Abstract: Cement kiln can be used for both destruction of hazardous organics and stabilization of heavy metals. This method provides alternative materials as raw materials or the fuel in the cement production process. However, the heavy metals in waste made the effect to cement properties. This study investigated the chemical and physical characteristics of various mixtures of cement clinker. Raw materials were mixed with different ratios of oxides of heavy metals: Namely, Cr (Cr2O3), Cu (CuO), Ni (NiO2), Zn (ZnO) for combinations of four kinds of heavy metals were between 0.05 to 2.0%. The mixtures were burned in a pilot-scale rotary cement kiln which the liquid petroleum gas fired. The results showed that the concentration of heavy metals in the clinker was higher than in the mixtures before combustion. Moreover, the heavy metals were not destroyed in the burning process and the crystals of heavy metals appeared in the clinker which were trapped to the C2S, C3S, C3A and C4AF. Improvement of early strength at the 1 day for 0.05% wt. of combination of heavy metals was more than 130 kg cm-2. The compressive strength at the 28 days of the clinker contained 0.05 and 0.3% wt. of combination of heavy metals were more than 400 kg cm-2. However, the leaching test have shown that the all of different ratio of heavy metals in clinker and cement mortar still in the range when compared with Thai standard. High concentration of heavy metals increased percent free lime but decreased percent C3S.
INTRODUCTION
The manufacturing industry generates both hazardous and non-hazardous wastes (Ract et al., 2003; Anonymous, 2006; Guner and Yucel, 2005). Therefore, the proper treatment of waste is essential, particularly of hazardous wastes which require complete and careful extermination. For example, some hazardous wastes need to undergo a destabilization process first, before being buried at a specific licensed, or secure landfill (Begum and Pereira, 2008; Minocha and Bhatnagar, 2007; Amu et al., 2005). However, the leachate from landfill will contain heavy metals such as Cd, Cr, Pb, Cu and Zn which can contaminate the soil (Chuangcham et al., 2008). Some hazardous wastes, such as used oil, solvents, isopropyl alcohol, thinner and acetone need to be incinerated at high temperatures. The treatment and disposal of hazardous waste and non-hazardous waste can be carried out by means of incineration employing cement kiln technology (Punmatharith et al., 2010; Anonymous, 2006; Ract et al., 2003; Dalton et al., 2004). Cement kilns which are operated at a high temperature of about 1450°C, are one of the well known methods for destruction of hazardous wastes and non-hazardous wastes and have been approved by the United States Environmental Protection Agency (US. Environmental Protection Agency, 1996) and the Department of Industrial Works (DIW) of the Ministry of Industry, Thailand (Anonymous, 2006). The use of hazardous waste incinerators and cement kilns are increasingly popular because they provide temperatures as high as 1450°C the heat level adequate for detoxifying any hazardous wastes (Dalton et al., 2004; Prodjosantoso and Kennedy, 2003). Cement kilns provide a new stability of cement products that leave no ash after incineration; the ash from the combustion of fuels or raw materials is incorporated into Clinker or Cement Kiln Dust (CKD) (Trezza and Scian, 2000; Barros et al., 2002; Dalton et al., 2004). Most cement kiln operators normally return the majority of the CKD to the kiln to complete the calcinations and burning processes (Dalton et al., 2004; Olaleye et al., 2005). Additionally, some wastes, such as wastewater sludge, chemical-contaminated soil and ash from the power plant, may be used as substitutes for raw materials and incinerated together with shale and limestone with high efficiency (Andrade et al., 2003; Ract et al., 2003; Punmatharith et al., 2010). Waste incineration in cement kilns is an exemplar model of environmental conservation and the added advantage of it being a low-cost incineration option has made it a popular waste treatment method worldwide (Punmatharith et al., 2010; Holcim and GTZ, 2006; Ract et al., 2003; Sinyoung et al., 2011).
The main objectives of this study were to analyze the components of heavy metals in the clinker after burning them in a pilot-scale rotary cement kiln and in the mortar. To find the application concentration values of heavy metals in raw meal that does not affect the quality of the clinker as well as to focus in the phase of clinker which contained heavy metals such as chromium, copper, nickel and zinc.
MATERIALS AND METHODS
Raw materials: Experimental raw meal (kiln feed) was collected from a Thailand’s local cement plant in, 2009 which had been heated in cement kiln of manufacturing for preheating at 900°C. The combinations of heavy metals (metal oxide) of 4 series are Cr (Cr2O3), Cu (CuO), Ni (NiO2) and Zn (ZnO). They were mixed with raw meal (called mixture) and a control one (without metal addition called control). The mixtures were homogenized and the concentration of heavy metals varied from 0.05, 0.3, 0.5, 1.0 and 2.0% by weight. The samples are referred to C0.05, C0.3, C0.5, C1 and C2, respectively while the reference sample (C0) means the pure cement raw meal or kiln feed from manufacturing.
The pilot scale rotary cement kiln: The pilot-scale rotary cement kiln was designed by author. It was prepared and constructed by the local cement plant in Thailand in 2008. The building has an area of about 72 m2 (12 m (length)x6 m (width)x6 m (height)). For the pilot-scale rotary cement kiln, it consisted of a rotary kiln, clinker cooler, burners, lifter and cooling tower. Its hot air outlet chamber has an inside circle diameter of about 0.60x0.60 m (length). The rotary kiln has an inside circle diameter of about 0.60x1.0 m (length), rotating at 0.5 rpm (revs per minute) and the fuel used was the liquid petroleum gas. The clinker cooler rectangular chamber has inside diameter of about 1.6 m (length)x1 m (width)x1.3 m (height). Natural air was used to decrease temperature of clinker. The CKD was trapped by water in cooling tower.
Methods: The pills of the mixtures were left in an oven at 100°C for 12 h and burned in a pilot scale rotary cement kiln of preheating at 700-900°C for 10 min from the time this temperature is reached. Finally, they were burned at 1450°C for 60 min and quenched in a clinker cooler; the final products of this experimental are clinkers. The clinkers were kept in a moisture-free container until testing. The application concentrations of each heavy metal in the mixture that can be co-processed by a cement kiln without affecting the quality of the clinker were considered. The maximum metal concentrations allowed in the Thai regulatory leaching procedure are health-based standards. Finally, the clinkers were analyzed the Total Threshold Limit Concentration (TTLC) which were digested by Milestone ETHOS SEL microwave digester following USEPA SW846- method 3052 and then by using the Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) via VARIAN model type VISTA-MPX Axial EL02086289 and the Soluble Threshold Limit Concentration (STLC) using Wet Extraction Test (WET) which 50 g of sample was weighted into polypropylene bottles. 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 (Anonymous, 2006). The chemical compositions of the clinkers were analyzed by X-ray Fluorescence Spectroscopy (XRF) and X-ray Diffraction (XRD) via Thermo electron corporation ARL9900 OASIS. The phases of clinker were analyzed by the Scanning Electronic Microscopy (SEM) via JEOL model type JSM-5800LV and the physical properties of cement were analyzed such as blaine testing and compressive strength (ASTM C109/C109M-05, 2005).
RESULTS AND DISCUSSIONS
Raw materials analysis: Kiln feed (C0) which was passed pre-calcinations process, was collected from the local cement plant in Thailand. The XRF was used to analyze the chemical composition in the kiln feed was showed in Table 1. Lime Saturation Factor (LSF), Silica Modulus (SM) and Alumina Modulus (AM) that serve as indication of combinability and burn ability of feed mixtures in burning at the pilot scale rotary cement kiln. The result shows that LSF, SM and AM of kiln feed were 75.38, 2.80 and 1.31%, respectively. However, the main chemical compositions of kiln feed which are important such as SiO2, Al2O3, Fe2O3 and CaO were equal to 18.04, 3.65, 2.79 and 42.69%, respectively. 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 (Hewlett, 2004; Amiri et al., 2008). The moisture content is very low only 0.09%. The Loss On Ignition (LOI) is good indicators to show percent loss of the weight of kiln feed and samples before burning at 1450°C, so the LOI of kiln feed was 30.56%.
| Table 1: | The chemical compositions of the collected kiln feed which was passed pre-calcinations process |
The concentrations of heavy metals in kiln feed, clinker and cement mortar: The results obtained from total concentration of heavy metals which were added in the kiln feed, the clinker and cement mortar were added with different ratios of Ni:Cr:Cu:Zn (Table 2, 3). The result showed that heavy metals which were added into the kiln feed have increased concentration in the clinker because during the burning process the compounds were reacted and some of them were evaporated such as CO2 and volatized compounds, so the volume and the weight of clinker have reduced lower than kiln feed are about 30-40%. However, the concentration of heavy metals in clinker must be not over Thai’s standard for classification of hazardous waste such as concentration of Ni, Cr, Cu and Zn in clinker must be less than 2000, 2500, 2500 and 5000 mg kg-1, respectively (Anonymous, 2006).
The heavy metals were found in the clinker and cement mortar at different ratio (C1-C5). The amounts of heavy metals increased and detected of initial concentrations of nickel doped in kiln feed were in range 141 to 5312 mg kg-1, whereas total concentration of nickel in clinker was increased in range 232 to 9175 mg kg-1. However, total concentration of nickel in cement mortar was decreased in range 78 to 2354 mg kg-1. Because of cement mortar was mixed with sand can be diluted the concentration of nickel in cement. Therefore, nickel was not destroyed in burning process and occurred in clinker. Because of nickel is moderate volatile element (Kolovoes et al., 2002) and high of melting of NiO2 and boiling point of Ni at 1455 and 2730°C, respectively.
| Table 2: | Analysis of the concentrations of heavy metal (combination of Ni:Cr:Cu:Zn = 1:1:1:1) in the kiln feed, the clinker and the cement mortar |
| *Standard limit Anonymous (2006) | |
| Table 3: | Analysis of the concentrations of heavy metal (combination of Ni:Cr:Cu:Zn = 1:3:5:7) in the kiln feed, the clinker and the cement mortar |
| *Standard limit Anonymous (2006) | |
Nickel oxides, when added to the clinker raw meal, do not evaporate during the burning process, at which 1% NiO, would not affect the emission ratio of these metals, even if the wastes have chlorides and are thus incorporated into the clinker (Barros et al., 2002). The TTLC of nickel by Thai (DIW) standard must be less than 2000 mg kg-1 to classify non hazardous material. Thus, the total concentration of nickel in kiln feed must be less than 1000 mg kg-1.
The initial concentrations of chromium doped in kiln feed were in range 145 to 5058 mg kg-1, whereas total concentration of chromium in clinker was increased in range 274 to 7573 mg kg-1. However, total concentration of chromium in cement mortar was decreased in range 95 to 2536 mg kg-1. Because of cement mortar was mixed with sand can be diluted the concentration of chromium in cement. Therefore, chromium was not destroyed in burning process and occurred in clinker. Because of chromium is moderate volatile element (Kolovoes et al., 2002) and high of melting and boiling point at 1857 and 2672°C, respectively. A large amount of chromium (84%) was trapped in Portland cement clinker (Murat and Sorrentino, 1996). Chromium oxides, when added to the clinker raw meal, do not evaporate during the burning process, by which 1% Cr2O3, would not affect the emission ratio of these metals, even if the wastes have chlorides and this oxide would be totally incorporated into the clinker (Barros et al., 2002). The TTLC of chromium by Thai (DIW) standard must be less than 2500 mg kg-1 to classify non hazardous material. Thus, the total concentration of chromium in kiln feed must be less than 1000 mg kg-1.
Considering the initial concentrations of Cu doped in kiln feed were in range 189 to 5025 mg kg-1, whereas total concentration of Cu in clinker was increased in range 268 to 5280 mg kg-1. However, total concentration of Cu in cement mortar was decreased in range 81 to 1611 mg kg-1. Because of cement mortar was mixed with sand can be diluted the concentration of Cu in cement. Therefore, Cu was not destroyed in burning process and occurred in clinker. Because of Cu is high of melting point and boiling point at 1084.62 and 2562°C, respectively. The same results of previous research led to the conclusion that additions of up to 2% wt. of a galvanic sludge containing 2.4% wt. Cu and 1.2% wt. Ni to clinker raw-material do not affect the clinkering reactions and that these metals are totally incorporated into the clinker (Ract et al., 2003). The TTLC of Cu by Thai (DIW) standard must be less than 2500 mg kg-1 to classify non hazardous material. Thus, the total concentration of Cu in kiln feed must be less than 1000 mg kg-1.
Moreover, Table 2 gives the initial concentrations of Zn doped in kiln feed were in range 265 to 5254 mg kg-1, whereas total concentration of Zn in clinker was increased in range 299 to 5621 mg kg-1, respectively. However, total concentration of Zn in cement mortar was decreased in range 85 to 1606 mg kg-1. Because of cement mortar was mixed with sand can be diluted the concentration of Zn in cement. Therefore, Zn was not destroyed in burning process and occurred in clinker. Among the added metals, Zn is more easily incorporated in C3S, followed by V and Pb (Andrade et al., 2003). Because of Zn is moderate volatile element, melting point and boiling point at 419°C and 907°C, respectively; however, Zinc Oxide (ZnO) is melting point at 1975°C. The TTLC of Zn by Thai (DIW) standard must be less than 5000 mg kg-1 to classify non hazardous material. Thus, the total concentration of Zn in kiln feed must be less than 5000 mg kg-1.
Leaching tests: The results of leaching tests by WET were shown that the concentrations of heavy metals in clinker and cement mortar were not over standard (Table 4) as same as the results of leaching tests were performed on samples: Cr2O3 0.8% wt. and NiO 1.0% wt. The results of the chemical analyses of the leaching solutions showed that Cr content in the solution was less than 0.55 mg L-1 and Ni content in the solution was 0.04 mg L-1. These values are far below the legal limits (Barros et al., 2002).
The chemical composition of clinker at different ratio of heavy metals
The content of free lime: The percent concentration of free lime
was increased when the different ratio of combination of Ni:Cr:Cu:Zn = 1:1:1:1
increased in kiln feed from 0.05 up to 2.0% wt. (Fig. 1a).
The percent concentration of free lime was gradually increased at the clinker
from ratio of combination of Ni:Cr:Cu:Zn were in range 0.660 to 1.210%. Although,
the highest concentration of combination ratio is 2.0% wt. but the free lime
content was not over 2%. Figure 1b shows the percent concentration
of free lime was decreased when the different ratio of combination of Ni:Cr:Cu:Zn
= 1:3:5:7 increased in kiln feed from 0.05 up to 1.0% wt. The percent concentration
of free lime was gradually decreased at the clinker from ratio of combination
of Ni:Cr:Cu:Zn were in range 0.78 to 0.61%. Although, the highest concentration
of combination ratio is 2.0% wt. the free lime was increased to 1.920% but the
free lime content was not over 2%. Some effects of heavy metals to clinker was
found high concentration of Ni and Zn can permanently reduced concentration
of free lime in clinker (Stephan et al., 1999a).
However, at 2.5% wt. of Cr was increased free lime content in the clinker around
3.2% (Odler and Schmidt, 1980).
| Table 4: | The leaching test of heavy metals (combination of Ni:Cr:Cu:Zn) in the clinker and the cement mortar by WET |
| *Standard limit Anonymous (2006), ND: Not detect | |
| Fig. 1(a-b): | Percent free lime in the clinker with different ratio of combination of Ni:Cr:Cu:Zn, (a) ratios is 1:1:1:1 and (b) ratios is 1:3:5:7 |
The content of chemical oxide and chemical compounds in the clinker: Figure 2a and b showed the comparison of chemical oxide and chemical compounds between in the clinker control and in clinker samples with different ratio of combination of heavy metal of Ni:Cr:Cu:Zn = 1:1:1:1, the result of clinker with different ratio of heavy metals, high concentration between 1.0 to 2.0% wt. of heavy metals in the clinker was increase concentration of C2S but reduces concentration of C3S. Same as the result of combination of heavy metal of Ni:Cr:Cu:Zn = 1:3:5:7 (Fig. 2c, d), showed that at high concentration of 2.0% wt. of heavy metals in the clinker was increase concentration of C2S but reduces concentration of C3S. Not only a large amount of chromium (84%) was trapped and 53% of zinc was fixed in Portland cement clinker but also ZnO addition increased the alite size crystals (Murat and Sorrentino, 1996). The results showed that 2.5% of Cr inhibited the C2S reacted with CaO to set C3S form, caused of found C3S only 21.9%. However, 2.5% of Zn small increased of C3S and as corresponding decrease of C2S (Stephan et al., 1999b). The chemical mineral value in clinker accepted to the range of basic standard, high Cr increased concentration of C2S into clinker. Similarly, to the studied of addition of Cr into the kiln feed sample caused of inhibit C2S to set C3S whereas, Ni didn’t made the effect to chemical mineral in the clinker (Barros et al., 2002). The concentration of C3S were reduced when added concentration of Cr around 1.5% wt. (Malozhon et al., 1971).
The increased concentration of C3S came from substitution of Zn for Ca (Odler and Schmidt, 1980). One percent of ZnO in raw meal didn’t make the effect in chemical mineral into clinker and the value quiet similar when compare to the chemical mineral in clinker that produced from pure kiln feed (Andrade et al., 2003). Moreover, high concentration of ZnO in raw meal can change clinker color from black to brown color (Kakali et al., 2003).
The phase of clinker at different ratio of heavy metals: The clinker phase that contained the combination of heavy metals at different ratio of Ni:Cr:Cu:Zn as shown in Figure 3 and the elements has shown in Table 5 and 6. At 2.0% wt. of heavy metals ratio 1:1:1:1 (Table 5) showed that C3S were contained Cr, Cu and Zn equal to 0.69, 1.19 and 0.71%, respectively. C2S were contained Cr, Cu and Ni equal to 0.57, 0.32 and 0.87%, respectively. Whereas, C4AF were contained Cr, Zn and Ni equal to 1.70, 0.37 and 0.51%, respectively. At 2.0% wt. of heavy metals ratio 1:3:5:7 (Table 6) showed that C3S were contained Cr, Cu and Zn equal to 0.46, 0.79 and 0.79%, respectively. C2S were contained Cr, Cu, Zn and Ni equal to 0.36, 1.24, 1.88 and 0.52%, respectively. Whereas, C4AF were contained Cr and Zn equal to 1.28 and 1.49%, respectively.
| Fig. 2(a-d): | Concentrations of chemical compounds in clinker at different ratios of combination of Ni:Cr:Cu:Zn |
| Fig. 3(a-l): | Analysis of clinker phase with added combination of Ni:Cr:Cu:Zn |
| Table 5: | Analysis of clinker phase with added the combination of Ni:Cr:Cu:Zn = 1:1:1:1 |
| Remark: CM: Clinker of manufacturing, C0: Control clinker, *S.: Spectrum no. 1, 2 and 3 | |
| Table 6: | Analysis of clinker phase with added the combination of Ni:Cr:Cu:Zn = 1:3:5:7 |
| Remark: *S.: Spectrum no. 1, 2 and 3 | |
The results showed that all heavy metals such as Cr, Cu, Ni and Zn were incorporated into the clinker phase during the burning process. Cr reacted with Al and Fe in the clinker phase and got new solution called calcium aluminate, calcium chloroaluminate, gehlenite calcium chromate and two types of ferrites (Murat and Sorrentino, 1996). Cr was concentrated in C3S and C2S and makes the size of them larger than usual (> 100 and 80-90 μm, respectively). Incorporation of Cr in C3S and C2S also caused of made green color in clinker (Opoczky and Gavel, 2004). Moreover, Cr reacted with K and got K2CrO4 and K2Cr2O7. Therefore, the high chromium concentrations have affected to alite and belite formed (Stephan et al., 1999b). Some effects of the chromium on the chemical composition in clinker were described by previous researches such as 1% wt. of CrO3 added in raw meal was burn at 1,450°C, chromium has affected to inhibit alite formation reaction, the alite (C3S) decomposed into belite (C2S) and CaO in addition, occurring a new compound such as Ca4Al6O12Cr4 and Ca6Al4Cr2O15 (Kolovoes et al., 2002). At 2.5% wt. of chromium, the content of belite was always higher than that of alite. In addition, due to alite is formed by the reaction between belite and CaO. When chromium presented in the raw material, chromium would inhibit the reaction between belite and CaO to form alite and the clinker with 2.5% wt. of Zn small increase of C3S and corresponding decrease C2S (Stephan et al., 1999a) and increased of C3S due to substitution of Zn for Ca (Odler and Schmidt, 1980). A large amount of chromium (84%) was trapped and 53% of zinc was fixed in Portland cement clinker. Also, ZnO addition increased the alite size crystals and Zn react with Fe in clinker and created large size in C3S (Murat and Sorrentino, 1996). The same results a concentration of 1.5 wt.% of Cr2O3 in the sample, a lower content of C3S in clinker when doped with chromium. This effect was result from decomposition of C3S (Malozhon et al., 1971). The Ni is not only incorporated into C3S and C2S and take up the place of the Ca2+ ion. Due to the size of Ni2+ ion smaller than Ca2+ ion, it does not cause of deformation of C3S and C2S lattice (Opoczky and Gavel, 2004) and also the Ni was reacted with Mg in burning process and got MgNiO2 solution in clinker phase (Stephan et al., 1999b). The additions of up to 2% wt. of a galvanic sludge containing 2.4% wt. Cu and 1.2% wt. Ni to clinker raw-material do not affect the clinkering reactions and all Ni and over 99% wt. of Cu were incorporated into the clinker during the burning process (Ract et al., 2003). Moreover, The Zn more easily incorporated in C3S and concentration of Zn in C3S also gave higher value than C2S (Andrade et al., 2003). At 2.0% wt. of the waste ammunition material which is Cu and Zn had appeared of both alite and belite crystals. Alite was developed in large compact crystals which tend to appear more prismatic and angular in shape (Kolovoes, 2006). Microstructural analysis of the synthesized Portland clinker revealed that the grinding sludge which is contented heavy metals promoted formation of secondary C2S as well as caused color change of C3S (Punmatharith et al., 2010).
The physical properties of clinker at different ratio of heavy metals
Compressive strength: At concentration of combination No. 1 of heavy
metals (Ni:Cr:Cu:Zn = 1:1:1:1) in clinker was increased from 0.05 up to 0.3%
wt. the result showed that it increased the performance of compressive strength
in cement mortar but when the concentration of heavy metals more than 0.5 up
to 2.0% wt., it decreased the performance of compressive strength in cement
mortar as shown in Table 7. The cement mortar contained 0.05%
wt. of combination of heavy metals was given highest compressive strength at
1 day, 3 days, 7 days and 28 days were 135.19, 241.24, 342.31 and 417.78 kg
cm-2, respectively. However, The cement mortar contained 2.0% wt.
of combination of heavy metals was given lowest compressive strength at 1 day,
3 days, 7 days and 28 days were 77.00, 168.67, 239.90 and 345.60 kg cm-2,
respectively. The results showed that the clinker contained 0.05% wt. of combination
of heavy metals improved early strength at the 1 day was more than 130 kg cm-2
and the compressive strength at the 28 days of the clinker contained 0.05 and
0.3% wt. of combination of heavy metals were more than 400 kg cm-2
and were met the standard, respectively.
| Table 7: | Compressive strength of cement mortar with different ratio of heavy metals |
At concentration of combination no. 2 of heavy metals (Ni:Cr:Cu:Zn = 1:3:5:7) in clinker was increased from 0.05 up to 0.3% wt., the result showed that it increased the performance of compressive strength in cement mortar but when the concentration of heavy metals more than 0.5 up to 2.0% wt., it decreased the performance of compressive strength in cement mortar as shown in Table 7. The cement mortar contained 0.05% wt. of combination of heavy metals was given highest compressive strength at 1 day, 3 days, 7 days and 28 days were 138.98, 251.35, 345.60 and 439.31 kg cm-2, respectively. However, The cement mortar contained 2.0 wt.% of combination of heavy metals was given lowest compressive strength at 1 day, 3 days, 7 days and 28 days were 89.67, 180.93, 240.86 and 331.02 kg cm-2, respectively. The results showed that the clinker contained 0.05% wt. of combination of heavy metals improved early strength at the 1 day was more than 130 kg cm-2 and the compressive strength at the 28 days of the clinker contained 0.05 and 0.3% wt. of combination of heavy metals were more than 400 kg cm-2 and were met the standard, respectively.
The previous researches showed that the effect from the Cr in cement mortar was described which both of 1.2 and 3.25% wt. of Cr deteriorated the 28 days compressive strength (Murat and Sorrentino, 1996). The compressive strength was reduced at the 2.5% wt. of Cr in cement mortar and the compressive strength of cement mortar contained 2.5% wt. of Ni higher than control after 28 days test and also found MgNiO2 from the reaction in burning process can improve the strength due to it reduced concentration of free Mg in the alite (Stephan et al., 1999b). A suitable amount of CuO can lower the clinkering temperature and improve the burn-ability of clinkers. It can also promote the formation of 3CaO°SiO2 and 3CaO°3Al2O3°CaSO4 minerals and facilitate the coexistence of the two minerals in the clinkers. But adding 1% CuO to the raw material can cause the decomposition of 3CaO°3Al2O3°CaSO4 (Ma et al., 2006). At the 1 days test, the compressive strength of cement mortar contained 2.5% wt. of Zn lower than control but after the compressive strength of cement mortar gave highest values when compared with control and other ratio (Stephan et al., 1999b). Finally, the residues of metals and metallic oxides incorporated to the clinker during the clinkering process through an alternative fuel (used oils from cars) do not seem to alter detrimentally the characteristics of the material; however, at 3 as well as at 28 days mechanical strengths of M0 (0% w/w of ashes) had lower values than those obtained for the M1-M4 (0.005-0.040% w/w of ashes) (Trezza and Scian, 2000).
CONCLUSIONS
The results of this study show that the heavy metals which were added into the kiln feed have increased concentration and incorporated to the clinker. Therefore, all heavy metals were not destroyed in burning process and occurred in the clinker and cement mortar at different ratios. Moreover, the results of leaching test by the WET were shown that the all concentrations at different ratios of heavy metals in clinker and cement mortar were not over Thai standard which did not present a leaching hazard to environment because they are trapped in the clinker.
The free lime was slightly increased when the concentrations of heavy metals mixture evenly increased. The color of cement was green due to the influence of Cr on clinker increased.
The phase of the clinker did not affect when the concentrations of heavy metals increased in range 0.05 to 0.5% wt. but if higher concentration of heavy metal from 1.0 to 2.0% wt. have resulted in lower amount of C3S which found that the amount of C2S>C3S>C4AF. The study also found that the crystal structures of heavy metals were included in the phase of the clinker which could be detected easily at the concentration of total 2.0% wt.
The compressive strengths of the mixture of heavy metals increased from 0.05 to 0.3% wt. are higher than the standards; especially at 0.05% wt. has the highest compressive strength at 1 day and 28 days.
Finally, the appropriate total concentrations of the heavy metals that can be mixed in the kiln feed should not exceed 3000 mg kg-1 without causing negative impact on standards of quality cement factory and the concentration in the clinker do not exceed standards of the DIW specified.
ACKNOWLEDGMENTS
The authors would like to thank the Siam City Cement Public Company Limited and THE 90th ANNIVERSARY OF CHULALONGKORN UNIVERSITY FUND (Ratchadaphiseksomphot Endowment Fund) Class 6 (2/2551) Research Fund for the grant, equipment, material and all involved information in this research. Support for this work was also provided by a grant from and the National Center of Excellence for Environmental and Hazardous Waste Management (NCE-EHWM) of Chulalongkorn University.