Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
RIETVELD STRUCTURE REFINEMENT OF CARBONATE AND
SULFITE ETTRINGITE
D.G. Grier1, E.L. Jarabek1, R.B. Peterson1, L.E. Mergen2, G.J. McCarthy1
1Department of Chemistry, North Dakota State University, Fargo, ND 58105-5516
2Department of Chemistry, Concordia College, Moorhead, MN 56562
ABSTRACT
Two sulfate ettringite analogs, sulfite and carbonate ettringite, were synthesized using
procedures based on the “saccharate method”. Phases with the ettringite structure occur as
hydration products of many cement and coal combustion by-products (CCBs) systems.
Structural refinements of carbonate and sulfite ettringite were performed using Rietveld
techniques. The present study was carried out to determine which starting model, sulfate
ettringite or thaumasite, gave better end results in the structure refinement of several variant
models of carbonate and sulfite ettringite. X-ray powder diffraction (XRPD), along with several
auxiliary characterization techniques, namely, thermogravimetric analysis (TGA), Fouriertransform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), were used to
characterize the synthetic ettringites. Rietveld results suggest sulfate ettringite is a more
appropriate starting model compared to thaumasite when modeling sulfite or carbonate ettringite.
In the case of sulfite ettringite, results show better pattern fitting when modeling for disorder;
while carbonate ettringite variants show very little deviation between the fully ordered variant
and those variants modeled for disorder.
INTRODUCTION
Phases with the ettringite structure occur as hydration products of many cements [1,2] and coal
combustion by-products (CCBs) [3,4]. The mineral ettringite, nominally Ca6Al2(SO4)3(OH)12 •
26H2O and here referred to as “sulfate ettringite”, exhibits at least limited solid solution behavior
with many other compositional end-members. Substitution may occur on the aluminum site by
metals such as iron, manganese, chromium, or silicon, in the case of thaumasite [5]. Substitution
may also occur on oxyanion sites by groups such as carbonate, sulfite, borate, and several other
possible divalent oxyanions available from the mineral’s surrounding chemical environment [6].
Sulfate ettringite crystallizes in the space group P31c, and is composed of columns of
Ca6[Al2(OH)12 • 24H2O]6+ oriented along the c-axis, with the inter-column channels filled with
three sulfate ions and the two remaining water molecules [7]. Thaumasite, nominally
Ca6Si2(SO4)2(CO3)2(OH)12 • 24H2O, is a related phase with a similar structure, and also occurs
in hydrated concretes and CCBs. Thaumasite crystallizes in the space group P63, with the Al:Si
substitution charge balanced by replacement of an additional oxyanion (carbonate) for the
channel water molecules. Tetrahedral sulfate and trigonal planar carbonate groups alternate
along the thaumasite inter-column channels [8]. With the sulfate ettringite unit cell referred to
hexagonal axes, both minerals have similar lattice dimensions, although details of the structure
lead to a doubling of the c-axis for sulfate ettringite [7,8].
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Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
Carbonate and sulfite ettringite have typically been characterized by XRPD using the sulfate
ettringite structure by analogy [9]. The primary differences between the ettringite and related
thaumasite structures include; space group, ordering of the sulfate tetrahedral orientations,
rotational ordering of stacked groups (oxyanions and/or water) in the channel sites, and
occupational disorder among sulfate, carbonate, and water on the channel sites [6]. Since these
slight differences noted in the single crystal determinations negate complete solid solution per se,
the extent of potential miscibility between the two structure models is still being debated [5,10].
To the authors’ knowledge, no structure data are available for carbonate or sulfite ettringite.
Moreover, cell determinations from Rietveld quantitative XRPD analyses of sulfate ettringite in
several hydrated CCBs suggest sulfite or carbonate substitution [11]. The present study was
therefore undertaken to provide appropriate structure models for the sulfite and carbonate end
members. Several potential models were tested by Rietveld structure refinement techniques
using the structural variations given above, along with X-ray powder diffraction patterns from
synthetic sulfite and carbonate ettringite.
EXPERIMENTAL PROCEDURES
Synthesis
Two sulfate ettringite analogs, sulfite and carbonate ettringite, were synthesized using
procedures based on the “saccharate method” of Carlson and Berman [12], which involves
precipitation from supersaturated solutions of CaO (dissolved to saturation in a 10% sucrose
solution) and the correct stoichiometric equivalents of sodium aluminate tetrahydrate (Na2Al2O4
• 4H20) and sodium sulfite (Na2SO3) in the case of sulfite ettringite or sodium aluminate
tetrahydrate (Na2Al2O4 • 4H20) and sodium carbonate (Na2CO3) in the case of carbonate
ettringite. The sulfite ettringite was found to be phase pure by XRPD analysis, while the
carbonate ettringite contained trace impurities of calcite and a measurable amorphous content.
Reagents were analyzed by XRPD methods and shown to be phase pure. To alleviate
atmospheric CO2 in solution, 600 ml of distilled, ionized water was boiled in a 1liter flask for 10
minutes and left to cool and stir under bubbling nitrogen at room temperature for approximately
24 hours. After the CaO dissolved, the CaO/sucrose solutions (pH: 12.5-13) were allowed to stir
for 15 minutes before inducing the formation of the precipitate with the advent of either
Na2Al2O4 • 4H20 and Na2SO3 solution, in the case of sulfite ettringite, or Na2Al2O4 • 4H20 and
Na2CO3 solution, in the case of carbonate ettringite. These solutions were titrated slowly into the
CaO/sucrose solution; upon which, a white precipitate formed. The pH of the initial solutions
were adjusted, as needed, using NaOH pellets to maintain a pH between 11.5-12.5.
Following precipitation, the two solutions were left to stir under nitrogen at ambient temperature
for 24 hours. The solutions were then vacuum-filtrated and washed with distilled water and
acetone. Preliminary (3-minute scan time) qualitative XRPD data were collected, followed by
higher quality, longer scan time data, to determine phase purity. The XRPD data showed the
crystalline component of the synthetic products to be phase pure sulfite and carbonate ettringite,
with exception to trace amounts of calcite, due to atmospheric carbonation of CaO, in the case of
carbonate ettringite.
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Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
Characterization Techniques
The synthetic products were initially tested for purity and characterized using qualitative XRPD,
Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). X-ray
powder diffraction data was collected using a Phillips PW3040 X’pert-MPD Multipurpose
Diffractometer in Bragg-Brentano geometry (Cu Kα radiation). This unit is equipped with a Cu
monochromater, programmable divergence, scatter and receiving slits, and soller slits on both
incident and diffracted beams. Qualitative variable slit data were collected over 7°-65° 2θ, using
a step size of 0.025 and a run time of 1s/step while quantitative fixed slit data were collected
over 20°-140° 2θ, using a step size of 0.03° at a rate of 2s/step. Structure refinements were
performed by the Rietveld method using GSAS software. Several models were tested using
disorder variants of analogous published structures for sulfate ettringite and thaumasite [7,8].
Thermal analyses were gathered using a Perkin Elmer Thermogravimetric Series 7 Analyzer.
TGA scan data, utilizing sample specimens of approximately 100mg, were collected between 25°
and 1200° at 10°/min.
Samples, prepared for scanning electron microscopy imaging, were mounted on aluminum
mounts and coated with gold using a Technics Hummer II sputter coater. Images were obtained
using a JEOL JSM-6300 Scanning Electron Microscope.
IR analyses were made using a Nexus 470 FT-IR Spectrometer equipped with a N2 purge
chamber. Scans were collected at 400-4000 wavenumbers following a 15 minute N2 purge.
RESULTS and DISCUSSION
Qualitative XRD Analysis
Figure 1A) Diffractogram of synthetic carbonate ettringite
Figure 2A) Diffractogram of synthetic sulfite ettringite
Quantitative XRD Analysis
Figure 1B) Rietveld fit to synthetic carbonate ettringite data
Figure 2B) Rietveld fit to synthetic sulfite ettringite data
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Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
197
GSAS, Rietveld Structure Refinements
A comparative study was carried out to determine which starting model, sulfate ettringite or
thaumasite, gave better end results in the structure refinement of several variant models of
carbonate and sulfite ettringite (see figures 3A and 3B). GSAS was used to refine six variant
models of both the carbonate and sulfite ettringite analogs. These variants were refined to
determine which model obtains the best end results when modeling for disorder on all or several
of the channel sites.
S
S
O O
S O
S
O O
S O
O O O
O O O
O O O
S
O O O
O O O
S
C
C
O O O
S
O O
S O
S
O O
S O
O O
S O
O O O
O O O
S
O O O
O O O
S
O O O
O O O
O O O
O O
S O
S
S
S
S
O O O
O O O
O O O
O O O
O O O
O O O
O O O
O O O
SO3Et-2
SO3Et-3
SO3Et-4
SO3Et-5
SO3Et-6
Figure 3a) Sulfite ettringite model variants refined,
including disorder on several channel sites. Only
channel constituents are shown for clarity.
Key:
S
O O O
O O
S O
S
O O O
Sulfite
Sulfite, disordered
O O O
O O O
O O O
Water
Water, disordered
O O O
O CO O
C
C
C
O O O
O O O
O O O
O O O
C
O O O
O O O
C
O O O
C
O O O
C
O O O
C
O O O
O O O
O O O
O O O
O O O
O O O
O O O
O O O
O O O
S
O O O
C
O O O
C
C
O O O
S
O O O
O O O
C
C
O O O
C
C
O O O
O O O
CO3Et-2
CO3Et-3
CO3Et-4
CO3Et-5
C
C
C
C
O O O
CO3Et-6
Figure 3b) Carbonate ettringite model variants
refined, including disorder on several channel sites.
Only channel constituents are shown for clarity.
Key:
C
C
OC
O O
O O O
O O O
Carbonate
CO3, disordered
O O O
O O O
O O O
Water
Water, disordered
The Rietveld results, in the case of carbonate ettringite, show little deviation between the final
Chi2 values of each variant model (see table 1). A slightly improved fit was achieved when
disorder was modeled for all of the carbonate ions or water molecules within the channel above
and below each z = n • (1/4) site (see figure 3b, CO3Et-6). Modeling for the trigonal planar,
position-disordered carbonate groups in the channel sites of carbonate ettringite produced very
little difference in the fits to observed data.
Table 1. Rietveld Structure Refinement Results
Chi 2
Sample
Carbonate Ettringite
CO3Et2
2.672
CO3Et3a
2.768
CO3Et3b
2.762
CO3Et4a
2.730
CO3Et4b
2.724
CO3Et5a
2.729
CO3Et5b
2.693
CO3Et6a
2.587
2.569
CO3Et6b
Sulfite Ettringite
SO3Et2
4.361
SO3Et3a
3.725
SO3Et3b
3.920
SO3Et4a
3.926
SO3Et4b
3.897
SO3Et5a
3.439
3.388
SO3Et5c
SO3Et6a
3.622
SO3Et6c
3.508
Figure 4) SEM photomicrograph of synthetic carbonate ettringite
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
198
The results, in the case of the sulfite ettringite, showed greater deviation between the final Chi2
values of each variant model (see table 1). The best fit was achieved using a model that
contained orientation-disordered sulfite groups in one of the three sulfite sites and positiondisordered water groups in the water site (see figure 3a, SO3Et-5). This seems to be an
appropriate model, explained by the electron repulsion of the apical electron pair of the sulfite
ions in response to their electronic environment.
TGA
120
1
100
0
0
-4
60
-5
50
80
-2
Weight %
-3
70
-1
Derivative Weight Percent
-2
80
Weight Percent
100
-1
60
-3
-4
40
-5
-6
40
20
-7
30
-6
-8
0
200
400
600
800
1000
Derivative Weight %
90
0
1200
-7
0
200
400
Temperature
600
800
1000
1200
Temperature
TGA and DTG thermograms of synthetic
carbonate ettringite
Table 2A) TGA Results
Total mass
Mass
Temperature
loss, %
range, °C
loss
TGA and DTG thermograms of synthetic
sulfite ettringite
Assign
-ment
Table 2B) TGA Results
Total
Mass
Temperature
mass loss
loss, %
range, °C
25-550
47.0
47.0
H2O,
OH
550-800
1.2
48.2
OH
800-925
0.9
49.1
OH (?)
925-1100
2.2
51.3
OH (?)
25-500
46.1
46.1
H2O,
OH
500-725
4.2
50.3
OH
725-860
4.8
55.1
CO2
860-925
6.1
61.2
CO2
925-1100
1.0
62.2
CO2
Assign
-ment
These TGA results suggest that both the carbonate and sulfite synthetic ettringites contained
three water molecules per formula unit in the channel sites, yielding a molecular formula of
Ca6Al2(SO3 or CO3)3(OH)12 • 27H2O. Previous literature reports that ettringite members are
capable of containing between 0 and 6 water molecules in their channels depending on the
method of preparation, the types of anions, and the relative humidity in the laboratory setting [9].
CONCLUSIONS
The synthesized ettringite analogs, sulfite and carbonate ettringite, were found to be phase pure
by XRPD and FTIR analysis with the exception of the trace contaminant calcite and a
measurable amorphous content, in the case of the carbonate ettringite.
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
Rietveld results, in the case of both carbonate and sulfite ettringite, were found to refine to more
appropriate values when using the sulfate ettringite starting model, instead of thaumasite. In the
case of carbonate ettringite, results suggest that the choice of starting model causes a relatively
insignificant difference between the observed patterns of each model. In the case of sulfite
ettringite, all of the models with disordered sulfite groups show a significant improvement in the
observed pattern (fitting) data relative to the fully ordered model. The best fit, observed for the
carbonate ettringite variants, was achieved using a model with disordered carbonate or water
groups above and below each z = n • (1/4) site. The best fit, observed for the sulfite ettringite
variants, was achieved using a model containing orientation-disordered sulfite groups in one of
three sulfite sites and position-disordered water groups on the water site.
TGA results suggest that both the carbonate and sulfite synthetic ettringites contained three water
molecules per formula unit in the channel sites.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
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G. J. McCarthy and J. K. Solem-Tishmack. “Hydration Mineralogy of Cementitious Coal
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R. A. Edge and H. F. W. Taylor. “Crystal Structure of Thaumasite, [Ca3Si(OH)6 •
12H2O](SO4)(CO3),” Acta Cryst., B27 (1971), 594.
H. Pöllmann, H. J. Kuzel, and R. Wenda. “Compounds with Ettringite Structures,” Neues
Jahrbuch Miner. Abh. 160: 133-158; Stuttgart 1989.
J. Bensted. “Thaumasite – Background and Nature in Deterioration of Cements, Mortars,
and Concretes,” Cement and Concrete Research, Volume 21, 117-121, 1999.
D. G. Grier, M.S. Thesis, Department of Chemistry, North Dakota State University, Fargo, ND,
96 pp. (2000).
E. T. Carlson and H. A. Berman, J. Res. NBS 64A, 333-341, 1960.
ACKNOWLEDGMENTS
We would like to thank Scott Payne for collection of SEM images. This work was supported financially
through the following grants: DOE-UCR #DE-FG22-96PC96207 & NSF-EPSCoR #EPS-987480
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