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Rate of alite-formation in clinker sandwiches
CEMENT and CONCRETERESEARCH. Vol. 8, pp. 693-702, 1978. PergamonPress, Inc Printed in the United States.
RATE OF A L I T E - F O R M A T I O N
N.H.
Christensen,
O.L.
IN C L I N K E R S A N D W I C H E S
J e p s e n and V. J o h a n s e n
F.L. Smidth & Co. A/S 77 V i g e r s l e v Alle, DK-2500 V a l b y D e n m a r k
(Communicated by J. Skalny) (Received July 27, 1978)
ABSTRACT The r e a c t i o n rate of the alite reaction, C + C2S ~ C3S , was m e a s u r e d by m e a n s of a s a n d w i c h technique, for e x p e r i m e n t a l c o n d i t i o n s p e r t i n e n t to b u r n a b i l i t y studies. The g r o w t h of the alite layer f o l l o w e d the p a r a b o l i c rate law, X 2 = k.t, where X is w i d t h of layer and t r e a c t i o n time. k (cm2 sec -I) was found to i n c r e a s e with i n c r e a s i n g m e l t c o n t e n t in the structure, w i t h i n c r e a s i n g temperature, and w i t h i n c r e a s i n g s u b s t i t u t i o n of Fe203 for AI203 in the melt. S u b s t i t u t i o n of MgO for AI203 up to M g O - s a t u r a t i o n had no s i g n i f i c a n t e f f e c t on k.
Die R e a k t i o n s g e s c h w i n d i g k e i t fur die A l i t - R e a k t i o n , C + C2S ~ C3S , w u r d e m i t t e l s einer D i f f u s i o n s p a a r technik u n t e r e x p e r i m e n t e l l e n B e d i n g u n g e n gemessen, die fur das S t u d i u m der B r e n n b a r k e i t yon I n t e r e s s e sind. Der Zuwachs der Dicke, X, der A l i t s c h i c h t folgt dem p a r a b o l i s c h e n Gesetz, X 2 = k x t, in dem t g l e i c h der R e a k t i o n s d a u e r ist. Es w u r d e festgestellt, dass mit a n s t e i g e n d e m G e h a l t an Schmelze in der Struktur, a n s t e i g e n d e r T e m p e r a t u r und a n s t e i g e n d e m Ersatz durch Fe203 fur A1203 in der S c h m e l z e der W e r t k (cm2 sec-l) anw~chst. Ersatz durch MgO fHr A 1 2 0 3 bis zur M g O - S i t t i g u n g h a t t e k e i n e n s i g n i f i k a n t e n E i n f l u s s auf k.
693
694
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen Introduction
The f a c i l i t y with w h i c h the c o m p o n e n t s of any p a r t i c u l a r P o r t l a n d cement raw mixture are combined, c o m m o n l y d e s i g n a t e d the burnability, may be d e t e r m i n e d in the l a b o r a t o r y by n o t i n g the p e r c e n t a g e of u n c o m b i n e d lime, w h i c h remains in the p r o d u c t a f t e r it has been s u b j e c t e d to a s p e c i f i e d t r e a t m e n t of c o m p a c t i n g and heating. It is b e l i e v e d that the critical step in such a t r e a t m e n t is the h e a t i n g at 1 4 0 0 - 1 5 0 0 o c a s s o c i a t e d with the alite reaction: C + C2S ~ C3S. Therefore, u n d e r s t a n d i n g of the k i n e t i c s and m e c h a n i s m of the alite r e a c t i o n may be the key to solving b u r n a b i l i t y problems. In this paper we report on rate data for alite f o r m a t i o n m e a s u r e d in s a n d w i c h couples, for v a r y i n g c o n t e n t of melt, for v a r y i n g c o n c e n t r a t i o n s of Fe203, MgO and AI203 in the melt, and for v a r y i n g r e a c t i o n temperature. Sandwich Technique A s a n d w i c h is a couple of two compact samples, also c a l l e d e n d - m e m b e r s , w h i c h are joined at a c o m m o n plane interface. The c o m p a c t s should be h o m o g e n e o u s and be e q u i l i b r a t e d s e p a r a t e l y before the joining. The s a n d w i c h is h e a t e d i s o t h e r m a l l y at Toc for t secs, then a i r - q u e n c h e d , s e c t i o n e d p e r p e n d i c u l a r l y to the contact plane, p r e p a r e d for m i c r o s c o p y and then i n s p e c t e d in the m i c r o s c o p e . A layer of r e a c t i o n products has u s u a l l y been formed at the c o n t a c t plane; t y p i c a l l y it has sharp b o u n d a r i e s towards the o r i g i n a l end-members, so that its w i d t h can be e a s i l y measured. The a d v a n t a g e of this technique, c o m p a r e d to p o w d e r techniques, is that p a r t i c l e size effects are eliminated. Moreover, k i n e t i c s are s i m p l e r and often e a s i e r to interpret. The mere joining of a C a O - c o m p a c t w i t h a C 2 S ~ c o m p a c t r e p r e s e n t s a s i t u a t i o n too d i s s i m i l a r to the alite r e a c t i o n in technology. Kondo and Choi (i) improved the t e c h n i q u e by i n t r o d u c i n g t w o - p h a s e e n d - m e m b e r s , a f i n e - g r a i n e d CaOc o m ? a c t with an i n t e r s t i t i a l melt, and a f i n e - g r a i n e d C 2 S + m e l t compact. A layer c o n s i s t i n g of C 3 S + m e l t was o b s e r v e d to grow at the joint. We have found it more c o n v e n i e n t to use 3-phase endmembers, C+C3S+melt, and C 3 S + C 2 S + m e l t (2), Fig. i. I n v a r i a n c e in t h r e e - c o m p o n e n t t h r e e - p h a s e regions is t h e r e b y achieved, w h i c h g r e a t l y s i m p l i f i e s the i n t e r p r e t a t i o n , since the d i s a p p e a r i n g regions now remain c o n s t a n t in c o m p o s i t i o n and chemical p o t e n t i a l s d u r i n g a k i n e t i c run. The same is not always true, w h e n t w o - p h a s e e n d - m e m b e r s are used, cf. the couples 1-2 and 3-4 in Fig. 2. In points 3 and 4 the c o m p o s i t i o n of the m e l t will change along the isotherms U-Y and V-Z d u r i n g the kinetic run, w h i l e in 1 and 2 the melts remain fixed at U and V. Even with fourand f i v e - c o m p o n e n t clinker sandwiches, the use of these t h r e e - p h a s e e n d - m e m b e r s helps to m i n i m i z e v a r i a t i o n s in the C a O - c o n c e n t r a t i o n of the d i s a p p e a r i n g regions.
Vol. 8, No. 6
695 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE
C3S*CaO ÷ Liq
| ~
1
C3S*C2S * Liq
%s A,2%F..,o3
1500°C
C1
C2
,
Liq
)
CaO
FIG.
'-I
I ~ X
FIG.
~
1
P r i n c i p l e of s a n d w i c h technique.
2
I s o t h e r m a l section (1500 o) of C-S-(A,F) p h a s e diagram, showing t w o - p h a s e fields C 2 S - Z - V , C 3 S - V - U and CaO-U-Y, and t h r e e - p h a s e fields C 2 S - C 3 S - V and C 3 S - C a O - U .
Model for S a n d w i c h K i n e t i c s Let X be the w i d t h of the p r o d u c t layer formed in a s a n d w i c h after t secs at T ° . Experience, and d e d u c t i o n b a s e d on a d i f f u s i o n c o n t r o l l e d m e c h a n i s m , show that the g r o w t h follows a p a r a b o l i c law X 2 = k-t
(i)
w h e r e k is a f u n c t i o n of T and e n d - m e m b e r
composition.
For a b i n a r y sandwich, whose c o n s t i t u e n t s only are A and B, it may be shown (e.g. by c o m b i n i n g e q a t i o n s (2c), (9a) and (gb) of ref. (3)) that k = 2D(ca-cb).H,
with H = i/(cl-ca)+i/(cb+c2)
(2),(3)
Here, D is the b i n a r y i n t e r d i f f u s i o n c o e f f i c i e n t at T for the p r o d u c t phase; c's are c o n c e n t r a t i o n s of e n d - m e m b e r s (Cl and c2) and of p r o d u c t phase, at the b o u n d a r y towards i, Ca, and at the b o u n d a r y towards 2, c b. In the c l i n k e r s a n d w i c h e s the e n d - m e m b e r s and the g r o w i n g layer have a m i c r o s t r u c t u r e of two or three phases, and the d i f f u s i o n is m u l t i c o m p o n e n t (4). The p r o b l e m may, however, be t r e a t e d as a p s e u d o - b i n a r y d i f f u s i o n problem, a s s u m i n g that the local c o m p o s i t i o n s lie on the s t r a i g h t c o n n e c t i o n line 1-2; therefore, on the v e r y short lines e c t i o n a-b in Fig. 2~). c's are now w e i g h t f r a c t i o n CaO in local m u ! t i p h a s e s t r u c t u r e s (in e n d - m e m b e r s , c I and c 2,
*) The c o m p o s i t i o n s l-a and b-2 d e g e n e r a t e of no spatial extension, cf. ref. (5).
to b o u n d a r i e s
696
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen and in alite layer, Ca and Cb). If the line a-b (or 1-2) w e r e p a r a l l e l to U - V (melts in e q u i l i b r i u m w i t h C + C 3 S and C 3 S + C 2 S , r e s p e c t i v e l y , at T), we w o u l d h a v e Ca-C b = a ( c u - c v ) , w h e r e a is w e i g h t f r a c t i o n of m e l t in a l i t e layer. T h e r e f o r e , in g e n e r a l Ca-C b = a . h . ~ m
(4)
w h e r e Am = CU-CV, and h a c o r r e c t i o n f a c t o r a c c o u n t i n g for n o n - p a r a l l e l l i t y b e t w e e n 1-2
(1.1-1.3) and U-V.
D of eq. (2) s h o u l d n o w be r e p l a c e d by an e f f e c t i v e b i n a r y d i f f u s i o n c o e f f i c i e n t for CaO in the t w o - p h a s e a l i t e s t r u c t u r e : DO. This, in turn, m a y be w r i t t e n DO = D~-F, w h e r e D~ is the e f f e c t i v e b i n a r y d i f f u s i o n c o e f f i c i e n t for CaO in the melt. D~ is e x p e c t e d to v a r y w i t h c h e m i c a l c o m p o s i t i o n of the m e l t (e.g. w i t h % A 1 2 0 3 / % F e 2 0 3 ) and w i t h T. It m a y a l s o v a r y w i t h the d i r e c t i o n of the c o m p o s i t i o n line 1-2, Fig. 2, (6). F is a s h a p e f a c t o r
(5)
F p r o b a b l y v a r i e s w i t h a, p o r o s i t y and p o r e size. The p o r o s i t y of these s a n d w i c h e s w a s low (~0.i). Pores were s m a l l e r in s a n d w i c h e s w i t h low m e l t c o n t e n t than in s a n d w i c h e s w i t h h i g h c o n t e n t of melt, 5-20 ~m v e r s u s 25-100 ~m. For e a c h s a n d w i c h H of p o i n t a was f o u n d i n t e r s e c t i o n of line For this p u r p o s e the linear interpolation C-S-F diagrams (7). the a l i t e - l a y e r was w h i c h was c a l c u l a t e d
and a w e r e c a l c u l a t e d . The c o m p o s i t i o n by an a n a l y t i c a l p r o c e d u r e s i m u l a t i n g 1-2 w i t h line C 3 S - U , cf. Fig. 2. c o m p o s i t i o n of U was d e t e r m i n e d by a of the r e a d i n g s f r o m the C - S - A and the The w e i g h t f r a c t i o n of melt, a, in i d e n t i f i e d w i t h the m e l t c o n t e n t for a, f r o m the e x p r e s s i o n
a = 3.05-aa+2.3-fa+m a
(6)
w h e r e a a, fa, and m a are the w e i g h t f r a c t i o n s of A, F and M for a. F o r m u l a (6) is an e x t r a p o l a t i o n to 1 5 0 0 ° C f r o m L e a a n d P a r k e r ' s f o r m u l a for 1400 ° and 1 4 5 0 o c (8). cb can now be d e r i v e d f r o m eq. (4), and H f r o m eq. (3). Experimental
Procedure
M o s t of the e x p e r i m e n t a l t e c h n i q u e was d e s c r i b e d e l s e w h e r e (2). Batches were prepared from analytical grade reagents w i t h k n o w n i g n i t i o n losses; the c o m p o s i t i o n g i v e n in the t a b l e s are t h o s e c a l c u l a t e d f r o m the w e i g h e d q u a n t i t i e s of reagents. A n a l y s e s of C - S - A - F m i x t u r e s g a v e 0.02% K2 O, 0.03% Na20, 0.03% MgO. The r e a c t i o n t e m p e r a t u r e T was c o n s t a n t w i t h i n ~ 3°C, but l o n g - t e r m v a r i a t i o n s and lack of r e p r o d u c i b i l i t y m a y be 7oc. The w i d t h of the alite layer, X, was t y p i c a l l y 2 0 0 - 1 5 0 0 ~. It c o u l d be m e a s u r e d w i t h 5% s t a n d a r d d e v i a t i o n . The h e a t i n g p e r i o d s t secs w e r e N I 0 0 0 , 3600, 9000 and 62000 secs. A f a i r l y s l o w h e a t i n g up from 1 2 5 0 o c to T was
Vol. 8, No. 6
697 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE
required 5 0 ° C / m i n ) in o r d e r to m i n i m i z e b r e a k a g e of s a n d wicnes. C o o l i n g f r o m T was m u c h f a s t e r . In o r d e r to a s s e s s t, a c o r r e c t i o n w a s i n t r o d u c e d to a c c o u n t for the s l o w h e a t i n g up. R e g r e s s i o n a n a l y s e s of t, X 2 g a v e the s l o p e k w i t h 5% s t a n d a r d d e v i a t i o n . The c a l c u l a t e d i n t e r s e c t i o n w i t h the t i m e axis w a s <30 secs. f r o m the origin. Thus, the p a r a b o l i c r a t e l a w w a s o b e y e d w i t h high accuracy. H o w e v e r , the r e p r o d u c i b i l i t y w a s p o o r in some cases, w h e n d i f f e r e n t b a t c h e s of raw m a t e r i a l s w e r e used. This may be c o n n e c t e d w i t h the f a c t t h a t H is v e r y s e n s i t i v e to c h a n g e s in c I and c 2. Thus, c I = 0.733, c a = 0 . 7 1 7 9 , c b = 0 . 7 1 6 2 and c 2 = 0 . 6 9 2 g i v e H = 107.5, but w i t h c I = 0.730 one g e t s H = 124.0. Results
and D i s c u s s i o n
In e x p e r i m e n t a l s e r i e s No. I, the m e l t f r a c t i o n a w a s the parameter varied. The c o m p o s i t i o n of the e n d - m e m b e r s , s o m e c a l c u l a t e d p a r a m e t e r s , a n d the m e a s u r e d v a l u e s for k at 1 5 0 0 0 are c o m p i l e d in T a b l e i.
TABLE
1
D a t a for S e r i e s w i t h V a r y i n g M e l t C o n t e n t a. V a l u e s S h o w n are: C o m p o s i t i o n s of e n d - m e m b e r s , on I g n i t e d Basis, as D e t e r m i n e d f r o m W e i g h t of R e a g e n t G r a d e C h e m i c a l s ; C a l c u l a t e d V a l u e s for Ca, Cb, a, a n d H; R e a c t i o n R a t e C o n s t a n t k as M e a s u r e d for T = 1 5 0 0 o c . Ca % CaO % SiO2 % A1203 % Fe203 cb a H
k. 108 cm2sec_ 1
70.78 66.45
19.48 22.37
6.49 7.46
3.25 3.73
0.6878 0.6828
0.292
104.6
65.0
72.12 67.90
20.91 24.07
4.65 5.35
2.32 2.68
0.7017 0.6983
0.210
103.1
48.4
72.87 68.71
21.71 25.03
3.62 4.17
1.81 2.09
0.7095 0.7069
0.163
102.6
34.5
73.34 69.23
22.22 25.64
2.96 3.42
1.48 1.71
0.7145 0.7124
0.134
102.7
20.5
73.67 69.59
22.57 26.06
2.51 2.90
1.25 I. 45
0.7179 0.7162
0.113
102.5
15.3
73.91 69.86
22.83 26.37
2.17 2.51
i. 09 1.26
0.7205 0.7190
0.098
102.8
9.4
74 .i0 70.06
22.02 26.61
1.92 2.22
0.96 i.ii
0.7224 0.7211
0.087
102.5
9.6
74.24 70.22
23.18 26.80
1.72 1.99
0.86 0.99
0.7239 0.7228
0.078
102.6
7.5
74.36 70.35
23.31 26.95
1.55 1.80
0.78 0.90
0.7252 0.7242
0.070
102.7
4.7
698
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen k is seen to i n c r e a s e m a r k e d l y w i t h i n c r e a s i n g a. A plot of k/(Ha) v e r s u s a, Fig. 3, shows that in the a - r a n g e 0 . 1 5 - 0 . 3 0 , w h i c h is t y p i c a l for P C - c l i n k e r , k/(Ha) is f a i r l y c o n s t a n t ; h o w e v e r , o u t s i d e this r a n g e a v a r i a t i o n is seen. S i n c e s e r i e s No. 1 was d e s i g n e d w i t h % A 1 2 0 3 / % F e 2 0 3 c o n s t a n t (= 2.0), m e l t c o m p o s i t i o n s h o u l d have b e e n c o n s t a n t , thus Am s h o u l d h a v e b e e n c o n s t a n t . The v a r i a t i o n m a y be c o n n e c t e d w i t h the o b s e r v e d v a r i a t i o n in p o r e - s i z e a f f e c t i n g F. In s e r i e s No. 2, T a b l e 2, e n d - m e m b e r c o m p o s i t i o n s w e r e 73% Cao, 21% SiO2, and 67% CaO, 27% SiO 2. The r e m a i n i n g 6% w e r e m a d e up by A I 2 0 3 and Fe203, w i t h the same c o n t e n t s for both end-members, k/(Ha) was c o m p u t e d and p l o t t e d v e r s u s i00-~, Fig. 4, w h e r e ~ = % F e 2 0 3 / ( % F e 2 0 3 + % A 1 2 0 3 ) . The p l o t is l i n e a r and m a y be r e p r e s e n t e d by 108-k/(Ha)
= 1.89-~+1.45
(7)
By d e f i n i n g y as the r a t i o b e t w e e n k/(Ha) for an a r b i t r a r y ~, and k/(Hal for ~ = 0, and by i n t r o d u c i n g M A = % A 1 2 0 3 / % Fe203, eq. (7) m a y be t r a n s f e r r e d to Y =
(M A + 2 . 3 ) / ( M A + i)
The o b s e r v e d v a r i a t i o n in D~ and in Am.
(8)
in k/(Ha)
TABLE
may
reflect
a variation
2
E f f e c t of F e 2 0 3 - A I 2 0 3 S u b s t i t u t i o n on k at 1500 ° . The F i r s t T w o C o l u m n s S h o w C o n t e n t s of Fe203 and A I 2 0 3 in E n d - m e m b e r s a n d A l i t e Layer. 108-k Ca % Fe203 % A1203 a Cb H cm2sec-i 0
6
0.183
0.5
5.5
0.179
1.0
5.0
1.176
2.0
4.0
0.168
3.0
3.0
0.161
3.6
2.4
0.156
0 0 0 0 0 0 0 0 0 0 0 0
7112 7134 7105 7127 7098 7121 7084 7108 7070 7095 7062 7087
76.2
20 3
74.7
21 2
73.3
22 8
70.8
25 1
68.8
26
67.9
27 4
3
In s e r i e s No. 3 M g O was s u b s t i t u t e d for AI203, in one c a s e for A I 2 0 3 + F e 2 0 3 ; o t h e r w i s e the c o m p o s i t i o n s w e r e as for s e r i e s No. 2. S i n c e U is d i f f i c u l t to a s s e s s in this case, no a t t e m p t has b e e n m a d e to c a l c u l a t e H, but H was p r e s u m a b l y f a i r l y c o n s t a n t t h r o u g h this series. From T a b l e 3 it w i l l be seen t h a t k is f a i r l y c o n s t a n t e x c e p t for 3% MgO, w h e r e the v a l u e is low, p r o b a b l y o w i n g to low m e l t c o n t e n t (since M g O - s a t u r a t i o n is r e a c h e d ) . In o r d e r to c o m p e n s a t e k for the e f f e c t s of c h a n g i n g a and y, k/(ya) was c a l c u l a t e d and f o u n d to be f a i r l y c o n s t a n t
Vol. 8, No. 6
699 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE TABLE
3
E f f e c t of M g O - A I 2 0 3 S u b s t i t u t i o n on k at 1500 ° . The F i r s t T h r e e C o l u m n s Show C o n t e n t of MgO, A1203 and Fe203 in e n d - m e m b e r s and alite layer. % MgO
% AI203
% Fe203
108.k c m 2 s e c -I
a
108.k/(ya) cm2sec-1
0 03
6.0
0
0.183
20.3
iii
0 25
5.75
0
0.178
18.3
103
05
5.5
0
0.173
18.1
105
075
5.25
0
0.168
17.2
102
15
4.5
0
0.152
19.6
129
30
3.0
0
0.107")
7.8
73
05
3.67
1.83
0.159
23.8
104
*) A s s u m i n g M g O - s a t u r a t i o n
at 1.5%.
through this series. It w o u l d a p p e a r that, p e r m i t t i n g a s l i g h t v a r i a t i o n in H, k/(Hya) m u c h i n f l u e n c e d by M g O - s u b s t i t u t i o n .
even w h e n c a n n o t be
For two s a n d w i c h c o m p o s i t i o n s the r e a c t i o n rate was m e a s u r e d at o t h e r t e m p e r a t u r e s than 15000; the data are shown in Table 4. Arrhenius' plots of these data are linear, see Fig. 2 in ref. (9), with a p p a r e n t a c t i v a t i o n e n e r g i e s of 44 and 50 kcal mole -1 , in r e a s o n a b l e a g r e e m e n t w i t h the v a l u e 42 kcal m o l e -I found by Kondo and Choi (i) for the alite r e a c t i o n with 15% melt. P r o v i d e d that C3A or C4AF do not p r e c i p i t a t e at the lower t e m p e r a t u r e s , Lea and P a r k e r ' s f o r m u l a for m e l t c o n t e n t s at 1400 ° and 1450o may be e x t r a p o l a t e d to 1375 ° and 1350 ° , and a has been c a l c u l a t e d a c c o r d i n g l y , a is seen to r e m a i n r a t h e r c o n s t a n t o v e r the t e m p e r a t u r e r a n g e observed. F r o m the v a l u e s of Table 4 it is c a l c u l a t e d that k/a is reduced by a factor 0.47, when T is changed from 1500 ° to 1400°C. TABLE 4 I n f l u e n c e of T e m p e r a t u r e on k for two S a n d w i c h Compositions, I: 73% CaO, 21% SiO2 (67% CaO, 27% CaO), 4% AI203, 2% Fe203; II: As I, but 3.3% A1203, 1.7% Fe203, 1% MgO. Temp.
II
oc
1350
1375
1450
1500
108 k cm2sec -1
n.d.
12.2
22.4
31.5
a
n.d.
0.161
108 k cm2sec -I
7.4
9.1
a
0.142
0.144
0.165 !8.0 0.147
0.168 26.6 0.150
700
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen 10skl(H.a:)
crn2$-I 0 0
0
0 0 0 0
0
0
Melt, weight fract~n ,
I
o]
,
I
a
Q2
FIG.
I
=
0.3
3
The i n f l u e n c e of melt fraction, ~, on k/(H~) for s a n d w i c h e s with % A1203/% Fe203 = 2.0.
at 1500oc
The m a j o r c o n t r i b u t i o n to the a p p a r e n t a c t i v a t i o n energy p r o b a b l y o r i g i n a t e s from D~. However, it must not be o v e r l o o k e d that Am must d e c r e a s e with falling temperature, since at the lower s t a b i l i t y point for C3 S, at 1200-1250oc, where C2S and C are in e q u i l i b r i u m , Am must be zero. Conclusion The r e a c t i o n r a t e c o n s t a n t for a l i t e f o r m a t i o n in s a n d w i c h couples increases with melt content. For m e l t c o n t e n t s t y p i c a l of P o r t l a n d c e m e n t c l i n k e r (15-30%) the c o n s t a n t is a l m o s t p r o p o r t i o n a t e to the m e l t c o n t e n t . S u b s t i t u t i o n of F e 2 0 3 for A 1 2 0 3 in the m e l t at 1500 ° c a u s e s a m o d e s t but d i s t i n c t i n c r e a s e in rate. S u b s t i t u t i o n of M g O for A 1 2 0 3 + F e 2 0 3 in the m e l t has no s i g n i f i c a n t e f f e c t on the rate. The t e m p e r a t u r e has an e f f e c t on the r a t e c o r r e s p o n d i n g to an a c t i v a t i o n e n e r g y of ~ 4 7 k c a l / m o l e . 108.k/(H.O~) cm2s-1 2.5
2.0
% FI203- su~sUtution
FIG.
4
The i n f l u e n c e of s u b s t i t u t i o n of F e 2 0 3 for A I 2 0 3 on k/(Ha) at 1 5 0 0 o c for s a n d w i c h e s w i t h s ~ 0.17.
Vol. 8, No. 6
701 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE
Acknowledgements The e f f i c i e n t technical a s s i s t a n c e of K i r s t e n Hansen, Kenn S v a n l u n d h and D o r t h e Lotz C l a s e n is g r a t e f u l l y a c k n o w l e d g e d . References 1. R. Kondo and S. Choi, pp. 163-171 in Part I, C h e m i s t r y of C e m e n t C l i n k e r Vol. I. Proc. 5th Internat. Symp. Chem. Cem., Tokyo, 1968. The C e m e n t A s s o c i a t i o n of Japan, Tokyo 1969. 2. N.H. C h r i s t e n s e n and O.L. Jepsen, Soc. 54 [4] 208-210 (1971).
J. Amer.
3. N.H. C h r i s t e n s e n , 54-58 (1977).
Soc.
J. Amer.
Ceram.
60
Ceram.
[1-2]
4. V. Johansen, O.L. Jepsen, N.H. C h r i s t e n s e n , J.C.G. C e m e n t and C o n c r e t e Research, 8~ 301-310 (1978). 5. N.H. C h r i s t e n s e n , 293-296 (1977).
J. Amer.
C e r a m Soc.
6. A.R. C o o p e r and A.K. Varshneya, [2] 103-106 (1968).
Hansen,
6__00[7-8]
J. Amer.
Ceram.
Soc.
5__!
7. E.F. O s b o r n and A. Muan, Phase E q u i l i b r i u m D i a g r a m s Oxide Systems, Plate 1 and i0. Amer. Ceram. Soc., Ohio 1960.
of
8. F. Lea and T.W. Parker, No. 16, L o n d o n 1935.
Paper
Building Research Technical
9. V. J o h a n s e n and N.H. Christensen,
and C o n c r e t e
Research,
1978.
paper
submitted
to Cement
RATE OF A L I T E - F O R M A T I O N
N.H.
Christensen,
O.L.
IN C L I N K E R S A N D W I C H E S
J e p s e n and V. J o h a n s e n
F.L. Smidth & Co. A/S 77 V i g e r s l e v Alle, DK-2500 V a l b y D e n m a r k
(Communicated by J. Skalny) (Received July 27, 1978)
ABSTRACT The r e a c t i o n rate of the alite reaction, C + C2S ~ C3S , was m e a s u r e d by m e a n s of a s a n d w i c h technique, for e x p e r i m e n t a l c o n d i t i o n s p e r t i n e n t to b u r n a b i l i t y studies. The g r o w t h of the alite layer f o l l o w e d the p a r a b o l i c rate law, X 2 = k.t, where X is w i d t h of layer and t r e a c t i o n time. k (cm2 sec -I) was found to i n c r e a s e with i n c r e a s i n g m e l t c o n t e n t in the structure, w i t h i n c r e a s i n g temperature, and w i t h i n c r e a s i n g s u b s t i t u t i o n of Fe203 for AI203 in the melt. S u b s t i t u t i o n of MgO for AI203 up to M g O - s a t u r a t i o n had no s i g n i f i c a n t e f f e c t on k.
Die R e a k t i o n s g e s c h w i n d i g k e i t fur die A l i t - R e a k t i o n , C + C2S ~ C3S , w u r d e m i t t e l s einer D i f f u s i o n s p a a r technik u n t e r e x p e r i m e n t e l l e n B e d i n g u n g e n gemessen, die fur das S t u d i u m der B r e n n b a r k e i t yon I n t e r e s s e sind. Der Zuwachs der Dicke, X, der A l i t s c h i c h t folgt dem p a r a b o l i s c h e n Gesetz, X 2 = k x t, in dem t g l e i c h der R e a k t i o n s d a u e r ist. Es w u r d e festgestellt, dass mit a n s t e i g e n d e m G e h a l t an Schmelze in der Struktur, a n s t e i g e n d e r T e m p e r a t u r und a n s t e i g e n d e m Ersatz durch Fe203 fur A1203 in der S c h m e l z e der W e r t k (cm2 sec-l) anw~chst. Ersatz durch MgO fHr A 1 2 0 3 bis zur M g O - S i t t i g u n g h a t t e k e i n e n s i g n i f i k a n t e n E i n f l u s s auf k.
693
694
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen Introduction
The f a c i l i t y with w h i c h the c o m p o n e n t s of any p a r t i c u l a r P o r t l a n d cement raw mixture are combined, c o m m o n l y d e s i g n a t e d the burnability, may be d e t e r m i n e d in the l a b o r a t o r y by n o t i n g the p e r c e n t a g e of u n c o m b i n e d lime, w h i c h remains in the p r o d u c t a f t e r it has been s u b j e c t e d to a s p e c i f i e d t r e a t m e n t of c o m p a c t i n g and heating. It is b e l i e v e d that the critical step in such a t r e a t m e n t is the h e a t i n g at 1 4 0 0 - 1 5 0 0 o c a s s o c i a t e d with the alite reaction: C + C2S ~ C3S. Therefore, u n d e r s t a n d i n g of the k i n e t i c s and m e c h a n i s m of the alite r e a c t i o n may be the key to solving b u r n a b i l i t y problems. In this paper we report on rate data for alite f o r m a t i o n m e a s u r e d in s a n d w i c h couples, for v a r y i n g c o n t e n t of melt, for v a r y i n g c o n c e n t r a t i o n s of Fe203, MgO and AI203 in the melt, and for v a r y i n g r e a c t i o n temperature. Sandwich Technique A s a n d w i c h is a couple of two compact samples, also c a l l e d e n d - m e m b e r s , w h i c h are joined at a c o m m o n plane interface. The c o m p a c t s should be h o m o g e n e o u s and be e q u i l i b r a t e d s e p a r a t e l y before the joining. The s a n d w i c h is h e a t e d i s o t h e r m a l l y at Toc for t secs, then a i r - q u e n c h e d , s e c t i o n e d p e r p e n d i c u l a r l y to the contact plane, p r e p a r e d for m i c r o s c o p y and then i n s p e c t e d in the m i c r o s c o p e . A layer of r e a c t i o n products has u s u a l l y been formed at the c o n t a c t plane; t y p i c a l l y it has sharp b o u n d a r i e s towards the o r i g i n a l end-members, so that its w i d t h can be e a s i l y measured. The a d v a n t a g e of this technique, c o m p a r e d to p o w d e r techniques, is that p a r t i c l e size effects are eliminated. Moreover, k i n e t i c s are s i m p l e r and often e a s i e r to interpret. The mere joining of a C a O - c o m p a c t w i t h a C 2 S ~ c o m p a c t r e p r e s e n t s a s i t u a t i o n too d i s s i m i l a r to the alite r e a c t i o n in technology. Kondo and Choi (i) improved the t e c h n i q u e by i n t r o d u c i n g t w o - p h a s e e n d - m e m b e r s , a f i n e - g r a i n e d CaOc o m ? a c t with an i n t e r s t i t i a l melt, and a f i n e - g r a i n e d C 2 S + m e l t compact. A layer c o n s i s t i n g of C 3 S + m e l t was o b s e r v e d to grow at the joint. We have found it more c o n v e n i e n t to use 3-phase endmembers, C+C3S+melt, and C 3 S + C 2 S + m e l t (2), Fig. i. I n v a r i a n c e in t h r e e - c o m p o n e n t t h r e e - p h a s e regions is t h e r e b y achieved, w h i c h g r e a t l y s i m p l i f i e s the i n t e r p r e t a t i o n , since the d i s a p p e a r i n g regions now remain c o n s t a n t in c o m p o s i t i o n and chemical p o t e n t i a l s d u r i n g a k i n e t i c run. The same is not always true, w h e n t w o - p h a s e e n d - m e m b e r s are used, cf. the couples 1-2 and 3-4 in Fig. 2. In points 3 and 4 the c o m p o s i t i o n of the m e l t will change along the isotherms U-Y and V-Z d u r i n g the kinetic run, w h i l e in 1 and 2 the melts remain fixed at U and V. Even with fourand f i v e - c o m p o n e n t clinker sandwiches, the use of these t h r e e - p h a s e e n d - m e m b e r s helps to m i n i m i z e v a r i a t i o n s in the C a O - c o n c e n t r a t i o n of the d i s a p p e a r i n g regions.
Vol. 8, No. 6
695 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE
C3S*CaO ÷ Liq
| ~
1
C3S*C2S * Liq
%s A,2%F..,o3
1500°C
C1
C2
,
Liq
)
CaO
FIG.
'-I
I ~ X
FIG.
~
1
P r i n c i p l e of s a n d w i c h technique.
2
I s o t h e r m a l section (1500 o) of C-S-(A,F) p h a s e diagram, showing t w o - p h a s e fields C 2 S - Z - V , C 3 S - V - U and CaO-U-Y, and t h r e e - p h a s e fields C 2 S - C 3 S - V and C 3 S - C a O - U .
Model for S a n d w i c h K i n e t i c s Let X be the w i d t h of the p r o d u c t layer formed in a s a n d w i c h after t secs at T ° . Experience, and d e d u c t i o n b a s e d on a d i f f u s i o n c o n t r o l l e d m e c h a n i s m , show that the g r o w t h follows a p a r a b o l i c law X 2 = k-t
(i)
w h e r e k is a f u n c t i o n of T and e n d - m e m b e r
composition.
For a b i n a r y sandwich, whose c o n s t i t u e n t s only are A and B, it may be shown (e.g. by c o m b i n i n g e q a t i o n s (2c), (9a) and (gb) of ref. (3)) that k = 2D(ca-cb).H,
with H = i/(cl-ca)+i/(cb+c2)
(2),(3)
Here, D is the b i n a r y i n t e r d i f f u s i o n c o e f f i c i e n t at T for the p r o d u c t phase; c's are c o n c e n t r a t i o n s of e n d - m e m b e r s (Cl and c2) and of p r o d u c t phase, at the b o u n d a r y towards i, Ca, and at the b o u n d a r y towards 2, c b. In the c l i n k e r s a n d w i c h e s the e n d - m e m b e r s and the g r o w i n g layer have a m i c r o s t r u c t u r e of two or three phases, and the d i f f u s i o n is m u l t i c o m p o n e n t (4). The p r o b l e m may, however, be t r e a t e d as a p s e u d o - b i n a r y d i f f u s i o n problem, a s s u m i n g that the local c o m p o s i t i o n s lie on the s t r a i g h t c o n n e c t i o n line 1-2; therefore, on the v e r y short lines e c t i o n a-b in Fig. 2~). c's are now w e i g h t f r a c t i o n CaO in local m u ! t i p h a s e s t r u c t u r e s (in e n d - m e m b e r s , c I and c 2,
*) The c o m p o s i t i o n s l-a and b-2 d e g e n e r a t e of no spatial extension, cf. ref. (5).
to b o u n d a r i e s
696
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen and in alite layer, Ca and Cb). If the line a-b (or 1-2) w e r e p a r a l l e l to U - V (melts in e q u i l i b r i u m w i t h C + C 3 S and C 3 S + C 2 S , r e s p e c t i v e l y , at T), we w o u l d h a v e Ca-C b = a ( c u - c v ) , w h e r e a is w e i g h t f r a c t i o n of m e l t in a l i t e layer. T h e r e f o r e , in g e n e r a l Ca-C b = a . h . ~ m
(4)
w h e r e Am = CU-CV, and h a c o r r e c t i o n f a c t o r a c c o u n t i n g for n o n - p a r a l l e l l i t y b e t w e e n 1-2
(1.1-1.3) and U-V.
D of eq. (2) s h o u l d n o w be r e p l a c e d by an e f f e c t i v e b i n a r y d i f f u s i o n c o e f f i c i e n t for CaO in the t w o - p h a s e a l i t e s t r u c t u r e : DO. This, in turn, m a y be w r i t t e n DO = D~-F, w h e r e D~ is the e f f e c t i v e b i n a r y d i f f u s i o n c o e f f i c i e n t for CaO in the melt. D~ is e x p e c t e d to v a r y w i t h c h e m i c a l c o m p o s i t i o n of the m e l t (e.g. w i t h % A 1 2 0 3 / % F e 2 0 3 ) and w i t h T. It m a y a l s o v a r y w i t h the d i r e c t i o n of the c o m p o s i t i o n line 1-2, Fig. 2, (6). F is a s h a p e f a c t o r
(5)
F p r o b a b l y v a r i e s w i t h a, p o r o s i t y and p o r e size. The p o r o s i t y of these s a n d w i c h e s w a s low (~0.i). Pores were s m a l l e r in s a n d w i c h e s w i t h low m e l t c o n t e n t than in s a n d w i c h e s w i t h h i g h c o n t e n t of melt, 5-20 ~m v e r s u s 25-100 ~m. For e a c h s a n d w i c h H of p o i n t a was f o u n d i n t e r s e c t i o n of line For this p u r p o s e the linear interpolation C-S-F diagrams (7). the a l i t e - l a y e r was w h i c h was c a l c u l a t e d
and a w e r e c a l c u l a t e d . The c o m p o s i t i o n by an a n a l y t i c a l p r o c e d u r e s i m u l a t i n g 1-2 w i t h line C 3 S - U , cf. Fig. 2. c o m p o s i t i o n of U was d e t e r m i n e d by a of the r e a d i n g s f r o m the C - S - A and the The w e i g h t f r a c t i o n of melt, a, in i d e n t i f i e d w i t h the m e l t c o n t e n t for a, f r o m the e x p r e s s i o n
a = 3.05-aa+2.3-fa+m a
(6)
w h e r e a a, fa, and m a are the w e i g h t f r a c t i o n s of A, F and M for a. F o r m u l a (6) is an e x t r a p o l a t i o n to 1 5 0 0 ° C f r o m L e a a n d P a r k e r ' s f o r m u l a for 1400 ° and 1 4 5 0 o c (8). cb can now be d e r i v e d f r o m eq. (4), and H f r o m eq. (3). Experimental
Procedure
M o s t of the e x p e r i m e n t a l t e c h n i q u e was d e s c r i b e d e l s e w h e r e (2). Batches were prepared from analytical grade reagents w i t h k n o w n i g n i t i o n losses; the c o m p o s i t i o n g i v e n in the t a b l e s are t h o s e c a l c u l a t e d f r o m the w e i g h e d q u a n t i t i e s of reagents. A n a l y s e s of C - S - A - F m i x t u r e s g a v e 0.02% K2 O, 0.03% Na20, 0.03% MgO. The r e a c t i o n t e m p e r a t u r e T was c o n s t a n t w i t h i n ~ 3°C, but l o n g - t e r m v a r i a t i o n s and lack of r e p r o d u c i b i l i t y m a y be 7oc. The w i d t h of the alite layer, X, was t y p i c a l l y 2 0 0 - 1 5 0 0 ~. It c o u l d be m e a s u r e d w i t h 5% s t a n d a r d d e v i a t i o n . The h e a t i n g p e r i o d s t secs w e r e N I 0 0 0 , 3600, 9000 and 62000 secs. A f a i r l y s l o w h e a t i n g up from 1 2 5 0 o c to T was
Vol. 8, No. 6
697 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE
required 5 0 ° C / m i n ) in o r d e r to m i n i m i z e b r e a k a g e of s a n d wicnes. C o o l i n g f r o m T was m u c h f a s t e r . In o r d e r to a s s e s s t, a c o r r e c t i o n w a s i n t r o d u c e d to a c c o u n t for the s l o w h e a t i n g up. R e g r e s s i o n a n a l y s e s of t, X 2 g a v e the s l o p e k w i t h 5% s t a n d a r d d e v i a t i o n . The c a l c u l a t e d i n t e r s e c t i o n w i t h the t i m e axis w a s <30 secs. f r o m the origin. Thus, the p a r a b o l i c r a t e l a w w a s o b e y e d w i t h high accuracy. H o w e v e r , the r e p r o d u c i b i l i t y w a s p o o r in some cases, w h e n d i f f e r e n t b a t c h e s of raw m a t e r i a l s w e r e used. This may be c o n n e c t e d w i t h the f a c t t h a t H is v e r y s e n s i t i v e to c h a n g e s in c I and c 2. Thus, c I = 0.733, c a = 0 . 7 1 7 9 , c b = 0 . 7 1 6 2 and c 2 = 0 . 6 9 2 g i v e H = 107.5, but w i t h c I = 0.730 one g e t s H = 124.0. Results
and D i s c u s s i o n
In e x p e r i m e n t a l s e r i e s No. I, the m e l t f r a c t i o n a w a s the parameter varied. The c o m p o s i t i o n of the e n d - m e m b e r s , s o m e c a l c u l a t e d p a r a m e t e r s , a n d the m e a s u r e d v a l u e s for k at 1 5 0 0 0 are c o m p i l e d in T a b l e i.
TABLE
1
D a t a for S e r i e s w i t h V a r y i n g M e l t C o n t e n t a. V a l u e s S h o w n are: C o m p o s i t i o n s of e n d - m e m b e r s , on I g n i t e d Basis, as D e t e r m i n e d f r o m W e i g h t of R e a g e n t G r a d e C h e m i c a l s ; C a l c u l a t e d V a l u e s for Ca, Cb, a, a n d H; R e a c t i o n R a t e C o n s t a n t k as M e a s u r e d for T = 1 5 0 0 o c . Ca % CaO % SiO2 % A1203 % Fe203 cb a H
k. 108 cm2sec_ 1
70.78 66.45
19.48 22.37
6.49 7.46
3.25 3.73
0.6878 0.6828
0.292
104.6
65.0
72.12 67.90
20.91 24.07
4.65 5.35
2.32 2.68
0.7017 0.6983
0.210
103.1
48.4
72.87 68.71
21.71 25.03
3.62 4.17
1.81 2.09
0.7095 0.7069
0.163
102.6
34.5
73.34 69.23
22.22 25.64
2.96 3.42
1.48 1.71
0.7145 0.7124
0.134
102.7
20.5
73.67 69.59
22.57 26.06
2.51 2.90
1.25 I. 45
0.7179 0.7162
0.113
102.5
15.3
73.91 69.86
22.83 26.37
2.17 2.51
i. 09 1.26
0.7205 0.7190
0.098
102.8
9.4
74 .i0 70.06
22.02 26.61
1.92 2.22
0.96 i.ii
0.7224 0.7211
0.087
102.5
9.6
74.24 70.22
23.18 26.80
1.72 1.99
0.86 0.99
0.7239 0.7228
0.078
102.6
7.5
74.36 70.35
23.31 26.95
1.55 1.80
0.78 0.90
0.7252 0.7242
0.070
102.7
4.7
698
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen k is seen to i n c r e a s e m a r k e d l y w i t h i n c r e a s i n g a. A plot of k/(Ha) v e r s u s a, Fig. 3, shows that in the a - r a n g e 0 . 1 5 - 0 . 3 0 , w h i c h is t y p i c a l for P C - c l i n k e r , k/(Ha) is f a i r l y c o n s t a n t ; h o w e v e r , o u t s i d e this r a n g e a v a r i a t i o n is seen. S i n c e s e r i e s No. 1 was d e s i g n e d w i t h % A 1 2 0 3 / % F e 2 0 3 c o n s t a n t (= 2.0), m e l t c o m p o s i t i o n s h o u l d have b e e n c o n s t a n t , thus Am s h o u l d h a v e b e e n c o n s t a n t . The v a r i a t i o n m a y be c o n n e c t e d w i t h the o b s e r v e d v a r i a t i o n in p o r e - s i z e a f f e c t i n g F. In s e r i e s No. 2, T a b l e 2, e n d - m e m b e r c o m p o s i t i o n s w e r e 73% Cao, 21% SiO2, and 67% CaO, 27% SiO 2. The r e m a i n i n g 6% w e r e m a d e up by A I 2 0 3 and Fe203, w i t h the same c o n t e n t s for both end-members, k/(Ha) was c o m p u t e d and p l o t t e d v e r s u s i00-~, Fig. 4, w h e r e ~ = % F e 2 0 3 / ( % F e 2 0 3 + % A 1 2 0 3 ) . The p l o t is l i n e a r and m a y be r e p r e s e n t e d by 108-k/(Ha)
= 1.89-~+1.45
(7)
By d e f i n i n g y as the r a t i o b e t w e e n k/(Ha) for an a r b i t r a r y ~, and k/(Hal for ~ = 0, and by i n t r o d u c i n g M A = % A 1 2 0 3 / % Fe203, eq. (7) m a y be t r a n s f e r r e d to Y =
(M A + 2 . 3 ) / ( M A + i)
The o b s e r v e d v a r i a t i o n in D~ and in Am.
(8)
in k/(Ha)
TABLE
may
reflect
a variation
2
E f f e c t of F e 2 0 3 - A I 2 0 3 S u b s t i t u t i o n on k at 1500 ° . The F i r s t T w o C o l u m n s S h o w C o n t e n t s of Fe203 and A I 2 0 3 in E n d - m e m b e r s a n d A l i t e Layer. 108-k Ca % Fe203 % A1203 a Cb H cm2sec-i 0
6
0.183
0.5
5.5
0.179
1.0
5.0
1.176
2.0
4.0
0.168
3.0
3.0
0.161
3.6
2.4
0.156
0 0 0 0 0 0 0 0 0 0 0 0
7112 7134 7105 7127 7098 7121 7084 7108 7070 7095 7062 7087
76.2
20 3
74.7
21 2
73.3
22 8
70.8
25 1
68.8
26
67.9
27 4
3
In s e r i e s No. 3 M g O was s u b s t i t u t e d for AI203, in one c a s e for A I 2 0 3 + F e 2 0 3 ; o t h e r w i s e the c o m p o s i t i o n s w e r e as for s e r i e s No. 2. S i n c e U is d i f f i c u l t to a s s e s s in this case, no a t t e m p t has b e e n m a d e to c a l c u l a t e H, but H was p r e s u m a b l y f a i r l y c o n s t a n t t h r o u g h this series. From T a b l e 3 it w i l l be seen t h a t k is f a i r l y c o n s t a n t e x c e p t for 3% MgO, w h e r e the v a l u e is low, p r o b a b l y o w i n g to low m e l t c o n t e n t (since M g O - s a t u r a t i o n is r e a c h e d ) . In o r d e r to c o m p e n s a t e k for the e f f e c t s of c h a n g i n g a and y, k/(ya) was c a l c u l a t e d and f o u n d to be f a i r l y c o n s t a n t
Vol. 8, No. 6
699 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE TABLE
3
E f f e c t of M g O - A I 2 0 3 S u b s t i t u t i o n on k at 1500 ° . The F i r s t T h r e e C o l u m n s Show C o n t e n t of MgO, A1203 and Fe203 in e n d - m e m b e r s and alite layer. % MgO
% AI203
% Fe203
108.k c m 2 s e c -I
a
108.k/(ya) cm2sec-1
0 03
6.0
0
0.183
20.3
iii
0 25
5.75
0
0.178
18.3
103
05
5.5
0
0.173
18.1
105
075
5.25
0
0.168
17.2
102
15
4.5
0
0.152
19.6
129
30
3.0
0
0.107")
7.8
73
05
3.67
1.83
0.159
23.8
104
*) A s s u m i n g M g O - s a t u r a t i o n
at 1.5%.
through this series. It w o u l d a p p e a r that, p e r m i t t i n g a s l i g h t v a r i a t i o n in H, k/(Hya) m u c h i n f l u e n c e d by M g O - s u b s t i t u t i o n .
even w h e n c a n n o t be
For two s a n d w i c h c o m p o s i t i o n s the r e a c t i o n rate was m e a s u r e d at o t h e r t e m p e r a t u r e s than 15000; the data are shown in Table 4. Arrhenius' plots of these data are linear, see Fig. 2 in ref. (9), with a p p a r e n t a c t i v a t i o n e n e r g i e s of 44 and 50 kcal mole -1 , in r e a s o n a b l e a g r e e m e n t w i t h the v a l u e 42 kcal m o l e -I found by Kondo and Choi (i) for the alite r e a c t i o n with 15% melt. P r o v i d e d that C3A or C4AF do not p r e c i p i t a t e at the lower t e m p e r a t u r e s , Lea and P a r k e r ' s f o r m u l a for m e l t c o n t e n t s at 1400 ° and 1450o may be e x t r a p o l a t e d to 1375 ° and 1350 ° , and a has been c a l c u l a t e d a c c o r d i n g l y , a is seen to r e m a i n r a t h e r c o n s t a n t o v e r the t e m p e r a t u r e r a n g e observed. F r o m the v a l u e s of Table 4 it is c a l c u l a t e d that k/a is reduced by a factor 0.47, when T is changed from 1500 ° to 1400°C. TABLE 4 I n f l u e n c e of T e m p e r a t u r e on k for two S a n d w i c h Compositions, I: 73% CaO, 21% SiO2 (67% CaO, 27% CaO), 4% AI203, 2% Fe203; II: As I, but 3.3% A1203, 1.7% Fe203, 1% MgO. Temp.
II
oc
1350
1375
1450
1500
108 k cm2sec -1
n.d.
12.2
22.4
31.5
a
n.d.
0.161
108 k cm2sec -I
7.4
9.1
a
0.142
0.144
0.165 !8.0 0.147
0.168 26.6 0.150
700
Vol. 8, No. 6 N.H. Christensen, O.L. Jepsen, V. Johansen 10skl(H.a:)
crn2$-I 0 0
0
0 0 0 0
0
0
Melt, weight fract~n ,
I
o]
,
I
a
Q2
FIG.
I
=
0.3
3
The i n f l u e n c e of melt fraction, ~, on k/(H~) for s a n d w i c h e s with % A1203/% Fe203 = 2.0.
at 1500oc
The m a j o r c o n t r i b u t i o n to the a p p a r e n t a c t i v a t i o n energy p r o b a b l y o r i g i n a t e s from D~. However, it must not be o v e r l o o k e d that Am must d e c r e a s e with falling temperature, since at the lower s t a b i l i t y point for C3 S, at 1200-1250oc, where C2S and C are in e q u i l i b r i u m , Am must be zero. Conclusion The r e a c t i o n r a t e c o n s t a n t for a l i t e f o r m a t i o n in s a n d w i c h couples increases with melt content. For m e l t c o n t e n t s t y p i c a l of P o r t l a n d c e m e n t c l i n k e r (15-30%) the c o n s t a n t is a l m o s t p r o p o r t i o n a t e to the m e l t c o n t e n t . S u b s t i t u t i o n of F e 2 0 3 for A 1 2 0 3 in the m e l t at 1500 ° c a u s e s a m o d e s t but d i s t i n c t i n c r e a s e in rate. S u b s t i t u t i o n of M g O for A 1 2 0 3 + F e 2 0 3 in the m e l t has no s i g n i f i c a n t e f f e c t on the rate. The t e m p e r a t u r e has an e f f e c t on the r a t e c o r r e s p o n d i n g to an a c t i v a t i o n e n e r g y of ~ 4 7 k c a l / m o l e . 108.k/(H.O~) cm2s-1 2.5
2.0
% FI203- su~sUtution
FIG.
4
The i n f l u e n c e of s u b s t i t u t i o n of F e 2 0 3 for A I 2 0 3 on k/(Ha) at 1 5 0 0 o c for s a n d w i c h e s w i t h s ~ 0.17.
Vol. 8, No. 6
701 ALITE FORMATION, BURNABILITY EXPERIMENTS, REACTION RATE
Acknowledgements The e f f i c i e n t technical a s s i s t a n c e of K i r s t e n Hansen, Kenn S v a n l u n d h and D o r t h e Lotz C l a s e n is g r a t e f u l l y a c k n o w l e d g e d . References 1. R. Kondo and S. Choi, pp. 163-171 in Part I, C h e m i s t r y of C e m e n t C l i n k e r Vol. I. Proc. 5th Internat. Symp. Chem. Cem., Tokyo, 1968. The C e m e n t A s s o c i a t i o n of Japan, Tokyo 1969. 2. N.H. C h r i s t e n s e n and O.L. Jepsen, Soc. 54 [4] 208-210 (1971).
J. Amer.
3. N.H. C h r i s t e n s e n , 54-58 (1977).
Soc.
J. Amer.
Ceram.
60
Ceram.
[1-2]
4. V. Johansen, O.L. Jepsen, N.H. C h r i s t e n s e n , J.C.G. C e m e n t and C o n c r e t e Research, 8~ 301-310 (1978). 5. N.H. C h r i s t e n s e n , 293-296 (1977).
J. Amer.
C e r a m Soc.
6. A.R. C o o p e r and A.K. Varshneya, [2] 103-106 (1968).
Hansen,
6__00[7-8]
J. Amer.
Ceram.
Soc.
5__!
7. E.F. O s b o r n and A. Muan, Phase E q u i l i b r i u m D i a g r a m s Oxide Systems, Plate 1 and i0. Amer. Ceram. Soc., Ohio 1960.
of
8. F. Lea and T.W. Parker, No. 16, L o n d o n 1935.
Paper
Building Research Technical
9. V. J o h a n s e n and N.H. Christensen,
and C o n c r e t e
Research,
1978.
paper
submitted
to Cement