Bergaya F. Handbook of Clay Science

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(Palmour and Johnson, 1967). The latter two approaches were subsequently devel-

oped by So

rensen (1978) who devised a special heating procedure called ‘Stepwise

Isothermal Analysis’ (SIA). Here isothermal sections are combined with constant

heating rate sections in such a way as to keep the rate of mass loss or of sintering

between two pre-determined values. This is to combine the interest of an appro-

ximately controlled rate of dehydration or sintering and that of an easy kinetic

processing of the isothermal steps by relatively standard equations.

Fig. 12.11.6 shows the principle by which the thermal behaviour of clay minerals

may be studied incorporating the three improvements indicated above (analytical,

preparative and kinetic) (Rouquerol, 1970), using the technique of ‘Controlled Rate

Evolved Gas Detection’ (CR–EGD).

Here, the clay mineral sample is placed on the bottom of a fused silica bulb and is

permanently evacuated through a diaphragm, D. Any water vapour released by

sample causes an increase in the residual pressure (or ‘‘vacuum’’) above the sample,

and therefore, a small pressure drop through the diaphragm. The heating control is

set so as to maintain a constant rate of water vapour release from the clay mineral;

this is obtained by keeping the pressure drop through the diaphragm constant, and

therefore, by simply keeping constant the residual pressure above the sample (since

vacuum is constant downstream). The residual pressure is measured by a vacuum

gauge, G (Pirani, thermocouple or Penning gauge, or capacitance manometer) whose

Fig. 12.11.5. Principle of Controlled Rate Thermal Analysis (CRTA, left), a part of the more

general method of Sample-Controlled Thermal Analysis (SCTA, right) where, nevertheless,

only the ‘‘X loop’’ is compulsory, not the ‘‘T loop’’ (Rouquerol, 2003).

12.11.3. Sample-Controlled Thermal Analysis 1009

signal directly feeds the heating control of the furnace (instead of the usual tem-

perature signal provided by a thermocouple or a resistance thermometer). As it will

be seen, this set up is used by several authors for the study of clay minerals.

Another set-up is that of the Constant Rate Thermogravimetry (CR-TG) pro-

posed by Paulik and Paulik (1971) under the name of ‘‘Quasi-Isothermal, Quasi-

Isobaric Derivatography’’ and specially used by Fo

ldvari (1991) for the studies of

clay minerals. The set-up consists of a thermobalance operating at atmospheric

pressure, and controlled in the CRTA mode by feeding the heating control of the

furnace with the derivative dm/dt of the mass signal.

12.11.4. APPLICATION OF THERMAL ANALYSIS TO CLAYS

Our aim is not to provide a comprehensive summary of the literature on the

thermal analysis of clay minerals; rather, we will describe the most promising tech-

niques and approaches for future work in this ﬁeld. In particular, we will show what

can be obtained with careful control of the experimental conditions, i.e., with Sam-

ple-Controlled Thermal Analysis. With the help of microprocessors and microcom-

puters, SCTA now reached an interesting state of maturity.

A. Analytical Aspects

The ﬁrst objective of a good thermal analysis exp eriment is of course to separate as

much as possible the successive steps of any thermal reaction. In the case of clay

Fig. 12.11.6. Principle of a simple set-up of Controlled Rate Evolved Gas Detection

(CR-EGD) operating with continuous evacuation (Rouquerol, 1970).

Chapter 12.11: Thermal Analysis1010

minerals, this means separating as much as possible the steps of water evolution, i.e.,

separating the various types of water initially present. The improvement brought by

the SCTA approach is illustrated in Fig. 12.11.7 for a natural sepiolite from Vallecas

(Spain).

The dashed curve corresponds to a conventional TG experiment (Rautureau and

Mifsud, 1977) whereas, the solid curve was obtained by Controlled Transformation

Rate Evolved Gas Detection. The four steps are mo re clearly separated on the

CR-EGD than the TG curve (Grillet et al., 1988). Step I corresponds to the de-

sorption of zeolitic water. This is followed by the release of water molecules bound to

Mg atoms exposed at layer edges in two successive steps (II and III), before ﬁnal

dehydroxylation of the mineral occurs (step IV).

The successive steps of the dehydroxylation of montmorillonite are also easily seen

on the CR-EGD trace in Fig. 12.11.8.(Laureiro et al., 1996). The ﬁrst step up to ca.

350 1C is due to water desorption from the surface of the mineral (BET surface

area ¼ 78 m

/g. The second step up to ca. 650 1C is due to dehydroxylation limited by

a two-dimensional diffusion mechanism, while the last step corresponds to sintering.

The separation power of SCTA is also illustrated in Fig. 12.11.9 (Villieras et al.,

1992) showing the CR-EGD curve obtained for a mixture of talc and chlorite. The

dehydroxylation of chlorite apparently occurs in two separate steps (1 and 2), while

talc dehydroxylates in a singl e step (between 800 and 1000

C). The SCTA curve can

be used to determine the proportion of chlorite and talc in the sample.

Fig. 12.11.7. CR-EGD curve of a sepiolite from vallecas (solid curve) and corresponding

conventional TG curve (dashed curve) (Grillet et al., 1988).

12.11.4. Application of Thermal Analysis to Clays 1011

SCTA can also be used to characterise the initial state of clay miner als and, more

particularly, to distinguish between similar miner al species.

This is illustrated in Fig. 12.11.10 for two kaolinites of different origin (Dion

et al., 1998). Both samples (one from Charentes, France, the other from Keokuk,

UK) have the same grain size and are submitted to the same conditions of therm-

olysis (same rate of dehydration of 4.5 mg h

1

, and the same residual water va-

pour pressure of 0.45 mbar). The large displacement of one curve from the other may

be ascribed to the larger proportion of defects in the Char entes sample as compared

0 200 400 600 800

Temperature / °C

Time / h

Water desorption

Diffusion

mechanism D2

Sintering

Fig. 12.11.8. CR-EGD trace for the thermolysis of montmorillonite (Laureiro et al., 1996).

0 200 400 600 800 1000

Temperature °C

Time / h

Talc

Chlorite

(1)

Chlorite

(2)

Fig. 12.11.9. CR-EGD traces for a mixture of talc with chlorite (Villieras et al., 1992).

Chapter 12.11: Thermal Analysis1012

with the Keokuk material. These structural defects are responsible for lowering the

dehydroxylation temperature.

The high resolution obtainable from a Sample-Controlled experiment allows im-

purities or very weak phenomena to be detected. Fig. 12.11.11 shows how the re-

crystallisation of metakaolinite can be detected, in a CR-EGD experiment. This

event is indicated by the simple perturbation of the pressure signal. This perturbation

is directly due to structural reorganisation allowing a very small amoun t (less than

0.1% of the initial water co ntent) of weakly bound water to escape at that inst ant.

B. Preparative Aspects

Sample-Controlled Ther mal Analysis allows samples of much greater homogeneity

to be prepared than is possible with standard temperature-progr ammed heating.

This is because the rate of dehydration can be controlled at a sufﬁciently low level for

low temperature and pressure gradients to be obtained. These conditions are re-

quired for homogeneity of the ﬁnal product. This approach is indeed shown to be

most efﬁcient to control the thermal transformation corresponding to the heating of

kaolinite (Dion, 1994; Massiot et al., 1995).

It is worth stressing that this SCTA approach lends itself to industrial application

simply by making use of the SCTA results recorded in the laboratory with a test

sample. One simply needs indeed to reproduce, in the industrial furnace, the same

T ¼ f(t) proﬁle to ensure a controlled rate of dehydration up to the selected

temperature.

0.2

0.4

0.6

0.8

300 400 500 600 700

Temperature / °C

Extent of Reaciton

Charentes

Keokuk

Fig. 12.11.10. CR-EGD traces for two kaolinites of different origin (Dion et al., 1998).

12.11.4. Application of Thermal Analysis to Clays 1013

C. Kinetic Aspects

The major advantages of using SCTA to study the kinetics of thermolysis for a solid

like a clay mineral are as follows:

(i) the temperature gradients can be reduced at will; the single temperature sensor

in good contact with the sample therefore measur es a ‘meaningful temperature’.

This is far from being the case in conventional (constant heating rate) thermal

analysis where tempe rature differences from 5 to 50 K are observed between

different points of the sample in obtaining a DTA signal (Liptay, 1973);

(ii) at any time during dehydration the energy of activation may be determined by

the ‘rate-jump method’ (Rouquerol and Rouquerol, 1972). In this method, the

rate of dehydration is made to swing between two values (say, in the ratio 1/3 or

1/4) and the resulting modulated T ¼ f(t) curve is recorded (Fig. 12.11.12). Each

modulation of the experiment provides a set of 2 rates of dehydration, r

and r

(imposed) and 2 sample temperatures, T

and T

(measured). During the ‘rate-

jump’ from one temperature to the next, the state of the sample is virtually

unchanged, since the experiment is usually carried out in such a way as to

increase the extent of dehydration by less than 0.1%. This allows Arrhenius’ law

to be applied whi le the dependence of the rate of reaction on its extent is

ignored, i.e., without assuming any f(a) law. Under these conditions, the method

can be called either ‘isoconversional’ or ‘assumptionless’;

(iii) provided the rate of dehydration is kept constant throughout the experiment,

each dehydration mechanism gives rise to a speciﬁc shape of the CRTA curve, as

shown in Fig. 12.11.13 (Criado et al., 1990). These curves allow the major

-50 50 150 250

350

Temperature / °C

Time / h

2.7 2.75 2.8 2.85 2.9

Pressure / a.u.

Fig. 12.11.11. CR-EGD traces (temperature trace and pressure trace with perturbations)

showing transformation from kaolinite to metakaolinite (Dion, 1994).

Chapter 12.11: Thermal Analysis1014

mechanisms to be differentiated; that is, where the rate of the dehydration is

controlled either by germination and growth of nuclei (saddle shaped, with

minimum), or by diffusion (S-sh aped) or by interfacial advancement (no min-

imum, no inﬂexion);

(iv) as reaction rates can be closely controlled CRTA uniquely provides a means of

passing progressively from a ‘thermodynamic regime’ (at very low rate of de-

hydration, under quasi- equilibrium conditions) to a ‘kinetic regime’ (at a higher

rate which is controlled by heat and/or mass transport phenomena);

(v) as CRTA can separate the successive steps in thermolysis and control the pro-

cess from beginning to end, rate laws within the entire reaction domain may be

established as was done for kaolinite (Nahdi et al., 2002a). This contrasts with

isothermal kinetics where either the ﬁrst or the ﬁnal part of the reaction, or both,

is skipped.

Fig. 12.11.12. Modulated T ¼ f(t) curve as recorded when applying the rate-jump procedure

to determine the activation energy.

Fig. 12.11.13. Three main shapes of isokinetic curves, corresponding to different basic mech-

anisms (Criado et al., 1990).

12.11.4. Application of Thermal Analysis to Clays 1015

(vi) ﬁnally, the reaction mechanism may be modelled for application to industrial

heating conditions as Nahdi et al. (2002b) done for kaolinite dehydroxylation.

REFERENCES

Caille

re, S., He

nin, S., 1948. L’analyse thermique et son interpre

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International Congress on Ceramics, Paris, pp. 137–151.

Criado, J.M., Ortega, A., Gotor, F., 1990. Correlation between the shape of controlled rate

thermal analysis curves and the kinetics of solid state reactions. Thermochimica Acta 157,

171–179.

Dion, P., 1994. De

shydroxylation de la kaolinite par analyse thermique a

vitesse de trans-

formation controˆ le

e. Etude de la me

takaolinite. Ph.D. thesis. Universite

d’Orle

ans, France.

Dion, P., Alcover, J.F., Bergaya, F., Ortega, A., Llewellyn, P., Rouquerol, F., 1998. Kinetic

study by controlled transformation rate thermal analysis of the dehydroxylation of ka-

olinite. Clay Minerals 33, 269–276.

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balances, in the case of stepwise isothermal heating. Hungarian Patent N 1 152197, reg-

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Geosciences. Springer, Berlin, pp. 84–100.

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Nahdi, K., Llewellyn, P., Rouquerol, F., Rouquerol, J., Ariguib, N.K., Ayadi, M.T., 2002b.

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Chapter 12.11: Thermal Analysis1016

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References 1017

Handbook of Clay Science

Edited by Bergaya, Theng and Lagaly

Developments in Clay Science, Vol. I

1019

Chapter 13

SOME OTHER MATERIALS RELATED TO CLAYS

F. BERGAYA

, B.K.G. THENG

AND G. LAGALY

CRMD, CNRS-Universite

d’Orle

ans, F-45071 Orle

ans Cedex 2, France

Landcare Research, Palmerst on North, New Zealand

Institut fu

r Anorganische Chemie, Universita

t Kiel, D-24118 Kiel, Germany

In this chapter the layered double hydroxides (LDH) called anionic clays

(Chapter 13.1), three-dimensional zeolites (Chapter 13.2), which share several com-

mon properties with two-dimensional phyllosilicates, and ﬁnally the hydrated ce-

ment phases that beneﬁt from the knowledge of clay mineral properties are discussed

(Chapter 13.3). Many other layer silicates and a large diversity of non-silicate layered

materials are not discussed even if the properties are sometimes quite similar

(Schwieger and Lagaly, 2004).

REFERENCE

Schwieger, W., Lagaly, G., 2004. Alkali silicates and crystalline silicic acids. In: Auerbach,

S.M., Carrado, K.A., Dutta, P.K. (Eds.), Handbook of Layered Materials. Marcel Dekker,

New York, pp. 541–629.

DOI: 10.1016/S1572-4352(05)01038-X