Journal of Minerals and Materials Characterization and Engineering, 2013, 1, 321-325
Published Online November 2013 (ht t p:/ / )
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Evaluation of the Refractory Properties of Nigerian
Ozanagogo Clay Deposit
Alexander Asanja Jock1*, Fatai Afolabi Ayeni1, Abdulkarin Salawu Ahmed2, Umar Adeiza Sullayman2
1National Metallurgical Development Centre, Jos, Nigeria
2Ahmadu Bello University, Zaria, Nigeria
Email: *
Received September 2, 2013; revised October 18, 2013; accepted October 29, 2013
Copyright © 2013 Alexander Asanja Jock et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this paper, the refractory properties of clay from Ozanagogo had been studied for possible utilization in refractory
production. The clay had a specific gravity of 2.57, linear firing shrinkage of 1.01%, 2.14 g/cm3 bulk density and poros-
ity of 20.4%. A cold crushing strength of 17.48 MN/m2 was obtained with modulus of rupture of 8.28 MN/m2. The
thermal shock resistance exceeded 30 cycles and the refractoriness was 1750˚C. The sample was analysed for its chemi-
cal composition, and it was revealed that it contained 38.07% alumina (Al2O3), 46.00% silica (SiO2) and iron impurities
(Fe2O3) of 0.78%. The results generally showed that Ozanagogo clay could be used as a refractory material.
Keywords: Ozanagogo; Clay; Porosity; Refractory
1. Introduction
Clays differ very considerably among themselves in struc-
ture, workability, plasticity, particle-size distribution, and
mineralogical composition. These differences lead to
such terms as flint clays, plastic clays, fireclays, kaolins,
ball clays and clay minerals are grouped into kaolinite,
montmorillonite and the illite. Clays have variety of ap-
plications such as in refractory making, as catalysts, ab-
sorbents, binders and fillers, and the useful characteris-
tics of the most versatile material are being appreciated.
The production of special grogs or aggregates, the puri-
fication, bleaching and organic modifications of clays are
all acquiring new emphasis at present.
Refractories are materials capable of withstanding
very high temperature without an undue deformation,
softening or change in composition [1]. They include
aluminosilicates, silica, magnesite, chrome, carbon, car-
bides, nitrides etc., and they are classified as acid, basic
and neutral refractories. Refractories are employed for
the construction of furnaces, kilns, crucibles, flues used
in high temperature operations. Refractory clays are
broadly grouped into fireclay and kaolins, both are based
on the mineral kaolinite (Al2O3·2SiO2·2H2O). Fireclays
are the most widely used refractory materials and about
70% of refractories are fireclay produced mainly from
clays with alumina content ranging from 25% to 45% [2].
Impurities in fireclay are pyrites, quartz, calcites, ferrous,
carbonates and some organic compounds. The organic
impurities impart plasticity to the clay while impurities
like quartz and iron redu ce their refractoriness.
Clay deposits are widely distributed in Nigeria [3-6].
In spite of the extensive use and the demand for clay in
industrial processes, Ozanagogo clay deposit in Delta
State is used mainly for local paint and building bricks,
and investigation of its refracto ry prop erties is being con-
ducted so as to reveal its other potentials as a refractory
material. The clay is chosen for evaluation mainly be-
cause of its bulk availability and proximity to Ajaokuta
Steel Complex [3].
2. Materials and Methods
2.1. Sample Collection and Preparation
The clay sample was randomly collected in lumps form
from the deposit site at Ozanagogo in Delta State, South
Nigeria. The lumps were sun dried for a week to reduce
moisture content and enhance grinding. The clay lumps
were then crushed, ground and sieved.
2.2. Chemical Composition Analysis
*Corresponding author. The ground clay sample was dried and the chemical com-
position in wt% of Al2O3, SiO2, Fe2O3, TiO2, CaO, Na2O
and K2O was determined using Energy Dispersive X-Ray
Fluorescence Spectrometer (ED-XRF) model PW1660,
XRA. The loss on ignitio n (LOI) of the clay (Mainly vol-
atile matters) was determined by measuring the weight
loss of a known mass of the sample after firing in furnace
at 1000˚C for 1 hour 30 minutes. Loss on ignition was
calculated using this relation :
LOI, %100
 (1)
where, Wi and Wf are initial and final weight respectively.
2.3. Specific Gravity
The specific gravity test is usually conducted on materi-
als that do not dissolve in or get attacked by water. The
test piece was cut from within the core of a refractory
shape moulded from the clay and crushed to a size not
exceeding 3 mm [7]. The crushed material was then
mixed and reduced to a 50 g sample by cone and quar-
tering method. The sample material obtained was dried at
110˚C to a constant weight, and then 10 g sample was
weighed using a glass stoppered weighing bottle. A py-
conometer with stopper was dried at 110˚C, cooled in a
desiccator and its weight (Wp) was noted. The pycono-
meter was filled with distilled water at room temperature;
with its stopper put in place, the weight (W1) was noted.
The test refractory sample was put in dry pyconometer,
covered with the stopper and weighed (W). The stopper
was removed and distilled water was added to the sample
to fill the pyconometer to its half capacity. This was gen-
tly boiled for 10 minutes to avoid loss of sample due to
popping. The weight (W2) of the pyconometer contain ing
the test sample and the water was recorded. The specific
gravity was calculated from the following:
Specific Gravit yp
 (2)
where, W is the weight of test sample and dry pycono-
meter with stopper, Wp is the weight of dry pyconometer
with stopper, W1 and W2 are the weights of pyconometer
with stopper filled with distilled water and pyconometer
with stopper containing the test sample and distilled wa-
ter respectively.
2.4. Brick Production
The sequence adopted in making refractory bricks is
crushing, grinding, sizing, mixing and forming, drying,
firing, and cooling [8,9].
The clay sample was dried and calcined by firing to
1200˚C for 8 hours in a muffle furnace (RHF 16/15 mo-
del), making the clay lose its plasticity by forming grog
(firesand). The grog was then crushed, ground and
screen-ed through sieves size of 2000 µm, 710 µm and
212 µm to represent the coarse, medium and fine frac-
tions respectively, necessary to improve the manufacture
of high packing density products. The grog aggregates
were blended with 10% unfired clay which was capable
of developing desire plasticity when mixed with 5% wa-
ter. The resulting mixture was compressed with a hy-
draulic press in a cylindrical mould of dimensions 100
mm × 50 mm × 25 mm. The pressed brick was left over-
night to air-dry and th en oven dried at 110˚C for 8 hours
before firing in a muffle furnace upto 1300˚C. Firing of
the brick sample was done gradually at the rate of
5˚C/minute.The sample was soaked at 1300˚C for 2
hours and allowed to cool gradually in the furnace over-
2.5. Firing Shrinkage
The firing shrinkage is found to be the most useful and
relevant property in the production of refr actory bricks. It
is determine by measuring the dimensional changes be-
tween the dried and fired bricks. In this work, the dis-
tance between the two ends of the sample was measured
with a venier caliper after the drying and firing processes.
The firing shrinkage was calculated thus:
Firing Shrinkage100
where, LB = Dry dimension; LD = fired dimension.
2.6. Bulk Density
The bulk density of a refractory body is represented by
the weight per unit volume including pore space. The
method used in determining the bulk density of the re-
fractory sample was boiling method. A moulded fired
brick specimen measuring 30 mm by 30 mm by 20 mm
was prepared. The brick was air dried for 24 hours and
oven dried at 110˚C to a constant weight (D). After
which it was transferred to a beaker and boiled with dis-
tilled water for 2 hours to assist in releasing trapped air.
It was then allowed to soak and the saturated weight free
of excess water (W) was taken. The specimen was then
suspended in water using a beaker and the suspended
weight (S) was taken. The bulk density was then calcu-
lated using the relationship [1],
Bulk densityw
where, D = Dried weight; W = Saturated weight; S =
suspended weight; ρw = Density of water.
2.7. Apparent Porosity
Porosity is the percentage relationship between the vol-
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A. A. JOCK ET AL. 323
ume of the pore space and the total volume of the sample.
Using boiling method, the test specimen measuring 30
mm × 30 mm × 20 mm was cut from the fired refractory
brick and then dried in an oven at 110˚C to a constant
weight (D). The dried specimen was suspended freely in
distilled water and boiled for 2 hours, cooled to room tem-
perature and its weight (S) noted. The specimen was re-
moved and the soaked or saturated weight in air (W) was
recorded. The apparent porosity was calculated using,
Apparent porosity100
where, W = soaked or saturated weight; D = Dried weight;
S = suspended weight.
2.8. Cold Crushing Strength
This is the load at which cracks appear in the specimen.
The test piece was cut from the fired brick in the form of
cubes of about 25 mm size. The test piece was marked to
indicate the direction in which forming pressure was ap-
plied and the two faces normal to this direction were
prepared as bearing faces. Cardboard was placed be-
tween the platens of the press and the bearing faces of the
test piece. The load was applied at a uniform rate until
the test piece failed to support the load. The maximum
recorded load was taken as the crushing load and the area
obtained from the size of the test piece before the appli-
cation of load was calculated and recorded. The cold
crush-ing strength was determined using the formula,
Cold crushing strengt hLA (6)
where, L = Maximum load (KN); A = Cross sectional area.
2.9. Modulus of Rupture (MOR)
This is a measure of the fracture strength (by bending) of
a refractory product against the forces of breakage or
crack due to application of certain amount of pressure.
The test piece at room temperature was cut from the fired
brick into 25 mm × 25 mm × 100 mm box. The test piece
was supported near its ends and loaded at the centre
(three-point-load) until failure occurs. The breaking load
(maximum load) was obtained using the motorized proc-
essing Monsanto Tensometer, type “W” and the MOR
was calculated by,
where, W = load at which the specimen failed (N); b =
width of specimen (m); h = height of specimen (m); L =
distance between the centre of the two supports (m).
2.10. Thermal Shock Resistance
Thermal shock resistance is the number of heating and
cooling (cycles) needed to cause conspicuous crack on
the sample [10]. This test was performed by heating the
specimen in a muffle furnace preset at 1200˚C for 10
minutes, after which, it was air cooled for another 10
minutes and observed for cracks. This process was re-
peated up till 30 cycles without crack being observed.
2.11. Refractoriness
The refractoriness is a measure of fusibility of material.
It indicates the temperature at which the material softens.
The Pyrometric Cone equivalent (PCE) method was used
to determine the r efractoriness on the ground Ozan agogo
clay sample [1,10]. The clay sample was dried and ground
to fines, and watered to make a plastic mass. The test
piece was then formed into a suitable mould of pyramid
shape with 12.7 mm × 12.7 mm base and 38 mm height.
British standard and test piece cones were fixed with
cement at the centre of a refractory plaque. Both cones
were placed in the furnace. The heating rate over the last
200˚C below the estimated fusion temperature was care-
fully controlled at 5˚C per minute and was observed by
the use of an optical pyrometer. The heating continued
till the tip of the test cone bent touching the refractory
plaque. The plaque bearing the test cone was removed
from the furnace and the test cone examined when cold.
Refractoriness of the test cone was determined by the
temperature equivalent on the standard cone that bent to
a large extent similar to the test cone.
3. Results and Discussion
3.1. Results
The results of chemical analysis, physical properties of
Ozanagogo clay and comparison of the properties with
established standard are as shown in Tables 1-3 respec-
3.2. Discussion of Results
The chemical composition of Ozanagogo clay presented
in Table 1 showed that the alumina (Al2O3) content was
38.07 wt%, silica (SiO2) 46.00 wt% and iron (Fe2O3)
content was 0.78 wt%. The alu mina in the clay was with-
in the range of 25% - 45% required for fireclay refracto-
ries (the higher the alumina content in clay, the higher
the refractoriness) [10]. The iron oxide (0.78 wt%) is
within the acceptable range of 0.5% - 2.4% for refractory
clays. Similarly the low values of the fluxing oxides such
as CaO and MgO further confirmed that the clay could
be used for refractory application since high amount of
these oxides are expected to cause adverse effects on the
refractory properties such as lowering the melting point
of the clay [1]. The loss on ignitio n (LOI) is 13.70% and
is a ls o w i th i n the range of 9% - 14% required in a firecla y
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Table 1. Chemical analysis of Ozanagogo clay.
Composition Al2O3 SiO2 Fe2O3 MgO CaO K2O Na2O TiO2 LOI
Percentage. (wt%) 38.07 46.00 0.78 0.09 0.14 0.02 0.07 1.32 13.70
Table 2. Physical properties of Ozanagogo clay.
Properties Values
Specific gravity 2.57
Linear firing shrinkage (%) 1.01
Bulk density, (g/cm3) 2.14
Apparent porosity, (%) 20.40
Cold crushing strength, (MN/m2) 17.48
Modulus of rupture, (MN/m2) 8.28
Thermal shock resistance, (cycles) 30
Refractoriness, (˚C) 1750
Table 3. Comparison of the determined properties of Ozanagogo clay with some established refractory standard.
Properties Ozanagogo clay
Specific gravity 2.57 2.6 - 2.7**
Linear shrinkage, (%) 1.01 7 - 10
Bulk density, (g/cm3) 2.14 1.71 - 2.1
Apparent porosity, (%) 20.4 20 - 30
Cold crushing strength, (MN/m2) 17.48 14 - 84**
Modulus of rupture, (MN/m2) 8.28
Thermal shock resisitance, (cycles) 30 25 - 30
Refractorines, (˚C) 1750 1500 - 1700
*Gilchrist (1977), **Misra (1975).
[11]. The loss on ignition (LOI) is the organic or com-
bustible or volatile matter loss on heating and this affect
the values of shrinkage and porosity to some extent.
3.3. Specific Gravity
The specific gravity result of Ozanagogo clay obtained was
2.57 and this was closed to range of 2.6 - 2.7 required for
high heat duty fireclay refractory as shown i n Table 3.
3.4. Linear Firing Shrinkage
The result of the linear firing shrinkage of the clay de-
termined was 1.01% and less than the recommended
range standard of 7% - 10% for refractory clays (Table
3). However, the low value obtained in this study could
be due to the fact that 90 % of the brick composition was
made of grog, which is thermally stable and also serves
as anti-shrinkage agent [10].
3.5. Bulk Density
The bulk density of the clay was 2.14 g/cm3 and this
value was slightly greater than the typical bulk density
for fireclay refractories which was 1.91 g/cm3 as sug-
gested by Gilchrist [11]. The high bulk density obtained
that was above the typical one could be explained by the
level of grittiness or coarse size of the grog fr actions used.
3.6. Apparent Porosity
Ozanagogo clay gave an apparent porosity of 20.40%,
which was within the range of 20% - 30% required for
firebrick clay as reported by Gilchrist [11] in Table 3.
3.7. Cold Crushing Strength
The cold crushing strength of the clay determined is
17.48 MN/m2, and is above the minimum recommended
value of 14 MN/m2 for dense fired brick reported as by
Misra [10]. This value, however, shows that Ozanagogo
clay can comfortably withstand impacts at low tempera-
tures as the cold crushing strength is an indicator of the
effect of firing on ceramic bond.
3.8. Modulus of Rupture
The modulus of rupture of the clay was 8.28 MN/m2.
A. A. JOCK ET AL. 325
This was quite good as fireclay refractories whose strength
ranged between 2 MN/m2 and 4 MN/m2 were believed to
have performed well [7].
3.9. Thermal Shock Resistance
The result of the thermal shock resistance displayed in
Table 2 shows that the number of cycles without failure
is 30. This value falls within the 20 - 30 number of cycles
recommended for fireclay refractories reported as by
Gilchrist [11].
3.10. Refractoriness
The result in Table 2 showed that the refractoriness of
the sample occurred at a temperature of 1750˚C. This
high temperature might have been due to the appreciable
amount of the alumina content (38.07%) in the clay. The
alumina in the clay was a strong indicator of its refracto-
riness and the higher the alumina, the higher the refracto-
riness [1].
4. Conclusion
Ozanagogo clay has b een investigated in this work for its
suitability as a refractory material. It was revealed that on
the basis of physio-chemical characteristics of this kao-
linitic deposit, it could be processed for use as refractory
material. The investigated properties gave excellent re-
sults for refractory application. Ozanagogo clay is found
to be a good substitute for imported refractories used for
lining of furnaces, kilns, crucibles ladles, soaking pits
and flues. The deposit site also has an advantage of close
proximity to Ajaokuta Steel Complex in Nigeria. This
work also suggested that further work should be carried
out to determine other important properties such as re-
fractoriness under load (RUL) and slag resistance. The
clay should be exploited not only for making refractory
bricks but also for chemical and paper industries.
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Delhi, 1986, pp. 55-63.
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