Kinetic study of the hydrothermal reaction between lime and rice-husk-ash silica

Kinetic study of the hydrothermal reaction between lime and rice-husk-ash silica

CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 577-588, 1992. Printed in the USA. 0008-8846/92. $5.00+00. Copyright © 1992 Pergamon Press Ltd. KINETIC ST...

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CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 577-588, 1992. Printed in the USA. 0008-8846/92. $5.00+00. Copyright © 1992 Pergamon Press Ltd.

KINETIC STUDY OF THE HYDROTHERMAL REACTION BETWEEN LIME AND RICE-HUSK-ASH SILICA

P.R.KHANGAONKAR AZMI RAHMAT University Sains Malaysia, Tronoh, Perak, Malaysia K.G. JOLLY KUTTY National Metallurgical Laboratory, Madras (Refereed) (Received Dec. 5, 1990; in f'mal form March 24, 1992)

ABSTRACT

The paper desribes a study of the reaction between lime and amorphous silica obtained from rice-husk (93-94% SiO2) under hydrothemal conditions, in the range 80-140oc. The progress of the reaction, conducted in stainless steel bombs was followed by analysing unreacted lime. It was observed that the reaction follows two-stage progress pattern similar to the one reported earlier for lime-quartz hydrothermal reactions. X-ray diffractometry and DTA on the reaction products of both stage 1 and stage 2 indicated the formation of CSH (1), calcium-silicate-monohydrate in both the stages. An earlier model by Bezjak and coworkers developed for two-stage transformation observed in lime-quartz hydrothermal reaction was examined for a possible application to the data from the present work. Calculations were made following the general assumptions of the model, which could be applied satisfactorily to the first stage, but not the second stage, possibly because of the relatively more rapid reaction between lime and amorphous silica in the first-stage, creating sluggishness in the second stage.

INTRODUCTION The reactions between lime and silica have been the subject of investigations by many researchers for more than half a century (1). The investigations about and the characterisation of tobermorites and the synthetic hydrates CSH (I)* and CSH (gel) have been reviewed by Snell earlier (2). Of late the reaction between lime and silica from rice-husk ash (RHA) has been the subject of a number of investigations (3-10). Rice husk is one of the major agricultural wastes, and contains over 20% silica. On controlled combustion it yields ash which is a good source of amorphous silica. An acid treatment of the ash removes most of the cationic impurites and makes it more reactive. Rice husk ash, on reaction with lime, forms gel compounds similar to the principal constituents of cement pastes. The present work is concerned with the kinetics of hydrothermal reaction between lime and rich-husk ash at C/S = 1, at various tempeatures. * C = CaO, S = SiO 2, H = H20 577

578

P.R. Khangaonkar et al.

Vol. 22, No. 4

EXPERIMENTAL Materials Rice husk ash was prepared from rice-husk from Chingleput district, Tamilnadu, India. The husk was washed to remove adhering clay impurities, dried and ignited at about 6 0 0 o c in a muffle furnace for 15 h. The pale white ash so obtained was treated with HCI for 1 hr, washed in hot water, filtered and dried. Fraction passing through B.S. 300 mesh sieve was taken for the experimental work. (Analysis in Table 1) Lime was prepared from lime-shells from pulicate lake, Tamilnadu, India, by calcination at 950°C for 4.5h. (Analysis in Table 1), and powdered in a pulverizer. Method The experiments were conducted to study the reaction in a mixture of lime and rice-husk-silica in closed bomb-type reactors in a temperature range 80-140oc under hydrothermal conditions. Pre-weighed calcined shell lime and rice-husk ash were mixed intimately by shaking and rolling and equal portions were closed in six stainless bombs of equal capacity. The bombs were placed in an a oil bath thermostatically controlled to + 0.5°C. The bombs were removed turn by turn at different intervals of time, and plunged into a large excess of water, so as to relieve the pressure built up inside. The sample inside the bomb was then quantitatively transferred into a conical flask, and analysed for free lime by rapid sugar method (11). The degree of completion of the reaction (~) was calculated from the free-lime obtained by analysis. TABLE 1 Material

Rice husk ash

SiO 2

93.91

AI203

2.10

Fe20 3

0.04

CaO

MgO

LOI

LOI

1000oc

300oc

0.40

0.36

2.54 44.25

Raw lime shell

0.34

0.23*

54.99

0.07

Calcined shell lime

0.57

0.51 *

98.38

0.07

1.17

*R20 3

The particle size distribution of rice-husk silica was measured microscopically and the results are shown in Table 2. In order to examine the effect for calcination (ignition) temperature on the reactive response of (rice husk ash) silica, separate samples were prepared by igniting (calcining) rice husk at 500oc, 600°C and 690oc, and were used in lime-silica reaction study at 139oc. The compressive strength of the pellets, with varying time of the binder-reaction was noted. The product of reaction was studied by DTA as well as by X-ray diffractometry for its crystal stucture using Philips micro-processor diffractometer and vertical goniometer at a scan rate of 2O/min.

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RESULTS The results of the experiments at 81.5oc, 100o and 139oc, for CaO/SiO 2 = 1, are shown in fig.~ as a plot of the degree of completion of reaction (~) with time. It will be seen that the reaction progresses at first, followed by a plateau of stagnation and later progresses again by way of a second stage. This notable feature of the kinetic behaviour of lime-silica reaction under hydrothermal conditions has been studied for lime-quartz system by earlier workers (13-16).

0.6

0,7

~

E

0./,

0"3

~*C I

J

°

0.1

ltO _

o

o

6o

L

,2o ,eo 2~o ,oo 36o ~2o . o

~ 7 7 _ ~ o 6oo 66o

TIME(rain)

FIG. 1 Variation of fractional conversion with time

O.$G

0.65

0"60 REACTIONTEMPERATURE =f39~( CoO/St01RATIO . I CALCINATIONTEMPERATUnF I~( i ---- ~. 500

Z 0"55 0"50

,1__

0

1.

t.

60

120

~

_1~

t _ ~

I80 2t, O 900

.t_

360

¢20

o..-----o

6oo

I~'--'-"A

6 9o

i

i

i

I

Id~O 5&O 600 660

TIMEfminl

FIG.2 Effect of calcination temperature. of rice-husk ash.

Fig. 2 shows the effect of temperature of calcination of rice-husk ash silica on its response to the reaction. It is seen that the rate of reaction is lowered as the calcination temperature increases. This may be due to loss of reactivity of rich-husk silica, due to calcination at higher temperature, with the loss of porosity and increase in crystallinity of the silica produced.Fig. 3 shows the increase in the strength of iron-ore bonded by lime-ricehusk silica reaction with the progress of the reaction.

The binding effect of the reaction product is clearly seen by the increase in the strength of the pellets. X-ray diffractometric study on the reacted samples, at all the temperatures studied, showed a prominent peak around -__3.03A° due to CSH (I). This was separately confirmed by a sample of synthetic CSH (I) which was given the same thermal treatment. The structure of the product was observed to be the same in stage I and II as well as for the products at the various temperatures studied. DTA study indicated an exothermic peak around 850°C confirming the presence of CSH (I) by comparative studies. DISCUSSION The two-stage progress of the reaction can be examined, from the point of view of the mechanism, following an earlier treatment by Bezjak and alujevic (14-16). It is necessary to consider the two stages separately, so as to analyse them somewhat independently. This treatment assumes that since the sample consists of range of particle sizes, (poly-size specimen) more than one mechanism may operate simultaneously, in different particle sizes (12).

580

P.R. Khangaonkar et al.

Vol. 22, No. 4

T A B L E 2: Particle size D i s t r i b u t i o n

SI No.

Particle Radii Ri(gm)

Wt.Fraction Wi

*SI

No.

Particle Radii Ri(gm)

Wt.Fraction

wi 1.

0.85

0.0037

24

3.15

0.0163

2.

0.95

0.0046

25.

3.25

0.016

3.

1.05

0.0055

26.

3.35

0.0166

4.

1.15

0.0063

27.

3.45

0.0166

5.

1.25

0.0072

28.

3.55

0.0166

6.

1.35

0.0079

29.

3.65

0.0166

7.

1.45

0.0086

30.

3.75

0.0166

8.

1.55

0.0093

31.

3.85

0.0164

9.

1.65

0.0100

32.

3.95

0.0163

10.

1.75

0.0106

33.

4.05

0.0161

11.

1.85

0.0113

34.

4.15

0.0160

1.95

0.0120

35.

4.25

0.0157

13.

2.05

0.0126

36.

4.35

0.0155

14.

2.15

0.0132

37.

4.45

0.0151

15.

2.25

0.0137

38.

4.55

0.0148

16.

2.35

0.0142

39.

4.65

0.0145

17.

2.45

0.0146

40.

4.75

0.0142

18.

2.55

0.0149

41.

4.85

0.0139

19.

2.65

0.0152

42.

4.95

0.0135

20.

2.75

0.0156

43.

5.05

0.0131

21.

2.85

0.0158

44.

5.15

0.0128

22.

2.95

0.0160

45.

5.25

0.0124

23.

3.05

0.0162

46.

5.35

0.0122

12.

Vol. 22, No. 4

LIME, RICE HUSK ASH, HYDROTHERMAL REACTION, STAGES

581

Table 2 - Continued ..... No.

Ri

47.

Wi

No.

Ri

Wi

5.45

0.0121

73.

8.05

0.0072

48.

5.55

0.0119

74.

8.15

0.0072

49.

5.65

0.0117

75.

8.25

0.0069

50.

5.75

0.0115

76.

8,35

0.0069

51.

5.85

0.0113

77.

8.45

0.0069

52.

5.95

0.0110

78.

8.55

0.0066

53.

6.05

0.0109

79.

8.65

0.0066

54.

6.15

0.0107

80.

8.75

0.0066

55.

6.25

0.0105

81.

8.85

0.0064

56.

6.35

0.0103

82.

8.95

0.0064

57.

6.45

0.0097

83.

7.05

0.0064

58.

6.55

0.0097

84.

9.15

0.0061

59.

6.65

0.0092

85.

9.25

0.0061

60.

6.75

0.0092

86.

9.35

0.0061

61.

6.85

0.0089

87.

9.45

0.0061

62.

6.95

0.0089

88.

9.55

0.0059

63.

7.05

0.0086

89.

9.65

0.0059

64.

7.15

0.0086

90.

9.75

0.0059

65.

7.25

0.0083

91.

9.85

0.0059

66.

73.5

0.0083

92.

9.95

0.0056

67.

7.45

0.0080

93.

10.05

0.0056

68.

7.55

0.0080

94.

10.15

0.0056

69.

7.65

0.0078

95.

10.25

0.0056

70

7.75

0.0078

71

7.85

0.0075

72.

7.95

0.0075

582

P.R. Khangaonkar et al.

Vol. 22, No. 4

For the well-known mechanisms of nucleation and growth, phase boundary interaction (i.e. surface area control or chemical reaction control) and diffusion through the product layer, for a single particle, the following equations apply. (o<) is the degree of completion of the reaction, t is time. Nucleation and growth :

f ( ~< ) = -In (1 - ~<) 1/3 = kn t ....... (1)

Phase boundary interaction:

f (o<) = _ (1 - ~ )

1/3 =

kc t ...... (2)

Diffusion : f (o<) = 1 - 2 ~ - (1- ~ ) 2/3 = kd t (Gunstling & Brownshtein) ..... (3) 3 ........................................... or [ 1 - (1- ~< ) 1/3 ]2 = k d t (Jander) ............ (4) The above functions, for the data of the present study, yield relationships as shown in figs. 4 to 6. The straight lines of the two periods are distinct, separated by a transtion zone. Such an effect has been observed earlier in studies of particle reaction kinetics, when samples with a range of particle sizes were examined.

'oo F" 80

c3

/

~

~

~

30

REACTION TEMPERATURE = 139"C

/

o

~

20

TO 0

i

l

1

i

|

|

t

i

i

I

0"50 0"52 0.51, 0"56 0-58 0"60 0.62 0.64 0"66 0"6,8 0.70 FRACTIONAL

CONVERSION (c~)

FIG. 3 Effect of progress of reaction on the strength of pellets.

The linear relationships yield the factors k n, k c and k d for the two acceleration periods, and can be called apparent reaction constants as shown in figs. 7 to 9. Table 3 states the activation energy values derived from the variation of k with 1/T for the data shown in figs. 7 to 9. The values, mostly between 15-20 kJ/mole, apparently indicate the possible predominance of diffusion, considering the categories of activation energy ranges observed earlier in such cases where more than one mechanism may be operating with varying degrees of effectiveness.

T A B L E 3: Arrhenius activation energy values calculated using nucleation, phase boundary interaction and diffusion model for both stages of the lime-silica reaction at CaO/SiO 2 = 1

Model Nucleation Phase boundary reaction Diffusion

Eal (KJ/Mole)

Ea2 (KJ/Mole)

7.61 14.77

15.83

18.19

20.89

Vol. 22, No. 4

LIME, RICE HUSK ASH, HYDROTHERMAL REACTION, STAGES

583

It m a y be assumed that there is a particle radius R L for which the transition from nucleation and growth controlled process to the process controlled by phase boundary interaction takes place at the degree o f of transformation ~c T. Similarity there is a particle radius R D for which the transition from nucleation and growth controlled process to diffusion controlled process takes place at the degree of transformation,~ T. It is further assumed that ~ T corresponds the degree of transformation, at which the transformation rate by nucleation and growth reaches maximum. T

= 1 - e -2/3 ..........................................

(5)

Let ~c li = the degree of hydration at which control by phase boundary reaction replaces the nucleation growth process. 2i = the degree of hydration at which diffusion process replaces the nucleation growth process. 3i = the degree of hydration at which diffusion process replaces the phase boundary reaction process. These can be determined from the following set of equations, derived from equations (1) to (4). ( 1 -*¢1i )1/3 [ _ In (1 - ~< li) 2/3 = RLRi-I(1-*CT) 1/3 [ - In ( 1 - ~ T ) ] 2/3

...........

(6)

(1-*<2i) 1/3 [1 - (1- *c2i) 1/3 ] [ - In (1 -~-2i)] 2/3 = RD2Ri-2 (1-~T)U3 [1 - (1-O~T)ln] [ - in (1--O(T)] 2/3 ........... (7) [1 - (1 - ~3i)] 113 = RD 2 R L -1Ri-1 [1 - (1 -~CT)1/3]

...........

(8)

The relative time corresponding to *
...........

where A = [ - In (1 -*(T)] 1/3 = 0.874

............. (10)

T 2i = A-1 [ - In (1 -*<2i)]1/3

............. (11)

"I'3i = B -1 RiRL- 1 [ (1 - *~li) 1/3 - (1- *¢3i) 1/3] + Tli

.......... (12)

Where B = A (1 - ~ T ) 1/3 [ - In (1 -C~T~2/3 = 0.534

(9)

............ (13)

Table 4 indicates the experimental degree of completion of reaction f,~), time ( t ) and relative time •3" obtained by using T = t/t o. t o is the time corresponding to ~
(1

- *~i) 1/3 ] -

[1 - (1 - 0¢ li)1/3 ] = BRLR-li ( "7--T li)

.............. (14) .............. (15)

584

P.R. Khangaonkar et al.

TABLE

4: D e g r e e s o f h y d r a t i o n

(o,),

0.4567 0.5144 0.5422 0.5878 0.5730 0.6008 0.6153 0.6333 0.6407

Vol. 22, No. 4

t i m e (t)

r e l a t i v e t i m e (y).

33 60 90 120 180 249 300 366 420

0.717 1.305 1.957 2.609 3.913 5.413 6.522 7.957 9.130

(16)

[1 - (1 -o~i)1/312 - [ 1 - (1 - '<2i)1/312= C RD2Ri-2 ( T - T2i)

..............

where C = 2A (1 - ~ T ) 1/3 [1- (1- ~
............. (17)

]~/5

or I 1- (1 _4 i)1/312 _ [1 - (1 - ~:3i)1/3] 2 = C RD2R1-2 (T - T3i )

0.9

~11

0.7

w

o.6 ,~1

T : [ oC) REACT ON TEMPERAI(RE

e~J

/

o----o

0.5

D~---E]

87 5 119

W------M f29 0

60

120 180 21,0 300

260 z,20 180 5&O 6DD 860

720

T/ME ( r a i n )

FIG. 4 Variation of o<- function (nucleation and growth).

Equations 14, 15 and 18 used to obtain o< i values. determined by using

.............. (18) Eq. 16 or 18 may be used depending on w h e t h e r diffusion follows nucleation and growth or phase boundary reaction. It can be seen firther that if li < c~2 i, nucleation and growth will be replaced by phase boundary interaction, f o l l o w e d by diffusion. If ~Zli >~x2i, nucleation-growth control will be replaced directly by diffusion control, without the intermediate stage of phase boundary, interaction. In the present case it was observed that '~li > ~ 2 i

The total degree of transformation was

.............. (19) The ~7(-r) values were compared with ~: -t plots and the relative time ( T ) corresponding to each of these was found out. The calculations were done with the help of a computer. The time (t) was plotted against T for each R L - R D combination. Various combinations like R L = 0.9, R D = 1.3, R L = 1.0, R D = 1.4, R L = 1.0 and R D =1.25 were used (Fig. 10). The slope of the line was taken as to, following the relation. t observed = to "-5"o~-Calcu. + ta .......... (20) The actual rate constants kn, k c and k d for the lime silica reaction (C/S= 1) carried out for 139oc

Vol. 22, No. 4

LIME, RICE HUSK ASH, HYDROTHERMAL REACTION, STAGES

585

were calculated using the relations k n = A/t o .............. (21) kc= BRL/to .............. (22)

0.32 O,JO

kd= CR D 2/t o

0.28

............. (23)

0.2#

For the first acceleration period, the values were found to be kn = 8.0 x 10-3 (min-1) kc= 4.89 x 10-3 (~tm min -1) and k d = 3.05 x 10-3 (t.tm2 min -1)

0"2~ 0,22 0.20

O•W

T

O'PI~ #'12 0"~

0.01

"5

0-04

O

H

1~

G02

0

60

I20

leo ~

~

.1610 /JO ~ fINE

SI,O ~

U O ;'20

Imln)

FIG. 5 Variation of ~<- function (phase-boundary reaction)

The analysis of the data of the second acceleration period was also carded out in a m a n n e r indicated by Bezjak and co-workers (13-16). The various steps involved are similar to those described under the first acceleration period, but the calculations are done using recalculated particle radii (R'i). R' i = R i [1 -
O.Otl

..............

The °~'i values were calculated according t6 equations 14, 15 and 18 (smceo(li was found to be lesser than ~4 2i) but instead of T, T' values were used. •

O,og





O.OS

13



"3-'= 7 - - ~V~

0"04

O.O.1

0,02

0 O ~ ~ L ~ _

00! • 0

(25)



0.07

60

f20

REACI'IONTEMPERATURE ('C / 0~0 e~.5 "- .~ ?0o Ho = = r39

t#O 2'40 300 360 ¢20

¢60

5,¢0

aO0

6aO ;'20

;'SO

P

......... (26)

where q'g = tg/to In the second accelaration period, prior to calculating thetotal degree of hydration ~ c a l c , the ~i values obtained frima the equations 13, 14 and 17 using R' i w e r e converted to °< i for R'i, using the relation. c< i , = ~ g i + °
rlMEImln)

FIG. 6 Variation of ~- function (Diffusion)

The above procedure was repeated with various R L and R D combinations and a linear

586

P.R. Khangaonkaret al.

Vol. 22, No. 4

Co0/5102 RATIO = !

-2"80

PERIOD

-2"85 -2.90 -295

I -3.oo -3.05

-3. ;0 -3.15 !

-3.20 20

2.2

2.!

2

2.1,,

2"5 2 6 l/Tx

2"7

2"

2"93

3'

3"2

~0"~(" k }

FIG. 7 Variation o f k n with temperature

-3'2

CctO/Nz02 R A llO = 1 •

7 ACCF[ERATION

-3"3

PF,'TIO[3

ELERAHON PERIOD

-3.4 -3

'5

-3 6 t~ 9.

-3.7 -38 -3.9

-4'0 -4 -! -1. "2

I

2.0

24

|

I

i

i

i

i

2.1

2.3

24.

2"5

2.6

2.7

2.8

29

i

t

3-0

3.r

I / r x1oJ ( ' k )

plot was obtained with R' L = 1.0 and R' D = 1.0. the to values were obtained from the slopes and the rate constants were calculated using the equations 21, 22 and 23.The rate constants thus calculated were found to be much larger than those obtained for the first acceleration period. This is c o n t r a d i c t o r y to the experimental observation of k n l > kn2, > kc2 and k d l > kd2.

Bezjak and co-workers, in their work, used quartz, with which lime reaction is much slower than with rice-husk silica, the initial reaction between lime and amorphous rice-husk silica apparently produces a gel-like coating of reaction product, which hinders the progress of the reaction, rendering it much slower. Keeping all the above observations in view, the mechanism suggested by Greenberg (18) seems to be applicable to the reaction studied in the present work. According to Greenberg, lime is chemisorbed on the surface of silica. The Si-O-Si bond in silica is then hydrolysed by OH- ions and H 2 0 to form H2SiO42-. The H2HiO42- ions react with Ca 2+ ions forming nuclei of calcium-silicate hydrate. This is then f o l l o w e d by the growth and crystallization of CSH. CONCLUSION

The reaction between lime and rice-husk silica was observed to exhibit two acceleration periods. The participation of various rate determining processes together was observed since the material was a polysize sample, the application of a mathermatical model for the two stage acceleration process was satisfactory for the first acceleration period but not for the second acceleration period. FIG. 8 Variation of kc with temperature

REFERENCES 1.

H.H. Steinour, "The system CaO-SiO2-H20 and the hydration of calcium silicates" chem. Rev. 40 (3) 391-460 (1947).

Vol. 22, No. 4

LIME, RICE HUSK ASH, HYDROTHERMAL REACTION, STAGES

587

2. D.S snell, Review of synthesis and properties of tobemorite, CSH (I) and C-S-H c.OlS,a , Rar,o . , gel" J. am. Ceram. soc. 5._.88(7 - 8) 292 - 5 • I ACCELER~aTION PERIO0 (1975) -3"5 0 I I ACCELERATION PERIOD 3. Naomichi Hara, Hideo Yamoda, Norihiro Inoue, "Calcium silicate hydrates by -3"7 hydrothermal reaction as refractory materials for thermal insulation". Brit. UK Pat. Appl. -3"9 G 8 2 , 106, 087 (cl. CO I B33/24) 07 Apr. -,L'O 19836pp. (C.A 99: 271739) 4. Jose James, M. subba Rao., "Reaction product of lime and silica from rice husk ash' Cem. con. Res. 16 (1), 67 - 73 (1986) 5. Jose James; M. subba Rao, "Reactivity of rice husk ash". Cem. Con. res. l___fi6(3)., -I,.5 0 296-302 (1986) -¢.6 6. D.J. cook. a discussion of the paper "Reaction product of lime and silica from rice -I,.7 husk ash" by James J. and subra Rao. M., -¢st I I I i i i I 2.2 2"3 24* 2"5 2"6 2"7 ~,8 2.9 cem. Con. Res. 1__.7_7(4), 685-6 (1987) l / T x '/OJl°k) 7. J.James, M. subba Rao; a reply to a discussion by D.J. Cook of the paper FIG. 9 Variation of k d with temperature. "Reaction product of lime and silica from rice husk ash" Cem. con. Res. 17 (4), 687-90 (1987) 8. H.E1-Didamony, M.G. Abd. E1 Wahed, K.M.Elewa, A.A. Amer., "Rice 16tl husk ash and its resistivity in the formation of dicalcium silicate (13.2.CaO.SiO2)", Arab 11,0 Gulf. J, Sci. Res. 5 (1), 45-57 (1987) (C.A. 120 108: 42894y and 109: 40218C). 9. Hiroaki Noma, Norihiro Inoue, Hideo 100 Yamada, Naomichi Hara, "Synthesis of ~ L ..y_ RD Xonotlite using rice hisk ash as a siliceous ? 80 material". Kyushu Kogyo Gijut-Su :t 1.25 Shikensho Ho Ko Ku 3___99,2531-40 (1987), 0---.0 i (C.A. 108: 97124C). O---,,,'O 0.9 1.3 10. J. James, M. Subba Rao, "Hydration H ! 7.1,. of rice husk ash-lime pastes", cemento 8_.A4 (4), 383-96 (1987) (C.A. 109: 13448Z). 11. Annual book of ASTM standards, Part I I | I I | 0 I I 13 - Cement, Lime, Gypsum, C25 p32 0.2 0-" 0 6 o , e , . o ,.2 , ~ ,-6 (1982). r {z=~lt,) 12. S. Anand. R.P. Das., "Treatment of extraction curves which may simultaneously FIG. 10 Correlation of t and "J'. follow both diffusion as well as chemically controlled models" Trans of IIM 4_.1_1(4) 335-341 (1988). 13. A. Bezjak, I. Jelemic, "On the determination of rate constants for hydration process in cement pastes". Cem. Con. Res. 1...00(4) 553-563 (1980) 14. A. Bezjak; V. Alujevic, "A kinetic study of hydrothermal reactions in C2S-Quartz system: I. Determination of rate constants for processes with two acceleration periods." Cem. Con, Res. "1__!(1) 19-27 (1981) 15. V. Alujevic, A. Bezjak, "A kinetic study of hydrothermal reaction in C2S - Quartz system: II. Influence of granulomety of quartz and of the treatment of samples" Cem. Con. Res. 13 (1) p 34-40 (1983). o

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V. Alujevic, A. Bezjak, A. Gillosnovic, "Kinetic study of the hydrothermal reactions in CaO-Quartz system", Cem. Con. Res. 16 (5) 695-699 (1986). H.G. Midgley; "Hydrothermal reactions in the lime-rich part of the system CaO-SiO2-H20" Magazine of Concrete Research 12 (34) p 19-26 (1960) S.A. Greenberg, J. Phys. Chem 61. (1957), 373.