Calcium Sulfoaluminate Sodalite (Ca
4
Al
6
O
12
SO
4
) Crystal Structure Evaluation
and Bulk Modulus Determination
Craig W. Hargis,
§,
*
,†
Juhyuk Moon,
§,‡
Barbara Lothenbach,
¶
Frank Winnefeld,
¶
Hans-Rudolf Wenk,
k
and Paulo J. M. Monteiro
§
§
Civil and Environmental Engineering, University of California at Berkeley, Berkeley, California 94720
¶
Empa, Laboratory for Concrete and Construction Chemistry, D€ ubendorf 8600, Switzerland
k
Earth and Planetary Science, University of California at Berkeley, Berkeley, California 94720
The predominant phase of calcium sulfoaluminate cement,
Ca
4
(Al
6
O
12
)SO
4
, was investigated using high-pressure synchro-
tron X-ray diffraction from ambient pressure to 4.75 GPa.
A critical review of the crystal structure of Ca
4
(Al
6
O
12
)SO
4
is
presented. Rietveld refinements showed the orthorhombic crys-
tal structure to best match the observed peak intensities and
positions for pure Ca
4
(Al
6
O
12
)SO
4
. The compressibility of
Ca
4
(Al
6
O
12
)SO
4
was studied using cubic, orthorhombic, and
tetragonal crystal structures due to the lack of consensus on
the actual space group, and all three models provided similar
results of 69(6) GPa. With its divalent cage ions, the bulk
modulus of Ca
4
(Al
6
O
12
)SO
4
is higher than other sodalites with
monovalent cage ions, such as Na
8
(AlSiO
4
)
6
Cl
2
or Na
8
(Al-
SiO
4
)
6
(OH)
2
H
2
O. Likewise, comparing this study to previous
ones shows the lattice compressibility of aluminate sodalites
decreases with increasing size of the caged ions. Ca
4
(Al
6
O
12
)
SO
4
is more compressible than other cement clinker phases
such as tricalcium aluminate and less compressible than
hydrated cement phases such as ettringite and hemicarboalumi-
nate.
I. Introduction
C
ALCIUM sulfoaluminate (CSA) clinker was first developed
in the 1960s at the University of California at Berkeley
by Alexander Klein.
1,2
CSA clinker generally contains a high
proportion of C
4
A
3
S,
††
which can be accompanied by a wide
variety of other phases (C
3
S, C
2
S, C
4
AF, C S, CA, and
C
12
A
7
) depending on the kiln feed and operating conditions.
3
CSA clinker can be used to make cements with a variety of
properties including: high early strength, rapid setting,
shrinkage compensating, or self stressing. CSA clinker can
also be blended with portland cement (PC) to make Type K
cement, which is expansive. The amount of expansion
induced by CSA cement can be controlled by varying the
water to cement ratio (w/c), amount of calcium sulfate
added, the particle size distribution, lime content, and the
C
4
A
3
S content.
4–7
By varying cement phase proportions and
the concrete mix proportions, a wide range of properties can
be developed including: self stressing, shrinkage compensat-
ing, nonexpansive, rapid setting, and high early strength.
1–10
Research in CSA cement has experienced a renaissance
because of its four main potential environmental and mone-
tary benefits. First, CSA clinker can be fired at approxi-
mately 1250°C–1350°C, which is about 100°C–200°C lower
than PC clinker, thus saving money and reducing CO
2
emis-
sions from burning fuel in the kiln.
5,11
Second, of the major
cement phases, C
4
A
3
S has one of the lowest CaO contents,
for instance, compare C
4
A
3
S (36.7%) to C
3
S (73.7%). The
lower CaO content equates to a lower CaCO
3
demand in the
kiln, which results in less CO
2
emissions during calcination.
Third, CSA clinker is more friable than PC; therefore, it
requires less energy to grind.
5
Finally, CSA clinker can be
manufactured from a wide variety of industrial byproducts
including: fly ash, flue gas desulfurization sludge, fluidized
bed ash, blast-furnace slag, phosphogypsum, incinerated
municipal waste, and red mud.
12–17
Although preliminary models for the structure of C
4
A
3
S
have been developed, there is ongoing research in this area
due to the complexity of the structure. At the current time, it
remains unclear if C
4
A
3
S is cubic,
18,19
orthorhombic,
20
or
tetragonal,
21,22
although there is a generally well agreed upon
cubic subcell with space group I 43m. In the following para-
graphs, we will chronologically go through the work that has
been done to determine the structure of C
4
A
3
S.
The synthesis of CSA was first reported by Ragozina in
1957.
23
Ragozina prepared the compound by heating trical-
cium aluminate (C
3
A) with gypsum (C
SH
2
) at 1200°C; the
composition was reported as 1.6–3.6(CA)C S. In 1958, during
the course of producing expansive cements, Klein and Trox-
ell reported composition estimates of C
5
A
2
S and C
9
A
4
S
3
.
24
They produced their samples by firing CH or C C, C SH
2
, and
bauxite or aluminum sulfate at 1350°C. In 1961, Fukuda cor-
rectly identified the composition of C
4
A
3
S after firing bauxite,
lime, and gypsum at 1350°C.
25
In 1962, Halstead and Moore suggested the cubic space
group I4
1
32 for C
4
A
3
S, based on systematic absences in their
powder patterns.
18
They determined the refractive index to
be 1.57 and the density to be 2.61 g/cm
3
. In addition, they
observe that all reflections that cannot be indexed on a body-
centered cubic cell (a = 9.195) are weak, and the strong
reflections are consistent with the space group I 43m. These
observations suggested that C
4
A
3
S is an end-member of the
sodalite–noselite–hauynite series with all the Na
+
replaced
by Ca
2+
and the Si
4+
replaced by Al
3+
.
Sodalites have the general formula M
8
(T
12
O
24
)X
2
, where
M is a relatively low charge caged cation (Na
+
,K
+
, Ca
2+
,
Sr
2+
, etc.,), T (usually Si
4+
or Al
3+
) is tetrahedrally coordi-
nated with oxygen to form the framework, and X is the
caged anion (either a single atom anion such as Cl
or a
J. Biernacki—contributing editor
Manuscript No. 33080. Received April 22, 2013; approved October 8, 2013.
*Member, The American Ceramic Society.
Based in part on the dissertation to be submitted by C. W. Hargis for the Ph.D.
degree in civil and environmental engineering, University of California, Berkeley, CA,
2013 (expected).
‡
Present address: Civil Engineering Program, Department of Mechanical Engineer-
ing, State University of New York at Stony Brook, Stony Brook, New York 11794.
†
Author to whom correspondence should be addressed. e-mail: Craig_Hargis
@Berkeley.edu
††
Cement chemistry notation used (C = CaO, S = SO
3
, A = Al
2
O
3
, F = Fe
2
O
3
,
S = SiO
2
, C = CO
2
&H = H
2
O)
892
J. Am. Ceram. Soc., 97 [3] 892–898 (2014)
DOI: 10.1111/jace.12700
© 2013 The American Ceramic Society
J
ournal