Production and characterization of glazed tiles containing incinerated sewage sludge

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Production and characterization of glazed tiles containing

incinerated sewage sludge

D.F. Lin

a,*

, W.C. Chang

b

, C. Yuan

c

, H.L. Luo

a

a_{Department of Civil and Ecological Engineering, I-Shou University, 1, Section 1, Hsueh-Cheng Road, Ta-Hsu Hsiang, Kaohsiung County 84008,}

Taiwan, ROC

b

Department of Civil Engineering, Kao Yuan University, No. 1821, Jhongshan Road, Lujhu Township, Kaohsiung County 82151, Taiwan, ROC

c

Department of Civil and Environmental Engineering, National University of Kaohsiung, No. 700, Kaohsiung University Road, Nan Tzu District, 811. Kaohsiung City, Taiwan, ROC

Accepted 16 January 2007 Available online 11 April 2007

Abstract

In this article, glaze with different colorants was applied to tile specimens manufactured by incinerated sewage sludge ash (ISSA) and clay. Improvements using different amounts of colorants, and glaze components and concentrations on tile bodies were investigated. Four different proportions of clay (by weight ratio) were replaced by ISSA. Tiles of size 12 cm· 6 cm · 1 cm were made and left in an electric furnace to make biscuit tiles at 800C. Afterwards, four colorants, Fe2O3(red), V2O5(yellow), CoCO3(blue), and MnO2

(purple), and four different glaze concentrations were applied on biscuit tile specimens. These specimens were later sintered into glazed tiles at 1050C. The study shows that replacement of clay by sludge ash had adverse effects on properties of tiles. Water absorption increased and bending strength reduced with increased amounts of ash. However, both water absorption and bending strength improved for glazed ash tiles. Abrasion of grazed tiles reduced noticeably from 0.001 to 0.002 g. This implies glaze can enhance abrasion resistance of tiles. Effects like lightfastness and acid–alkali resistance improved as different glazes were applied on tiles. In general, red glazed tiles showed the most stable performance, followed by blue, yellow, and purple.

1. Introduction

The government in Taiwan has been very positive in the construction of sanitary sewer and wastewater treatment plants. House connections to sewer systems increased about 16% from 1997 to 2003. More house connections are to be completed in the next 6 years. With this increases in house connections, the amounts of wastewater and sew-age sludge will likely increase as well. It is becoming harder to find land to be used as sanitary landfill for dry sludge cakes in Taiwan. As a result, how to efficiently reclaim from sewage sludge is important. Tay and Show (1992)

pointed out that incinerations of sewage sludge, with some

help from techniques of de-odorization and disinfection, would not produce a chemical reaction and could reduce amounts of sewage sludge.Tay (1987)also mixed dry sew-age sludge with clay to manufacture building bricks, with 40% of dry sludge seen as the optimum amount.Lin and

Weng (2001)mixed diﬀerent amounts of incinerated

sew-age sludge ash (ISSA) with clay to make building bricks. They found that properties such as bending strength and abrasion of ash bricks met the standards set by CNS. They concluded that using ash bricks was a way of reclamation for sewage sludge.Liew et al. (2004)applied sewage sludge as a raw material to make clay bricks. Sewage sludge was discovered to have poor eﬀects on the surface quality of clay bricks. When more than 30% of sludge was added to clay bricks, and due to characteristics of diﬃculties in melt-ing and poor quality surface caused by sewage sludge, qual-ities and properties of sludge bricks were found to be way

*

Corresponding author. Tel.: +886 7 6577711x3320; fax: +886 7 6577461.

E-mail address:dﬂ[email protected](D.F. Lin).

www.elsevier.com/locate/wasman Waste Management 28 (2008) 502–508

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below average. Using sewage sludge as construction mate-rials are commonly seen in research. However, high-priced tiles manufactured by sewage sludge and clay were studied less. Weng and Lin (2000) applied bio-solid ash to make tiles. They showed that water absorption and bending strength met the requirements of CNS speciﬁcations. But when diﬀerent amounts of ISSA were added to clay to manufacture biscuit tiles, shortcomings such as higher water absorption, abrasion, and excessive pores were noticed. In order to solve such shortcomings, Lin et al.

(2005)mixed ISSA with clay to make glazed tiles. Results

indicated that application of glaze to biscuit tiles could improve drawbacks such as water absorption and abrasion. It also slightly increased the bending strength of tiles. In addition, bending strengths of tiles with various amounts of ash added when sintered at same temperature also met the standards set by CNS.

Glaze is commonly applied to ceramic art productions in Taiwan. When glazes were applied on the surface of ash tiles, a hard dense impermeable layer formed after being fired in a kiln at high temperature. This layer pro-vided glazed ash tiles with better resistance in physical and chemical erosions. Moreover, glaze with application of different colorants potentially make ash tile more color-ful and meet specific requirements for tile usages. Hence, glaze not only improves beauty and increases the economic value of ash tiles, but also improves the extent and uses for reclamation of sewage sludge. In this study, improvements on the appearances and properties of biscuit ash tiles through applications of different colorants are investigated. It is hoped that more properties of glazed ash tiles would be understood with the help of this study.

2. Methods and materials

Dewatered sewage sludge samples were obtained from a local municipal wastewater treatment plant in Taiwan and then transported to a brick kiln plant. Before being incin-erated in a tunnel kiln at about 800C to remove organic materials as shown in Fig. 1, these samples were dried at room temperature. Incinerated sludge ash passing a #200 sieve was collected and properties such as unit weight (2.71 g/cm3), specific weight (2.67), specific surface area (4860 cm2/g), and pH values (5.97–6.02) were obtained. The specific weight (2.52) and specific surface area (5398 cm2/g) of clay were also determined. The plasticity index of clay, which is decided by the Atterberg Limits test, was 19.11 and reduced to 16.94 when 30% of ash was mixed with clay. This indicated that ash can lower the plasticity of mixture. Further, both chemical components of clay and ISSA are shown inTable 1. As seen in the table, the quan-tities of each component in ISSA were more than those in clay, with the exceptions of Si. Toxic characteristic leaching procedure (TCLP) test results of ash for As, Cd, Cr, Cu, Ni, Pb, and Zn were 1.67, 0.4, 0.67, 174.5, 0.66, 0.54, and 34 mg/kg, respectively, well below the standard regulated by ROC-TCLP criteria.

In this study, the procedure of making glazed ash tile is shown diagrammatically in Fig. 2. Four diﬀerent propor-tions of ash, 0%, 15%, 30%, and 45% were prepared for mixing with clay. The optimum amount of water for a mix-ture was determined by a standard AASHTO compaction test. Before tiles of size 12 cm· 6 cm · 1 cm were made, air in the mixture was expelled using a de-airing vacuum pug mill to knead the mixture for about 16 min and then pressed by a pressing machine with an averaged vertical pressure of 300 kg/cm2to make strips with a thickness of 1 cm. Next, tile specimens were left in an electric furnace to make biscuit tiles at 800C. Temperatures in the furnace were raised at a rate of 2C/min before reaching 800 C, and at 1C/min thereafter. A sintering temperature of 1050C was determined based on previous experiments: (1) when temperature is higher than 1000C, pure clay tile body has a better bending strength; and (2) when tempera-ture is more than 1100C, melting phenomenon was observed for tiles with 30% ash added and large strains in tiles were noticed when 45% ash was added. The wet glaze can easily, uniformly, and eﬀectively sinter into the surface of ash tiles at 1050C. Besides temperature, the choice of glaze is another controlled experimental variable. In this study, glaze was formulated by mixing base glaze

Fig. 1. Sludge samples left in brick kiln plant for incineration. (a) Before incineration. (b) After incineration.

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together with different glaze colorants. Four different glaze concentrations of 0, 0.03, 0.06, and 0.1 g/cm2were applied on biscuit ash tiles. In order to investigate the influences of different glaze colorants on properties of ash tiles, four glaze colorants with different amounts (in percentage) were applied: 2% of iron oxide Fe2O3(red colorant), 6% of

vana-dium oxide V2O5(yellow colorant), 0.5% of cobalt

carbon-ate CoCO3 (blue colorant), and 4% of manganese

carbonate MnO2(purple colorant).

To meet the speciﬁcations regulated by the ROC CNS9737-R1018 standards (CNS, 2000) for ceramic tiles, a series of tests and inspections such as appearance and dimensions, warp measurement, shrinkage measurement,

water absorption test, weight loss on ignition test, bending resistance, abrasion resistance, and acid–alkali resistance tests were performed to determine properties of glazed ash tiles. In addition, microstructure analyses including scanning electron microscope (SEM) and energy dispersive spectroscopic analysis (EDS) were applied.

3. Results and discussion 3.1. Penetration test

To help investigate the eﬀects of ash replacement on water when used in making a mixture of ash clay paste,

Table 1

Results of EDS analysis for diﬀerent colorants, clay, sludge ash, and clay with 30% sludge ash added (in %)

Colorants Red Yellow Blue Purple Clay Sludge ash Clay with 30% sludge ash added Components Al 7.5 10.05 8.11 9.12 17.54 21.3 21.81 Si 46.48 45.12 44.8 50.27 38.64 14.35 28.56 O 36.04 28.95 34.4 29.47 38.64 30.1 24.47 Fe 1.13 – – – 8.16 22.26 13.48 Na 1.43 1.91 2.14 2.29 0.67 6.45 2.75 Ca 3.72 10.63 6.28 4.06 0.76 1.01 1.49 Mg – – – – 1.53 3.29 4.76 K 3.70 3.34 4.27 4.8 4.41 – 2.69

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penetration tests were performed on specimens of mixture (150 mm in height) with 0% and 30% ash added. If the amount of water applied was higher than 45%, liquiﬁed phenomena were noticed in the mixtures and 150 mm of penetration depth was easily reached, as shown in Fig. 3. The depth of penetration for the mixture with 30% ash added was less when the same amount of water was used. This implies that the replacement of ash could reduce the plasticity of mixture and lead specimens to a semi-solid state.

3.2. Tile water absorption

Water absorption is used to estimate the pore ratio of tile specimens. High water absorption in tile is character-ized by a high pore ratio. CNS requires the water absorp-tion of tile earthenware be less than 16%. Water absorption of tiles increased by about 3–4% with the addi-tion of ash, as can be seen inFig. 4. However, glaze formed a thin film, which helped protect the surface and reduce water absorption of ash tiles.Fig. 4also shows that water absorption for 30% ash tile specimens was 13% before glaze was applied, and became less than 10% with the application of glaze. It also indicates that water absorption decreased with the increase of glaze concentration application. Fur-thermore, all glaze colors tested helped reduce more than 2–3% of water absorption. Performances of different colors were closely related to components of glaze as well as degrees of crystallization during the firing process. The purple color performed poorly since glaze with purple col-orant contained great amounts of Si, which had a higher melting temperature. Hence, when the sintering tempera-ture was set at 1050C for all colorants, the degree of melt crystallization for the purple color was worse than the other colors.

3.3. Warp measurement

Fig. 5(a) displays the relationships among warpage, colo-rants, and concentrations for tiles with 0% and 30% ash added. Since, shrinking force increased with glaze thickness in per unit area, the increment of warpage of specimens

Fig. 3. Relationship between depth of penetration and water applied to mixture with 0% and 30% ash.

Fig. 4. Eﬀects of glaze concentration and colorant on the water absorption for glazed tiles with 0% and 30% of ash.

Fig. 5. Relationships among warpage, colorants, and glaze concentrations for tiles with diﬀerent amounts of ash added. (a) Glaze concentrations (g/ cm2), (b) substitution of sludge ash (%).

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became larger with higher glaze concentration, as displayed inFig. 5(a). Among all glazed tiles tested, warpage of purple glazed tiles had the greatest increment, which was caused by the larger coeﬃcient of shrinkage in purple colorant. On the contrary, red glazed tile had the least increment. InFig. 5(b), the addition of ash also increased the warpage of tiles and the amounts increased ranged from 0.3 to 1.0 mm. Therefore, ash can improve the melting property of tile bodies. This melting crystallization can even make tiles denser. These phenomena of warpage became more noticeable as greater amounts of ash were added.

3.4. Abrasion of tile surface

The durability and hardness of tiles are determined by abrasion, which is influenced by daily human activities. Abrasion types such as drag motion, friction, and impact are commonly seen in daily life. Hence, requirements for tile abrasion are decided by locations and frequencies of different activities. CNS requires the abrasion of tile earth-enware be less than 0.1 g. The abrasion of tile with 30% ash added was twice that of the controlled group (clay tile with no glaze), as displayed inFig. 6. This indicates that simply replacing part of the clay with ash in tiles was insufficient for resisting abrasion and weathering process. After appli-cation of different glazes on ash tiles, abrasion resistance of tiles improved about 10–20 times more than ash tiles with-out glaze applied. This result matched closely to tiles with glaze applied in the controlled group. It can be seen that application of glazes to ash tiles was important in abrasion resistance. Further, different glaze colors had similar effects on abrasion resistance, even though the red color was slightly better.

3.5. Tile weight loss on ignition

Weight loss on ignition is to measure the weight diﬀer-ences in tile specimens before and after ﬁring. As stated before, sewage sludge was incinerated at about 800C before applying it to manufacture tiles. In this manner,

organic materials in sludge samples were burnt away. The only source of organic materials came from the clay used in making ash tiles. Hence, weight loss on ignition reduced with an increase in amount of ash added, and the amount reduced was about 2–3%, as shown inFig. 7. Further, glaze concentrations had a slight inﬂuence on weight loss on igni-tion. This was due to glaze composed of high-temperature oxides such as SiO2and Al2O3. Most oxides in glaze can be

burnt, but not incinerated at 1050C. The reduction in weight loss on ignition caused by glaze is about 0.3–0.5%, and rose with the increase of glaze concentrations. In addi-tion, differences in the reduction of weight loss on ignition among various glaze colors are little, about 0.05–0.1%. This indicates that different colorants have a very small effect on weight loss on ignition.

3.6. Bending strength of tile

Bending strength is affected by the pore distributions and the vitrification level of the tile body. Different bending strengths of tiles are listed in regulations according to the location and frequency of application. For example, bend-ing strengths of wall tile earthenware range from 60 to 100 kgf/cm2as set by CNS, and 100 kgf/cm2for floor tile earthenware. Theoretically, if glazes were applied to tiles, bending strength would reduce with increased thickness. However, as shown inFig. 8, bending strengths of glazed tiles improved to about 5–10 (kgf/cm2). Glazes melted tightly into tile bodies in the sintering process at high tem-peratures. After crystallizations were rearranged, melted glazes formed a hard layer on the surfaces of tile, which could improve the bending strength of tiles. By comparing the effects of different glaze colors on bending strength, red glaze gives a better performance. Since, red colorant con-tains iron oxide, which could lower sintering temperature, interface adhesion between red glaze and tile bodies improved most. Hence, its bending strength increased. Moreover, results of EDS analysis indicate that ash con-tained aluminum component after being sintered at 800C. This aluminum would raise sintering temperature of ash tiles and lead to a phenomenon of under firing.

Fig. 6. Eﬀects of glaze concentration and colorant on the abrasion resistance for glazed tiles with 0% and 30% of ash added.

Fig. 7. Eﬀects of glaze concentration and colorant on the weight loss on ignition for glazed tiles with 0% and 30% of ash added.

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Therefore, the bending strengths of tiles with ash added were less than tiles without ash added.

3.7. Ageing resistance of tile

Lack of glassy gloss and emergence of ﬁne cracks are two criteria used to evaluate the quality of glazed surface after exposure to ultraviolet light. Ageing resistance can provide information for the consistency and quality of sur-faces of glazed tiles. In this study, red glazed tiles were the most stable in ageing resistance tests, followed by blue, yel-low, and purple, as can be seen inTable 2. This indicates that red colorant composed of iron oxide had a low melting temperature and made glaze crystallization better. An excellent lightfastness of tile surfaces was produced. On the other hand, purple glaze with manganese carbonate as colorant showed the worst lightfastness among all glaze colors. When glaze concentration was at 0.03 g/cm2, all glazed tiles failed ageing tests, except for red glazed tiles; at 0.06 g/cm2, only purple glazed tiles failed the tests. This implies that an increase in concentrations of glaze can improve the lightfastness of glazed tiles. Therefore, quanti-ties of ash added had little eﬀect on ageing resistance of glazed ash tiles.

3.8. Acid–alkali resistance of tiles

Tiles are easily eroded by activated chemical solutions such as grease and detergent. The object of acid–alkali

resistance test is to examine any discoloring or other abnor-mal reactions that occurred to the surface of glazed tile.

Table 2also shows the results of acid–alkali resistance tests for glazed ash tiles. Na, one of the glaze components, dis-solves in acid easily, possibly leading to discoloring of the glazed surface. Table 2 indicates that applying concentra-tion equivalent to or less than 0.03 g/cm2caused the glazed surface of tiles to fail in acid–alkali resistance tests. This was due to the eﬀect of Na. A higher concentration of glaze was needed for tiles to show better performance in acid– alkali resistance tests. At 0.06 g/cm2, all yellow glazed tile specimens performed well in acid–alkali resistance tests, implying that vanadium oxide in the yellow colorant was better in resisting acid–alkali. However, due to the high content of Na in the purple colorant, purple glazed tile specimens performed the poorest among all glazed tiles in resisting acid–alkali corrosions.

3.9. EDS analysis

Glaze is composed of different oxides, and ratios of oxides in glaze influence kiln process and final products. For exam-ple, Si and Al affect both the melting temperature and crys-talline state of glaze. Na can increase expansion coefficients and Ca is helpful to hardness. In order to understand the dis-tributions of chemical components for tile specimens, EDS analysis was performed in this study.Table 1shows a com-parison of components among different glaze colorants, clay, ash, and clay with 30% ash replacement. Red colorant had the smallest amount of Na among all colorants, and when expansion coefficients of glaze were larger than tile bodies, red glaze was better in resisting the effects of warpage on tile bodies. Hence, increments of warpage for red glaze were less than other glaze colors. Further, purple colorant contained more Si and Al than others and these components would lead to a higher fusion temperature. Hence, degrees of fusion between purple glaze and tile bodies were relatively worse than other colorants. This explains why performances such as bending strength and abrasion of purple glazed tiles were less impressive when compared to other glazed tiles. More-over,Table 1shows that the quantity of Ca, Fe, and Mg in ash was higher than those in clay. This implies that the addi-tion of ash to tiles lowered the melting temperature of tiles, since Ca, Fe, and Mg were characterized as good flux. These metal components were helpful to improve the co-melting and coupling between glazes and tile bodies. When compo-nents of tile bodies with 0% and 30% ash added were com-pared, it is found that 30% ash tiles bodies had more Al, thereby raising the melting point temperature. Therefore, 30% ash tile bodies were less mature which led to poorer per-formances than tiles without ash added.

3.10. SEM analysis

SEM analysis was carried out to understand more about crystalline states of tile bodies in this study.Fig. 9displays

Fig. 8. Eﬀects of glaze concentration and colorant on the bending strength for glazed tiles with 0% and 30% of ash added.

Table 2

Eﬀects of glaze concentrations and diﬀerent colorants on the ageing resistance test and acid–alkali resistance test

Concentration (g/cm2)

Ageing resistance test Acid–alkali resistance test Red Yellow Blue Purple Red Yellow Blue Purple

0.03 · · · ·

0.06 · ·

0.1

For ‘‘good’’. · For ‘‘not good’’.

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pictures obtained from SEM analysis for ash tiles with applications of diﬀerent colorants at glaze concentration of 0.06 g/cm2. Melting states between glazes and tile parti-cles are seen in pictures. Among all glaze colorants applied, red colorant was the best in inducing the melting state. Also, more pores at the interface between glaze and tile bodies were noticed. With the aid of this inducing property from ash at high temperature, glaze displayed a phenome-non of downward co-melting crystallization and could den-sely fuse with tile bodies. In addition, glassy matrixes agglomerated by mullite crystallizations were noticed in tile bodies; this was clearly observed in red glazed ash tile bodies.

4. Conclusions

In this study, results obtained on the basis of experimen-tal data are directed towards comparing various amount of ISSA added to the tile body, as well as diﬀerent colorants and quantities of glaze concentration applied to the surface of biscuit tile bodies. The results are summarized as follows:

1. Test results indicate that glaze formed a thin film that could protect the surface and reduce water absorption in ash tiles. They also show that water absorption decreased with an increase in glaze concentration applied. However, different colorants had very small effects on weight loss on ignition.

2. Comparison of the eﬀects of diﬀerent glaze colors on bending strength showed that red glaze gave a better performance. Since red colorant contains iron oxide thereby lowering the sintering temperature, interface adhesion between red glaze and tile bodies improved most. As a result, bending strength also increased. 3. In this study, red glazed ash tiles were most stable in

ageing resistance tests, followed by blue, yellow, and purple. With the help of iron oxide, red glaze makes

glaze crystallization better. A more excellent lightfast-ness of ash tiles surfaces was produced. On the other hand, purple glaze with manganese carbonate as color-ant showed the worst lightfastness among all glaze colors.

4. Applying a concentration of less than or equal to 0.03 g/cm2 caused the glazed surface of ash tiles to fail in the acid–alkali resistance tests due to the eﬀect of Na. A higher glaze concentration was needed for ash tiles to show better performance in acid–alkali resistance tests. At 0.06 g/cm2, all yellow glazed ash tiles specimens performed well in acid–alkali resistance tests, which imply that vanadium oxide in yellow colorant was better in resisting acid–alkali.

References

CNS (Chinese National Standards) Catalog, 2000. Ceramic Industry, Pottery Wares, Bureau of Standards, Metrology and Inspection. Bureau of Standards, Metrology and Inspection, Ministry of Eco-nomic Aﬀairs, Republic of China (ROC).

Liew, A.G., Idris, A., Wong, C.H.K., Samad, A.A., Noor, M.J.M.M., Baki, A.M., 2004. Incorporation of sewage sludge in clay brick and its characterization. Waste Management and Research 22 (4), 226– 233.

Lin, D.F., Weng, C.H., 2001. Use of sewage sludge ash as brick material. Journal of Environmental Engineering, ASCE 127 (10), 922–927.

Lin, D.F., Luo, H.L., Sheen, Y.N., 2005. Glazed tiles manufactured from incinerated sewage sludge ash and clay. Journal of the Air and Waste Management Association 55 (2), 163–172.

Tay, J.H., 1987. Bricks manufacture from sludge. Journal of Environ-mental Engineering, ASCE 113 (2), 278–283.

Tay, J.H., Show, K.Y., 1992. Utilization of municipal wastewater sludge as building and construction materials. Resources, Conservation and Recycling, 191–204.

Weng, C.H., Lin, D.F., 2000. Utilization of bio-solid ash as tile material. IAWQ Specialty Conference of Chemical and Petrochemical Industries Group, Critical Technologies to the World in 21st Century: Pollution Control and Reclamation in Process Industries, Beijing, P.R. China, September 18–20, pp. 142–151.

Fig. 9. Pictures of interface between glaze and tile bodies for ash tiles with applications of diﬀerent colorants at glaze concentration of 0.06 g/cm2. (a) Red colorant, (b) yellow colorant, (c) blue colorant, and (d) purple colorant.