Preparation, characterization, and physical properties of lightweight ceramics by using sewage sludge ash

Abstract

In the present study, sewage sludge ash (SSA) was added to clay to prepare lightweight ceramics for sustainable construction materials. The characterization and the effect of different concentrations of SSA on the physical and mechanical properties of the samples were studied. The results showed that the organic matter in SSA facilitated the combustion process. SSA addition reduced the bulk density from (1.94 to 1.32 g/cm³). Otherwise, the water absorption, the apparent porosity and the loss on ignition increased with the increase in SSA concentration. The addition of SSA lowered the compression strength but still within the standard range of the construction materials at concentration up to (30 wt.%). Furthermore, heavy metals are solidified inside the sintered samples, since Cu, Cd, Fe, Zn, Cr, Mn, Ni, and Pb, concentrations in the leachate met the range of Egyptian standard specification.

Full text

Int J Appl Ceram Technol. 2020;00:1–10. wileyonlinelibrary.com/journal/ijac
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© 2019 The American Ceramic Society
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INTRODUCTION
At present, the processing of waste is one of the most important
challenges in the waste management everywhere.1 Increasing
the growth of cities and industry resulted in a massive increase
in the volume of wastewater over the world.2 Furthermore, by
using suitable handling methods, a considerable amount of
materials and energy can be retrieved from waste.1 Sewage
sludge ash (SSA) is a kind of semisolid waste that is produced
as a byproduct during the sewage treatment of the industrial
or the municipal wastewater treatment plants (WTP). SSA en-
closes a great number of pollutants such as organic contami-
nants, heavy metals, and pathogenic microorganisms, which
represent serious negative environmental impact.3
So far, there are three main methods for the disposing of
SSA such as sea dumping, soil application, and landfilling.4
These methods are accompanied by some problems and hence
negative environmental impact.5 Problems of landfilling result
from the high moisture state and high content of volatile solids
in the sludge.2 On the other hand, sea dumping causes water
pollution and toxicity of sea biology.2,6 The heavy metals con-
tained in SSA cause the problems of soil application.2 So, there
must be novel methods for the disposing of this type of waste.
The reusing of SSA with the addition to clay to produce
lightweight ceramic products is economically and environ-
mentally promising. It reduces the consumption of clay and
saves traditional raw materials, also provides appropriate
disposing of the water treatment plant sludge so minimize
the negative impacts of WTP residues on the environment.7,8
SSA belongs to the renewable pore-forming agent since it has
high organic matter content.2,9 The break down of these com-
pounds during the firing step of ceramics lead to the creation
of pores that decrease ceramics density.9 Using SSA to pro-
duce ceramic products can cut down the cost of the additional
energy.2 Therefore, the objective of the current research is to
use SSA (containing a large amount of organic matter) with
clay to prepare lightweight ceramic having good physical
properties for sustainable construction materials.
Received: 26 November 2019
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Revised: 26 November 2019
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Accepted: 12 December 2019
DOI: 10.1111/ijac.13456
ORIGINAL ARTICLE
Preparation, characterization, and physical properties of
lightweight ceramics by using sewage sludge ash
Asmaa A.Negm1
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Tarek Y.Elrasasi2
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Hanem A.Khoder2
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Fatma M.Metawe1
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Mabrouk K.El-Mansy2
1Basic Science Department, Faculty of
Engineering at Shoubra, Benha University,
Cairo, Egypt
2Physics Department, Faculty of Science,
Benha University, Benha, Egypt
Correspondence
Asmaa A. Negm, Basic Science
Department, Faculty of Engineering at
Shoubra, Benha University, Cairo, Egypt.
Email: asmaanegm@yahoo.com.
Abstract
In the present study, sewage sludge ash (SSA) was added to clay to prepare light-
weight ceramics for sustainable construction materials. The characterization and the
effect of different concentrations of SSA on the physical and mechanical properties
of the samples were studied. The results showed that the organic matter in SSA fa-
cilitated the combustion process. SSA addition reduced the bulk density from (1.94
to 1.32g/cm3). Otherwise, the water absorption, the apparent porosity and the loss on
ignition increased with the increase in SSA concentration. The addition of SSA low-
ered the compression strength but still within the standard range of the construction
materials at concentration up to (30 wt.%). Furthermore, heavy metals are solidified
inside the sintered samples, since Cu, Cd, Fe, Zn, Cr, Mn, Ni, and Pb, concentrations
in the leachate met the range of Egyptian standard specification.
KEYWORDS
ceramic engineering, characterization, porous materials, waste disposal

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NEGM Et al.
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EXPERIMENTAL PROCEDURE
2.1
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Sewage sludge and clay samples
The SSA used in this work was collected from the water treat-
ment plant in Benha (Egypt). The obtained SSA was dried at
105°C for 4hours and then crushed by mortar to pass 212μm
sieves. The used clay was obtained from the Egyptian deserts.
The samples of clay were milled and sieved to 212μm particles
in order to get a uniform particle size. The particle size distribu-
tion of both clay and SSA powders was determined by analyti-
cal shaker sieve (AS 200 control-RETSCH). The two curves of
clay and SSA show particle sizes between 20 and 200μm as
shown in Figure 1. It is also seen that the average particle size,
d50, is 100 and 109μm for clay and SSA respectively.
2.2
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Preparation of ceramic samples
Four different weight percentages of SSA to clay were evalu-
ated: (0, 10, 20, 30, and 40 wt.%). The materials were finely
mixed using a mixer with dropwise of water (15% by weight)
to induce some plasticity since water content is a key fac-
tor that can influence the shaping and the drying of the final
product.9,10 The mixtures were molded by cylindrical steel
die with a diameter of 14.5mm and a height of 17mm by
uniaxial pressing at a pressure of 105MPa. An example of
the green prepared samples is shown in Figure 2.
Green prepared samples were left at room temperature for
24hours, followed by an oven drying at 105°C for another
one hour so that the weight was maintained constant. After
drying, samples were transferred to a laboratory electric
furnace for sintering. Heating followed a system of grad-
ual increase in temperature to avoid cracking since it takes
90minutes until it reaches 600°C at which it is left for 1hour
then rises to the maximum temperature of 800°C and re-
mains for 30minutes. Finally, the prepared samples were left
to cool down gradually. An example of the sintered sample
is shown in Figure 3.
2.3
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Characterization
As the major properties of the ceramic materials are connected
to their mineralogical composition, the raw materials and the
prepared samples were analyzed by X-ray diffraction (XRD)
with a Philips X-ray diffractometer using monochromatized
CuKα1 radiation of wavelength 1.54056 Å from a fixed
source operated at 45kV and 9mA, and the scanning electron
microscope (SEM), Model Quanta 250 Field Emission Gun
attached with EDX Unit (energy dispersive X-ray analyses),
with accelerating voltage 30 kV. The differential thermal
analysis (DTA) and thermo gravimetric analysis (TGA) using
(STA 504) was used to investigate the thermal behavior dur-
ing sintering in flowing nitrogen atmosphere at a heating rate
20°C/min in the range between room temperature and 900°C
to obtain mass decomposition and phases transformation.
2.4
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Physical properties
Some physical parameters were measured on clay/SSA sam-
ples. These parameters include; bulk density (B), apparent
porosity (P), water absorption (A), loss on ignition (LOI),
FIGURE 1 Particle size distribution
curves of A: clay, B: sewage sludge ash
respectively
FIGURE 2 An example of the green
prepared sample: A is clay/SSA (0 wt.%)
and B-D are clay/SSA (20 wt.%). SSA,
sewage sludge ash
(A) (B) (C) (D)

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NEGM Et al.
and linear firing shrinkage which were performed according
to (ASTM C373-88 and ASTM C20-00).
Prepared samples for each concentration of clay/SSA
were dried at 105°C to constant weight and the dry weight
was determined (D) in grams. The test specimens were
placed in distilled water and boiled for 2hours. Afterwards,
the test specimens were cooled to room temperature while
still completely immersed in water for 24hours. The sus-
pended weight (S) of each test specimen after boiling and
while suspended in water was determined. Then, the satu-
rated weight (W) was determined by weighing in air.
The bulk density B (g/cm3), apparent porosity P (%),
and water absorption A (%) were estimated by the following
equations’:
The linear firing shrinkage were expressed as a percent-
age and calculated according to the following formula:9
LOI was determined by measuring the mass loss of the
sample between the drying and firing steps. They are ex-
pressed as a percentage and calculated according to the fol-
lowing formula:9
2.5
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Leaching test
Due to the presence of heavy metals in sewage sludge, their
leachability from SSA and clay/SSA 30 wt.%) samples were
determined according to the literature.11 500mL of distilled
water which was acidified with 1N HNO3 to pH= 4, was
added to 5g of ground-up samples which had particle size
below 125μm. Samples prepared in this way have been stir-
ring by a magnetic stirrer for 5hours, and the pH of solu-
tions throughout the process was kept constant by using 1N
HNO3. After leaching, the suspensions were centrifuged. The
obtained clear solutions were filtered, and the concentration
of heavy metals of the remaining elements was detected with
Atomic Absorption Spectrometer (Thermo Scientific ICE
3300).11
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RESULTS AND DISCUSSION
3.1
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Characterization
3.1.1
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Chemical composition of raw
clay and SSA
Table 1 shows the analysis of the chemical composition of
raw clay and SSA materials. It can be noticed from the table
that SiO2 represents the high content of the clay (49.82 wt.%)
followed by Al2O3 (2.63 wt.%) then Fe2O3 (10.98 wt.%) and
traces of other oxides such as K2O, Na2O, CaO, TiO2, and
MnO. The presence of these main compounds in the raw
materials of the ceramic facilitates the densification in the
prepared samples since, these oxides lead to liquid phase,
transformed into glass phase up on cooling and so decrease
the optimum firing temperature.11,12
The LOI of clay is due to the vaporization of water con-
tained in its boundaries during the ignition as well as the
(1)
B=
D
∕(
w
−
s
)
(2)
P
=
W−D
W−S
×
100
(3)
A=
W−D
D
×
100
(4)
Linear firing shrinkage (%)
=
L
dried
−L
fired
L
dried
×
100
(5)
Loss on ignition (%)
=
W
dried
−W
fired
W
dried
×
100
FIGURE 3 An example of the
sintered samples: A is clay/SSA (0 wt.%)
and B-D are clay/SSA (20 wt.%). SSA,
sewage sludge ash
(A)(B) (C
)(
D)
TABLE 1 The chemical composition of clay raw material and SSA by XRF
Chemical
constituen SiO2Fe2O3Al2O3TiO2MnO MgO CaO Na2O K2O P2O5Cl SO3LOI
(wt.%) Clay 49.82 10.98 24.63 1.09 0.05 1.52 0.32 0.56 0.81 0.14 0.05 0.01 9.79
SSA 10.28 7.78 2.29 0.88 0.12 0.13 8.80 0.03 0.60 1.66 0.16 5.78 61.12
Abbreviations: LOI, loss on ignition; SSA, sewage sludge ash.

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NEGM Et al.
organic matter in this clay.13 For SSA, it can be seen that, only
about 38 wt.% of SSA is inorganic since it showed high value
for LOI (61.12 wt.%) which represents the organic matter that
releases during firing.
3.1.2
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Mineralogy of the prepared samples
Figure 4 shows the XRD analysis of both clay raw mate-
rial and SSA to establish the inorganic crystalline compo-
sition. The main mineral components of raw clay which
were identified include quartz (SiO2) (code: 01-074-3485)
and gypsum (CaSO4.2H2O) (code: 01-074-1904), fol-
lowed by kaolinite (Al2Si2O5(OH)4) (code: 00-001-0527)
and montmorillonite (Na0.3(Al, Mg)2Si4O10 (OH)2.xH2O)
(code: 00-058-2010) as shown in Figure 4A. For SSA,
Figure 4B shows that SSA contains quartz and magnesium
doped calcite (Mg0.06 Ca0.94) (CO3) (code: 01-089-1305)
and followed by Tamarugite (Na Al (SO4)2(H2O)6), (code:
01-071-2385).
Figure 5 shows the XRD pattern for clay/SSA (0, 10, 20,
30, and 40 wt.%) fired ceramic samples. It can be noticed
that the Kaolinite peaks disappeared in all fired samples and
the Quartz peak at (2Ɵ=26.7°) is more stable since it ap-
pears in all fired samples, but its intensity decreased. Also,
it can be noticed that the Montmorillonite converted to Illite
(2K2O3·MgO·Al2O24·3SiO12·2H2O) (code: 00-002-0050)
upon firing. Also, Gypsum converted to -Anhydrite (CaSO4)
(code: 00-003-0162) due to the release of water molecules.
Albite (NaAlSi3O8) (code: 00-001-0739) appeared and there
was an increase in the anhydrite peak height as the fraction
of SSA increased which confirm X-ray fluorescence (XRF)
analysis. Also, the Illite decrease in higher SSA samples may
be due to the interaction with the calcite of SSA.14
3.1.3
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Thermal analysis
Figure 6A shows the DTA and TGA curves of the clay. It can be
noticed from this figure that there are four peaks, two peaks are
endothermic and the other two are exothermic. The first endo-
thermic peak at 150°C is due to the removal of physical-bound
water (dehydration) from the clay with a weight loss of (3.5
wt.%) of the sample. The second endothermic change occurs
with the most prevailing peak at about 550°C is due to the re-
moval of chemical-bound water. Also, this reaction, is known
as dehydroxylation, whereby part of the clay structure (hy-
droxyl groups) is destroyed. the XRD confirms the disappear-
ance of the kaolinite peaks in all fired samples.15 The weight
loss of approximately 6 wt.% because of dehydroxylation is
shown on the TGA curve. As for the exothermic reactions,
peaks recorded at 400 and 750°C are caused by burn-off of
carbonaceous organic matter, which mainly derives from plant
and animal fossils in the clay samples.15
Figure 6B shows the DTA and TGA curves of SSA. There
is a clear broad endothermic effect, beginning at 50°C and
ending at 277°C and the maximum occurs at 120°C which
FIGURE 4 X-ray diffraction patterns
of A: clay raw material and B: sewage
sludge ash respectively
FIGURE 5 X-ray diffraction pattern for clay/sewage sludge
ash (0, 10, 20, 30, and 40 wt.%)

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NEGM Et al.
is attributed to the removal of physical-bound water (dehy-
dration). About (5.3 wt.%) of the weight was lost during this
effect.11 The endothermic effect goes to an exothermic effect
with three peaks at 380, 470, and 542°C which finished around
700°C. A weight loss of about 41.9 wt.% of the initial sample
is associated with this effect. In the range of 277 to 380°C, the
sample has lost (16.4 wt.%) which most likely corresponds
to the depolymerization reactions of biodegradable matter,
and at the second step from 380 to 522°C, the weight loss of
(24.1 wt.%) should be ascribed to the degradation reactions of
non-biodegradable organic matter.16 The end of this process
appears in the temperature of 650-670°C and is characterized
by the sudden disappearance of the exothermic. At the final
step of the heating process (>700°C), a clear endothermic
effect with a maximum at 771°C, during which the residual
carbonaceous material and ash experienced the mass loss at
slow rates.12 Since SSA has a relatively high firing sensitivity
below 650°C, it is necessary to lower the heating rate and/or
to hold for some time at 650°C or higher constant temperature,
in order to fully destroy the organic matter from SSA and to
eliminate their impact on the sintering of ceramic samples.
The effect of SSA addition on the firing process of the
samples
Figure 7A shows the TGA curves for clay/SSA (10, 20, and
30 wt.%). It can be noticed from the figure that the decom-
position process can be divided into two stages. The first
stage corresponds to the dehydration in the temperature
range from 80-180ºC, during which; the TGA curves for
different concentrations of SSA are approximately similar.
As previously discussed, due to hydroxylation, the second
stage became in the range of 200-600ºC instead of in the
range of 400 to 600ºC with the addition of SSA to clay.
This drop-in mass loss is due to the decomposition of the
organic matter of SSA. The addition of SSA led to a de-
crease in the initial temperature of decomposition at this
stage as shown in Figure 7B for DTA curves which means
that SSA works as internal fuel during the combustion pro-
cess of ceramic samples and the so retrieve the energy of
SSA.
Kinetic analysis
The knowledge of the process which takes place during the
thermal treatment of the clay is necessary to understand the
sintering behavior of ceramic material and the influence of
additives on the process. Coats-Red fern method was used
to analyze the obtained data.17 The obtained results with
Coats-Red fern are presented in Table 2. Figure 8 shows
the typical plots of ln (−ln (1−χ)/T2) vs 1/T for clay/SSA
10 and 30 (wt.%) samples as an example in the two steps
of the decomposition. The figure shows that decomposi-
tion is a first-order reaction. It can be noticed from Table
2 that the decomposition energy, E, depends on the SSA
ratio and tends to decrease as this ratio increase. The or-
ganic matter in SSA seemed to facilitate the combustion
process since hydroxylation shifted to a lower temperature
FIGURE 6 TGA and DTA for
A: clay and B: sewage sludge ash
respectively
FIGURE 7 A, TGA and B: DTA for
clay/sewage sludge ash (10, 20, 30 wt.%)
samples

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NEGM Et al.
for higher concentration which confirmed the decrease in
the consumed energy.
3.1.4
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Microstructure analysis
Figure 9 shows the SEM of the different prepared samples.
It can be noticed from the figure that there are differences
in the pore structure and the cracks in the prepared sam-
ples. Generally, the samples exhibited an increase in the
numbers of pores and cracks with increasing SSA concen-
tration. Also, it is noted that the nature of the samples dif-
fered, especially for the SSA ratio above (20 wt.%) since
the samples became rougher as shown in Figure 9D. This
is due to the increase in organic matter of SSA, which gets
burning during the sintering of the samples and leaves be-
hind many pores.2 This is clear from EDX in Table 3 since
the carbon element increases with increasing SSA ratio.
Also, it is clear from Table 3 that the ratio of CaO increases
and SiO2 decreases with increasing SSA ratio in the sam-
ples which assert the XRD analysis.
3.2
|
Physical properties of
lightweight ceramics
3.2.1
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Bulk density and apparent porosity
Figure 10 shows the dependence of the bulk density and
the apparent porosity on SSA concentration. It is clear that
TABLE 2 Decomposition energy for clay/SSA (0, 10, 20 and 30 wt.%) (Coats-Red fern method)
SSA concentration.
(wt.%) 0 10 20 30
Range of temperature
T (°C)
94-140 492-584 103-186 288-437 450-578 106-194 296-409 424-560 94-176 306-369 369-510
Decomposition
energy (KJ/mole)
66.3 179.2 47.3 46.1 140.8 44.6 65.9 111.2 21.6 28.6 13.5
Abbreviation: SSA, sewage sludge ash.
FIGURE 8 Plots of ln (−ln
(1−χ)/T2) vs 1/T: A-B for clay/SSA
(10 wt.%) sample in first and second
decomposition regions, respectively, and
C-D clay/SSA (30 wt.%) sample in first and
second decomposition regions respectively.
SSA, sewage sludge ash

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NEGM Et al.
the bulk density of the samples is inversely proportional to
the concentration of SSA. The total organic matter in SSA
is quite high as seen from thermal and XRF analyses which
have been released from the ceramic products during firing
leaving various pores and cracks so the porosity of the ce-
ramic body will be increased as shown in Figure 10B and
hence the density will be decreased.2,8
3.2.2
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The water absorption
The water absorption is increased with increasing SSA con-
centrations as shown in Figure 11. During the firing of the
samples, SSA makes porous structure in the ceramic body so
the increase in water absorption may be due to the growth in
open porosity of grains and bigger heterogeneity of the grain
shape as seen from Figure 9.11
3.2.3
|
Weight LOI and linear
firing shrinkage
Figure 12 shows the weight LOI and linear firing shrinkage vs
SSA concentration. It can be noticed that the increase in the SSA
concentration resulted in an increase in ceramic weight LOI. As
seen from the thermal and XRF analysis of SSA that about (61
wt.%) of its weight is an organic matter which releases during
the firing and contributes to the increase in the loss in weight
of the ceramic during the firing process.7 Also, it can be seen
that the change in the linear firing shrinkage can be attributed to
the evaporation of the added water during the preparation. One
can see that the change in the values of linear firing shrinkage is
within the experimental error which verifies that this property is
independent of SSA concentration in the mixture.18 The quality
of ceramic products can be classified according to the degree
FIGURE 9 Scanning electron
microscope of clay/sewage sludge ash A:
(0 wt.%), B: (10 wt.%), C: (20 wt.%) and D:
(30 wt.%)
TABLE 3 EDX of clay/SSA (0, 10, 20 and 30 wt.%) samples
Element
SSA concentration (wt.%)
0 10 20 30
C 3.93 4.46 6.08 9.88
O 31.03 32.38 32.21 30.9
Na 1.08 0.84 0.87 1.07
Mg 1.35 1.27 1.45 1.36
Al 15.91 14.36 14.47 15.02
Si 33.57 31 30.11 23.93
Cl 0.21 0.28 0.88 0.39
K 1.44 1.28 1.35 1.25
Ca 0.55 2.42 1.82 4.06
Ti 0.98 0.93 0.88 1.05
Fe 9.95 9.11 9.88 9.47
Abbreviations: EDX, energy dispersive X-ray; SSA, sewage sludge ash.

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NEGM Et al.
of firing shrinkage. Normally, for bricks, a good quality one
exhibits shrinkage below 8%.7 All of the prepared samples have
shrinkage below 4%.
3.3
|
Mechanical properties
3.3.1
|
Compression test
Figure 13 shows the effect of SSA concentration on the
compressive strength of the fired prepared samples. It can
be noticed that compressive strength depends on the con-
centration of SSA in the samples. Compressive strength
is usually affected by the porosity and the pore size so
the observable reduction in strength was mainly due to
the increase in porosity and consequently the decrease in
density.19 Also, one can notice that up to (30 wt.%), the
ceramic samples met the Egyptian requirement for building
materials.20 Samples with greater than (30 wt.%) of SSA
addition are not suitable for use since they are fragile and
easily deform.
3.4
|
Leachability test
Leachability measurements of elements in water extracts
showed that the presence of toxic metals in SSA is relatively
FIGURE 10 A, Bulk density and B:
Apparent porosity with SSA concentration.
SSA, sewage sludge ash
FIGURE 11 Water absorption with SSA concentration. SSA,
sewage sludge ash
FIGURE 12 The weight loss on ignition and the linear firing
shrinkage vs SSA concentration. SSA, sewage sludge ash
FIGURE 13 The effect of SSA concentration on the
compressive strength of the fired samples. SSA, sewage sludge ash

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NEGM Et al.
higher than the threshold values as prescribed in (Egyptian
standard specification of drinking water 2007) as shown in
Table 4. On the other hand, clay/SSA (30 wt.%) sample has
shown dropping in the presence of those metals. This means
that the metal elements solidified in the sintered samples and
are harmless to the environment.21
4
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CONCLUSIONS
SSA was used with clay to obtain lightweight ceramics.
The chemical composition of SSA was close to clay. The
densities of the fired samples varied between 1.94 and
1.32 g/cm3, which means a decrease in 38% compared
to the density of the sample without SSA. The results
showed that SSA enhances the combustion process of the
samples. Small shrinkage occurred in the samples due
to dehydration. Water absorption and apparent porosity
were increased with the increase in SSA concentration.
The porosity and shape of pores have a noticeable effect
on the mechanical strength. The compressive strength of
the samples decreased with SSA concentration but, it is
still higher than the Egyptian standard strength values. It
can be concluded from the above results that lightweight
ceramics with acceptable compressive strength were ob-
tained with a reduction in the consumed energy in the fir-
ing process.
ACKNOWLEDGMENT
The authors thank the Water Treatment Plant in Benha
(Egypt) for their help in providing SSA for this research. In
addition, the authors thank the staff members of the physics
department faculty of science at Benha University, Egypt for
accessing their laboratories.
ORCID
Asmaa A. Negm https://orcid.org/0000-0002-0450-6805
Tarek Y. Elrasasi https://orcid.
org/0000-0002-2133-7724
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TABLE 4 Concentrations of heavy metals in SSA and clay/SSA
(30 wt.%) samples
Heavy metal
(mg/L)
SSA
(mg/L)
Clay/SSA 30
wt.% (mg/L) Standard
Cu 0.081 0.009 2
Cd 0.023 Nd 0.003
Fe 0.415 Nd 0.3
Zn 3.454 0.637 3
Cr Nd Nd 0.05
Mn 1.150 0.203 0.1
Pb Nd Nd 0.01
Ni 0.11 0.124 0.02
Abbreviations: Nd: not detected; SSA, sewage sludge ash.

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How to cite this article: Negm AA, Elrasasi TY,
Khoder HA, Metawe FM, El-Mansy MK. Preparation,
characterization, and physical properties of
lightweight ceramics by using sewage sludge ash. Int
J Appl Ceram Technol. 2020;00:1–10. https ://doi.
org/10.1111/ijac.13456