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Studies on expansive cement II. Hydration kinetics, surface properties and microstructure
CEMENT and CONCRETERESEARCH. Vol. 7, pp. 351-358, 1978. PergamonPress, Inc Printed in the United States•
STUDIES ON EXPANSIVE CEMENT HYDRATIONKINETICS, SURFACEPROPERTIESAND MICROSTRUCTURE
II.
H. EI-Didamony and M.Y. Haggag Faculty of Science, Zagazig University, Zagazig, Egypt S.A. Abo-EI-Enein Faculty of Science, Ain Shams University, Cairo, Egypt (Communicated by H.F.W. Taylor) (Received Feb. 16, 1978) ABSTRACT Hydration kinetics were followed by measuring non-evaporable water and free sulphate contents. Surfaceareas, total pore volumes and the microstructure of the hardened expansive cement pastes are discussed. Nitrogen and water vapour were used as adsorbates in the measurement of the surface areas and pore volumes and the results obtained are compared with each other. Scanningelectron microscopy was employed to study the microstructures of the hardened pastes. Die Kinetik der Hydratation wurde durch Messung des chemischen Verbindungswassergehalts und des freien Sulphatgehalts bestimmt Spezlflsche Oberflache, gesamtes Porenvolumen und M1krostruktur von geharteten Quellzementen werden dlskutlert. SpezlflscheOberflache und Porenvolumen wurden durch Adsorptionsmessungen von Stickstoff und Wasserdampf festgelegt. Die Resultate wurden mit einander vergleicht. Die Mikrostruktur von geh~rteten Pasten wurde mit Hilfe eines REM untersucht. .
.
351
•
°
°
II
352
Vol. 8, No. 3 H. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein
Introduction Expansive cement clinkers have been usually made using bauxite as a raw material, but Nakamura et al (1) and EI-Didamony and Henning (2) examined the p o s s i b i l i t y of replacing bauxite with kaolin. Expansive cements (both shrinkage-compensating and self-stressing) were discussed by Mehta and Polivka (3). Three types of expansive cements are known: type K, containing C,A3S; type M containing CA and C~2A7; and type S, containing a high - C3A portland cement clinker. A natural alunite-derived expansive cement of somewhat different properties has recently been produced (4). Other expansive cements based on hydration of CaO (5)orMgO(mixed with CaF2 as mineralizer) (6) have also been reported. Bentur and Ish-Shalom (7) reported in a series of papers the properties of type K expansive cement made with pure components. The results of paste hydration experiments were presented and a mechanism of e t t r i n g i t e formation and expansion in unrestrained paste was also proposed. Matousekand Sauman (8) investigated the hydration of expansive cement prepared from portland cement clinker, metakaolinite and CaSO,.2H20. Some physicochemical characteristics of the hardened paste were also discussed. EI-Didamony, Mostafa and Mostafa (9) reported the strengths and expansion characteristics of cements prepared, using varying proportions of portland cement expansive clinkers made using kaolin. In the present paper, we present results on hydration kinetics of the cements and the surface properties and microstructures of the hardened pastes. The nomenclature of the clinkers and cements is that used in the e a r l i er paper (9), which also gives details of the preparation and curing of the pastes. Experimental After each hydration period, two representative samples were ignited in platinum crucibles for one hour at 1050°C. The total water content, Wt, was calculated on an ignited weight basis. Two further representative samples, of about lO g each, were each placed in a beaker containing 20 m~ acetone and 20 m~ methanol, then stirred mechanically for one hour. The mixture was f i l t e r e d through a Gooch crucible, G4, and washed with acetone-methanol mixture and then several times with ether. The solid was then dried at 40 - 50°C for 30 minutes to complete evaporation of the ether. The dried solids were weighed in platinum crucibles and then ignited for one hour at 1050°C. The chemically-combined water, Wn, was taken as the loss calculated on an ignited weight basis. The free or evaporable water content, We, was calculated as Wt - Wn. Free CaS04 in the dried paste was determined by Forsen's method (lO). One g of dried sample and 300 m~ of half saturated lime water were stirred mechanically for one hour in a beaker and then f i l t e r e d . The residue was washed with lime water several times. The free CaSO, was assumed to dissolve and was determined in the f i l t r a t e gravimetrically. The bulk density, dp, was determined by weighing a certain weight of the hardened paste in air and in water. The total pore volume, Pt, of the paste was taken as the weight of evaporable water, We, in the saturated paste multiplied by 0.99, which is the specific volume of the evaporable water (ll,12). Hence the porosity, ~, (pore volume in unit volume of the paste) is 0.99 We.dp/(l +Wt).
Vol. 8, No. 3
353 HYDRATION, KINETICS, EXPANSIVECEMENT
Water adsorption isotherms were measured gravimetrically at 35°C on pastes of expansive cement Kz 20% cured for 0.5 h, 1 2 ~ 3d, and 28d. Each D-dried sample was outgassed for 30 hours at 5 x " mm Hg before any adsorption run. A minimum of 12 days was allowed between successive measurements to attain equilibrium. Nitrogen adsorption - desorption isotherms at liquid nitrogen temperature were measured volumetrically on hardened pastes of expansive cements Kz 20%, Kl 15%, Kz I0%, K2 20%, K2 15%, K2 I0%, K~ 20%, K~ 15% and K~ I0% after 28 days hydration. Additional isotherms were also measured on the Kz 20% pastes after l h and 6 h hydration. Equilibrium could be attained f a i r l y rapidly and successive measurements were taken at intervals of 45 minutes. For SEM examination, unbroken specimens were dried in Nz (free from COz) at 60% for ~4 h to remove free water. Theywere then fractured, coated with a 300 - 400 A layer of gold, and stored in a desiccator before SEM examination using a Leitz - AMR lO00 SEM. Results and Discussion Hydration Kinetics The results in Fig. l show that, for a given expansive cement, the nonevaporable water content increases not only with curing time, but also with the amount of expansive component. This is due to the formation of increasing amounts of ettringite, which contains more chemically-combined water (about 45%) than calcium silicate hydrate (about 23%). The results in Fig. 2 show that the free sulphate content of the hydrated samples can also be taken as a measure of the extent of hydration. The f a l l in free SO~ content is rapid in the f i r s t hour and slower thereafter; additional results for the clinker Kz with 20% expansive component showed a value of 4.2% at l hour. A
i
FIG. 1
0.2
............
Chemically combined water as a function of curing age for the hardened
~ c
0.1
>,
g,
. f
•
1 5
o
10"/,,
"Io
'
I
I
'
~
I
3
?
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3
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2B
Curing age,(days)
354
Vol. 8, No. 3
H. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein The DTA curves of a l/4 h old paste (Fig. 3) show low-temperature endotherms at 180°C, 220% and 2800C. The f i r s t could be due to the decomposition of ettringite and C-S-H, and the other two peaks to monosulphate decomposition. The endotherms at 450-550°C and 800-9000C are attributed to the decomposition of Ca(OH)2 and calcium carbonate, respectively. As hydration proceeds the amount of ettringite increases and that of monosulphate decreases. 20% After three days, monosulphate disappears completely. The o t 0 % amount of ettringite does not Q.6 1.A change between 3 and 28 days. 0.4 1.2
Bulk Density and Porosity
o
0.2
1,0 0
The results in Fig. 4 show that the density of portland cement paste increases with the curing time, but that the density of the expansive cement pastes decreases. As hydration proceeds, the fraction of the hydrated portland cement increases and these hydrates precipitate in the pores of the paste. The fact that the density of the expansive cement paste nevertheless decreases with curing time is attributed to the increase in volume. As indicated from this and another previous investigation (9) the paste which gives higher expansion has a lower density. The results in Fig. 5 indicate that the porosity of hardened expansive cement paste depends on the age of the paste and on its composition. For any paste, porosity decreases as hydration progresses. Immediately after mixing the cement with water the porosity is at its maximum. At early ages, expansive cement pastes have higher porosities than do portland cement pastes, but with continued hydration the reverse occurs. This is attributed to the large specific volume of ettringite
i
,
¢, L
O.8
0 . 6 ~
~I
0.6 O J,
0.2
0.2
01,,
I
L 0 28 3 ? Curing age (days)
,
3
7
28
FIG. 2 Free sulphate content as a function of curing age for the hardened pastes
E
r~
200
/*O0 600 B00 TernpePature °C
FIG.
1000
3
DTA curves at various ages for the hardened pastes using K120% clinker
Vol. 8, No. 3
355 HYDRATION, KINETICS, EXPANSIVECEMENT
.a
K1
>~ 2.22
L/
"~
lO "1.
-.=ec 2.18 L
ts'l.
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,
k I
20 %
K2 ~
15 % "
-
-
/
?
28
28 Cur;rig a g e , (days)
1 3
?
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28
?
FIG. 4 Bulk density as a function of curing age 0.4
0.4
0.3
0.3
0.4
W
~\ '~
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O.
a2
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7
28
0.2 ±
3
7
28
Curing
age.
0.2i
I I 3
I 7
15%
°1 I 28
(days)
FIG. 5 Total porosity as a function of curing age (about 202% that of the unhydrated calcium sulphoaluminate). Accordingly, the hydration products will f i l l a largerpart of the original water f i l l e d space on continued hydration than for a normal portland cement paste. These factors, namely, the decrease in total porosity, the increase in degree of hydration and the formation of ettringite, lead to an increase in the compressive strength and the dimensional changes of the hardened expansive cement pastes as the curing age is increased. Surface Properties The water isotherms for the hardened Ki 20% expansive cement pastes cured for 0 . 5 h , 12h, 3d and 28d are shown in Fig. 6. Theseisotherms seem to belong to type II of Brunauer's classification (13), but the " sigmoid" character decreases continuously with increasing paste age. The BET water surface areas, based on the ignited sample weights, are 44-56 m~/g. (Table I). This small variation in the water areas for samples cured for 0.5 h to 28d is surprising, especially as the degree of hydration increases tremendously during this period.
356
Vol. 8, No. 3 H. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein
The total pore volumes (Vp), as calculated from the saturation points of the water isotherms, are also given in Table I. In some samples the Vp values measured with water are actually smaller than those measured with nitrogen (vide infra), particularly for the sample cured for 28 days. Apparently, water does not measure the total pore volume of these pastes, especially when ettringite is allowed to age and to recyrtsallize into large crystals. The change in the nature of the surface during and after the recrystallization process might lead to some changes in the mode of interaction with the polar water molecules, and the strong interaction between the water molecules and the polar surface of ettringite affects the nature of the measured isotherm (Fig. 6). The use of polar molecules in measuring the total pore volume from the saturation values of the isotherms was discussed in detail by de Boer (14).
j/
29 d
~0,',
0.02
J
0 O.Ot, 0.02 o,
0
o, 0.04 cn
12 h
O.02 0 O.06 O.OL. 112 h
o.o21 i
0
n2
i
i
i
O~
0.6
0.8
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FIG. 6 Adsorption isotherms of water vapour. TABLE I Some Surface Characteristics of Four Expansive Cement Pastes from Water Vapour Adsorption Sample K~ 20%(0.5h) Kz 20%(12h) K~ 20%(3d) K~ 20%(28d)
SBET
Vp
(mZlg)
(mllg)
44 48 56 50
O.lll7 0.0979 0.0917 0.0661
I~
Nitrogen adsorption isotherms were measured on the hardened expansive cement pastes K~ 20%, Kl 15%, Kz I0%, K2 20%, K2 15%. K2 I0%, K~ 20%, K~ 15% and K4 I0% after 28 days hydration and also on the Kz 20% paste after l and 6 hours hydration. The nitrogen adsorption isotherms show common characteristics and Fig. 7 shows a typical adsorption desorption cycle for the adsorption of nitrogen gas at liquid nitrogen temperature on the Kz 20% expansive cement paste cured for 28 days. The BET nitrogen areas,-based on the ignited weights of the samples, are given in Table I I . They are very small. Evidently, the nitrogen molecules are inaccessible towards a major part of the pore system of these pastes. I t is presumed that the inaccessible pores are narrow or of "ink-bottle" type, and that Nz, unlike H20, cannot enter them because of its weak quadrupole moment and relatively large size. Table II also gives the total pore volumes, based on the ignited sample weights. The isotherm in Fig. 7 is typical for a material with "inkbottle" pores. The radius of the narrow neck, r l , as well-as the radius of the wide body, r2, of the ink-bottle pores can be calculated fro~ Fig. 7,gand are found to be 15 A and 59 A respectively. X-Ray Diffraction~ Morphology and Microstructure X-ray diffraction patterns for the Kz, K2 and K~ clinkers showed the presence of CaSO~ (anhydrite),
Vol. 8, No. 3
357
HYDRATION, KINETICS, EXPANSIVECEMENT I
CwA3S and 2(C=S).CS. Very weak lines of CaO were also observed. SEM's showed large crystals of the principal phase, C~A3S, together with prismatic crystals of 2(CzS).CS, spherical crystals of CaO, and small euhedral crystals of CaSOw. The latter phase was found to enclose the C~A3{ and other compounds (Fig. 8). The ferrite and silicate phases had the appearance of groups of minute crystals with some irregular characteristics. SEM's were obtained for the Kz 20% pastes, which were considered representative of the pastes in general. After 3 days hydration, partly rolled, semicrystalline foils and rod-like particles of ettringite $0 were the main hydration product observed (Fig. 9). The nearly amorphous calcium silicate hydrates, some hexagonal crystals of calcium 6O hydroxide and round crystals of gypsum appeared also in the structure with the remaining _ unhydrated C~A~Sand 2(CzS).CS grains. ~0 The microstructure after 7 and 28 days hydration exhibited a dense structure of large rods of ettringite, nearly amorphous 2O calcium silicate hydrates and calcium hydroxide crystals as the dominant hydration products. The 0 f -, , , round gypsum crystals were not seen 0.2 O~ O.6 1.0 after 7 days. X-ray diffraction P/P o patterns of these pastes showed ettringite as the main crystalline FIG 7 hydration product. The characterAdsorption-Desorption Isotherms of istic peaks for this compound Nitrogen on Kz 20% Paste (28 d) appeared after 6 h hydration. The peaks of Ca(OH)~ were clearly detected. The gypsum was found to be consumed after 7 days. TABLE II Some Surface Characteristics of the Hardened Pastes from Nitrogen Adsorption Sample
SBET (mZ/g)
Vp (mZ/g)
Kz 20% (l h) Kz 20% (6h) Kz 20% (28d) Kz 15% (28d) Kz I0% (28d) K2 20% (28d) Kz 15% (28d) Kz I0% (28d) K~ 20% (28d) K~ 15% (28d) K~ 10% (28d)
4.3 5.5 If.2 8.3 9.6 ll.6 5.8 I0.9 I0.8 14.2 4.3
0.0045 0.0072 0.1565 0.1274 0.0707 0.0935 0.0142 0.0103 0.]307 0.0221 0.0512
358
Vol. 8, No. 3 R. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein
FIG. 8
FIG. 9
Microstructure of Anhydrous KI Clinker
Microstructure of the Hardened K120% Paste (3d)
References . .
3. .
N. Nakamura, G. Sudoh and S. Akaiwa, Proc. 5th Int. Symp. Chem. Cement, Tokyo, 1968, 4, 351 (1969). H. EI-Didamony and D. Henning, Silikattech. 24, 279 (1973). P.K. Mehta and M. Polivka, Proc. 6th Int. Symp. Chem. Cement, Moscow, 1974, 3, 158 (1976). V.V. Volkov, V.P. Kolyovski and Ya.D. Yanev, ibid, 3, 182 (1976).
5.
T. Kawano, K. Hitotsuya and T. Mori,
6.
Yu.V. Nikiforov, R.A. Zozulya and N.M. Ivanova,
7.
A. Bentur and M. Ish-Shalom, Cement Concr. Res. 4, 519 (1974); 4, 709 (1974); 5, 139 (1975); 5, 597 (1975),
°
9. lO. II. 12. 13. 14.
ibid, 3, 179 (1976). ibid, ~, 113 (1976).
M. Matousek and Z. Sauman, Cement Concr. Res. 4, 113 (1974). H. EI-Didamony, H. Mostafa and M.Z. Mostafa, Cement Concr. Res. 6, 707 (1976). L. Forsen, Proc. 2nd Int. Symp. Chem. Cement, Stockholm, 325 (1938). L.E. Copeland and T.C. Hayes, J. Amer. Concr. Inst. 52, 633 (1956). L.E. Copeland, J. Amer. Concr. Inst. 5__22,836 (1956). S. Brunauer and P.H. Emmett, J. Amer. Chem. Soc. 5j_7, 1754 (1935). J.H. de Boer, "The Dynamical Character of Adsorption", Oxford, I s t Ed. (1953), 2nd Ed. (1968),
STUDIES ON EXPANSIVE CEMENT HYDRATIONKINETICS, SURFACEPROPERTIESAND MICROSTRUCTURE
II.
H. EI-Didamony and M.Y. Haggag Faculty of Science, Zagazig University, Zagazig, Egypt S.A. Abo-EI-Enein Faculty of Science, Ain Shams University, Cairo, Egypt (Communicated by H.F.W. Taylor) (Received Feb. 16, 1978) ABSTRACT Hydration kinetics were followed by measuring non-evaporable water and free sulphate contents. Surfaceareas, total pore volumes and the microstructure of the hardened expansive cement pastes are discussed. Nitrogen and water vapour were used as adsorbates in the measurement of the surface areas and pore volumes and the results obtained are compared with each other. Scanningelectron microscopy was employed to study the microstructures of the hardened pastes. Die Kinetik der Hydratation wurde durch Messung des chemischen Verbindungswassergehalts und des freien Sulphatgehalts bestimmt Spezlflsche Oberflache, gesamtes Porenvolumen und M1krostruktur von geharteten Quellzementen werden dlskutlert. SpezlflscheOberflache und Porenvolumen wurden durch Adsorptionsmessungen von Stickstoff und Wasserdampf festgelegt. Die Resultate wurden mit einander vergleicht. Die Mikrostruktur von geh~rteten Pasten wurde mit Hilfe eines REM untersucht. .
.
351
•
°
°
II
352
Vol. 8, No. 3 H. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein
Introduction Expansive cement clinkers have been usually made using bauxite as a raw material, but Nakamura et al (1) and EI-Didamony and Henning (2) examined the p o s s i b i l i t y of replacing bauxite with kaolin. Expansive cements (both shrinkage-compensating and self-stressing) were discussed by Mehta and Polivka (3). Three types of expansive cements are known: type K, containing C,A3S; type M containing CA and C~2A7; and type S, containing a high - C3A portland cement clinker. A natural alunite-derived expansive cement of somewhat different properties has recently been produced (4). Other expansive cements based on hydration of CaO (5)orMgO(mixed with CaF2 as mineralizer) (6) have also been reported. Bentur and Ish-Shalom (7) reported in a series of papers the properties of type K expansive cement made with pure components. The results of paste hydration experiments were presented and a mechanism of e t t r i n g i t e formation and expansion in unrestrained paste was also proposed. Matousekand Sauman (8) investigated the hydration of expansive cement prepared from portland cement clinker, metakaolinite and CaSO,.2H20. Some physicochemical characteristics of the hardened paste were also discussed. EI-Didamony, Mostafa and Mostafa (9) reported the strengths and expansion characteristics of cements prepared, using varying proportions of portland cement expansive clinkers made using kaolin. In the present paper, we present results on hydration kinetics of the cements and the surface properties and microstructures of the hardened pastes. The nomenclature of the clinkers and cements is that used in the e a r l i er paper (9), which also gives details of the preparation and curing of the pastes. Experimental After each hydration period, two representative samples were ignited in platinum crucibles for one hour at 1050°C. The total water content, Wt, was calculated on an ignited weight basis. Two further representative samples, of about lO g each, were each placed in a beaker containing 20 m~ acetone and 20 m~ methanol, then stirred mechanically for one hour. The mixture was f i l t e r e d through a Gooch crucible, G4, and washed with acetone-methanol mixture and then several times with ether. The solid was then dried at 40 - 50°C for 30 minutes to complete evaporation of the ether. The dried solids were weighed in platinum crucibles and then ignited for one hour at 1050°C. The chemically-combined water, Wn, was taken as the loss calculated on an ignited weight basis. The free or evaporable water content, We, was calculated as Wt - Wn. Free CaS04 in the dried paste was determined by Forsen's method (lO). One g of dried sample and 300 m~ of half saturated lime water were stirred mechanically for one hour in a beaker and then f i l t e r e d . The residue was washed with lime water several times. The free CaSO, was assumed to dissolve and was determined in the f i l t r a t e gravimetrically. The bulk density, dp, was determined by weighing a certain weight of the hardened paste in air and in water. The total pore volume, Pt, of the paste was taken as the weight of evaporable water, We, in the saturated paste multiplied by 0.99, which is the specific volume of the evaporable water (ll,12). Hence the porosity, ~, (pore volume in unit volume of the paste) is 0.99 We.dp/(l +Wt).
Vol. 8, No. 3
353 HYDRATION, KINETICS, EXPANSIVECEMENT
Water adsorption isotherms were measured gravimetrically at 35°C on pastes of expansive cement Kz 20% cured for 0.5 h, 1 2 ~ 3d, and 28d. Each D-dried sample was outgassed for 30 hours at 5 x " mm Hg before any adsorption run. A minimum of 12 days was allowed between successive measurements to attain equilibrium. Nitrogen adsorption - desorption isotherms at liquid nitrogen temperature were measured volumetrically on hardened pastes of expansive cements Kz 20%, Kl 15%, Kz I0%, K2 20%, K2 15%, K2 I0%, K~ 20%, K~ 15% and K~ I0% after 28 days hydration. Additional isotherms were also measured on the Kz 20% pastes after l h and 6 h hydration. Equilibrium could be attained f a i r l y rapidly and successive measurements were taken at intervals of 45 minutes. For SEM examination, unbroken specimens were dried in Nz (free from COz) at 60% for ~4 h to remove free water. Theywere then fractured, coated with a 300 - 400 A layer of gold, and stored in a desiccator before SEM examination using a Leitz - AMR lO00 SEM. Results and Discussion Hydration Kinetics The results in Fig. l show that, for a given expansive cement, the nonevaporable water content increases not only with curing time, but also with the amount of expansive component. This is due to the formation of increasing amounts of ettringite, which contains more chemically-combined water (about 45%) than calcium silicate hydrate (about 23%). The results in Fig. 2 show that the free sulphate content of the hydrated samples can also be taken as a measure of the extent of hydration. The f a l l in free SO~ content is rapid in the f i r s t hour and slower thereafter; additional results for the clinker Kz with 20% expansive component showed a value of 4.2% at l hour. A
i
FIG. 1
0.2
............
Chemically combined water as a function of curing age for the hardened
~ c
0.1
>,
g,
. f
•
1 5
o
10"/,,
"Io
'
I
I
'
~
I
3
?
28
3
?
2B
Curing age,(days)
354
Vol. 8, No. 3
H. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein The DTA curves of a l/4 h old paste (Fig. 3) show low-temperature endotherms at 180°C, 220% and 2800C. The f i r s t could be due to the decomposition of ettringite and C-S-H, and the other two peaks to monosulphate decomposition. The endotherms at 450-550°C and 800-9000C are attributed to the decomposition of Ca(OH)2 and calcium carbonate, respectively. As hydration proceeds the amount of ettringite increases and that of monosulphate decreases. 20% After three days, monosulphate disappears completely. The o t 0 % amount of ettringite does not Q.6 1.A change between 3 and 28 days. 0.4 1.2
Bulk Density and Porosity
o
0.2
1,0 0
The results in Fig. 4 show that the density of portland cement paste increases with the curing time, but that the density of the expansive cement pastes decreases. As hydration proceeds, the fraction of the hydrated portland cement increases and these hydrates precipitate in the pores of the paste. The fact that the density of the expansive cement paste nevertheless decreases with curing time is attributed to the increase in volume. As indicated from this and another previous investigation (9) the paste which gives higher expansion has a lower density. The results in Fig. 5 indicate that the porosity of hardened expansive cement paste depends on the age of the paste and on its composition. For any paste, porosity decreases as hydration progresses. Immediately after mixing the cement with water the porosity is at its maximum. At early ages, expansive cement pastes have higher porosities than do portland cement pastes, but with continued hydration the reverse occurs. This is attributed to the large specific volume of ettringite
i
,
¢, L
O.8
0 . 6 ~
~I
0.6 O J,
0.2
0.2
01,,
I
L 0 28 3 ? Curing age (days)
,
3
7
28
FIG. 2 Free sulphate content as a function of curing age for the hardened pastes
E
r~
200
/*O0 600 B00 TernpePature °C
FIG.
1000
3
DTA curves at various ages for the hardened pastes using K120% clinker
Vol. 8, No. 3
355 HYDRATION, KINETICS, EXPANSIVECEMENT
.a
K1
>~ 2.22
L/
"~
lO "1.
-.=ec 2.18 L
ts'l.
3
,
k I
20 %
K2 ~
15 % "
-
-
/
?
28
28 Cur;rig a g e , (days)
1 3
?
3
28
?
FIG. 4 Bulk density as a function of curing age 0.4
0.4
0.3
0.3
0.4
W
~\ '~
•
O.
a2
3
7
28
0.2 ±
3
7
28
Curing
age.
0.2i
I I 3
I 7
15%
°1 I 28
(days)
FIG. 5 Total porosity as a function of curing age (about 202% that of the unhydrated calcium sulphoaluminate). Accordingly, the hydration products will f i l l a largerpart of the original water f i l l e d space on continued hydration than for a normal portland cement paste. These factors, namely, the decrease in total porosity, the increase in degree of hydration and the formation of ettringite, lead to an increase in the compressive strength and the dimensional changes of the hardened expansive cement pastes as the curing age is increased. Surface Properties The water isotherms for the hardened Ki 20% expansive cement pastes cured for 0 . 5 h , 12h, 3d and 28d are shown in Fig. 6. Theseisotherms seem to belong to type II of Brunauer's classification (13), but the " sigmoid" character decreases continuously with increasing paste age. The BET water surface areas, based on the ignited sample weights, are 44-56 m~/g. (Table I). This small variation in the water areas for samples cured for 0.5 h to 28d is surprising, especially as the degree of hydration increases tremendously during this period.
356
Vol. 8, No. 3 H. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein
The total pore volumes (Vp), as calculated from the saturation points of the water isotherms, are also given in Table I. In some samples the Vp values measured with water are actually smaller than those measured with nitrogen (vide infra), particularly for the sample cured for 28 days. Apparently, water does not measure the total pore volume of these pastes, especially when ettringite is allowed to age and to recyrtsallize into large crystals. The change in the nature of the surface during and after the recrystallization process might lead to some changes in the mode of interaction with the polar water molecules, and the strong interaction between the water molecules and the polar surface of ettringite affects the nature of the measured isotherm (Fig. 6). The use of polar molecules in measuring the total pore volume from the saturation values of the isotherms was discussed in detail by de Boer (14).
j/
29 d
~0,',
0.02
J
0 O.Ot, 0.02 o,
0
o, 0.04 cn
12 h
O.02 0 O.06 O.OL. 112 h
o.o21 i
0
n2
i
i
i
O~
0.6
0.8
PIPo
FIG. 6 Adsorption isotherms of water vapour. TABLE I Some Surface Characteristics of Four Expansive Cement Pastes from Water Vapour Adsorption Sample K~ 20%(0.5h) Kz 20%(12h) K~ 20%(3d) K~ 20%(28d)
SBET
Vp
(mZlg)
(mllg)
44 48 56 50
O.lll7 0.0979 0.0917 0.0661
I~
Nitrogen adsorption isotherms were measured on the hardened expansive cement pastes K~ 20%, Kl 15%, Kz I0%, K2 20%, K2 15%. K2 I0%, K~ 20%, K~ 15% and K4 I0% after 28 days hydration and also on the Kz 20% paste after l and 6 hours hydration. The nitrogen adsorption isotherms show common characteristics and Fig. 7 shows a typical adsorption desorption cycle for the adsorption of nitrogen gas at liquid nitrogen temperature on the Kz 20% expansive cement paste cured for 28 days. The BET nitrogen areas,-based on the ignited weights of the samples, are given in Table I I . They are very small. Evidently, the nitrogen molecules are inaccessible towards a major part of the pore system of these pastes. I t is presumed that the inaccessible pores are narrow or of "ink-bottle" type, and that Nz, unlike H20, cannot enter them because of its weak quadrupole moment and relatively large size. Table II also gives the total pore volumes, based on the ignited sample weights. The isotherm in Fig. 7 is typical for a material with "inkbottle" pores. The radius of the narrow neck, r l , as well-as the radius of the wide body, r2, of the ink-bottle pores can be calculated fro~ Fig. 7,gand are found to be 15 A and 59 A respectively. X-Ray Diffraction~ Morphology and Microstructure X-ray diffraction patterns for the Kz, K2 and K~ clinkers showed the presence of CaSO~ (anhydrite),
Vol. 8, No. 3
357
HYDRATION, KINETICS, EXPANSIVECEMENT I
CwA3S and 2(C=S).CS. Very weak lines of CaO were also observed. SEM's showed large crystals of the principal phase, C~A3S, together with prismatic crystals of 2(CzS).CS, spherical crystals of CaO, and small euhedral crystals of CaSOw. The latter phase was found to enclose the C~A3{ and other compounds (Fig. 8). The ferrite and silicate phases had the appearance of groups of minute crystals with some irregular characteristics. SEM's were obtained for the Kz 20% pastes, which were considered representative of the pastes in general. After 3 days hydration, partly rolled, semicrystalline foils and rod-like particles of ettringite $0 were the main hydration product observed (Fig. 9). The nearly amorphous calcium silicate hydrates, some hexagonal crystals of calcium 6O hydroxide and round crystals of gypsum appeared also in the structure with the remaining _ unhydrated C~A~Sand 2(CzS).CS grains. ~0 The microstructure after 7 and 28 days hydration exhibited a dense structure of large rods of ettringite, nearly amorphous 2O calcium silicate hydrates and calcium hydroxide crystals as the dominant hydration products. The 0 f -, , , round gypsum crystals were not seen 0.2 O~ O.6 1.0 after 7 days. X-ray diffraction P/P o patterns of these pastes showed ettringite as the main crystalline FIG 7 hydration product. The characterAdsorption-Desorption Isotherms of istic peaks for this compound Nitrogen on Kz 20% Paste (28 d) appeared after 6 h hydration. The peaks of Ca(OH)~ were clearly detected. The gypsum was found to be consumed after 7 days. TABLE II Some Surface Characteristics of the Hardened Pastes from Nitrogen Adsorption Sample
SBET (mZ/g)
Vp (mZ/g)
Kz 20% (l h) Kz 20% (6h) Kz 20% (28d) Kz 15% (28d) Kz I0% (28d) K2 20% (28d) Kz 15% (28d) Kz I0% (28d) K~ 20% (28d) K~ 15% (28d) K~ 10% (28d)
4.3 5.5 If.2 8.3 9.6 ll.6 5.8 I0.9 I0.8 14.2 4.3
0.0045 0.0072 0.1565 0.1274 0.0707 0.0935 0.0142 0.0103 0.]307 0.0221 0.0512
358
Vol. 8, No. 3 R. EI-Didamony, M.Y. Haggag, S.A. Abo-EI-Enein
FIG. 8
FIG. 9
Microstructure of Anhydrous KI Clinker
Microstructure of the Hardened K120% Paste (3d)
References . .
3. .
N. Nakamura, G. Sudoh and S. Akaiwa, Proc. 5th Int. Symp. Chem. Cement, Tokyo, 1968, 4, 351 (1969). H. EI-Didamony and D. Henning, Silikattech. 24, 279 (1973). P.K. Mehta and M. Polivka, Proc. 6th Int. Symp. Chem. Cement, Moscow, 1974, 3, 158 (1976). V.V. Volkov, V.P. Kolyovski and Ya.D. Yanev, ibid, 3, 182 (1976).
5.
T. Kawano, K. Hitotsuya and T. Mori,
6.
Yu.V. Nikiforov, R.A. Zozulya and N.M. Ivanova,
7.
A. Bentur and M. Ish-Shalom, Cement Concr. Res. 4, 519 (1974); 4, 709 (1974); 5, 139 (1975); 5, 597 (1975),
°
9. lO. II. 12. 13. 14.
ibid, 3, 179 (1976). ibid, ~, 113 (1976).
M. Matousek and Z. Sauman, Cement Concr. Res. 4, 113 (1974). H. EI-Didamony, H. Mostafa and M.Z. Mostafa, Cement Concr. Res. 6, 707 (1976). L. Forsen, Proc. 2nd Int. Symp. Chem. Cement, Stockholm, 325 (1938). L.E. Copeland and T.C. Hayes, J. Amer. Concr. Inst. 52, 633 (1956). L.E. Copeland, J. Amer. Concr. Inst. 5__22,836 (1956). S. Brunauer and P.H. Emmett, J. Amer. Chem. Soc. 5j_7, 1754 (1935). J.H. de Boer, "The Dynamical Character of Adsorption", Oxford, I s t Ed. (1953), 2nd Ed. (1968),