Surface area and pore structure of hydrothermal reaction products of granulated blast furnace slag

CEMENT and CONCRETERESEARCH. Vol. 8, pp. 151-160, 1978. Pergamon Press, Inc Printed in the United States.

SURFACE AREA AND PORESTRUCTUREOF HYDROTHERMAL REACTION PRODUCTSOF GRANULATED BLAST FURNACESLAG S. A. Abo-EI-Enein and R. Sh. Mikhail Ain Shams University, Cairo, Egypt M. Daimon and R. Kondo Tokyo Institute of Technology, Tokyo, Japan

(Refereed) (Received May 3; in final form Nov. lO, 1977) ABSTRACT Specific surface areas and pore structure studies were carried out on autoclaved slag-lime and slag-quartz-lime pastes. Autoclaving temperatures were 181 and 213°C. Water and nitrogen adsorption isotherms were measured and their results are comparatively discussed. The specific surface areas measured by water were appreciably higher than the nitrogen surface areas, whereas the total pore volumes measured by water were found to be smaller. A probable explanation was given to this unexpected behaviour, based mainly on the interaction between the polar water molecule and the ionic surface, which takes place in a pore structure with narrower entrance way than inside pore size. This interaction might interfere with the further adsorption, blocking the entrances to further capillary condensation of water vapour. F~r die hydrothermale Reaction von Schlacke-Ca(OH)2 und SchlackeQuartz-Ca(OH)2 Pasten, wurden die spezifische Obergl~che und die Porenstruktur untersucht. Die Reaction~temperaturen waren 181 und 213 C° . Die Adsorptions-isothermen yon Stlckstoff und yon Wasserdampf wurden gemessen. Die spezifische Oberfl~che von Wasserdampf war gr~sser als die von Stichstoff, dagegen die Gesamteporen volumen von Wasserdampf war sleiner. Eine mBglische Erkl~rung dieses unerwarteten Benehmens wurde gegeben. Sie berUht vorwiegend auf die Zusammen-wirkung von den polaren WassermBIBkulen und der Ionen-Oberfl~che, die in einer Porenstruktur mit engerem Eingangsweg als in Inneren, erfolgt. Diese Zusammenwirkung kBnnte die weitere Adsorption stBren; sie blockiert den Eingang zu weiterer Kapillarcondensation von Wasserdampf. 151

152

Vol. 8, No. 2 S. A. Abo-EI-Enein, R. Sh. Mikhail, M. Daimon, R. Kondo

Introduction Previous papers of this series deal with the hydrothermal reaction during the hydration of two main systems, namely, ( i ) granulated blast furnace slag-lime system, and ( i i ) slag-quartz-lime system. The kinetics, the mechanism of hydration as well as the identification of the formed phases, were discussed for the autoclaved samples in the f i r s t paper, including a method for the determination of the uncombined slag content in the hydrothermal reaction (1), while results of the morphological studies concerned with the microstructure of the autoclaved specimens were reported in the second paper (2). However, the microstructure depends primarily on two parameters, a solid fraction as well as its distribution in space. A study of both parameters is essential for the understanding of the microstructure of the pastes under investigation. The specific surface might be taken as an estimate of the f i r s t parameter, namely the solid fraction of the structure, while the pore volume and pore volume distribution might be taken as an estimate of the second parameter, namely the space fraction of the structure. In the present investigation adsorption isotherms of the two classical adsorbates, namely water and nitrogen were measured on the autoclaved samples and the parameters derived from both are c r i t i c a l l y compared with each other. Experimental Details of the preparation of the autoclaved slag-lime and slag-quartz-lime specimens were given in an earlier publication (1). The autoclaving temperatures were 180.5 and 213,1°C which co~respond to water steam pressures of lO and 20 atm., respectively. The autoclavinq times were 0.5, 2, 6, 12 and 24 hours. The percentage oxide composition of the slag used in this study is: SiO2, 34.3; Al203, 15.5; Fe203, 0.4; MnO, l . l ; CaO, 44.0; MgO, 3.4, respectively. The slag-lime samples were prepared with an i n i t i a l slag:calcium hydroxide weight ratio of 80:20, while the slag-quartz-lime samples were prepared with an i n i t i a l slag:quartz:calcium hydroxide weight ratio of 50:30:20, respectively. The degree of hydration can be determined from the reaction ratios of each constituent in the specimen. The free lime, free silica and free slag contents were determined according to the methods previously reported in an earlier publication (1); and from the reaction ratios of these constituents the degree of hydration of the total mixtures could be calculated. Surface areas and pore volumes were measured from the adsorption of both water and nitrogen. Prior to any adsorption run each sample was subjected to D-drying by equilibrating i t at the vapour pressure of ice at -78°C; 5xlO-~ mm Hg. Water vapour adsorption isotherms were determined gravimetrically for several samples with the aid of a number of silica spring balances with sensitivities ranging between 40 and 50 cm/g. The adsorption of water vapour on these autoclaved samples was found to be very slow, and a minimum period of 5 days is required in order to attain equilibrium conditions and for some points much longer periods of time were required. All the adsorption isotherms were measured at 35°C. Adsorption-desorption isotherms of nitrogen were measured volumetrically on all samples investigated. Equilibrium could be attained f a i r l y rapidly and successive measurements were taken at intervals of 30 minutes. The nigrogen isotherms were all measured at liquid nitrogen temperature (-195,8°C). Resultsan d Discussions I. The adsorption isotherms of water vapour were determined on lO of the autoclaved specimens and are graphically shown in Figs. l and 2; Fig. l for slaglime pastes and Fig. 2 for slag-quartz-lime pastes, both are autoclaved at lO atm.

Vol. 8, No. 2

153 SURFACE AREA, PORESTRUCTURE, AUTOCLAVEDSLAGS

0.I0

O.Oc:

/

O.Oa

0.03 Q06 0.0~ ~

0.02

°o,

0 0 0

0 0 0

0

Q~

~20

0.~

0~0

O~

OJO

0.70 0.80 0.~

0 I/

I

0

0.10

t

I

I

I

I

I

0.20 0.30 0 . 4 0 0.50 0.60

1~

I

0.70

I

O.aO 0 . 9 0 1.00

P"Po

FIG. 2 Adsorption isotherms of water vapour on autoclaved slag-quartz-lime pastes at 181°C.

FIG. 1 A d s o r p t i o n i s o t h e r m s o f w a t e r vapour

on autoclaved slag-lime pastes at 18I°C.

of saturated water steam for several durations. long to type II of Brunauer's classification.

All the isotherms seem to be-

Adsorption-desorption isotherms of nitrogen were also measured on all samples investigated, and one representative run is shown in Fig. 3 obtained for slagquartz-lime specimen autoclaved for 6 hours at lO arm. of saturated water vapour. The adsorption of nitrogen at liquid nitrogen temperature is characterized by being irreversible, with closed hysteresis loops which are illustrated in Fig. 3. 2. From the adsorption isotherms, the specific surface areas could be evaluated by applying the BET equatio~p and by u§~ng the values of ll.4A- and 16.2A~ to represent the area covered by a single adsorbate molecule of water and nitrogen respectively. The BET surface areas are designated as SBET in m2/g calculated on ignited weight bases; those obtained by nitrogen and water vapour are shown in columns 3 and 8 of Table I, respectively. Comparison of the two columns shows that the surface areas measured by water vapour are larger than those measured by nitrogen; this is a consequence of the fact that nitrogen molecules are unable to penetrate into all the pores of these adsorbents (3).

80

7O

0.0~

8O

0.0e

so

o,o~

/

/I

/ /

/P~

J:

|

.P~""

/

r

//

~ 40

.° c .? ~ d

3(1

>

20

0 0

I 010

I 020

I 0.10

I 0.60

I 0.50

I 0.60

, I 0.70

I OJO

L 0.90

1.00

P/Po

FIG. 3 Adsorptton-desorption isotherms as well as the Vl-t plot for slag-quartz-lime paste autoclaved for 6 hours from nitrogen adsorption.

Degree of

hydration

Autoclaving

time (hrs.).

St

%ET

34.9 50.9 58.1 65.3 76.3

19.8 27.1 46.5 65.3 72.3

14.0 20.6 27.3 29.5 28.8

: 12 24

0.5

23.8 29.4 30.8 29.1 38.2

38.4

61.3 56.9 67.3 73.6

Slag-quartz-lime pastes at 20 atm.

0.5 2 6 12 24

Slag-quartz-lime pastes at 10 atm.

0.5 2 6 12 24

14.4 12.1 14.4 16.1 15.4

17.4

75.6

24

Slag-lime pastes at 20 atm.

10.9 15.7 16.6 14.0

26.7 30.2 44.8 64.5

0.5 2 15

28.8 29.7 29.7 38.7

23.7

14.2 20.5 27.3 29.4 27.9

14.6 12.0 14.5 16.1 15.2

17.4

10.8 15.3 16.4 13.8

(m2/g)

4

3

(m2/g)

Slag-lime pastes at 10 atm.

2

1

From nitrogen adsorption

P

0.1397 0.1887 0.1444 0.1350 0.2181

0.0714 0.1443 0.1474 0.1633 0.1474

0.1022 0.0927 0.1086 0.1148 0.1304

0.0621 0.0900 0.0620 0.0771 0.0858

(ml/g)

V

5 car

62.0 51.7 50.2 43.2 51.9

70.9 76.1 58.6 45.2 39.9

41.3 23.8 24.8 24.7 20.2

40.8 52.0 34.3 25.7 23.0

(m2/g)

S

6 car

0.3639 0.3317 0.2355 0.2005 0.2963

0.3615 0.5327 0.3171 0.2500 0.2040

0.2931 0.1822 0.1871 0.1758 0.1708

0.2325 0.2981 0.1384 0.1196 0.1133

(ml/g)

V

7

36.9 41.4 57.5 74.5 67.4

31.9 38.1 33.5 36.5 34.4

(m2/g)

'BET

8

36.4 41.1 58.0 72.8 63.9

30.8 38.7 33.7 36.3 35.4

(m2/g)

St

9

0.0401 0.0448 0.0980 0.1247 0.0791

0.0513 0.0444 0.0599 0.0498 0.0378

186.8 152.9 123.6 114.0 93.3

119.3 126.1 74.7 56.6 45.4

(m2/g)

car

V (ml/Ps)

11

10 S

From water vapour adsorption

Some Surface Characteristics of Autoclaved Specimens

Table 1

car

0.2032 0.1654 0.2108 0.1908 0.1094

0.1922 0.1470 0.1323 0.0772 0.0500

(ml/g)

V

12

Vol. 8, No. 2

155 SURFACE AREA, PORESTRUCTURE, AUTOCLAVEDSLAGS

3. Whenthe saturation vapour pressure of the adsorbate is reached, the vapour condenses, and all of the pores become f i l l e d with liquid adsorbate. Thus, the adsorption at the saturation pressure measures the total pore volume of the adsorbent. I t is assumed that the density of the capillary condensed water is the same as bulk water and is equal to one. The total pore volumes in ml/g, are designated Vp, and these are shown in columns 5 and 10 of Table l for nitrogen and water vapour, respectively. A comparison of the two columns shows that the total pore volume obtained by water vapour adsorption for the various samples investigated is considerably lower than those estimated from nitrogen adsorption. This result has also been obtained previously in the study of low porosity slag-lime pastes hydrated at 20°C. (4) This result is surprising and unexpected since several studies of water and nitrogen adsorption on hardened portland cement pastes have led to the conclusion that water pore volumes should actually be much higher than n i t rogen pore volumes. The relatively low values of water pore volumes obtained in the present investigation might be related with both the specific interactions (4) between the highly polarized water molecules and the ionic surface of the present adsorbents, and the predominance of pores of particular shape, namely ink bottle pores (4). The particular pore structure of the present adsorbents, combined with the specific character of the adsorption of water, might be responsible for the depressed values of the total pore volumes as measured by water. This point will be discussed again in this investigation, after presenting some more results related with the nature of this specific interaction and its effect on the adsorption process. Nitrogen, however, interacts with almost non-specific forces with the solid surface, consequently capillary condensation takes place normally in the available pores at high pressures, following the expected behaviour. 4. Since the specimens are not completely hydrated, and that the degree of hydration varies with the autoclaving time, the specific surface areas presented in columns 3 and 8 of Table l obrained by nitrogen and water vapour, respectively, would be more meaningful i f some correction is made to calculate the specific surface areas of the hydration products themselves. In making this correction, the main assumption is that the area of the unhydrated materials is negligible as compared with that of the hydration products. The BET nitrogen areas as well as the total pore volumes are summarized in Figs.4 and 5, while those of water are shown in Fig. 6. In each figure, curves I and II show the "non-corrected" 60

I

i

I

/'"~

i

I

0.30

60

161°C 0.25

•

"\

SO

025

/i Ob

4C

0.30

213°G

;"~ ..;

0.20

\

--

t,,O

0.15 ~

~

30

o.~o ~

~

20

O.lO ~

0.05

I0

0.05

0

0

0I

0.20

A

3c

"..~ " ~ .

~n 2C 10

ol Autoclaving t;me

(hours)

,

\

Q1S "

I

I

Aut~&lWing

time

(hours)

FIG. 4 Variation of nitrogen surface area and total pore v o l u m e with autoclaving time for slag-lime pastes.

156

Vol. 8, No. 2 S. A. Abo-EI-Enein, R. Sh. M i k h a i l ,

M. Daimon, R. Kondo

,

/A

1Bl°C

i

70

"-.

6C

\~,

I)'

2 ~ 3°C

035

3,50

100

If'/'/

\

80

O.&O

60

3.30 ~-

4o

o.2o ~

20

0.10

I 2

Autoclav;n 9

I 6

time

I

I')*

~

0.30 Q25

&O

ozo

30

o.,s

20

O.tO

I0

~05

m

I 0.5

t

"....,_._

SO

E

\

214

12

( hours )

0

l 0.S

I 2

i 6

Autoctaving

time

t t2

0

(hours)

FIG. 5 Variation of nitrogen surface area and total pore volume with autoclaving time for slag-quartz-lime pastes surface areas and total pore volumes per gram ignited weight, whereas curves I' and I I ' represent the "corrected" areas and volumes of the hydration products respectively. For nitrogen adsorption on autoclaved slag-lime samples the "corrected" surface area and pore volume show f i r s t an increase with the processing time until the end of the dormant period as suggested by the authors.(1) This is due to the fact that nearly amorphous CSH i n i t i a l l y formed has a low nitrogen .area, and as a thicker envelope of more stable crystalline CSH is formed, the nitrogen area is increased (cf. Fig. 4). After the dormant period, nitrogen adsorption is hindered as a result of the formation of new inner hydration products, so that nitrogen becomes unable to penetrate into all the pore system; this accounts for the continuous decrease of the nitrogen areas and pore volumes after the dormant period, which starts after about 6 hours autoclaving. j

j

i

i

i

~

180

sl ,Ig - lirn (l

160

0.20 I~0

i

t60

i

,

i

slag'quartz'lime ~ 020

\

I,'0

\ 120

O.v6

~ ¢~E 100

t2o

o16

E 0.12 ~

m m

-

~

80

~

o.

:" 80

0.08

~

tOC

m u~

8C ; 60

>~

0.08

&Q 0.0~, 2¢

I

z,O

U

00~. 20

0

l 0.5

I 2 Autoclaving

I 6 time

112 (hours)

1 2-'

i

0 0

05

I 2 Autocl&v~ng

8i time

FIG. 6 Variation of water surface area and total pore volume with autoclaving time at 181°C.

112 (hours)

2~

0

Vol. 8, No. 2

157 SURFACE AREA, PORESTRUCTURE, AUTOCLAVED SLAGS

The water areas obtained on the same pastes, autoclaved at I0 atm. of saturated water steam, show a value of I19-126 m~/g characteristic of the i n i t i a l l y formed CSH during the f i r s t two hours of the hydrothermal reaction (predormant and dormant periods). The total pore structure is accessible to the water molecules around the monolayer region, an unlike nitrogen, and later accumulation of the hydration product should not lead to obstruction of water penetration. The situation is different in the high pressure range as w i l l be shown later. Beyond the dormant period (starting from 6 hours of autoclaving), however, the appearance and later growth of hydrogarnet (C3ASH4) is responsible for the consequent decrease in the area in the final stages (cf. Fig. 6). Hydrogarnet has a considerably smaller area than CSH. On the other hand, water pore volumes show a less significant change since they are underestimated values as w i l l be discussed later in this investigation. In slag-quartz-lime hydrothermal reactions, the adsorption of nitrogen leads to an increase of the "corrected" surface area and pore volume within the f i r s t stage of autoclaving. During this stage amorphous high lime calcium s i l i cate hydrate is the main product. Crystallization of the high lime product and consequent transformation into low lime product causes a decrease in the surface area measured by nitrogen as a result of increasing the amount of hydration products which f i l l the pores and hinder the penetration of nitrogen (cf. Fig. 5). At 20 atm of saturated water steam, the nitrogen surface area and pore volume show the same variation described earlier for curing at IO arm, with only one difference; namely the abrupt increase in surface and volume noticed for 24 hrs. autoclaving at 20 atm (cf. Fig. 5). This is probably due to the creation of new binding centers and to the tight packing of the hydration products during the final stages of the hydrothermal process. The parallel variations in surface areas and pore volumes is a further evidence that nitrogen measures only the area of one kind of pores, namely the wider pores. For water vapour adsorption on the same pastes autoclaved at lO arm, the "corrected" surface area indicates f i r s t a high value of 187 m2/g characteristic of the high lime product i n i t i a l l y formed during 0.5 hours autoclaving, then the area decreases as a result of stabilization and crystallization of the high lime product. After complete consumption of all free lime during 12 hours autoclaving, the high lime product starts to transform into low lime product. This results in further marked reduction of the water area after 12 hours autoclaving at IO atm. In this system no hydrogarnet is noticed to develop (1). The results of nitrogen and water adsorption are in good agreement with the mechanism illustrated earlier for the hydrothermal solidification processes (1). 5. The Vl-t plots were constructed for the adsorption of both water vapour and nitrogen on the various autoclaved specimens~ The reference t-curve used for water was that published by Mikhail et al (5), which is mainly based on the measurements of Hagymassy et al (6). The nitrogen reference t-curve used was that suggested by de Boer et al (7). For nitrogen adsorption the indication gained from the Vl-t plots is that nitrogen is only accessible to the wide pores present, as shown by the upward deviation from the i n i t i a l atraight line which passes through the origin. The i n i t i a l straight line could be used as a measure of the surface area, known as St , and these values are summarized in column 4 of Table l in m2/g. The close agreement between the surface area obtained from the Vl-t plot (St ) and the BET surface area is a definite criterion for the correctness of the t-curve used in the analysis. A typical Vl-t plot for nitrogen adsorption is shown in Fig. 3 for the slag-quartz-lime sample autoclaved at IO atm for 6 hours. In the case of water vapour adsorption, the Vl-t plots are shown in Fig. 7.

158

Vol. 8, No. 2 S. A. Abo-EI-Enein, R. Sh. Mikhail, M. Daimon, R. Kondo

0.04 0.0~ 0.03

~ I

OOq O.OZ

.., ~ ..~

~

2& h

~ -

0.0~

/

0.0;

C 0.0; E >

>--

C 0.02

oZ "

0.01

-

2h

G

/" ).02

0

001 3.02

0

i

[

1

i

2

L,

6

e,

i

~,

i

I

10

12

14

16

t(1)

18

0 2

z.

6

8

10

12

14

16

18

t(1)

FIG. 7 V l - t plots for autoclaved pastes at 181°C from water adsorption. The f i r s t experimental points, i n i t i a l l y lying on a straight line, is a measure of the specific surface area,S t m2/g (8). Beyond the i n i t i a l straight line the points begin to deviate negatively, indicating the existence of micropores. The existence of meso-pores is also indicated by the upward inflection which takes place at higher relative pressures. The downward deviations undoubtedly indicate the existence of micropores, with the f i l l i n g of these pores by the adsorbate according to the views originally expressed by de Boer et al (8). However, specific interactions might also lead to a downward deviation in the V l - t plot, and this has already been discussed, even in wide pore samples (9). The downward deviation noticed in the present situation represents, thus, a combined effect of both microporosity and specific interactions. 6. The situation with water vapour as i t seems, would lead to certain d i f ficulties in computing the total pore structure of these samples. Thus, the existence of some specific interactions between the polar water molecules and the ionic surface of the present samples might interfere seriously with the analysis of the micropores present by the "MP-method" (lO), and would also interfere indirectly with the analysis of the mesopores by the "corrected modelless method" ( l l ) , at least in determining the region at which analysis of micropores should terminate, and at which the analysis of meso-pores should commence. A major point should be stated in this respect, namely that in the presence of such specific interactions, the specific surface areas of water reported in this paper, which are based on a model of close packing between the adsorbate molecules, should be considered s t r i c t l y as minimum values. The specific interactions between the water molecules and the present adsorbents, might explain the low values taken at the saturation vapour pressure. Thus regarding the microstructure as composed of "ink-bottle" pores, with narrow "necks" and wide "bodies," and assuming that the specific interaction taking

Vol. 8, No. 2

SURFACE AREA, PORESTRUCTURE, AUTOCLAVEDSLAGS

159

place around the "neck" would eventually block the neck, either by i t s e l f or through the building of multilayers on top of i t , this would eventually isolate the "body" of the pore, and make i t unavailable for further condensation to take place inside the body. This assumption, however, is awaiting for further evidence and confirmation. 7. From nitrogen adsorption, the analysis of wide pores (meso-pores) is possible by the application of the "corrected modelless method" ( l l ) . All the analysis was based on the desorption branches of the isotherms; and was continued down t i l l the closure of the hysteresis loops. Judging from the agreement between SBET (or S ) and the cumulative surface area obtained from the analysis Scum, as well as t~e agreement between the cumulative pore volume Vcum and the total pore volume Vp, i t was decided in favour of the parallel plate idealization for the pore shapes. Results of the analysis are given in Table 2, and the pore volume distribution curves are shown in Figs. 8 and 9 for slag-lime and slag-quartz-lime systems autoclaved at I0 arm of saturated water steam pressure, respectively. Obviously, nitrogen gives a part of the pore structure, namely that fraction of the total pore system which is accessible to the nitrogen molecules (3). Table 2 Cumulative Areas and Volumes Versus the Measured Values from Nitrogen Adsorption l

2

3

4

Autoclaving

SBET

Spp cum (m219)

vpp cm (mllg)

(m]/9)

Slag-lime pastes at lO atm: 0.5 I0.9 2 15.7 6 14.0 12 16.6 24 17.4

13.3 17.6 15.4 18.4 19.1

0.0636 0.0912 0.0616 0.0803 0.0865

0.0621 0.0900 0.0620 0.0771 0.0858

Slag-quartz-lime 0.5 2 6 12 24

14.3 23.5 27.0 31.9 29.3

0.0708 0.1472 0.1308 0.1623 0.1457

0.0714 0.1443 0.1474 0.1633 0.1474

"(m219)

at lO atm: 14.0 20.6 27.3 29.5 28.8

5 V P

All the distribution curves indicate the pr)sence of a group of meso-pores having a most probable hdyraulic radius of 14-16A. The height of the distribution curves (the number of pores having the most probable radius indicated in the figures from the location of the maxima) is sensibly varied with the change of both the degree of hdyration (autoclaving time) and the nature of hydration products. Thus, in slag-lime samples the marked increase in height after 12 hours autoclaving is mainly due to the appearance and later accumulation of hydrogarnet (cf. Fig. 8). In slag-quartz-lime specimens, the considerable decrease in height after 12 hours autoclaving is mainly attributed to the conversion of the high lime CSH intoolow lime CSH. In this system more groups of wider pores, extending to about 200A in radius, are also present and these have an appreciable contribution to the pore structure of these specimens (cf. Fig. 9).

160

Vol. 8, No. 2 S. A. Abo-EI-Enein, R. Sh. Mikhail, M. Daimon, R. Kondo

2,(. h 0.003

" •~

0.002

i!

2~

o.s.

!i

~I

ii

!~

6h

FIG. 8 Pore size distribution curves for autoclaved slag-lime pastes at 181°C from nitrogen adsorption.

'

I

I'\ 'o

0,00;

[

\ •e, i

/ 0

..

~

./

10

0

L.'/,

o I0

0

I0

0

10

0

10

20

30

I ~0

50

6h Qo04

0,003

FIG. 9 ~ore size distribution :urves for autoclaved slag-quartz-lime pastes ~t 181°C from nitrogen Idsorption.

2h O.5h

o~ 12h

•

~4 h

~- 0002 4

0.001

/ I0

\

i •

O

10

/ 0

/ ;0

" 0

i 10

J 0

Fh(pp){~)

IO

20

30

z.O

50

References I. 2. 3. 4, 5. 6. 7. 8. 9. I0. II.

R. Kondo, S.A. Abo-EI-Enein, M. Daimon, Bull. Chem. Soc. of Japan, 48, 222 (1975). S. Coto, S.A. Abo-El-Enein, R. Kondo, sumitted for publication. R. Sh. Mikhail, L. E. Copeland, S. Brunauer, Can. a. Chem. 42, 426 (1964). S. A. Abo-EI-Enein, M. Daimon, S. Ohsawa, R. Kondo, Cem. ConE. Res. 4, 299 (1974). R. Sh. Mikhail A. M. Kamel S.A. Abo-EI-Enein, J. App. Chem. 19, 325 (1969) J. Hagymassy, i . Brunauer, R. Sh Mikhail, J. Colloid and Interface Sci. 29, 485 (1969). B. C. Lippens, B. C. Linsen, J. H. de Boer, J. Catalysis, 3, 32 (1964). B. C. Lippens, J. H de Boer, J. Catalysis 4, 319 (1965). R. Sh. Mikhail, F. A. Shebl, J. Colloid and Interface Sci. 34, 65 (1970). R. Sh. Mikhail, S. Brunauer, E. E. Bodor, J. Colloid and Int-erface Sci. 26, 45 (1968). S. Brunauer, R. Sh. Mikhail, E. E. Bodor, J. Colloid and Interface Sci. 2_44, 451 (1967).