One of the most important environmental issues in developing countries around
the world is poor waste management (Tchobanoglous et al.,
1993; Begum and Joy Jacqueline, 2008). Traditional
waste management includes illegal dumping of wastes at unsuitable locations,
or disposal in ill-designed or mismanaged landfills (Shin
et al., 2005; Chuangcham et al., 2008).
Compositions of wastes contain chemicals that may be both nontoxic and dangerous
compounds (Tchobanoglous et al., 1993; LaGrega
et al., 2001). Highly toxic substances can cause serious contamination
of soil, water and the atmosphere that lead to endangerment of all living organisms.
More specifically, they can enter the food chain and affect to human and animals
(LaGrega et al., 2001; Minocha
and Bhatnagar, 2007).
Increased concern regarding waste management in Thailand has been placed on
hazardous waste management. The new waste management notification was enacted
by Department of Industrial Works, Ministry of Industry Work (DIW) on 2006.
This notification increased liability of waste generator, waste transporter
and waste treatment company (Anonymous, 2006). Both non-hazardous
and hazardous wastes have been addressed in this notification using six-digit
waste codes to classify wastes into groups and identify proper management alternatives
Traditional hazardous waste disposal in a secure landfill has its downfalls.
First, it faces the problem of attaining public approval, which limits the amount
of area available for this method (Espinosa and Tenorio,
2000; Lin and Lin, 2005; Shin
et al., 2005). Furthermore, the operation of a secure landfill incurs
high investment costs, which makes its hazardous waste management costs relatively
high. Many industries are, therefore, in need of a lower cost alternative or
one that could generate economic benefit (Raupp-Pereira
et al., 2008; Tsakiridis et al., 2008).
Alternative wast e treatment technologies such as incineration and co-processing
help to address these issues. Despite the fact that both techniques employ a
burning process to reduce the amount and volume of hazardous waste, they are
significantly different. Incineration produces highly toxic ash that requires
a further treatment process, such as stabilization/solidification, before it
can be dumped into a secure landfill (Shin et al.,
2003; Trezza and Scian, 2007). Co-processing, on
the other hand, works toward waste minimization and saves natural resources
by utilizing wastes or by-products as raw materials or fuel in production processes
(Mokrzycki et al., 2003; Dalton
et al., 2004; Holcim and GTZ, 2006). This technology
was first utilized by the cement industry in the 1970s (Kleppinger,
1993); some of the wastes were used as substitutes for main chemical compounds
in the raw materials, such as silica, alumina and iron (Trezza
and Scian, 2000, 2005; Kaantee
et al., 2004; Pipilikaki et al., 2005;
Frias et al., 2006; Navia
et al., 2006; Kolovoes, 2006). The use of
co-processing in the cement industry continues to this day and allows for easy
destruction of organic contaminants such as used oil, contaminated soil, scrap
tires and expired chemicals because of high temperatures of more than 1400°C
in cement production and be suitable for air pollution technology (Espinasa
and Tenório, 2000; Kurdowski, 2002; Holcim
and GTZ, 2006).
In recent years, there has been an increasing interested in burning of hazardous
waste in cement production. The serious discussions of co-processing of hazardous
waste in cement kiln were addressed to the impacts of heavy metals on cement
product and environmental risk (Espinasa and Tenorio, 2000;
Shin et al., 2005; Kolovoes,
2006; Trezza and Scian, 2007). Thus, the research
to date has tended to focus on effect of utilization of real hazardous waste
rather than pure chemical oxides.
The main purpose of this study was to investigate the potential of partially substituting ordinary cement raw meal with grinding sludge as alternative raw materials in Portland cement clinker production. The substitution of grinding sludge in the cement raw meal will introduce varying concentration of minor metal elements such as cadmium, chromium, nickel and zinc.
MATERIALS AND METHODS
Ordinary cement raw meal and grinding sludge analysis: The ordinary
cement raw meal was collected from a Thailands local cement plant in,
2007. The chemical compositions of the ordinary cement raw meal were analyzed
by Thermo electron corporation ARL9900 OASIS X-ray fluorescence spectroscopy
(XRF). The chemical composition of raw meal was compared with ideal chemical
composition, minimum composition and maximum composition of raw meal (Peter,
Grinding sludge from the iron forging industry was received from a waste collector
(roughly 50 kg). It was dried at 105°C for 24 h in a laboratory electrical
oven. The pH of grinding sludge was detected via HACH sension1pH meter. The
chloride content of grinding sludge was determined by chloride potentiometer
titration and the sulfur content was tested by sulfur analyzer. The main chemical
composition of grinding sludge was measured via XRF. The grinding sludge was
digested by Milestone ETHOS SEL microwave digester following USEPA SW846- method
3052 and then the concentration of heavy metals were analyzed via Varian Vista
MPX Axial EL02086289 Inductively Coupled Plasma spectroscopy (ICP). The concentration
of heavy metals was employed to classify kinds of waste (Anonymous,
Sample preparation: 1, 2 and 3% by weight of grinding sludge were mixed
with cement raw meal in order to produce the raw meal to be tested for the cement
production. The above samples are referred to M1, M2 and M3 respectively, while
the reference sample (M0) means the pure cement raw meal. Homogeneity was ascertained
by dosing the added grinding sludge on the mixtures.
Burning procedure:All samples including M0, M1, M2 and M3 were placed
in a high alumina crucible and burnt at 900°C for 30 min and subsequently
burnt at 1400 °C for 1 h in a high temperature electric furnace. Once done,
they were cooled rapidly in the air and kept strictly in dry condition. After
burning process, the raw meal samples were called the synthesized Portland Cement
(PC) clinker (Altun, 1999).
Experimental procedures: The basic clinker analyses consisted of free
lime content, chemical composition, phase identification and microstructure
of sample. The purpose of these basic clinker analyses is to understand the
effects of this waste on cement product. Moreover, the applied clinker analysis
containing incorporation of heavy metals and regulatory leaching test were applied
to evaluate the environmental risk of cement product as well.
Free lime content: The effect on the burning activity was evaluated
on the basis of the unreacted lime content or free lime content in synthesized
PC clinker. The free lime content can also change when CaO takes place in a
reaction that leads to a complete cement phase. This parameter is the first
favorite parameter in order to control a cement quality and be employed to measure
a burning condition in cement rotary kiln. The synthesized PC clinkers were
grounded. Ethylene glycol method was employed to determine the free lime content
in the synthesized PC clinkers (Kakali el al., 2003,
2005; Kolovoes, 2006).
Chemical composition of the synthesized PC clinker: To control the quality
of cement product in cement industry, the main parameters generally apply from
the proportion of main oxide in PC clinker. They are called Modulus equations.
The modulus equations are comprised of Lime Saturation Factor (LSF), Silica
Ration (SR) and Alumina Ration (AR). The modulus parameters have limited value
of LSF, SR and AR at 90-101, 1.4-4.2 and 0.6-4.2, respectively (Kurdowski,
2002). The modulus parameters were expressed as Eq. 1-3.
Moreover, the reaction in cement product depends upon the main chemical compounds.
There are four main complex chemical compounds including tri-calcium silicate
(C3S) or alite, di-calcium silicate (C2S) or belite, tri-calcium
aluminate (C3A) or celite and tetra-Calcium aluminate ferrite (C4AF)
or browmillerite. The calculation potential composition of this compound is
referred to Bogue equations. The Bogue equations were expressed in Eq.
The chemical compositions of synthesized PC clinker were determined via XRF.
The results were reported in percentage of oxide. These results usually put
into Bogue and the modulus equations in order to roughly calculate the predictable
amount of main complex chemical compounds and the important parameters (Neville,
Phase identification of the synthesized PC clinker: This technique was
used in order to identify the mineralogical phases formed during the sintering
of the clinker and find out the differentiation caused by grinding sludge. The
X-ray Diffraction Spectroscopy (XRD) is the scientific instrument to identify
the crystalline compound in cement phases. Its pattern looks like a fingerprint
of crystalline compound. XRD was performed on synthesized PC clinker samples
using X Pert-PRO PW 3040/60 with Cu-Ka radiation, 30 kV-30 mA and 1 second
per step in the range of 2θ from 5° to 80°. The XRD result is commonly
compared to crystalline pattern of PC clinker from a local company.
Microstructure of synthesized PC clinker: The microstructure of synthesized
PC clinker samples were measured by Olimpas BX51 optical microscopy. The clinker
sample were cracked and put in epoxy resin for polished section. A section of
material has been ground and plane polished on one face for examination under
an optical microscope. After that, the crystalline phase in sample was looked
and taken photos at 5x, 50x and 100x. The Optical Microscopy (OM), which is
the light method, was used in order to study the effect of the added oxides
on the texture of the produced clinker. The OM analysis explained the crystal
size and distribution and other features of clinker to assess productions conditions.
The information obtained can be used to predict the likely performance of cement
made from the clinker, or perhaps indicated the cause of production difficulties
such as poor combination.
Incorporation of heavy metals in synthesized PC clinker: In order to
verify the heavy metals really entrapped in the cement structure, the residual
of heavy metals in synthesized PC clinker were determined to characterize the
incorporation of heavy metals in synthesized PC clinker. The amount of heavy
metals in clinker and raw material samples were digested following US EPA SW846
method 3052 and analyzed via ICP-AES.
About 0.5 g of samples was digested in 9 mL of concentrated nitric acid and
3 mL of hydrofluoric acid for 15 min using a laboratory microwave digester.
A specific temperature profile was programmed such that the temperature of 180±5°C
must be reached in approximately less than 5.5 min and maintained for 9.5 min
for the completion of the reaction. After cooling, 37 mL of 0.87 M H3BO3
was added the vessel. The specific temperature profile for second digestion
was programmed such that the temperature of 160±5°C must be reached
in approximately less than 13 min and maintained for 6 min for the completion
of the reaction. Finally, the cooled samples in the vessel were filtered through
0.45 μm filter papers, diluted to volume and measured by ICP-AES (US.
Environmental Protection Agency, 1996).
Regulatory leaching test of synthesized PC clinker: The leaching tests
were performed according to Thailand regulatory and US regulatory. The Thailand
regulatory was enacted by department of industry work (Anonymous,
2006). The main acid leaching solution used in Wet Extraction Test (WET)
was citric acid at pH 5 while the US EPA method employed acetic acid at pH 2.88.
The WET and TCLP were applied with synthesized PC clinker in order to confirm
the stability of heavy metals in co-processed cement product and apply the leaching
experiment as environmental risk tool. The interesting heavy metals in this
section were only Thai regulate heavy metals such as Cd, Cr, Cu, Ni, Pb, V and
Zn. But, there are no regulatory standard for Cu, Ni, V and Zn in TCLP method.
Wet extraction test (WET): About 50 g of sample was weighted into polypropylene
bottles. The 500 mL of 0.2 M sodium citrate solution (adjusted to pH 5±0.1
with 4.0 M NaOH) was added and agitated at room temperature for 48 h. The extracted
solution was filtered through a 0.45 μm membrane filter, preserved by HNO3
and stored at 4°C. The concentration of heavy metals was analyzed by ICP-AES
Toxicity characteristic leaching procedure (TCLP): The TCLP test employed
in this study followed the standard procedure described by the US
EPA (US. Environmental Protection Agency, 1992). About 10 g of the sample
was weighed and placed into each of the polypropylene bottles. About 200 mL
of the TCLP No. 2 leachant (0.1 M HOAc at pH 2.88) was added. The bottles were
tumbled at 29±1 rpm in a rotary extractor at room temperature for 18
h. At the end of the extraction, the leachate was filtered with GF/C glass fiber
filter paper. The pH of the filtrate was measured and the leachate was acidified
by a small amount of concentrated nitric acid to a pH of less than 2 before
subsequent analysis by ICP-AES.
RESULTS AND DISCUSSION
Raw materials analysis: The chemical compositions of the collected raw
meal were illustrated in Table 1 together with those from
the literature as a comparison. Quality of Portland cement clinker depends on
its raw meal chemical composition. The contents of oxides in the raw meal obtained
from the experiment are approximately 15% of SiO2, 3% of Al2O3,
3% of Fe2O3 and 42 % of CaO. The ideal chemical composition
was 14% of SiO2, 4.1% of Al2O3, 1.6% of Fe2O3
and 43.2 % of CaO. The typical ranges of chemical compositions for raw meal
were 6.9-15.9 % SiO2, 1.9-4.7% Al2O3, 0.6-1.9%
of Fe2O3 and 41.7-49.0% CaO (Peter,
2004). It is apparent that the chemical compositions of the raw meal were
appropriate for synthesizing clinker in this research.
Grinding sludge analysis: A local waste collector supplied the grinding
sludge for using in this research. This sludge, a residue from grinding process,
was generated by iron forging industry. The chemical characteristics of the
grinding sludge were determined and shown in Table 2. The
pH value of this sludge was 8.85. The main chemicals composition of grinding
sludge consisted of 5% of Al2O3, 0.3% of CaO, 75% of Fe2O3
and 15% of SiO2. The minor chemical compositions such as MgO, K2O,
Na2O and SO3 were found less than 1% by weight. For the
chloride content, it was not found in the sample.
||Chemical composition of the cement raw meal obtained from
|*Peter (2004) NA: Not available
||Chemical characteristics of the grinding sludge obtained from
iron forging industry
|ND: Not detected
||Heavy metals in the grinding sludge compared with Thai regulations
set by the Department of Industrial Works, Ministry of Industry
|ND: Non detected
Desired alternative raw materials for cement production must contain compounds
that consist mainly of silica, alumina and iron whereas undesired compounds
are sulfur, chloride and heavy metals (Kolovos et al.,
2001; Pollmann, 2002). The Thai cement company controls
pH, sulfur content and chloride content. The pH should be less than 4, while
the sulfur and chloride content must be less than 2.5% (w/w) and 0.5 %(w/w),
respectively. According to Thailand regulations set by the Department of Industrial
Works, under the Ministry of Industry, this sludge is characterized as a hazardous
waste and has the waste code number of 12 08 18 HA (metal sludge (grinding,
honing and lapping sludge) containing oil) (Anonymous, 2006).
As shown in Table 3, the concentrations of heavy metals such
as cadmium, chromium, mercury and nickel in this sludge were reported higher
than the Thai regulation values. The highest concentrations of iron and manganese
were observed. For the local cement company, an interesting alternative raw
material must contain more than 50% of iron (w/w). It is, however, corresponding
to many researchers reporting that the reactivity of cement increases at high
heavy metal contents (Trezza and Scian, 2000; Kolovoes
et al., 2002; Shin et al., 2005; Kolovoes,
2006). Therefore, the high heavy metals contents in the sludge have the
potential to be applied as an alternative raw material in the cement industry.
Mercury (Hg) was excluded because volatile heavy metals such as mercury do not
become incorporated into clinker.
Free lime content and chemical compositions in synthetic PC clinker:
The results of free lime content and chemical compositions of PC clinker were
displayed on Table 4. The chemical compositions were used
to calculate LSF, SR, AR, C3S, C2S, C3A and
As listed in Table 4, the free lime contents of M0, M1, M2
and M3 were 1.27, 0.83, 0.71 and 0.69, respectively. Free calcium oxide in small
amounts (usually below 1 wt.%) is a regular constituent of Portland clinker,
but larger amounts may be present if the maximum temperature in the production
of the clinker is too low, the burning time is too short, or the CaO content
in the raw material exceeds the acceptable range (lime saturation factor >100).
Large amounts may cause expansion, strength loss and cracking of the hardened
paste. It is due to a delayed hydration of free calcium oxide to calcium hydroxide.
Thus, excessive amounts of free calcium oxide in clinker must be avoided (Raupp-Pereira
et al., 2008). The free lime levels below 1% in an ideal clinker
(Trezza and Scian, 2000; Lin and
Lin, 2005; Raupp-Pereira et al., 2008) and
1.5% in a real clinker (Potgieter et al., 2002)
are considered acceptable. However, free lime content in the synthesized PC
clinker decreased when the substitution of grinding sludge increased. It indicated
that the formation of cement phases was complete reaction. Moreover, decreasing
in free lime means an improvement in the burnability.
The chemical compounds were composed of major compounds and minor compounds.
The major compound considered on CaO, SiO2, Al2O3
and Fe2O3, while the minor compounds were MgO, K2O,
Na2O3, SO3, P2O5, Cl,TiO2
and Mn2O3. The contents of CaO, SiO2,
Al2O3 and Fe2O3 in M0 were approximately
66, 22, 5 and 4%, respectively and the sum of minor compound was about 2.7%.
According to the M1, M2 and M3, the SiO2 content remained between
20 and 22% and the Al2O3 content were about 5.2, 4.9 and
4.4%, respectively while the Fe2O3 content was found at
5.1, 6.7 and 8.5%, respectively. The content of Al2O3
slightly decreased whereas the content of Fe2O3 slightly
increased. The sum of minor compounds of M1, M2 and M3 remained about 3%. From
these results, it can be noted that the major compounds still remained in the
range of cement product except the content of Fe2O3 in
M3. Moreover, the sums of minor compound in all synthesized PC clinker were
less than 5%. It associated with the limitation of minor components that must
be commonly under 5%.
The LSF, SR and AR were calculated from the modulus equations. Amount of LSF in M0 to M3 were found to be approximately 92.5, 91.5, 91.5 and 92.9%, respectively. The SR contents in M0 to M3 were about 2.6, 2.1, 1.8 and 1.7%, respectively. And, the AR contents in M1 to M3 were 1.5, 1.0, 0.7 and 0.5%, respectively. The results of study from Table 4 can be compared with the range of cement product. The percentage ranges of cement product for LSF, SR and AR were varied in the percentage ranges of 90-101, 1.4-4.2 and 0.6-4.2%, respectively. It was found that LSF and SR rations of all synthesized PC clinker remained in the range of cement product except the AR of M3 which below the range of cement product.
The amount of C3S, C2S, C3A and C4AF
were roughly estimated by the proportion of main chemical oxide with Bogue equations.
Amount of C3S in M0 to M3 were found to be 57.57, 54.85, 54.93 and
58.79%, respectively. The C2S in M0 to M3 were 20.54, 21.18, 19.18
and 13.32%, respectively. The C3A in M1 to M3 were 7.60, 5.04, 1.68
and -2.65%, respectively. And, the C4AF in M1 to M3 were 10.74, 15.46,
20.40 and 25.85%, respectively. It was obvious that the amount of C3S
and C2S of M0 to M2 fell within the range of cement product values.
Moreover, C3A had gone down but the C4AF had gone up when
the utilization of sludge increased. This waste promoted the formation of C4AF
and inhibited the formation of C3A. These results associated with
Tsakiridis et al. (2008). They studied on the
utilization of steel slag for Portland cement clinker production and found that
the increasing of C4AF in clinker occurred when there is the presence
of impurity ions (Fe). Moreover, they indicated that the iron in sludge was
able to combine with calcium and alumina to produce the ferrite phase upon cooling
from the melt.
Generally, C4AF reacts at a slower rate than C3A and
contributes little to the strength of the cement at any age. It is thought to
form both high and low sulphate forms from tetracalcium aluminoferrites during
its hydration, in the same manner of tricalcium aluminate. The change of C4AF
phase affects only the rate of hydration; as the iron content is raised, hydration
reaction becomes slower. High C4AF in cement are much more resistant
to sulfate attack (Neville, 2003). This sulfate attack
is a typical of attack by solutions of sodium sulfate or potassium sulfate.
Phase identification of the synthesized PC clinker: The phase identification
of a commercial Portland cement was shown in Fig. 1 whereas
Fig. 2a-d illustrated the phase identification
of synthesized sample M0 to M3. From the observation, it was interesting to
note that all XRD patterns of synthesized PC clinker were similar to the phase
identification of a commercial Portland cement. It can be implied that, their
crystalline phases of all synthesized PC clinker were similar to the main crystalline
phases in a commercial Portland cement. Moreover, the main mineralogical phases
(C3S, C2S and C4AF or browmillerite) in a commercial
Portland cement were well formed. The findings of this study can be suggested
that the utilization of grinding sludge did not influence crystalline formation
of cement product.
|| X-ray powder diffraction patterns of commercial PC clinker
from a local cement company
||X-ray powder diffraction patterns of synthesized Portland
cement clinker at different percent. (a) M0 at 1400°C 60 min, (b) M1
at 1400°C 60 min, (c) M2 at 1400°C 60 min and (d) M3 at 1400°C
Nonetheless, the intensity of commercial PC clinker was higher than the intensity
of synthesized PC clinker. The high amount of C4AF indicated that
the iron oxide was able to combine with calcium and alumina to produce the ferrite
phase upon cooling from liquid phase (Tsakiridis et al.,
2008; Shin et al., 2005). Stephan
et al. (1999) pointed out that heavy metal such as Ni, Cr and Zn
have no influence on the formation of clinker phases, even at concentrations
that are 10 to 20 times higher than the concentrations observed in normal clinkers.
Microstructure of synthesized PC clinker: Clinker microstructure was
examined by optical microscopy in polished sections. The utilization of grinding
sludge as alternative raw material in the clinker burning did not seem to affect
its microstructure and the formation of its characteristics mineralogical phases.
The synthesized PC clinker of M0 and M3 were compared with the commercial PC
clinker. They were displayed on Fig. 3a-i.
The M0, M3 and commercial PC clinker were examined at 5X, 50X and 100X. The
magnification at 5X showed the surface of samples. In the optical microscopy,
the belite was observed as bluish rounded crystals or blue color oval shape
and the alite is the brownish color in hexagonal shape. According to 50X, the
intensity of belite color in M0 and M3 was observed higher than in the commercial
PC clinker. The belite size of M0 was the same as commercial PC clinker whereas
the belite size of M3 was remarked bigger than M0 and the commercial PC clinker.
The modification at 100X was applied to investigate the C3S or alite.
The C3S color of M0 and M3 is darker than the commercial PC clinker.
However, the alite size of M3 is smaller than the commercial PC clinker. Both
of M0 and M3 appeared the secondary belite on alite. These results can be explained
by considering that during the cooling stage, the molten phase goes to a glass
or, if cooling is slow, the C3A crystallizes out and in extreme cases
the alite dissolved back into the liquid phase and reappeared as secondary belite
||Optical microscope examinations of commercial PC clinker and
synthesized PC clinker (M0 and M3) at 5X, 50X and 100X. (a) 5X M0 (b) 5X
M3 (c), 5X Commercial PC clinker, (d) 50X M0, (e) 50X M3, (f) 50X Commercial
PC clinker, (g) 100X M0, (h) 100X M3 and (i) 100X Commercial PC clinker
Incorporation of heavy metal in clinker: The incorporation percentages
of the foreign elements or heavy metals such of Cd, Cr, Cu, Ni, Pb and Zn were
obtained by comparing the total heavy metals of the raw materials to the synthesized
Portland cement clinkers. The results were provided in Table 5.
The results were found that the Cd and Pb incorporation degree in synthesized
PC clinker were roughly 50-52 and 40-56%, respectively. It indicated that the
Cd and Pb were evaporating during the burning process. The Cd incorporation
degree found in this synthesized PC clinker agrees with Barros
et al. (2004). They reported a Cd incorporation degree of 51%, when
studying a system with no chlorine. Meanwhile, the Pb incorporation degree associated
with Barros et al. (2004) and Kolovoes
(2006). Thus, Cd and Pb seem to be volatilized at all.
In contrast, the incorporation degree for Cu and Ni were about 92-96 and 92-96%,
respectively. The incorporation degree for Cr and Zn fluctuated between 88-
92 and 77-85%, sequentially. As many previous researches, the incorporation
degree of Cu, Ni, Cr and Zn were reported at least 90% (Espinosa
and Tenorio, 2000; Ract et al., 2003; Barros
et al., 2002, 2004; Shin
et al., 2005; Kolovoes, 2006).
||Incorporation degree (%) of Cd, Cr, Cu, Ni, Pb, V and Zn in
Hence, these heavy metals (Cu, Ni, Cr and Zn) seem to be entrapped into cement
Moreover, these results agree with the study of the volatility of minor elements
during the burning process of cement production (Kolovoes
et al., 2002). They expressed that the foreign cation can classify
into three groups, high volatile (like Cd and Pb), moderate volatile (like Ni
and Cu) and low volatile (like Cu). The high volatile and moderate volatile
heavy metals generally evaporated and jointed cement kiln dust. The cement kiln
dust must recycle into cement rotary kiln. This recirculation can accumulate
the heavy metals in cement product or increase the incorporation degree.
|| Leaching test results of synthesized PC clinker
Regulatory leaching test of synthesized PC clinker: The results of WET
and TCLP leaching tests were listed in Table 6. The WET concentration
of Cd, Cr, Cu, Ni, Pb, V and Zn in synthesized Portland cement clinker were
varied in the concentration ranges of 0.003-0.005, 0.054-1.281, 2.237-4.016,
0.200-0.725; 0.256-0.654, 0.367-0.477 and 0.209-0.125 mg L-1, respectively.
The TCLP concentrations of Cd and V in all samples were 0.001 and 0.002 mg L-1,
sequentially. The TCLP concentrations of Cr, Cu, Ni, Pb and Zn in synthesized
Portland cement clinker were varied in the concentration ranges of 0.011-0.315,
0.008-0.017, 0.007-0.008; 0.141-0.465 and 0.017-0.029 mg L-1, respectively.
The concentration of heavy metals from TCLP method was found less than the WET
analysis. However, the concentration of heavy metals both in WET and TCLP methods
were found below the standard limits. It is confirmed that the heavy metals
in synthesized PC clinker were encapsulate into clinker matrix. These results
associated with many previous researches (Ract et al.,
2003; Barros et al., 2004; Shin
et al., 2005; Kolovoes, 2006; Tsakiridis
et al., 2008).
From these findings, they were found to be feasible to use grinding sludge as alternative raw materials. The following conclusions can be employed to explain the effects of grinding sludge replacement during the clinker phase and the release of heavy metals in the environment from co-processed clinker.
A high amount of iron oxide in grinding sludge decreases the silica and alumina ratio, which hinders the occurrence of C3A and promotes the formation of brownmillerite or C4AF in clinker. The increasing of C4AF helped to promote the sulfate attack.
The utilization of grinding sludge in cement production reduced the free lime content. It confirmed that this waste can improve the burnability of clinker.
The utilization of grinding sludge in synthesized PC clinker did not effect on phase identification of synthesized clinker. The synthesized PC clinker phase is similar to the commercial PC clinker.
This sludge has potential applications in cement production when it included at a value of 3% or less in the raw meal because C3A disappears at 3% of sludge replacement. The disappearance of C3A reduce the phenomenon of flash set (instantaneous set) and a large amount of heat is generated. It can cause spontaneous over-heating in large masses of concrete.
The results of the WET and TCLP methods showed that the incorporated heavy metals cannot be released because they are trapped in the clinker matrix. These did not present a leaching hazard to environment.
In summary, the utilization of grinding sludge in cement production has a good potential of being used as an alternative raw material. It could be a promising alternative for the waste management.
This research has been funded by THE 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund) with grant number No. 3/2550, Chulalongkorn University and the National Center of Excellence for Environmental and Hazardous Waste Management (NCE-EHWM) of Chulalongkorn University.