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Current Inorganic Chemistry, 2015, 5, 00-00
1
Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates
Guillaume Renaudin1,2,*,#, Adel Mesbah1,3, Belay Zeleke Dilnesa4, Michel Francois5 and
Barbara Lothenbach4
1
Clermont Université, ENSCCF, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, France; 2CNRS, UMR 6296, ICCF, F-63171 Aubière, France; 3ICSM, UMR5257
CEA/CNRS/UM2/ENSCM, Site de Marcoule, Bât. 426, BP 17171, F-30207 Bagnol/Cèze cedex,
France; 4Empa, Laboratory for Concrete & Construction Chemistry, Überlandstrasse 129, CH-8600
Dübendorf, Switzerland; 5Université de Lorraine, Institut Jean Lamour – UMR 7198, BP 70239, 54506
Vandœuvre-lès-Nancy Cedex, France
Abstract: The crystal structure of the three main Fe-containing AFm phases (Al2O3-Fe2O3-mono:
family of lamellar calcium alumina-ferrite hydrates) encountered in cement hydration process are
characterized and compared with their Al-analogs. This includes AFm phases containing sulfate
(which is present in Portland cement to regulate the hydration kinetic), carbonate (which is present in Portland cements, or
originates from atmospheric carbon oxide) and chloride (either from the water used or from the environment). The results
show that Fe-AFm and Al-AFm compounds are not (or rarely) isostructural. Iron in AFm phases does not simply substitute aluminium. Fe-carbonate has a rhombohedral symmetry whereas Al-carbonate has a triclinic symmetry, with carbonate anions located in different crystallographic sites in both compounds. Fe-Friedel’s salt corresponds to a 3R polytype
whereas Al-Friedel’s salt corresponds to a 6R polytype. Both compounds have a temperature dependent transition with
two different HT- and two different LT-polymorphs descriptions (HT: high-temperature, LT: low-temperature). Only Fesulfate and Al-sulfate are isostructural. Despite this isostructural feature, only limited solid solutions have been observed
between both sulfate end-members. In a general way, this system (when considering sulfate, carbonate and chloride with
aluminium and iron) leads to extremely complicated subsystems with limited solid solutions. The crystallographic studies
and comparisons developed here have been fully completed by thermodynamic characterisations in order to make possible
thermodynamic modelling of the hydrates assemblage during the hydration process and the aging of Portland concrete.
Keywords: Cement hydrates, crystal structure, iron, solid solution, X-ray diffraction.
1. INTRODUCTION
The hydrates formed during the hydration of Portland
cement are mainly C-S-H (calcium silicate hydrate), portlandite (Ca(OH)2), ettringite (the AFt, Al2O3-Fe2O3-tri,
family with composition Ca6Al2(OH)6·(SO4)3·26H2O) and
lamellar AFm (the Al2O3-Fe2O3-mono family: lamellar calcium alumina-ferrite hydrates) phases. AFm hydrates
are obtained from the C3A (3CaO·Al2O3) and C2(A,F)
(‘4CaO·Al2O3·Fe2O3’) – the two calcium aluminate anhydrous compounds present in clinker – hydration process in
the presence of sulfate, carbonate, chloride and hydroxide.
The general formula of AFm phases is Ca2Al(OH)6·X·nH2O.
X represents a single charged or half a double charged anion
located in the interlayer region of these lamellar hydrates.
The layered structure of AFm compounds is based on two
layers; a positively charged rigid main layer [Ca2Al(OH)6] +
and a negatively charged [X·nH2O]- interlayer corresponding
to the hydrocalumite type structure (an ordered modification
*Address correspondence to this author at the ENSCCF, Institut de Chimie
de Clermont-Ferrand, Clermont-Ferrand, France; Tel: 04-73-40-73-36; Fax:
04-73-40-73-33; E-mail: [email protected]
#
Present address: ICSM, UMR5257 CEA/CNRS/UM2/ENSCM, Site de
Marcoule, Bât. 426, BP 17171, F-30207 Bagnol/Cèze cedex, France
1877-9441/15 $58.00+.00
from the large LDH family described by the hydrotalcite
type structure in which bivalent and trivalent cations are disordered in a unique crystallographic site). Different anions
and number of water molecules can be present in the
interlayer region depending on the relative humidity and
temperature. The most important anions in cement chemistry
are OH- (pH around 12.5), SO42- (presence of gypsum to
regulate the cement hydration kinetic), CO32- (from atmospheric contamination, cement addition or environment) and
Cl- (from water or environment). The main layers are composed of Ca(OH)6 octahedra (portlandite type) with every
third Ca2+ substituted by Al3+ and/or Fe3+ in an ordered way.
Crystal chemistry, solubility products and thermodynamic
data are well-known for the Al-containing AFm hydrates as
comprehensive experimental studies were carried in the past
[1-3]. Due to the low Fe-content in Portland cement (between 2 and 5 weight percent of iron oxide, mainly present
in C2(A,F)) the first series of experimental data on Fecontaining AFm hydrates have been determined quite recently [4-8]. Thermodynamic studies on Fe-monosulfate [6],
Fe-monocarbonate [7] and Fe-hydrogarnet [8] have been
published last years. The fate of Fe during the hydration of
cement depends on the cement. In the absence of other
clinker phases, C2(A,F) reacts to form Fe-containing ettrin© 2015 Bentham Science Publishers
2 Current Inorganic Chemistry, 2015, Vol. 5, No. 2
gite, monosulfate or monocarbonate, depending on the presence of calcium, sulfate or carbonate. In many studies, the
presence of amorphous iron hydroxide and the solid solution
formation with their Al-containing analogous were assumed
[9-14]. The almost equivalent ionic radii in octahedral environment of Al3+ (0.55 Å) and Fe3+ (0.65 Å) [15] and the
well-known octahedral Al to Fe substitution in clays minerals have suggested the (Al,Fe)-AFm solids solutions formation. Whether and to what extent the Fe-containing phases
and their solid solutions formed in PC (Portland cement) is
presently poorly understood. Investigations on the fate of Fe
in cementitious materials are complicated by the fact that
identification of the Fe-containing minerals in hydrated cement using standard techniques, such as X-ray diffraction
(XRD), thermogravimetric analysis (TGA) and Scanning
Electron Microscopy (SEM), is difficult due to significant
overlap of the signals from the Fe-containing phases with
those of the respective Al analogs. Further, detection of
amorphous Fe-containing phases in hydrated cement is difficult using standard techniques. Nowadays the development
of new cement formulations [16,17] enhances the necessity
to have a full and detailed understanding of each element
that acts during cement hydration. The present study exposes
recent structural investigations of the main Fe-based AFm
phases. Crystallographic data on Al- and Fe-based compounds show that solids solutions are highly limited for carbonate, sulfate and also chloride compounds. Despite similar
crystal chemistry for all AFm phases, the existence of different space groups clearly evidences the absence of complete
solids solutions. This has already been described for the different anions observed in Al-based AFm hydrates: R3 for
monosulfoaluminate [18], R3c for hemicarboaluminate
[19], P1 or P1 for monocarboaluminate [20,21], C2/c for
Friedel’s salt [22], P3c1 for nitrate [23]. The comparison of
the crystal structures of Al-based AFm compounds with that
of Fe-based AFm compounds can indicate whether the formation of solids solutions based on Al to Fe substitution are
possible. In the present paper the three most important anions in cement chemistry have been considered; i.e. sulfate,
carbonate and chloride. In the complex cement matrix, the
amount of Fe incorporated in AFm phases will influence the
AFt/AFm ratio and thus the hydrates volume and finally the
hydrated cement properties. A complete understanding of the
characteristics of the hydrates is important since the material
properties of cement-based materials are related to the
chemical environment and properties of the hydrated phases.
Renaudin et al.
with a liquid over solid ratio ~20. CaO was prepared by
heating CaCO3 at 1000°C. Ca2Fe2O5 was prepared as
previously described. More details are given in thermodynamic study of the compound [7].
-
PE-bottles at different temperatures (20, 50 and 80°C)
were used to store the samples. Ageing effect (or reaction
time) has been considered to improve the crystallinity of the
powder. The solid and liquid phases were separated by vacuum filtration through 0.45 μm nylon filter after equilibration. All manipulations were done in a nitrogen filled glove
box to minimize atmospheric CO2 contamination.
2.2. Powder X-ray Diffraction
-
Fe-monocarbonate Ca2Fe(OH)6·(CO3)·nH2O and FeFriedel’s salt Ca2Fe(OH)6·Cl·nH2O: synchrotron powder
diffraction data were collected at Swiss-Norwegian Beam
Line (SNBL) at the European Synchrotron Radiation
Facility (ESRF), Grenoble, France. The powder material
was introduced into glass capillaries (0.5 mm diameter).
Data collection was performed at 295 K at a wavelength
of = 0.72085 Å using a MAR345 image plate detector
with the highest resolution (34503450 pixels with a
pixel size of 100 μm). The calculated absorption coefficient mμR (m = powder packing factor, μ = linear absorption coefficient, R = radius of the capillary) was estimated at 0.65. Three sample-to-detector distances were
used (150, 250 and 350 mm) in order to combine the advantages of high resolution and extended 2 range. The
detector parameters and the wavelength were calibrated
with NIST LaB6. The exposure time was 60 s with a rotation of the capillary by 60°. The two-dimensional data
were integrated with the Fit2D program which produced
the correct intensity in relative scale. This 2D detector
was used in order to perfectly define the background, to
observe very weak diffraction peaks, and to improve the
accuracy of the integrated intensities by achieving a better powder average. Uncertainties of the integrated intensities were calculated at each 2-point applying Poisson
statistics to the intensity data, considering the geometry
of the detector. The Instrument Resolution Function was
determined from the LaB6 data.
-
Fe-monosulfate Ca2Fe(OH)6·(SO4)·nH2O: X-ray powder
diffraction measurements were carried out using CuK
radiation on a PANalytical X’Pert Pro MPD diffractometer in a –2 configuration at room temperature in the interval 3° < 2 < 120°, with a step size of 2 = 0.0167°
and a counting time of 500 s for each data value. A total
counting time of ~8 h was used for each sample.
-
Structure
transition
of
Fe-Friedel
salt’s
Ca2Fe(OH)6·Cl·nH2O: low temperature measurements
2. EXPERIMENTAL METHOD
2.1. Syntheses
-
-
Synthesis of Fe-monosulfate Ca2Fe(OH)6·(SO4)·nH2O:
addition of the appropriate amount of Ca2Fe2O5 (C2F),
gypsum and CaO to 50 mL of 0.4 M KOH solution with
a liquid over solid ratio ~20. CaO was prepared by heating CaCO3 at 1000°C. Ca2Fe2O5 was prepared by mixing
CaCO3 with Fe2O3 and burning at 1400°C and 1350°C
for 24 h respectively. More details are given in thermodynamic study of the compound [6].
Synthesis of Fe-monocarbonate Ca2Fe(OH)6·(CO3)·
nH2O: addition of the appropriate amount of Ca2Fe2O5
(C2F), calcite and CaO to 50 mL of 0.1 M KOH solution
Synthesis of Fe-Friedel’s salt Ca2Fe(OH)6·Cl·nH2O: addition of the appropriate amount of reactants at a liquid/solid ratio ~20. Three different synthesis protocols
have been used: a) FeCl3.6H2O and CaO in 0.1M KOH
(pH = 11.94), b) C2F, CaCl2.2H2O and CaO in distilled
water (pH = 12.39) and c) C2F, CaCl2.2H2O, and CaO in
0.1 M KOH (pH = 12.84). More details are given in [4].
CaO was prepared by heating CaCO3 at 1000°C.
Ca2Fe2O5 was prepared as previously described.
Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates
were realized in order to investigate the structure transition of the Fe-Friedel’s salt compound. A series of diffraction patterns was recorded in the temperature range
+25 °C to -50°C at interval of -5°C and in the range and 60°C – -130°C at interval of -10°C, using X-Pert Pro diffractometer (Panalytical) with Bragg-Brentano geometry,
a Ni filtered CuK radiation, a Multistrip Detector Xcelerator (Panalytical) and a TTK 450 HT chamber (Anton
Paar). The data are recorded between 10-50° (2) in step
of 0.0167° (2) and a measuring time of 21 seconds per
step.
3. RESULTS
Crystal structure description of Al-based AFm hydrates
have been extensively studied, and are today well described
for sulfate, carbonate, chloride and nitrate anions [18-22].
Boron-containing AFm-phase has also been recently described [24]. Table 1 gathered crystallographic data of the
mains Al-AFm phases. Solid solutions in these Al-AFm systems have also been extensively investigated. Contrary to
former expectations of a unique crystallographic description
with extensive anionic exchange, recent results indicated that
solids solutions are limited in the sulfate-carbonate-chloride
systems [25-27] due to the different anion-dependent space
groups encountered.
The crystal structure of monosulfoaluminate was first
solved by Allmann in 1977 in the trigonal R3 space group
[18]. This description, with the sulfate anion in the center of
the interlayer region linked to main layer via hydrogen bonding, was enlarged for the whole AFm family during several
decades. During the years 2000-2010, crystallographic studies have shown that each anion has its own environment in
the interlayer region leading to distinct space groups (from
triclinic to rhombohedral description) [20-23]. Ten years
after, studies on bi-anionic compounds have allowed to obtain much more crystallographic data of AFm compounds
[25,26]. These studies have shown restricted solids solutions
in cement system between sulfate, carbonate and chloride
[27]. For a long time, aluminum to iron substitution was
simply assumed [28]. Recent studies allowed exploring the
miscibility between Al3+ and Fe3+ in these systems [4]. The
obtained results allow now investigating the formation of
solids solution in the (Al,Fe)-AFm system taking into account crystallographic considerations.
3.1. The Ca2(Al,Fe)(OH)6·(CO3)·nH2O System: Monocarboaluminate Versus Monocarboferrate
The monocarboferrate (Fe-Mc) sample was prepared at
room temperature. The kinetic of reaction of Ca2Fe2O5 ferrite
phase was slow and 3 years equilibration time was necessary
to obtain Fe-Mc only with some traces of calcite (i.e. to observe the disappearance of Fe-hemicarbonate). Higher temperature (50°C and 80°C) increased the kinetic of formation
but destabilized Fe-Mc to portlandite and hematite [7]. High
quality diffraction data, using synchrotron measurement, was
used to solve and refine the structure of Fe-Mc. The crystal
data and multi-pattern refinement (using data from the different samples-to-detector distances) parameters have been
previously published with the thermodynamic study [7].
Monocarboaluminate (Al-Mc) has a triclinic symmetry while
Current Inorganic Chemistry, 2015, Vol. 5, No. 2
3
Fe-Mc
exhibits
a
trigonal
symmetry,
and
3CaO·Al2O3·CaCO3·11H2O contains less water than
3CaO·Fe2O3·CaCO3·12H2O. The main difference is in the
carbonate anion environment: CO32- is directly linked to
main layer in Al-Mc (connected to Ca2+), whereas CO32- is
located at the center of the interlayer in Fe-Mc (with hydrogen bonding only). Fig. (1) illustrates these two distinct crystallographic situations. The general representation (Fig. 1,
top) show the layered structure usually encountered for AFm
phases with the main layer composed of sevenfold coordinated Ca2+ together with six-fold coordinated trivalent
cations (Al3+ or Fe3+). The [Ca2Al(OH)6]+ and [Ca2Fe(OH)6] +
main layers are similar. The Ca2+ cations are shifted out of
the main layer central plane (defined by Al3+ cations) to a
quite extent: ± 0.57 Å for Al-Mc (average value) [20] and ±
0.54 Å for Fe-Mc. The 0.03 Å difference can be attributed to
the different carbonate connection mode or to the higher
ionic radius of Fe3+ (0.65 Å) versus 0.55 Å for Al3+ [15],
which induces a smaller distortion in the ordered rigid main
layer due to smaller Ca2+/Fe3+ radius ratio. The interlayer
region contains anionic species and water molecules. The
slightly larger ionic radii of Fe3+ (compared to Al3+) enlarges
the basal a lattice parameter (considering the hexagonal lattice), which is close to 5.92 Å for Fe-Mc (Table 2) compared
to ~5.76 Å for the hexagonal lattice of Al-based AFm phases
(Table 1). Carbonate anions are directly connected to the
main layer in Al-Mc with an ionic bonding between one
oxygen atom belonging to the carbonate and to a Ca2+ cation.
The case of Fe-Mc is totally different with CO32- located at
the center of the interlayer connected to main layer by a network of hydrogen bonds only. Details of the structure around
carbonate anions (Fig. 1, bottom) illustrate the two different
carbonate environments. For Fe-Mc, each carbonate anion is
linked to six Ca-polyhedron via six hydrogen bonds between
oxygen atoms belonging to carbonate and water molecule
linked to Ca2+. Each oxygen atom from the carbonate group
shares two hydrogen bonds with the two adjacent main layers. The structure of Al-Mc is different; the carbonate groups
are also connected by three hydrogen bonds with one main
layer, but with an ionic bond (see the red bond in Fig. 1, bottom left) and a hydrogen bond with the octahedrally coordinated Al3+ cation with the adjacent main layer. In addition,
the carbonate groups share also hydrogen bonds with water
molecules in the interlayer region. It is surprising to observe
such a different carbonate environment when exchanging
Al3+ cation by Fe3+. Previous study on the (Al,Fe)-Mc system
has clearly evidenced the absence of iron to aluminum substitution in Al-Mc, as well as the absence of aluminum to
iron substitution in Fe-Mc [7]. The Al- and Fe-end members
have different symmetry with different environment for carbonate anion, such that not even a limited solid solution occurs in this system, contrary to previous reports [29]. As already reported for Al-AFm phases, Raman spectroscopy can
easily distinguish between carbonate bonded to main layer
via ionic bonding to Ca2+ (symmetry stretching mode at 1068
cm-1) and carbonate located at the center of interlayer bonded
with hydrogen bonds only (symmetric stretching mode at
1086 cm-1) [26]. Raman spectra realized on Fe-Mc, in
agreement with structural description, have shown the carbonate symmetric stretching at 1085 cm-1 corresponding to
hydrogen bonding only [7].
4 Current Inorganic Chemistry, 2015, Vol. 5, No. 2
Table 1.
Renaudin et al.
Crystallographic data of Al-AFm phases. The Z number is calculated assuming the motif [Ca2Al(OH)6]·[X·nH2O].
Compound name
Chemical composition
Symmetry, space group
Z
Lattice parameters
Unit cell volume
Year,
Reference
Monosulfoaluminate
3CaO·Al2O3 ·CaSO 4·12H2O
Trigonal, R3
3
a = 5.7586 (3) Å,
b = 26.7946 (12) Å,
V = 769.51 (1) Å3.
1977,
Allmann [18]
Triclinic, P1
2
a = 5.775 (1) Å,
b = 8.469 (1) Å,
c = 9.923 (3) Å,
= 64.77 (2) °,
= 82.75 (2) °,
= 81.43 (2) °,
V = 433.0 (2) Å3.
1998,
François et al. [20]
Disordered monocarboaluminate
D-3CaO·Al2O3 ·CaCO 3·11H2O
Triclinic, P1
2
a = 5.7422 (4) Å,
b = 5.7444 (4) Å,
c = 15.091 (3) Å,
= 92.29 (1) °,
= 87.45 (1) °,
= 119.547 (7) °,
V = 432.5 (41) Å3.
1999,
Renaudin et al. [21]
AFm-(NO3-)
3CaO·Al2O3 ·Ca(NO3) 2·10H2O
Trigonal, P3c1
2
a = 5.7445 (8) Å,
b = 17.235 (5) Å,
V = 492.55 (16) Å3.
1999,
Renaudin et al. [23]
a = 9.960 (4) Å,
b = 5.7320 (2) Å,
c = 16.268 (7) Å,
= 104.471 (2) °,
V = 432.5 (41) Å3.
2002,
Rapin et al. [22]
Ordered monocarboaluminate
O-3CaO·Al2O3 ·CaCO 3·11H2O
Friedel’s salt
3CaO·Al2O3 ·CaCl2·10H2O
Monoclinic, C2/c
2
AFm-(Cl-,CO3 2-)
3CaO·Al2O3 ·CaCO3 ·CaCl2·11H2O
Trigonal, R3c
6
a = 5.7557 (1) Å,
b = 46.947 (1) Å,
V = 1346.90 (5) Å3.
2011,
Mesbah et al. [26]
Kuzel’s salt
3CaO·Al2O3 ·CaSO 4·CaCl2·11H2O
Trigonal, R3
6
a = 5.7508 (2) Å,
b = 50.418 (3) Å,
V = 1444.04 (11) Å3.
2011,
Mesbah et al. [25]
Hemicarboaluminate
3CaO·Al2O3 ·CaCO3 ·Ca(OH)2 ·10H2O
Trigonal, R3c
6
a = 5.77570 (10) Å,
b = 48.812 (2) Å,
V = 1410.15 (8) Å3.
2012,
Runcevski et al. [19]
3.2. The Ca2(Al,Fe)(OH)6·(SO4)·nH2O System: Monosulfoaluminate Versus Monosulfoferrate
The kinetic of monosulfoferrate (Fe-Ms) precipitation increases with temperature. Both at 50°C and at 80°C Fe-Ms is
formed within 7 days, but at 80°C Fe-Ms decomposes to
portlandite and hematite after 28 days. Crystal structure investigations were performed on two well-crystalline samples: one synthesized at room temperature for two years and
one synthesized at 50°C for one year [6]. The crystal structure of Fe-Ms is isostructural to monosulfoaluminate (AlMs); the first AFm crystal structure solved by Allmann [18]
in 1977. The representation of its structure in Fig. 2a is valid
for both Fe-Ms and Al-Ms compounds. The structure is
trigonal, described in the R3 space group (Table 2), with
two kinds of disorder around the anionic sulfate group: i)
statistical disorder between one sulfate and three water
molecules and ii) orientation up/down disorder of the sulfate
anion. In agreement with the larger ionic radii of Fe3+, the a
basal lattice parameter increases from Al-Ms (5.76 Å) to FeMs (5.89 Å). In contrast the interlayer distance (or the hexagonal c lattice parameter) decreases from 8.932 Å for AlMs to 8.875 Å for Fe-Ms, although the same amount of water has been observed in the interlayer region:
3CaO·Al2O3·CaSO4·12H2O and 3CaO·Fe2O3·CaSO4·12H2O.
The decrease of interlayer distance indicates stronger hydrogen bonding around SO42- in the case of Fe-Mc. The
strengthening of hydrogen bond network around sulfate
group (greys dotted links in Fig. 2a) in Fe-Ms is illustrated
by Raman spectroscopy (Fig. 2b) by the shift of the [SO4] 1
mode toward higher Raman shift (992 cm-1 for Fe-Ms, compared to 982 cm-1 for Al-Ms) and a smaller Raman shift for
the intense hydrogen bonds vibration (3640 cm-1 for Fe-Ms,
compared to 3688 cm-1 for Al-Ms). Despite the same crystal
Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates
Current Inorganic Chemistry, 2015, Vol. 5, No. 2
5
Fig. (1). General representations (Top) of the crystal structure of Al-Mc (left) and Fe-Mc (right), and details showing the carbonate anion
environment (bottom).
Fig. (2). Representation of the crystal structure of Fe-Ms (a) and parts of the Raman spectra for Al-Ms and Fe-Ms (b). Hydrogen bonds are
represented by grey dotted links.
structure of both end members, a miscibility gap has been
evidenced in the range 0.45 < Al/(Al+Fe) < 0.95 [6], again
contrary to previous findings indicating an ideal solid solution [30]. Aluminum to iron substitution is only observed in
Fe-Ms with the following general formulae [Ca2(Fe1xAlx)(OH)6]·[(SO4)·3H2O] with 0 x < and only a very
limited iron to aluminium substitution is possible for the AlMs end member. The origin of the existence of the miscibility gap is difficult to explain as both solids present the same
crystallographic structure. Whatever, such observation
clearly indicates that iron cannot simply be considered as an
aluminium substitution in cement hydrates.
3.3. The Ca2(Al,Fe)(OH)6·Cl·nH2O System: Al-Friedel’s
Salt Versus Fe-Friedel’s Salt
Fe-Friedel’s salt started to form from C2F after 7 days
equilibration time. Portlandite and Fe-hydroxide coprecipitate with Fe-Friedel’s salt as observed for the two
previous Fe-AFm phases. The presence of portlandite and
calcite is detected on powder diffraction patterns after 3
years of equilibration time. The sharp peaks of the FeFriedel’s salt indicate the presence of well crystalline solid.
Fig. (3a) shows that the XRD patterns of Fe-Friedel’s salt
synthesized at different pH values (pH = 11.94, 12.39 and
6 Current Inorganic Chemistry, 2015, Vol. 5, No. 2
Table 2.
Renaudin et al.
Crystallographic data of Fe-AFm phases. The Z number is calculated assuming the motif: [Ca2Fe(OH)6]·[X·nH2O].
Compound name
Chemical composition
Symmetry, space group
Z
Lattice parameters
Unit cell volume
Year,
Reference
Fe-sulfate
3CaO·Fe2O3 ·CaSO 4·12H2O
Trigonal, R3
3
a = 5.8864 (2) Å,
c = 26.614 (2) Å,
V = 798.62 (6) Å3.
2012,
Dilnesa et al. [6]
Fe-carbonate
3CaO·Fe2O3 ·CaCO3 ·12H2O
Trigonal, R3c
6
a = 5.9196 (1) Å,
c = 47.8796 (10) Å,
V = 1453.01 (4) Å3.
2011,
Dilnesa et al. [7]
Fe-Friedel’s salt
3CaO·Fe2O3 ·CaCl2·10H2O
Trigonal, R3
3
a = 5.8262 (5) Å,
c = 23.417 (1) Å,
V = 688.4 (1) Å3.
2002,
Rousselot et al. [31]
Fe-Friedel’s salt
3CaO·Fe2O3 ·CaCl2·10H2O
Trigonal, R3
3
a = 5.9000 (3) Å,
c = 23.740 (6) Å,
V = 715.7 (3) Å3.
2001,
Rapin thesis [32]
Fig. (3). a: X-ray powder pattern ( = 1.5418 Å) from Fe-Friedel’s salt synthesized at different pH values, b: Representation of the crystal
structure of Fe-Friedel’s salt, and c: Rietveld plot ( = 0.697751 Å) together with Raman spectrum showing the carbonate contamination.
Hydrogen bonds are represented by grey dotted links.
12.84) are very similar. At higher pH values, however, a
slight peak shift of the main diffraction peak towards higher
2 values is observed, which could indicate the uptake of
additional hydroxide – or carbonate contamination – in the
interlayer of Fe-Friedel’s salt. The solid synthesized at pH =
12.4 equilibrated for 500 days was used for the crystallographic investigation. Multipattern Rietveld refinement was
performed (see Rietveld plot in Fig. 3c) to solve the structure
of Fe-Friedel’s salt. In agreement with previous studies, the
crystal structure of Fe-Friedel’s salt was described in the
rhombohedral R3 space group [31,32] with refined lattice
parameters a = 5.8567 (2) Å and c = 23.314 (1) Å (V =
692.57 (5) Å3). A general representation of the structure is
shown in Fig. (3b), the structure parameters are gathered in
Table 3. The Fe-Friedel’s salt is composed of a positively
charged main layer [Ca2Fe(OH)6]+ with a negatively charged
interlayer [Cl·2H2O]- corresponding to the refined composi-
tion 3CaO·Fe2O3·CaCl2·10H2O (similarly to the well-known
Al-Friedel’s salt). The large Biso value observed for the
interlayer species (chloride and water molecules) can be explained either by dynamical disorder within the interlayer
region or by carbonate contamination which imply carbonate
to chloride substitution in the 3b site. The carbonate contamination has been evidenced by Raman spectroscopy (Fig.
3c top) with the [CO3] symmetric stretching observed at
1088 cm-1 corresponding to carbonate anion located at the
center of the interlayer region [26].
The structure of Al-Friedel’s salt has been investigated
by Terzis et al. [33] with the monoclinic C2/c symmetry, and
revisited by Rapin et al. [22,34] with the same symmetry.
This monoclinic description corresponds to the low temperature LT-polymorph of Friedel’s salt which transforms to the
rhombohedral R3c HT-polymorph above 37 °C [22,34].
The temperature of transition is dependent on the carbonate
Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates
Table 3.
Current Inorganic Chemistry, 2015, Vol. 5, No. 2
7
Refined structure parameters of Fe-Friedel’s salt (standards deviation are indicated in parentheses): a = 5.8567 (2) Å and
c = 23.314 (1) Å (V = 692.57 (5) Å3).
Atom
Wyckoff
x
y
z
Biso (Å3 )
Occupancy
Ca
6c
2/3
1/3
0.0255 (2)
1.2 (1)
1
Fe
3a
0
0
0
1.2 (1)
1
O (OH)
18f
0.270 (1)
-0.053 (1)
0.0445 (3)
0.9 (2)
1
Cl
3b
0
0
1/2
7.0 (4)
1
O (H2O)
6c
2/3
1/3
0.1355 (5)
4.0 (4)
1
Fig. (4). In-situ low temperature PXRD measurement from +25°C down to -130°C on Fe-Friedel’s salt sample ( = 1.5418 Å).
contamination; the transition temperature decreases if carbonate substitutes chloride in the interlayer [35]. Small carbonate contamination during synthesis (from atmospheric
CO2) is sufficient to observe at room temperature the HTrhombohedral polymorph. The co-precipitation method to
prepare Al-Friedel’s salt resulted in a new R3 description
(instead of R3c ) [31]. These two polytypes, labeled 3R and
6R for the R3 and the R3c descriptions, have also been
observed for the bromide analog [36]. Whereas the coprecipitation method was not use here to prepare FeFriedel’s salt, its structure corresponds to the 3R HTpolytype.
Low temperature in-situ XRD measurements have been
performed (between +25°C and -130°C) in order to complete
the comparison between Fe- and Al-Friedel’s salt. Fig. (4)
shows the 24 temperature dependent powder patterns. At
room temperature the sample is mainly composed of Fe-
Friedel’s salt (about 90 wt %) with impurities of calcite and
portlandite. Below -45°C ice is visible in the powder pattern,
which becomes more and more important down to -130°C.
The ice formation is due to an anomalous leak in the low
temperature chamber, nevertheless did not perturb our sample characterization. Careful analysis of powder patterns
shown in Fig. (4) evidences small modifications during the
temperature decrease; especially diffraction peaks at about
38.5° and 42.5° showing enlargement and shift when the
temperature decreases below -15°C. The details shown in
Fig. (5) illustrate the modification appearing at around 15°C: a shift towards lower angles together with a broadening of the diffraction peaks during cooling. The transition is
not well described by the appearance of new superstructure
diffraction peaks. Nevertheless the sharp shift toward low
angles, indicating the increase of unit cell volume during
cooling, can be explained only by a structural transition. A
8 Current Inorganic Chemistry, 2015, Vol. 5, No. 2
Renaudin et al.
Fig. (5). Details on the in-situ low temperature PXRD measurement from +25°C down to -130°C showing the evolution of the intense diffraction peaks (003), (006) and (104) of Fe-Friedel’s salt ( = 1.5418 Å). The bold line corresponds to the powder pattern recorded at -15°C,
where the transition is observed during cooling.
Fig. (6). Temperature dependences of the (pseudo-)hexagonal a (top left) and c (top right) lattice parameters, the corresponding unit cell volume (bottom left) and the interlayer distance (bottom left).
Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates
DSC measurement on the sample, not shown here, has
shown an endothermic signal at around -15°C. Rietveld refinements of the lattice parameters, assuming a continuous
hexagonal lattice, were performed to follow their temperature dependence (Fig. 6). The two curves illustrating the
changes during heating and cooling show the reversible feature of the transition. The c lattice parameter clearly increases during cooling at around -15°C leading to an increase of ~0.06 Å of the interlayer distance around the temperature of transition. The evolution of the a lattice parameter is not so distinct, although a small increase is also observed during cooling with an incident around -20°C. These
observations on the lattice show a structural transition at
around -15°C, although the resolution of the powder patterns
did not allow elucidating the symmetry of the low temperature Fe-Friedel’s salt polymorph. By comparison with observation
made
on
the
iodide
AFm
compound
[Ca2Al(OH)6]·[I·2H2O] [36], we assumed for Fe-Friedel’s
salt a triclinic LT-polymorph. Previous crystallographic studies performed on a series of halide Al-AFm (Cl-, Br- and I-)
have shown that all R3c HT-polymorphs (6R) present a
monoclinic C2/c LT-polymorph, whereas the R3 HTpolymorph (3R) of the iodide compound presents a triclinic
LT-polymorph (structure not solved yet). High quality low
temperature synchrotron measurements are needed to pursue
the crystallographic study of this triclinic LT-polymorph.
Present results on Fe-Friedel’s salt show that the temperature
dependent phase transition, well known for Al-Friedel’s salt,
remains when iron substitutes aluminum. This phase transition corresponds to an order-disorder transition of the hydrogen bond network around the halide anion [36]. Nevertheless
the reason of the existence of two couples of polymorphs
( R3c that transforms in C2/c, and R3 that transforms in
P 1 ) is not yet clear; these two couples of polymorphs
seems to be synthesis dependent, halide anion dependent,
and also trivalent cation (Al3+/Fe3+) dependent.
Current Inorganic Chemistry, 2015, Vol. 5, No. 2
mined the hydrogen bond network and the cohesion between
the main layers; i.e. the interlayer region commands the crystal symmetry (from high rhombohedral symmetry to low
triclinic symmetry). Present observations on Fe-AFm compounds show now that the nature of the trivalent cation,
which is contained at the center of the rigid main layer, has
also an impact on the crystal symmetry of the compound.
This is certainly also due to interaction with the hydrogen
bond network, but is less evident to understand as the size
difference between Al3+ and Fe3+ cations is relatively small.
The present paper shows the complexity of the crystallographic description of the AFm phases. All the phases discussed here could be present during the hydration of Portland
cements as well as various solid solutions. This complexity
has an impact not only on the hydration process, but also on
the relative fraction of AFm and AFt phases and thus on the
final properties of the concrete. This indicates the importance
to well understand such structural differences to correctly
predict the hydrates assemblage of concrete over time.
Thermodynamic calculations of such complicated systems
need reliable thermodynamic data of the different hydrates
formed [4] based on good description and understanding of
the different structures.
CONFLICT OF INTEREST
The authors confirm that this article content has no conflict of interest.
ACKNOWLEDGEMENTS
The authors thank SNBL-ESRF (Yaroslav Filinchuk) for
the in-house beam time allocation used for the characterization of Fe-carbonate and Fe-Friedel’s salt (HT-polymorph).
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CONCLUSION
The present paper illustrates the crystal chemical complexity of the AFm family (hydrocalumite type structure).
Behind the apparent simplicity of the main structural features
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shown that the interlayer region is not at all that flexible. The
interlayer description depends on the anion which deter-
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Revised: January 15, 2015
Accepted: January 16, 2015