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Effect of the Particle Size of the Grog on the Properties and Microstructure of Bricks
C. M. F. Vieira*,1, J. Alexandre2, S. N. Monteiro1
State University of the North Fluminense Darcy Ribeiro - UENF
1
Advanced Materials Laboratory - LAMAV
2
Civil Engineering Laboratory - LECIV
Av. Alberto Lamego – 2000, 28013-602, Campos dos Goytacazes, Brazil,
Phone/fax: 55 22 27261533
*
[email protected]
Keywords: Brick, Grog, Microstructure, Recycling, Properties.
Abstract: In the present paper crushed fired brick waste, known as grog, screened at two
different particle size, 840 (20 mesh) and 420 µm (80 mesh), was used in mixtures with
clayey body to make typical red ceramics for bricks. The effect of the grog addition up to 20
wt.% on the properties and microstructure of bricks fired at 700oC was evaluated. The results
indicated that both the particle size and the amount of grog addition changed the fired
properties of the clayey body. Additions above 5 wt.% of grog with a coarser particle size,
decreased the mechanical strength of both the dry body and the fired ceramic pieces. On the
other hand grog with a finer particle size may be used up to 10% wt. without deteriorate the
properties and corresponding microstructure of the clayey body.
Introduction
Non-plastic materials have an important function on red ceramic bodies due to the
possibility of adjusting the plasticity. Moreover, these materials facilitate the drying stage as
well as the breakdown reactions that occur during the firing stage [1]. Thus, for very plastic
clays it may be necessary to add non-plastic material to improve the process conditions.
Crushed powder, obtained from fired refractory clays or fired red ceramic wastes, also known
as grog, is a non-plastic material usually used in mixture with clay to make ceramics for civil
construction sector [2-4]. In the case of highly plastic clays, the grog usually enhances some
characteristics of the unfired body, such as the permeability, which facilitates the drying
stage. However, its addition can be detrimental to the porosity and mechanical strength of the
final ceramic product. This being the case, the characterization of the grog and the limit for
the amount to be added into clayey body that will be convenient for both the processing and
the quality of the ceramic product, become important aspects.
In the municipal area of Campos dos Goytacazes, located at the northern region of the
state of Rio de Janeiro, southeast of Brazil, there is a great production of red ceramics, mainly
perforated bricks, estimated in 70 million pieces/month. Bricks are normally fired at
temperatures around 600oC in Hoffmann type furnace, using firewood as fuel. Considering
only the firing stages, 900 tons of fired brick wastes are generated every month by the local
industries.
In previous works [5,6] grog from bricks fired at 600oC was added up in quantities up
to 20 wt.% in roofing tile bodies that were than fired at 970oC. It was observed that this
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addition enhances the pre-firing parameters and practically did not change the porosity of the
fired pieces. This was attributed to the sintering reaction that the grog suffered when fired
above its original production temperatures.
In the present paper, the effect of the same type of grog from bricks produced at
600°C, added into a clayey body before firing at 700oC was studied. The objective was to
investigate the influence of the grog’s particle size on the properties and microstructure of the
fired clayey ceramic.
Materials and Methods
The raw materials used were awaste from bricks produced at 600oC and a typical
kaolinitic plastic clayey body employed for red ceramic production in the county of Campos
dos Goytacazes, State of Rio de Janeiro, Brazil. The brick waste was jaw crushed and
screened to 20 mesh (840 µm) and 80 mesh (420 µm) generating a powder material, which is
here denoted as the grog. Figure 1 shows the two particle size distribution curves of the grog,
sieved at 20 mesh and 80 mesh, obtained by screening and sedimentation techniques [7]. It is
observed that the grog screened at 80 mesh, G#80, shows a mean particle size of 40 µm, that is
smaller than that screened at 20 mesh, G#20, with a mean particle size of 160 µm.
Additions of 5, 10 and 20 wt.% of grog to the clayey body were carried out in a dry
mixer. The clayey body was actually composed of two local kaolinitic clays. The
compositions produced, including that of the grog-free body, CB, formed only by the mixture
of the two clays, are shown in Table 1.
100
Cumulative mass percent finer
Coordenação
Grog(#20)
Grog(#80)
80
60
40
20
0
1
10
100
1000
Equivalent spherical diameter (µm)
Figure 1. Particle size distributions of the grog screened at 20 and 80 mesh.
Table 1. Elaborated compositions (wt.%).
Raw
materials
Clayey body
Grog
CB
100
0
CB5G20
95
5
CB5G80
95
5
Compositions
CB10G20 CB10G80 CB20G20 CB20G80
90
90
80
80
10
10
20
20
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Cylindrical test specimens, with a diameter of 31 mm and 11 mm thick, were prepared
by uniaxial pressing at 20 MPa, with 8 wt.% water. The specimens were dried at 110ºC, until
constant weight was achieved, and then fired in an electric laboratory furnace at 700oC, with
60 minutes at the soaking temperature. The heating/cooling rate was 3oC/min. Tests to obtain
linear shrinkage, water absorption and mechanical strength were carried out. Linear shrinkage
was assessed as the difference between the diameter of the dry and fired specimens divided by
the diameter of the dry specimen. Water absorption was calculated by measuring the weight
gained by the test specimens after placing them in boiling water for two hours. The
mechanical strength, was determined by the diametrical compression, the so-called Brazilian
disk test [8], performed on an universal testing machine, Instron, model 5582, with 0.075
mm/min cross head displacement.
The microstructure of the fracture surface of selected fired samples was studied by
scanning electron microscopy, SEM, using a Zeiss model DSM 962 equipment. The pore size
distribution between 0.00648 to 8.8884 µm was obtained by mercury intrusion porosimetry,
using a contact angle of 140o, in an Autoscan 33 Quantachrome Porosimeter.
Results and Discussion
Table 2 shows the bulk density, ρs, and the mechanical strength, σs, of the dried,
unfired, compositions. It is observed a decrease in the bulk density of the ceramic body with
additions above 5 wt.% of grog, screened at 20 mesh, and above 10 wt.% of grog, screened at
80 mesh. Except for the composition CB5G80, the mechanical strength of the compositions
did not practically change up to 10 wt.% of brick waste additions. This property is highly
influenced by the porosity and atomic bonding among the particles [9]. Since the degree of
packing associated with the bulk density of the compositions with 20% of brick waste,
CB20G20 and CB20G80, is smaller than that of the ceramic body, the porosity is probably
responsible for the decrease in the strength of these compositions. On the other hand, the
higher mechanical strength of the unfired composition CB5G80 may be attributed to the
elevated dry bulk density.
Table 2. Dried technological properties of the compositions.
Properties
ρs
(g/cm3)
σs
(MPa)
CB
1.97
±0.01
1.23
±0.11
CB5G20
1.95
±0.01
1.21
±0.03
CB5G80
2.01
± 0.01
1.69
±0.10
Compositions
CB10G20 CB10G80 CB20G20 CB20G80
1.92
1.96
1.92
1.90
±0.01
± 0.01
±0.01
±0.01
1.17
1.38
0.89
1.03
±0.04
±0.06
±0.04
±0.04
Figure 2 shows the water absorption, linear shrinkage and the diametral compression
of the fired compositions as a function of grog addition with different particle size. One
should observe that the water absorption does not practically change for the ceramic body and
compositions with grog screened at 20 mesh. It is also observed that the additions of 5 wt.%
of grog with lower particle size, screened at 80 mesh, decreased the water absorption as
compared with that of the ceramic body. Additions of 10% did not change the water
absorption,, however, higher amounts of grog increased the water absorption.
These results show that the best values of water absorption obtained, Fig. 2, for the
composition CB5G80 are probably associated with its higher dry bulk density. The results also
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26
5
25
4
3
2
Water absorption (%)
6
(a)
1.5
24
23
1.0
22
Water absorption
21
1
20
0
19
0.5
Linear shrinkage
Linear shrinkage (%)
Diametrical compression (MPa)
indicate that the grog produced at low temperatures suffers sintering reactions when fired at
elevate temperatures. The results in Fig. 2 also indicate that, within the statistical error, the
grog additions did not change the linear shrinkage of the ceramic body. The low temperature
used in this work, 700oC, was insufficient to promote densification and, consequently,
promote substantial closing of the porosity. With respect to the mechanical strength, it is
important to notice that the particle size of the grog influence the mechanical behavior of the
compositions. Additions above 5 wt.% of grog screened at 20 mesh decreased the mechanical
strength of the ceramic body. Conversely, a finer grog, i.e., that screened at 80 mesh,
decreased the mechanical strength only when added in an amount of 20 wt.%. This can be
attributed to the higher surface area of the finer grog that make it possible a stronger particle
consolidation with the clay. Another factor that justifies these results could be the bulk
density of the compositions, as previously discussed.
Diametrical compression
0
5
0.0
10
20
Grog - 20 mesh - (%)
26
5
25
4
3
2
1
0
Water absorption (%)
6
(b)
1.5
24
23
1.0
22
21
20
19
0.5
Water absorption
Linear shrinkage
Linear shrinkage (%)
Diametrical compression (MPa)
Coordenação
Diametrical compression
0
5
10
0.0
20
Grog - 80 mesh - (%)
Figure 2. Fired technological properties of the compositions. (a) grog screened at 20 mesh
(840 µm). (b) grog screened at 40 mesh (420 µm).
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Fig. 3 presents the porosimetric curves for the fired bodies CB, CB20G20 and
CB20G80. The behavior of these curves indicates that, for a firing temperature of 700°C, the
compositions show practically the same amount of accumulated intrusion volume of mercury.
This indicates that additions of grog with two different particle size did not change the open
porosity of the ceramic body CB. Figure 3 also reveals that a grog addition of 20 wt.% tends
to coarsen the pores. This can be associated with the lower dry bulk density of this
composition in comparison with that of the ceramic body CB.
3
3
Accumulated intrusion volume x 10 (cm /g)
Coordenação
100
CB
CB20G20
CB20G80
80
60
40
20
0
0.001
0.01
0.1
1
10
Pore diameter (µm)
Figure 3. Porosimetric curves of the compositions.
SEM analysis performed on the fracture surface of the industrial ceramic body CB and
on those of compositions with 20% of grog addition, CB20G20 and CB20G80, are shown in
Fig. 4. It can be seen that all bodies present a rough fractured surface with loosen particles and
interconnected pores. An apparently lower amount of pores is observed, Fig. 4(a), for the
industrial ceramic body, CB, in comparison with that of the compositions CB20G20 and
CB20G80, figs. 4(b) and 4(c), respectively.
(a)
(b)
(c)
Figure 4. SEM micrographs of the fractured region of the compositions fired at 700oC. (a)
CB; (b) CB20G20; (c) CB20G80.
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Conclusion
The addition of a brick waste, also known as grog, with two different particle size and
produced at low temperatures, of the order of 600°C, into a red ceramic body that was then
fired at 700oC, led to the following results:
• The grog, screened at 20 mesh, which was added in amounts above 5 wt.%, worsen the
packing of the particles and so reduced the dry mechanical strength of the compositions. The
same behavior was obtained with a finer grog, screened at 80 mesh, only for 20 wt.%
addition.
• According to the evaluated properties, additions of grog screened at 20 mesh may be, in
practice, performed up to 5 wt.%. With a finer particle size, screened at 80 mesh, additions
may be done up to 10 wt.% without compromising the quality of the ceramic. Therefore,
according to the decrease in water absorption and the increase in mechanical strength, it is
suggested that the best addition would be that of 5 wt.% of grog, screened at 80 mesh.
• The results showed that the incorporation of this type of waste could be advantageous for
the processing and quality of the ceramic, aggregating value to a waste material. Moreover, it
can also be considered an environmentally correct solution.
.
Acknowledgements
The authors wish to thank the Brazilian federal agencies, CNPq (Process 150444/20036) and CAPES, for supporting this investigation as well as the Rio de Janeiro state agencies,
FAPERJ and FENORTE/TECNORTE, for providing scientific initiation scholarships and
technical grants.
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