Recycling CMP sludge as a resource in concrete

Construction and Building Materials 30 (2012) 243–251

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Recycling CMP sludge as a resource in concrete Tzen-Chin Lee a,⇑, Kae-Long Lin b, Xun-Wei Su a, Kuo-Kang Lin a a b

Department of Civil and Disaster Prevention Engineering, National United University, Miao-Li 360-03, Taiwan, ROC Department of Environmental Engineering, National Ilan University, I-Lan 260-41, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 3 July 2011 Received in revised form 18 November 2011 Accepted 25 November 2011 Available online 29 December 2011 Keywords: Chemical mechanical polishing Nano-particles CMP sludge-concrete Compressive strength Resource

a b s t r a c t Sludge, a byproduct of chemical mechanical polishing (CMP) used in integrated-circuit (IC) industries, is composed of hazardous materials such as nano-particles of SiO2, Al2O3, CaF2, and unknown organics. These nano-particles are potentially dangerous if released into the environment. Preliminary studies are carried out on sludge-blended cement concrete (SBCC) formed into specimens of cylindrical concrete with 10 cm diameter and 20 cm height. They are with 10 wt.% cement replaced by CMP sludge powder. Three water/cementitious (W/C) ratios were selected to mold the specimens for setting time, XRD analysis, TCLP analysis and compressive strength and stress–strain tests, respectively. TCLP test results revealed that the concentrations of leached heavy metals were substantially below the regulatory thresholds. The strengths of the SBCC specimens were comparable to those of the ordinary Portland cement concrete (OPCC) specimens with 3-day curing and about 22–38% much higher than those of OPCC cylindrical concrete specimens at 7–91 days of age. These results demonstrate the feasibility of recycling CMP sludge into construction materials, a positive step for sustainable development. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction CMP sludge is the waste generated by integrated-circuit (IC) factories during the planarization processing of silicon wafers. Because planarization uses considerable amounts of water, large quantities of wastewater and sludge containing nano-particles are potentially dangerous if released into the environment. In 2010, 12 million tons of this wastewater was generated in Taiwan, a country known for its IC industries. In an IC factory, the etching process, which uses hydrofluoric acid, usually employs CMP to remove silicon oxide and other unwanted debris from wafers. The waste generated by CMP is then neutralized with sodium hydroxide (NaOH); water-soluble calcium dichloride (CaCl2) is added to precipitate fluoride ions such as CaF2. Thus, CMP waste contains numerous chemicals. Such waste is simply called CMP sludge. Improving the treatment of semiconductor wastewater for recovery and reuse has caught the interest of many researchers. Lin and Yang treated CMP wastewater using chemical coagulation and reverse osmosis to reduce suspended SiO2 particles [1], and electrocoagulation has been used to remove nano-particles from CMP wastewater [2–6]. CMP sludge contains over 200 types of organic and inorganic materials [7]. The organic materials include oxidizing agents, additives, and dispersing agents. The inorganic materials consist mainly of SiO2 and CaF2 nano-particles typically 20–300 nm in size. However, the disposal or recycling of CMP ⇑ Corresponding author. Tel.: +886 37 382358; fax: +886 37 382367. E-mail address: [email protected] (T.-C. Lee). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.11.019

sludge has not been investigated to the knowledge of the current authors. Brar et al. concluded that engineered nano-particles in wastewater and wastewater sludge are a significant environmental risk that can contaminate the aquatic and soil environment and likely affect human health [8]. Huang et al. showed that the internalization of mesoporous silica nano-particles had a significant effect on the transient protein response and osteogenic signal in human mesenchymal stem cells and argued that the effects of nano-particles on diverse aspects of cellular activities should be carefully evaluated [9]. Eom and Choi studied the potentially harmful effects of exposure to fumed and porous silica nano-particles using human bronchial epithelial cells. The silica nano-particles exerted toxicity via oxidative stress, a mechanism potentially implicated in many human diseases. Cells exposed to porous silica nano-particles showed a more sensitive response than those exposed to silica fumes [10]. Moore pointed out that in aquatic environment nano-materials may harm the cells and tissues of aquatic animals after uptake via cell endocytotic routes and consequently damage human health. It also threatens ecological food chains [11]. Moore concluded that precautions must be taken when evaluating the effects of new nano-materials on health and the environment. Improper disposal of the huge amounts of nanoparticles in sludge, 313,000 tons annually in Taiwan, can be disastrous. When dumping sites are close to reservoirs or underground water sources, these nano-particles can pollute drinking water and cause lung-related diseases, among others [9–11]. Because human health is vulnerable to the presence of nano-particles in sludge, the disposal of potentially hazardous sludge by immobilizing it as a

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useful resource warrants careful research. Lee and Liu [12] found that CMP sludge can be utilized as a useful resource to replace a portion of the cement in cement mortar; they also found that through the hydration reaction and pozzolanic reaction the originally hazardous nano-particles of CMP sludge became part of the hydration compounds after curing for more than 28 days and could not be released to pollute the environment, resulting in no release of heavy metal ions according to TCLP tests. To the best of our knowledge, the recycling of CMP sludge in concrete has not been proposed. Herein, we conduct preliminary studies on CMP sludge-blended cement concrete (SBCC) for construction engineering. 2. Materials and Methods

of 23 ± 1.7 °C; the chamber containing this solution could be programmed to a certain temperature for 1–91 days. All concrete cylinder specimens were subjected to compressive strength testing and stress–strain analyses. 2.7. Fresh concrete tests Slump tests were performed according to ASTM 143 [17] to record slump data for both the SBCC and OPCC (reference) groups. The air content in fresh concrete was measured according to ASTM C231 [18]. The variation in the volume of concrete due to pressure changes was measured with a pressure-type air content meter (Forney, USA). The unit weight of fresh concrete was measured as specified in ASTM C138 [19]. The chloride content was measured using a chloride content meter (CL-1B, Rikin, Japan) according to Taiwan National Standard CNS 3090 [20]. The setting time was measured using a concrete penetration meter (H-4133, Humboldt Mfg. Co.) according to ASTM C403 [21].

2.1. Materials

2.8. Hardened concrete tests

CMP sludge was supplied by an IC factory located at a science-based technology park in Taichung, Central Taiwan. The sludge was dried and ground into particles smaller than 75 lm (sieve #200) with a specific gravity (SG) of 2.41. The cement was Type I ordinary Portland cement (OPC) obtained from the Taiwan Cement Company. The OPC exhibited a specific gravity of 3.14 and a fineness of 3520 cm2/g, and its physical and chemical properties met the ASTM C150 [13] specifications. Fine aggregate (sand) was obtained from the middle region of the Ta-an River in Miao-Li, Taiwan. The sand had a specific gravity of 2.57, and the fineness modulus was 2.83. Coarse aggregate was also obtained from the middle region of the Ta-an River. Large stones were crushed, and those measuring 10 mm and 20 mm were mixed in a certain ratio and then used as coarse aggregate. The coarse aggregate had a specific gravity of 2.59, and its fineness modulus was 6.72.

The OPCC and SBCC cylinder specimens were prepared according to ASTM C39 [16] and cured for 1–91 days. At each curing assessment stage, three cylinder specimens were tested for compressive strength and subjected to stress–strain analyses. The stress–strain curves were obtained while the concrete specimens underwent compressive strength testing. Data from the linear variable differential transformer (LVDT) apparatus and from strain gauges on the specimens were captured by a Data Logger (TDS-7130, TML, Japan) connected to a personal computer, from which appropriate stress–strain curves could be plotted. The relationship between stress and strain in concrete is not always linear. According to ASTM C469 codes [22], the elastic modulus is defined as the slope of the line drawn from a stress of f1 to a compressive stress of 0.40 fc0 (fc0 is the compressive stress corresponding to ultimate load). Thus, the secant modulus obtained from the straight line connecting the point of 0.40 fc0 to the f1 point on the stress– strain curve was used as the elastic modulus. Hence, the elastic modulus equation is as follows:

2.2. Los Angeles abrasion test for the coarse aggregate

0

Ec ¼ All tests were performed according to ASTM C131 [14]. Aggregates of crushed stones were impacted and abraded with steel balls in a Los Angeles testing machine to determine the weight losses associated with the coarse aggregates.

0:40fc  f1

e0  0:000050

ð1Þ

where fc0 represents the stress at the ultimate load in Mpa, f1 is indicates the stress in Mpa, which associated with the longitudinal strain of 50 millionths, e0 is the longitudinal strain at the stress of 0.40 fc0 .

2.3. SEM/EDS analysis of CMP sludge powder 2.9. Measurements of mass growth in specimens The samples of CMP sludge were oven-dried at 100 °C for 24 h and cooled in a desiccator. After being coated with platinum, the samples were analyzed with a field-emission scanning electron microscope (FE-SEM) (JEOL, JSM6700F, Japan). Elemental analysis was conducted with an energy dispersive X-ray spectrometer (EDS) (INCAX-sight) that was attached to the FE-SEM to analyze the chemical composition of the materials. 2.4. XRD analysis of CMP and OPC cement paste Powder X-ray diffraction (XRD) analysis was carried out using an X-ray diffractometer (Rigaku, D/Max-2200, Japan) with Cu Ka1 radiation and a 2h scanning range between 5° and 70°. The XRD scans were run at 0.05° per step with a counting time of one second. 2.5. TCLP analysis of CMP sludge and mortars The leaching of the CMP sludge and mortars was analyzed by TCLP testing according to ‘‘Toxicity Characteristic Leaching Procedures (TCLPs): SW 846-1311 [15] (USA)’’. Heavy metal concentrations were then measured based on SW-8467131A for Cd, SW 846-7421 for Pb, SW 846-7951 for Zn, SW 846-7211 for Cu and SW 846-7191 for Cr. 2.6. Molding and testing of cylindrical concrete specimens Concrete cylinder specimens 10 cm in diameter by 20 cm in height were molded according to ASTM C39/C39M03 [16]. The target water-to-cementitious ratios were 0.45, 0.55 and 0.65; the slump metric was 12.5 cm. The largest aggregates measured 25 mm, and the design proportions of both ordinary Portland cement concrete (reference specimens; OPCC) and sludge-blended cement concrete (experiment specimens; SBCC) specimens are listed in Table 1. According to our previous study [12], 10 wt.% cement replacement in the sludge-blended cement mortar with CMP sludge powder is the best proportion, so we used this formula for the SBCC specimens. The fresh concrete was used to test for slump, chloride content, air content and both the initial and final setting times. All of the specimens were cured in a saturated calcium hydroxide (Ca(OH)2) solution at a temperature

The specimens were weighed with a precision electronic balance (HJ–21KE, Shinko Denshi Corporation, Japan). The maximum reading was 21 kg, and the minimum was 0.1 g. Both groups of OPCC and SBCC specimens each comprised of 6 specimens (10 cm diameter and 20 cm height cylindrical concrete), were cured for 1, 3, 7, 14, 28, 56 and 91 days to measure weight variations during the curing period. Specimens were weighed under the saturated-surface-dry moisture condition. The original weights of the OPCC and SBCC concrete cylinder specimens were in the range of 3681.2–3821.8 g; thus, the measurement precision was approximately 1/36000, or 28 ppm, and the micro-growth of mass could be measured.

3. Results and discussion 3.1. SEM/EDS analysis of CMP sludge A sample of CMP sludge was diluted and dispersed with water, dried and observed with scanning electron microscopy (SEM). Aggregates measuring 2–30 lm were observed under 1000 magnification, and spherical nano-particles 30–100 nm in diameter were observed under 50,000 magnification; The large 2–30 lm particles were in fact agglomerates of nano-particles, as shown in Fig. 1a and b. Table 2 lists the chemical compositions of the particles according to EDS analysis; SiO2, CaF2, Al2O3, and CaO were the principal components, as shown in Table 2. The weight percentages of non-calcium oxides, namely, SiO2 and Al2O3, in CMP sludge were, respectively, 35.95%, 7.18%, which occurred in the form of nano-SiO2 and nano-Al2O3 spherical particles. The SiO2 and Al2O3 nano-particles can fill the spaces between particles of gel of calcium-silicate-hydrates (C–S–H), acting as a nano-filler, promoted hydration, thereby improving the microstructure of concrete in the SBCC specimens [23].

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T.-C. Lee et al. / Construction and Building Materials 30 (2012) 243–251 Table 1 Constituent contents of the ordinary Portland cement concrete (OPCC) and CMP sludge-blended cement concrete (SBCC) (design proportion weights per 0.06 m3). Specimen a

OPCC(0.45) SBCC(0.45)b OPCC(0.55) SBCC(0.55) OPCC(0.65) SBCC(0.65) a b

Cement (kg)

CMP (kg)

Water (kg)

Fine aggregates

Coarse aggregates

26.28 23.65 21.48 19.33 18.18 16.36

– 2.63 – 2.15 – 1.82

11.82 11.82 11.82 11.82 11.82 11.82

41.40 41.40 45.42 45.42 48.12 48.12

60.66 60.66 60.66 60.66 60.66 60.66

Ordinary Portland cement concrete; the ratio in parentheses is the water to cementitious ratio. Sludge-blended cement concrete; the ratio in parentheses is the water to cementitious ratio.

Table 2 Chemical compositions of cement and CMP sludge. Composition (wt.%)

Cement

CMP sludge

Na2O MgO Al2O3 SiO2 SO3 Cl– K2O CaO Fe2O3 CaF2

– 2.71 5.29 21.21 2.08 – – 63.71 3.12 –

0.62 – 7.18 35.95 0.61 – – 14.60 – 40.53

though the intensity of this peak is very low due to poor crystallization. The peak value at 18.05° and 34.11° of crystal Ca(OH)2 increases with the increasing age of OPC paste from 1 to 91 days, as shown in Fig. 2. However, the peak values of CMP (10%) decrease with increasing cement paste curing age after 7 days, which reveals that Ca(OH)2, a hydration product in cement, dwindles during reactions in the paste. This phenomenon is similar to a pozzolanic reaction. 3.3. Los Angeles abrasion test In this study, the average abrasive loss of coarse aggregate was 19.07%, which meets the regulation of ASTM C131 (requiring that this metric be no more than 40%) [14]. 3.4. Results of fresh concrete test

Fig. 1. SEM micrographs of (a) CMP sludge under 1000 magnification and (b) CMP sludge under 50,000 magnification.

3.2. XRD analysis of OPC and CMP cement paste Fig. 2 shows the X-ray diffractograms for the OPC and CMP sludge-blended cement paste. The spectra show that calcium-containing substances, including C3S, C–S–H and Ca(OH)2, are the most prominent crystals in CMP cement paste. According to the Joint Committee on Powder Diffraction Standards (JCPDS), diffraction peaks of crystal Ca(OH)2 appear at 18.05°, 34.11°, 47.13°, and 50.84° [24]; those of C3S appear at 32°, 33.4°, 41°, and a diffraction peak for C–S–H colloid appears at 29.06°, 31.94°, and 49.79° [24],

The unit weights of fresh concrete are shown in Table 3. The specific gravity of CMP is lower than that of cement, so it is somewhat less than that of the ordinary Portland cement concrete group (reference group). The slumps associated with the OPCC specimens with water/cementitious ratios of 0.45, 0.55, and 0.65 were 12.2, 12.0, and 11.7 cm, respectively, and those of the SBCC specimens were 12.0, 11.9, and 11.7 cm, respectively; the averages were 11.9 cm and 12.0 cm for the OPCC and SBCC specimens, respectively. Slumps in both groups were similar and close to the target slump of 12.5 ± 1.0 cm. The average air contents of the OPCC and SBCC specimens were 0.67% and 1.05%, respectively; these metrics meet the relevant ACI design proportion limits (less than 1.5%). The air contents of the SBCC specimens were somewhat larger than those of the OPCC specimens because the CMP sludge contained many more small particles than cement. According to the Taiwan national standard CNS 3090, the chloride content of concrete must not exceed 0.3 kg/m3 in reinforced concrete or 0.15 kg/m3 in prestress concrete. In this study, the chloride contents of OPCC and SBCC specimens were in the range of 0.0153–0.0290 and 0.0380– 0.0477 kg/m3, respectively, far below the regulation limits. The results of the concrete setting tests are shown in Table 4. Both the initial and final setting times of the SBCC concrete

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1.Ca(OH)2

1.Ca(OH)2

2.CSH1

2.CSH1

3.C3S+C2S 3

1

1

91-day 60-day 28-day 7-day

5

1

2

31 3

3

Intensity (a.u.)

1

3 3 2 1

Intensity (a.u.)

3.C3S+C2S

1

1

91-day 60-day 28-day 7-day

3-day

3-day

1-day

1-day

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

2θ

5

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

(a)

2θ

(b)

Fig. 2. X-ray diffractograms of (a) OPC paste and (b) CMP (10%) paste.

Table 3 Slump, air content, chloride content and unit weight of the fresh concrete. Specimen

OPCC concrete

W/C ratio Slump (cm) Air content (%) Chloride content (kg/m3) Unit weight (kg/m3)

0.45 12.2 0.6 0.0200 2.402

SBCC concrete 0.55 12.0 0.6 0.0153 2.368

0.65 11.7 0.8 0.0290 2.364

Table 4 Setting times (in min) of ordinary Portland cement concrete (OPCC) and CMP sludgeblended cement concrete (SBCC). Setting time

Initial Final

W/C = 0.45

W/C = 0.55

W/C = 0.65

OPCC

SBCC

OPCC

SBCC

OPCC

SBCC

370 462

364 458

375 474

365 460

383 480

368 464

samples were somewhat shorter than those of the OPCC concrete samples. Because the fineness of the specific area of CMP sludge is 13,630 cm2/g and that of cement is 3520 cm2/g, CMP sludge absorbs more water than cement at the same water-to-cementitious ratio; this decreases the whole consistency of concrete paste. Thus, the setting times of SBCC concrete were 6–15 min shorter than those of OPCC concrete; the final setting times were 4–16 min shorter. This minor difference in setting time has little impact on practical construction engineering. 3.5. Toxicity characteristic leaching procedure (TCLP) test CMP sludge from science technology parks may contain heavy metals and nano-particles. When CMP sludge is exposed to strong acids and strong alkalis, it is easily deflocculated and leached out to contaminate water sources or groundwater. According to Lee and Liu [12], CMP nano-particles react with Ca(OH)2 to form colloids of C–S–H and C–A–S–H, and the nano-particles disappear in forming a hydration product. Fig. 3 depicts the nano-particles deformed by a pozzolanic reaction in the structure of mortar. The SiO2 and Al2O3 nano-particles could be clearly observed in the samples cured for 1 day and 3 days (Fig. 3a and b), and then through the pozzolanic reaction became part of the hydration products, which could not be released to pollute the environment after 28 days of curing; the structure of the resulting material is typical of hardened cement mortar with a network of hydration compounds

0.45 12.0 1.0 0.0380 2.330

0.55 11.9 0.85 0.0390 2.317

0.65 11.7 1.3 0.0477 2.310

(Fig. 3c and d). The results of the TCLP tests for samples of CMP sludge powder, OPCC and SBCC specimens are shown in Table 5. All of the leaching concentrations of heavy metals (Zn, Cd, Cr, Cu and Pb) from the samples are far below the regulatory thresholds. 3.6. Compressive strength test Fig. 4 shows the results of the compressive strength test. The compressive strengths of 1-day-old SBCC concrete cylinder specimens were all less than those of OPCC concrete cylinder specimens. The compressive strengths of both concrete types at 3 days of age were comparable. For samples at 7 days of age, the compressive strengths of SBCC concrete were 122–137% of those of OPCC concrete; for samples at 14, 28, 56, and 91 days of age, SBCC concrete compressive strengths were 124–138%, 127–137%, 126–134%, and 124–133%, respectively, of those of OPCC concrete. These test results are in agreement with previous compressive strength results from the cement–mortar test [12]. Fig. 5 shows that the strength activity index of the SBCC specimens relative to the OPCC specimens increases with curing age. The strength activity indices of SBCC (0.45), SBCC (0.55) and SBCC (0.65) specimens are weaker than those of the OPCC specimens during the early stages (1 day) of curing. Thus, the strength activity indices of these SBCC specimens are close to or higher than those of the OPCC specimens at 3 days and eventually exceed those of the OPCC specimens after 7 days. The greater the water-to-cementitious ratio is, the higher the strength activity index of the SBCC specimens becomes due to a stronger pozzolanic reaction. Relevant variations are shown in Fig. 5. The results reveal that the SiO2 nano-particles in CMP sludge behave as filler and as an activator promoting hydration, thereby improving the microstructure of concrete in SBCC specimens [25–27]. The nanoSiO2 particles can absorb hydrated Ca(OH)2 crystals, reducing their amount and pore size and thus increasing the density of binding in the paste matrix [28]. The phenomenon of reduced cement with higher strength growth is reminiscent of a pozzolanic reaction, as illustrated by the results of the XRD analysis.

T.-C. Lee et al. / Construction and Building Materials 30 (2012) 243–251

247

Fig. 3. SEM micrographs showing the nano-particles deformed in structure of mortar cured for (a) 1 day, (b) 3 days, (c) 28 days and (d) 91 days.

Table 5 TCLP leaching concentrations from the CMP sludge, OPCC, and SBCC specimens (in mg/l). TCLP (mg/l)

Zn

Cd

Cr

Cu

Pb

CMP sludge OPCC specimens SBCC specimens Regulatory limits

1.12 ND ND –

ND ND ND 1.00

0.13 ND 0.01 5.00

3.13 0.02 0.07 15.00

0.21 0.01 0.02 5.00

3.7. Measurement of mass growth in cylindrical concrete specimens Hydration products in cement fill the voids in cylindrical concrete specimens and make the structures denser, possibly improving the compressive strength as the curing age increases. To demonstrate this phenomenon, six cylindrical concrete specimens were cast and cured for weight measurement at curing ages of 1, 3, 7, 14, 28, 56, and 91 days. According to regulations, the specimens were placed under saturated-surface-dry moisture before being weighed. The weight of each specimen after 1 day of curing age was normalized to 1.0000, and the weights at other ages were normalized by dividing the weight of the specific curing age by that after 1 day of curing age. The results listed in Table 6 are the averages of 6 specimens. Table 6 shows that the weight of every specimen increased with increased concrete curing age. The weights of OPCC concrete aged for 28 days were 1.0040–1.0080 times those observed after curing for 1 day, while after 91 days they were 1.0054–1.0088 times those observed after 1 day. The weights of SBCC concrete aged for 28 days and 91 days were 1.0046–1.0088 and 1.0057–1.0097, respectively, which are somewhat higher than those of OPCC concrete. This phenomenon of there being less cement with higher mass growth is reminiscent of a pozzolanic reaction. Thus, the reason for these weight gains is related to the increased volume of hydration products, which is extended with

curing time. The results also reveal that the mass increase of the SBCC concrete specimens is greater than that of the OPCC specimens and coincides with the stronger compressive strength of SBCC concrete. Fig. 6a and b depicts the evolutionary curves of compressive strength and normalized weight against curing time for both OPCC and SBCC concrete, showing that the evolutionary trends of the growth of compressive strength and normalized weight are closely related. Fig. 7 shows the linear regression lines of normalized weight vs. the normalized compressive strength of the cylindrical concrete specimens. The values of the correlation coefficient (R2) are as high as 0.94–0.99. Furthermore, the increasing rates of compressive strength with respect to mass growth for all of the SBCC specimens are obviously higher than those of the OPCC specimens. This phenomenon is consistent with the test results regarding compressive strength. 3.8. Stress–strain analysis of concrete Measurements of longitudinal stress–strain were taken during compressive strength testing. Fig. 8 depicts the curves of compressive strength vs. strain for curing times of 1, 3, 7, 14, 28, 56, and 91 days for OPCC and SBCC concrete with a water-to-cementitious ratio of 0.65. Fig. 8 also shows that the compressive strength of SBCC concrete increased rapidly from 3 days to 14 days of curing time, a more rapid increase than that observed for OPCC concrete. This phenomenon coincides with the decreasing Ca(OH)2 content indicated by XRD analysis and the pozzolanic reaction phenomenon. The elastic moduli of the concrete specimens were measured according to ASTM C469. The relationships between the elastic modulus (Ec) and the water-to-cementitious ratio (W/C) and the pffiffiffiffiffi0 square root of compressive strength ( fc ) are depicted in Fig. 9a and b, respectively. We plotted EcW/C for the OPCC and SBCC

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T.-C. Lee et al. / Construction and Building Materials 30 (2012) 243–251 Table 6 Evolution of normalized mass of OPCC and SBCC specimens.

80

50 40 30 20

Normalized weight 1-day

3-day

7-day

14-day

28-day

56-day

91-day

OPCC (0.45) SBCC (0.45) OPCC (0.55) SBCC (0.55) OPCC (0.65) SBCC (0.65)

1.0000 1.0000 1.0000 1.0000 1.0000 1.0000

1.0030 1.0037 1.0029 1.0051 1.0016 1.0020

1.0044 1.0059 1.0051 1.0071 1.0026 1.0032

1.0053 1.0069 1.0068 1.0079 1.0033 1.0041

1.0061 1.0079 1.0080 1.0088 1.0040 1.0046

1.0069 1.0085 1.0086 1.0095 1.0049 1.0054

1.0075 1.0090 1.0088 1.0097 1.0054 1.0057

OPCC (0.45) OPCC (0.55) OPCC (0.65)

20

40

60

80

100

Curing time (days)

80

(a)

70

Compressive strength (Mpa)

0

80

Compressive strength (Mpa)

70 60 50 40

1.012 1.010

60 1.008

50 40

1.006

30

1.004

20 Compressive strength

10

30 0

0

20

20

40

60

80

Curing time (days)

80

(b)

70

SBCC(0.45) SBCC(0.55) SBCC(0.65) OPCC

180 160

80

1.000 100

(a)

Fig. 4. Evolution of compressive strength for (a) OPCC cylindrical concrete specimens and (b) SBCC cylindrical concrete specimens.

200

60

100

Compressive strength (Mpa)

0

40

Curing time (days)

SBCC (0.45) SBCC (0.55) SBCC (0.65)

10

1.012 1.010

60 1.008

50

1.006

40 30

1.004

20 10

Compressive strength

140

1.002

Normalized weight

0

120

0

20

40

60

80

100

Curing time (days)

80

(b)

60

1.000 100

Fig. 6. Evolution of compressive strength and normalized weight in (a) OPCC (0.45) cylindrical concrete specimens and (b) SBCC (0.45) cylindrical concrete specimens.

40 20

1.002

Normalized weight

20

0

Normalized weight

10 0

Strength activity index (%)

Specimens

60

Normalized weight

Compressive strength (Mpa)

70

0

7 14 21 28 35 42 49 56 63 70 77 84 91

Curing time (days)

OPCC concrete : Ec ðGPaÞ ¼ 4:6107 

Fig. 5. Relationship between the strength activity index of slag-cement concrete and curing age.

groups (see Fig. 9a). The four linear regression equations are as follows, in which R2 is the goodness of fit.

OPCC concrete : Ec ðGPaÞ ¼ 8:2835  ðW=CÞ þ 29:945

ð4Þ

qffiffiffiffiffi 0 fc

ð5Þ

R2 ¼ 0:9189 SBCC concrete : Ec ðGPaÞ ¼ 4:1342 

ð2Þ R2 ¼ 0:9399

R2 ¼ 0:9625 SBCC concrete : Ec ðGPaÞ ¼ 8:0734  ðW=CÞ þ 31:151

qffiffiffiffiffi 0 fc

ð3Þ

R2 ¼ 0:9683 pffiffiffiffiffi0 Our plots of Ec— fc are shown in Fig. 9b; the regression equations are

where fc0 is the compressive stress (Mpa) at 28 curing days. The four regression equations above imply that the effects of the water-to-cementitious ratio (W/C) and the compressive pffiffiffiffiffi0 strength ( fc ) on the elastic modulus in both concrete groups are quite significant. The four regression equations show correlation coefficients greater than 92% and should yield suitable predictions for the elastic moduli of both of the concrete groups studied

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T.-C. Lee et al. / Construction and Building Materials 30 (2012) 243–251

Normalized strength

26 R2 = 0.9829

SBCC (0.45) OPCC (0.45) SBCC (0.55) OPCC (0.55) SBCC (0.65) OPCC (0.65)

21 16

R2 = 0.9759

11

R2 = 0.9903 R2 = 0.9904

6

R2 = 0.9849 R2 = 0.9418

1 1.000

1.002

1.004

1.006

1.008

1.010

Normalized weight Fig. 7. Linear correlations between normalized compressive strength and normalized weight of the OPCC and SBCC cylindrical concrete specimens.

in this work. The test results also show that the elastic moduli of both OPCC and SBCC concrete are positively related to the compffiffiffiffiffi 0 pressive strength ( fc ) and inversely related to the water-topffiffiffiffiffi0 cementitious ratio (W/C). Thus, the compressive strength ( fc ) and water-to-cementitious ratio (W/C) significantly affect the elastic moduli (Ec) of both OPCC and SBCC concrete. Fig. 10 shows that the evolution of the elastic moduli in OPCC and SBCC specimens was similar. For samples at 7 days of age, the elastic moduli of SBCC concrete were 105–112% those of OPCC concrete; for samples cured for 28, 56, and 91 days, the elastic moduli of SBCC concrete were 104–108%, 102–103%, and 102– 105%, respectively, of those of OPCC specimens. The test results reveal that the elastic moduli increased with the curing age of the specimens, which is consistent with compressive strength growth; those of the SBCC specimens were slightly higher than those of the OPCC specimens. These results are beneficial to practical construction engineering.

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pffiffiffiffiffi0 fc .

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(c) Fig. 10. Evolution of elastic moduli for (a) OPCC (0.45) and SBCC (0.45) cylindrical concrete specimens, (b) OPCC (0.55) and SBCC (0.55) cylindrical concrete specimens, and (c) OPCC (0.65) and SBCC (0.65) cylindrical concrete specimens.

4. Conclusions In this study, cylindrical specimens of CMP sludge-blended cement concrete (SBCC; experimental group) were cast with 10 wt.% cement replacement by calcined CMP sludge particles smaller than sieve #200; ordinary Portland cement concrete (OPCC) cylindrical specimens of the same size (reference group) were cast for comparison. The specimens were subjected to compressive strength tests and related analyses. The following conclusions can be drawn: 1. The results of the toxicity characteristic leaching procedure (TCLP) test of CMP mortar reveal that the leaching concentrations of heavy metals were far below regulatory thresholds. Thus, CMP sludge can be safely used to partially replace cement in practical applications. 2. The compressive strengths of SBCC concrete at curing times of 7, 14, 28, 56, and 91 days were 122–138% of those of OPCC concrete, demonstrating that the CMP sludge enhanced the compressive strength of SBCC concrete after 3 days of age. 3. The properties of fresh SBCC and OPCC concrete were similar: the slumps, unit weights, air content, and chloride content were comparable; the initial setting time of SBCC concrete was somewhat less than that of OPCC concrete (6–15 min shorter), and the final setting time of SBCC concrete was 4–16 min shorter than that of OPCC concrete. The minor differences in properties between the two have little impact on practical applications. 4. The stress–strain curves of both SBCC and OPCC concrete show that the strength growth rate of SBCC concrete in the age range of 3–91 days is higher than that of OPCC concrete, revealing the presence of a pozzolanic reaction.

5. The weight measurements of concrete specimens reveal that the weights increased with curing age. The weight increase of SBCC concrete was greater than that of OPCC concrete, which is indicative of a pozzolanic reaction, and coincided with increased compressive strength. 6. The elastic modulus was observed to increase with the curing age of the specimens in accordance with the increased compressive strength. The elastic moduli of the SBCC specimens were slightly higher than those of the OPCC specimens, which is beneficial to practical construction engineering usability. In conclusion, this study confirms the feasibility of partial cement replacement with CMP sludge. It is highly possible that CMP sludge could be recovered as a material that can be used to strengthen cement concrete. The result would be a reduction in hazardous waste, which is beneficial to our environment and meets the need for sustainable development. Acknowledgments The authors thank the National Science Council of the Republic of China, Taiwan, for fully supporting this research under Contract No. NSC 99-2221-E-239-029. The authors would also like to thank the Central Taiwan Science Park Bureau for its assistance. References [1] Lin SH, Yang CR. Chemical and physical treatments of chemical mechanical polishing wastewater from semiconductor fabrication. J Hazard Mater 2004;B108:103–9. [2] Lai CL, Lin SH. Electrocoagulation of chemical mechanical polishing (CMP) waste water from semiconductor fabrication. Chem Eng J 2003;95:205–11.

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