- Email: [email protected]
Mechanochemical synthesis of hydrated calcium silicates by room temperature grinding
SOLID STATE ELSEVIER
Solid State Ionics 101-103 (1997) 37-43
IONII
Mechanochemical synthesis of hydrated calcium silicates by room temperature grinding •
Fumio Salto
a~ " , Guomin
.a Mt
,
Mitsuo Hanada b
~lnstitute .[br Advanced Materials Processing. Tohoku Universio'. 2-1-1, Katahira. Aoba-ku. Sendai 980-77. Japan Yoshtzawa Lime Co. Ltd.. Kuzuu. Aso-gun. Tochigi 327-05. Japan h
.
Abstract
Room temperature grinding of a mixture of calcium hydroxide and silicagel with different water contents was conducted in a planetary ball mill. Afwillite was synthesized mechanochemically by controlling the water weight rate at about 0.23-0.30. Two-hours grinding of the mixture containing with a water weight rate of more than 0.38 enabled the synthesis of other different types of calcium silicate hydrates. On the other hand, tobermorite was synthesized by three-hours grinding of the mixture having a Ca/Si molar ratio of about one and a water rate at about 0.8, while calcium silicate hydrates-(B) were mechanochemically synthesized by grinding during about 1.5 h. The water amount in the mixture plays a significant role to achieve the mechanochemical synthesis of these compounds. Kevwords: Hydrated calcium silicates; Afwillite: Tobermorite: Mechanochemistry; Wet grinding; Calcium hydroxide; Silicagel; Planetary
ball mill Materials: Ca(OH)_, (calcium hydroxide); SiO_~(silicagel); Ca~(SiO (OH))2.2H_,O (afwillite); Ca~(OH),SirO,~' .4H_,O (tobermorite)
1.
Introduction
Afwillite [Ca3(SiO3(OH)) 2 • 2H20], tobermorite [ C a s ( O H ) 2 S i r O ~ 6 - 4 H 2 0 ] and xonotlite [Car(Si6OlT)(OH)2[, which are categorized as useful calcium silicate hydrates (CSHs), are utilized as building materials and some other functional materials [1]. These compounds have low density and are heat insulating and incombustible. Such characteristic properties enable us to use them potentially for engineering applications [2]. It is known that afwillite is usually synthesized by hydrothermal reaction Corresponding author. Tel. and fax: +81-22 217 5135; e-mail: [email protected]
of lime and silica in the presence of water at above 373 K in an autoclave [3]. Similarly, mostly tobermorite has been artificially synthesized by an autoclave method. Nevertheless, it was reported that afwillite is obtained by hydration of tricalcium silicate induced by its ball milling at room temperature [2,4]. Furthermore, few investigations have been reported on the mechanochemical synthesis of CSHs from its constituents by grinding in atmospheric conditions [5-8], and no attempt has been carried out for the direct synthesis of tobermorite a n d / o r xonotlite mechanochemically. The main purpose of this paper is to provide information on the direct synthesis of afwillite and tobermorite by room temperature grinding of a
0167-2738/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved, PII S0167-2738(97)00278-6
38
F. Saito et al. / Solid State tonics 101-103 (1997) 3 7 - 4 3
mixture composed of calcium hydroxide and silicagel with water.
2. E x p e r i m e n t a l
gravimetric and differential thermal analysis (Rigaku, TAS-200). They were also observed by a scanning electron microscope (Hitachi, S-430) with energy dispersive X-ray spectroscopy (Hitachi, S430-EDS).
2.1. Samples 3. R e s u l t s and d i s c u s s i o n
Samples used in this experiment were calcium hydroxide [Ca(OH)2] and silicagel (SiO:) (Kanto Chemical Co., Japan) powders. Their representative particle sizes, ds0, which were determined from the size distribution curves, were 4.8 Ixm and 58 Ixm, respectively. For the synthesis of afwillite, both samples were mixed to keep a Ca(OH)2/SiO 2 molar ratio of 1.5. When tobermorite was synthesized, the two kinds of samples were put together with different Ca/Si ratios, ranging from 0.5 to 3.0, and were mixed in an agate mortar for 10 min in order to prepare a starting mixture. Tobermorite sample synthesized by an autoclave method was also used as a reference.
2.2. Grinding and characterization A planetary ball mill (Fritsch, Pulverisette-7) with two agate pots of 45 c m 3 in volume was used for room temperature grinding of the mixtures with different amounts of distilled water. The grinding procedure was as follows: 5 g of mixture were put into the mill pot with 7 steel balls of 15 mm in diameter. In the experiment of afwillite synthesis, distilled water in various amounts was added into the mixture. For tobermorite synthesis, a water rate [ = water/(water + mixture) in weight] was fixed constant at 0.8. Rotational speed of the mill was kept constant at 700 rpm, and the grinding time ranged from 15 rain to 9.5 h. The grinding operation was stopped at every 15 min in order to prevent an excess increment of sample temperature during grinding. The starting samples mixed in an agate mortar for 10 min were called the unmilled mixture (0-minutes mixture) in this paper. X-ray diffraction (XRD) using Cu-Ke~ radiation was performed to identify the phase composition of the ground mixtures using an XRD analyzer (Rigaku, RAD-B system). The mixtures were subjected to drying operation and were investigated by thermo-
3.1. Mechanochemical synthesis of afwillite and its thermal behavior Fig. 1 shows the XRD patterns of the 2-hour ground mixtures with different water weight rates ranged from 0.074 to 0.375. In the range below 0.138 in water weight rate, only calcium hydroxide was detected in the patterns. However, new peaks corresponding to afwillite appeared in the patterns of the ground mixtures with the water weight rates ranging from 0.167 to 0.286. When the rate became about 0.375, peaks of different types of calcium silicate hydrates (CSHs) appeared, while the peaks of afwillite disappeared. Although grinding is performed at room temperature, high pressure and
....
~
I
....
....
~ ....
I '''''r
....
I ....
] ....
I ....
[ 0 074
*
•
I
Afwinite ~ C S Hs
J
.................................................................................................................. 0.138
~ ....
• Ca(On)~
J.~a-~S-
•
-4 _ 0.167
•
~
•
'J
0.231
4z
I).286
10
*
~
20
•
•
-
• •
"~¢c
30
4(I
50
60
20(degree, CuKc0 Fig. 1. XRD patterns of the 2-h ground mixtures with various weight rates of water.
39
F. Saito et al. / Solid State lonics 101-103 (1997) 37-43
temperature may exist in a pulp having a definite ratio of solid particles and liquid [6]. If the conditions for the hydraulic continuity of the liquid in the pulp are violated and the value of mechanical pulse is sufficiently high, the following reaction takes place, forming afwillite: 3Ca(OH)~ + 2SiO 2 --~ Ca 3[ S i O 3 ( O H ) ] 2 2 H 2 0 .
( 1)
According to the thermodynamic data [3], calculation of the free energy change of formation, AG, in the above reaction gives - 4 . 1 kcal/mol. If the above equation is considered, water hardly contributes to forward the reaction. However, experimental results show that water appears to be essentially necessary and plays a significant role in the mechanochemical formation of afwillite and CSHs by room temperature grinding. At this moment, it is difficult to conclude that water is involved as a catalyst in this reaction. Further investigation would be needed. Fig. 2 shows the XRD patterns of the mixtures ground for different times, with the weight rate of water kept at 0.286. Peaks of afwillite are detected in the patterns of the mixtures ground over 60 rain. The
'"f 15
~'~
....
I ....
rn[n
i
45
t'tlin
60
inin
120
° lf)
I ....
A
' ....
I ....
' ....
I ....
' ....
• Ca(OH)z ~ Afwillite
*
I I [ ]
]
•
~ : : . . . 90
' ....
peak intensity was found to increase with increasing grinding time. This suggests that the amount of afwillite increases as grinding is progressing. Fig. 3 shows a SEM micrograph of the mixture ground for 120 rain, with the same weight rate of water at 0.286. It is observed that the mixture is composed mainly of aggregates of very fine afwillite particles of below 0.5 t~m with small amounts of Ca(OH) 2 and silicagel particles. According to the result obtained by EDS analysis, the Ca/Si composition ratio in the mixture is about 1.6, which is almost equivalent to the composition ratio in the starting samples. Further, the values of the standard deviation of Si and Ca in the mixture are 4.2 and 2.6%, respectively. This means that the composition of the mixture is almost equivalent to that of afwillite and constituents become quite uniform. Fig. 4 shows the DTA curves of the mixtures ground for various times with the same weight rate of water at 0.286. The first endothermic peak of the ground mixtures is detected in each curve at about 355 K and is attributed to the dehydration of the
;.. :.....'L. ;
~
min
min
.~
........ 20
7 2 30
:
..... ° 413
50
60
20(degree, CuKct) Fig. 2. XRD patterns of the mixtures ground for different times, with a weight rate of water at 0.286.
0.5
lxm
Fig. 3. SEM photograph of the mixture ground for 120 min, with the same weight rate of water as in Fig. 2.
F, Saito et al. I Solid State lonics 101-103 (1997) 37-43
40
I, '1
I
'
I
i......... T(; 10%
0
I I
' -
-
D1~
'
'~
I
....
I ....
~ ....
I ....
~ ....
I ....
Tobermorite CSH(B)
lO~V
15 rain
~'""f
....
~ ....
1 ....
~ ....
[
• Qa(()H)2 I
45 min
743 "K ........................................................... O~ ' " . . . . v
60 mln •
"" ~
.............................................................. L;2
o
"v
. . . . . . . . . .
•
1.5 h
N
3 h
.... ....
°
120 min
II "
A ,It
5~ 8 K[
4c~
'1~
""=................................................................................
I
I
~
I
~
6o0 8oo Temperature (K)
I 1~]
~
I
12oo l0
Fig. 4. TG-DTAcurves of the mixtures ground for various times, with the same weight rate of water as in Fig. 2.
20
30
41)
50
60
20(degree, CuKa) Fig. 5. X R D patterns o f the m i x t u r e s g r o u n d for d i f f e r e n t t i m e s , w i t h a w e i g h t rate o f w a t e r at 0.8.
mixture. The second endothermic peak appears at about 743 K in the curves of the mixtures ground less than 90 min, This peak is due to the decomposition of calcium hydroxide. The peak intensity decreases as grinding progresses until 90 rain and almost disappears in the curve of the mixture ground for 120 rain. Furthermore, a very weak exothermic peak is detected at about ! 100 K in the curves of the mixtures ground from 15 to 90 min. The peak intensity is observed to increase rapidly in the curve of the mixture ground for 120 min. These results suggest that most of Ca(OH) 2 in the mixture reacts with silicagel within 120 min of grinding.
3.2. Mechanochemical synthesis of tobermorite and its thermal behavior Fig. 5 shows the XRD patterns of the mixtures ground for different times, keeping a molar Ca/Si ratio of 5:6 and a water weight ratio of 0.8. The XRD pattern of tobermorite sample synthesized by an autoclave method is also shown in this figure as a reference. Very weak peaks of calcium silicate hydrate-B [CSH(B)] appear in the pattern of the mixture ground for 15 rain. The peak intensity increases with increasing grinding time, at the same
time the peak intensity of Ca(OH) 2 decreases. Subsequently, very weak peaks of CaCO 3 are observed in the pattern of the mixture ground for 1.5 h. This compound is assumed to be formed by exposing the ground Ca(OH)2 fine particles to air. Noticeable peaks of tobermorite are observed in the pattern of the 3-h ground mixture and more clearly as grinding progresses. The reaction is given as follows: 5Ca(OH) 2 + 6SiO 2 --~ Cas(OH)2Si6OI64H20.
(2)
The free energy change for this reaction is calculated as A G = - 7 2 . 5 kcal/mol [3]. But the reaction does not occur when water is absent. Practically, water addition to the mixture is necessary and water also plays a significant role in the mechanochemical formation of tobermorite by room temperature grinding. No clear explanation can be proposed on this question at this moment and further investigations are needed. Consequently, tobermorite can be synthesized by wet grinding of the mixture under the given conditions. By comparing with the pattern of the autoclaved sample, however, it can be deduced
41
F. Saito et al. / Solid State lonics 101-103 (1997) 37-43
"
....
I .... •
' ....
I ....
~ ....
I ....
' ....
I ....
' ....
I ....
mixtures with different Ca/Si ratios, keeping the weight rate of water at 0.8. As seen from the figure, tobermorite is mechanochemically synthesized when the molar Ca/Si ratio is set at one. On the contrary, no peaks of tobermorite are observed in the case of the other (Ca/Si) molar ratios. This implies that the control of the molar ratio at about one is a very important key to synthesize tobermorite mechanochemically. This is well consistent with the thermodynamic data [3]. In this experiment, temperature inside the mill pot was measured below 343 K, which is much lower by about 50-130 K than that in the autoclave method. According to Boldyrev's report [6], even though it is room temperature grinding, high temperature and pressure could exist in a pulp with a definite ratio of solid particles and liquid due to mechanochemical effect. It is deduced that these conditions are likely achieved during the wet grinding, resulting in the mechanochemical synthesis of tobermorite. Fig. 7 shows SEM micrographs of the 3-h ground mixtures (A) and (B), together with the autoclaved sample (C). The particles shown in Fig. 7A are identified as tobermorite by SEM-EDS. Relatively large and cubic particles shown in Fig. 7B are recognized as CaCO 3, due to exposure of fine Ca(OH)~ particles to air. The particle sizes of the
' ....
Ca(OH)z
•
~'~
Ca/Si=Z
Ca/Si=3/2
"~
4{ T o b e r m o r i t c
4{
.
4{
10
20
30 40 20(degree, CuKct)
Ca/~Si= 4{~
50
I -
60
Fig. 6. XRD patterns of the 3-h ground mixtures with different Ca/Si ratios and the weight rate of water at 0.8.
that tobermorite synthesized mechanochemically is poorly crystallized. Fig. 6 shows the XRD patterns of the 3-h ground
5[t,trn
5ptm
5~trn
Fig. 7. SEM photographs of the 3-h ground mixtures (A) and (B) together with the tobermorite sample (C) synthesized by the hydrothermal reaction.
42
F. Saito et al. / Solid State lonics 1 0 1 - 1 0 3 (1997) 3 7 - 4 3
autoclaved sample shown in Fig. 7C are much larger than those of the tobermorite particles synthesized mechanochemically. Furthermore, the autoclaved sample particles seem to be round shaped aggregates composed of flaky fine particles. Thus, morphology and size of the tobermorite particles synthesized mechanochemically are totally differentiated from those of the autoclaved sample. Fig. 8 shows the TG-DTA curves of the dried mixtures after wet grinding for different times, together with that of the autoclaved tobermorite sample. In the DTA curve of the 15-min ground mixture, the first endothermic peak at around 355 K indicates the dehydration of free water in the mixture. The second endothermic peak at around 743 K is due to the decomposition of calcium hydroxide, which remains unreacted in the mixture. At about 1120 K, a weak exothermic peak is detected, indicating the decomposition of the mixture containing CSHs and the formation of e-wollastonite CaOSiO 2. In case of the 1.5-h ground mixture, the endothermic peaks appearing at about 355 and 743 K become small and an exothermic peak appears significantly at around 1120 K. In the curves of the 3-h ground mixture and made of tobermorite particles synthesized mechanochemically, no noticeable peaks can be '
I
'
"...,,.,
~.
'
'
'
I
'
15 rain
355lg.
742.6 K
O
~"
=
~
Z ol
"""..........
1.5 h
~'"'"'"'""'""'"..
k___
3 h
112o.8 K
seen until 1120 K, except a very weak endothermic peak detected at about 950 K. The endothermic peak at about 950 K is attributed to the decomposition of CaCO 3 formed during and after grinding. Thus, the mechanochemically synthesized tobermorite particles show thermal stability in the temperature range till about 1100 K. The thermal stability of the mechanochemically synthesized tobermorite is lower than that of the autoclaved sample.
4. Concluding remarks The experimental results of this work can be summarized as follows: (1) Afwillite, Ca3[SiO3(OH)]z.2H20, can be synthesized mechanochemically by room temperature grinding of a powder mixture of Ca(OH) 2 and silicagel with a Ca/Si ratio of one and a water weight rate of 0.23-0.30. Without water addition to the powders mixture, no mechanochemical reaction between the samples takes place. The synthesized afwillite particles are stable thermally until about 1000 K. Other different types of calcium silicate hydrates are synthesized mechanochemically by grinding of the mixture with more than 38 wt.% water. (2) Tobermorite, Cas(OH)2Si60~6.4H20, is synthesized mechanochemically by grinding of the mixture with a (Ca/Si) ratio of about one and a water weight rate at 0.8. The synthesized tobermorite particles are thermally stable in the range till about 1100 K. Thermal stability of the mechanochemically synthesized tobermorite is lower than that of the autoclaved sample. (3) The amount of water in the mixture plays a significant role to achieve the mechanochemical synthesis of these hydrated calcium silicates.
A i''i"'" .......................... ,:... : r . j . . - - - - ~ ~ O
Acknowledgements
... Autoclaved sample(as a reference)
......... ;............ i............i ........... i ........... i............ ;............ i .........
I,v, 500
1000
Temperature (K) Fig. 8. TG-DTA curves of the dried mixtures after wet grinding for various times.
One of the authors (G.M.) is grateful to the Ministry of Education, Science and Culture of Japan (Monbusho) for the financial support provided through the Monbusho Scholarship Program. This work was supported by the Proposal-Based Advanced Industrial Technology R and D Program from
F. Saito et al. / Solid State lonics 101-103 (1997) 37-43
the
New
Energy
and
Development
Organization
( N E D O ) o f Japan.
References [1] T. Mitsuda, Advances in hydrated calcium silicate materials, Gypsum and Lime 229 (1990) 464. [2] T. Mitsuda, Application of Construction System, in: H. Saito (Ed.), Utilization of Fine Ceramics, Taiga Shuppan Co. Ltd., Tokyo, 1986, p. 208. [3] V.I. Babushin, G.M. Matveyev, O.EM. Petrossyan, Thermodynamics of Silicates, Springer, Tokyo, 1985, pp. 1-100.
43
[4] H. Ishida, K. Sasaki, A. Mizuno, Y. Okada, T. Mitsuda, High reactivity beta-dicalcium silicate-IV, ball milling and static hydration at room temperature, J. Am. Ceram. Soc. 75 (1992) 2779. [5] K. Tkacova, Mechanical Activation of Minerals, Elsevier, Amsterdam, 1989, pp. 1-100. [6] VTV.Boldyrev, Mechanical Activation of Solids, Proc. of the Ist INCOME, Kosice (Slovakia), 23-26 March, 1993, Cambridge Inter-Science Publishing, p. 1. [7l E.G. Avvakumov, E.T. Devyatkina. N.V. Kosova, Mechanochemical reactions of hydrated oxides, J. Solid State Chem. 113 (1994) 379. [8] E.M. Gutman, Mechanochemistry of Solid Surfaces, World Scientific, New Jersey, 1994, pp. 1-100.
Solid State Ionics 101-103 (1997) 37-43
IONII
Mechanochemical synthesis of hydrated calcium silicates by room temperature grinding •
Fumio Salto
a~ " , Guomin
.a Mt
,
Mitsuo Hanada b
~lnstitute .[br Advanced Materials Processing. Tohoku Universio'. 2-1-1, Katahira. Aoba-ku. Sendai 980-77. Japan Yoshtzawa Lime Co. Ltd.. Kuzuu. Aso-gun. Tochigi 327-05. Japan h
.
Abstract
Room temperature grinding of a mixture of calcium hydroxide and silicagel with different water contents was conducted in a planetary ball mill. Afwillite was synthesized mechanochemically by controlling the water weight rate at about 0.23-0.30. Two-hours grinding of the mixture containing with a water weight rate of more than 0.38 enabled the synthesis of other different types of calcium silicate hydrates. On the other hand, tobermorite was synthesized by three-hours grinding of the mixture having a Ca/Si molar ratio of about one and a water rate at about 0.8, while calcium silicate hydrates-(B) were mechanochemically synthesized by grinding during about 1.5 h. The water amount in the mixture plays a significant role to achieve the mechanochemical synthesis of these compounds. Kevwords: Hydrated calcium silicates; Afwillite: Tobermorite: Mechanochemistry; Wet grinding; Calcium hydroxide; Silicagel; Planetary
ball mill Materials: Ca(OH)_, (calcium hydroxide); SiO_~(silicagel); Ca~(SiO (OH))2.2H_,O (afwillite); Ca~(OH),SirO,~' .4H_,O (tobermorite)
1.
Introduction
Afwillite [Ca3(SiO3(OH)) 2 • 2H20], tobermorite [ C a s ( O H ) 2 S i r O ~ 6 - 4 H 2 0 ] and xonotlite [Car(Si6OlT)(OH)2[, which are categorized as useful calcium silicate hydrates (CSHs), are utilized as building materials and some other functional materials [1]. These compounds have low density and are heat insulating and incombustible. Such characteristic properties enable us to use them potentially for engineering applications [2]. It is known that afwillite is usually synthesized by hydrothermal reaction Corresponding author. Tel. and fax: +81-22 217 5135; e-mail: [email protected]
of lime and silica in the presence of water at above 373 K in an autoclave [3]. Similarly, mostly tobermorite has been artificially synthesized by an autoclave method. Nevertheless, it was reported that afwillite is obtained by hydration of tricalcium silicate induced by its ball milling at room temperature [2,4]. Furthermore, few investigations have been reported on the mechanochemical synthesis of CSHs from its constituents by grinding in atmospheric conditions [5-8], and no attempt has been carried out for the direct synthesis of tobermorite a n d / o r xonotlite mechanochemically. The main purpose of this paper is to provide information on the direct synthesis of afwillite and tobermorite by room temperature grinding of a
0167-2738/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved, PII S0167-2738(97)00278-6
38
F. Saito et al. / Solid State tonics 101-103 (1997) 3 7 - 4 3
mixture composed of calcium hydroxide and silicagel with water.
2. E x p e r i m e n t a l
gravimetric and differential thermal analysis (Rigaku, TAS-200). They were also observed by a scanning electron microscope (Hitachi, S-430) with energy dispersive X-ray spectroscopy (Hitachi, S430-EDS).
2.1. Samples 3. R e s u l t s and d i s c u s s i o n
Samples used in this experiment were calcium hydroxide [Ca(OH)2] and silicagel (SiO:) (Kanto Chemical Co., Japan) powders. Their representative particle sizes, ds0, which were determined from the size distribution curves, were 4.8 Ixm and 58 Ixm, respectively. For the synthesis of afwillite, both samples were mixed to keep a Ca(OH)2/SiO 2 molar ratio of 1.5. When tobermorite was synthesized, the two kinds of samples were put together with different Ca/Si ratios, ranging from 0.5 to 3.0, and were mixed in an agate mortar for 10 min in order to prepare a starting mixture. Tobermorite sample synthesized by an autoclave method was also used as a reference.
2.2. Grinding and characterization A planetary ball mill (Fritsch, Pulverisette-7) with two agate pots of 45 c m 3 in volume was used for room temperature grinding of the mixtures with different amounts of distilled water. The grinding procedure was as follows: 5 g of mixture were put into the mill pot with 7 steel balls of 15 mm in diameter. In the experiment of afwillite synthesis, distilled water in various amounts was added into the mixture. For tobermorite synthesis, a water rate [ = water/(water + mixture) in weight] was fixed constant at 0.8. Rotational speed of the mill was kept constant at 700 rpm, and the grinding time ranged from 15 rain to 9.5 h. The grinding operation was stopped at every 15 min in order to prevent an excess increment of sample temperature during grinding. The starting samples mixed in an agate mortar for 10 min were called the unmilled mixture (0-minutes mixture) in this paper. X-ray diffraction (XRD) using Cu-Ke~ radiation was performed to identify the phase composition of the ground mixtures using an XRD analyzer (Rigaku, RAD-B system). The mixtures were subjected to drying operation and were investigated by thermo-
3.1. Mechanochemical synthesis of afwillite and its thermal behavior Fig. 1 shows the XRD patterns of the 2-hour ground mixtures with different water weight rates ranged from 0.074 to 0.375. In the range below 0.138 in water weight rate, only calcium hydroxide was detected in the patterns. However, new peaks corresponding to afwillite appeared in the patterns of the ground mixtures with the water weight rates ranging from 0.167 to 0.286. When the rate became about 0.375, peaks of different types of calcium silicate hydrates (CSHs) appeared, while the peaks of afwillite disappeared. Although grinding is performed at room temperature, high pressure and
....
~
I
....
....
~ ....
I '''''r
....
I ....
] ....
I ....
[ 0 074
*
•
I
Afwinite ~ C S Hs
J
.................................................................................................................. 0.138
~ ....
• Ca(On)~
J.~a-~S-
•
-4 _ 0.167
•
~
•
'J
0.231
4z
I).286
10
*
~
20
•
•
-
• •
"~¢c
30
4(I
50
60
20(degree, CuKc0 Fig. 1. XRD patterns of the 2-h ground mixtures with various weight rates of water.
39
F. Saito et al. / Solid State lonics 101-103 (1997) 37-43
temperature may exist in a pulp having a definite ratio of solid particles and liquid [6]. If the conditions for the hydraulic continuity of the liquid in the pulp are violated and the value of mechanical pulse is sufficiently high, the following reaction takes place, forming afwillite: 3Ca(OH)~ + 2SiO 2 --~ Ca 3[ S i O 3 ( O H ) ] 2 2 H 2 0 .
( 1)
According to the thermodynamic data [3], calculation of the free energy change of formation, AG, in the above reaction gives - 4 . 1 kcal/mol. If the above equation is considered, water hardly contributes to forward the reaction. However, experimental results show that water appears to be essentially necessary and plays a significant role in the mechanochemical formation of afwillite and CSHs by room temperature grinding. At this moment, it is difficult to conclude that water is involved as a catalyst in this reaction. Further investigation would be needed. Fig. 2 shows the XRD patterns of the mixtures ground for different times, with the weight rate of water kept at 0.286. Peaks of afwillite are detected in the patterns of the mixtures ground over 60 rain. The
'"f 15
~'~
....
I ....
rn[n
i
45
t'tlin
60
inin
120
° lf)
I ....
A
' ....
I ....
' ....
I ....
' ....
• Ca(OH)z ~ Afwillite
*
I I [ ]
]
•
~ : : . . . 90
' ....
peak intensity was found to increase with increasing grinding time. This suggests that the amount of afwillite increases as grinding is progressing. Fig. 3 shows a SEM micrograph of the mixture ground for 120 rain, with the same weight rate of water at 0.286. It is observed that the mixture is composed mainly of aggregates of very fine afwillite particles of below 0.5 t~m with small amounts of Ca(OH) 2 and silicagel particles. According to the result obtained by EDS analysis, the Ca/Si composition ratio in the mixture is about 1.6, which is almost equivalent to the composition ratio in the starting samples. Further, the values of the standard deviation of Si and Ca in the mixture are 4.2 and 2.6%, respectively. This means that the composition of the mixture is almost equivalent to that of afwillite and constituents become quite uniform. Fig. 4 shows the DTA curves of the mixtures ground for various times with the same weight rate of water at 0.286. The first endothermic peak of the ground mixtures is detected in each curve at about 355 K and is attributed to the dehydration of the
;.. :.....'L. ;
~
min
min
.~
........ 20
7 2 30
:
..... ° 413
50
60
20(degree, CuKct) Fig. 2. XRD patterns of the mixtures ground for different times, with a weight rate of water at 0.286.
0.5
lxm
Fig. 3. SEM photograph of the mixture ground for 120 min, with the same weight rate of water as in Fig. 2.
F, Saito et al. I Solid State lonics 101-103 (1997) 37-43
40
I, '1
I
'
I
i......... T(; 10%
0
I I
' -
-
D1~
'
'~
I
....
I ....
~ ....
I ....
~ ....
I ....
Tobermorite CSH(B)
lO~V
15 rain
~'""f
....
~ ....
1 ....
~ ....
[
• Qa(()H)2 I
45 min
743 "K ........................................................... O~ ' " . . . . v
60 mln •
"" ~
.............................................................. L;2
o
"v
. . . . . . . . . .
•
1.5 h
N
3 h
.... ....
°
120 min
II "
A ,It
5~ 8 K[
4c~
'1~
""=................................................................................
I
I
~
I
~
6o0 8oo Temperature (K)
I 1~]
~
I
12oo l0
Fig. 4. TG-DTAcurves of the mixtures ground for various times, with the same weight rate of water as in Fig. 2.
20
30
41)
50
60
20(degree, CuKa) Fig. 5. X R D patterns o f the m i x t u r e s g r o u n d for d i f f e r e n t t i m e s , w i t h a w e i g h t rate o f w a t e r at 0.8.
mixture. The second endothermic peak appears at about 743 K in the curves of the mixtures ground less than 90 min, This peak is due to the decomposition of calcium hydroxide. The peak intensity decreases as grinding progresses until 90 rain and almost disappears in the curve of the mixture ground for 120 rain. Furthermore, a very weak exothermic peak is detected at about ! 100 K in the curves of the mixtures ground from 15 to 90 min. The peak intensity is observed to increase rapidly in the curve of the mixture ground for 120 min. These results suggest that most of Ca(OH) 2 in the mixture reacts with silicagel within 120 min of grinding.
3.2. Mechanochemical synthesis of tobermorite and its thermal behavior Fig. 5 shows the XRD patterns of the mixtures ground for different times, keeping a molar Ca/Si ratio of 5:6 and a water weight ratio of 0.8. The XRD pattern of tobermorite sample synthesized by an autoclave method is also shown in this figure as a reference. Very weak peaks of calcium silicate hydrate-B [CSH(B)] appear in the pattern of the mixture ground for 15 rain. The peak intensity increases with increasing grinding time, at the same
time the peak intensity of Ca(OH) 2 decreases. Subsequently, very weak peaks of CaCO 3 are observed in the pattern of the mixture ground for 1.5 h. This compound is assumed to be formed by exposing the ground Ca(OH)2 fine particles to air. Noticeable peaks of tobermorite are observed in the pattern of the 3-h ground mixture and more clearly as grinding progresses. The reaction is given as follows: 5Ca(OH) 2 + 6SiO 2 --~ Cas(OH)2Si6OI64H20.
(2)
The free energy change for this reaction is calculated as A G = - 7 2 . 5 kcal/mol [3]. But the reaction does not occur when water is absent. Practically, water addition to the mixture is necessary and water also plays a significant role in the mechanochemical formation of tobermorite by room temperature grinding. No clear explanation can be proposed on this question at this moment and further investigations are needed. Consequently, tobermorite can be synthesized by wet grinding of the mixture under the given conditions. By comparing with the pattern of the autoclaved sample, however, it can be deduced
41
F. Saito et al. / Solid State lonics 101-103 (1997) 37-43
"
....
I .... •
' ....
I ....
~ ....
I ....
' ....
I ....
' ....
I ....
mixtures with different Ca/Si ratios, keeping the weight rate of water at 0.8. As seen from the figure, tobermorite is mechanochemically synthesized when the molar Ca/Si ratio is set at one. On the contrary, no peaks of tobermorite are observed in the case of the other (Ca/Si) molar ratios. This implies that the control of the molar ratio at about one is a very important key to synthesize tobermorite mechanochemically. This is well consistent with the thermodynamic data [3]. In this experiment, temperature inside the mill pot was measured below 343 K, which is much lower by about 50-130 K than that in the autoclave method. According to Boldyrev's report [6], even though it is room temperature grinding, high temperature and pressure could exist in a pulp with a definite ratio of solid particles and liquid due to mechanochemical effect. It is deduced that these conditions are likely achieved during the wet grinding, resulting in the mechanochemical synthesis of tobermorite. Fig. 7 shows SEM micrographs of the 3-h ground mixtures (A) and (B), together with the autoclaved sample (C). The particles shown in Fig. 7A are identified as tobermorite by SEM-EDS. Relatively large and cubic particles shown in Fig. 7B are recognized as CaCO 3, due to exposure of fine Ca(OH)~ particles to air. The particle sizes of the
' ....
Ca(OH)z
•
~'~
Ca/Si=Z
Ca/Si=3/2
"~
4{ T o b e r m o r i t c
4{
.
4{
10
20
30 40 20(degree, CuKct)
Ca/~Si= 4{~
50
I -
60
Fig. 6. XRD patterns of the 3-h ground mixtures with different Ca/Si ratios and the weight rate of water at 0.8.
that tobermorite synthesized mechanochemically is poorly crystallized. Fig. 6 shows the XRD patterns of the 3-h ground
5[t,trn
5ptm
5~trn
Fig. 7. SEM photographs of the 3-h ground mixtures (A) and (B) together with the tobermorite sample (C) synthesized by the hydrothermal reaction.
42
F. Saito et al. / Solid State lonics 1 0 1 - 1 0 3 (1997) 3 7 - 4 3
autoclaved sample shown in Fig. 7C are much larger than those of the tobermorite particles synthesized mechanochemically. Furthermore, the autoclaved sample particles seem to be round shaped aggregates composed of flaky fine particles. Thus, morphology and size of the tobermorite particles synthesized mechanochemically are totally differentiated from those of the autoclaved sample. Fig. 8 shows the TG-DTA curves of the dried mixtures after wet grinding for different times, together with that of the autoclaved tobermorite sample. In the DTA curve of the 15-min ground mixture, the first endothermic peak at around 355 K indicates the dehydration of free water in the mixture. The second endothermic peak at around 743 K is due to the decomposition of calcium hydroxide, which remains unreacted in the mixture. At about 1120 K, a weak exothermic peak is detected, indicating the decomposition of the mixture containing CSHs and the formation of e-wollastonite CaOSiO 2. In case of the 1.5-h ground mixture, the endothermic peaks appearing at about 355 and 743 K become small and an exothermic peak appears significantly at around 1120 K. In the curves of the 3-h ground mixture and made of tobermorite particles synthesized mechanochemically, no noticeable peaks can be '
I
'
"...,,.,
~.
'
'
'
I
'
15 rain
355lg.
742.6 K
O
~"
=
~
Z ol
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1.5 h
~'"'"'"'""'""'"..
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3 h
112o.8 K
seen until 1120 K, except a very weak endothermic peak detected at about 950 K. The endothermic peak at about 950 K is attributed to the decomposition of CaCO 3 formed during and after grinding. Thus, the mechanochemically synthesized tobermorite particles show thermal stability in the temperature range till about 1100 K. The thermal stability of the mechanochemically synthesized tobermorite is lower than that of the autoclaved sample.
4. Concluding remarks The experimental results of this work can be summarized as follows: (1) Afwillite, Ca3[SiO3(OH)]z.2H20, can be synthesized mechanochemically by room temperature grinding of a powder mixture of Ca(OH) 2 and silicagel with a Ca/Si ratio of one and a water weight rate of 0.23-0.30. Without water addition to the powders mixture, no mechanochemical reaction between the samples takes place. The synthesized afwillite particles are stable thermally until about 1000 K. Other different types of calcium silicate hydrates are synthesized mechanochemically by grinding of the mixture with more than 38 wt.% water. (2) Tobermorite, Cas(OH)2Si60~6.4H20, is synthesized mechanochemically by grinding of the mixture with a (Ca/Si) ratio of about one and a water weight rate at 0.8. The synthesized tobermorite particles are thermally stable in the range till about 1100 K. Thermal stability of the mechanochemically synthesized tobermorite is lower than that of the autoclaved sample. (3) The amount of water in the mixture plays a significant role to achieve the mechanochemical synthesis of these hydrated calcium silicates.
A i''i"'" .......................... ,:... : r . j . . - - - - ~ ~ O
Acknowledgements
... Autoclaved sample(as a reference)
......... ;............ i............i ........... i ........... i............ ;............ i .........
I,v, 500
1000
Temperature (K) Fig. 8. TG-DTA curves of the dried mixtures after wet grinding for various times.
One of the authors (G.M.) is grateful to the Ministry of Education, Science and Culture of Japan (Monbusho) for the financial support provided through the Monbusho Scholarship Program. This work was supported by the Proposal-Based Advanced Industrial Technology R and D Program from
F. Saito et al. / Solid State lonics 101-103 (1997) 37-43
the
New
Energy
and
Development
Organization
( N E D O ) o f Japan.
References [1] T. Mitsuda, Advances in hydrated calcium silicate materials, Gypsum and Lime 229 (1990) 464. [2] T. Mitsuda, Application of Construction System, in: H. Saito (Ed.), Utilization of Fine Ceramics, Taiga Shuppan Co. Ltd., Tokyo, 1986, p. 208. [3] V.I. Babushin, G.M. Matveyev, O.EM. Petrossyan, Thermodynamics of Silicates, Springer, Tokyo, 1985, pp. 1-100.
43
[4] H. Ishida, K. Sasaki, A. Mizuno, Y. Okada, T. Mitsuda, High reactivity beta-dicalcium silicate-IV, ball milling and static hydration at room temperature, J. Am. Ceram. Soc. 75 (1992) 2779. [5] K. Tkacova, Mechanical Activation of Minerals, Elsevier, Amsterdam, 1989, pp. 1-100. [6] VTV.Boldyrev, Mechanical Activation of Solids, Proc. of the Ist INCOME, Kosice (Slovakia), 23-26 March, 1993, Cambridge Inter-Science Publishing, p. 1. [7l E.G. Avvakumov, E.T. Devyatkina. N.V. Kosova, Mechanochemical reactions of hydrated oxides, J. Solid State Chem. 113 (1994) 379. [8] E.M. Gutman, Mechanochemistry of Solid Surfaces, World Scientific, New Jersey, 1994, pp. 1-100.