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The substitution of alkalies in tricalcium silicate
CEMENTand CONCRETERESEARCH. Vol. 9, pp. 701-71l, 1979. Printed in the U.S.A. 0008-8846/79/06070]-11502.00/0 Copyright (c) 1979 Pergamon Press, Ltd.
THE SUBSTITUTION OF ALKALIES IN TRICALCIUM SILICATE + E. Woermann, Th. Hahn and W. Eysel Institut f~r Kristallographie der RWTH Aachen 51OO Aachen, Germany
(Refereed) (Received March 14; in final form Aug. 27, ]979)
ABSTRACT In tricalcium silicate at 15OO°C up to 1.4 % K20 , 1.4 % Na20 and 1.2 % Li20, respectively, can be incorporated. The following structural substitution types were found: K replaces Ca with valency balance by removal of oxygen; Na replaces Ca and occupies interstitial sites, in addition some oxygen atoms are removed. Li replaces Ca as well as Si and occupies interstitial voids. Only modifications T I and TII are stabilized by alkalies. The decomposition temperature of pure Ca3SiO 5 was determined to be 1180 + IOOC and increases strongly with K20- and slightly with Na20-content. ' In Tricalciumsilikat werden bei 15OO°C bis zu 1.4 % K20, 1.4 % Na20 oder 1.2 % Li20 eingebaut. Als Substitutionstypen treten auf: K ersetzt Ca mit Ladungsausgleich durch Sauerstoffdefizit. Na ersetzt Ca und besetzt leere Gitterp!Atze, wobei ebenfalls Sauerstoffatome entfernt werden. Li ersetzt Ca- und Si-Positionen sowie ZwischengitterplHtze Die Alka!ien stabilisieren die Modifikationen TI und TII. Die ermlttelte Zersetzungstemperatur des reinen Ca3Si05 yon 1180 + IOOC steigt stark mit dem K20- und wenlg mit dem Na20-Gehalt. Introduction In view of the growing attention to the influence of alkalies on hydration reactions of portland cement, as well as on reactions between cement paste and aggregate, it is important to gain a better understandlng of the role of alkalies in portland cement clinker. This includes information about: I) Solid solution of alkalies in clinker phases: + Presented by Th. Hahn (1) at the 54th meeting of the Deutsche Mineraloglsche Gesellschaft at Braunschweig, Sept. 15th, 1976. 70]
702
Vol. 9, No. 6 E. Woermann, et al. a) type of s u b s t i t u t i o n b) limit of solid solution and its t e m p e r a t u r e c) s t a b i l i z a t i o n of p o l y m o r p h i c forms.
2) F o r m a t i o n
of d i s c r e t e
alkali
3) Influence
on phase equilibria,
dependence,
and
phases. in particular:
a) d e p r e s s i o n of solidus t e m p e r a t u r e s and b) shift of field boundaries, w i t h special emphasis on the influence on the p r i m a r y phase field of free lime, w h i c h controls the c a p a b i l i t y of the s y s t e m to c o m b i n e CaO. In this paper the influence of s o d i u m and p o t a s s i u m on the properties of t r i c a l c i u m silicate is investigated. Some additional experiments were p e r f o r m e d on the i n c o r p o r a t i o n of lithium in order to compare the effects of the v a r i o u s alkali species. Previous
work
A n a l y s e s of t e c h n i c a l clinker indicate that alkalies are inc o r p o r a t e d in C a 3 S i O 5 in small but m e a s u r a b l e amounts. Y a m a g u c h i and Takagi (2) report c o n c e n t r a t i o n s of around O . 1 % Na20 and 0.15 % K20, w h i l e M i d g l e y (3, 4) found up to 0.5 % N a 2 0 and 0.25 % K20. In the past m u c h w o r k has been p e r f o r m e d to gain i n f o r m a t i o n on the i n c o r p o r a t i o n of alkalies in d i c a l c i u m silicate and tricalcium aluminate. The influence of a l k a l i e s on t r i c a l c i u m silicate, however, has been i n v e s t i g a t e d by only a few authors. Kr~mer and zur S t r a s s e n (5) r e p o r t e d that t r i c a l c i u m silicate is stable in the presence of s o d i u m oxide, but is d e c o m p o s e d to KC23S12 and CaO by p o t a s s i u m oxide. Y a m a g u c h i & U c h i k a w a (6) found that tric a l c i u m silicate can form a solid s o l u t i o n i n c o r p o r a t i n g up to O . 7 1 % Na20. At 0.33 % Na20 a change of the m o d i f i c a t i o n of tric a l c i u m silicate was observed. No q u a n t i t a t i v e i n f o r m a t i o n on the influence of p o t a s s i u m is available. Experimental
Methods
s t a r t i n g Materials A mixture, c o n s i s t i n g of 1.25 % free CaO and 98,75 % of s t o i c h i o m e t r i c Ca3SiO 5 was used as s t a r t i n g m a t e r i a l for all experiments. For the p r e p a r a t i o n of this s t a r t i n g m i x t u r e 74.0 % CaO (CaCO 3, Merck No. 2066) and 26.0 % SiO 2 (Aerosil 200, Degussa) w e r e thoroughly m i x e d and h e a t e d for 24 hours at 15OO°C. After c o o l i n g in air and grinding, the free lime content was determined, a p p l y i n g the F r a n k e (7) method. A f t e r four further heating and g r i n d i n g cycles a c o n s t a n t free lime c o n c e n t r a t i o n was analyzed, indicating e q u i l i b r i u m conditions. Synthesis To batches of this starting m i x t u r e potassium, s o d i u m or l i t h i u m w e r e added in small increments as K 2 C O 3 (Merck No. 4928), N a 2 C O 3 (Merck No. 6392) or Li2CO 3 (Merck No. 5680). Because of the volatility of alkalies at high t e m p e r a t u r e s the samples w e r e t i g h t l y w e l d e d in p l a t i n u m capsules. S u f f i c i e n t free space was p r o v i d e d
Vol. 9, No. 6
703 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
for the c a r b o n d i o x i d e to be l i b e r a t e d d u r i n g the reaction. The samples w e r e h e a t e d at 15OOOC for 24 hours and q u e n c h e d in air. The cold c a p s u l e s w e r e c h e c k e d for leaks a f t e r the h e a t t r e a t m e n t by d r o p p i n g t h e m into hot water. O n l y c a p s u l e s that did not develop b u b b l e s from e x p a n d i n g gas were used for f u r t h e r analyses. Analytical Methods To d e t e r m i n e the a 1 k a 1 i c o n t e n t s , some of the samples w e r e a n a l y z e d by flame p h o t o m e t r y and by a t o m i c absorption s p e c t r o m e t r y . The results c o n f i r m e d that w i t h i n a n a l y t i c a l error the a l k a l i e s a d d e d r e m a i n e d in the samples d u r i n g the h e a t i n g process. For a q u a n t i t a t i v e e v a l u a t i o n , however, the analytical data w e r e not p r e c i s e enough. Thus the o r i g i n a l w e i g h i n g s w e r e a c c e p t e d as the c o n c e n t r a t i o n s of alkalies. F r e e 1 i m e w a s d e t e r m i n e d b y t h e F r a n k e (7) method. It has b e e n shown (Woermann, Hahn and Eysel, 8, 9) that this m e t h o d does not a t t a c k t r i c a l c i u m s i l i c a t e nor its solid solutions. F r o m r e p e a t e d a n a l y s e s of 0.7 g samples its a c c u r a r y w a s e s t i m a t e d to be b e t t e r than + 0.05 % CaO. W i t h i n the p r e s e n t e x p e r i m e n t a l series, however, T h e data s c a t t e r in a m u c h w i d e r range. This inc r e a s e in a n a l y t i c a l error is m a i n l y due to the s m a l l e r size of the s a m p l e s (O.1 g) w h i c h was s e l e c t e d in order to m i n i m i z e the e x p e n s e s of platinum. All samples w e r e i n v e s t i g a t e d by X - r a y p o w d e r d i f f r a c t o m e t r y after a d d i t i o n of NaCI as an i n t e r n a l s t a n d a r d u s i n g C u K e - r a d i a t i o n w i t h Ni-filter. Due to the e x t r e m e b r i t t l e n e s s of the samples o p t i c a 1 i n v e s t i g a t i o n s of p o l i s h e d s e c t i o n s w e r e less sat i s f a c t o r y than in a l k a l i - f r e e Ca3Si05, a l i t e s a m p l e s or p o r t l a n d c e m e n t clinker. C o n s e q u e n t l y , this m e t h o d was only e m p l o y e d for q u a l i t a t i v e i n s p e c t i o n of the samples. The S u b s t i t u t i o n of A l k a l i e s
in C a 3 S i O 5
Theoretical Considerations The i n c o r p o r a t i o n of m o n o v a l e n t a l k a l i ions in C a 3 S i O 5 c a n n o t be a c h i e v e d by the s u b s t i t u t i o n of d i v a l e n t Ca or t e t r a v a l e n t Si alone. In o r d e r to m a i n t a i n electroneutrality, e i t h e r the o c c u p a t i o n of i n t e r s t i t i a l sites of s u i t a b l e size or the c r e a t i o n of v a c a n cies in the o x y g e n s u b l a t t i c e is required. T h e o r e t i c a l l y only three s u b s t i t u t i o n a l types m a y be deduced, each r e p r e s e n t e d by a c h a r a c t e r i s t i c f o r m u l a and a r a t i o Ca/M, w h i c h are g i v e n in T a b l e I. A n y a l k a l i s u b s t i t u t i o n can be i n t e r p r e t e d e i t h e r in terms of one of the above p r i n c i p a l m o d e l s or of t h e i r s u p e r p o s i t i o n . A l k a l i s u b s t i t u t i o n s are thus r e s t r i c t e d to C a / M - r a t i o s f r o m -1.O tO + 0 . 7 5 . Experimental Results The type of s u b s t i t u t i o n of a l k a l i ions in C a 3 S i O 5 is determined by a n a l y s i n g the free CaO c o n t e n t of the sample before and a f t e r the r e a c t i o n w i t h a l k a l i oxides. The c h a n g e in free CaO (~fr. CaO) is p l o t t e d a g a i n s t the c o n c e n t r a t i o n of a l k a l i e s in Figs. I - 3. The r e s u l t i n g c u r v e s d e f i n e the C a / M - r a t i o s of the s u b s t i t u t i o n of the ion species.
704
Vol. 9, No. 6 E. Woermann, et al.
TABLE Substitution
models
Substitutional
1
for m o n o v a l e n t
ca%io~s
model
in C a 3 S i O 5 Hypothetical end member
oJ'
co si%
%
c%.~v, col co,
Ca/M
-l.O
I~>
s~ ~" 05
M~s~o~
-0.5
<21
S 05
co M,%
+0.75
131
M
=
monovalent
alkali
VI
=
octahedral
lattice
ion
VI'
=
o c t a h e d r a l i n t e r s t i t i a l site. s i t e s VI' p e r f o r m u l a unit.
IV
=
tetrahedral
lattice
site
[]
=
unoccupied
lattice
site
Ca/M
=
r a t i o o f c h a n g e o f C a - i o n s to a l k a l i ions: S u b s t i t u t i o n of C a by H l e a d s to n e g a t i v e r a t i o s , c o m b i n a t i o n C a d u e to s u b s t i t u t i o n of Si by M l e a d s to p o s i t i v e r a t i o s .
site The
s t r u c % u r e of C a 2 S i O 5 c o n t a i n s
three empty
of a d d i t i o n a l
(~t/~" //~,"~
o~30." / 2.0- ° ~
o
"20"~ 1.0"
tO-
io
go
Y~
Wt % No20
1,0
2~0 3.0 Wt %K20 FIG. l
Substitution of K÷ in Ca3SiO5. Liberation of free lime (Afr. CaO) as a function of K20 added
FIG. 2 Substitution of Na+~in Ca3SiO5.
Liberation of free iime (Afr. CaO) as a function of Na20 added . Circles: Substitution of Na+ in pure Ca3SiO~; Triangles: Substitution of Na+ in Al-saturated Ca3Si+~O~; Squares: Substitution of Na+ in Fe .-saturated Ca3SiO5.
Vol. 9, No. 6
705 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
The results for potassium substitution are shown in Fig. I where the Ca/K ratio is -1.0. According to the substitution model (I) this ratio indicates the substitution of K + for Ca ++ and a simultaneous removal of oxygen from the structure. For sodium substitution (Fig. 2) the ratio Ca/Na is -0.75. This ratio cannot be explained by any single substitution model. It can however, be interpreted as a superposition of models (I) and (2~: Substitution of Na for Ca ++ , incorporation of Na + in interstitial sites VI' and a simultaneous removal of some oxygen atoms. Additional experiments with AI- and with Fe-saturated Ca3SiO 5 indicate that the influence of these ions on the substitution of sodium is negligible. For lithium substitution (Fig. 3) the ratio Ca/Li is -0.25. This can be explained by the superposition of models (2) and (3): substitution of Li ÷ for Ca ++ , incorporation of Li + in interstitial sites and additional substitution of Li + for Si ++++. Alternatively, a superposition of models (I) and (3) is possible. The Limit of Substitution of Alkalies in Tricalcium Silicate In Fig. I the data points follow the slope with the ratio Ca/K = -1.O up to a concentration of 1.4 % K20. At this point a discontinuity indicates the limit of solubility of potassium in Ca3SiO 5. Beyond this point a third phase in addition to Ca3SiO 5 and free CaO appears, the composition of which is now controlling the Ca/K-ratio. X-ray data of quenched samples indicate that the "third phase" is an e'-Ca2SiO 4 solid solution ("KC23S12"). The schematic isothermal section of the ternary system K20-CaO-SiO 2 has been constructed accordingly (Fig. 4). It is in agreement with the observations by Kr~mer & zur Strassen (5) who claimed that the dicalcium silicate phase is stabilized by potassium, thus preventing the formation of alite.
,
R/
I
o
2:o wt'/.L O FIG. 3 Substitution of Li + in Ca3SiO5. Liberation of free lime (fr.CaO) as a function of Li20 added.
coo
K20 FIG. 4
Schematic isothermal section of the system K20-CaO-SiO2 at temperatures above the IBwer stabiTity limit of Ca3SiO5.
706
Vol. 9, No. 6 E. Woermann, et al.
T[ ' C ]
I~00-
/ II
•
•
c..s,o,.
,(.,'c sio 7/ ,',0
1300
1200
1100
•
.
S o / i z/
o
l0
I
t
.CaO
o icoo ,
l
l
I.
!
k
Wt %K20
cao
FIG. 5 S t a b i l i t y range of tricalcium s i l i c a t e solid solutions in the pseudobinary section Ca3SiO5 "K3Si03.5". Solid dots: Ca3SiO5 solid solutions. Open circles: Ca2SiO4 solid solutions + CaO.
...... FIG. 6
Schematic isothermal section of the system Na20 - CaO - SiO 2 at temperatures above the lower s t a b i l i t y l i m i t of Ca3SiO5.
The c o e x i s t e n c e of C a 3 S i O 5 s s + C a 2 S i O 4 s s + CaO at 14OO°C in the ternary s y s t e m K 2 0 - C a O - S i O 2 is c h e m i c a l l y a n a l o g o u s to the e q u i l i b r i u m C a 3 S i O 5 + C a 2 S i O 4 + CaO in the b i n a r y s y s t e m CaO-SiO 2. From this it can be c o n c l u d e d that the lower limit of s t a b i l i t y of alite is d r a s t i c a l l y raised by the s u b s t i t u t i o n of potassium. A q u a n t i t a t i v e e v a l u a t i o n of this effect is given in Fig. 5 w h i c h shows the lower stability t e m p e r a t u r e of alite as a function of K20-concentration. Similarly, the d i s c o n t i n u i t y in the C a / N a - s l o p e of Fig. 2 defines the limit of solubility of Na20 in C a 3 S i O 5 at 1.4 % N a 2 0 at 15OO°C. The "third phase" in this case is N a 2 C a S i O 4. The isothermal section of the N a 2 0 - C a O - S i O 2 phase d i a g r a m (Fig. 6) thus differs in essential points from the K 2 0 - C a O - S i O 2 diagram. A c c o r d i n g to Fig. 7 the t e m p e r a t u r e of the lower s t a b i l i t y limit of s o d i u m c o n t a i n i n g alite increases, but to a lesser e x t e n t than for potassium. The S t a b i l i z a t i o n
of P o l y m o r p h i c
Forms
At room t e m p e r a t u r e pure t r i c a l c i u m silicate is triclinic in the form T I. By i n c o r p o r a t i o n of alkalies the form TII is stabilized (Table 2). It is impossible, however, to s t a b i l i z e higher forms - M I, MII or R - by a d d i t i o n of a single alkali species. In c o m b i n a t i o n with AI203 and Fe203, however, w h i c h by themselves are s t a b i l i z i n g form TII only, high c o n c e n t r a t i o n s of alkalies lead to M I. F r o m Table 2 it can be seen that at least 0.7 % Na20 are r e q u i r e d to stabilize TII and that t r i c a l c i u m silicate can incorporate a m a x i m u m of 1.4 % Na20. This is c o n s i d e r a b l y more than
Vol. 9, No. 6
707 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
/~00-
.,~CaSiO~÷CxTO
C~,m05,, FIG. 7
1300 e e
Stability range of tricalcium
silicate solid solutions in the pseudobinary section Ca3SiO5 ,,Na3NaSi04,,"
o o 0
1200
~
0 0 o
1100
,
~
C~5iO~.÷CuO ,
,
~
C025i0~. ÷N~CoSiO~ ÷CaO
,
,
,
lO Wt %Na20 the corresponding concentrations of 0.33 and 0 . 7 1 % Na20, given by Yamaguchi and Uchikawa (6). The results given in this work differ from those of Yamaguchi and Uchikawa by a factor of 2. This discrepancy is also evident from a comparison of the corresponding Ca/Na-ratios. Although Yamaguchi and Uchikawa themselves did not relate the free lime of their experiments to the sodium concentration, this information may be obtained ~rom the data in their Table I. The resulting values follow closely a line with the slope Ca/Na = - 3/2. Their results differ from our ratio Ca/Na = - 3/4 again by a factor of exactly 2. The crystal chemical deductions (Table I), however, postulate Ca/Na = -1.O (model I) as the smallest possible ratio. Thus, the ratio Ca/Na = - 3/2 of Yamaguchi and Uchikawa cannot be explained structurally.
TABLE Stabilization
Ca3SiO 5 +
(wt.
of Ca3SiO 5 polymorphs
%)
2
by i n c o r p o r a t i o n
Phases
of various
(20°C)
ions
Limit
of
solid
solution TI
(15OO°C)
TII
Na20
O - 0.7
0.7
- 1.4
-
1.4
% Na20
K20
O - 1.3
1.3
- 1.4
-
1.4
% K20
Li20
0 - 0.5
0.5
- 1.2
-
1.2
% Li20
AI203
0 - 0.45
0.45
- 1.0
-
1.O % A I 2 0 3
Fe203
0 - 0.9
0.9
- 1.1
-
1.1%
Fe203
AI203
(I %)
+ Na20
-
O
- 0.8
0.8
- 1.4
1.4
% Na20
Fe203
(I %)
+ Na20
-
0
- 0.8
0.8
- 1.4
1.4
% Na20
Fe203
(I %)
+ Li20
-
O
- 0.8
?
% Li20
?
708
Vol. 9, No. 6 E. Woermann, et al.
The shift of the lattice p a r a m e t e r s of t r i c a l c i u m silicate by i n c o r p o r a t i o n of alkalies is very small and w i t h i n the limits of error of precise measurements. Discussion F r o m the e x p e r i m e n t a l results it is evident that each alkali species has a c h a r a c t e r i s t i c C a / M - r a t i o and thus exhibits a specific type of substitution. model
The ratio Ca/K = -I.O c o r r e s p o n d s exactly (I) w i t h the s u b s t i t u t i o n pattern: (Ca 3 - xKx )VI
[3VI'
siIVo~~ - ~x
[]x
to the t h e o r e t i c a l (A)
w i t h O <~ x ~< 0.068 The K + - i o n is s u b s t i t u t e d for a Ca++-ion. E v i d e n t l y it is too large for i n c o r p o r a t i o n into the other sites. The C a / K - r a t i o proves for the first time that a s u b s t i t u t i o n results in vacancies in the o x y g e n sublattice of C a 3 S i O 5. The structure of C a 3 S i 0 4 + I (Jeffery, 10, 11) contains two types of o x y g e n atoms. One type is s t r o n g l y bonded in SiO 4 tetrahedra w h i l e the other is c o o r d i n a t e d to Ca only w i t h a considerably w e a k e r b o n d i n g strength. It is a s s u m e d that o x y g e n vacancies are c r e a t e d only by the latter, C a - c o o r d i n a t e d oxygens. The s u b s t i t u t i o n Ca/Na = -0.75 requires a s u p e r p o s i t i o n of models (I) and (2) in the ratio I : I leading to an ultimate s u b stitution p a t t e r n . VI'y siIVo5 y D y (Ca. 3y Na 3y) vI Na o- 4 4 4 -4 4
(B)
w i t h 0 ~ y < 0.103 Clearly, the smaller size of the s o d i u m ion p e r m i t s its introduction into o c t a h e d r a l l y c o o r d i n a t e d i n t e r s t i t i a l positions, but, in addition, some oxygens c o n t i n u e to be removed from the structure. On the other hand the size of Na + p r o h i b i t s its s u b s t i t u t i o n for Si 4+ in t e t r a h e d r a l sites and thus excludes p a r t i c i p a t i o n of model 3 in the s u b s t i t u t i o n scheme. Strictly, however, the ratio Ca/Na = -0.75 is v a l i d only for t r i c a l c i u m silicate solid solutions in e q u i l i b r i u m w i t h free lime. It is c o n c e i v a b l e that in S i - s a t u r a t e d C a 3 S i O 5 c o e x i s t i n g w i t h d i c a l c i u m silicate the superp o s i t i o n ratio of m 6 d e l s (1) and (2) w i l l be shifted in favour of model (I). F r o m Table I it follows that model (I) is the limiting case. This e f f e c t w o u l d lead to a d i v a r i a n t phase field of the t r i c a l c i u m silicate phase in the s y s t e m Na20-CaO-Si02. A c o m p a r a b l e single phase field for t r i c a l c i u m silicate solid solutions has been o b s e r v e d b e f o r e in the s y s t e m C a O - A I 2 0 3 - S i O 2 , CaO-MgO-AI203SiO2 and C a O - M g O - F e 2 0 3 - S i O 2 (Woermann, Eysel and Hahn, 9, 12). A field was not c o n f i r m e d e x p e r i m e n t a l l y for the s o d i u m samples since the free lime m e t h o d is r e s t r i c t e d to C a - s a t u r a t e d solid solutions. Moreover, the b r i t t l e n e s s of the samples p r e c l u d e d optical investigations. The ratio Ca/Li = -0.25 can formally be i n t e r p r e t e d either as a s u p e r p o s i t i o n of m o d e l s (2) and (3) in the ratio 4 : 1, leading
Vol. 9, No. 6
709 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
to the substitution (Ca3_ ~
formula
Li ~ ) V I
Li -~11zvI' (Si1_~)IV
05
(C)
with 0 ~< z ~ O. 18 or of models formula
(I) and
(3) in the ratio 4 : 3 with the substitution
(Ca~_4z' Li 4z')VI Li 9z 'VI' (Si1_3z' Li 3z')Iv O=2z' --7 -72-~ 2-~ ~2-~
O2Z'
(D)
2-3-
with O ~< z' <~ O. 18 In each case, however, model (3) must be considered which involves the substitution of Li + for Si ++++. This model seems possible since a tetrahedrally coordinated Li + is well established in many compounds. Even though, the substitution of a monovalent ion for a tetravalent one is quite unusual. For the given Ca/Li-ratio of -0.25 the substitution formula (C) (models (2) and (3)) avoids the formation of vacancies in the oxygen sublattice. Furthermore, the substitution of Li + for Si ++++ is lower in this substitution formula than in (D) (models (1) and (3)) for equal amounts of total Li. For crystal chemical reasons, therefore, the substitution pattern (C) is preferred to (D). The ratio Ca/Li = -0.25, however, is again only valid for Ca-saturated tricalcium silicate solid solutions. Towards Ca-undersaturated compositions a single-phase range may exist due to a shift of the superposition ratio 4 : I of models (2) and (3) in favour of model (2). The substitution patterns of the various alkali species in tricalcium silicate thus appear to be closely controlled by their size. On the other hand the stability of the tricalcium silicate solid solutions is influenced by the deficit in the oxygen lattice created by the incorporation of the rather large alkali ions potassium and sodium. The lower limit of stability is not only shifted to higher temperatures but, in addition, the rate of decomposition increases with the amount of alkalies. Thus, while it is possible to determine the lower limit of stability of alkali solid solutions with satisfactory precision it has not yet been possible to determine accurately the equilibrium point Ca3SiO 5 = Ca2SiO 4 + CaO in the system CaO-SiO 2, due to the extreme sluggishness of this reaction. An extrapola£ion of the data obtained in the present work to the alkali-free end member results in a reliable e s t i m a t e of the stability point of pure Ca3SiO 5. Extrapolations from both experimental series, the potassium as well as the sodium solid solutions, arrive at an equilibrium point of 1180 + IOOC. +) This is in definite contrast to the stability limit of 125oOc which is accepted by many authorities - e.g. Levin, Robbins and McMurdie (Fig. 237 in ref. 13). The temperature of 1180°C compares favourably with the decompositon point of Fe2+-saturated CasSiO= at 1183°C determined by Woermann (14). ~ The phase diagrams +) Reference
in Figs.
temperature:
4 - 7 are helpful
in explaining
melting point of gold at 1064.5°C.
710
Vol. 9, No. 6 E. Woermann, et al.
the d i f f e r e n t b e h a v i o u r of t r i c a l c i u m s i l i c a t e w i t h r e s p e c t to pot a s s i u m or s o d i u m oxide, as o b s e r v e d by K r ~ m e r and zur S t r a s s e n (5). The c o n c e n t r a t i o n s of K20 and N a 2 0 in a l k a l i - s a t u r a t e d tric a l c i u m s i l i c a t e are of c o m p a r a b l e m a g n i t u d e (1.4 % K20 or 1.4 % N a 2 0 at 15OO°C). A l k a l i c o n c e n t r a t i o n s b e y o n d this s a t u r a t i o n limit r e s u l t in t h r e e - p h a s e - a s s e m b l a g e s w h i c h are c h a r a c t e r i s t i c of each system. F r o m Figs. 4 and 5 it is evident that in the s y s t e m K 2 0 - C a O SiO 2 this a s s e m b l a g e is C a 3 S i O 5 s s + C a 2 S i O 4 s s + CaO. The a m o u n t of the t r i c a l c i u m s i l i c a t e p h a s e is d e t e r m i n e d by the lever rule. Since the d a t a p o i n t s in Fig. 5 do not indicate a d e f i n i t e range of a t h r e e - p h a s e - f i e l d , the latter seems to be e x t r e m e l y narrow. Thus, only a slight e x c e s s of K20 w o u l d be s u f f i c i e n t to pass over the C a 2 S i O 4 s s - CaO join w h e r e the t r i c a l c i u m s i l i c a t e p h a s e disappears. In contrast, the c o r r e s p o n d i n g t h r e e - p h a s e - a s s e m b l a g e in the s y s t e m N a 2 0 - C a O - S i O 2, as shown in Figs. 6 and 7, is C a 3 S i O 5 s s + N a 2 C a S i O 4 + CaO. A c c o r d i n g to the lever rule the t r i c a l c i u m silicate p h a s e is zero in samples w h i c h are located on or b e y o n d the N a 2 C a S i O 4 - C a O join. On the line C a 3 S i O 5 - N a 2 0 this r e q u i r e s a minim u m s o d i u m c o n c e n t r a t i o n of 21.4 % Na20. In all c o m p o s i t i o n s w i t h lower N a 2 0 - c o n c e n t r a t i o n s the s o d i u m - s a t u r a t e d t r i c a l c i u m silicate solid s o l u t i o n is a stable phase. It s e e m e d p r o b a b l e that a d d i t i o n a l c o m p o n e n t s , e.g. A1203 and Fe203, may e x e r t some i n f l u e n c e on the type as w e l l as on the limit of s u b s t i t u t i o n of alkalies. S i n c e the solid s o l u t i o n of C a 3 S i O 5 w i t h N a 2 0 is c h a r a c t e r i z e d by the s u p e r p o s i t i o n of two d i f f e r e n t s u b s t i t u t i o n m o d e l s it will p r o b a b l y react m o r e sensit i v e l y to f o r e i g n c o m p o n e n t s than solid s o l u t i o n s w i t h K20, w h i c h r e p r e s e n t a l i m i t i n g case. Fig. 2 shows that C a / N a - r a t i o s and limits for the i n c o r p o r a tion of N a 2 0 in pure C a 3 S i O 5 as w e l l as in t r i c a l c i u m s i l i c a t e sat u r a t e d w i t h AI203 or w i t h Fe203 are i d e n t i c a l w i t h i n the limits of e x p e r i m e n t a l error. The i n f l u e n c e s of A 1 3 + and of Fe 3+ on the s u b s t i t u t i o n of N a 2 0 are thus n e g l i g i b l y small. Thus it is c o n c l u d e d that the r e s u l t s on the i n c o r p o r a t i o n of a l k a l i e s are v a l i d not only for pure t r i c a l c i u m s i l i c a t e but also r e p r e s e n t c l o s e l y the p r o p e r t i e s of the alite phase in t e c h n i c a l p o r t l a n d c e m e n t clinker. Acknowledgements We are i d e b t e d to the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t the V e r e i n D e u t s c h e r Z e m e n t w e r k e for support of this work.
and
References
I.
Th. Hahn, E. W o e r m a n n and W. Eysel. h e f t 1, 27 - 29 (1976).
Fortschr.
2.
G. Y a m a g u c h i and S. Takagi. Proc. F i f t h Intl. Cem., Tokyo, Vol. I.,181 - 225 (1969).
3.
H. G. Midgley. Proc. F i f t h Intl. Vol. I, 226 - 233 (1969).
Symp.
Chem.
Min. 54, BeiSymp.
Chem.
Cem., Tokyo,
Vol. 9, No. 6
71l C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
4.
H. G. Midgley.
5.
H. K r ~ m e r and H. zur Strassen. Proc. F o u r t h Intl. Chem. Cem., W a s h i n g t o n , Vol. I, 32 - 33 (1962).
6.
G. Y a m a g u c h i (1961).
7.
B. Franke.
8.
E. W o e r m a n n , Th. Hahn and W. Eysel. 370 - 375 (1963).
Z e m e n t - K a l k - G i p s 16,
9.
E. W o e r m a n n , W. E y s e l and Th. Hahn. 385 - 391 (1967).
Z e m e n t - K a l k - G i p s 20,
10. J
Mag.
Concr.
Res. 20,
and H. Uchikawa. Z e m e n t 30,
W. Jeffery.
401
(1968). Symp.
Z e m e n t - K a l k - G i p s 14,
497 - 504
(1941).
A c t a Cryst. 5,
11. J. W. Jeffery. Proc. 30 - 48 (1952).
41 - 44
26 - 35
T h i r d Intl.
Symp.
12. E. W o e r m a n n , W. E y s e l and Th. Hahn. 241 - 251 (1968).
(1952). Chem.
Cem.
London,
Z e m e n t - K a l k - G i p s 21,
13. E. M. Levin, C. R. Robbins and H. F. McMurdie. Phase D i a g r a m s for C e r a m i s t s , The A m e r i c a n C e r a m i c Society, Columbus, O h i o (1964). 14. E. Woermann. Proc. F o u r t h Intl. Vol. I, 119 - 129 (1962).
Symp.
Chem.
Cem., W a s h i n g t o n ,
THE SUBSTITUTION OF ALKALIES IN TRICALCIUM SILICATE + E. Woermann, Th. Hahn and W. Eysel Institut f~r Kristallographie der RWTH Aachen 51OO Aachen, Germany
(Refereed) (Received March 14; in final form Aug. 27, ]979)
ABSTRACT In tricalcium silicate at 15OO°C up to 1.4 % K20 , 1.4 % Na20 and 1.2 % Li20, respectively, can be incorporated. The following structural substitution types were found: K replaces Ca with valency balance by removal of oxygen; Na replaces Ca and occupies interstitial sites, in addition some oxygen atoms are removed. Li replaces Ca as well as Si and occupies interstitial voids. Only modifications T I and TII are stabilized by alkalies. The decomposition temperature of pure Ca3SiO 5 was determined to be 1180 + IOOC and increases strongly with K20- and slightly with Na20-content. ' In Tricalciumsilikat werden bei 15OO°C bis zu 1.4 % K20, 1.4 % Na20 oder 1.2 % Li20 eingebaut. Als Substitutionstypen treten auf: K ersetzt Ca mit Ladungsausgleich durch Sauerstoffdefizit. Na ersetzt Ca und besetzt leere Gitterp!Atze, wobei ebenfalls Sauerstoffatome entfernt werden. Li ersetzt Ca- und Si-Positionen sowie ZwischengitterplHtze Die Alka!ien stabilisieren die Modifikationen TI und TII. Die ermlttelte Zersetzungstemperatur des reinen Ca3Si05 yon 1180 + IOOC steigt stark mit dem K20- und wenlg mit dem Na20-Gehalt. Introduction In view of the growing attention to the influence of alkalies on hydration reactions of portland cement, as well as on reactions between cement paste and aggregate, it is important to gain a better understandlng of the role of alkalies in portland cement clinker. This includes information about: I) Solid solution of alkalies in clinker phases: + Presented by Th. Hahn (1) at the 54th meeting of the Deutsche Mineraloglsche Gesellschaft at Braunschweig, Sept. 15th, 1976. 70]
702
Vol. 9, No. 6 E. Woermann, et al. a) type of s u b s t i t u t i o n b) limit of solid solution and its t e m p e r a t u r e c) s t a b i l i z a t i o n of p o l y m o r p h i c forms.
2) F o r m a t i o n
of d i s c r e t e
alkali
3) Influence
on phase equilibria,
dependence,
and
phases. in particular:
a) d e p r e s s i o n of solidus t e m p e r a t u r e s and b) shift of field boundaries, w i t h special emphasis on the influence on the p r i m a r y phase field of free lime, w h i c h controls the c a p a b i l i t y of the s y s t e m to c o m b i n e CaO. In this paper the influence of s o d i u m and p o t a s s i u m on the properties of t r i c a l c i u m silicate is investigated. Some additional experiments were p e r f o r m e d on the i n c o r p o r a t i o n of lithium in order to compare the effects of the v a r i o u s alkali species. Previous
work
A n a l y s e s of t e c h n i c a l clinker indicate that alkalies are inc o r p o r a t e d in C a 3 S i O 5 in small but m e a s u r a b l e amounts. Y a m a g u c h i and Takagi (2) report c o n c e n t r a t i o n s of around O . 1 % Na20 and 0.15 % K20, w h i l e M i d g l e y (3, 4) found up to 0.5 % N a 2 0 and 0.25 % K20. In the past m u c h w o r k has been p e r f o r m e d to gain i n f o r m a t i o n on the i n c o r p o r a t i o n of alkalies in d i c a l c i u m silicate and tricalcium aluminate. The influence of a l k a l i e s on t r i c a l c i u m silicate, however, has been i n v e s t i g a t e d by only a few authors. Kr~mer and zur S t r a s s e n (5) r e p o r t e d that t r i c a l c i u m silicate is stable in the presence of s o d i u m oxide, but is d e c o m p o s e d to KC23S12 and CaO by p o t a s s i u m oxide. Y a m a g u c h i & U c h i k a w a (6) found that tric a l c i u m silicate can form a solid s o l u t i o n i n c o r p o r a t i n g up to O . 7 1 % Na20. At 0.33 % Na20 a change of the m o d i f i c a t i o n of tric a l c i u m silicate was observed. No q u a n t i t a t i v e i n f o r m a t i o n on the influence of p o t a s s i u m is available. Experimental
Methods
s t a r t i n g Materials A mixture, c o n s i s t i n g of 1.25 % free CaO and 98,75 % of s t o i c h i o m e t r i c Ca3SiO 5 was used as s t a r t i n g m a t e r i a l for all experiments. For the p r e p a r a t i o n of this s t a r t i n g m i x t u r e 74.0 % CaO (CaCO 3, Merck No. 2066) and 26.0 % SiO 2 (Aerosil 200, Degussa) w e r e thoroughly m i x e d and h e a t e d for 24 hours at 15OO°C. After c o o l i n g in air and grinding, the free lime content was determined, a p p l y i n g the F r a n k e (7) method. A f t e r four further heating and g r i n d i n g cycles a c o n s t a n t free lime c o n c e n t r a t i o n was analyzed, indicating e q u i l i b r i u m conditions. Synthesis To batches of this starting m i x t u r e potassium, s o d i u m or l i t h i u m w e r e added in small increments as K 2 C O 3 (Merck No. 4928), N a 2 C O 3 (Merck No. 6392) or Li2CO 3 (Merck No. 5680). Because of the volatility of alkalies at high t e m p e r a t u r e s the samples w e r e t i g h t l y w e l d e d in p l a t i n u m capsules. S u f f i c i e n t free space was p r o v i d e d
Vol. 9, No. 6
703 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
for the c a r b o n d i o x i d e to be l i b e r a t e d d u r i n g the reaction. The samples w e r e h e a t e d at 15OOOC for 24 hours and q u e n c h e d in air. The cold c a p s u l e s w e r e c h e c k e d for leaks a f t e r the h e a t t r e a t m e n t by d r o p p i n g t h e m into hot water. O n l y c a p s u l e s that did not develop b u b b l e s from e x p a n d i n g gas were used for f u r t h e r analyses. Analytical Methods To d e t e r m i n e the a 1 k a 1 i c o n t e n t s , some of the samples w e r e a n a l y z e d by flame p h o t o m e t r y and by a t o m i c absorption s p e c t r o m e t r y . The results c o n f i r m e d that w i t h i n a n a l y t i c a l error the a l k a l i e s a d d e d r e m a i n e d in the samples d u r i n g the h e a t i n g process. For a q u a n t i t a t i v e e v a l u a t i o n , however, the analytical data w e r e not p r e c i s e enough. Thus the o r i g i n a l w e i g h i n g s w e r e a c c e p t e d as the c o n c e n t r a t i o n s of alkalies. F r e e 1 i m e w a s d e t e r m i n e d b y t h e F r a n k e (7) method. It has b e e n shown (Woermann, Hahn and Eysel, 8, 9) that this m e t h o d does not a t t a c k t r i c a l c i u m s i l i c a t e nor its solid solutions. F r o m r e p e a t e d a n a l y s e s of 0.7 g samples its a c c u r a r y w a s e s t i m a t e d to be b e t t e r than + 0.05 % CaO. W i t h i n the p r e s e n t e x p e r i m e n t a l series, however, T h e data s c a t t e r in a m u c h w i d e r range. This inc r e a s e in a n a l y t i c a l error is m a i n l y due to the s m a l l e r size of the s a m p l e s (O.1 g) w h i c h was s e l e c t e d in order to m i n i m i z e the e x p e n s e s of platinum. All samples w e r e i n v e s t i g a t e d by X - r a y p o w d e r d i f f r a c t o m e t r y after a d d i t i o n of NaCI as an i n t e r n a l s t a n d a r d u s i n g C u K e - r a d i a t i o n w i t h Ni-filter. Due to the e x t r e m e b r i t t l e n e s s of the samples o p t i c a 1 i n v e s t i g a t i o n s of p o l i s h e d s e c t i o n s w e r e less sat i s f a c t o r y than in a l k a l i - f r e e Ca3Si05, a l i t e s a m p l e s or p o r t l a n d c e m e n t clinker. C o n s e q u e n t l y , this m e t h o d was only e m p l o y e d for q u a l i t a t i v e i n s p e c t i o n of the samples. The S u b s t i t u t i o n of A l k a l i e s
in C a 3 S i O 5
Theoretical Considerations The i n c o r p o r a t i o n of m o n o v a l e n t a l k a l i ions in C a 3 S i O 5 c a n n o t be a c h i e v e d by the s u b s t i t u t i o n of d i v a l e n t Ca or t e t r a v a l e n t Si alone. In o r d e r to m a i n t a i n electroneutrality, e i t h e r the o c c u p a t i o n of i n t e r s t i t i a l sites of s u i t a b l e size or the c r e a t i o n of v a c a n cies in the o x y g e n s u b l a t t i c e is required. T h e o r e t i c a l l y only three s u b s t i t u t i o n a l types m a y be deduced, each r e p r e s e n t e d by a c h a r a c t e r i s t i c f o r m u l a and a r a t i o Ca/M, w h i c h are g i v e n in T a b l e I. A n y a l k a l i s u b s t i t u t i o n can be i n t e r p r e t e d e i t h e r in terms of one of the above p r i n c i p a l m o d e l s or of t h e i r s u p e r p o s i t i o n . A l k a l i s u b s t i t u t i o n s are thus r e s t r i c t e d to C a / M - r a t i o s f r o m -1.O tO + 0 . 7 5 . Experimental Results The type of s u b s t i t u t i o n of a l k a l i ions in C a 3 S i O 5 is determined by a n a l y s i n g the free CaO c o n t e n t of the sample before and a f t e r the r e a c t i o n w i t h a l k a l i oxides. The c h a n g e in free CaO (~fr. CaO) is p l o t t e d a g a i n s t the c o n c e n t r a t i o n of a l k a l i e s in Figs. I - 3. The r e s u l t i n g c u r v e s d e f i n e the C a / M - r a t i o s of the s u b s t i t u t i o n of the ion species.
704
Vol. 9, No. 6 E. Woermann, et al.
TABLE Substitution
models
Substitutional
1
for m o n o v a l e n t
ca%io~s
model
in C a 3 S i O 5 Hypothetical end member
oJ'
co si%
%
c%.~v, col co,
Ca/M
-l.O
I~>
s~ ~" 05
M~s~o~
-0.5
<21
S 05
co M,%
+0.75
131
M
=
monovalent
alkali
VI
=
octahedral
lattice
ion
VI'
=
o c t a h e d r a l i n t e r s t i t i a l site. s i t e s VI' p e r f o r m u l a unit.
IV
=
tetrahedral
lattice
site
[]
=
unoccupied
lattice
site
Ca/M
=
r a t i o o f c h a n g e o f C a - i o n s to a l k a l i ions: S u b s t i t u t i o n of C a by H l e a d s to n e g a t i v e r a t i o s , c o m b i n a t i o n C a d u e to s u b s t i t u t i o n of Si by M l e a d s to p o s i t i v e r a t i o s .
site The
s t r u c % u r e of C a 2 S i O 5 c o n t a i n s
three empty
of a d d i t i o n a l
(~t/~" //~,"~
o~30." / 2.0- ° ~
o
"20"~ 1.0"
tO-
io
go
Y~
Wt % No20
1,0
2~0 3.0 Wt %K20 FIG. l
Substitution of K÷ in Ca3SiO5. Liberation of free lime (Afr. CaO) as a function of K20 added
FIG. 2 Substitution of Na+~in Ca3SiO5.
Liberation of free iime (Afr. CaO) as a function of Na20 added . Circles: Substitution of Na+ in pure Ca3SiO~; Triangles: Substitution of Na+ in Al-saturated Ca3Si+~O~; Squares: Substitution of Na+ in Fe .-saturated Ca3SiO5.
Vol. 9, No. 6
705 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
The results for potassium substitution are shown in Fig. I where the Ca/K ratio is -1.0. According to the substitution model (I) this ratio indicates the substitution of K + for Ca ++ and a simultaneous removal of oxygen from the structure. For sodium substitution (Fig. 2) the ratio Ca/Na is -0.75. This ratio cannot be explained by any single substitution model. It can however, be interpreted as a superposition of models (I) and (2~: Substitution of Na for Ca ++ , incorporation of Na + in interstitial sites VI' and a simultaneous removal of some oxygen atoms. Additional experiments with AI- and with Fe-saturated Ca3SiO 5 indicate that the influence of these ions on the substitution of sodium is negligible. For lithium substitution (Fig. 3) the ratio Ca/Li is -0.25. This can be explained by the superposition of models (2) and (3): substitution of Li ÷ for Ca ++ , incorporation of Li + in interstitial sites and additional substitution of Li + for Si ++++. Alternatively, a superposition of models (I) and (3) is possible. The Limit of Substitution of Alkalies in Tricalcium Silicate In Fig. I the data points follow the slope with the ratio Ca/K = -1.O up to a concentration of 1.4 % K20. At this point a discontinuity indicates the limit of solubility of potassium in Ca3SiO 5. Beyond this point a third phase in addition to Ca3SiO 5 and free CaO appears, the composition of which is now controlling the Ca/K-ratio. X-ray data of quenched samples indicate that the "third phase" is an e'-Ca2SiO 4 solid solution ("KC23S12"). The schematic isothermal section of the ternary system K20-CaO-SiO 2 has been constructed accordingly (Fig. 4). It is in agreement with the observations by Kr~mer & zur Strassen (5) who claimed that the dicalcium silicate phase is stabilized by potassium, thus preventing the formation of alite.
,
R/
I
o
2:o wt'/.L O FIG. 3 Substitution of Li + in Ca3SiO5. Liberation of free lime (fr.CaO) as a function of Li20 added.
coo
K20 FIG. 4
Schematic isothermal section of the system K20-CaO-SiO2 at temperatures above the IBwer stabiTity limit of Ca3SiO5.
706
Vol. 9, No. 6 E. Woermann, et al.
T[ ' C ]
I~00-
/ II
•
•
c..s,o,.
,(.,'c sio 7/ ,',0
1300
1200
1100
•
.
S o / i z/
o
l0
I
t
.CaO
o icoo ,
l
l
I.
!
k
Wt %K20
cao
FIG. 5 S t a b i l i t y range of tricalcium s i l i c a t e solid solutions in the pseudobinary section Ca3SiO5 "K3Si03.5". Solid dots: Ca3SiO5 solid solutions. Open circles: Ca2SiO4 solid solutions + CaO.
...... FIG. 6
Schematic isothermal section of the system Na20 - CaO - SiO 2 at temperatures above the lower s t a b i l i t y l i m i t of Ca3SiO5.
The c o e x i s t e n c e of C a 3 S i O 5 s s + C a 2 S i O 4 s s + CaO at 14OO°C in the ternary s y s t e m K 2 0 - C a O - S i O 2 is c h e m i c a l l y a n a l o g o u s to the e q u i l i b r i u m C a 3 S i O 5 + C a 2 S i O 4 + CaO in the b i n a r y s y s t e m CaO-SiO 2. From this it can be c o n c l u d e d that the lower limit of s t a b i l i t y of alite is d r a s t i c a l l y raised by the s u b s t i t u t i o n of potassium. A q u a n t i t a t i v e e v a l u a t i o n of this effect is given in Fig. 5 w h i c h shows the lower stability t e m p e r a t u r e of alite as a function of K20-concentration. Similarly, the d i s c o n t i n u i t y in the C a / N a - s l o p e of Fig. 2 defines the limit of solubility of Na20 in C a 3 S i O 5 at 1.4 % N a 2 0 at 15OO°C. The "third phase" in this case is N a 2 C a S i O 4. The isothermal section of the N a 2 0 - C a O - S i O 2 phase d i a g r a m (Fig. 6) thus differs in essential points from the K 2 0 - C a O - S i O 2 diagram. A c c o r d i n g to Fig. 7 the t e m p e r a t u r e of the lower s t a b i l i t y limit of s o d i u m c o n t a i n i n g alite increases, but to a lesser e x t e n t than for potassium. The S t a b i l i z a t i o n
of P o l y m o r p h i c
Forms
At room t e m p e r a t u r e pure t r i c a l c i u m silicate is triclinic in the form T I. By i n c o r p o r a t i o n of alkalies the form TII is stabilized (Table 2). It is impossible, however, to s t a b i l i z e higher forms - M I, MII or R - by a d d i t i o n of a single alkali species. In c o m b i n a t i o n with AI203 and Fe203, however, w h i c h by themselves are s t a b i l i z i n g form TII only, high c o n c e n t r a t i o n s of alkalies lead to M I. F r o m Table 2 it can be seen that at least 0.7 % Na20 are r e q u i r e d to stabilize TII and that t r i c a l c i u m silicate can incorporate a m a x i m u m of 1.4 % Na20. This is c o n s i d e r a b l y more than
Vol. 9, No. 6
707 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
/~00-
.,~CaSiO~÷CxTO
C~,m05,, FIG. 7
1300 e e
Stability range of tricalcium
silicate solid solutions in the pseudobinary section Ca3SiO5 ,,Na3NaSi04,,"
o o 0
1200
~
0 0 o
1100
,
~
C~5iO~.÷CuO ,
,
~
C025i0~. ÷N~CoSiO~ ÷CaO
,
,
,
lO Wt %Na20 the corresponding concentrations of 0.33 and 0 . 7 1 % Na20, given by Yamaguchi and Uchikawa (6). The results given in this work differ from those of Yamaguchi and Uchikawa by a factor of 2. This discrepancy is also evident from a comparison of the corresponding Ca/Na-ratios. Although Yamaguchi and Uchikawa themselves did not relate the free lime of their experiments to the sodium concentration, this information may be obtained ~rom the data in their Table I. The resulting values follow closely a line with the slope Ca/Na = - 3/2. Their results differ from our ratio Ca/Na = - 3/4 again by a factor of exactly 2. The crystal chemical deductions (Table I), however, postulate Ca/Na = -1.O (model I) as the smallest possible ratio. Thus, the ratio Ca/Na = - 3/2 of Yamaguchi and Uchikawa cannot be explained structurally.
TABLE Stabilization
Ca3SiO 5 +
(wt.
of Ca3SiO 5 polymorphs
%)
2
by i n c o r p o r a t i o n
Phases
of various
(20°C)
ions
Limit
of
solid
solution TI
(15OO°C)
TII
Na20
O - 0.7
0.7
- 1.4
-
1.4
% Na20
K20
O - 1.3
1.3
- 1.4
-
1.4
% K20
Li20
0 - 0.5
0.5
- 1.2
-
1.2
% Li20
AI203
0 - 0.45
0.45
- 1.0
-
1.O % A I 2 0 3
Fe203
0 - 0.9
0.9
- 1.1
-
1.1%
Fe203
AI203
(I %)
+ Na20
-
O
- 0.8
0.8
- 1.4
1.4
% Na20
Fe203
(I %)
+ Na20
-
0
- 0.8
0.8
- 1.4
1.4
% Na20
Fe203
(I %)
+ Li20
-
O
- 0.8
?
% Li20
?
708
Vol. 9, No. 6 E. Woermann, et al.
The shift of the lattice p a r a m e t e r s of t r i c a l c i u m silicate by i n c o r p o r a t i o n of alkalies is very small and w i t h i n the limits of error of precise measurements. Discussion F r o m the e x p e r i m e n t a l results it is evident that each alkali species has a c h a r a c t e r i s t i c C a / M - r a t i o and thus exhibits a specific type of substitution. model
The ratio Ca/K = -I.O c o r r e s p o n d s exactly (I) w i t h the s u b s t i t u t i o n pattern: (Ca 3 - xKx )VI
[3VI'
siIVo~~ - ~x
[]x
to the t h e o r e t i c a l (A)
w i t h O <~ x ~< 0.068 The K + - i o n is s u b s t i t u t e d for a Ca++-ion. E v i d e n t l y it is too large for i n c o r p o r a t i o n into the other sites. The C a / K - r a t i o proves for the first time that a s u b s t i t u t i o n results in vacancies in the o x y g e n sublattice of C a 3 S i O 5. The structure of C a 3 S i 0 4 + I (Jeffery, 10, 11) contains two types of o x y g e n atoms. One type is s t r o n g l y bonded in SiO 4 tetrahedra w h i l e the other is c o o r d i n a t e d to Ca only w i t h a considerably w e a k e r b o n d i n g strength. It is a s s u m e d that o x y g e n vacancies are c r e a t e d only by the latter, C a - c o o r d i n a t e d oxygens. The s u b s t i t u t i o n Ca/Na = -0.75 requires a s u p e r p o s i t i o n of models (I) and (2) in the ratio I : I leading to an ultimate s u b stitution p a t t e r n . VI'y siIVo5 y D y (Ca. 3y Na 3y) vI Na o- 4 4 4 -4 4
(B)
w i t h 0 ~ y < 0.103 Clearly, the smaller size of the s o d i u m ion p e r m i t s its introduction into o c t a h e d r a l l y c o o r d i n a t e d i n t e r s t i t i a l positions, but, in addition, some oxygens c o n t i n u e to be removed from the structure. On the other hand the size of Na + p r o h i b i t s its s u b s t i t u t i o n for Si 4+ in t e t r a h e d r a l sites and thus excludes p a r t i c i p a t i o n of model 3 in the s u b s t i t u t i o n scheme. Strictly, however, the ratio Ca/Na = -0.75 is v a l i d only for t r i c a l c i u m silicate solid solutions in e q u i l i b r i u m w i t h free lime. It is c o n c e i v a b l e that in S i - s a t u r a t e d C a 3 S i O 5 c o e x i s t i n g w i t h d i c a l c i u m silicate the superp o s i t i o n ratio of m 6 d e l s (1) and (2) w i l l be shifted in favour of model (I). F r o m Table I it follows that model (I) is the limiting case. This e f f e c t w o u l d lead to a d i v a r i a n t phase field of the t r i c a l c i u m silicate phase in the s y s t e m Na20-CaO-Si02. A c o m p a r a b l e single phase field for t r i c a l c i u m silicate solid solutions has been o b s e r v e d b e f o r e in the s y s t e m C a O - A I 2 0 3 - S i O 2 , CaO-MgO-AI203SiO2 and C a O - M g O - F e 2 0 3 - S i O 2 (Woermann, Eysel and Hahn, 9, 12). A field was not c o n f i r m e d e x p e r i m e n t a l l y for the s o d i u m samples since the free lime m e t h o d is r e s t r i c t e d to C a - s a t u r a t e d solid solutions. Moreover, the b r i t t l e n e s s of the samples p r e c l u d e d optical investigations. The ratio Ca/Li = -0.25 can formally be i n t e r p r e t e d either as a s u p e r p o s i t i o n of m o d e l s (2) and (3) in the ratio 4 : 1, leading
Vol. 9, No. 6
709 C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
to the substitution (Ca3_ ~
formula
Li ~ ) V I
Li -~11zvI' (Si1_~)IV
05
(C)
with 0 ~< z ~ O. 18 or of models formula
(I) and
(3) in the ratio 4 : 3 with the substitution
(Ca~_4z' Li 4z')VI Li 9z 'VI' (Si1_3z' Li 3z')Iv O=2z' --7 -72-~ 2-~ ~2-~
O2Z'
(D)
2-3-
with O ~< z' <~ O. 18 In each case, however, model (3) must be considered which involves the substitution of Li + for Si ++++. This model seems possible since a tetrahedrally coordinated Li + is well established in many compounds. Even though, the substitution of a monovalent ion for a tetravalent one is quite unusual. For the given Ca/Li-ratio of -0.25 the substitution formula (C) (models (2) and (3)) avoids the formation of vacancies in the oxygen sublattice. Furthermore, the substitution of Li + for Si ++++ is lower in this substitution formula than in (D) (models (1) and (3)) for equal amounts of total Li. For crystal chemical reasons, therefore, the substitution pattern (C) is preferred to (D). The ratio Ca/Li = -0.25, however, is again only valid for Ca-saturated tricalcium silicate solid solutions. Towards Ca-undersaturated compositions a single-phase range may exist due to a shift of the superposition ratio 4 : I of models (2) and (3) in favour of model (2). The substitution patterns of the various alkali species in tricalcium silicate thus appear to be closely controlled by their size. On the other hand the stability of the tricalcium silicate solid solutions is influenced by the deficit in the oxygen lattice created by the incorporation of the rather large alkali ions potassium and sodium. The lower limit of stability is not only shifted to higher temperatures but, in addition, the rate of decomposition increases with the amount of alkalies. Thus, while it is possible to determine the lower limit of stability of alkali solid solutions with satisfactory precision it has not yet been possible to determine accurately the equilibrium point Ca3SiO 5 = Ca2SiO 4 + CaO in the system CaO-SiO 2, due to the extreme sluggishness of this reaction. An extrapola£ion of the data obtained in the present work to the alkali-free end member results in a reliable e s t i m a t e of the stability point of pure Ca3SiO 5. Extrapolations from both experimental series, the potassium as well as the sodium solid solutions, arrive at an equilibrium point of 1180 + IOOC. +) This is in definite contrast to the stability limit of 125oOc which is accepted by many authorities - e.g. Levin, Robbins and McMurdie (Fig. 237 in ref. 13). The temperature of 1180°C compares favourably with the decompositon point of Fe2+-saturated CasSiO= at 1183°C determined by Woermann (14). ~ The phase diagrams +) Reference
in Figs.
temperature:
4 - 7 are helpful
in explaining
melting point of gold at 1064.5°C.
710
Vol. 9, No. 6 E. Woermann, et al.
the d i f f e r e n t b e h a v i o u r of t r i c a l c i u m s i l i c a t e w i t h r e s p e c t to pot a s s i u m or s o d i u m oxide, as o b s e r v e d by K r ~ m e r and zur S t r a s s e n (5). The c o n c e n t r a t i o n s of K20 and N a 2 0 in a l k a l i - s a t u r a t e d tric a l c i u m s i l i c a t e are of c o m p a r a b l e m a g n i t u d e (1.4 % K20 or 1.4 % N a 2 0 at 15OO°C). A l k a l i c o n c e n t r a t i o n s b e y o n d this s a t u r a t i o n limit r e s u l t in t h r e e - p h a s e - a s s e m b l a g e s w h i c h are c h a r a c t e r i s t i c of each system. F r o m Figs. 4 and 5 it is evident that in the s y s t e m K 2 0 - C a O SiO 2 this a s s e m b l a g e is C a 3 S i O 5 s s + C a 2 S i O 4 s s + CaO. The a m o u n t of the t r i c a l c i u m s i l i c a t e p h a s e is d e t e r m i n e d by the lever rule. Since the d a t a p o i n t s in Fig. 5 do not indicate a d e f i n i t e range of a t h r e e - p h a s e - f i e l d , the latter seems to be e x t r e m e l y narrow. Thus, only a slight e x c e s s of K20 w o u l d be s u f f i c i e n t to pass over the C a 2 S i O 4 s s - CaO join w h e r e the t r i c a l c i u m s i l i c a t e p h a s e disappears. In contrast, the c o r r e s p o n d i n g t h r e e - p h a s e - a s s e m b l a g e in the s y s t e m N a 2 0 - C a O - S i O 2, as shown in Figs. 6 and 7, is C a 3 S i O 5 s s + N a 2 C a S i O 4 + CaO. A c c o r d i n g to the lever rule the t r i c a l c i u m silicate p h a s e is zero in samples w h i c h are located on or b e y o n d the N a 2 C a S i O 4 - C a O join. On the line C a 3 S i O 5 - N a 2 0 this r e q u i r e s a minim u m s o d i u m c o n c e n t r a t i o n of 21.4 % Na20. In all c o m p o s i t i o n s w i t h lower N a 2 0 - c o n c e n t r a t i o n s the s o d i u m - s a t u r a t e d t r i c a l c i u m silicate solid s o l u t i o n is a stable phase. It s e e m e d p r o b a b l e that a d d i t i o n a l c o m p o n e n t s , e.g. A1203 and Fe203, may e x e r t some i n f l u e n c e on the type as w e l l as on the limit of s u b s t i t u t i o n of alkalies. S i n c e the solid s o l u t i o n of C a 3 S i O 5 w i t h N a 2 0 is c h a r a c t e r i z e d by the s u p e r p o s i t i o n of two d i f f e r e n t s u b s t i t u t i o n m o d e l s it will p r o b a b l y react m o r e sensit i v e l y to f o r e i g n c o m p o n e n t s than solid s o l u t i o n s w i t h K20, w h i c h r e p r e s e n t a l i m i t i n g case. Fig. 2 shows that C a / N a - r a t i o s and limits for the i n c o r p o r a tion of N a 2 0 in pure C a 3 S i O 5 as w e l l as in t r i c a l c i u m s i l i c a t e sat u r a t e d w i t h AI203 or w i t h Fe203 are i d e n t i c a l w i t h i n the limits of e x p e r i m e n t a l error. The i n f l u e n c e s of A 1 3 + and of Fe 3+ on the s u b s t i t u t i o n of N a 2 0 are thus n e g l i g i b l y small. Thus it is c o n c l u d e d that the r e s u l t s on the i n c o r p o r a t i o n of a l k a l i e s are v a l i d not only for pure t r i c a l c i u m s i l i c a t e but also r e p r e s e n t c l o s e l y the p r o p e r t i e s of the alite phase in t e c h n i c a l p o r t l a n d c e m e n t clinker. Acknowledgements We are i d e b t e d to the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t the V e r e i n D e u t s c h e r Z e m e n t w e r k e for support of this work.
and
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71l C3S, ALKALIES, LOW TEMPERATUREDECOMPOSITION
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