The thermal decomposition of Ca3SiO5 at temperatures below 1250o

The thermal decomposition of Ca3SiO5 at temperatures below 1250o

CEMENT and CONCRETERESEARCH. Vol. 7, pp. 269-276, 1977. Pergamon Press, Inc. Printed in the United States. THE THERMAL DECOMPOSITION TEMPERATURES ...

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CEMENT and CONCRETERESEARCH. Vol. 7, pp. 269-276, 1977. Pergamon Press, Inc. Printed in the United States.

THE

THERMAL

DECOMPOSITION

TEMPERATURES PART

2.

THE OXIDES

K.

BELOW

INFLUENCE ON T H E

Mohan

and

OF

OF C a 3 S i 0 5

AT

1250 ° Mg,

Fe,

A6

and N a

DECOMPOSITION

F.P.

Glasser

Department of C h e m i s t r y , U n i v e r s i t y of A b e r d e e n M e s t o n Walk, 01d A b e r d e e n A B 9 2UE, S c o t l a n d

(Communicated by H. F. W. Taylor) (Received February I, 1977)

ABSTRACT The decomposition k i n e t i c s of C a 3 S i 0 5 s o l i d s o l u t i o n s c o n t a i n i n g A6, Mg, Fe, N a and (Mg + Fe) h a v e b e e n s t u d i e d by d e t e r m i n i n g the r a t e at w h i c h free l i m e forms during isothermal annealing. Ag has l i t t l e effect, Fe m a r k e d l y a c c e l e r a t e s d e c o m p o s i t i o n , w h i l e N a 2 0 has a s l i g h t r e t a r d i n g e f f e c t and M g m a r k e d l y retards decomposition. In the p r e s e n c e of b o t h M g and Fe, the a c c e l e r a t i n g e f f e c t of Fe d o m i n a t e s .

RESUMEN .I

Se h a e s t u d i a d o la d e s c o m p o s i c l o n c i n e t i c a de s o l u c i o n e s s o l i d a s de C a 3 S i 0 5 c o n t e n i e n d o A6, Mg, Fe, Nay (Mg + Fe), d e t e r m i n a n d o v e l o c i d a d de f o r m a c i ~ n de la cal f i b r e d u r a n t e c o c i m i e n t o s isot~rmicos. A6 t i e n e p o c o e f e c t o , el Fe a c e l e r a m a r c a d a m e n t e la d e s c o m p o s i c i ~ n , m i e n t r a s N a 2 0 t i e n e un l i g e r o e f e c t o r e t a r d a n t e y el M g r e t a r d a m a r c a d a m e n t e la d e s c o m p o s i c i ~ En p r e s e n c i a de ambos M g y Fe, d o m i n a el e f e c t o a c e l e r a d o r del Fe.

269

270

Vol. 7, No. 3 K. Mohan, F. P. Glasser

Introduction In part I of this paper (1) we determined and analyzed the kinetics of decomposition of C3S and compared the results with those reported in the literature. We concluded that decomposition below 1250 ° is initiated by surface nucleation and that it is catalyzed by both of the products (Ca0 and C2S ) . A review of the literature regarding the effects of other ions liable to be present in cement clinkers revealed that the data were conflicting or inconclusive. We now report data on the effects of Mg, A6, Na and Fe on the decomposition of C~S. Influence

of

M60

Figs. I and 2 show data for and Fe in solid solution, which a control sample of pure C3S. were the same as those described retards the decomposition while However, these results can best compositions and thermal histories taken into account.

and

Iron

Oxide

decomposition of C~S with Mg are contrasted with data for The experimental methods used in Part I. At 1175 °, Mg0 Fe markedly accelerates it. be understood if the phase of the preparations are

Adding Mg0 to C3S results in the formation of Ca0 during subsequent refiring. This is in accord with predictions made from the CaO-Mg0-SiO 2 phase diagram (2). B u t i f M g O is instead a d d e d as 'M3S' - w i t h M g O a n d S i O 2 i n t h e r a t i o 3:1 free lime should not be produced at e q u i l i b r i u m . In practise, it w a s n e c e s s a r y to f i r e at a b o u t 1 5 5 0 ° to a c h i e v e a satisfactory degree of homogenization of Mg within the C3S phase in reasonable time. Sample A was made by adding 2.0% o f M g 0 to C 3 S a n d r e f i r i n g at 1 5 5 0 o ; the free Ca0 content was 1.5%, w h i c h agrees with the phase diagram. In samples B and C, 2 . 0 % o f M g O w a s a d d e d t o g e t h e r with an equivalent amount of SiO 2 in accordance with the formula M3S, and no free lime was detectable. T h e M g O w a s a d d e d as o x i d e f o r s a m p l e B a n d as nitrate for sample C. The source of the Mg did not prove to be a significant variable, so that a common curve has been fitted in Fig. I to the points for B and C. This curve most correctly represents the effect of Mg 2+ in solid solution: that is, tMgO' has been compensated by Si02 additions so as not also to produce free Ca0. Mg is thus seen to have a significant retarding effect. The interpretation of curve A is more complicated, as two competing effects operate: the retarding effect of Mg in solid solution and the accelerating effect resulting from the presence of free Ca0. Microscopic examination of this preparation showed that the CaO grains tended to collect into clusters at the intersections of C3S grain margins; this matrix isolation of the CaO grains helps greatly to reduce its effectiveness as a nucleating agent. While we think that these same textural relationships will occur in clinkers in which a liquid phase is also present, a different textural distribution of CaO might more effectively accelerate the decomposition. Thus reports in the literature claiming that Mg -added as Mg0 -accelerates decomposition are not impossibly at variance with our findings.

Vol. 7, No. 3

271 C3S DECOMPOSITION, KINETICS, IMPURITY EFFECT

25

25

20

2O

///c3s

B,C gJ m~

"u 15

,~ 15

uJ tU riM.

5

0

i

~o

4~o 600 ~o lO'OO 1200 ' 1400 '

80 1(~0 2~10 3:20 4 ~ 480 5~0 640 720 TIME (hours)

TIME (hours)

FIG.

I

FIG.

2

Fig.

I. Decomposition of C 3 S and M g 0 - c o n t a i n i n g C3S solid s o l u t i o n s (samples A, B and C) at 1175 °. See text for additional details. S a m p l e s B and C h a d s p e c i f i c s u r f a c e s (Blaine) of ~I00 m2/kg.

Fig.

2. Decomposition of C 3 S and i r o n - o x i d e c o n t a i n i n g C 3 S s o l i d s o l u t i o n s ~ a m p l e s A, B and C) at 1175 °. See text for additional details. S a m p l e s B and C h a d s p e c i f i c s u r f a c e s (Blaine) of N53 m2/kg

At 1550 ° , C ~ S is s a t u r a t e d by a p p r o x i m a t e l y I% M g 0 (as M3S): we f i n d n~ s i g n i f i c a n t d i f f e r e n c e b e t w e e n the d e c o m p o s i t i o n r a t e s of p r e p a r a t i o n s containing I% and 2% Mg0. Fig. 2 s h o w s d a t a o b t a i n e d f r o m the i s o t h e r m a l d e c o m p o s i t ion of i r o n o x i d e - c o n t a i n i n g preparations at 1175 °. Fe m a r k e d ly a c c e l e r a t e s d e c o m p o s i t i o n . T h e e x t e n t to w h i c h d e c o m p o s i t i o n is e n h a n c e d d e p e n d s b o t h u p o n the t o t a l i r o n c o n t e n t in the range

0

-

4.0

wt.

% iron

(as

Fe203)

,

as

well

as

upon

the

t h e r m a l h i s t o r y of the p r e p a r a t i o n . H o w e v e r , the rate d e p e n d e n c e of the d e c o m p o s i t i o n on the i r o n c o n t e n t is not m a r k e d up to 50% C 3 S d e c o m p o s i t i o n , only b e c o m i n g more i m p o r t a n t at l a t e r s t a g e s . Fig. 2 s h o w s t h o s e d e c o m p o s i t i o n curves w h i c h are p r o b a b l y m o s t r e l e v a n t to c l i n k e r i n g : the C 3 S was s a t u r a t e d in Fe by a d d i n g 1.1 wt. % F e 2 0 3 . In the p r e p a r a t i o n designated (A), i r o n w a s a d d e d as C 2 F and the p r e p a r a t i o n h o m o g e n i z e d by f i r i n g at 1375 - 1400 ° in air, that is, just b e l o w the s o l i d u s t e m p e r a t u r e of C 3 S - C 2 F m i x t u r e s . Preparations (B) and (C) a l s o c o n t a i n e d 1.1% t o t a l i r o n oxide~ in the former, it w a s a d d e d as C 2 F , and in the l a t t e r as F e 2 0 3 ; h o w e v e r (B) and (C)

272

Vol. 7, No. 3 K. Mohan, F. P. Glasser

were fired at 1550 ° . None of the three samples developed any free Ca0 during firing. The Ca0/Fe203 ratio did not prove to be a significant variable: however, the firing temperature did prove to be important. The data obtained on preparations (B) and (C) are probably more relevant to clinkering because iron was present in excess of that required to saturate the

C3S and a liquid phase developed during firing. °C

°C

1250

125C o%

120(:

1200

l

115G

110(:

1050

1150

1100

1050

1000

1000

950

\

~

I

~

161

i

71

i

950

61

3 £'a TIME (hours)

P.n TIME (hours)

FIG.

3

Time-temperature-transformation of C3S solid solutions. decomposed. Fig.

3. as

Fig.

4o (as

Solid M~S) at

solution 1550 ° .

Solid solution C2F ) at 1550 ° .

FIG. curves Percentages

4

for the decomposition are the weight ~ of

made

by

firing

C~S

made

by

firing

at

with C3S

2~ with

Mg0 I.]~

C3S

(added Fe203

The effects of Mg and Fe on the decomposition at temperatures other than 1175 ° are shown in Figs. 3 and % by T-T-T curves These are derived from isothermal decompositions made at temperatures between 1025 and 1220 °. The shapes of the T-T-T curves are similar to those obtained from pure C3S; see Fig. 2, part 1. There is, however, some suggestion tha% the otherwise smooth T-T-T curves exhibit a slight discontinuity at ~105 °o . On account of the long time which is required for partial decomposition, we have but few data points in this temperature range. It seems reasonable that a discontinuity could exist because rhombohedral C3S and its solid solutions transform to lowersymmetry phases at about this temperature. The existence of a discontinuity has therefore been suggested by breaking the T-T-T curves at ~I050 °.

Vol. 7, No. 3

273

C3S DECOMPOSITION, KINETICS, IMPURITY EFFECT

Influence

of A 6 2 ~ 3

25

200

400 TIME (hours)

FIG.

6oo

5

Decomposition at 1175 ° of C3S solid solutions containing A6203. See text f o r e x p l a n a t i o n of symbols. Influence

As w i t h MgO, a d d i t i o n of A6 m a y c a u s e f r e e l i m e to f o r m w h e n preparations are s u b s e q u e n t l y fired. Fig. 3 s h o w s some typical results. Open triangles show data points obtained from a preparation c o n t a i n i n g 2% A g 2 0 3 as C 3 A and f i r e d at 1550 ° . D u r i n g f i r i n g . it d e v e l o p e d 0 . 7 % free lime. T h i s m a y be c o m p a r e d w i t h the b e h a v i o u r of a p r e p a r a t i o n c o n t a i n i n g I% A 6 2 0 3 a d d e d as 'C2A' ( c r o s s e s ) w h i c h , a f t e r firing, contained essentially z e r o free lime. C o m p a r i n g this w i t h the C 3 S c o n t r o l s a m p l e (open c i r c l e s ) , a l u m i n a a d d e d as C 2 A is s e e n to h a v e v i r t u a l l y no e f f e c t on the r a t e of decomposition. The increased r a t e of d e c o m p o s i t i o n in the p r e p a r a t i o n c o n t a i n i n g C 2 A is a t t r i b u t e d to its f r e e l i m e content; the i n c r e a s e im r e l a t i v e l y s l i g h t b e c a u s e of the l i m i t e d c o n t a c t b e t w e e n C a 0 and C3S grains. of N a 2 ~

It is d i f f i c u l t to m a k e a C 3 S s o l i d s o l u t i o n h a v i n g a homogeneous distribution of a l k a l i ions. During firing, alkali escapes preferentially f r o m the s u r f a c e s of g r a i n s and w e s u s p e c t that s i n t e r e d s a m p l e s w e r e n o t e n t i r e l y h o m o g e n e ous. Two preparations were made ; analysis after firing s h o w e d that the a v e r a g e N a 2 0 c o n t e n t of one w a s 0 . 6 5 % N a 2 0 and that of the o t h e r , 5 . 0 % Na20 ( b o t h as "N3S" ). The former h a d 0 . 8 3 % f r e e Ca0, the l a t t e r 0.09%. A correction factor h a s b e e n a p p l i e d to r e s u l t s o b t a i n e d f r o m the 5% N a 2 0 s a m p l e to a l l o w for the fact t h a t it c o n t a i n e d i n i t i a l l y l e s s t h a n 100% C3S. Compared with pure C3S , which attains 50% decomposition in ~ 2 8 0 h at 1175 °, the 0 . 6 5 and 5 . 0 % Na20 preparations required, respectively, ~ 4 0 0 and ~ 4 5 0 h to reach half decomposition. T h u s , t h e s u b s t a n t i a l free l i m e is r e l a t i v e l y i n e f f e c t i v e in c o u n t e r a c t i n g the r e t a r d i n g a c t i o n of N a b e c a u s e the C a 0 g r a i n s c l u s t e r at the i n t e r s e c t i o n s of C3S grains. S u f f i c i e n t d a t a w e r e o b t a i n e d at o t h e r t e m p e r a t u r e s to s h o w firstly, that the m a x i m u m r a t e of d e c o m p o s i t i o n o c c u r r e d at ~ 1 1 7 5 ° and s e c o n d l y , t h a t a l k a l i m a r k e d l y r e t a r d s decomposition at o t h e r t e m p e r a t u r e s b e t w e e n 1025 ° a n d 1210 ° . T h e d a t a o b t a i n e d w e r e not, h o w e v e r , s u f f i c i e n t l y c o m p l e t e to c o n s t r u c t a f u l l set of T - T - T c u r v e s .

274

Vol. 7, No. 3

K. Mohan, F. P. Glasser

Influence 25

T

r

of

both

1.1{ Fe203 (as C2~)o

Fe.

After

f i r i n g at 1 5 5 0 ° it c o n t a i n e d C3S and liquid, and analysis gave 0.0~ free CaO. Decomposition d a t a at 1025 ° and 1175 ° are shown in Fig. 6. We calculate that complete decomposition of the C3S would give ~ 22~ free CaO. The rate of decomposition of this solid solution does not differ significantly at either temperature from the rate obtained in the presence of Fe only (see Fig. 2).

15

.J Ill ~J (¢ U.

Mechanism I

I

100

J

J

200

300

TIME (hours)

FIG.

6

Decomposition at 1 0 2 5 ° crosses) and 1175 ° open circles) of a C9S solid solution cSntaining both Mg and Fe203 .

I

and

We studied one preparation in detail with beth a retarder (Mg) a n d an a c c e l e r a t o r (Fe) ; it c o n t a i n e d 2 ~ M g O (as M 3 S ) a n d

20

LU

Mg

i

of

the

Decomposition

The application of the Avrami equations to these data and the significance of the parameter n was discussed in the first part of this paper. Values of n calculated for C3S solid solutions are shown in Table I. The nucleation mechanism is, in each case, surface nucleation.

Discussion

Empirical observations have shown that some oxides in solid TABLE I solution enhance the decomposition of C]S while othersinhibit it. ~3S Composition T-T-T curves show that the lower Pure 2.6 stability l i m i t o f 0 3 S is n o t Iron saturated 2. 5 appreciably altered by solid solution, and that the temperature A6 2 0 3 " 2 .5 MgO " 2 o2 of the maximum rate of decomposition - ~1175 ° does not vary Na20 " 2° I significantly. H e n c e it is appropriate to refer to some oxides as tretarders' and others as ' a c c e l e r a t o r s ' . However w e a r e n o t , as y e t , a b l e to o f f e r an explanation of why iron oxide, either alone or in conjunction with others, acceleratesdecomposition, while other oxides are either neutral or inhibitory. Iron oxide may differ from the others because of changes which can occur in its oxidation state. It m a y b e r e c a l l e d t h a t t h e i r o n is f i x e d i n the C 3 S s t r u c t u r e by firing in a band of high temperatures, typically 1 4 0 0 - 1 5 5 0 °, w h i l e d e c o m p o s i t i o n is m e a s u r e d in lower-temperature band. At the constant oxygen pressure u~ed in these experiments(air : PO~ = 0 21 a r m ), c h a n g e s i n o x i d a t i o n s t a t e m i g h t o c c u r b e t w e e~n t h e" t w o r a"n g e s of temperature.

Vol. 7, No. 3

275

C3S DECOMPOSITION, KINETICS, IMPURITY EFFECT On a c c o u n t of the s m a l l q u a n t i t y of i r o n w h i c h d i s s o l v e s in C 3 S , it is d i f f i c u l t d i r e c t l y to d e t e r m i n e the o x i d a t i o n s t a t e of the i r o n b y c h e m i c a l a n a l y s i s . We have now prepared C3S solid solutions containing F e 5 7 a n d are e x a m i n i n g these by M~ssbauer spectroscopy. D e s p i t e the q u a n t i t y of d a t a thus f a r a c c u m u l a t e d , w e do n o t y e t t h i n k it a p p r o p r i a t e to d r a w a n y c o n c l u s i o n s about the d e c o m p o s i t i o n of C 3 S or a l i t e in a c l i n k e r , b e c a u s e two more important c h e m i c a l v a r i a b l e s h a v e y e t to be a s s e s s e d : the s u l p h a t e c o n t e n t of the c l i n k e r a n d the w a t e r v a p o u r p r e s s u r e o f the a t m o s p h e r e in c o n t a c t w i t h the s a m p l e . Part 3 o~ this paper will provide data showing how sulphate and water influence the d e c o m p o s i t i o n . Acknowledgement Miss area

T h e a u t h o r s are g r a t e f u l f o r the a s s i s t a n c e of A.E. M o o r e and Mr. J.A. D a l z i e l w h o m a d e the s u r f a c e determinations r e p o r t e d in p a r t s I and II of t h i s p a p e r . References

1 •

Ko

Mohan

and

F.P.

2.

R.W. Ricker and 37, 133 (1954).

Glasser,

E.F.

Cement

Osborn,

J.

Concr.

Amero

Res.

Ceram.

~, I (1977). Soc.,