Stability of Monosulfate in the Presence of Iron
Belay Zeleke Dilnesa,
‡,†
Barbara Lothenbach,
‡
Guillaume Renaudin,
§,¶
Adrian Wichser,
‡
and Erich Wieland
k
‡
Laboratory for Concrete & Construction Chemistry, Empa, U
¨
berlandstrasse 129, CH-8600 Du¨bendorf, Switzerland
§
Clermont Universite´, ENSCCF, ICCF, BP 10448, 63000 Clermont-Ferrand, France
¶
CNRS, UMR 6296, ICCF, 63177 Aubie`re, France
k
Paul Scherrer Institute, Nuclear Energy and Safety Department, Laboratory for Waste Management,
5232 Villigen PSI, Switzerland
Monosulfate (3CaO·(Al
x
Fe
1x
)
2
O
3
·CaSO
4
·12H
2
O) is an AFm
phase that can be formed during the hydration of cement.
Fe-containing monosulfate and (Al,Fe) mixed monosulfate were
synthesized and characterized. Fe-monosulfate is composed of
a positively charged main layer [Ca
2
Fe(OH)
6
]
+
and a
negatively charged interlayer [
1
/
2
SO
4
·3H
2
O]
-
, crystallizes in
the trigonal R
3 symmetry, and is isotypic with Al-monosulfate.
The solubility product at 25°C was determined to be 31.57.
The formation of solid solution due to Al-Fe substitution in the
main layer structure of monosulfate was observed. Based on
the evolution of the unit cell parameters and the thermody-
namic investigations, a presence of solid solution from 0.0 to
0.45 Al/(Al + Fe) ratio and a miscibility gap in the range
0.45 < Al/(Al + Fe) ratio <0.95 is suggested.
I. Introduction
T
HE formation of monosulfate (Ca
4
(Al,Fe)
2
(OH)
12
·SO
4
·
6H
2
O) in cement paste has been extensively studied.
1–4
Monosulfate is an AFm (Al
2
O
3
–Fe
2
O
3
-mono) phase that can
be represented as Ca
2
(Al,Fe)(OH)
6
X·nH
2
O, where X denotes
a single charged or half of a double charged anion which
occupies the interlayer sites. Among possible anions are
OH
, SO
4
2
, CO
3
2
, and Cl
for which substitution has
been reported.
3,5–11
Cationic substitution of Al(III) by Fe
(III) in the main layer structure of AFm phases is also
possible.
12–14
The formation of Al-monosulfate (monosulfoaluminate;
Ca
4
Al
2
(OH)
12
SO
4
6H
2
O or C
4
AsH
12
using cementitious
notation
1
) from the hydration of the aluminate phase (C
3
A)
in presence of gypsum, as well as the formation of ettringite
(Al–AFt phase of composition Ca
6
Al
2
(OH)
12
·3SO
4
·26H
2
O),
are mainly reported in the cementitious system. Al-monosul-
fate can also be formed from the reaction of ferrite (2CaO
(Al,Fe)
2
O
3
or C
2
(A,F)). Moreover, the presence of iron in
C
2
(A,F) might result in the formation of Fe-monosulfate
(Ca
4
Fe(OH)
12
·SO
4
·6H
2
O or C
4
FsH
12
using cementitious
notation) or the mixed (Al,Fe)-monosulfate. The formation
of Fe-monosulfate has been reported in aqueous systems
containing only calcium, iron, sulfate, and alkalis; i.e., in the
absence of cements.
13,15
The similarity in the ionic radii of
Al
3+
(0.54 Å) and Fe
3+
(0.65 Å)
16
and the commonly
encountered Al to Fe substitution in octahedral environment
(in clay minerals as example) suppose the formation of (Al,
Fe)-AFm solid solutions. Mo¨schner et al.
17
observed the
formation of a solid solution between Al- and Fe-ettringite
with a miscibility gap between X
Al,total
= 0.3–0.6. In contrast,
no solid solution was found between Al- and Fe-monocar-
bonate due to the different symmetries of Al-monocarbonate
(triclinic)
18,19
and Fe-monocarbonate (rhombohedral)
12
and
the different bonding of carbonate anion (bonded to Ca
2+
in
Al-monocarbonate and located at the center of the interlayer
in Fe-monocarbonate). To which extent Fe and Al form
solid solution in monosulfate is unclear. Kuzel et al.
14
found
that C
4
AsH
12
and C
4
FsH
12
form a continuous solid solution
at 100°C, but at 25°C or 50°C, miscibility is incomplete. At
25°C, C
4
FsH
12
was found to accommodate up to 50% Al
and C
4
AsH
12
up to 10% Fe. Ecker et al.
13
pointed out the
existence a continuous of solid solution at room temperature.
The solubility of Al-monosulfate has been determined.
7,20
For Fe-monosulfate, only a rough estimation of the solubil-
ity was reported from experiments designed to study Fe-
ettringite formation (where the formation of Fe-monosulfate
was observed at high pH values).
15
In addition, the crystal
structure of Fe-monosulfate is poorly understood.
In this study, Fe-containing monosulfate and (Al,Fe)
mixed monosulfate were synthesized and characterized. Their
crystal-chemical and thermodynamics characteristics were
investigated.
II. Materials and Methods
(1) Synthesis of Fe-Containing Phases
3CaO·Al
2
O
3
(C
3
A) and 2CaO·Fe
2
O
3
(C
2
F) clinkers were used
as starting materials for the synthesis. C
3
A and C
2
F were
prepared by mixing appropriate amounts of CaCO
3
with
Al
2
O
3
and Fe
2
O
3
powders and burned at 1400°C and 1350°
C, respectively, for 24 h. The powders were ground to
63 lm. X-ray powder diffraction (XRPD) analysis indicated
that no other solids than C
3
A or C
2
F were present. CaO was
synthesized by burning CaCO
3
at 1000°C.
Fe-monosulfate was synthesized by the addition of appro-
priate amounts of C
2
F, CaSO
4
·2H
2
O, and CaO to 50 mL of
0.4 M KOH solution (pH = 13.6) at a liquid/solid ratio ~20.
The overall stoichometric reaction is given by:
2CaO.Fe
2
O
3
þ CaSO
4
:2H
2
O þ CaO þ 10H
2
O
! 3CaO.Fe
2
O
3
:CaSO
4
:12H
2
O
0.4 M KOH solution was used to mimic the high pH
present in the pore solution of Portland cement in which
H. Jennings—contributing editor
Manuscript No. 31014. Received January 29, 2012; approved June 01, 2012.
†
Author to whom correspondence should be addressed. e-mail: belay.dilnesa@
gmail.com
1
Throughout this article cement short hand notation is used: A, Al
2
O
3
; C, CaO; c,
CO
2
; F, Fe
2
O
3
; H, H
2
O, s, SO
3.
1
J. Am. Ceram. Soc., 1–12 (2012)
DOI: 10.1111/j.1551-2916.2012.05335.x
© 2012 The American Ceramic Society
J
ournal