Microscopic study on the polymorphism of Ca3SiO5

CEMENT and CONCRETE RESEARCH. Vol. 8, pp. 407-414, 1978. Printed in the United States.

Pergamon Press, Inc

MICROSCOPIC STUDY ON THE EOL~IORPHISH OF Ca3SiO 5 lwao Maki Nagoya Institute of Technology Gokiso-cho, Showa-ku, Nagoya, 766 Japan and Stanislav Chrom~ Research Institute of Building Materials Brno-Kom~rov, Hn~vkovsk~ho 65, Czechoslovakia

(Communicated by H.F.W. T a y l o r ) (Received March 21, 1978) ABSTRACT The polymorphism of Ca3SiO 5 has been studied m i c r o s c o p i c a l l y by f o l l o w i n g changes in o p t i c p r o p e r t i e s and modes of t w i n n i n g o f the c r y s t a l as a f u n c t i o n of temperature. Besides the s i x m o d i f i c a t i o n s already e s t a b l i s h e d , a h i t h e r t o - u n i d e n t i f i e d m o n o c l i n i c phase MS, which can be c h a r a c t e r i z e d o n l y by microscopy at present, has been found to e x i s t j u s t below the rhombohedral phase (R). The t r a n s i t i o n s T2~-~T 3 and MI+-~M 2 t h a t give c l e a r thermal e f f e c t s on the DTA curve show no corresponding change under the microscope.

Le polymorphisme de Ca3SiO 5 a ~t~ ~tudi~ en suivant les changements de propri~t~s optiques et de modes de maclage du cristal en fonction de la temperature. Outre les six vari~t~s d~j~ ~tablies, l'existence d'une phase monoclinique encore inconnue M 3 qui, jusqu'~ present, ne ~tre caract~ris~e que par microscopie, a ~t~ mise en ~vidence juste au dessous de la phase rhombo~drique (R). Les transitions T2+-~T 3 et MI,--~M 2, qui donnent des effets thermiques bien marquis en A.T.D., ne montrent pas de changements d'aspect correspondants sous le microscope.

407

4 ,~o ~

Vol..3,

NO. "a

.. Maki, S. Cnromy

Introduction The allotropic phase transition of C3S has been explored intensively by means of both differential thermal analysis and X-ray powder diffraction at elevated temperatures; the precise work on the pol>~orphism of C3S was carried out in this way by Bigar~ et al (1), who confirmed the occurrence of six modifications up to the decomposition temperature (1250°C), i.e. three triclinic (TI, T2, T3) , two monoclinic (~II, q2), and one rhombohedral (R) modifications. 620 920 98~ 993 1050 TI * ' T~ ' -, T 3 ' ' ~I I ~---~I~_ . . . . . R A l l the above t r a n s i t i o n s are r e v e r s i b l e , i n d i c a t i n g the smallness o f the d i f f e r e n c e in s t r u c t u r e between the m o d i f i c a t i o n s . The e x i s t e n c e o f these s i x polymorphs has been g e n e r a l l y accepted since 1968 ( 2 ) . In the present work the p o l > ~ o r p h i c t r a n s f o r m a t i o n of C3S was f o l l o w e d d u r i n g h e a t i n g under the microscope. In some o f the phase t r a n s i t i o n s o f C3S, i n v e r s i o n twins are newly introduced i n t o the c r y s t a l or t w i n n i n g s t r u c t u r e s change a b r u p t l y upon t r a n s i t i o n ; and an i n s i g h t i n t o the mechanism o f the twin f o r m a t i o n or o f the change in the mode o f t w i n n i n g can throw l i g h t on the nature o f the phase t r a n s f o r m a t i o n in q u e s t i o n . Furthermore, o p t i c measurement o f t e n has the advantage o f being more s u s c e p t i b l e to minor s t r u c t u r e changes than X-ray measurement. As the r e s u l t , a h i t h e r t o - u n i d e n t i f i e d monoc l i n i c phase bl3 has been confirmed to e x i s t j u s t below R or between M2 and R. Experimental C r y s t a l s of the s i z e and q u a l i t y able to use f o r a d e t a i l e d m i c r o s c o p i c study were prepared by employing the method o f Nurse ( 3 ) , g e n e r a l l y being t h i n basal p l a t e s o f hexagonal o u t l i n e w i t h d i s t i n c t p y r a m i d a l faces. For h e a t i n g c r y s t a l s under the microscope, the L e i t z ' H e a t i n g Stage 1350' was employed. The temperature scale was c a l i b r a t e d f o r the m e l t i n g p o i n t s o f Ba(NO3)2(593:C), NaCI(800.4°C), Na2SO4(884°C), and K2SO4(I067°C). The t r a n s i t i o n temperatures obtained by Mazi~re (4) from DTA are used in t h i s paper f o r the f i r s t f o u r t r a n s i t i o n s as there was no a p p r e c i a b l e d i f f e r e n c e between the two sets o f e x p e r i m e n t a l v a l u e s f o r both TI~-~T 2 and T3+-~M I . Light intensity change during heating was recorded automatically with a microscope photometer together with the birefringence measurement by means of the Babinet compensator. Photometry, very quick in response, can be used to find small or fast changes difficull to detect with naked eyes under the microscope. With a single crystal it is possible to photometrically obtain a real birefringence change by" constantly bringing the crystal to a 4S-degree position which gives the maximum light intensity. Light intensity can be easily reduced into birefringence provided absorption is negligible (5). Results

blorphology o f C3S In o r d e r t o make the s u b s e q u e n t m i c r o s c o p i c s t u d i e s more s i g n i f i c a n t , it is essential to learn the exact crystallographic orientation of C3S. It is evident, from the external symmetry of the crystal, that the 3-fold axis (the o-axis) is perpendicular to the basal plane (0001) and that the three a-axes lie in this plane. A precession photograph of the zone {hkC} was taken to determine the directions of the a-axes with reference to the hexagonal outline of the crystal on (0001). Although the crystal gives a trigonal Dseudomorph at ambient temperature, the basic character of the original structure is retained. The result showed that the a-axes lie Darallel to the three diagonals of the hexagonal outline of the crystal, so that the indexing of the crystal faces given by Guttmann and Gille (6) and by Ono (7) has been proved

Vol. 8, No. 4

409 C3S, POLYMORPHISM,MICROSCOPY

0001 FIG.

I

A crystal habit of C3S

to be correct

(Fig.l).

The Miller indLces and optic orientations that appear subsequently in this paper are those referred to the trigonal cell given by Jeffery (~=7.0, o=25.0~) (8). Description

of

the

optic

character

of

each

modification

of C3S

I) TI(%620°C): C3S crystals grown from the melt with CaCl~ as a flux are characterized by three sets of twinning band on (000i) which, crossing each other at an angle of 120 or 60 ° , extend parallel to (i120} or at right angles to the hexagonal outline of the crystal. An area with striations developing in one direction defines a domain on (0001); and the crystal is usually made up of a few domaLns (Fig 3 a and b)

The LndicatrLces in two neighboring domains shm, a 120 ° difference in orientation around the common o-axis perpendicular to (¢00!) (Fig.2a). The domains are separated from each other by a narrow zone showing wavy extinction or by a different type of twin with the composition olane bisecting the twinning lines of the two adjacent domains. Fig.2b shows stereographically the optic orientations for alternating twinning bands in each domain, which are related to one another by reflection. The angle bet~,een the acute bisectrix X and the trigonal e-axis is ca.18 ° and the Y-axis lies almost in {10i0} olane.

a3

a3

-al -

-aI I. x ~l

X'~ Yz~"" I i

a

2

...... \

~ ~ 2 Y I

1

-a3 (a)

~

-a3 FIG. 2

Stereograms sho~
(b) for Tl (in the sane

-al

,oi.

~'i: !. Fhe o p t i c

axial

angle

2V i s

~, ?;o. 4

Maki, S. Cnromy ~>5.:~ ~" f o r

?;aD " i g h t

',,LIP. d l - s t i n c :

.[isperston

r:,~.

Fig.4 shows the temDera:ure dependence of the extinction direction of the fast tight ( x ' ) m e a s u r e d f r o : : :':-e : . , - n n i n ~ L i n e on " , : ' I : . 7?',,, e x t i r , : : / . : . . a n g l e t h u s d e f ~ . n e d ~i.ec:'eas._'s 2 : - : ; J ' : t ' , i v '.,it?: i n c r e a s i z z te:n:'.::':~.-:::'.-" .!~'.,.,: passing t h r o u g h : e r e a t t ' : e :c::~per-~t:~re o f : r a n . ~ i : i . ' : :~, : 2 , L: :'.;-~i:s :.; c:~.2}; ~,ith a different s i ; z o f e x : : . n c : i . ~ : a a:;,[ r e m a i R s :it=.-- E c : ? n s : : t : : : :.:" ~3 . a q : ~ : a : ;,hich the transizio:: t ~ ?.!, ::~;:es p l a c e . '.?:e : ~ i r e f : ' ! : : ~ . - ' - ' c e .,:: '". f:'.'st fails rapidl.v, the;'., s l o ; , i ' . , .z:-d
a.

b.

TI(20°C),

T1(20°C)

another

crystal.

~.

T2(6aO°Cl

e.

~!2(I030°C]

I

c.

TI(600°C)

=. !15[!065°.7. ", FI(;.

Photomicrographs sho'.,tr,.~ t h e ',,' i t h Ce..m.DeF ~t'cU F9.

change

S in c r v s t a ' . ! - - . e

te:
Vol. 8, No. 4

411 C3S, POLYMORPHISM,MICROSCOPY

40 °

FIG.

-

= 30o-~

4

E x t i n c t i o n angle ( x ' ~ 1 1 2 0 ) as a f u n c t i o n o f t e m p e r a ture.

~

20° "~

LJ

10 o 0 0

¢N I

T1

I,

,

,

200

.~" 1

, 400

I

R

:M

1

•

I

600

I

800

II

i

i000

I

1200

Temperature (*C) tion to T 2. The transverse section also shows the similar trend of birefringence change with temperature (Fig. S, a and b). 2) T2(620~920°C), T3(920~980°C): No appreciable optic change could be seen until the transition to MI took place. The twinning structure o£ the crystal also remained as it was immediately after the transition to T2. It is noteworthy that there is no change in optic properties at around 920°C in spite of a big, reversible peak observed on the DTA curve due to the transition between T2 and T3. xlO-~

3.0

(a) ¢QJ c~ ¢-

•~,

2. s

FIG. S Birefringence as a funct i o n o f temperature on sections cut perpendicul a r Ca) and p a r a l l e l (b) to (0001).

e~

M3

210

M1 T2 , T3

Tl

R

1.0 M2

= o

•~

0. S (b)

~w

5o O 0 '

!

200

A

|

400

.

I

600

800

Temperature (°C)

1000

1200

412

Vol. 8, No. 4

I. Maki, S. Chromy

FIG. 6 Optic orientations for the monoclinic forms of C3S. (a) MI and M2

(b) M3 Broken lines represent twinning planes.

k\

// '\

m

//

// X\ z \\ /

(a)

J

'\\ Y

(b)

3} MI(980~990°C), M2(990~ I060°C): Upon transition to M 1 the birefringence changes remarkably in both of the sections parallel and perpendicular to (00Of) (Fig.5). The twinning bands in the triclinic phase persist even after the transition to the monoclinic phase; but on being cooled from the stability region of R or M3, to be described below, the crystal occurs in fine, polysynthetic twins proper to both M I_ and M 2 (Fig.3e). This newly" formed twin has {I120} as the composition plane and is characterized by dense, parallel alignments of very fine twinning bands tapering out at both ends in the host. This type of twin disappears upon transition to T3 and the twin peculiar to the triclinic phase reappears instead. The extinction angle shows marked hysteresis. During heating it increases gradually from ca.20 °, the extinction angle of T3, to 30°; whereas during cooling it remains exactly 30 ° over the whole stability region of these two phases. The optic orientation of M I and M 2 is X lle, Yil a, Z ± a with the optic plane parallel to {i120}, so that in twins the adjacent optic planes are at angles of 120 ° (Fig.6a). The distinction between M1 and M2 comes from an extremely small ~eak at 990°C on the DTA curve; the stability range of M1 is only 10°C with pure C3S. The corresponding change in the unit cell dimension is the displacement of the 8 angle from 89.88 ° in M I to 90 ° in M2 (I). However, no distinction could be drawn optically between these two phases. 4) M3(I060~ I070=C): The most remarkable optic change that distinguishes ~I3 from M2 is the rotation of the indicatrix through 120 = in either direction around X normal to (0001) with y and B interchanged, i.e. the optic orientation changes from Xilc, Ylla, Z ± a in M 2 to XII c, Y i a , Zlla in M 3. The optic plane is parallel to {10i0} (Fig.6b). It is diagnostic under the microscope that in M 3 the slower light y vibrates at an angle of 30 ° with the twinning line on (0001) as compared with 60 ° in M2. M 3 is twinned on the same law as M 2 and MI. Sometimes it happens that the vibration directions on (0001) locally remain unchanged upon transition though ¥ and B are interchanged with each other. This can be explained as a twin counterpart of an area where the foregoing 120 ° ratation of the indicatrix with the transition is realized; both twin individuals are of the same crystallographic orientation before transition (Fig.7). The twin boundaries, as a matter of course, move laterally from their previous positions. It is usual, however, that the change in optic orientation is accompanied by the simultaneous 120 ° rotation of twinning lines, causing the complete rearrangement of the twinning pattern throughout the crystal (Fig.3f). The birefringence in the transverse section, which shows the highest value of all the modifications of C3S immediately after the transition, falls sharply with temperature and the inflection of the curve at around I070°C indicates the transition to R together with the disappearance of birefringence on (0001)

Vo]. 8, No. 4

413 C3S, POLYMORPHISM, MICROSCOPY Twinning

plane in M 2 FIG.

Twinning

plane in M 3

7 Y(M3)

Explanatory diagram of the lateral movement of twinning lines upon transition.

x (M3)

x (M3)

(M2)

mCM2)/

I

\ BCM3) (M2)

S(M2)

I

I I

~3(M23

(Fig.5). The photometric record of light intensity change shows that the birefringence on (0001) increases slightly upon transition to M 3 though falls very quickly thereafter. 5) R(1070=C~): The birefringence on (0001) disappears upon transition to R and the crystal becomes uniaxial negative. The maximum birefringence (e-w) falls with temperature. Discussion The rhombohedral phase is the highest-temperature modification of C3 S. It was for this phase that Jeffery (8) proposed an approximate crystal structure based on the space group R3m. Upon transition to M 3 birefringence appears on (0001) and two sets of twin are simultaneously introduced into the crystal. One is the cyclic twin or domain structure caused by the disappearance of the 3-fold symmetry. The other is a fine, polysynthetic twin whose composition plane is parallel to {I120} or at right angles to the hexagonal boundaries of the crystal on (0001). The formation of this twin can be interpreted as follows. The rhombohedral phase has three sets of mirror plane parallel to {i120} according to its trigonal symmetry 3m; whereas in the monoclinic phase, whose point group symmetry m is the subgroup of 3m in the rhombohedral phase, only one mirror plane out of the three remains as a real symmetry plane and the other two become pseudo-symmetry planes, able to act as the twinning plane. Thus fine, polysynthetic twins are formed with either one of these two pseudomirror planes as a composition plane; and the original trigonal structure is approximately retained in each domain. In M3, 8 vibrates parallel to m and ¥ perpendicular to it; they respectively form angles of 60 and 30 ° with the twinning line on (0001). Because of the marked pseudo-trigonal character of the M 3 phase, even very small displacements of atoms in structure can cause the transition to M2, i.e. the 120 = rotation of the structure or, more pertinently, of the symmetry plane with the resulting interchange of y and B in the indicatrix. In b12, therefore, it is y that vibrates parallel to m, which makes an angle of 60 or 120 ° with the twinning line on (0001). Naturally a complete rearrangement in twinning results from the foregoing changes in structure. The optic orientation remains unchanged upon transition to MI; further, the difference in structure between M 2 and M l is so small that no distinction can be drawn optically between these two phases. Upon transition to T 3 the twin proper to both M 2 and M 1 is replaced by a different type of twin with the disappearance of the mirror plane in the triclinic phase. The transition between T 3 and T 2 gives no optic change at all in spite of its biggest heat of transformation of all the phase transitions of C3S. No well-grounded explanation for this is available at present; but this

414

Vol. 8, No. 4 I. Maki, S. Chromy

transition may be attributed to such a kind of structure change as has no influence on megascopic quantities like optic properties, e.g. the formation of a modulation structure (superstructure). Summary i. M3, a hitherto-unidentified monoclinic modification of C3S, has been found to exist just below R or between M 2 and R. M 3 can be clearly distinguished from M 2 under the microscope by the difference in optic orientation. 2. The transitions T3+-~MI, M2+-~M3, and M3~-~R cause the change in twinning and TI+-+T2, T3+-+M I, M2+-+M 3, and M3+-~R the change in optic properties such as optic orientation and birefringence. 3. Neither the mode of twin nor optic character changes for the transitions T2+-+T 3 and M 1 ÷ ~ M 2 despite the clear thermal effects observed on the DTA curve. Acknowledgments We thank the Japanese Government to complete this work at the Research Czechoslovakia. Thanks are also due tory of Highways and Bridges, Paris,

for the support to I.M. which allowed him Institute of Building Materials in Brno, to Monsieur Deloye of the Central Laborafor preparing the French abstract.

References I.

M. Bigar~, A. Guinier, C. Mazi~re, M. Regourd, N. Yannquis, W. Eysel, Th. Hahn, and E. Woermann, J. Am. Ceram. Soc., 50, 609(1967).

2.

A. Guinier and M. Regourd, "Proc. 5th Intnl. Symp. Chemistry of Cement, Tokyo (1968)" Part I, p.l, Cement Association of Japan, Tokyo.

3.

R.W. Nurse, J. Sci.

4.

C. Mazi~re, Anal. Chem., 36, 602(1964).

5.

S. Chrom~, Silik~ty 18, 105(1974).

6.

A. Guttmann and F. Gille, Zement 22, 383(1933).

7.

G. Yamaguchi and Y. Ono, Zement-Kalk-Gips

8.

J.W. Jeffery, Acta Cryst.,

Instr.

Phys.

Ind., 26, 102(1949).

5, 26(1952).

20, 390(1966).