- Email: [email protected]
The CaO - Al2O3 - CaSO4 - H2O system equilibrium states
Cement and Concrete Research, Vol. 24, No. 2, pp. 259-266, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0008-8846/94 $6.00 + .00
Pergamon
THE
CaO - AI203 - CaSO~ - H20
SYSTEM EQUILIBRIUM STATES
I.Ner~d, S . ~ u ~ o v ~ and L. ~tevula Institute of Inorganic Chemistry, Slovak Academy of Sciences Ddbravsk~ cesta 9, 842 36 Bratislava, Slovakia (Communicated by Z. Sauman) (Received April 27, 1993)
ABSTRACT The equilibrium dissociation temperature of ettringite 3CaO. AI20 m. 3CaSO~.32H20 in the presence of liquid water as well as those of monosulphate 3CaO. AI203.CaSO~.I2H20 in the presence of water vapor in relation to pressure were examined. At pressure from 150 up to 900 kPa ettringite in the presence of its saturated aqueous solution is thermally stable at temperature lower than I07 to III °C, respectively. 3CaO. AI203.CaSO~.I2H20 and predominantly CaSO~ are the decomposition products formed under conditions studied. The phases 3CaO. AI20. CaSO~.IOH20(s), 3CaO. AI203.6H20(s), CaSO~(s) and H20(g) coexist in the equilibrium at the thermal dissociation of monosulphate under conditios employed. The 3CaO. AI203. CaSO~.IOH20 thermal dissociation equlibrium temperatures reach values from 150 °C to 177 °C at the pressures of water vapor from I00 kPa to 900 kPa, respectively.
INTRODUCTION The CaO-AI203-CaSO~-H20 system is important in studying the processes related to the application of some types of inorganic binders. It is well known that quaternary phases of this system 3CaO. AI203.3CaSO~.32H~O (CsAS3H32) ettringite and 3CaO. AI203.CaSO~.I2H20 (C~ASH±2) monosulphate are formed in the initial stages of the hydration of portland cement. The controlled crystallization of these phases is the basis for utilization of some expanded cements. These phases are also main components of concretes produced on the basis of sulphate-aluminous cements. Furthermore, they play an important role in corrosion caused by the action of sulphate solutions on concretes. Althought closely related to the possibility to evaluate the applicability of concrete composites neither the problems dealing with the thermal stability of hydrated sulphate aluminous phases nor phase relations in the systems formed by their thermal dissociation, have not been satisfactorily solved yet. The purpose of this work is to establish relationship between pressure and thermal dissociation temperatures of hydrated sulphate-aluminous phases in the presence of their aqueous solutions and water vapor, respectively.
259
260
I. Ner~d et al.
Vol. 24, No. 2
THEORETICAL From chemical point of view, the above mentioned compounds belong to a large group of hydrated calcium-aluminium hydroxy-salts. According to their structure, morphology of crystals and some other properties they can be ~tvided into two groups. The first one includes compounds of general formula 3CaO. AI203.3CaX2.nH20, where X is the i-th portion of i-valent anion. They crystallize in the form of hexagonal needles. Included to the first group 2compounds containing SO~ anion as wel as those with OH[ CI03, CrOCi etc. anions were described in the literature. The secound group consists of the compounds of general formula 3CaO. AI2Os. CaX2.nH20 which crystallize in the form of hexagonal platelets. In addition to sulphates some chlorides, carbonates, nitrates, etc. are known in the second group. Detailed description of the synthesis of the compounds discussed and some of their structural parameters can be already found in the papers of Feitknecht and Buser [1,2,3]. A structure of ettringite has been determined by Moore and Taylor [4,5,6]. The most important characteristics of its structure is the arrangement of columns and channels parallel to the c axis of prisms. The surface of cylindrical columns which consist of the Cas[Al(OH)s]2.24H20s+ cations is formed from water molecules. Channels are filled with [(SO~)3.nH20] s- anions (n ~ 2) bonded to columns by hydrogen bridges. The hybrid layer structure of monosulphate has been lastly refined by Allmann [7]. The composition of brucite-like main layer and that of the interlayer corresponds to [Ca2Al(OH)s] + and [I/2S0~.3H20] [ respectively. Two thirds of the water molecules of the interlayer are tightly bound to Ca. By this, Ca becomes 7-coordinated and the Ca-atoms are shifted by 0.057 nm out of the centers of their (OH)s-°ctahedrao in direction to the H20 molecules. The other part of the interlayer (SO~- anions and one third of the water molecules) is disordered. The SO~-groups only occur in every second cell and can occupy there two possible positions. Where SO~- ions are missing, two H20 molecules take the place. This HmO molecules are only space filling. They are not involved in the hydrogen bond network and seem to be unnecessary for the crystal structure. Zeolitically bonded water filling up the spaces between columns is released from ettringite practically at room temperature as soon as its crystals lost contact with their saturated aqueous solution. The equilibrium temperature at corresponding pressure of saturated water vapor teq(psat) = (III ±_~) °C and enthalpy change AHdlss[CsAS3Hs2,384 K, psat) = (188 ± 4.2) kJ mol for the reaction according to the scheme CsAS3H32(s)
= C~ASHIm(S) + 2CSH0.s(s) + 19H(I)
(i)
has been determined by the DHA method by ~atava [8]. The stability of ettringite in dependence on temperature over pressure range from 690 up to 4100 kPa has been determined by Ogawa and Roy [9]. They found that the decomposition temperature was between 145 to 150 °C at pressures higher than 2760 kPa. This was slightly lowered at lower pressure and reached 130 °C at 690 kPa. The main phases formed by the decomposition of ettringite are monosulphate and CaSO~.I/2H20. At 150 °C and pres§ures of 2760 and 4140 kPa, respectively a small quantity of anhydrous CS were determined by X-ray diffraction analysis in ettringite decomposition products. Many of the C~ASH x phases hydrated to various degree have been described [10]. Most frequently phases with x = 12, I0 and 8 which can be distinguished according to the rtg. diffraction patterns corresponding to the basal distances of 0.89, 0.815 and 0.795 nm, respectively are referred [II].
Vol. 24, No. 2
CaO-AI203-CaSO4-H20, EQUILIBRIUM,PRESSURE
261
The thermal decomposition of monosulphate _in hydrothermal conditions yields hydrogarnet 3CaO. AI203.6H20 (C3AH s) and CS. The equilibrium temperature of the process expressed by following reaction scheme C~ASHt2(s) = C3AHs(s) + CS( s)
+ 6H(1)
(2)
at corresponding pressure of saturated water vapor t e q ( p s a t ) = (177 ± 2) °C as well as the e n t h a l ~ y of the reaction (2) AHdIss(C~ASHim,450 K, psat) = ( 5 0 . 6 6 ± 4 . 1 9 ) k J mol h a s b e e n d e t e r m i n e d by ~ a t a v a [ 8 ] . At t h e p r e s s u r e s o f s a t u r a t e d w a t e r vapor of 3 - 90 kPa t h e thermal decomposition of hydrogarnet resulting i n 12CaO. 7A120 a (C~2A 7) and Ca(OH) 2 (CH), p r o c e e d s a t t e m p e r a t u r e s f r o m 160 up t o 220 °C [ 1 2 ] .
EXPERIMENTAL Sample preparation Monosulphate was prepared according to Kuzel [13]. Suspension consisting of four portions (by weight) of water and one portion of stoichiometrical mixture composed of CmA and CSH m was transferred to an autoclave and allowed to react at 150 °C for 4 days. The suspension in autoclave was shaken occasionally. Reaction product still hot, was sepa_rated by filtration and washed with methanol. Except of main product C~ASHxm traces of CS were indicated by X-ray phase analysis. Total weight loss of the sample determined by thermal analysis after ignltlon at 950 C refers to 1 1 .7 5 formal mol of H20 for a formula mol of monosulphate. A sample of ettringite was prepared from a mixture of CmAH s with CgH 2 (molar ratio 1:3) and water. The slurry has been continuously stirred at room temperature in a closed vessel for 30 days. A product of the reaction was washed with aceton and ethyleter and allowed to dry at room temperature and relative humidity of 52-66 %. Ettringite was the main component of the sample, but a small amount of CSH m has been detected by X-ray phase analysis. Total weigh loss at 950 °C refers to 30.7 formal mol of water for one formula mol of ettringite. .
.
.
.
0
Operating technique Stability of ettringite
in the presence of liquid water
A slightly modified method described by Ogawa and Roy [9] was utilized for the investigation of stability of ettringite in its saturated aqueous solution. For each selected couple of temperature and pressure, behavior of reactants and potential reaction products were investigated. Ettringite and a stoichiometric mixture of its decomposition products was suspended in distilled and decarbonated water. Both suspensions placed in teflon vessels were transferred to a high pressure reactor equipped with manometer and highpressure valve. Nitrogen was introduced to the reactor until the desired pressure was reached. Heating then started. Pressure was maintained constant by gradual releasing of the gas from the autoclave. Samples have been kept in the reactor at the selected pressure and temperature for 2 - 41 days. The content of teflon vessels was then filtered and a solid phase was dried. The same procedure of drying as above was used. The phase composition of the samples was determined by X-ray diffraction analysis and electron microscopy.
262
I. Nereid et ~.
Vol. 24, No. 2
Stability of monosulphate in relation to the water vapor pressure Similarly as i n the case of ettringite both directions of reaction were investigated in order to determine the thermal stability of monosulphate. The thermal dissociation of monosulphate proceeds in the interval of temperature in which the water vapor coexists with solid phases in phase diagram studied. For this reason monosulphate as well as a stoichiometric mixture of its reaction products were placed dry into teflon vessels. In order to prevent the samples from having contact with liquid water, only a part of volume of the reactor was filled with distilled and decarbonated water. The autoclave was then heated to the temperature at which the pressure of saturated water vapor corresponds to a chosen pressure. Water vapor was then slowly and carefully liberated until the whole volume of liquid water in the autoclave was evaporated. In order to reach the desired temperature at the chosen constant pressure, the excess of water vapor was released from the autoclave contemporarily increasing the temperature. After finishing the experiments the samples were checked for the absence of condensed water.
RESULTS AND DISCUSSION
X-ray d i f f r a c t i o n phase analysis Phase diagrams in Fig. I show the equilibrium between ettringite and its thermal dissociation products in the presence of liquid water. The results were obtained by the investigation of the reaction in both directions. Part a demonstrates dissociation of ettringite to the monosulphate and CS. Empty circles correspond to undecomposed ettrin~ite, filled ones represent the conditions under which the presence of C~ASHIm were unambiguously proved in reaction products. In part b a formation of ettringite from a mixture composed of C~ASHI2 and CSH 2 is demonstrated. Filled circles illustrate the experiments in which
kPa
o Io 800
o
I
800
I 600
I
I
I
600
, 400;
f f
oo! 0 (
/
/
(D/ ,
J
' Ii
I
I
I I 2o0~ 0 do /
/ •
//
, OI
90
Io
110 G
130
t T
,
90
110 130
B
"C
FIG. I Stability of ettringite in the presence of liquid water, a - thermal decomposition of ettringite: (o) undecomposed ettringite, (@) C~ASHi2 + CS. b - formation of ettrlngite from its dissociation products: (o) ettrlngite as a newformed phase,(@) C~ASHI2 + CS. The equilibrium conditions for reaction (I) accordimg to ~ t a v a [8] (D) and Ogawa end Roy [9] (V).
Vol. 24, No. 2
CaO-A1203-CaSO4-H20, EQUILIBRIUM, PRESSURE
263
the presence of ettringite in solid products was not proved by X-ray diffraction analysis. Empty circles refer to the conditions under which ettringite was unambiguously proved in solid reaction products. All the values were measured only at pressures of I00, 200, 400, 700 and 900 kPa. Repeated measurements obtained under conditions identical with those under which the lowest value of corresponding n-termed sequence was measured are manifested by the points located on the upper right side. Where equivalent symbols are used in both parts of the figure, the same phase composition of the system in both directions of reaction was proved. The equilibrium conditions according to reaction (I) as obtained by ~atava [8] are ilustrated by square. Triangel and a solid line drawn from it corresponds to the equilibrium in the system consisting of ettringite, monosulphate and CaSO~.I/2HmO as found by Ogawa ~nd Roy [9]. The equilibrium between liquid and gaseous water is indicated by dashed and dotted line. As follows from Fig. I C~ASH~2 and almost exclusively anhydrous calcium sulphate CS were detected by X-ray diffraction analysis as the decomposition products. The thermal decomposition equilibrium temperatures as determined by our method were by about 20 °C lower than those obtained by Ogawa and Roy [9] in the common pressure range. In spite of substantial difference between the methods used, comparatively good agreement with experiments performed by ~atava was achieved at low pressures. The equilibrium of monosulphate phases and their dissociation products in the presence of water vapor in both directions of the reaction, are illustrated in Fig. 2. In part a the dissociation of monosulphate gizIng the rise to C3AH s and CS is shown. Undecomposed monosulphate C~ASH~2 is represented by empty circles. The conditions under which the presence of monosulphate phase C~ASHx0, but not that of C3AH G was proved, are illustrated by partially filled circles. Filled circles refer to the experiments in which the presence of the C=AH s in solid reaction products was unambiguously proved. In part b a formation of monosulphate from the mixture of C3AH s and CSH= is demonstrated. In experiments illustrated by filled circles CsAH G and CS are present while formation of monosulphate has not been found. The presence of monosulphate C~ASH~0 but not that of C~ASHx2 has been proved at temperatu-
Stability of monosulphate in the presence of dec•mwater FlG. 2thermal vapor, a position of monosulphate: (O) undecomposed C~ASHI2 , [~) C~ASHIn, [0) C3AH ~ + CS. b - Formation of monosulphate from its dissociation products: (o) C~ASH12 as a new-formed phase, (0) C~ASHIo as a new-formed phase, (o) C3AH 6 + CS, (D) the equilibrium conditions of the reaction [i) accor-
ding to ~ t a v a
[9].
//
//
800F
80O
/ //" '~ • / /
[ 600 t
/
6o
/~O
/ / •
/
/
•
•
•
•
0
"C
OI
,
,
/
2ool/0
l
150 1~0 160 180 O
/
/
/
/ 200 / 0 0 ~0 ~ 0
°
/I
/ /
/ 400
/~ /t
• ~
,
•
120 1~0 160 180 .2__ "C b
264
I. Nerfidet al.
Vol. 24, No. 2
res and pressures represented by p a r t i a l l y filled circles. Empty o n e s r e f e r to the conditions u n d e r w h i c h m o n o s u l p h a t e CaASH~2 h a s b e e n f o u n d i n s o l i d reaction products formed. Similarly as in Fig. 1 where equivalent symbols are used in both parts of the figure, t h e same p h a s e c o m p o s i t i o n of the system was c o n f i r m e d in both directions of reaction. The e q u i l i b r i u m conditions according to reaction ( 2 ) a s m e a s u r e d by ~ a t a v a [9] a r e e x p r e s s e d b y s q u a r e . Repeated measurements are illustrated analogically as in Fig. 1. The equilibrium o f l i q u i d a n d g a s e o u s w a t e r i s d e m o n s t r a t e d by t h e d a s h e d a n d dotted line. As f o l l o w s f r o m F i g . 2 t h e e q u i l i b r i u m t e m p e r a t u r e o f m o n o s u l p h a t e t h e r mal d i s s o c i a t i o n is within the interval o f 150 - 177 °C o v e r t h e w a t e r v a p o r p r e s s u r e o f 100 - 900 kPa. U n d e r t h e e q u i l i b r i u m c o n d i t i o n s the C~ASH~o(S), C 3 A H s ( s ) , C S ( s ) a n d H20(g) p h a s e s c o e x i s t i n t h e s y s t e m . Microstructure
and morphology of samples
The r e s u l t s of X-ray diffraction phase analysis where confirmed by electron microscopy. Typical arrangement of rod-like crystals of undecomposed ettringite a f t e r 30 d a y s t r e a t m e n t a t 95 °C a n d 900 kPa i n w a t e r s u s p e n s i o n i s i n F i g . 3. I n F i g . 4 we c a n s e e a h e x a g o n a l c r y s t a l s of monosulphate and small lats of anhydrite as a result o f 18 d a y s t r e a t m e n t of stoichiometric m i x t u r e o f m o n o s u l p h a t e a n d gypsum a t l i 5 °C a n d 900 kPa i n w a t e r s u s p e n s i o n .
Ettringite at 95 °C
FIG. 3 a f t e r 30 d a y s t r e a t m e n t a n d 900 k P a in water suspension.
FIG. 4 Mixture of monosulphate and gypsum after 18 days treatment at llS °C and 900 kPa in water suspension.
Fig. 5 show the thermal decomposition products of ettringite after 18 days treatment at I15 °C and 900 kPa in water suspension. Starting compound is no more present, only decomposition products are seen, i. e. great idiomorphic hexagonal crystals of monosulphate and relative very small laths of anhydrite. The products of interaction of monosulphate with gypsum in water suspension at 9S °C and 900 kPa after 30 days are shown in Fig. 6. Unusually view to ettringite with shorter robust rods, sprinkled with extremly fine sticks or laths of probably unreacted anhydrite.
Vol. 24, No. 2
CaO-AI203-CaSO4-H20, EQUILIBRIUM, PRESSURE
20H rn
FIG. S Ettringite after 18 days treatment at IIS UC and 900 kPa in water suspension.
265
~
FIG. 6 Mixture of monosulphate and gypsum after 30 days treatment at 95 °C and 900 kPa in water suspension.
CONCLUSIONS The equilibrium dissociation temperature of ettringite 3CaO. AI203. 3CaSO~.S2H20 in the presence of liquid water as well as those of monosulphate 3CaO. AI~O3.CaSO~.I2H20 in the presence of water vapor in relation to pressure were investigated using X-ray diffraction and electron microscopy. The equilibrium temperatures were determined on the basis of studying the dissociation processes proceeding in the above mentioned phases as well as reactions leading to the formation of these phases from their dissociation products. At pressure from ISO up to 900 kPa, ettringite in the presence of its saturated aqueous solution is thermally stable at temperatures lower than 107 to ill °C, respectively. 3CaO. AI203.CaSO~.I2H20 and predominantly CaSO~ are the decompositiom products formed under conditions studied. The phases 3CaO. AI203.CaSO~.IOH20(s), 3CaO. AI203.6H20(s), CaSOa(s) and H20(g) coexist in the equilibrium at the thermal dissociation of monosulphate under conditions employed. The 3CaO. AI203.CaSO~.IOH20 thermal dissociation equlibrium temperature reaches values from 150 °C to 177 °C at the pressures of water wapour from i00 kPa to 900 kPa. Acknowledgment The study was supposed by GAS of the Slovak Academy of Sciences. The authors are grateful to Mrs M. D. Fr£alov~ for carrying out of electron microscopy and useful discussion of its interpretation, as well as to Mr. J. Horv~th for performing a number of X-ray analysis and to Mrs. A. Celkov~ for translation of the paper into English.
REFERENCES I. W. Feitknecht and H. W. Buser, Helv. Chim. Acta 22, 2298 (1949). 2. W. Feitknecht and H. W. Buser, Helv. Chim. Acta 24, 128 (195]).
266
I. Nereidet al.
Vol. 24, No. 2
3. W. Feitknecht, K. Michel and H. W. Buser, Helv. Chim. Acta 24, 119 (19SI). 4. A. Moore and H. F. W. Taylor, Nature 218, 1048 (1968). 5. A. Moore and H. F. W. Taylor, Acta Cryst. B 28,part 4, 386 (1970). 6. H. F. W. Taylor, Miner. Mag. 304, 377, (1973). 7. R. Allmann N. Jb. Miner. Mh. H. 3, 138-144 (1977). 8. V. ~atava and O. J. Vep~ek, J. Amer. Cer. Soe. 68, 37S (197S). 9. K. Ogawa and D. M. Roy, Cem. Conc. Res. 11, 741 (1981). 10. H. E. Schwiete and V. Ludwig, Proc. 5th Int. Symp. Chem. Cement 2, 84, Tokyo 1968 p. 3 7 - 6 7 . 11. W. D o s c h , H. K e l l e r a n d H. z u r S t r a s s e n , P r o c . S t h I n t . Symp. Chem. C e m e n t 2, 84, Tokyo 1968 p. 7 3 - 7 8 . 12. I . P r o k s , I. Ner&d a n d L. K o s a , S i l i k ~ t y 22, 125 ( 1 9 7 8 ) . 13. H. J . N. K u z e l , N. J b . M i n e r . Mh. 1 9 3 - 1 9 7 ( 1 9 6 S ) .
Pergamon
THE
CaO - AI203 - CaSO~ - H20
SYSTEM EQUILIBRIUM STATES
I.Ner~d, S . ~ u ~ o v ~ and L. ~tevula Institute of Inorganic Chemistry, Slovak Academy of Sciences Ddbravsk~ cesta 9, 842 36 Bratislava, Slovakia (Communicated by Z. Sauman) (Received April 27, 1993)
ABSTRACT The equilibrium dissociation temperature of ettringite 3CaO. AI20 m. 3CaSO~.32H20 in the presence of liquid water as well as those of monosulphate 3CaO. AI203.CaSO~.I2H20 in the presence of water vapor in relation to pressure were examined. At pressure from 150 up to 900 kPa ettringite in the presence of its saturated aqueous solution is thermally stable at temperature lower than I07 to III °C, respectively. 3CaO. AI203.CaSO~.I2H20 and predominantly CaSO~ are the decomposition products formed under conditions studied. The phases 3CaO. AI20. CaSO~.IOH20(s), 3CaO. AI203.6H20(s), CaSO~(s) and H20(g) coexist in the equilibrium at the thermal dissociation of monosulphate under conditios employed. The 3CaO. AI203. CaSO~.IOH20 thermal dissociation equlibrium temperatures reach values from 150 °C to 177 °C at the pressures of water vapor from I00 kPa to 900 kPa, respectively.
INTRODUCTION The CaO-AI203-CaSO~-H20 system is important in studying the processes related to the application of some types of inorganic binders. It is well known that quaternary phases of this system 3CaO. AI203.3CaSO~.32H~O (CsAS3H32) ettringite and 3CaO. AI203.CaSO~.I2H20 (C~ASH±2) monosulphate are formed in the initial stages of the hydration of portland cement. The controlled crystallization of these phases is the basis for utilization of some expanded cements. These phases are also main components of concretes produced on the basis of sulphate-aluminous cements. Furthermore, they play an important role in corrosion caused by the action of sulphate solutions on concretes. Althought closely related to the possibility to evaluate the applicability of concrete composites neither the problems dealing with the thermal stability of hydrated sulphate aluminous phases nor phase relations in the systems formed by their thermal dissociation, have not been satisfactorily solved yet. The purpose of this work is to establish relationship between pressure and thermal dissociation temperatures of hydrated sulphate-aluminous phases in the presence of their aqueous solutions and water vapor, respectively.
259
260
I. Ner~d et al.
Vol. 24, No. 2
THEORETICAL From chemical point of view, the above mentioned compounds belong to a large group of hydrated calcium-aluminium hydroxy-salts. According to their structure, morphology of crystals and some other properties they can be ~tvided into two groups. The first one includes compounds of general formula 3CaO. AI203.3CaX2.nH20, where X is the i-th portion of i-valent anion. They crystallize in the form of hexagonal needles. Included to the first group 2compounds containing SO~ anion as wel as those with OH[ CI03, CrOCi etc. anions were described in the literature. The secound group consists of the compounds of general formula 3CaO. AI2Os. CaX2.nH20 which crystallize in the form of hexagonal platelets. In addition to sulphates some chlorides, carbonates, nitrates, etc. are known in the second group. Detailed description of the synthesis of the compounds discussed and some of their structural parameters can be already found in the papers of Feitknecht and Buser [1,2,3]. A structure of ettringite has been determined by Moore and Taylor [4,5,6]. The most important characteristics of its structure is the arrangement of columns and channels parallel to the c axis of prisms. The surface of cylindrical columns which consist of the Cas[Al(OH)s]2.24H20s+ cations is formed from water molecules. Channels are filled with [(SO~)3.nH20] s- anions (n ~ 2) bonded to columns by hydrogen bridges. The hybrid layer structure of monosulphate has been lastly refined by Allmann [7]. The composition of brucite-like main layer and that of the interlayer corresponds to [Ca2Al(OH)s] + and [I/2S0~.3H20] [ respectively. Two thirds of the water molecules of the interlayer are tightly bound to Ca. By this, Ca becomes 7-coordinated and the Ca-atoms are shifted by 0.057 nm out of the centers of their (OH)s-°ctahedrao in direction to the H20 molecules. The other part of the interlayer (SO~- anions and one third of the water molecules) is disordered. The SO~-groups only occur in every second cell and can occupy there two possible positions. Where SO~- ions are missing, two H20 molecules take the place. This HmO molecules are only space filling. They are not involved in the hydrogen bond network and seem to be unnecessary for the crystal structure. Zeolitically bonded water filling up the spaces between columns is released from ettringite practically at room temperature as soon as its crystals lost contact with their saturated aqueous solution. The equilibrium temperature at corresponding pressure of saturated water vapor teq(psat) = (III ±_~) °C and enthalpy change AHdlss[CsAS3Hs2,384 K, psat) = (188 ± 4.2) kJ mol for the reaction according to the scheme CsAS3H32(s)
= C~ASHIm(S) + 2CSH0.s(s) + 19H(I)
(i)
has been determined by the DHA method by ~atava [8]. The stability of ettringite in dependence on temperature over pressure range from 690 up to 4100 kPa has been determined by Ogawa and Roy [9]. They found that the decomposition temperature was between 145 to 150 °C at pressures higher than 2760 kPa. This was slightly lowered at lower pressure and reached 130 °C at 690 kPa. The main phases formed by the decomposition of ettringite are monosulphate and CaSO~.I/2H20. At 150 °C and pres§ures of 2760 and 4140 kPa, respectively a small quantity of anhydrous CS were determined by X-ray diffraction analysis in ettringite decomposition products. Many of the C~ASH x phases hydrated to various degree have been described [10]. Most frequently phases with x = 12, I0 and 8 which can be distinguished according to the rtg. diffraction patterns corresponding to the basal distances of 0.89, 0.815 and 0.795 nm, respectively are referred [II].
Vol. 24, No. 2
CaO-AI203-CaSO4-H20, EQUILIBRIUM,PRESSURE
261
The thermal decomposition of monosulphate _in hydrothermal conditions yields hydrogarnet 3CaO. AI203.6H20 (C3AH s) and CS. The equilibrium temperature of the process expressed by following reaction scheme C~ASHt2(s) = C3AHs(s) + CS( s)
+ 6H(1)
(2)
at corresponding pressure of saturated water vapor t e q ( p s a t ) = (177 ± 2) °C as well as the e n t h a l ~ y of the reaction (2) AHdIss(C~ASHim,450 K, psat) = ( 5 0 . 6 6 ± 4 . 1 9 ) k J mol h a s b e e n d e t e r m i n e d by ~ a t a v a [ 8 ] . At t h e p r e s s u r e s o f s a t u r a t e d w a t e r vapor of 3 - 90 kPa t h e thermal decomposition of hydrogarnet resulting i n 12CaO. 7A120 a (C~2A 7) and Ca(OH) 2 (CH), p r o c e e d s a t t e m p e r a t u r e s f r o m 160 up t o 220 °C [ 1 2 ] .
EXPERIMENTAL Sample preparation Monosulphate was prepared according to Kuzel [13]. Suspension consisting of four portions (by weight) of water and one portion of stoichiometrical mixture composed of CmA and CSH m was transferred to an autoclave and allowed to react at 150 °C for 4 days. The suspension in autoclave was shaken occasionally. Reaction product still hot, was sepa_rated by filtration and washed with methanol. Except of main product C~ASHxm traces of CS were indicated by X-ray phase analysis. Total weight loss of the sample determined by thermal analysis after ignltlon at 950 C refers to 1 1 .7 5 formal mol of H20 for a formula mol of monosulphate. A sample of ettringite was prepared from a mixture of CmAH s with CgH 2 (molar ratio 1:3) and water. The slurry has been continuously stirred at room temperature in a closed vessel for 30 days. A product of the reaction was washed with aceton and ethyleter and allowed to dry at room temperature and relative humidity of 52-66 %. Ettringite was the main component of the sample, but a small amount of CSH m has been detected by X-ray phase analysis. Total weigh loss at 950 °C refers to 30.7 formal mol of water for one formula mol of ettringite. .
.
.
.
0
Operating technique Stability of ettringite
in the presence of liquid water
A slightly modified method described by Ogawa and Roy [9] was utilized for the investigation of stability of ettringite in its saturated aqueous solution. For each selected couple of temperature and pressure, behavior of reactants and potential reaction products were investigated. Ettringite and a stoichiometric mixture of its decomposition products was suspended in distilled and decarbonated water. Both suspensions placed in teflon vessels were transferred to a high pressure reactor equipped with manometer and highpressure valve. Nitrogen was introduced to the reactor until the desired pressure was reached. Heating then started. Pressure was maintained constant by gradual releasing of the gas from the autoclave. Samples have been kept in the reactor at the selected pressure and temperature for 2 - 41 days. The content of teflon vessels was then filtered and a solid phase was dried. The same procedure of drying as above was used. The phase composition of the samples was determined by X-ray diffraction analysis and electron microscopy.
262
I. Nereid et ~.
Vol. 24, No. 2
Stability of monosulphate in relation to the water vapor pressure Similarly as i n the case of ettringite both directions of reaction were investigated in order to determine the thermal stability of monosulphate. The thermal dissociation of monosulphate proceeds in the interval of temperature in which the water vapor coexists with solid phases in phase diagram studied. For this reason monosulphate as well as a stoichiometric mixture of its reaction products were placed dry into teflon vessels. In order to prevent the samples from having contact with liquid water, only a part of volume of the reactor was filled with distilled and decarbonated water. The autoclave was then heated to the temperature at which the pressure of saturated water vapor corresponds to a chosen pressure. Water vapor was then slowly and carefully liberated until the whole volume of liquid water in the autoclave was evaporated. In order to reach the desired temperature at the chosen constant pressure, the excess of water vapor was released from the autoclave contemporarily increasing the temperature. After finishing the experiments the samples were checked for the absence of condensed water.
RESULTS AND DISCUSSION
X-ray d i f f r a c t i o n phase analysis Phase diagrams in Fig. I show the equilibrium between ettringite and its thermal dissociation products in the presence of liquid water. The results were obtained by the investigation of the reaction in both directions. Part a demonstrates dissociation of ettringite to the monosulphate and CS. Empty circles correspond to undecomposed ettrin~ite, filled ones represent the conditions under which the presence of C~ASHIm were unambiguously proved in reaction products. In part b a formation of ettringite from a mixture composed of C~ASHI2 and CSH 2 is demonstrated. Filled circles illustrate the experiments in which
kPa
o Io 800
o
I
800
I 600
I
I
I
600
, 400;
f f
oo! 0 (
/
/
(D/ ,
J
' Ii
I
I
I I 2o0~ 0 do /
/ •
//
, OI
90
Io
110 G
130
t T
,
90
110 130
B
"C
FIG. I Stability of ettringite in the presence of liquid water, a - thermal decomposition of ettringite: (o) undecomposed ettringite, (@) C~ASHi2 + CS. b - formation of ettrlngite from its dissociation products: (o) ettrlngite as a newformed phase,(@) C~ASHI2 + CS. The equilibrium conditions for reaction (I) accordimg to ~ t a v a [8] (D) and Ogawa end Roy [9] (V).
Vol. 24, No. 2
CaO-A1203-CaSO4-H20, EQUILIBRIUM, PRESSURE
263
the presence of ettringite in solid products was not proved by X-ray diffraction analysis. Empty circles refer to the conditions under which ettringite was unambiguously proved in solid reaction products. All the values were measured only at pressures of I00, 200, 400, 700 and 900 kPa. Repeated measurements obtained under conditions identical with those under which the lowest value of corresponding n-termed sequence was measured are manifested by the points located on the upper right side. Where equivalent symbols are used in both parts of the figure, the same phase composition of the system in both directions of reaction was proved. The equilibrium conditions according to reaction (I) as obtained by ~atava [8] are ilustrated by square. Triangel and a solid line drawn from it corresponds to the equilibrium in the system consisting of ettringite, monosulphate and CaSO~.I/2HmO as found by Ogawa ~nd Roy [9]. The equilibrium between liquid and gaseous water is indicated by dashed and dotted line. As follows from Fig. I C~ASH~2 and almost exclusively anhydrous calcium sulphate CS were detected by X-ray diffraction analysis as the decomposition products. The thermal decomposition equilibrium temperatures as determined by our method were by about 20 °C lower than those obtained by Ogawa and Roy [9] in the common pressure range. In spite of substantial difference between the methods used, comparatively good agreement with experiments performed by ~atava was achieved at low pressures. The equilibrium of monosulphate phases and their dissociation products in the presence of water vapor in both directions of the reaction, are illustrated in Fig. 2. In part a the dissociation of monosulphate gizIng the rise to C3AH s and CS is shown. Undecomposed monosulphate C~ASH~2 is represented by empty circles. The conditions under which the presence of monosulphate phase C~ASHx0, but not that of C3AH G was proved, are illustrated by partially filled circles. Filled circles refer to the experiments in which the presence of the C=AH s in solid reaction products was unambiguously proved. In part b a formation of monosulphate from the mixture of C3AH s and CSH= is demonstrated. In experiments illustrated by filled circles CsAH G and CS are present while formation of monosulphate has not been found. The presence of monosulphate C~ASH~0 but not that of C~ASHx2 has been proved at temperatu-
Stability of monosulphate in the presence of dec•mwater FlG. 2thermal vapor, a position of monosulphate: (O) undecomposed C~ASHI2 , [~) C~ASHIn, [0) C3AH ~ + CS. b - Formation of monosulphate from its dissociation products: (o) C~ASH12 as a new-formed phase, (0) C~ASHIo as a new-formed phase, (o) C3AH 6 + CS, (D) the equilibrium conditions of the reaction [i) accor-
ding to ~ t a v a
[9].
//
//
800F
80O
/ //" '~ • / /
[ 600 t
/
6o
/~O
/ / •
/
/
•
•
•
•
0
"C
OI
,
,
/
2ool/0
l
150 1~0 160 180 O
/
/
/
/ 200 / 0 0 ~0 ~ 0
°
/I
/ /
/ 400
/~ /t
• ~
,
•
120 1~0 160 180 .2__ "C b
264
I. Nerfidet al.
Vol. 24, No. 2
res and pressures represented by p a r t i a l l y filled circles. Empty o n e s r e f e r to the conditions u n d e r w h i c h m o n o s u l p h a t e CaASH~2 h a s b e e n f o u n d i n s o l i d reaction products formed. Similarly as in Fig. 1 where equivalent symbols are used in both parts of the figure, t h e same p h a s e c o m p o s i t i o n of the system was c o n f i r m e d in both directions of reaction. The e q u i l i b r i u m conditions according to reaction ( 2 ) a s m e a s u r e d by ~ a t a v a [9] a r e e x p r e s s e d b y s q u a r e . Repeated measurements are illustrated analogically as in Fig. 1. The equilibrium o f l i q u i d a n d g a s e o u s w a t e r i s d e m o n s t r a t e d by t h e d a s h e d a n d dotted line. As f o l l o w s f r o m F i g . 2 t h e e q u i l i b r i u m t e m p e r a t u r e o f m o n o s u l p h a t e t h e r mal d i s s o c i a t i o n is within the interval o f 150 - 177 °C o v e r t h e w a t e r v a p o r p r e s s u r e o f 100 - 900 kPa. U n d e r t h e e q u i l i b r i u m c o n d i t i o n s the C~ASH~o(S), C 3 A H s ( s ) , C S ( s ) a n d H20(g) p h a s e s c o e x i s t i n t h e s y s t e m . Microstructure
and morphology of samples
The r e s u l t s of X-ray diffraction phase analysis where confirmed by electron microscopy. Typical arrangement of rod-like crystals of undecomposed ettringite a f t e r 30 d a y s t r e a t m e n t a t 95 °C a n d 900 kPa i n w a t e r s u s p e n s i o n i s i n F i g . 3. I n F i g . 4 we c a n s e e a h e x a g o n a l c r y s t a l s of monosulphate and small lats of anhydrite as a result o f 18 d a y s t r e a t m e n t of stoichiometric m i x t u r e o f m o n o s u l p h a t e a n d gypsum a t l i 5 °C a n d 900 kPa i n w a t e r s u s p e n s i o n .
Ettringite at 95 °C
FIG. 3 a f t e r 30 d a y s t r e a t m e n t a n d 900 k P a in water suspension.
FIG. 4 Mixture of monosulphate and gypsum after 18 days treatment at llS °C and 900 kPa in water suspension.
Fig. 5 show the thermal decomposition products of ettringite after 18 days treatment at I15 °C and 900 kPa in water suspension. Starting compound is no more present, only decomposition products are seen, i. e. great idiomorphic hexagonal crystals of monosulphate and relative very small laths of anhydrite. The products of interaction of monosulphate with gypsum in water suspension at 9S °C and 900 kPa after 30 days are shown in Fig. 6. Unusually view to ettringite with shorter robust rods, sprinkled with extremly fine sticks or laths of probably unreacted anhydrite.
Vol. 24, No. 2
CaO-AI203-CaSO4-H20, EQUILIBRIUM, PRESSURE
20H rn
FIG. S Ettringite after 18 days treatment at IIS UC and 900 kPa in water suspension.
265
~
FIG. 6 Mixture of monosulphate and gypsum after 30 days treatment at 95 °C and 900 kPa in water suspension.
CONCLUSIONS The equilibrium dissociation temperature of ettringite 3CaO. AI203. 3CaSO~.S2H20 in the presence of liquid water as well as those of monosulphate 3CaO. AI~O3.CaSO~.I2H20 in the presence of water vapor in relation to pressure were investigated using X-ray diffraction and electron microscopy. The equilibrium temperatures were determined on the basis of studying the dissociation processes proceeding in the above mentioned phases as well as reactions leading to the formation of these phases from their dissociation products. At pressure from ISO up to 900 kPa, ettringite in the presence of its saturated aqueous solution is thermally stable at temperatures lower than 107 to ill °C, respectively. 3CaO. AI203.CaSO~.I2H20 and predominantly CaSO~ are the decompositiom products formed under conditions studied. The phases 3CaO. AI203.CaSO~.IOH20(s), 3CaO. AI203.6H20(s), CaSOa(s) and H20(g) coexist in the equilibrium at the thermal dissociation of monosulphate under conditions employed. The 3CaO. AI203.CaSO~.IOH20 thermal dissociation equlibrium temperature reaches values from 150 °C to 177 °C at the pressures of water wapour from i00 kPa to 900 kPa. Acknowledgment The study was supposed by GAS of the Slovak Academy of Sciences. The authors are grateful to Mrs M. D. Fr£alov~ for carrying out of electron microscopy and useful discussion of its interpretation, as well as to Mr. J. Horv~th for performing a number of X-ray analysis and to Mrs. A. Celkov~ for translation of the paper into English.
REFERENCES I. W. Feitknecht and H. W. Buser, Helv. Chim. Acta 22, 2298 (1949). 2. W. Feitknecht and H. W. Buser, Helv. Chim. Acta 24, 128 (195]).
266
I. Nereidet al.
Vol. 24, No. 2
3. W. Feitknecht, K. Michel and H. W. Buser, Helv. Chim. Acta 24, 119 (19SI). 4. A. Moore and H. F. W. Taylor, Nature 218, 1048 (1968). 5. A. Moore and H. F. W. Taylor, Acta Cryst. B 28,part 4, 386 (1970). 6. H. F. W. Taylor, Miner. Mag. 304, 377, (1973). 7. R. Allmann N. Jb. Miner. Mh. H. 3, 138-144 (1977). 8. V. ~atava and O. J. Vep~ek, J. Amer. Cer. Soe. 68, 37S (197S). 9. K. Ogawa and D. M. Roy, Cem. Conc. Res. 11, 741 (1981). 10. H. E. Schwiete and V. Ludwig, Proc. 5th Int. Symp. Chem. Cement 2, 84, Tokyo 1968 p. 3 7 - 6 7 . 11. W. D o s c h , H. K e l l e r a n d H. z u r S t r a s s e n , P r o c . S t h I n t . Symp. Chem. C e m e n t 2, 84, Tokyo 1968 p. 7 3 - 7 8 . 12. I . P r o k s , I. Ner&d a n d L. K o s a , S i l i k ~ t y 22, 125 ( 1 9 7 8 ) . 13. H. J . N. K u z e l , N. J b . M i n e r . Mh. 1 9 3 - 1 9 7 ( 1 9 6 S ) .