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Hydration products of calcium aluminoferrite
CEMENT and CONCRETE RESEARCH. Vol. 13, pp. 696-702, 0008-8846/83 $3.00 + 00. Pergamon Press, Ltd.
1983.
Printed in the U S A
HYDRATION PRODUCTS OF CALCIUM ALUMINOFERRITE
J.M. Fortune Irish Cement Ltd, Research and Development Laboratory, Drogheda, Ireland and J.M°D. Coey Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland
(Communicated by H.F.W. Taylor) (Received Feb. 28, 1983) ABSTRACT Hydration products of Ca2AIFe05, steam cured at 72 C, have been identified using X-ray diffraction and Mbssbauer spectroscopy. Ironbearing phases are hydrogarnet Ca3Fe.AI 2 .(OH)f2 with y = 0.22, and non-crystalllne ferrlhydrlte Fe(OH)3~nH20~ These two are readily distinguished in ~dssbauer spectra at 4.2 K where they give, respectively, a single unspllt absorbtlon llne and a magnetically split hyperflne pattern with Hhf = 501 kOe. Addition of 5 wt% lime to the calcium alumlnoferrlte increases the proportion of iron In the hydrogarnet to y = 0.32 while leaving the ferrlhydrlte unchanged, whereas 5 wt% gypsum eliminates iron from the hydrogarnet entirely'and produces an even more disordered non-crystalline ferric hydrate.
The main Iron-bearlng phase in Portland cement is generally a member of the solid solution series Ca2Al2_xFexO 5 having x close to I. Hydration of the pure calcium alumlnoferrlte Ca2AIFe05, alternatively known as C4AF or brownmillerlte, has been studied by many workers [ 1-5] but there is still no complete picture of its hydration products. Part of the difficulty arises because the reaction with water is a sensitive function of temperature and time ~4], and may be significantly modified by the presence of small amounts of second phases such as llme or gypsum [3,5]. However, the major obstacle to a detailed quantitative understanding of the reaction is the non-crystalllne nature of some of the products. The main crystalline product in the temperature range 20-80 C has been identified by X-ray diffraction as hydrogarnet Ca3AI2 .Fe ~3(OH)12[I-4]. Flls used in the formula to indicate that the tetrahedra~ s~tes, generally occupied by Si in natural garnets, are vacant in this material. However, other reaction products include non-crystalline hydroxides which do not show up clearly in X-ray diffraction. They can be observed by scanning electron microscopy, but this technique does not permit their quantitative analysis. A spectroscopic technique, specific to iron, which can be used for quantitative analysis of that element in crystalline or amorphous phases is M~ssbauer spectroscopy ~6]. The valence 696
Vol. 13, No. 5 Ca2AIFe05,
697 HYDRATION,
STEAM CURING, PRODUCTS,
MOSSBAUER SPECTRA
state of the iron can be determined, and different iron sites can often be resolved. First applied to the study of Portland cement and its hydration products by Pobell and Wittmann [7], the method was subsequently applied to study C4AF hydration by V~rtes et al [I, 2] and Rogers and Aldridge [4]. On the basis of poorly-resolved room-temperature spectra, Tam~s and V~rtes [I] suggested that all the iron in C4AF is incorporated into hydrogarnet according to the reaction 6Ca2AIFe05 + 30H20 + 3Ca3Fe2(OH)I 2 + Ca3AI2(OH)I2
+ 4AI(OH)3
The AI(OH) 3 was thought to be amorphous, but these authors considered that no amorphous Pc(OH)3 was present. Rogers and Aldridge subsequently used the electron microprobe to establish that no finely divided phase contained alumlnlum without calcium or iron. They inferred from the lattice parameter that the hydrogarnet phase was alumlnlum-rlch with y = 0.24 ± 0.20, and further critized the above equation (I) on grounds of the known instability of Ca3Fe2(OH)12 [8,9]. Although unable to distinguish hydrogarnet from other iron phases, including amorphous hydrated ferric oxlde, on the basis of their MSssbauer spectra, they nevertheless concluded that the latter was a major reaction product, a conclusion supported by other workers 15]. On the other hand it has been claimed by Harchand et al [i0] that the presence of Pc(OH) 3 or its gel in hydrated cement is excluded, again on the basis of room temperature Mossbauer data. The aim of the present work was to re-examlne the hydration of C4AF , exploiting more fully the potential of the MSssbauer technique by including measurements in the liquid helium temperature range where noncrystalllne hydrous ferric oxide (ferrlhydrlte) exhibits characteristic magnetic behavlour 111,12]. It is thus possible to distinguish it from other oxides more dilute in iron. We have used this method, together with X-ray diffraction, to determine the iron phases produced on hydrating pure C4AF under standard conditions and have examined how the hydration products are influenced by the presence of small amounts of llme (an accelerator) or gypsum (a retarder) [5]. The calcium alumlnoferrlte starting material was prepared by standard ceramic procedures. A mixture of high purity (> 99 %) CaC03, AI203 and Fe203 was first sintered in air at 1380 C for one hour. It was reground and reflred until the product was single phase in X-ray diffraction and there was no free llme. The analytically pure C4AF was finally air quenched, ground to < 60~m in size and stored in plastic containers until ready for use. The cation distribution over octahedral and tetrahedral sites in the brownmillerlte structure was determined from ~6ssbauer spectra. The material is magnetically ordered at room temperature with overlapping slx-llne patterns from the two sites, but the patterns are quite clearly resolved at 80 K (figure 1). Relative areas were obtained as 70:30 by the standard least-squares computer analysis using a programme of Teillet and Varret. The corresponding formula is therefore Ca2{A10.3Fe0.7}~Fe0.TAI0.3]Os where {} and [] denote octahedral and tetrahedral sites respectively. Prom the temperature-dependence of the hyperflne splitting, we deduce that the magnetic ordering temperature T.N is 340±10 K, in agreement with earlier reports [13]. Slight variations of T~ among different samples are to be expected insofar as the cation distribution depends on the thermal treatment during preparation.
698
Vol. 13, No. 5 J.M. Fortune, J.M.D. Coey 1
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Mossbauer spectra of brownmillerite at 29OK and 80K.
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Hydrated material was prepared by mixing C4AF with an excess of distilled water, 50 wt%, in one case (sample A). Two other samples were prepared where, in addition, 5 wt% llme (sample B) or 5 wt% gypsum (sample C) was added to the mixture. The resulting slurries were packed into alr-tight plastic containers to prevent loss of water vapour, and then steam cured for 6 hours at 72 C after a 24 hour curing period at room temperature. The hydrated material, which was reddlsh-brown in colour, was then ground to a fine powder for X-ray and M~ssbauer analysis. X-ray diffraction was carried out on a Phillps diffractometer using CuK radiation and a scan speed of 2°/m. In all three samples a series of diffraction peaks was observed which can be attributed to the hydrogarnet phase. The only other lines visible were for sample C where the lines of the hydrogarnet pattern were somewhat broadened, and a small amount of portlandlte, Ca(OH)2, was detected. The observation of hydrogarnet as the major crystalline phase is in accord with other work where a similar hydration procedure was used ~1,4]. Lattice parameters obtained for samples A, B and C using a Debye-Scherrer camera were 12.57, 12.58 and 12.55 ~ respectively.
TABLE 1
Iron-Bearing Phases in Hydrated Calcium Aluminoferrite. Hydrogarnet
Sample hydrated with
Lattice Parameter
(~)
Isomer shift at 290K
Isomer shift at 4.2K
(w/s)
(~m/s)
Ferrlhydrite Iron fraction
y
(%)
Isomer shift at 290K
Quadrupole splitting a t 290K
(z~/s)
(mm/s)
4.2K (~¢n/s)
4.2K (kOe)
A
Isomer shift
Hyperflne field a t
Iron fraction
(%)
water
12.57(1)
0.31(I)
0.47(2)
12.5(10)
O.22
0.32(1)
0.62(2)
0.46(1)
501(5)
87.5(I)
B water + 5% llme
12.58
0.32
0.48
19.4
0.32
0.32
0.62
0.46
501
80.6
0%
0.00
0.32
0.66
0.47
491
I00
C water + 5% gypsum
12.55
Vol. 13, No. 5 Ca2AIFe05, HYDRATION,
699 STEAM CURING, PRODUCTS, MOSSBAUER SPECTRA
M~ssbauer spectra are shown Ln figure 2 for the three samples at 290, 80 and 4.2 K, and details of the room-temperature doublet appear Ln f~gure 3. All M~ssbauer data were obtained using a constant acceleration spectrometer w~th a source of 15 mCL of 57Co in Rh, but isomer sh~fts are quoted relative to an ~ron metal absorber• There are broad s i m i l a r i t i e s among the data for the three samples, but also some s ~ g n L f L c ~ a n t quant~tatSve differences. In the flrst place, by comparing the room-temperature spectra Ln figures i and 2, ~t ls evident that the original brownm~llerlte has been almost totally transformed by the hydratlon procedure. Traces of unreacted mater~al can be seen, particularly $n the spectrum of sample C, which Ls consistent w~th the known retardant action of gypsum on the hydratlon reaction [3,5]. The quadrupole doublet, shown on an expanded scale Ln figure 3, Ls best resolved for sample C but worst resolved for sample ~, for reasons that w~ll emerge ~n a moment. As the temperature Ls lowered, the doublet g~ves way progressively to a magnetic hyperfLne pattern. Th~s process begins at about 200 K, and ~s fairly well advanced for all three samples at 80 K. It ~s typical of superparamagnet~c fine partlcles with a broad dlstrLbut~on of p a r t i c l e s~ze [14,15]. By about 50
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M~ssbauer spectra of the hydration products of brownmillerlte at 290, 80 and 4.2Ko For sample A the reaction was carried out with water only, but for samples B or C 5 wt% of lime or gypsum was added.
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Vol. 13, No. 5 J.M. Fortune, J.M.D. Coey
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K the absorbtlon would be equally divided between the doublet and the magnetic hyperfine pattern, and this may be taken as the average blocking temperature. At 4.2 K the doublet has disappeared, except for a trace of it remaining in sample C. However in samples A and B there appears an unspllt central peak whose presence was masked by the doublet at higher temperatures. Its existence, with a different relative intensity in the three samples, accounts for the variation in the room-temperature spectra of figure 3. Fitting these spectra to a doublet and a single line gives isomer shifts 6 of 0.32 mm/s and O.31 mm/s respectively for the two components, and a quadrupole splitting A of 0.63 mm/s for the doublet. Linewidths £ (full width at half maximum) are 0.52 and 0.40 mm/s respectively. Results of fitting the 4.2 K spectra are included in Table i. In summary, the M~ssbauer data show the presence of two Iron-bearing phases in addition to traces of unreacted C4AF. One gives an unspllt absorbtion line at all temperatures whereas the other gives a doublet at room temperature, which gives way progressively to a magnetically split pattern as the temperature Is lowered. The two phases are Identifled as follows. The unsplit peak is associated with a phase which is relatively dilute in iron because there is no evidence of magnetic ordering at 4.2 K. The spectrum indicates undistorted oetahedral coordination of the Fe S+ ion. Its relative intensity increases in the order C < A < B which is precisely the order of increasing lattice parameter for the crystalline hydrogarnet, so it is therefore associated with iron in this material. The second phase also involves iron in octahedral coordination, but it is sufficiently concentrated in iron to order magnetically. No crystalline iron hydroxide was identified by X-ray diffraction, although a weak and very broad band centred at d = 4.4 ~ was seen for samples A and B . This suggests partially microcrystalline rather than truely amorphous ferric hydrate, and the Mossbauer parameters for samples A and B are close to those of some of the ferrlhydrltes described by Murad and Schwertmann [12]. The formula may be generally written as Fe(OH)s.nH20, without commitment to any particular ratio of 02- and (OH)- ions in the structure. The ferric hydrate is identical in samples A and B from the viewpoint of Mossbauer spectra, but in sample C the
Vol. 13, No. 5 701 Ca2SIFe05, HYDRATION, STEAM CURING, PRODUCTS, MOSSBAUER SPECTRA
hyperfine field at 4.2 K is somewhat lower and the lines broader, which suggest an even more disordered structure. If, following Rogers and Aldrldge [4], we exclude the possibility of alumlnium hydroxide and allow for the presence of some calcium hydroxide, the hydration reaction of pure C4AF may then be written, using the ratio of iron in hydrogarnet and ferrihydrite obtained from the 4.2 K Mossbauer spectrum, as Ca2AIFe05 + 5 H20 + 0.56 Ca3{AII.78Fe0.22}(0H)I 2 + 0.88 Fe(0H)3 + 0.31 Ca(0H) 2 In the same way, for sample B, Ca2AIFe05 + 0.22 Ca0 + 5.22 H20 + 0.60 Ca3{AII.68Fe0.32}(0H)I 2 + 0.81 Fe(OH)3 + 0.42 Ca(0H)2 These reactions show the minimum amount of water needed to write the equations, but some excess is almost certainly present in the ferric and calcium hydroxides. For sample C, M6ssbauer results show that the hydrogarnet phase is essentially free of iron and that all the iron is present as ferrihydrite, but we have no way of determining in which of the products the sulphate ions are incorporated. The difference in the ferrihydrite spectrum for sample C compared with the other two suggests that they might be incorporated there. In conclusion, we have established that M~ssbauer spectra at liquid helium temperature can be used for quantitative determination of the distribution of iron between the hydrogarnet and noncrystalline ferrihydrite phases in hydrated calcium aluminoferrite. Our results show that relatively little iron, i0 - 20% of the trivalent cations, appears in the hydrogarnet when it is produced by the action of water at 72 C, or by water and lime. When water and gypsum is used, the reaction is slower and the hydrogarnet is purely aluminous. Acknowledgements: We are grateful to A. Meagher for taking the spectra at 4.2 K, and to D.H. Ryan for help with the computer fitting. Our thanks are also due to Mr. B. O'Kelly (Head of Research and Development, Irish Cement Ltd) for his invaluable assistance and encouragement.
REFERENCES i) 2) 3) 4) 5) 6) 7) 8) 9) iO)
F.D. Tam~s and A. Vertes, Cem. Conc. Res. 3 575 (1973). A. V6rtes and H. Ranogajec-Komor, Proc 6th--Int. Congr. Chem. of Cements Moscow (1974) Vol 2. p3. I. Jawed, S. Goto and R. Kondo, Cem. Concr. Res. 6 441 (1976). D.E. Rogers and L.P. Aldridge, Cem. Concr. Res. 7 399 (1977). M. Fukuhara, S. Goto, K. Asaga, M. Daimon and R. Kondo, Cem. Concr. Res. ii, 407 (1981). N.N. Greenwood and R.C. Gibb, "M6ssbauer Spectroscopy" (Chapman and Hall, London) 1971). F. Pobell and F. Wittmann Phys. Letters 19, 175 (1965), Z. Angew. Physik 2 0 488 (1966): Z. Naturforsch. 21 831 (1966). H.J. Kuzel, Neues Jahrb. Mineral. Monatsh. 87 (1968). H.F.W. Taylow, Proc. 6th Int. Congr. Chem. Cements, Moscow (1974) vol. 1 p3. K.S. Harchand, Vishwamittar and K. Chandra, Cem. Concr. Res. iO, 243 (1980).
702
Vol. 13, No. 5 J.M. Fortune, J.M.D. Coey
ll) 12) 13) 14)
15)
J.M.D. Coey and P.W. Readman. Nature 246 471 (1973) ; Ear~'_h Planet. Sci. Lett., 21 45 (1973). E. Murad and U. Schwertmann, Amer. Mineral. 65 1044 (1980). R.W. Grant, S. Geller, H. Wiedersich, U. Gonser and L.D. Fullmer, J. Appl. Phys. 39 1122 (1968). S. Morup, J.A. Dumesic and H. Topsoe in 'Applications of M6ssbauer Spectroscopy, Vol. 2' R.L. Cohen (editor) Academic Press, New York (1980). J.L. Dormann, Rev. Phys. Appl. 16 275 (1981).
1983.
Printed in the U S A
HYDRATION PRODUCTS OF CALCIUM ALUMINOFERRITE
J.M. Fortune Irish Cement Ltd, Research and Development Laboratory, Drogheda, Ireland and J.M°D. Coey Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland
(Communicated by H.F.W. Taylor) (Received Feb. 28, 1983) ABSTRACT Hydration products of Ca2AIFe05, steam cured at 72 C, have been identified using X-ray diffraction and Mbssbauer spectroscopy. Ironbearing phases are hydrogarnet Ca3Fe.AI 2 .(OH)f2 with y = 0.22, and non-crystalllne ferrlhydrlte Fe(OH)3~nH20~ These two are readily distinguished in ~dssbauer spectra at 4.2 K where they give, respectively, a single unspllt absorbtlon llne and a magnetically split hyperflne pattern with Hhf = 501 kOe. Addition of 5 wt% lime to the calcium alumlnoferrlte increases the proportion of iron In the hydrogarnet to y = 0.32 while leaving the ferrlhydrlte unchanged, whereas 5 wt% gypsum eliminates iron from the hydrogarnet entirely'and produces an even more disordered non-crystalline ferric hydrate.
The main Iron-bearlng phase in Portland cement is generally a member of the solid solution series Ca2Al2_xFexO 5 having x close to I. Hydration of the pure calcium alumlnoferrlte Ca2AIFe05, alternatively known as C4AF or brownmillerlte, has been studied by many workers [ 1-5] but there is still no complete picture of its hydration products. Part of the difficulty arises because the reaction with water is a sensitive function of temperature and time ~4], and may be significantly modified by the presence of small amounts of second phases such as llme or gypsum [3,5]. However, the major obstacle to a detailed quantitative understanding of the reaction is the non-crystalllne nature of some of the products. The main crystalline product in the temperature range 20-80 C has been identified by X-ray diffraction as hydrogarnet Ca3AI2 .Fe ~3(OH)12[I-4]. Flls used in the formula to indicate that the tetrahedra~ s~tes, generally occupied by Si in natural garnets, are vacant in this material. However, other reaction products include non-crystalline hydroxides which do not show up clearly in X-ray diffraction. They can be observed by scanning electron microscopy, but this technique does not permit their quantitative analysis. A spectroscopic technique, specific to iron, which can be used for quantitative analysis of that element in crystalline or amorphous phases is M~ssbauer spectroscopy ~6]. The valence 696
Vol. 13, No. 5 Ca2AIFe05,
697 HYDRATION,
STEAM CURING, PRODUCTS,
MOSSBAUER SPECTRA
state of the iron can be determined, and different iron sites can often be resolved. First applied to the study of Portland cement and its hydration products by Pobell and Wittmann [7], the method was subsequently applied to study C4AF hydration by V~rtes et al [I, 2] and Rogers and Aldridge [4]. On the basis of poorly-resolved room-temperature spectra, Tam~s and V~rtes [I] suggested that all the iron in C4AF is incorporated into hydrogarnet according to the reaction 6Ca2AIFe05 + 30H20 + 3Ca3Fe2(OH)I 2 + Ca3AI2(OH)I2
+ 4AI(OH)3
The AI(OH) 3 was thought to be amorphous, but these authors considered that no amorphous Pc(OH)3 was present. Rogers and Aldridge subsequently used the electron microprobe to establish that no finely divided phase contained alumlnlum without calcium or iron. They inferred from the lattice parameter that the hydrogarnet phase was alumlnlum-rlch with y = 0.24 ± 0.20, and further critized the above equation (I) on grounds of the known instability of Ca3Fe2(OH)12 [8,9]. Although unable to distinguish hydrogarnet from other iron phases, including amorphous hydrated ferric oxlde, on the basis of their MSssbauer spectra, they nevertheless concluded that the latter was a major reaction product, a conclusion supported by other workers 15]. On the other hand it has been claimed by Harchand et al [i0] that the presence of Pc(OH) 3 or its gel in hydrated cement is excluded, again on the basis of room temperature Mossbauer data. The aim of the present work was to re-examlne the hydration of C4AF , exploiting more fully the potential of the MSssbauer technique by including measurements in the liquid helium temperature range where noncrystalllne hydrous ferric oxide (ferrlhydrlte) exhibits characteristic magnetic behavlour 111,12]. It is thus possible to distinguish it from other oxides more dilute in iron. We have used this method, together with X-ray diffraction, to determine the iron phases produced on hydrating pure C4AF under standard conditions and have examined how the hydration products are influenced by the presence of small amounts of llme (an accelerator) or gypsum (a retarder) [5]. The calcium alumlnoferrlte starting material was prepared by standard ceramic procedures. A mixture of high purity (> 99 %) CaC03, AI203 and Fe203 was first sintered in air at 1380 C for one hour. It was reground and reflred until the product was single phase in X-ray diffraction and there was no free llme. The analytically pure C4AF was finally air quenched, ground to < 60~m in size and stored in plastic containers until ready for use. The cation distribution over octahedral and tetrahedral sites in the brownmillerlte structure was determined from ~6ssbauer spectra. The material is magnetically ordered at room temperature with overlapping slx-llne patterns from the two sites, but the patterns are quite clearly resolved at 80 K (figure 1). Relative areas were obtained as 70:30 by the standard least-squares computer analysis using a programme of Teillet and Varret. The corresponding formula is therefore Ca2{A10.3Fe0.7}~Fe0.TAI0.3]Os where {} and [] denote octahedral and tetrahedral sites respectively. Prom the temperature-dependence of the hyperflne splitting, we deduce that the magnetic ordering temperature T.N is 340±10 K, in agreement with earlier reports [13]. Slight variations of T~ among different samples are to be expected insofar as the cation distribution depends on the thermal treatment during preparation.
698
Vol. 13, No. 5 J.M. Fortune, J.M.D. Coey 1
i
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Mossbauer spectra of brownmillerite at 29OK and 80K.
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Hydrated material was prepared by mixing C4AF with an excess of distilled water, 50 wt%, in one case (sample A). Two other samples were prepared where, in addition, 5 wt% llme (sample B) or 5 wt% gypsum (sample C) was added to the mixture. The resulting slurries were packed into alr-tight plastic containers to prevent loss of water vapour, and then steam cured for 6 hours at 72 C after a 24 hour curing period at room temperature. The hydrated material, which was reddlsh-brown in colour, was then ground to a fine powder for X-ray and M~ssbauer analysis. X-ray diffraction was carried out on a Phillps diffractometer using CuK radiation and a scan speed of 2°/m. In all three samples a series of diffraction peaks was observed which can be attributed to the hydrogarnet phase. The only other lines visible were for sample C where the lines of the hydrogarnet pattern were somewhat broadened, and a small amount of portlandlte, Ca(OH)2, was detected. The observation of hydrogarnet as the major crystalline phase is in accord with other work where a similar hydration procedure was used ~1,4]. Lattice parameters obtained for samples A, B and C using a Debye-Scherrer camera were 12.57, 12.58 and 12.55 ~ respectively.
TABLE 1
Iron-Bearing Phases in Hydrated Calcium Aluminoferrite. Hydrogarnet
Sample hydrated with
Lattice Parameter
(~)
Isomer shift at 290K
Isomer shift at 4.2K
(w/s)
(~m/s)
Ferrlhydrite Iron fraction
y
(%)
Isomer shift at 290K
Quadrupole splitting a t 290K
(z~/s)
(mm/s)
4.2K (~¢n/s)
4.2K (kOe)
A
Isomer shift
Hyperflne field a t
Iron fraction
(%)
water
12.57(1)
0.31(I)
0.47(2)
12.5(10)
O.22
0.32(1)
0.62(2)
0.46(1)
501(5)
87.5(I)
B water + 5% llme
12.58
0.32
0.48
19.4
0.32
0.32
0.62
0.46
501
80.6
0%
0.00
0.32
0.66
0.47
491
I00
C water + 5% gypsum
12.55
Vol. 13, No. 5 Ca2AIFe05, HYDRATION,
699 STEAM CURING, PRODUCTS, MOSSBAUER SPECTRA
M~ssbauer spectra are shown Ln figure 2 for the three samples at 290, 80 and 4.2 K, and details of the room-temperature doublet appear Ln f~gure 3. All M~ssbauer data were obtained using a constant acceleration spectrometer w~th a source of 15 mCL of 57Co in Rh, but isomer sh~fts are quoted relative to an ~ron metal absorber• There are broad s i m i l a r i t i e s among the data for the three samples, but also some s ~ g n L f L c ~ a n t quant~tatSve differences. In the flrst place, by comparing the room-temperature spectra Ln figures i and 2, ~t ls evident that the original brownm~llerlte has been almost totally transformed by the hydratlon procedure. Traces of unreacted mater~al can be seen, particularly $n the spectrum of sample C, which Ls consistent w~th the known retardant action of gypsum on the hydratlon reaction [3,5]. The quadrupole doublet, shown on an expanded scale Ln figure 3, Ls best resolved for sample C but worst resolved for sample ~, for reasons that w~ll emerge ~n a moment. As the temperature Ls lowered, the doublet g~ves way progressively to a magnetic hyperfLne pattern. Th~s process begins at about 200 K, and ~s fairly well advanced for all three samples at 80 K. It ~s typical of superparamagnet~c fine partlcles with a broad dlstrLbut~on of p a r t i c l e s~ze [14,15]. By about 50
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M~ssbauer spectra of the hydration products of brownmillerlte at 290, 80 and 4.2Ko For sample A the reaction was carried out with water only, but for samples B or C 5 wt% of lime or gypsum was added.
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Vol. 13, No. 5 J.M. Fortune, J.M.D. Coey
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FIG. 3 Room-temperature spectra on an expanded scale.
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2
Velocity(mm/s)
K the absorbtlon would be equally divided between the doublet and the magnetic hyperfine pattern, and this may be taken as the average blocking temperature. At 4.2 K the doublet has disappeared, except for a trace of it remaining in sample C. However in samples A and B there appears an unspllt central peak whose presence was masked by the doublet at higher temperatures. Its existence, with a different relative intensity in the three samples, accounts for the variation in the room-temperature spectra of figure 3. Fitting these spectra to a doublet and a single line gives isomer shifts 6 of 0.32 mm/s and O.31 mm/s respectively for the two components, and a quadrupole splitting A of 0.63 mm/s for the doublet. Linewidths £ (full width at half maximum) are 0.52 and 0.40 mm/s respectively. Results of fitting the 4.2 K spectra are included in Table i. In summary, the M~ssbauer data show the presence of two Iron-bearing phases in addition to traces of unreacted C4AF. One gives an unspllt absorbtion line at all temperatures whereas the other gives a doublet at room temperature, which gives way progressively to a magnetically split pattern as the temperature Is lowered. The two phases are Identifled as follows. The unsplit peak is associated with a phase which is relatively dilute in iron because there is no evidence of magnetic ordering at 4.2 K. The spectrum indicates undistorted oetahedral coordination of the Fe S+ ion. Its relative intensity increases in the order C < A < B which is precisely the order of increasing lattice parameter for the crystalline hydrogarnet, so it is therefore associated with iron in this material. The second phase also involves iron in octahedral coordination, but it is sufficiently concentrated in iron to order magnetically. No crystalline iron hydroxide was identified by X-ray diffraction, although a weak and very broad band centred at d = 4.4 ~ was seen for samples A and B . This suggests partially microcrystalline rather than truely amorphous ferric hydrate, and the Mossbauer parameters for samples A and B are close to those of some of the ferrlhydrltes described by Murad and Schwertmann [12]. The formula may be generally written as Fe(OH)s.nH20, without commitment to any particular ratio of 02- and (OH)- ions in the structure. The ferric hydrate is identical in samples A and B from the viewpoint of Mossbauer spectra, but in sample C the
Vol. 13, No. 5 701 Ca2SIFe05, HYDRATION, STEAM CURING, PRODUCTS, MOSSBAUER SPECTRA
hyperfine field at 4.2 K is somewhat lower and the lines broader, which suggest an even more disordered structure. If, following Rogers and Aldrldge [4], we exclude the possibility of alumlnium hydroxide and allow for the presence of some calcium hydroxide, the hydration reaction of pure C4AF may then be written, using the ratio of iron in hydrogarnet and ferrihydrite obtained from the 4.2 K Mossbauer spectrum, as Ca2AIFe05 + 5 H20 + 0.56 Ca3{AII.78Fe0.22}(0H)I 2 + 0.88 Fe(0H)3 + 0.31 Ca(0H) 2 In the same way, for sample B, Ca2AIFe05 + 0.22 Ca0 + 5.22 H20 + 0.60 Ca3{AII.68Fe0.32}(0H)I 2 + 0.81 Fe(OH)3 + 0.42 Ca(0H)2 These reactions show the minimum amount of water needed to write the equations, but some excess is almost certainly present in the ferric and calcium hydroxides. For sample C, M6ssbauer results show that the hydrogarnet phase is essentially free of iron and that all the iron is present as ferrihydrite, but we have no way of determining in which of the products the sulphate ions are incorporated. The difference in the ferrihydrite spectrum for sample C compared with the other two suggests that they might be incorporated there. In conclusion, we have established that M~ssbauer spectra at liquid helium temperature can be used for quantitative determination of the distribution of iron between the hydrogarnet and noncrystalline ferrihydrite phases in hydrated calcium aluminoferrite. Our results show that relatively little iron, i0 - 20% of the trivalent cations, appears in the hydrogarnet when it is produced by the action of water at 72 C, or by water and lime. When water and gypsum is used, the reaction is slower and the hydrogarnet is purely aluminous. Acknowledgements: We are grateful to A. Meagher for taking the spectra at 4.2 K, and to D.H. Ryan for help with the computer fitting. Our thanks are also due to Mr. B. O'Kelly (Head of Research and Development, Irish Cement Ltd) for his invaluable assistance and encouragement.
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