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Synergistic effect of fly ash and blast furnace slag on the
mechanical strength of traditional porcelain tiles
Kausik Dana
a
, Jayanta Dey
b
, Swapan Kumar Das
a,
*
a
Central Glass and Ceramic Research Institute, Kolkata 700032, India
b
College of Ceramic Technology, Kolkata 700010, India
Received 1 December 2003; received in revised form 17 February 2004; accepted 10 April 2004
Available online 7 July 2004
Abstract
Fly ash and blast furnace slag, the by-products of Indian thermal power plant and steel plant, were gradually added to a traditional kaolin—
quartz—feldspar based porcelain tile compositions singly and in combination by replacing a part of quartz and feldspar, respectively. The
effects of such additions were studied by measuring the linear shrinkage, bulk density, water absorption and flexural strength of the samples
heated in the temperature range of 1150–1200 8C. The results reveal that additions of both the by-products have beneficial effects towards
mechanical strength. The strength improvement was mainly due to the presence of microcrystalline components such as mullite and quartz in
fly ash and alkaline earth oxides in blast furnace slag. Slag containing compositions with or without fly ash vitrified at an early temperature
(1175 8C) and achieved highest strength at this temperature compared to 1200 8C for normal and fly ash bearing compositions. Specific
combinations of fly ash and slag in the ratio of 1:1 and 1:2 have shown the development of highest flexural strength (>70 MPa) at 1175 8C due
to their lowest R
2
O:R0O (R = K, Na and R0= CaO, MgO) and moderately higher SiO
2
:Al
2
O
3
ratio. Beyond this temperature, the strength
drastically reduced due to the formation of glassy phases at higher temperatures. XRD studies were also carried out to find out the differences
in phase evolution due to addition of fly ash and blast furnace slag to traditional porcelain composition.
ß2004 Elsevier Ltd and Techna S.r.l. All rights reserved.
Keywords: D. Porcelain; Fly ash; Blast furnace slag; Ceramic tile
1. Introduction
A traditional porcelain batch consists of kaolin, quartz
and feldspar. The role of each of these raw materials on the
physico-mechanical behaviour of such triaxial porcelain
bodies has been studied in detail by many authors [1–9].
Due to the gradual depletion of naturally occurring minerals,
there is a recent trend to substitute these minerals by
alternative source of raw materials, which are abundantly
available as overburden wastes. In such attempts, few
authors have utilized fly ash, a by-product of thermal power
plant as alternative source of alumino-silicate raw material
by replacing a part of naturally occurring kaolinitic clays in
porcelain tile compositions [10–13]. They found that repla-
cement of 25–30 wt.% of kaolinitic clay by fly ash is
beneficial as it has close resemblance with clay in their
chemistry and inherently contain some micro crystalline
components such as quartz and mullite. Dana et al. [14] in
their recent studies substitute a part of quartz by fly ash in
triaxial porcelain tile compositions. They observed higher
mullitization in fly ash containing samples that resulted in
maximum strength at 1300 8C. Some authors also reported
use of iron and steel slag in porcelain tile compositions [15–
20]. They observed that alkaline earth oxides present in slag
favours early maturing of porcelain bodies due to their
strong fluxing action. Dana et al. [21,22] also found some
beneficial effect of blast furnace slag in triaxial porcelain
composition by replacing a part of feldspar. The research
work of the above authors mainly dealt with an individual
effect of fly ash and slag on the physico-mechanical beha-
vior in porcelain compositions. Combined effect of fly ash
and slag in traditional triaxial porcelain composition by
replacing a part of quartz and feldspars is rarely reported.
www.elsevier.com/locate/ceramint
Ceramics International 31 (2005) 147–152
* Corresponding author. Fax: +91 33 2473 0957.
E-mail address: swapan@cgcri.res.in (S.K. Das).
0272-8842/$30.00 ß2004 Elsevier Ltd and Techna S.r.l. All rights reserved.
doi:10.1016/j.ceramint.2004.04.008
In the present investigation, the combined effect of fly ash
and blast furnace slag is studied by replacing a part of quartz
and feldspars respectively in a triaxial traditional porcelain
tile composition. The physico-mechanical properties such as
shrinkage, bulk density, water absorption and flexural
strength are compared individually with the traditional,
fly ash and slag-based compositions. The differences in
their strength behaviour and phase evolution are also studied
and discussed in the paper.
2. Experimental
Two kilograms of each batch (NP, FP, SP, FSP-1, FSP-2
and FSP-3) were prepared according to batch composition
presented in Table 1.
All the batches were mixed separately and wet ground in
a pot mill for a duration of 14 h to get the desired fineness
(residue less than 1.5% on 200 mesh BS sieve). The slurry
obtained was screened, dried at 110 8C, powdered to break
the agglomerate and granulated to small particles for better
compaction using 6–7% moisture. Samples of 60 mm
14 mm 5 mm were hydraulically compacted using uni-
axial pressing at 30–35 MPa. The shaped samples were dried
at 110–120 8C for 24 h till the moisture content reduced to
less than 0.5%. The dried samples were fired at three
different temperatures (1150, 1175 and 1200 8C) for a
soaking period of 120 min in an electrically operated labora-
tory furnace using on/off control system. A constant heating
rate of 8 8C/min was maintained in each firing. Finally, the
fired samples were subjected to physical tests such as linear
shrinkage (L.S.), water absorption (W.A.) and bulk density
(B.D.). Bulk density and % water absorption of the fired
samples were measured using the conventional liquid dis-
placement method according to Archimedes principle. Flex-
ural strength (three point bending) of the fired samples was
determined by universal testing machine (INSTRON
5500R). For X-ray diffraction experiment, Philips ‘X-Pert
Pro diffraction unit, attached with secondary monochroma-
tor, automatic divergence slit and nickel filter, was used to
get monochromatic Cu-Karadiation. Vitrified tile sample
was also tested for polishing with diamond paste after initial
grinding with SiC powder. Since, incorporation of fly ash
and B.F. Slag to normal porcelain composition in place of
quartz and feldspar adds some extra amount of colouring
oxides such as Fe
2
O
3
and TiO
2
, colour measurement was
also carried on vitrified tile surfaces. The resulting vitrified
samples were investigated by colour measurement following
ASTM-C609-71, a colorimeter instrument type Hunter Lab
with attached standard colours was used.
3. Results and discussion
All the raw materials were chemically analyzed by the
known method [23] and the results are given in Table 2.
The analysis shows that kaolinitic clay, quartz and feld-
spar used in the present study are of normal type. Fly ash
contains SiO
2
and Al
2
O
3
as major oxide constituents with
TiO
2
, CaO and Fe
2
O
3
slightly in higher side. Blast furnace
slag has SiO
2
, CaO and Al
2
O
3
as major oxide constituents.
An earlier XRD study of the authors [16] confirms the
presence of mullite and quartz in fly ash, whereas slag
shown only a diffused hallow around 308(2u) indicating
its glassy nature. The experimental compositions are exhib-
ited in the form of a triangle (Fig. 1)
Table 3 provides the chemical composition of the experi-
mental bodies. The major oxide constituents present are
shown in Fig. 2. Traditional porcelain (NP) is characterized
by high SiO
2
:Al
2
O
3
ratio and alkaline oxide (K
2
O+Na
2
O),
whereas fly ash porcelain (FP) is characterized by moderate
SiO
2
:Al
2
O
3
ratio and higher alkaline oxide. Slag porcelain
bodies without/with fly ash (SP and FSP series) are char-
K. Dana et al. / Ceramics International 31 (2005) 147–152148
Table 1
Batch compositions (wt.%)
Batches Kaolinitic clay Feldspar Quartz Fly ash B.F. Slag
NP 45 30 25 0 0
FP 45 30 10 15 0
SP 45 15 25 0 15
FSP-1 45 27 19 6 3
FSP-2 45 24 22 3 6
FSP-3 45 22 18 7 7
Table 2
Chemical analysis of the raw materials
Major chemical constituents (wt.%) Kaolinitic clay Feldspar Quartz Fly ash B.F. Slag
SiO
2
45.41 66.48 98.66 59.26 35.35
Al
2
O
3
34.39 17.29 0.39 27.97 19.15
Fe
2
O
3
1.13 0.14 0.07 4.00 0.60
TiO
2
0.89 0.02 0.01 2.92 0.77
CaO 1.07 0.31 0.10 2.63 36.56
MgO 0.76 0.03 0.02 0.56 2.99
Na
2
O 0.87 2.94 0.09 0.54 0.81
K
2
O 0.42 11.95 0.12 0.60 1.29
MnO ––––2.09
L.O.I. 14.67 0.71 0.37 1.12 0.21
acterized by moderately higher SiO
2
:Al
2
O
3
ratio and higher
alkaline earth oxides (CaO + MgO).
The variation in % L.S. with temperature is shown in
Fig. 3. As normally observed, the shrinkage increases with
heating temperature for all the compositions. The only slag
containing body (SP) with higher amount of alkaline earth
oxides (lowest R
2
O/R0O) show highest shrinkage at 1175 8C
compared to NP, FP and FSP series. At 1150 8C, although
there is no distinct relationship observed between shrinkage
and alkaline earth oxide content, a correlation may be seen
from the Fig. 3 that shrinkage value increases with increase
in alkaline earth oxide content beyond 1150 8C.
In contrast to the present observation, Marghussion and
Yekta [15] reported that alkaline earth oxide cause less
shrinkage in comparison to alkali oxides, but at below
1100 8C, particularly for wall tile compositions. The nature
of Fig. 3 in the present study agrees with this observation at
below 1150 8C showing a tendency to cause less shrinkage
in slag containing composition.
Variation in bulk density (Fig. 4) and percent water
absorption (Fig. 5) also follow the similar trend. Slag
containing bodies (SP and FSP series) shown higher B.D.
in the temperature range of 1175–1200 8C followed by less
than 0.5% W.A. It may also be observed that FSP-3, contain-
ing fly ash and slag in the ratio of 1:1, achieved highest
densification (2.71 gm cm
3
) and minimum W.A. (0.14%)
at 1200 8C compared to others.
An interesting feature may be noted from Fig. 5 that
water absorption of SP and FSP series of compositions has
been drastically reduced to less than 0.5% at 1175 8C while
K. Dana et al. / Ceramics International 31 (2005) 147–152 149
Fig. 1. Classical triangular representation of the experimental porcelain tile
compositions.
Table 3
Chemical compositions of the experimental bodies
Oxide constituents
(wt.%)
Batches
NP FP SP FSP-1 FSP-2 FSP-3
SiO
2
70.06 63.80 64.99 66.11 66.37 62.11
Al
2
O
3
22.36 26.86 22.65 24.45 23.51 24.95
Fe
2
O
3
0.61 1.25 0.69 0.89 0.75 0.87
TiO
2
0.44 0.91 0.56 0.70 0.65 0.73
CaO 0.65 1.06 6.50 2.01 3.14 3.32
MgO 0.38 0.47 0.86 0.77 1.12 1.15
Na
2
O 1.40 1.47 1.05 1.29 1.18 1.11
K
2
O 4.10 4.18 2.37 3.62 3.23 2.98
MnO ––0.34 0.07 0.14 0.15
Fig. 2. Chemical composition of the experimental bodies, SiO
2
:Al
2
O
3
—
alkaline oxides (K
2
O+Na
2
O)—alkaline earth oxides (CaO + MgO). Fig. 4. Variation in bulk density with heating temperature.
Fig. 3. Variation in % linear shrinkage with heating temperature.
normal porcelain (NP) and fly ash porcelain (FP) bodies did
not achieve full vitrification (W.A. <0.5%) even at 1200 8C.
Presence of alkaline earth oxides in SP and FSP series of
compositions might have assisted speedier formation of
glassy phases and promoted the reaction with clay minerals
present at this temperature. Harms [24] also observed similar
effect of alkaline earth oxide in a porcelain body.
Fig. 6 illustrates the flexural strength behaviour of all the
compositions at various temperatures. It may be seen that
slag containing compositions with or without fly ash devel-
oped higher strength at 1175 8C compared to others, beyond
which the strength decreases due to formation of more
glassy phases. Combination of fly ash and slag in the ratio
of 1:2 (FSP-2) and 1:1 (FSP-3) have shown highest strength
development (>70 MPa) at 1175 8C. Addition of fly ash and
slag singly in traditional porcelain composition although
improved the mechanical strength, the combined effect was
found to be more effective. Such synergistic composition of
porcelain bodies containing both fly ash and slag shall be
useful to produce high-strength ceramic tiles for industrial as
well as domestic applications.
Strength variations of all the experimental bodies at
1175 8C with respect to R
2
O:R0O and SiO
2
:Al
2
O
3
ratio is
exhibited in Fig. 7. FSP-2 and FSP-3 are amongst the top in
strength values (around 70 MPa) due to their lowest
R
2
O:R0O and moderately higher SiO
2
:Al
2
O
3
ratio followed
by FSP-1and SP, the second highest (around 60 MPa). NP
and FP with higher R
2
O:R0O ratio possesses moderate
strength (30 and 45 MPa, respectively).
K. Dana et al. / Ceramics International 31 (2005) 147–152150
Fig. 6. Variation in flexural strength with heating temperature.
Fig. 7. Variation in flexural strength of the experimental bodies at 1175 8C
with respect to R
2
O:R0O (R = K and Na, R0= Ca and Mg) and SiO
2
:Al
2
O
3
.
Fig. 8. XRD pattern of 1175 8C heated samples: (a) NP, (b) FP, (c) FSP-3,
(d) SP [, mullite; *, quartz; *, anorthite; ~, gehlenite].
Fig. 5. Variation in % water absorption with heating temperature.
Some of the selected samples (NP, FP, SP, FSP-3) heated
at 1175 8C were subjected to XRD study. The XRD pattern
confirms the presence of quartz and mullite in NP and FP
samples (Fig. 8a and b) as major phases, while FSP-3 and
SP (Fig. 8c and d) contain quartz, anorthite and mullite as
major phases with minor amount of gehlenite. Mullite
content in SP was significantly less than FSP-3 and that
resulted lower strength in SP compared to FSP-3. The
vitrified tile samples were also tested for polishing. A
surface finishing of 1 mm was achieved and no surface
defects were noted.
In modern day technology for the production of vitrified
porcelain ware, particularly for tiles, greater attention is paid
to the industrial development of white base body. The
resulting vitrified porcelain tile samples obtained in the
present investigation were subjected to colour measurement
using the opponent-colour coordinate system developed by
Hunter [25]. In this system, the third coordinate describes
the lightness of colour and is usually denoted by L. The
results are given in Table 4.
It may be noted that the normal porcelain tile composition
(NP) showed higher L-value (whiter) due to the presence of
lower amounts of Fe
2
O
3
and TiO
2
compared to the fly ash
and B.F. Slag based porcelain tiles. However a tile body with
L-value of 75–80 range can also be used for the production
of glazed ceramic floor tile.
4. Conclusion
Addition of fly ash and blast furnace slag in a traditional
triaxial porcelain composition in the proportion of 1:1 and
1:2 was found to be beneficial towards improvement in
mechanical strength and early vitrification at 1175 8C.
Presence of microcrystalline components of quartz and
mullite in fly ash and alkaline earth oxides in B.F. slag
were responsible to develop anorthite and mullite phases
which ultimately improved the mechanical strength. Such
type of synergistic porcelain composition may find poten-
tial applications to manufacture high strength ceramic floor
tiles for industrial as well as domestic buildings. Further,
part substitution of natural minerals (quartz and feldspar)
by overburden industrial by-products (flyashandblast
furnace slag) reduce the cost of raw materials, thermal
energy without altering the requisite physico-mechanical
properties.
Acknowledgements
The authors would like to thank Dr. H.S. Maiti, Director,
Central Glass and Ceramic Research Institute, Kolkata,
India for his kind permission to publish this research work,
Prof. N.K. Mitra, Calcutta University, India for his valuable
guidance during the course of this work. The authors also
acknowledge the sponsorship of this research work by H&R
Johnson (India) Ltd.
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Table 4
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