Application of water treatment sludge in the manufacturing of lightweight aggregate

Construction and Building Materials 43 (2013) 174–183

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Application of water treatment sludge in the manufacturing of lightweight aggregate Chung-Ho Huang a, Shun-Yuan Wang b,⇑ a b

Department of Civil Engineering and Environmental Resources Management, Dahan Institute of Technology, Hualien, Taiwan, ROC Department of Civil Engineering, National Chung-Hsing of University, Taichung, Taiwan, ROC

h i g h l i g h t s " We propose a procedure to manufacture of lightweight aggregates from water treatment sludges. " Both structural and non-structural LWAs can be produced using a rotary kiln. " The resulted LWAs meet the requirements of ASTM C330. " The properties of concrete made from the LWA comply with the requirements of structural lightweight concrete.

a r t i c l e

i n f o

Article history: Received 28 October 2012 Received in revised form 16 February 2013 Accepted 18 February 2013 Available online 15 March 2013 Keywords: Lightweight aggregate Mixture proportioning Mechanical properties Rotary kiln

a b s t r a c t This study assesses the possible use of water treatment sludge for the production of lightweight aggregate (LWA), and focuses on the engineering properties of concrete made from this LWA. The experiments in this study involve 10 sludges from ten water treatment plants in Taiwan. All sludges can be used to manufacture LWAs in the laboratory, and exhibit a particle density (qa) of 0.65–2.05 g/cm3 and water absorption of 0.5–15%. Five sludges are suitable for manufacturing both structural LWA (qa = 1.2–1.8 g/ cm3) and non-structural LWA (qa < 1.0 g/cm3), and the other five sludges are only suitable for manufacturing structural LWA. The sludge collected from the Hsing-Zu plant was successfully used to produce both structural and non-structural LWA on a large scale using a commercial rotary kiln. The resulting aggregates possessed a particle density of 1.35 g/cm3 or 0.98 g/cm3 and a bulk density of 726 kg/m3 or 518 kg/m3 for the structural LWA and non-structural LWA, respectively. The structural LWA meets the ASTM C330 requirements, with a bulk density less than 880 kg/m3 for light coarse aggregate, and is a suitable LWA for structural concrete. The engineering properties of the concrete made from the structural LWA comply with the requirements of structural lightweight concrete. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Water treatment sludge (WTS) is generated from water treatment plants during the water treatment process of chemical coagulation, flocculation, sedimentation, and rapid sand filtering. Taiwan produced more than 200,000 tons of WTS in 2010, and the amount is increasing by an average rate of 5% every year. The disposal of WTS requires careful consideration if it is to be managed in an environmentally acceptable and sustainable manner. So far, WTS has primarily been disposed of using two methods: landfilling and soil application [1,2]. Landfill disposal is not ideal, and involves tightening legislation and increasing costs [2]. For soil

⇑ Corresponding author. Address: Department of Civil Engineering, National Chung-Hsing University, 250, Kuo-Kuang Road, Taichung 40227, Taiwan, ROC. Tel.: +886 4 22859390; fax: +886 4 22855610. E-mail address: [email protected] (S.-Y. Wang). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.02.016

application in agriculture, WTS can be used as a liming agent on acidic soil. Despite its agronomic benefits, however, it raises concerns over heavy metal contents, which cause the problems of human, aquatic, and terrestrial toxicity [3–5]. This situation provides increasing incentive to develop alternative and economically viable reuse and recycling options. These alternatives include the use of WTS in building material and construction. WTS can be used as a partial replacement for clay in the manufacture of brick and cement [6–10]. However, the proportion of sludge must be limited to be less than 30%. Several studies have investigated the use of sludge in mortar and concrete, including controlled low strength material (CLSM) and ready-mixed soil material (RMSM) [11–15]. Other studies have proposed manufacturing lightweight aggregate (LWA) from sewage sludge, reservoir sediment, expanded clay and zeolitie tuffs [16–26]. And the sludge from water supply treatment plant and sawdust can be used in constructions and buildings and reduce the environmental degradation

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100

Fractional weight percent (%)

caused by irregular disposition of these wastes and by the use of the natural aggregate [27,28]. Because the main constituents of WTS are inherently similar to those of sewage sludge and reservoir sediment, it is a good idea to investigate the technology of manufacturing LWA using WTS as raw material. Because of the huge supply of WTS in Taiwan, and because it is not yet effectively reused, this study investigates the effects of using WTSs as raw materials for LWA. This study investigates the feasibility of manufacturing LWA with WTS and related factors, including strength, density, and water absorption of the resulting LWAs. This study also presents an experiment of mass production technology using a commercial rotary kiln. The engineering properties of concrete made from the proposed LWA are subsequently assessed.

90 80 S1 (Shin-San plant) S2 (Ban-Shin plant) S3 (Pin-Tzen plant) S4 (Shin-Zu plant) S5 (Feng-Yuan plant) S6 (Lin-Nai plant) S7 (Nan-Huar plant) S8 (Pin-Din plant) S9 (Cau-Tang plant) S10 (Tzien-Chin Lake plant)

70 60 50 40 30 20 10 0 10

2. Composition of water treatment sludge

0.1

0.01

1E-3

Particle size (mm)

WTS is an inevitable by-product during the water treatment process. Table 1 shows the WTS samples collected from ten water treatment plants of Taiwan Water Corporation. The samples were previously dewatered mechanically via belt presses and dehydrated in air. This section presents the physical, chemical, and mineralogical characteristics of these samples. 2.1. Physical characteristics The physical test of sludge samples included particle size analysis, soil classification, and specific gravity. Table 1 shows the physical test results of the 10 samples. Fig. 1 shows the particle size distribution of all samples. The particle size of the 10 sludges mainly ranges from 0.75 to 0.0015 mm, and the fractions of particle size smaller than 75 lm are more than 96%. This indicates that the sludge may be classified as silt or clay. The plasticity index (PI) of the 10 samples ranges from 9 to 27 (Table 1). According to the Unified Soil Classification System, all the sludges are classified as inorganic clay of low to medium plasticity. In addition, the 10 sludges have a specific gravity of 2.60–2.75, similar to that of the general soil. 2.2. Chemical characteristics The chemical compositions of the WTSs were analyzed by energy dispersive X-ray fluorescence (ED-ERF). Table 2 presents the test results. The 10 samples have approximately similar ingredients: SiO2 (62–67%) is the main ingredient, followed by Al2O3 (19–23%), and Fe2O3 (4.9–11%). The presence of Fe2O3 liberates O2 and CO2 at the glass transition temperature. The presence of fluxing elements (Na2O, K2O, CaO, MgO, and Fe2O3) ensures the Table 1 Physical test results of water treatment sludge (WTS). Sample

Specific gravity

PI (%)

D50 (mm)

Ingredient (wt.%) Sand

Silt

Clay

S1 (Hsing-San plant) S2 (Ban-Hsing plant) S3 (Pin-Tzen plant) S4 (Hsing-Zu plant) S5 (Feng-Yuan plant) S6 (Lin-Nai plant) S7 (Nan-Huar plant) S8 (Pin-Din plant) S9 (Kau-Tang plant) S10 (Tzien-Chin plant)

2.60 2.62 2.75 2.69 2.60

9 10 13 17 13

0.002 0.005 0.003 0.005 0.009

1 1 3 2 3

25 46 30 53 68

74 53 67 55 29

2.75 2.63 2.7 2.64 2.63

11 13 27 26 14

0.004 0.004 0.007 0.003 0.008

1 4 2 1 3

47 38 68 31 58

52 58 30 68 39

Note: PI = plasticity index; D50 = median diameter.

1

Fig. 1. Particle size distribution of water treatment sludges (WTSs).

development of high-temperature glassy phases with sufficient viscosity. According to theories of the bloating phenomenon, bloating materials should contain chemical compositions that are within the limits of the expandable region on Riley’s Ternary diagram (Fig. 2) [21,22]. The results of the 10 samples shown in Fig. 2 demonstrate that the composition of all sludges fell within the limit region of Riley’s Ternary diagram. These results show that all the tested sludges are feasible for generating LWA.

2.3. Mineralogical characteristic X-ray diffraction was used to determine the crystalline phase of WTSs. The 10 samples exhibited similar X-ray diffraction patterns. As shown in the typical diffractogram (Fig. 3) of the sludge from the S9 sample (Kau-Tang plant), the major crystalline phases in the as-received sludge were quartz, illite, chlorite, and feldspar. These results are in general agreement with the mineralogy of expanded clay and expanded shale [23–26]. Consequently, there appears to be potential to manufacture LWA from the WTS. 3. Production of lightweight aggregate using WTSs The laboratory experiments in this study were conducted in two phases. The first phase assessed the feasibility of manufacturing LWA from the WTS, and empirically determined an optimal thermal cycle (maximum temperature, soaking time, and soften temperature corresponding to the optimal bloating). The second phase investigates the particle density of aggregates and their engineering properties. The following paragraphs describe the process of manufacturing synthetic LWA in detail.

3.1. Preparation of raw pellets The WTS collected from the plants was dewatered mechanically, and dehydrated in air until reaching the desired moisture content of approximately 25%. The air-dried sludge was crushed, milled, and screened into fractions with fairly similar sizes. The homogenized raw material was then blended with an adequate amount of water and grained by an extrusion machine. This process formed cylindrical green pellets with a diameter of approximately 8–12 mm (Fig. 4). Prior to the sintering process, the green pellets were dried in a stove (105 ± 10 °C) for 24 h to strengthen the pellets and prevent them from cracking or exploding during high-temperature sintering.

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Table 2 Chemical composition of water treatment sludge. Sample

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SiO3

Total

LOI (%)

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

64.3 63.6 63.8 64.0 62.3 66.9 62.3 54.8 65.6 63.7

21.2 22.2 20.6 23.0 22.0 20.6 21.4 23.0 22.4 19.6

10.40 10.70 8.32 8.15 11.00 7.00 11.00 11.30 4.91 9.60

2.05 0.72 1.64 1.35 2.62 1.82 1.22 4.26 1.72 2.91

1.06 1.73 3.15 1.78 1.10 2.53 2.47 3.40 4.15 2.51

0.17 0.12 0.31 0.10 0.08 0.14 0.14 1.99 0.15 0.27

0.79 0.83 2.08 1.55 0.79 0.86 1.42 1.14 1.03 1.10

0.09 0.17 0.17 0.05 0.11 0.16 0.14 0.08 0.10 0.39

100 100 100 100 100 100 100 100 100 100

8.04 11.90 6.96 7.75 10.80 4.26 10.4 3.77 13.00 11.00

Note: LOI = loss on ignition.

SiO2 100%

FeO, Fe2 O3 CaO, MgO K2 O, Na2O 50%

Al2O3 50%

80.0%

Following the related reports of Wang and Chen [19,21] who discussed the manufacturing technologies of LWA from reservoir sediments, the first phase processed dried pellets in the oven using a sequential process. The process began with preheating at 500 °C for 10 min, followed by sintering at temperatures of 1150–1250 °C for 10 min, and then quenching in air. After this drying process, the resulting LWA was examined by measuring its bloating index (BI) and particle density. The bloating index represents the volume change after firing:

60.0%

BI ¼ ðV a  V o Þ=V u

Fig. 2. Chemical composition of WTS on the Riley’s triaxial diagram.

100.0%

Intensity

Fig. 4. Cylindrical green pellets.

40.0% 20.0% 0.0% 0

5

10 15 20 25 30 35 40 45 50 55 60 65 70

2θ Fig. 3. Typical X-ray diffraction pattern of WTS (S9 sample).

ð1Þ

where Va is the volume of the aggregate, and Vo is the initial volume of pellet. Table 3 shows the results of all sludges. The first phase investigation confirmed the feasibility of manufacturing LWA from the WTS. Similar to the first phase experiment, the second phase of the experiment subjected the dried pellets to a sequential process beginning with preheating at 500 °C for 0 min, 7.5 min, and 15 min, followed by sintering at a softening temperature (from first phase experiment, Table 4) for 10 min, and finally cooling down to room temperature. The second phase also measured the particle density, bloating index, and water absorption of the resulting LWAs.

3.2. Sintering 4. Results and discussion The main apparatus used in these experiments was an electrically heated oven consisting of two parts: a preheating oven and a sintering oven. To generate LWA by bloating and sintering the pellets in an oven, dried pellets were placed in the preheating oven with a pre-set temperature of 500 °C for 5–15 min. The pellets were then transferred by a carrier to the sintering oven for the bloating/sintering reactions in atmospheric air at 1100–1250 °C for 7.5–15 min. After the sintering process, the pellets were moved to in a cooling room to cool down to room temperature.

4.1. Manufacturing LWA in the laboratory In the first phase of the experiment, during the sintering process, the pellets were fired at temperatures ranging from 1150 °C to 1275 °C. The materials of the raw pellet softened, start melting, and released gases. The gases causing expansion came from the thermally instable materials of the minerals, such as CO and CO2 from the combustion of organic matter at 400–800 °C; CO2 from

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C.-H. Huang, S.-Y. Wang / Construction and Building Materials 43 (2013) 174–183 Table 3 Firing conditions and technological properties of lightweight aggregates made from WTS (first phase experiment).

a b c d

Sintering conditiona

Floating particle rateb (%)

Sticky phenomenon

Particle density (g/cm3)

BId (%)

10

0 0 0 0 0 0

     

1.29 1.31 1.34 1.38 1.42 1.47

104.4 103.2 101.0 97.7 95.3 91.8

1275 1250 1225 1200 1175 1150

10

0 0 0 0 0 0

     

1.28 1.31 1.34 1.42 1.50 1.63

104.2 101.5 99.2 93.9 88.7 81.6

S3

1275 1250 1225 1200 1175 1150

10

0 0 0 0 0 0

     

1.57 1.60 1.61 1.83 1.89 2.10

102.0 100.1 99.5 87.5 84.8 76.1

A4

S4

1275 1250 1225 1200 1175 1150

10

100.0 100.0 100.0 100.0 100.0 7.7

O     

– 0.65 0.71 0.73 0.87 1.10

– 224.9 204.2 198.7 167.4 132.7

A5

S5

1275 1250 1225 1200 1175 1150

10

0 0 0 0 0 0

     

1.31 1.37 1.49 1.55 1.56 1.59

111.0 105.8 97.3 93.7 93.2 91.5

A6

S6

1275 1250 1225 1200 1175 1150

10

100.0 100.0 100.0 100.0 46.2 7.7

O O O   

– – – 0.78 1.07 1.40

– – – 162.2 117.5 90.2

A7

S7

1275 1250 1225 1200 1175 1150

10

0 0 0 0 0 0

     

1.29 1.30 1.34 1.39 1.56 1.69

101.8 100.9 98.1 94.4 83.7 77.7

A8

S8

1275 1250 1225 1200 1175 1150

10

100.0 100.0 100.0 100.0 23.1 0

O O O   

– – – 0.74 1.08 1.45

– – – 228.1 156.2 115.8

A9

S9

1275 1250 1225 1200 1175 1150

10

100.0 100.0 100.0 100.0 38.5 0

O O O   

– – – 0.75 1.02 1.35

– – – 223.4 163.6 123.7

A10

S10

1275 1250 1225 1200 1175 1150

10

100.0 100.0 100.0 100.0 53.8 0

O O O   

– – – 0.73 0.96 1.22

– – – 181.8 137.7 108.1

Sample

WTS

Temp. (°C)

Soaking time (min)

A1

S1

1275 1250 1225 1200 1175 1150

A2

S2

A3

c

The preheating condition is selected at a temperature of 500 °C for 10 min. Floating particle rate = percentage of particle floats on water. Sticky phenomenon = material softened and deformed to stick together. BI = bloating index.

the dissociation of carbonates at 700–950 °C; O2 and CO2 formed from the reduction of ferric iron at 1000–1300 °C. This processes

transformed the green pellets into lightweight ceramic granules containing a significant glassy phase with isolated and irregular

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Table 4 Optimal sintering temperature of WTS for LWA manufacturing. Sample

Bloating temp. (°C)

Soften temp. (°C)

Optimal sintering temp. (°C)

Expansion mode

S1

1225–1275

>1275

1275

S2

1250–1275

>1275

1275

S3

1250–1275

>1275

1275

S4

1150–1250

1275

1250

S5

1250–1275

>1275

1275

S6

1150–1200

1225

1200

S7

1250–1275

>1275

1275

S8

1150–1200

1225

1200

S9

1150–1200

1225

1200

S10

1150–1200

1225

1200

Low expansion Low expansion Low expansion High expansion Low expansion High expansion Low expansion High expansion High expansion High expansion

pores (Fig. 5). The resulting LWAs generally exhibit spherical granules and present regular and round shapes. Table 3 shows the test results of firing conditions and the technological properties of the LWAs. Results show that most of the sludges effectively produced LWAs with the six selected temperature levels of 1150, 1175, 1200, 1225, 1250, and 1275 °C, except for the S6, S8, S9, and S10 samples, which inconsistently formed LWAs at sintering temperatures of 1225, 1250, and 1275 °C. This is because of the sticky phenomenon. In addition, the particle densities of the resulting LWAs ranged from 0.65 to 1.89 g/cm3, meeting the general requirement of the particle densities less than 2.0 g/cm3. To determine the optimal ranges of the bloating temperature and softening temperature, this study measures the bloating index (BI), floating particle rate, and sticky phenomenon of the resulting LWA. Table 3 shows these results for all 10 sludge samples. According to the BIs and the floating rates in Table 3, the tested sludges may be divided into two categories of low-expansive sludge and high-expansive sludge, with particle floating rates of 0% and 100%, respectively. The former includes S1 (max BI = 104.4%), S2 (max BI = 104.2%), S3 (max BI = 102.2%), S5 (max BI = 111.0%), and S7 (max BI = 101.8%) sludges. The latter includes S4 (max BI = 224.9%), S6 (max BI = 162.2%), S8 (max BI = 228.1%), S9 (max BI = 223.4%), and S10 (max BI = 181.8%) sludges.

(a) Cross section of particle

A temperature corresponding to a BI value larger than 100% was defined as the lower limit of bloating temperature, whereas the upper limit of the softening temperature was defined as the sticky phenomenon. Table 4 shows the bloating temperature and softening temperature for the tested sludges. The low-expansive sludges expanded at a higher bloating temperature (P1225 °C) and higher softening temperature (>1275 °C). The softening temperature of the high-expansive sludges, except for the S4 sample (Hsing-Zu plant), had lower value of 1225 °C and expanded at a lower temperature of 1150 °C. Consequently, based on the limited softening temperature (Table 4) and the upper temperature limit of 1300 °C generally used in commercial production, the optimal sintering temperatures are proposed as listed in fourth column of Table 4. The second phase of the experiment produced the LWA by preheating at 500 °C for 0, 7.5, or 15 min and sintering with the temperatures proposed in Table 4 for 10 min. Table 5 shows the properties of the resulting LWAs. Figs. 6 and 7 show the particle density and expansion rate of all aggregates with various preheating times. Table 5 shows that various LWA products were produced with controlled operating conditions, and particularly the preheating time. The particle densities of the LWAs ranged from 0.65 to 2.05 g/cm3. Fig. 6 shows that, for each sludge sample, the LWA particle density is significantly affected by the preheating time, and that increasing the preheating time increases the particle density. Fig. 7 shows that increasing the preheating soaking time decreases the bloating rate. These situations may be due to the heating mechanism of the pellets that longer preheating time will release more expansion gases, making the pellets to be denser by sintering and decreasing the expansibility, thus, increase the particle density and decrease the bloating rate. The results in Table 5 show that the water absorption rates of the LWAs are mostly less than 11%, except for the LWA obtained from S5 sludge. This shows that the water absorption rate decreases as the preheating time increases. This is because increasing the preheating time may improve the homogeneity of the raw pellet, enhancing the heating efficiency. This in turn increases the thickness of the hard ceramic shell, and decreases the permeability of the LWA. These results can then be explicated in the typical Fig. 8 (from S6 sample) that increasing the preheating time may decrease the water absorption and increase the particle density of the LWA. The results of the second phase of the experiment show that the 10 sludges are suitable for manufacturing LWAs with the appropriate thermal cycle (preheating temperature, sintering temperature, and soaking time) shown in Table 5. The resulting LWAs exhibit particle densities ranging from 0.65 g/cm3 to 2.05 g/cm3, and are mostly under 1.8 g/cm3, whereas the water absorption ranges from

(b) SEM x 100

Fig. 5. Cross section and SEM micrograph of sintered sludge LWA.

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C.-H. Huang, S.-Y. Wang / Construction and Building Materials 43 (2013) 174–183 Table 5 Engineering properties of lightweight aggregates made from WTS (Second phase experiment). Particle density (g/cm3)

Water absorption (%)

BI (%)

10

1.29 1.33 1.40

11.3 10.6 9.6

104.4 101.4 96.5

1275

10

1.29 1.39 1.65

11.3 7.3 3.7

102.9 95.7 80.5

0 7.5 15

1275

10

1.50 1.57 1.89

5.0 4.5 2.9

106.5 102.0 84.5

500

0 7.5 15

1250

10

0.65 1.74 1.88

9.0 5.7 3.6

224.9 83.8 77.5

S5

500

0 7.5 15

1150

10

1.31 1.43 1.52

15.0 12.2 8.7

110.7 101.4 95.4

A6

S6

500

0 7.5 15

1200

10

0.78 1.46 2.38

6.2 5.0 1.3

162.2 86.2 53.0

A7

S7

500

0 7.5 15

1275

10

1.26 1.29 1.58

7.4 4.9 2.7

104.0 101.6 82.8

A8

S8

500

0 7.5 15

1200

10

0.74 1.67 2.05

7.7 4.9 0.5

228.1 100.6 82.0

A9

S9

500

0 7.5 15

1200

10

0.75 1.28 2.03

8.8 5.2 1.1

223.4 130.5 82.1

A10

S10

500

0 7.5 15

1200

10

0.73 1.19 1.97

8.6 6.2 2.7

181.8 166.2 66.9

Sample

WTS

Preheating condition

Sintering condition

Temp. (°C)

Soaking time (min)

Temp. (°C)

Soaking time (min)

A1

S1

500

0 7.5 15

1275

A2

S2

500

0 7.5 15

A3

S3

500

A4

S4

A5

300

2.40

Preheating time 0 min. Preheating time 7.5 mins. Preheating time 15 mins.

250

Bloating Index, BI (%)

Particle density (g/cm3)

Preheating time 0 min. Preheating time 7.5 mins. Preheating time 15 mins.

2.35

1.8 g/cm3

2.30 1.8 1.6 1.4 1.2

200

100

1.0 0.8 0.6

50 S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

Samples Fig. 6. Particle density of LWAs with varied preheating times for various sludges (Second phase experiment).

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

Samples Fig. 7. Bloating index (BI) of LWAs with varied preheating soaking times for various sludges (Second phase experiment).

4.2. Mass production of LWA in a rotary kiln 0.5% to 15%, and are mostly under 11%. In general, the LWAs in structural concrete and non-structural concrete should have particle densities of 1.2–1.8 g/cm3 and less than 1.0 g/cm3, respectively. Thus, the LWAs reported in this study can be classified into two groups: one is suitable for use in both structural and non-structural concrete (A4, A6, A8, A9, and A10), and the other five samples (A1, A2, A3, A5, and A7) are only adequate for use in structural concrete.

In this study, the WTS of the S4 sample from Hsing-Zu plant, which can manufacture both structural and non-structural LWA, was selected for a mass-production experiment. A commercial rotary kiln with an outer diameter of 2 m and a length of 39 m (Fig. 9) was used to produce LWA. The kiln is a refractory cylinder with a slight incline rotating about its longitudinal axis. The kiln is also equipped with a rotary cooler measuring 1.5 m in diameter

C.-H. Huang, S.-Y. Wang / Construction and Building Materials 43 (2013) 174–183

2.6

1.6

2.4

1.4

2.2 1.2 2.0 1.0

1.8 1.6

Particle density Water absorption

0.8

1.4

0.6

1.2 0.4

Water absorption (%)

Particle density (g/cm3)

180

1.0 0.2

0.8

0.0

0.6 0.0

7.5

15.0

Preheating time (min) Fig. 8. Effects of preheating time on the particle density and water absorption of LWA (S6 sample).

and 14.3 m in length to recover the heat contained in the aggregate discharged from the kiln. The process of producing LWA from the sludge includes three steps: treatment of wet sludge, preparation of raw pellets, and sintering. The sludge collected from the water treatment plant had a relatively high water content of up to 75%. It was first dehydrated in air until reaching the desired moisture content of approximately 25%. The air-dried sludge was crashed and blended into a homogeneous mixture, and then grained by an extrusion machine that chopped the sludge into cylinder-shaped pellets (Fig. 10). The raw pellets were then transferred by a conveyer belt into the rotary kiln. During the journey through the kiln, the pellets went through a sequential process beginning with drying at 105 ± 10 °C for 15–25 min, followed by preheating at 400–600 °C for 10–15 min, then firing at 1000–1300 °C for 10–20 min. The processes for producing structural LWA and non-structural LWA were approximately 45 min and 40 min, respectively. The previously indicated manufacturing process and firing conditions successfully produced synthetic LWAs. The aggregates were tested for density, water absorption, strength, and concentration of heavy metals by Toxicity Characteristic Leaching Procedure (TCLP). Table 6 presents a summary of the measured results. Table 7 shows the TCLP-results of heavy metals in the produced LWA. The structural LWAs exhibited an average particle density of 1.35 g/cm3 and bulk density of 726 kg/m3. These values are significantly lower than those of normal density aggregate, and meet the requirements of ASTMC330 with a bulk density less than 880 kg/m3 for coarse aggregate. The non-structural LWA also exhibited appropri-

Fig. 9. Rotary kiln.

ate relative density properties, with a particle density of 0.98 g/ cm3and bulk density of 518 kg/m3. As for the water absorption, the structural LWAs presented a smaller value (6.9%) than the non-structural LWA (9.7%). This implies that the high absorption rate at 24 h corresponds to a low density. The aggregates were also tested for crushing strength following GB 2842-81 (China National Standard Test method for LWAs). Oven-dried samples of the LWA were placed in a steel cylinder with an internal diameter of 115 mm and a height of 145 mm. The strength of the samples was than measured under a specific compression when a steel plunger reached a prescribed distance of 20 mm: crushing strength r = P/A, where P = compression force, A = press area of the plunger. Table 6 presents a summary of the results, showing that the crushing strengths of structural and nonstructural aggregates were 12.0 MPa and 5.5 MPa, respectively. The density of the structural aggregate was higher than that of the non-structural aggregate, implying that a greater bulk density increases aggregate strength. Overall, the WTSs in this study can be used as a resource material for the mass production of synthetic lightweight aggregates, and comply with the requirements for LWAs. This study uses the produced synthetic aggregates in a concrete mix to assess the feasibility of making a concrete mixture. 5. Lightweight aggregate concrete made with sintered LWA 5.1. Materials and concrete mixture proportions This experiment used normal sand-lightweight aggregate structural concrete for technical investigation. The ingredients of specimens included Type I Portland cement, natural sand, coarse LWA, and superplasticizer HICON HPC 1000 conforming to ASTM C-494 Type G. Natural sand has a specific gravity of 2.56, water absorption of 1.33%, and fineness modulus (F.M.) of 2.75. The LWA with a particle density of 1.35 was selected as coarse aggregate. This LWA has the relative properties shown in Table 6 and a grain size of 4.76–12.5 mm with F.M. = 6.60. The main variables considered in the mixture proportions included the compressive strength of the lightweight aggregate concrete (LWAC) and the water-to-cement ratio. A unified slump of 120 mm and six water/cement (W/C) ratios were selected for the mixture design. The concrete was mixed according to the specifications of ACI 211.2. Table 8 lists the mix proportions of the LWAC. 5.2. Preparation and testing of concrete mixture To prepare LWAC mixtures, this study tests the amount of water absorbed by the LWA during mixing and placement. In the mixing process, a pre-measured amount of water was slowly added to the LWA without presoaking. The amount of added water was computed based on 30 min absorption of the same aggregate tested previously. The mixing started by blending cement, sand, and coarse aggregate for approximately 90 s, followed by the addition of a premixed water/superplasticizer solution and continuing the mixing process for approximately 90 s until producing a uniform mixture. Freshly mixed concrete was then taken from each mixture to cast the specimens. Samples included three cylinders measuring 100 mm in diameter and 200 mm height for the compressive strength test, three cylinder with a 150 mm diameter and 300 mm height for the splitting tensile strength test, and two 25  25  285 mm prismatic specimens for the drying shrinkage test. The prepared specimens were used to test the fresh concrete and hardened concrete properties. The slump, unit weight, air content, and setting time of the concrete mixtures were measured in

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Step 1: Air-dried sludge

Step 2: Crushing and blending

Step 3: Graining with an extrusion machine

Step 4: Raw pellets

Fig. 10. Raw pellets prepared from sludge.

Table 6 Physical and mechanical properties of LWA produced using a rotary kiln. Lightweight aggregate type

Particle density (g/ cm3)

Bulk density (kg/m3)

Water absorption after 24 h (%)

Crushing strength (MPa)

Structural Nonstructural

1.35 0.98

726 518

6.9 9.7

12.0 5.5

Heavy metals

Content of trace elements(mg/L) Structural LWA

Non-structural LWA

Extraction test standarda

Cu Cd Pb Cr Cr6+ As Hg Se Ba

0.035 0.446

Table 7 TCLP-results of heavy metals in LWAs.

Table 8 Mixture proportion of lightweight aggregate concrete. Mixture no.

W/C

L40 L45 L50 L55 L60 L65

0.40 0.45 0.50 0.55 0.60 0.65

Batch quantities (kg/m3) Water

Cement

Natural sand

LWA

Superplasticizer

168 168 168 175 175 175

419 372 335 318 292 269

717 732 742 738 836 845

550 562 569 566 523 528

1.13 0.97 0.77 0.70 0.61 0.54

5.3. Test results

15.0 1.0 5.0 5.0 2.5 5.0 0.2 1.0 100.0

MDL = minimum detection limit. a The standard for hazardous wastes, Environmental Protection Administration, Taiwan. (EPA regulation, NIEA R201.14C).

accordance with ASTM C39, ASTM C138, and ASTM C403, respectively. For the hardened properties of the concrete mixtures, the compressive strength, splitting tensile strength, and drying shrinkage were tested in accordance with ASTM C39, ASTM C496, and ASTM C157, respectively.

Table 9 presents the basic properties of the fresh LWAC mixtures. The slump of the fresh concrete mixtures ranged from 110–135 mm, complying with the target value of 120 mm and indicating that these mixtures have good workability. The fresh concrete had a unit weight ranging from 1770 to 1880 kg/m3, which is less than that of normal weight concrete of approximately 2300 kg/m3. After exposure to a relative humidity of 50 ± 5% and a temperature of 23 ± 2 °C for 28 d, the density (unit weight) of the concrete mixtures ranged from 1694 to 1789 kg/m3 (Table 9). These values comply with the requirements of structural LWAC. The concrete mixtures had an initial setting time of 4–5.5 h and a final setting time of 6–8 h. These times are approximately equivalent to the setting times of normal weight concrete. This indicates that LWA does not significantly affect the setting behavior of concrete. The compressive strength and splitting tensile strength of the concrete mixtures were determined at an age of 28 d. Table 10 shows the results, showing that all the mixtures had an average compressive strength greater than 21 MPa. The compressive strength increased with the density, but decreased with the W/C ratio (Fig. 11). Table 10 shows the splitting tensile strengths of

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Table 9 Properties of fresh LWAC mixtures. Mixtures no.

Slump (mm)

Unit weight (kg/ m3) Fresh

Hardened

L40 L45 L50 L55 L60 L65

115 125 110 135 110 110

1800 1795 1790 1780 1775 1770

1789 1755 1730 1714 1709 1694

Air content (%)

1.90 2.10 2.40 2.60 2.70 2.70

Setting time (h:min) Initial

Final

04:08 04:28 05:11 04:28 05:07 05:28

06:04 06:52 07:07 06:44 07:52 07:51

Table 10 Compressive strength and splitting tensile strength of LWAC. W/C

Compressive strength (MPa)

Splitting tensile strength (MPa)

Density (kg/m3)

L40 L45 L50 L55 L60 L65

0.40 0.45 0.50 0.55 0.60 0.65

43.2 39.9 39.0 33.8 29.0 24.8

4.6 4.1 3.8 3.3 2.9 2.5

1789 1755 1730 1714 1709 1694

The WTS samples collected from 10 water treatment plants in Taiwan can be used to produce LWAs, even without any gas-releasing additive. The aggregates manufactured in a laboratory had particle densities ranging from 0.65 g/cm3 to 2.05 g/cm3 and water absorption ranging from 0.5% to 15%. These values are comparable to the requirements for high-quality LWA. Among the 10 sludges, five samples (S4, S6, S8, S9, and S10) are suitable for manufacturing both structural and non-structural

0.70

1800

0.65

1775

0.60

1750

0.55 1725 0.50 1700

0.45

700

1675

Comp. Strength

0.40

6. Conclusion

Density (kg/cm3)

Water/Cement ratios

Mixture No.

density 0.35

the concrete mixtures. This table shows property relationships similar to the compressive strength, implying that the density and W/C ratio of the concrete mixtures also affected the splitting tensile strength. To assess the quality of the LWAC made using the sludge LWA; Table 11 compares the density and strength of the LWACs with the ASTM C330 specifications. The compressive strengths and splitting tensile strengths all significantly exceed the values required by ASTM C330. This indicates that the sludge LWA is suitable for producing LWAC for practical use. The drying shrinkage characteristics of the concrete mixtures of L45, L55, and L65 were compared with ASTM C157. The specimens were cured at 23 °C and 100% RH for 7 d prior to drying at 23 °C and 55% RH. Fig. 12 shows the results. The shrinkage strains of all mixtures was relatively low (190–225  106) after a short period (28 d), but attained a general value of approximately 600– 680  106 at 360 d. Increasing the water-to-cement ratio tends to increase the drying shrinkage.

Drying Shrinkage strain (×10−6)

182

600 500 400

L45 L55

300

L65 200 100

1650 20

30

40

0

50

0

Compressive strength (MPa)

50

100

150

200

250

300

350

400

Duration of drying (days) Fig. 11. Effect of density and water/cement ratio on the compressive strength of LWAC.

Fig. 12. Drying shrinkage of LWAC after initial moist curing of 7 d.

Table 11 Comparison between measured results and code requirements of the density and strength of sand – LWAC. Mixture no.

– L40 – L45 L50 L55 L60 L65 –

Density (kg/m3)

Compressive strength (MPa)

Splitting tensile strength (MPa)

Measured

ASTM C-330 (maximum)

Measured

ASTM C-330 (minimum)

Measured

ASTM C-330 (minimum)

– 1789 – 1755 1730 1714 1709 1694 –

1840 – 1760 – – – – – 1680

– 43.2 – 39.9 39.0 33.8 29.0 24.8 –

27.5 – 20.7 – – – – – 17.3

– 4.6 – 4.1 3.8 3.3 2.9 2.5 –

2.3 – 2.1 – – – – – 2.1

C.-H. Huang, S.-Y. Wang / Construction and Building Materials 43 (2013) 174–183

LWAs (qa = 1.2–1.8 g/cm3 and qa < 1.0 g/cm3) with preheating temperature of 500 °C for 0–15 min. The other five samples (S1, S2, S3, S5, and S7) are only suitable for manufacturing structural LWA (qa = 1.2–1.8 g/cm3) with preheating temperature of 500 °C for 0–15 min and sintering temperature of 1150–1275 °C for 10 min. The S4 sludge sample (from the Hsing-Zu plant) was successfully used to produce large amounts of LWA using a commercial rotary kiln. This mass-production experiment successfully produced both structural and non-structural LWAs. The resulting structural LWA exhibited an average particle density of 1.35 g/ cm3 and bulk density of 726 kg/m3. These values are significantly lower than those of normal density aggregate, and meet the requirements of ASTM C330 with a bulk density less than 880 kg/ m3. Conversely, the non-structural LWA had an average particle density of 0.98 g/cm3 and bulk density of 518 kg/m3, and can be classified as extra-LWA with thermal insulation. The LWAC samples made from the produced sludge aggregate exhibited densities ranging from 1694 to 1789 kg/m3. The drying shrinkage strains of the LWAC mixtures were relatively low (190–225  106) at a short period (28 d), but reached a standard value of approximately 600–680  106 at 360 d. The compressive strength and splitting tensile strength of the concrete mixtures exceeded those of the minimum requirement of ASTM C330. Both strengths increased with density, but decreased with the water-tocement ratio. These results confirm the feasibility of using sludge LWA to produce LWAC, and creating new opportunities for the commercial use of WTS. References [1] Sanchez-Monedero MA, Modini C, Nobili MD, Leita L, Roig A. Land application of biosolids. Soil response to different stabilization degree of the treated organic matter. Waste Manage 2004;24(4):325–32. [2] Kim EH, Jin CK, Soobin Y. Digested sewage sludge solidification by converter slag for landfill cover. Chemosphere 2005;59(3):387–95. [3] Singh RP, Agrawal M. Potential benefits and risks of land application of sewage sludge. Waste Manage 2008;28(2):347–58. [4] Zang FS, Yamasaki S, Nanzyo M. Waste ashes for use in agricultural production: I. Liming effect, contents of plant nutrients and chemical characteristics of some metals. Sci Total Environ 2002;284(1–3):215–25. [5] Zang FS, Yamasaki S, Kimura K. Waste ashes for use in agricultural production: II. Contents of minor and trace metals. Sci Total Environ 2002;286(1–3):111–8. [6] Anderson M, Skerratt RG, Thomas JP, Clay SD. Case study involving using fluidised bed incinerator sludge ash as a partial substitute in brick manufacture. Water Sci Technol 1996;34(3–7):507–15.

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