- Email: [email protected]
Factors influencing the phase constitution of alite in portland cement clinker
CEMENT and CONCRETE RESEARCH. 0008-8846/82/030301-08503.00/0
Vol° 12, pp. 301-308, 1982. Printed in the U S A Copyright (c) Pergamon Press, Ltd.
FACTORS INFLUENCING THE PHASE CONSTITUTION OF ALITE IN PORTLAND CEMENT CLINKER
I. Maki and K. Goto Nagoya I n s t i t u t e o f T e c h n o l o g y G o k i s o - c h o , Showa-ku, Nagoya, 466 J a p a n
(Communicated by H.F.W. Taylor) (Received Nov. 2, 1981)
ABSTRACT The p h a s e c o n s t i t u t i o n o f a l i t e i n p o r t l a n d cement c l i n k e r h i g h l y depends n o t o n l y on t h e c h e m i c a l c o m p o s i t i o n o f a raw mix b u t a l s o on the kinetics of crystallization from t h e i n t e r s t i t i a l melt. From t h e m e l t o f h i g h s u p e r s a t u r a t i o n a l i t e can be c r y s t a l l i z e d so q u i c k l y as t o t a k e up f o r e i g n i o n s i n q u a n t i t i e s i n f a v o r o f t h e o c c u r r e n c e o f M3 a t a m b i e n t t e m p e r a t u r e . The r e v e r s e i s t h e c a s e f o r t h e c r y s t a l l i z a t i o n from t h e m e l t low i n s u p e r s a t u r a t i o n , where M1 t e n d s t o be formed. The f o r m a t i o n o f zoned a l i t e can be e x p l a i n e d by t h e c h a n g e in the degree of supersaturation during crystallization. The p h a s e c o n s t i t u t i o n o f a l i t e can be c o r r e l a t e d w i t h i t s c r y s t a l s i z e i n terms of the s u p e r s a t u r a t i o n of the melt. The SO3 i n c l i n k e r c o n s i d e r a b l y lowers the v i s c o s i t y of the i n t e r s t i t i a l m e l t and t h u s e n c o u r ages t h e f o r m a t i o n o f M1 a l o n g w i t h t h e g r a i n growth o f a l i t e . The r e l a t i o n b e t w e e n t h e p h a s e c o n s t i t u t i o n o f a l i t e and t h e q u a l i t y o f c e m e n t s has b e e n remarked i n c o n j u n c t i o n w i t h t h e O n o ' s method.
301
302
Vol.
12, No. 3
I. Maki, K. Goto
Introduction Alite in portland cement clinker from the modern manufacturing process occurs in two forms, M 1 and M 3 (1-4). Alite, which at the clinkering temperature crystallizes as R from the interstitial melt, undergoes phase differentiation during cooling according to its chemical composition -- the quantity and species of foreign ions in solid solution. Mg ions, in particular, play an essential role in the phase differentiation of alite (1,2). The SO 3 in clinker also significantly changes the phase constitution of alite through encouraging the occurrence of M l (2). The heating history as well as the chemical composition of a raw mix exerts great influence on the phase constitution of alite. Concerning this point, however, no systematic study has ever been made. This investigation has been undertaken to establish a viewpoint for comprehensive understanding of those factors which determine the characters of alite, inclusive of its phase constitution, and thus influence the hydraulic properties of cements. Materials The basic chemical composition of the raw mix employed throughout ti~is investigation has been chosen as follows: SiO 2 22.94%, AI203 5.46%, Fe203 3.04%, CaO 66.56%, MgO 0.60%, SO 3 0.40%, Na20 0.40%, K20 0.60%. This composition was modified appropriately when necessary. The raw mixes were prepared using industrial raw materials -- limestone and clay - ground to a residue of 8.0% on 240 mesh. Chemical reagents were added for adjusting such components as SiO2, Fe203, MgO, SO 3 and alkali oxides. SO 3 was added in the form of gypsum. The raw mixes prepared were calcined at 1000°C for 2hrs, unless other wise noted in the text, before heating to clinkering temperatures. Results 0
!
o
1. Effect of chemical composition~ of raw mixes
I0
0
I 3,0 - - - - 0 - 0 o
o
M3 i
v ,E
2.0
.
o
o
0
o
?1 0
1.0
o-"
.-"
0
~
o--
.
.
/
" .
.~
. . . . . . . . . . . . . . . .
o~
I(] ~ ~'~ tl O/~ Be
~3"~1 . /
.
•
M1
•
10 SO 3 in clinker (°1.)
20
FIG. 1 Relation between the phase constitution of alite and the amounts of MgO and SO 3 in clinker. Commercial cement clinkers.
In Fig.l is plotted the phase constitution of alite against the amounts of MgO and SO 3 in clinker for each of the several tens of works cement clinkers from various localities of the world. This figure confirms, as already indicated in our previous papers (1,2), that the phase differentiation of alite depends primarily on the amount of MgO in clinker; that is, with an increase in Mg@ content the phase constitution of aline changes from M l to M 3 through the hybrid state of both phases. It is also seen from this figure that the SO 3 in clinker encourages the formation of M 1 . 2. Effect of clinkering temperature Despite the foregoing, the ph:~s~ constitution of alite in laborator~ clinkers prepared under specially ~ontrolled conditions can show wide de~i ations from the relation given in Fig. l. Fig.2 is the photomicro~ra~:n
Vol. 12, No. 3 ALITE, PHASE CONSTITUTION,
FIG. 2 Alite showing a preponderance of M 1 . Fired at 1450°C for 30min. Transmitted light. Crossed polars.
303 RAW MIX, COMPOSITION,
COOLING RATE
FIG. 3 Afire made up solely of M 3. Fired at 1600°C for 10min. Transmitted light. Crossed polars.
of a clinker prepared by firing the raw mix at 1450°C for 30min. The alite is made up essentially of M 1 in accordance with the relation in Fig.l. By contrast, when the same raw mix is fired at 1600°C for 10min, the alite comes to be made up exclusively of M 3 in spite of the low MgO content in the raw mix
(Fig.3). Another remarkable difference lies in the crystal size of alite. At 1600°C the precipitation of alite can be completed within a very short time and the crystal in general remains small in size; whereas at 1450°C the nucleation of alite is so limited that the crystal can develop large with prolonged firing. The alite formed at 1450°C are characterized by small liquid inclusions, each of which is observed as the intergrowth of fine ferrite and aluminate crystals at ambient temperature. The zonal structure tends to be formed when alite is crystallized over a wide range of temperatures (Fig.4). With alite in works cement clinkers it is usual that M 3 overgrows M 1 initially formed at lower temperatures. This type of zonation will be called 'normal zoning' in contrast with 'reverse zoning' to be described later. With works cement clinker the zonal structure of alite
FIG. 4 Zoned alite (M3+MI). Fired at 1400°C for lhr, and then at 1600°C for 10min. Crossed polars.
FIG. S Afire in clinker with 1.0% of SO 3. Fired at 1600°C for 10min. Crossed polars. (cf. Fig.3).
304
Vol. 12, No. 3 I. Maki, K. Goto
(a) FIG. 6 (b) C o m p a r i s o n o f t h e p h a s e c o n s t i t u t i o n o f a l i t e i n two c l i n k e r s w i t h d i f f e r e n t SO 3 c o n t e n t s . (a) 0.2%, (b) 1.0%. Both c l i n k e r s c o n t a i n 0.9% o f MgO. C r o s s e d p o l a r s . c a n g e n e r a l l y be formed when raw mix i s s l o w l y b r o u g h t t o t h e f i r i n g zone o f the kiln. Under such c o n d i t i o n s t h e p r e c i p i t a t i o n o f a l i t e can a l m o s t be completed at relatively low t e m p e r a t u r e s , e . g . j u s t beyond t h e l i q u i d f o r m a t i o n t e m p e r a t u r e , i n f a v o r o f t h e f o r m a t i o n o f M1. With a r i s e i n t e m p e r a t u r e alite crystals partly dissolve in the interstitial m e l t a n d , on c o o l i n g from elevated temperatures, are recrystallized w i t h M3 o v e r g r o w i n g Ml , The M1 zone o f a c r y s t a l u s u a l l y c o i n c i d e s w i t h such an a r e a as c h a r a c t e r i z e d by l i q u i d inclusions, i n d i c a t i n g t h a t t h e zone b o u n d a r y b e t w e e n M1 and M3 i s j u s t t h e solid-liquid i n t e r f a c e on r e m e l t i n g . The M3 z o n e , on t h e c o n t r a r y , i s g e n e r a l l y free from liquid inclusions and appears smooth under the microscope. Sometimes a more simple sector structure appears in place of the typical zonal structure as mentioned above. From the foregoing it foIlows that the relation given in Fig. l applies only to such clinkers as manufactured under normal technical conditions. The phase constitution of alite is most susceptible to the heating history especially for clinkers with intermediate MgO contents.
FIG. 7 Preference for M l and anomalous grain growth in the presence of large particles of quartz. Fired at 1600°C for lOmin. (cf. Fig.3). Crossed polars.
FIG. 8 Afire with M 3 predominating over MI. Fired at 1450°C for 30min without calcination at IO00°C. (cf. Fig.2%. Crossed polars.
Vol. 12, No. 3
305 ALITE, PHASE CONSTITUTION,
RAW MIX, COMPOSITION,
COOLING RATE
FIG. 10 An example o f r h y t h m i c z o n i n g of a l i t e in commercial c l i n k er. Crossed polars.
FIG. 9 Alite showing reverse zoning. Fired at 1600°C for 10min without preliminary heat treatment. ( c f . Fig.3). 3. E f f e c t o f SO3
The SO3 i n c l i n k e r c a n a l s o c h a n g e t h e p h a s e c o n s t i t u t i o n o f a l i t e t o a great extent. As s e e n i n F i g . l , t h e minimum amount o f MgO i n c l i n k e r n e c e s s a r y f o r s t a b i l i z i n g a l i t e e n t i r e l y as M3 a t a m b i e n t t e m p e r a t u r e i n c r e a s e s w i t h an i n c r e a s i n g amount o f SO3. F i g . S shows a l i t e i n c l i n k e r p r e p a r e d by f i r i n g a t 1600°C f o r 10min a raw mix whose SO3 c o n t e n t was i n c r e a s e d t o t h e amount o f 1.0%. The i n c r e a s e b y 0.6% d e f i n i t e l y p r o m o t e s t h e f o r m a t i o n o f M1 as w e l l as t h e g r a i n growth o f a l i t e . F i g . 6 compares t h e p h a s e c o n s t i t u t i o n o f a l i t e i n two c o m m e r c i a l c l i n k e r s . Both c l i n k e r s , b u r n t i n t h e same k i l n , have v e r y s i m i l a r c h e m i c a l c o m p o s i t i o n s e x c e p t f o r t h e SO3 c o n t e n t s . E v i d e n t l y M1 p r e d o m i n a t e s i n t h e c l i n k e r r i c h i n SO3 ( F i g . 6 b ) . By c o n t r a s t , a l i t e c o n s i s t s o f M3 i n t h e c l i n k e r by f a r l e s s i n t h e SO3 c o n t e n t ( F i g . 6 a ) . 4. E f f e c t o f p a r t i c l e
s i z e and c a l c i n a t i o n
o f raw m i x e s
Ono (5) i n v e s t i g a t e d e x t e n s i v e l y t h e r e l a t i o n b e t w e e n t h e p a r t i c l e s i z e o f raw mixes and t h e c r y s t a l s i z e o f a l i t e u s i n g q u a r t z as a s o u r c e o f s i l i c a . He e s t a b l i s h e d t h a t a l i t e grows l a r g e w i t h an i n c r e a s i n g p a r t i c l e s i z e o f quartz employed. Fig.7 shows alite in clinker from a raw mix in which reagent silicic anhydride was replaced by coarse-grained quartz ranging in diameter between 88 and 104~m. For comparison the raw mix was fired at 1600°C for 10min. The alite crystals have grown huge in accordance with the Ono's findings. But what is more interesting is that M 1 is a predominant constituent of alite. Prolonged heating of a raw mix below the liquid formation temperature (oa. 13300C) also enhances the occurrence of M 1 as well as the grain growth of alite. This is evidenced by the observation that without the calcination at 1000°C alite remains small in size and is rich in M 3 even though the raw mix is fired at 14500C (Fig.8). The reverse zoning, in which M 1 overgrows M3, is liable to appear when alite is precipitated extremely quickly from the melt. A raw mix of the basic composition was prepared using fine-grained chemical reagents as the starting materials and fired at 1600°C for 10min without any preliminary heat treatment. Fig.9 gives an example of the reverse zoning of alite in clinker thus prepared. Discussion The nucleation and crystal growth of alite from the interstitial melt in clinker are analogous to those from solution or flux. With the rise of tem-
306
Vol.
12, No. 3
I. Maki, K. Goto
p e r a t u r e CaO and CiS, formed i n l a r g e q u a n t i t i e s a t t h e r e l a t i v e l y e a r l y s t a g e s o f t h e c l i n k e r i n g r e a c t i o n , d i s s o l v e in t h e i n t e r s t i t i a l melt. If the product [CaO] and [CzS] i n t h e m e l t l o c a l l y e x c e e d s t h e s o l u b i l i t y p r o d u c t f o r C3S, a i i t e comes t o be p r e c i p i t a t e d from t h e m e l t . F u r t h e r d i s s o l u t i o n and d i f f u s i o n o f CaO and CiS make up f o r t h e d e c r e a s e i n s u p e r s a t u r a t i o n due t o t h e precipitation of alite. G e n e r a l l y t h e r a t e o f d i s s o l u t i o n o f b o t h CaO and C2 S i n c r e a s e s more r a p i d l y w i t h a r i s e i n t e m p e r a t u r e t h a n does t h e s o l u b i l i t y product. With c l i n k e r i n g a t h i g h t e m p e r a t u r e s , t h e r e f o r e , h i g h s u p e r s a t u r a t i o n i s c a u s e d o v e r l a r g e volumes o f t h e melt and many n u c l e i can be formed i n f a v o r of small alite crystals. The p r e c i p i t a t i o n o f a l i t e can be c o m p l e t e d w i t h i n a very short period of time. With c l i n k e r i n g a t low t e m p e r a t u r e s , on t h e o t h e r hand, t h e s u p e r s a t u r a t i o n o f t h e m e l t g e n e r a l l y r e m a i n s so low t h a t t h e n u c l e ation rate of alite is very limited. Consequently individual alite crystals can a f f o r d t o grow l a r g e w i t h d u r a t i o n o f f i r i n g . B e l i t e i n c l u s i o n s commonly found in a l i t e c r y s t a l s c o n f i r m t h a t the r a t e o f d i s s o l u t i o n o f b e l i t e i s s m a l l at low temperatures. The MgO in clinker, as already described, is of primary importance to the phase differentiation of alite. The minimum amount of MgO in alite necessary for stabilizing M 3 at ambient temperature is estimated 0.6- 0.8% (2), though depends also on the amount of Fei03 and A1203 coexisting in solid solution. During clinkerization MgO is mostly concentrated in the interstitial melt and can be taken up in alite to a certain extent upon crystallization; the ratio of the MgO concentration in alite to that in the liquid defines the effective distribution coefficient. That is why there exists a linear relationship between the amount of MgO in clinker and that in alite, though the slope is somewhat different according to the authors (6,7,8). Such correlation has been found also for FeiO 3. The EPMA data by Kristmann (8), however, gave no clear correlation for AIiO 3. It seems that alite is almost saturated with AIiO 3. The incorporation of Mg, Fe and A1 as well as other minor elements is influenced to a great extent also by the growth rate of alite. In melt growth segregation takes place, at any growth rate, at solid-liquid interfaces and the effective distribution coefficient can vary from the value at equilibrium, as obtained from the corresponding phase diagram, to l at very high growth rates (9). The analogous behavior can be expected in solution or flux growth. The mass rate of crystal growth in bulk crystallization, for example, is most conveniently expressed in terms of the supersaturation AC as Kg(AC) g where the order, g, of the growth process is generally between 1 and 2 (lO). Thus the state of supersaturation exerts great influence not only on the crystal size but also on the phase constitution of alite. The high concentration of foreign ions in alite associated with an increased distribution coefficient is favorable for the stabilization of M 3. This is the case for alite rapidly grown from highly supersaturated interstitial melts. By contrast, M 1 is predominant for alite grown from the melt of low supersaturation, i.e. under equilibrium conditions in which the growth rate is very small. The concentration of foreign ions in alite, therefore, is not necessarily proportional to that in the melt but subject to a great extent to the growth kinetics of alite. In general, the phase constitution of alite in clinkers with poor to intermediate MgO contents is very susceptible to the kinetics of crystallization from the melt; whereas in clinkers with more than 1.5% of MgO, for example, alite consists essentially of M 3. Clinker A in Fig.l is exceptional in this respect; the alite is zoned despite the MgO content in clinker as high as 1.9%. This suggests the anomalous manufacturing process for this clinker that the raw mix was kept so long at low temperatures as to enable the alite to crystallize primarily as M 1 . Alite once formed as M 1 carl hardly be inverted to M~ even if clinker is refired, after fine grinding, at
Vol. 12, No. 3 307 ALITE, PHASE CONSTITUTION, RAW MIX, COMPOSITION, COOLING RATE
as h i g h a t e m p e r a t u r e as 1600°C; a d d i t i o n o f MgO can no l o n g e r i n c r e a s e proportion of M3 significantly, either.
the
Varying supersaturations give rise to the concentration gradient of foreign ions within a crystal of alite, thus leading to the formation of the zonal structure. The state of supersaturation can vary according to the local environment of growing crystals during ¢linkerization. The rhythmic zoning of alite as shown in Fig.10 can be explained by an oscillatory change in supersaturation. The high reactivity of a raw mix, in general, is favorable for the attainment of high supersaturations at relatively low temperatures. The reactivity depends not only on the chemical composition of a raw mix but also on its mineral composition, particle size, the crystallinity of individual minerals and other many factors. In this respect the presence in quantities of large particles of quartz, the least reactive component, in a raw mix gives a detrimental effect on the formation of M 3 (Fig.7). Prolonged heating of a raw mix below the liquid formation temperature promotes both the recrystallization of CaO and grain growth of C25 and remarkably reduces the degree of supersaturation to be attained at the clinkering temperature. This is favorable for both the grain growth of alite and the occurrence of M 1 at ambient temperature. However, once fine grains of C35 are formed in large quantities by solid state reaction, alite remains small in size on subsequent firing at much higher temperatures. Very rapid crystallization of alite from the melt of exceedingly high supersaturation subsequently causes a significant drop in supersaturation, thus occasionally leading to the formation of the reverse zoning as shown in Fig.9. The SO 3 in clinker is concentrated at the clinkering temperature in the interstitial melt and significantly lowers its viscosity. According to Butt et. aZ. (Ii), the eutectic melt (CaO 54.8%, SiO 2 6.0%, AI203 22.7%, Fe203 16.5%) with m.p. of 1538°C, for example, gives the viscosity of 1.6poise at 1450°C. In the presence of 2.5% of SO 3 in the melt the same viscosity value can be reached at 1410°C. This implies that in clinker with plenty of 503 the formation of alite can take place at much lower temperatures than usual. This view is evidenced by many small belite grains taken up during crystallization. The decrease in viscosity, on the other hand, accelerates the diffusion or removal of those components rejected from the growing front of alite upon crystallization. This as well as the depression of the alite formation temperature reduces the effective distribution coefficient and thus encourages the occurrence of M I. SO 3 causes also an abnormal grain growth of alite. The alkalies in clinker counteract such effects of SO 3 on the phase constitution and grain growth of alite through preferentially combining with 503. Applications The results of this investigation can immediately be applied to some practical problems concerning the evaluation of the quality of cements. Ono (12) derived statistically the equations to give the strength development of mortars in terms of certain characters of the silicates in clinker as well as the hydraulic modulus(HM), the percentage of free lime etc. He attaches much importance to the characters of the clinker constituent minerals. As for alite the birefringence and mean grain size are chosen as parameters. He regards that the birefringence of slite in clinker varies from 0.002 to 0.010. According to our preceding paper (4), however, the maximum birefringence (y-e) ranges from 0.005 to 0.006 for M3, while it is 0.005 and essentially constant for M I. In the Ono's method cement powders are used for estimating the birefringence of alite and as the result no distinction can be drawn between M 1 and M 3. The intermediate birefringence between 0.005 and 0.005, therefore, presumably
308
Vol.
12, No.
3
I. Maki, K. Goto
originates from the hybrid state (zonation) of M 1 and M 3. If this is really the case, the birefringence of alite, the core of the Ono's method, can be reduced to the occurrence ratio of M 3 to M I. Further, the crystal size of alite, another important parameter, can be correlated with its phase constitution as described in the preceding section. Generally speaking, alite shows a tendency to occur in M 3 with a decrease in crystal size. The relation between the grain size of alite and the quality of cements, e.g. 28 days strength, which has been the subject of continuing technical interest, is explicable in terms of the phase constitution of alite. Thus the present study can provide to a certain extent a theoretical background for the Ono's method together with some restrictions to its application. This method seems available when the amount of MgO in clinker is moderate and the phase constitution of alite can vary significantly according to the technical conditions of cement manufacturing. Alite, as compared with belite, shows only slight differences in structure between the modifications; so that it must be emphasized that with regard to the hydraulic properties of cements the phase state is important in t h a t i t r e p r e s e n t s t h e c h e m i c a l c o m p o s i t i o n o f a l i t e as a whole r a t h e r t h a n i n itself. S u l f u r i n f u e l s a l s o e x e r t s i n f l u e n c e on t h e phase c o n s t i t u t i o n o f a l i t e t h r o u g h t h e f o r m a t i o n o f SO? and t h u s m o d i f i e s t h e q u a l i t y o f c e m e n t s .
Acknowledgment T h i s work was s u p p o r t e d by t h e G r a n t - i n - A i d f o r S c i e n t i f i c R e s e a r c h o f t h e J a p a n e s e M i n i s t r y o f E d u c a t i o n , S c i e n c e and C u l t u r e (No.00547047). Thanks a r e due t o Dr. C a m p b e l l o f t h e P o r t l a n d Cement A s s o c i a t i o n , U . S . A . , Mr. M u s i k a s o f t h e C e n t r a l L a b o r a t o r y o f O r i g n y Cement, F r a n c e , Mr. Lee of t h e Taiwan Cement C o r p o r a t i o n , and P r o f . Massazza o f t h e C e n t r a l L a b o r a t o r y o f I t a l y Cement f o r k i n d l y p r o v i d i n g many c l i n k e r s a m p l e s used in t h i s i n v e s tigation. References 1.
I . Maki and S. Chromg,
2.
I . Maki,
3.
I.
4.
I. Maki and K. Kato, Cement and Concrete Research 12, 93 (1982).
5.
Y. Ono, Doctor thesis,
6.
G. Yamaguchi and S. T a k a g i , p . 1 8 1 , Tokyo, 1968.
7.
P. T e r r i e r ,
8.
M. K r i s t m a n n , Cement and C o n c r e t e R e s e a r c h 8,
9.
J.A.
I 1 C e m e n t o 76,
I 1 C e m e n t o 75, 167 ( 1 9 7 9 ) .
M a k i , T. Ogiwara and S.
ibid.,
2a7 ( 1 9 7 8 ) .
Chrom~,
1 1 C e m e n t o 78, 53 /1981).
University of Tokyo,
\:ol.9,
"Proc.
5th I n t .
p.246,
1965.
Symp. Chem. Cement", Vol. I
p.278.
B u r t o n , R.C. Prim and W.P. S l i c h t e r ,
J.
93 ( 1 9 7 8 i . Chem. P h y s . ,
21,
1987
(1953). 10.
J.W. b l u l l i n , 1980.
i n B.R. P a m p l i n , " C r y s t a l
ll.
Y.H. B u t t and V.V. T i m a s h e v , P r i n c t p a l Cement, Moscow, 1974.
12.
Y. Ono, " P r o c .
7th I n t .
Growth", p . 5 2 9 ,
Pergamon P r e s s ,
paper,
Syrup. Chem.
6th
Int.
Syrup. Chem. C e m e n t " , Vol. II, 1-206, P a r i s ,
1980.
Vol° 12, pp. 301-308, 1982. Printed in the U S A Copyright (c) Pergamon Press, Ltd.
FACTORS INFLUENCING THE PHASE CONSTITUTION OF ALITE IN PORTLAND CEMENT CLINKER
I. Maki and K. Goto Nagoya I n s t i t u t e o f T e c h n o l o g y G o k i s o - c h o , Showa-ku, Nagoya, 466 J a p a n
(Communicated by H.F.W. Taylor) (Received Nov. 2, 1981)
ABSTRACT The p h a s e c o n s t i t u t i o n o f a l i t e i n p o r t l a n d cement c l i n k e r h i g h l y depends n o t o n l y on t h e c h e m i c a l c o m p o s i t i o n o f a raw mix b u t a l s o on the kinetics of crystallization from t h e i n t e r s t i t i a l melt. From t h e m e l t o f h i g h s u p e r s a t u r a t i o n a l i t e can be c r y s t a l l i z e d so q u i c k l y as t o t a k e up f o r e i g n i o n s i n q u a n t i t i e s i n f a v o r o f t h e o c c u r r e n c e o f M3 a t a m b i e n t t e m p e r a t u r e . The r e v e r s e i s t h e c a s e f o r t h e c r y s t a l l i z a t i o n from t h e m e l t low i n s u p e r s a t u r a t i o n , where M1 t e n d s t o be formed. The f o r m a t i o n o f zoned a l i t e can be e x p l a i n e d by t h e c h a n g e in the degree of supersaturation during crystallization. The p h a s e c o n s t i t u t i o n o f a l i t e can be c o r r e l a t e d w i t h i t s c r y s t a l s i z e i n terms of the s u p e r s a t u r a t i o n of the melt. The SO3 i n c l i n k e r c o n s i d e r a b l y lowers the v i s c o s i t y of the i n t e r s t i t i a l m e l t and t h u s e n c o u r ages t h e f o r m a t i o n o f M1 a l o n g w i t h t h e g r a i n growth o f a l i t e . The r e l a t i o n b e t w e e n t h e p h a s e c o n s t i t u t i o n o f a l i t e and t h e q u a l i t y o f c e m e n t s has b e e n remarked i n c o n j u n c t i o n w i t h t h e O n o ' s method.
301
302
Vol.
12, No. 3
I. Maki, K. Goto
Introduction Alite in portland cement clinker from the modern manufacturing process occurs in two forms, M 1 and M 3 (1-4). Alite, which at the clinkering temperature crystallizes as R from the interstitial melt, undergoes phase differentiation during cooling according to its chemical composition -- the quantity and species of foreign ions in solid solution. Mg ions, in particular, play an essential role in the phase differentiation of alite (1,2). The SO 3 in clinker also significantly changes the phase constitution of alite through encouraging the occurrence of M l (2). The heating history as well as the chemical composition of a raw mix exerts great influence on the phase constitution of alite. Concerning this point, however, no systematic study has ever been made. This investigation has been undertaken to establish a viewpoint for comprehensive understanding of those factors which determine the characters of alite, inclusive of its phase constitution, and thus influence the hydraulic properties of cements. Materials The basic chemical composition of the raw mix employed throughout ti~is investigation has been chosen as follows: SiO 2 22.94%, AI203 5.46%, Fe203 3.04%, CaO 66.56%, MgO 0.60%, SO 3 0.40%, Na20 0.40%, K20 0.60%. This composition was modified appropriately when necessary. The raw mixes were prepared using industrial raw materials -- limestone and clay - ground to a residue of 8.0% on 240 mesh. Chemical reagents were added for adjusting such components as SiO2, Fe203, MgO, SO 3 and alkali oxides. SO 3 was added in the form of gypsum. The raw mixes prepared were calcined at 1000°C for 2hrs, unless other wise noted in the text, before heating to clinkering temperatures. Results 0
!
o
1. Effect of chemical composition~ of raw mixes
I0
0
I 3,0 - - - - 0 - 0 o
o
M3 i
v ,E
2.0
.
o
o
0
o
?1 0
1.0
o-"
.-"
0
~
o--
.
.
/
" .
.~
. . . . . . . . . . . . . . . .
o~
I(] ~ ~'~ tl O/~ Be
~3"~1 . /
.
•
M1
•
10 SO 3 in clinker (°1.)
20
FIG. 1 Relation between the phase constitution of alite and the amounts of MgO and SO 3 in clinker. Commercial cement clinkers.
In Fig.l is plotted the phase constitution of alite against the amounts of MgO and SO 3 in clinker for each of the several tens of works cement clinkers from various localities of the world. This figure confirms, as already indicated in our previous papers (1,2), that the phase differentiation of alite depends primarily on the amount of MgO in clinker; that is, with an increase in Mg@ content the phase constitution of aline changes from M l to M 3 through the hybrid state of both phases. It is also seen from this figure that the SO 3 in clinker encourages the formation of M 1 . 2. Effect of clinkering temperature Despite the foregoing, the ph:~s~ constitution of alite in laborator~ clinkers prepared under specially ~ontrolled conditions can show wide de~i ations from the relation given in Fig. l. Fig.2 is the photomicro~ra~:n
Vol. 12, No. 3 ALITE, PHASE CONSTITUTION,
FIG. 2 Alite showing a preponderance of M 1 . Fired at 1450°C for 30min. Transmitted light. Crossed polars.
303 RAW MIX, COMPOSITION,
COOLING RATE
FIG. 3 Afire made up solely of M 3. Fired at 1600°C for 10min. Transmitted light. Crossed polars.
of a clinker prepared by firing the raw mix at 1450°C for 30min. The alite is made up essentially of M 1 in accordance with the relation in Fig.l. By contrast, when the same raw mix is fired at 1600°C for 10min, the alite comes to be made up exclusively of M 3 in spite of the low MgO content in the raw mix
(Fig.3). Another remarkable difference lies in the crystal size of alite. At 1600°C the precipitation of alite can be completed within a very short time and the crystal in general remains small in size; whereas at 1450°C the nucleation of alite is so limited that the crystal can develop large with prolonged firing. The alite formed at 1450°C are characterized by small liquid inclusions, each of which is observed as the intergrowth of fine ferrite and aluminate crystals at ambient temperature. The zonal structure tends to be formed when alite is crystallized over a wide range of temperatures (Fig.4). With alite in works cement clinkers it is usual that M 3 overgrows M 1 initially formed at lower temperatures. This type of zonation will be called 'normal zoning' in contrast with 'reverse zoning' to be described later. With works cement clinker the zonal structure of alite
FIG. 4 Zoned alite (M3+MI). Fired at 1400°C for lhr, and then at 1600°C for 10min. Crossed polars.
FIG. S Afire in clinker with 1.0% of SO 3. Fired at 1600°C for 10min. Crossed polars. (cf. Fig.3).
304
Vol. 12, No. 3 I. Maki, K. Goto
(a) FIG. 6 (b) C o m p a r i s o n o f t h e p h a s e c o n s t i t u t i o n o f a l i t e i n two c l i n k e r s w i t h d i f f e r e n t SO 3 c o n t e n t s . (a) 0.2%, (b) 1.0%. Both c l i n k e r s c o n t a i n 0.9% o f MgO. C r o s s e d p o l a r s . c a n g e n e r a l l y be formed when raw mix i s s l o w l y b r o u g h t t o t h e f i r i n g zone o f the kiln. Under such c o n d i t i o n s t h e p r e c i p i t a t i o n o f a l i t e can a l m o s t be completed at relatively low t e m p e r a t u r e s , e . g . j u s t beyond t h e l i q u i d f o r m a t i o n t e m p e r a t u r e , i n f a v o r o f t h e f o r m a t i o n o f M1. With a r i s e i n t e m p e r a t u r e alite crystals partly dissolve in the interstitial m e l t a n d , on c o o l i n g from elevated temperatures, are recrystallized w i t h M3 o v e r g r o w i n g Ml , The M1 zone o f a c r y s t a l u s u a l l y c o i n c i d e s w i t h such an a r e a as c h a r a c t e r i z e d by l i q u i d inclusions, i n d i c a t i n g t h a t t h e zone b o u n d a r y b e t w e e n M1 and M3 i s j u s t t h e solid-liquid i n t e r f a c e on r e m e l t i n g . The M3 z o n e , on t h e c o n t r a r y , i s g e n e r a l l y free from liquid inclusions and appears smooth under the microscope. Sometimes a more simple sector structure appears in place of the typical zonal structure as mentioned above. From the foregoing it foIlows that the relation given in Fig. l applies only to such clinkers as manufactured under normal technical conditions. The phase constitution of alite is most susceptible to the heating history especially for clinkers with intermediate MgO contents.
FIG. 7 Preference for M l and anomalous grain growth in the presence of large particles of quartz. Fired at 1600°C for lOmin. (cf. Fig.3). Crossed polars.
FIG. 8 Afire with M 3 predominating over MI. Fired at 1450°C for 30min without calcination at IO00°C. (cf. Fig.2%. Crossed polars.
Vol. 12, No. 3
305 ALITE, PHASE CONSTITUTION,
RAW MIX, COMPOSITION,
COOLING RATE
FIG. 10 An example o f r h y t h m i c z o n i n g of a l i t e in commercial c l i n k er. Crossed polars.
FIG. 9 Alite showing reverse zoning. Fired at 1600°C for 10min without preliminary heat treatment. ( c f . Fig.3). 3. E f f e c t o f SO3
The SO3 i n c l i n k e r c a n a l s o c h a n g e t h e p h a s e c o n s t i t u t i o n o f a l i t e t o a great extent. As s e e n i n F i g . l , t h e minimum amount o f MgO i n c l i n k e r n e c e s s a r y f o r s t a b i l i z i n g a l i t e e n t i r e l y as M3 a t a m b i e n t t e m p e r a t u r e i n c r e a s e s w i t h an i n c r e a s i n g amount o f SO3. F i g . S shows a l i t e i n c l i n k e r p r e p a r e d by f i r i n g a t 1600°C f o r 10min a raw mix whose SO3 c o n t e n t was i n c r e a s e d t o t h e amount o f 1.0%. The i n c r e a s e b y 0.6% d e f i n i t e l y p r o m o t e s t h e f o r m a t i o n o f M1 as w e l l as t h e g r a i n growth o f a l i t e . F i g . 6 compares t h e p h a s e c o n s t i t u t i o n o f a l i t e i n two c o m m e r c i a l c l i n k e r s . Both c l i n k e r s , b u r n t i n t h e same k i l n , have v e r y s i m i l a r c h e m i c a l c o m p o s i t i o n s e x c e p t f o r t h e SO3 c o n t e n t s . E v i d e n t l y M1 p r e d o m i n a t e s i n t h e c l i n k e r r i c h i n SO3 ( F i g . 6 b ) . By c o n t r a s t , a l i t e c o n s i s t s o f M3 i n t h e c l i n k e r by f a r l e s s i n t h e SO3 c o n t e n t ( F i g . 6 a ) . 4. E f f e c t o f p a r t i c l e
s i z e and c a l c i n a t i o n
o f raw m i x e s
Ono (5) i n v e s t i g a t e d e x t e n s i v e l y t h e r e l a t i o n b e t w e e n t h e p a r t i c l e s i z e o f raw mixes and t h e c r y s t a l s i z e o f a l i t e u s i n g q u a r t z as a s o u r c e o f s i l i c a . He e s t a b l i s h e d t h a t a l i t e grows l a r g e w i t h an i n c r e a s i n g p a r t i c l e s i z e o f quartz employed. Fig.7 shows alite in clinker from a raw mix in which reagent silicic anhydride was replaced by coarse-grained quartz ranging in diameter between 88 and 104~m. For comparison the raw mix was fired at 1600°C for 10min. The alite crystals have grown huge in accordance with the Ono's findings. But what is more interesting is that M 1 is a predominant constituent of alite. Prolonged heating of a raw mix below the liquid formation temperature (oa. 13300C) also enhances the occurrence of M 1 as well as the grain growth of alite. This is evidenced by the observation that without the calcination at 1000°C alite remains small in size and is rich in M 3 even though the raw mix is fired at 14500C (Fig.8). The reverse zoning, in which M 1 overgrows M3, is liable to appear when alite is precipitated extremely quickly from the melt. A raw mix of the basic composition was prepared using fine-grained chemical reagents as the starting materials and fired at 1600°C for 10min without any preliminary heat treatment. Fig.9 gives an example of the reverse zoning of alite in clinker thus prepared. Discussion The nucleation and crystal growth of alite from the interstitial melt in clinker are analogous to those from solution or flux. With the rise of tem-
306
Vol.
12, No. 3
I. Maki, K. Goto
p e r a t u r e CaO and CiS, formed i n l a r g e q u a n t i t i e s a t t h e r e l a t i v e l y e a r l y s t a g e s o f t h e c l i n k e r i n g r e a c t i o n , d i s s o l v e in t h e i n t e r s t i t i a l melt. If the product [CaO] and [CzS] i n t h e m e l t l o c a l l y e x c e e d s t h e s o l u b i l i t y p r o d u c t f o r C3S, a i i t e comes t o be p r e c i p i t a t e d from t h e m e l t . F u r t h e r d i s s o l u t i o n and d i f f u s i o n o f CaO and CiS make up f o r t h e d e c r e a s e i n s u p e r s a t u r a t i o n due t o t h e precipitation of alite. G e n e r a l l y t h e r a t e o f d i s s o l u t i o n o f b o t h CaO and C2 S i n c r e a s e s more r a p i d l y w i t h a r i s e i n t e m p e r a t u r e t h a n does t h e s o l u b i l i t y product. With c l i n k e r i n g a t h i g h t e m p e r a t u r e s , t h e r e f o r e , h i g h s u p e r s a t u r a t i o n i s c a u s e d o v e r l a r g e volumes o f t h e melt and many n u c l e i can be formed i n f a v o r of small alite crystals. The p r e c i p i t a t i o n o f a l i t e can be c o m p l e t e d w i t h i n a very short period of time. With c l i n k e r i n g a t low t e m p e r a t u r e s , on t h e o t h e r hand, t h e s u p e r s a t u r a t i o n o f t h e m e l t g e n e r a l l y r e m a i n s so low t h a t t h e n u c l e ation rate of alite is very limited. Consequently individual alite crystals can a f f o r d t o grow l a r g e w i t h d u r a t i o n o f f i r i n g . B e l i t e i n c l u s i o n s commonly found in a l i t e c r y s t a l s c o n f i r m t h a t the r a t e o f d i s s o l u t i o n o f b e l i t e i s s m a l l at low temperatures. The MgO in clinker, as already described, is of primary importance to the phase differentiation of alite. The minimum amount of MgO in alite necessary for stabilizing M 3 at ambient temperature is estimated 0.6- 0.8% (2), though depends also on the amount of Fei03 and A1203 coexisting in solid solution. During clinkerization MgO is mostly concentrated in the interstitial melt and can be taken up in alite to a certain extent upon crystallization; the ratio of the MgO concentration in alite to that in the liquid defines the effective distribution coefficient. That is why there exists a linear relationship between the amount of MgO in clinker and that in alite, though the slope is somewhat different according to the authors (6,7,8). Such correlation has been found also for FeiO 3. The EPMA data by Kristmann (8), however, gave no clear correlation for AIiO 3. It seems that alite is almost saturated with AIiO 3. The incorporation of Mg, Fe and A1 as well as other minor elements is influenced to a great extent also by the growth rate of alite. In melt growth segregation takes place, at any growth rate, at solid-liquid interfaces and the effective distribution coefficient can vary from the value at equilibrium, as obtained from the corresponding phase diagram, to l at very high growth rates (9). The analogous behavior can be expected in solution or flux growth. The mass rate of crystal growth in bulk crystallization, for example, is most conveniently expressed in terms of the supersaturation AC as Kg(AC) g where the order, g, of the growth process is generally between 1 and 2 (lO). Thus the state of supersaturation exerts great influence not only on the crystal size but also on the phase constitution of alite. The high concentration of foreign ions in alite associated with an increased distribution coefficient is favorable for the stabilization of M 3. This is the case for alite rapidly grown from highly supersaturated interstitial melts. By contrast, M 1 is predominant for alite grown from the melt of low supersaturation, i.e. under equilibrium conditions in which the growth rate is very small. The concentration of foreign ions in alite, therefore, is not necessarily proportional to that in the melt but subject to a great extent to the growth kinetics of alite. In general, the phase constitution of alite in clinkers with poor to intermediate MgO contents is very susceptible to the kinetics of crystallization from the melt; whereas in clinkers with more than 1.5% of MgO, for example, alite consists essentially of M 3. Clinker A in Fig.l is exceptional in this respect; the alite is zoned despite the MgO content in clinker as high as 1.9%. This suggests the anomalous manufacturing process for this clinker that the raw mix was kept so long at low temperatures as to enable the alite to crystallize primarily as M 1 . Alite once formed as M 1 carl hardly be inverted to M~ even if clinker is refired, after fine grinding, at
Vol. 12, No. 3 307 ALITE, PHASE CONSTITUTION, RAW MIX, COMPOSITION, COOLING RATE
as h i g h a t e m p e r a t u r e as 1600°C; a d d i t i o n o f MgO can no l o n g e r i n c r e a s e proportion of M3 significantly, either.
the
Varying supersaturations give rise to the concentration gradient of foreign ions within a crystal of alite, thus leading to the formation of the zonal structure. The state of supersaturation can vary according to the local environment of growing crystals during ¢linkerization. The rhythmic zoning of alite as shown in Fig.10 can be explained by an oscillatory change in supersaturation. The high reactivity of a raw mix, in general, is favorable for the attainment of high supersaturations at relatively low temperatures. The reactivity depends not only on the chemical composition of a raw mix but also on its mineral composition, particle size, the crystallinity of individual minerals and other many factors. In this respect the presence in quantities of large particles of quartz, the least reactive component, in a raw mix gives a detrimental effect on the formation of M 3 (Fig.7). Prolonged heating of a raw mix below the liquid formation temperature promotes both the recrystallization of CaO and grain growth of C25 and remarkably reduces the degree of supersaturation to be attained at the clinkering temperature. This is favorable for both the grain growth of alite and the occurrence of M 1 at ambient temperature. However, once fine grains of C35 are formed in large quantities by solid state reaction, alite remains small in size on subsequent firing at much higher temperatures. Very rapid crystallization of alite from the melt of exceedingly high supersaturation subsequently causes a significant drop in supersaturation, thus occasionally leading to the formation of the reverse zoning as shown in Fig.9. The SO 3 in clinker is concentrated at the clinkering temperature in the interstitial melt and significantly lowers its viscosity. According to Butt et. aZ. (Ii), the eutectic melt (CaO 54.8%, SiO 2 6.0%, AI203 22.7%, Fe203 16.5%) with m.p. of 1538°C, for example, gives the viscosity of 1.6poise at 1450°C. In the presence of 2.5% of SO 3 in the melt the same viscosity value can be reached at 1410°C. This implies that in clinker with plenty of 503 the formation of alite can take place at much lower temperatures than usual. This view is evidenced by many small belite grains taken up during crystallization. The decrease in viscosity, on the other hand, accelerates the diffusion or removal of those components rejected from the growing front of alite upon crystallization. This as well as the depression of the alite formation temperature reduces the effective distribution coefficient and thus encourages the occurrence of M I. SO 3 causes also an abnormal grain growth of alite. The alkalies in clinker counteract such effects of SO 3 on the phase constitution and grain growth of alite through preferentially combining with 503. Applications The results of this investigation can immediately be applied to some practical problems concerning the evaluation of the quality of cements. Ono (12) derived statistically the equations to give the strength development of mortars in terms of certain characters of the silicates in clinker as well as the hydraulic modulus(HM), the percentage of free lime etc. He attaches much importance to the characters of the clinker constituent minerals. As for alite the birefringence and mean grain size are chosen as parameters. He regards that the birefringence of slite in clinker varies from 0.002 to 0.010. According to our preceding paper (4), however, the maximum birefringence (y-e) ranges from 0.005 to 0.006 for M3, while it is 0.005 and essentially constant for M I. In the Ono's method cement powders are used for estimating the birefringence of alite and as the result no distinction can be drawn between M 1 and M 3. The intermediate birefringence between 0.005 and 0.005, therefore, presumably
308
Vol.
12, No.
3
I. Maki, K. Goto
originates from the hybrid state (zonation) of M 1 and M 3. If this is really the case, the birefringence of alite, the core of the Ono's method, can be reduced to the occurrence ratio of M 3 to M I. Further, the crystal size of alite, another important parameter, can be correlated with its phase constitution as described in the preceding section. Generally speaking, alite shows a tendency to occur in M 3 with a decrease in crystal size. The relation between the grain size of alite and the quality of cements, e.g. 28 days strength, which has been the subject of continuing technical interest, is explicable in terms of the phase constitution of alite. Thus the present study can provide to a certain extent a theoretical background for the Ono's method together with some restrictions to its application. This method seems available when the amount of MgO in clinker is moderate and the phase constitution of alite can vary significantly according to the technical conditions of cement manufacturing. Alite, as compared with belite, shows only slight differences in structure between the modifications; so that it must be emphasized that with regard to the hydraulic properties of cements the phase state is important in t h a t i t r e p r e s e n t s t h e c h e m i c a l c o m p o s i t i o n o f a l i t e as a whole r a t h e r t h a n i n itself. S u l f u r i n f u e l s a l s o e x e r t s i n f l u e n c e on t h e phase c o n s t i t u t i o n o f a l i t e t h r o u g h t h e f o r m a t i o n o f SO? and t h u s m o d i f i e s t h e q u a l i t y o f c e m e n t s .
Acknowledgment T h i s work was s u p p o r t e d by t h e G r a n t - i n - A i d f o r S c i e n t i f i c R e s e a r c h o f t h e J a p a n e s e M i n i s t r y o f E d u c a t i o n , S c i e n c e and C u l t u r e (No.00547047). Thanks a r e due t o Dr. C a m p b e l l o f t h e P o r t l a n d Cement A s s o c i a t i o n , U . S . A . , Mr. M u s i k a s o f t h e C e n t r a l L a b o r a t o r y o f O r i g n y Cement, F r a n c e , Mr. Lee of t h e Taiwan Cement C o r p o r a t i o n , and P r o f . Massazza o f t h e C e n t r a l L a b o r a t o r y o f I t a l y Cement f o r k i n d l y p r o v i d i n g many c l i n k e r s a m p l e s used in t h i s i n v e s tigation. References 1.
I . Maki and S. Chromg,
2.
I . Maki,
3.
I.
4.
I. Maki and K. Kato, Cement and Concrete Research 12, 93 (1982).
5.
Y. Ono, Doctor thesis,
6.
G. Yamaguchi and S. T a k a g i , p . 1 8 1 , Tokyo, 1968.
7.
P. T e r r i e r ,
8.
M. K r i s t m a n n , Cement and C o n c r e t e R e s e a r c h 8,
9.
J.A.
I 1 C e m e n t o 76,
I 1 C e m e n t o 75, 167 ( 1 9 7 9 ) .
M a k i , T. Ogiwara and S.
ibid.,
2a7 ( 1 9 7 8 ) .
Chrom~,
1 1 C e m e n t o 78, 53 /1981).
University of Tokyo,
\:ol.9,
"Proc.
5th I n t .
p.246,
1965.
Symp. Chem. Cement", Vol. I
p.278.
B u r t o n , R.C. Prim and W.P. S l i c h t e r ,
J.
93 ( 1 9 7 8 i . Chem. P h y s . ,
21,
1987
(1953). 10.
J.W. b l u l l i n , 1980.
i n B.R. P a m p l i n , " C r y s t a l
ll.
Y.H. B u t t and V.V. T i m a s h e v , P r i n c t p a l Cement, Moscow, 1974.
12.
Y. Ono, " P r o c .
7th I n t .
Growth", p . 5 2 9 ,
Pergamon P r e s s ,
paper,
Syrup. Chem.
6th
Int.
Syrup. Chem. C e m e n t " , Vol. II, 1-206, P a r i s ,
1980.