AP04-Bittnar1.vp

1 Introduction
Calcined gypsum is a historical binder that was used

already several thousands of years ago. It has been found in
the binder of buildings in the territory of present-day Syria
dated 7000 B. C., and it was also used in the Cheops pyramid
2650 B. C. Calcined gypsum is used in many technological
modifications, which aim to improve its properties, in partic-
ular as a binder of rendering mortars, for the production of
stuccowork and also for plasters. In interior applications and
in the binder of rendering mortars, calcined gypsum is still
employed, due to easy processing and low energy consump-
tion in the production process.

In the second half of the 20th century, technologies were
developed for desulfurization of flue gases in power stations
and heating plants. These methods are based on the reaction
of sulfur(II) oxide formed during combustion of brown coal
with a high content of sulfur with limestone CaCO3. Although
it seemed that these methods are suitable from the point of
view of protection of the environment, there is currently
opposition to these technologies. It has been pointed out that
the price of desulfurization equipment is too high, and that
a large amount of high quality limestone is consumed while a
huge amount of flue gas desulfurization (FGD) gypsum is
formed as a waste product.

According to the information given in the Mining Year-
book 2000 the amount of sulfur in higher quality brown coal
for households ranges from 0.9 % to 1.78 %. The coal for
energy production contains up to 2.5 % of sulfur. Flue gas
desulfurization of one power station block creates up to 20 t of
FGD gypsum per hour.

FGD gypsum is produced in great quantities, but is in-
sufficiently used. Calcined gypsum is produced from FGD
gypsum in only one power station in the Czech Republic,
while the remaining production ends with gypsum that is
used only partially as an additive for retarding the setting of
cement.

Calcined gypsum is mostly used for the production of
gypsum plasterboard. Gypsum, that is not utilized is de-
posited as waste. Therefore, it is very desirable to pay
attention to utilization of calcined gypsum also in those appli-
cations where it has not previously been used, i.e. in exteriors.
Utilization of binders with minimal energetic demand is
in accordance with the current trend in production, when

building materials including binders should be produced
with a minimized impact on the environment, i.e., with mini-
mal or no production of CO2 and minimal demands on
energy. Examples of such binders are belitic cements, binders
based on silicate waste products and also calcined gypsum.

Calcined gypsum as a low-energy material can be
produced from waste FGD gypsum, by dehydrating it at tem-
peratures between 110 and 150 °C. Then, �-form of calcined
gypsum is formed according to the equation

CaSO4� 2H2O � CaSO4� H2O + 1 H2O. (1)

The solid structure of calcined gypsum is created by re-
verse hydration when gypsum CaSO4� 2H2O is again formed.
This compound is relatively soluble in water, its solubility
being 0.256 mg in 100 g of water at 20°C. Therefore, it can-
not be utilized in exterior applications, as rain water could
dissolve this the product that should safeguard the mechani-
cal properties of the material.

In order to use gypsum elements in exteriors, it is neces-
sary to modify them so that they will exhibit more suitable
properties and a longer service life. Modifications of gypsum
are usually performed using polymer materials. Bijen and van
der Plas [1] reinforced gypsum with E-glass fibers, and modi-
fied the binder by using acrylic dispersion in a mixture with
melamine. The results show that this material had higher
flexural strength and higher toughness than glass fiber rein-
forced concrete after 28 days. A disadvantage of polymers
based on the carbon chain is a decrease in the fire resistance of
calcined gypsum elements.

Generally it can be stated that the resistance of hardened
gypsum to water has not yet been resolved in a satisfactory
way. In the literature, only applications of lime and artificial
resins (polyvinylacetate, urea formaldehyde and melamine
formaldehyde) have been studied, some inorganic substances
such as fluorosilicates, sulfates and silicates were found to in-
crease their surface hardness and impermeability, see Schulze
et al. [2].

Therefore, our primary aim is to adjust basic technologies
for the production of modified gypsum, particularly from
the point of view of hydrophobization and improving its me-
chanical, hygric and thermal properties. In this paper, we
present reference measurements of the mechanical, thermal
and hygric properties of common FGD gypsum, which will be

Acta Polytechnica Vol. 44 No. 5 – 6/2004

Mechanical, Hygric and Thermal
Properties of Flue Gas Desulfurization

Gypsum
P. Tesárek, J. Drchalová, J. Kolísko, P. Rovnaníková, R. Černý

The reference measurements of basic mechanical, thermal and hygric parameters of hardened flue gas desulfurization gypsum are carried
out. Moisture diffusivity, water vapor diffusion coefficient, thermal conductivity, volumetric heat capacity and linear thermal expansion
coefficient are determined with the primary aim of comparison with data obtained for various types of modified gypsum in the future.

Keywords: flue gas desulfurization (FGD) gypsum, calcined gypsum, mechanical properties, thermal properties, hygric properties.

utilized for a comparison with various types of modified gyp-
sum in the future.

2 Experimentals methods

2.1 Bending strength and compressive strength
The measurement of bending strength was performed

according to the Czech standard ČSN 72 2301 [3] on
40×40×160 mm prisms. The specimens were demolded
15 minutes after the final setting time and stored in the test-
ing room. Each specimen was positioned in such a way that
the sides that were horizontal during the preparation were in
the vertical position during the test. The experiment was
performed as a common three-point bending test using a
WPM 50 kN device. The distance of the supporting cylinders
was 100 mm. The bending strength was calculated according
to the standard evaluation procedure. The measurements
were done 2 hours, 1 day, 3 days, 7 days, 14 days and 28 days
after mixing.

Compressive strength was determined in accordance with
the Czech standard ČSN 72 2301 on the halves of the speci-
mens left over after the bending tests. The specimens were
placed between the two plates of the WPM 100 kN device in
such a way that their lateral surfaces adjoining during the
preparation to the vertical sides of the molds were in contact
with the plates. In this way, the imprecision of the geometry
on the upper cut off side did not have a negative effect on the
experiment. The compressive strength was calculated as the
ratio of the ultimate force and the load area.

2.2 Moisture diffusivity

2.2.1 Determination of the apparent moisture diffusivity
from a water sorption experiment

A common water sorption experiment was carried out.
The specimen was water and vapor-proof insulated on four
lateral surfaces and the face side was immersed 2 mm in water.
A constant water level in the tank was achieved using a bottle
placed upside down. The known water flux into the specimen
during the suction process was then employed to determine
the water absorption coefficient. The samples were tested in
constant temperature conditions.

To calculate the apparent moisture diffusivity Dw [m
2s�1],

the following approximate relation was employed:

D
A

w
w

c
�
�

�
�
�

�

�
�
�

(2)

where A is the water absorption coefficient [kgm�2s1/2], and wc
is the saturated moisture content [kgm�3].

2.2.2 Determination of moisture diffusivity from moistures
profiles

The capacitance method [4] was employed to measure
the moisture content, and the measuring frequency was
250–350 kHz. The parallel electrodes of the capacitance
moisture meter had dimensions 20×40 mm.

The moisture profiles were determined using a common
capillary suction 1-D experiment in the horizontal position,
and the lateral surfaces of the specimens were water and va-

por-proof insulated. A moisture meter reading along the
specimen was taken every 5 mm. The calibration curve was
determined after the last moisture meter reading, when the
moisture penetration front was about one half of the length of
the specimen, using this last reading and the standard gravi-
metric method after cutting the specimen into 1 cm wide
pieces. The final calibration curve for the material was con-
structed from the data of 6 samples. The moisture profiles
were then calculated from the calibration curve. The mea-
surements were done at 25 °C ambient temperature. Moisture
diffusivity was determining by the Matano method [5].

2.3. Water vapor diffusion coefficient

2.3.1 Standard cup methods
In the standard cup methods (dry and wet), the water va-

por diffusion coefficient D was calculated from the measured
data according to the equation

D
m d R T

S M pp

�

��
, (3)

where D is the water vapor diffusion coefficient [m2s�1], �m
the amount of water vapor diffused through the sample [kg],
d the sample thickness [m], S the specimen surface being in
contact with the water vapor [m2], � the period of time corre-
sponding to the transport of mass of water vapor �m [s], �pp
the difference between the partial water vapor pressure in the
air below and above the specimen [Pa], R the universal gas
constant [J mol�1 K�1], M the molar mass of water [kg mol�1],
T the absolute temperature [K].

On the basis of the diffusion coefficient D, the water vapor
diffusion resistance factor � was determined:

�
D
D

a , (4)

where Da is the diffusion coefficient of water vapor in the
air [m2s�1].

In the dry cup method, a sealed cup containing silica gel
was placed in a controlled climate chamber with 50 % relative
humidity and weighed periodically. For the wet cup method,
a sealed cup containing water was placed in an environment
with a temperature of about 25 °C and relative humidity about
50 %. The measurements were done at 25 °C over a period of
two weeks.

The steady state values of mass gain or mass loss deter-
mined by linear regression for the last five readings were used
to determine the water vapor transfer properties.

2.3.2 Transient method
In the transient method designed in [6], the measuring

device consists of two airtight glass chambers separated by a
board-type specimen of the measured material. In the first
chamber, a state near to 100 % relative humidity is main-
tained (achieved with the help of a cup of water), while in the
second chamber, there is a state close to 0 % relative humidity
(established using some desiccant, in our case a silica gel).
Alternately, saturated salt solutions establishing defined rela-
tive humidity conditions can be placed in either the wet or the
dry chamber, or in both of them. The change in the mass of
water in the cup and the mass of the desiccant are recorded by
an automatic balance in dependence on time. If steady-state

84 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 44 No. 5 – 6/2004

measurements are also required, the validity of the condition
that the change in the mass of water equals in absolute values
the change in the mass of the desiccant is tested, and the ex-
periment continues until this condition is realized. The ex-
periment is carried out under isothermal conditions, as in the
case of standard cup methods.

2.4 Thermal conductivity and volumetric heat
capacity

Thermal properties were measured using ISOMET 2104
(Applied Precision, Ltd., SK). This is a multifunctional instru-
ment for measuring thermal conductivity � [W m�1 K�1], volu-
metric heat capacity C [J m�3 K�1] and temperature [°C] of a
wide range of materials. The thermal diffusivity a [m2s�1] is
calculated by the device from the formula

a
C

�

. (5)

The measurements were done using surface probes with
samples, which were placed at laboratory conditions of 25 °C
and about 50 % relative humidity. The relative moisture con-
tent by mass of the samples was about 18 %.

2.5 Linear thermal expansion coefficient
The linear thermal expansion coefficient �T was deter-

mined in a common way using the measured length changes
(Carl Zeiss optical contact comparator with a precision of
�0.5 �m) between two different temperatures: 25 °C and
80 °C. It was calculated from the formula

a
l

l
T

T
T

d
d

, (6)

where l0, T is the length at a reference temperature.

3 Material and samples
The material, used for reference measurements was

�-form of calcined gypsum with purity higher than 98 % of
FGD gypsum, produced at the electric power station at Poče-
rady, CZ. The water/gypsum ratio was 0.627. The samples
were mixed according to Czech standard ČSN 72 2301.

For the measurements of particular mechanical, thermal
and hygric parameters, we used the following samples: bend-
ing strength and compressive strength – 8 sets of 3 specimens
each 40×40×160 mm, moisture diffusivity – capacitance
method – 6 specimens 20×40×300 mm, apparent moisture
diffusivity – 4 specimens 50×50×23–25 mm, water vapor dif-
fusion coefficient – 12 cylinders with the diameter 105 mm
and thickness 10–22 mm, thermal conductivity and volumet-

ric heat capacity – 6 specimens 70×70×70 mm, linear ther-
mal expansion coefficient – 5 specimens 40×40×160 mm.

The samples for determining moisture diffusivity were
insulated on all lateral surfaces by water- and vapor-proof
plastic foil, the samples for measuring water vapor diffusion
coefficient were also water- and vapor-proof insulated on the
lateral surfaces by Epoxy resin.

4 Experimental results
The basic properties of the studied material for its charac-

terization are shown in Table 1. In addition to these measure-
ments, which are commonly performed for all porous build-
ing materials, we also made a classification of FGD gypsum
according to Czech standard ČSN 72 2301. This classification
consists in determining the grinding fineness using the 0.2
mm sieve residue, initial and final setting times using a Vicat
device and compressive strength for a period of two hours af-
ter mixing. The results are summarized in Table 2. According
to these results, FGD gypsum can be classified as G-13 B III.

The dependence of compressive strength and bending
strength on time for the first 28 days after mixing is given in
Fig. 1. We can see that both strengths decrease slightly for
approximately 3 days, but then they begin to increase rapidly
and the maximum strengths are achieved after 14 days. These

Acta Polytechnica Vol. 44 No. 5 – 6/2004

Bulk density
[kgm�3]

Matrix density
[kgm�3]

Open porosity
[% by volume]

1019�1.5 % 2530�2.0 % 60�3.4 %

Table 1: Basic properties of FGD gypsum

Compressive strength
[MPa ]

Initial setting time
[min]

Final setting time
[min ]

0.2 mm sieve residue
[%]

Measured values 13.3 9 13 1.79

Limiting values according to
ČSN

Minimum
13.0

Earliest time
6

Latest time
30

Maximum
2

Classification according to ČSN G-13 B III

Table 2: Classification of FGD gypsum using ČSN 72 2301

0 5 10 15 20 25 30

Time [days]

S
tr

e
n

g
th

[M
P

a
].

compressive strength

bending strength

Fig. 1: Compressive strength and bending strength of FGD
gypsum

changes are apparently related to the change of moisture
content in the specimens. While the moisture content for the
2-hour specimens was 67% kg/kg, for the 28-day specimens
it was only 24 %. So, both compressive strength and bend-
ing strength were significantly improved by drying of the
specimens.

Fig. 2 shows typical moisture profiles determined by the
capacitance method. Fig. 3 presents the dependence of mois-
ture diffusivity on the moisture content calculated using the
moisture profiles and the apparent moisture diffusivity de-
termined on the basis of the water absorption coefficient.
Clearly, the agreement of both measurements is very good,
the value of apparent moisture diffusivity being equal to the
moisture diffusivity determined from moisture profiles for
83% of the capillary water saturation value.

Table 3 presents the basic thermal properties of FGD gyp-
sum. Table 4 shows the results of measurements of the water
vapor diffusion resistance factor using the dry cup and the wet
cup methods and also the transient method. We can see that
the results obtained by the transient method are very close to
the results of the wet cup method, the difference being within
the error range of both methods. This seems to indicate that
the boundary condition on the wet side with the relative hu-
midity close to 100% affected the measurements more signifi-
cantly than the condition on the dry side.

5 Discussion
The possibilities of comparing of the material parameters

of FGD gypsum measured in this paper with the parameters
determined by other scientists, at least for common gypsum,
are very limited. As for FGD gypsum, no data at all were
found in common sources.

Among the basic properties, Klein and von Ruffer [7]
found porosity of 55 % for gypsum with a water to gypsum
ratio of 0.67–0.72. Mrovec and Perková [8] indicate the bulk
density of cast gypsum blocks to be between 840 kgm�3and

86 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 44 No. 5 – 6/2004

0 000.

0 100.

0 200.

0 300.

0 400.

0 500.

0.600

0 000. 0 020. 0 040. 0 060. 0 080. 0 100. 0 120. 0 140. 0 160. 0 180.

Position [ m ]

M
o

is
tu

re
c
o

n
te

n
t

[k
g

k
g

-1
]

t1=300 s

t2=900 s

t3=1800 s

t4=2400 s

t5=3300 s

t6=6000 s

t7=7800 s

t8=9600 s

t9=11400 s

Fig. 2: Typical moisture profiles in FGD gypsum specimens

Thermal conductivity
[Wm�1K�1]

Volumetric heat capacity
[Jm�3K�1]

Thermal diffusivity
[m2s�1 ]

Linear thermal expansion coefficient
[K�1]

0.47�10 % (1.60�10 %) E�6 (0.29�10 %) E�6 (7.22�15 %)E�6

Table 3: Basic thermal properties of FGD gypsum

1 0E-08.

1 0E-07.

1 0E-06.

1.0E-05

0 15. 0 20. 0 25. 0 30. 0 35. 0 40. 0 45. 0 50.

Moisture content [kg kg
-1

]

M
o

is
tu

re
d

if
fu

s
iv

it
y

[m
2
s

-1
]

Moisture
profiles

Water sorption
experiment

Fig. 3: Moisture diffusivity of FGD gypsum

Water vapor diffusion resistance factor

Cup methods Transient method

Dry cup Wet cup

17.3 � 15 % 5.44 � 15 % 5.3 � 5 %

Table 4 Water vapor diffusion resistance factor

1130 kgm�3. The bulk density of FGD gypsum falls between
these limits. According to ČSN 73 0540-3 [9] the bulk density
of plasterboard is 750 kgm�3.

As for the mechanical properties, Klein and von Ruffer [7]
determined compressive strength 20 MPa and the bending
strength 4 MPa for �-gypsum with a water to gypsum ratio of
0.67–0.72. Singh and Garg [10] determined the compressive
strength of raw gypsum to be 12–14 MPa in dependence on
pH. For gypsum with a water to gypsum ratio of 0.6, Tazawa
[11] measured compressive strength of 18.2 MPa, bending
strength of 5.59 MPa. We can see that particularly the com-
pressive strength of FGD gypsum is much higher than the
mentioned reference data.

For thermal properties, Mehaffey et al. [12] indicate ther-
mal conductivity of gypsum of 0.25 Wm�1K�1. Sultan [13]
gives thermal conductivity of 0.25 Wm�1K�1 for gypsum in
the temperature range of 20–100 °C. Mrovec and Perková [8]
indicate thermal conductivity of gypsum as 0.20 Wm�1K�1. In
a comparison with these data, the thermal conductivity of
FGD gypsum is about two times higher.

Among the hygric parameters, Hanusch [14] determined
the water vapor diffusion resistance factor m in depend-
ence on the thickness of the plasterboard. For a thickness of
9.5 mm he obtained � 10 (for 0 and 50 % of relative humid-
ity) and � 6.5 (for 50 and 100 % of relative humidity), for a
thickness of 18 mm � 8.5 (for 0 and 50 % of relative humid-
ity) and � 5.5 (for 50 and 100% of relative humidity). As we
can see, the results of the wet cup measurements with FGD
gypsum in this paper correspond reasonably well with the
data in [14] but the dry cup data are higher than in [14].

For a detailed and more serious comparison of the data
obtained for FGD gypsum in this paper with the results mea-
sured by other scientists we unfortunately suffer a lack of
more detailed information in the above sources. The authors
usually make just references to national standards and re-
quirements that apply to the production and processing of
the specimens, and to the testing methods, which are not
easily accessible. This complicates a possible comparison. In
addition, some of the authors used plasterboard as the stud-
ied material, i.e., a gypsum board covered by a paper surface.
From the technological point of view, special additives such as
setting retarders, additives increasing the fire resistance etc.
are used in the production of plasterboard. Therefore, it is an
open question whatever we can talk of a common, unmodi-
fied gypsum in this case.

As follows from the above considerations, any comparison
with reference data can only be approximate. However, even
from such a rough comparison it is quite clear that the FGD
gypsum analyzed in this paper had significantly higher com-
pressive strength, which is the most important parameter for
cast gypsum blocks. This indicates that the overall quality of
FGD gypsum was much higher than the gypsum studied in
the above reference papers.

6 Conclusions
The main aim of the work done in this paper was to obtain

a reference data set for unmodified FGD gypsum without any
additives. This data set is relatively extensive, including not
only mechanical properties but also thermal and hygric prop-

erties. These parameters will help in simulating the processes
in the material, for instance in contact with water, air humid-
ity, due to changes of temperature, or due to some other load.

It also follows from the results obtained here that the basic
material will have to be subjected to substantial modifica-
tions primarily due to the worsening of its properties with
increasing moisture content. Protection against water and
air humidity will have to be provided with the use of hydro-
phobization additives. Also, some modifications need to be
made, aimed at increasing compressive strength and bending
strength, for instance using plasticizers, need made. Improve-
ment of thermal properties also seems to be an important
topic for the future modifications. However, all these modifi-
cations will have to be cross-checked for possible negative
effects on other parameters than those intended to be
improved.

The modified gypsum that will be developed in future
research will be used in the production of cast blocks for
application in envelope parts of building structures. Using
the measured data it will be possible to simulate both the
mechanical and the hygrothermal performance of the de-
signed structure of the envelope and to predict its long-term
behavior and service life.

Acknowledgment
This research has been supported by the Czech Science

Foundation, under grant No. 103/03/0006.

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Praha, 1990.

[3] ČSN 72 2301 Gypsum binding materials, Czech stan-
dard (in Czech), Vydavatelství Úřadu pro normalizaci
a měření, Praha 1979.

[4] Semerák P., Černý R.: “A Capacitance Method for Mea-
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[5] Drchalová J., Černý R.: “Non-Steady-State Methods for
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[6] Černý R., Hošková Š., Toman J.: “A Transient Method
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on Moisture Problems in Building Walls, V. P. de Freitas,
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[7] Klein D., von Ruffer C.: “Grundlagen zur Herstellung
von Formengips.” Keramische Zeitschrift, Vol. 49 (1997),
p. 275–281.

[8] Mrovec J., Perková J.. Pokyny pro projektanty. Tech-
nické podklady firmy GYPSTREND (2003).

[9] ČSN 73 0540–3. Tepelná ochrana budov, Část 3: Výpoč-
tové hodnoty veličin pro navrhování a ověřování, s. 15.

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Vydavatelství Úřadu pro normalizaci a měření, Praha,
1994.

[10] Singh M., Garg M.: “Retarding Action of Various Chem-
icals on Setting and Hardening Characteristics of Gyp-
sum Plaster at Different pH.” Cement and Concrete
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[11] Tazawa E.: “Effect of Self-Stress on Flexural Strength of
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[12] Mehaffey J. R., Cuerrier P., Carisse G.: “A Model for Pre-
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-Stud Walls Exposed to Fire.” Fire and Materials, Vol. 18
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[13] Sultan M. A.: “Model for Predicting Heat Transfer
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[14] Hanusch H.: “Übersicht über Eigenschaften und An-
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Ing. Pavel Tesárek
phone: +420 224 355 436
e-mail: tesarek@fsv.cvut.cz

Department of Structural Mechanics

RNDr. Jaroslava Drchalová, CSc.
phone: +420 224 354 586
e-mail: drchalov@fsv.cvut.cz

Department of Physics

Czech Technical University in Prague
Faculty of Civil Engineering
Thákurova 7

166 29 Praha 6, Czech Republic

Ing. Jiří Kolísko, Ph.D.
phone: +420 224 353 537
e-mail: kolisko@klok.cvut.cz

Czech Technical University in Prague
Klokner Institute
Šolínova 7
166 08 Prague 6, Czech Republic

Doc. RNDr. Pavla Rovnaníková, CSc.
phone: +420 541 147 633
e-mail: chrov@fce.vutbr.cz

Institute of Chemistry

University of Technology Brno
Faculty of Civil Engineering
Žižkova 17
662 37 Brno, Czech Republic

Prof. Ing. Robert Černý, DrSc.
phone: +420 224 354 429
e-mail: cernyr@fsv.cvut.cz

Department of Structural Mechanics

Czech Technical University in Prague
Faculty of Civil Engineering
Thákurova 7
166 29 Praha 6, Czech Republic

Acta Polytechnica Vol. 44 No. 5 – 6/2004