. 16, 103-113.
PUERTAS F. (1993) Escorias de alto homo: composici6n
y comportamiento hidr~ulico. Materiales de
construcci6n, 43, n ~ 229.
RODRIGUEZ-REINOSO F., RAMIREZ SAENZ A., LOPEZGONZALEZ J.D., VALENZUELA-CALAHORRO C.
ZURITA-HERRERA L. (1981) Activation of a sepiolite
with dilute solutions of HNO3 and subsequent heat
treatments: III. Development of porosity. Clay
Miner. 16, 315-323.
SERNA C., AHLRICHS J.L. & SERRATOSA J.M. (1975)
Folding in sepiolite crystals. Clays Clay Miner. 23,
452 -457.
SERRATOSA J.M. (1978) Surface properties of fibrous
clay minerals (palygorskite and sepiolite). Proc. Int.
Clay Conf. Oxford, 99-109.
WARRENB.E.(1969) X-ray Diffraction. Addison-Wesley,
Reading, Mass., USA.