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1. Introduction

beend harbely keeludgesy sludh andrecycli

and others (Chen et al., 2010; Mueller et al., 2008; Chiou et al.,2006; Cheeseman et al., 2005; Hossain, 2004; Mladenovic et al.,2004) and because of the increasing public requirement on greenenvironment, there is an increasing trend to use wastes as a substi-tute for the natural resources.

The water absorption capacity of the LWAs prepared in previousstudy is relatively high (6.0%14.0%) (Wei et al., 2008). Thus, an at-

man et al., 2002). Based on the principle that waste is a misplacedresource, the present study recycles the harbor sludge, with/with-out addition of waste glass powder, as lightweight aggregates(LWAs) via ring for considerably less process time, as comparedwith what required for manufacturing bricks and porcelain.

2. Experimental methods

The harbor sediment was collected from Taichung Harborlocated in central Taiwan. This harbor has very serious sediment

Corresponding author. Tel.: +886 42 359 1368; fax: +886 42 359 6858.

Marine Pollution Bulletin 63 (2011) 135140

Contents lists availab

ut

l seE-mail address: yulin@thu.edu.tw (Y.-L. Wei).resources is preferred. In a previous study, dredged sludge of theharbor Taiwan was successfully manufactured into lightweightaggregates (LWAs) after mixing with a reservoir sediment at differ-ent ratios and ring at 10501150 C (Wei et al., 2008).

LWAs have various applications. They can be used in roompartition to lessen the weight of building with great sonic and reresistance and ease the shocking wave caused by earth quake.LWAs can also be used in bridge construction, gardening, and soiland environmental engineering (Cheeseman et al., 2005). Inproducing LWAs, however, because of the gradual depletion of nat-ural resources, such as shale, pumice, volcanic ash, siliceous rock,

LWAs absorb considerable amount of water from cement, thepozzolanic reaction activity inside the cement matrix will be inu-enced and becomes difcult to control.

Traditionally, waste glass is recycled into secondary glass prod-uct and porcelain in Taiwan; however, these paths have lost theircompeting capability against imported products due to relativelyhigh labor cost. Thus, new approaches in recycling waste glassare desired. To enhance the sintering reaction occurring in varioushigh-temperature processes, addition of waste glass into rawmaterials at different ratios has been investigated (Ducman andMirtic, 2009; Nemes and Jzsa, 2006; Corinaldesi et al., 2005; Duc-Taichung Harbor of Taiwan hasdeposition problem in ship routes anreduces the water depth. To effectivcondition, regularly dredging the sthe cost of nal disposal of the maswan, the population density is higwaste landll is very limited. Thus0025-326X/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.marpolbul.2011.01.037troubled with sludgeor basin. The depositionp the harbor in usableis necessary althoughge is expensive. In Tai-the land available forng the sludge as useful

tempt is made in present study to improve the quality of LWAsregarding water sorption capacity. LWAs have frequently beenused as a ller to prepare lightweight cement (LWC) for construc-tion sector. It is highly desirable if the LWAs incorporated in LWCdo not absorb the water which is always added to cement for awell-controlled pozzolanic reaction to form calcium silicate andcalcium aluminosilicate hydrates during cement hydration. Thecement hydration may take as long as four weeks to reach desiredlong-termed strength and hardness. Conversely, if the incorporatedPreparation of low water-sorption lightwsediment added with waste glass

Yu-Ling Wei a,, Chang-Yuan Lin a, Kuan-Wei Ko a, HaDepartment of Environmental Science and Engineering, Tunghai University, Taichung CbDepartment of Environmental Engineering, National Cheng-Kung University, Tainan Cic Sustainable Environment Research Center, National Cheng-Kung University, Tainan Cit

a r t i c l e i n f o

Keywords:Lightweight aggregateSinteringSediment recycleBloatingWaste glass recycle

a b s t r a c t

A harbor sediment is succeaddition of waste glass powviscosity viscous phases duples. Water sorption capacwith the addition of wasteecial for preparing lightwseized by lightweight aggr

Marine Poll

journal homepage: www.ell rights reserved.ght aggregates from harbor

ul Wang b,c

07, Taiwan7, Taiwan1, Taiwan

lly recycled at 1150 C as low water-absorption lightweight aggregate via. Sodium content in the waste glass is responsible for the formation of low-ring process to encapsulate the gases generated for bloating pellet sam-f the lightweight products can be considerably reduced from 5.6% to 1.5%ss powder. Low water-absorption property of lightweight products is ben-t concrete because the water required for curing the cement would not bete ller, thus preventing the failure of long-term concrete strength.

2011 Elsevier Ltd. All rights reserved.

vier .com/locate /marpolbulle at ScienceDirect

ion Bulletin

deposition problem resulting from the minerals brought in byincoming sea wave, consequently the sediment has to be regularlydredged; otherwise the ship routes would become too shallow tofunction normally. After sampling, the sediment was dried at105 C in an oven for 24 h and ground to pass a 50-mesh sieve. Itwas end-to-end mixed for 6 h with waste glass powders at variousweight ratios (sediment/waste glass powder = 100:0, 100:10,100:20, 100:30, 100:40, 100:50). The glass powders were suppliedby a local glass manufacturing plant that has been worrying abouttheir nal disposal. The mixture was again dried and ground to

14.21% Na2O that can reduce sintering temperature because of itsrelatively low melting point. The ux content (dened as theweight sum of Fe2O3, Na2O, K2O, CaO, and MgO) in the sedimentis 11.23%, while being 24.35% in the waste glass. This fact indicatesthat addition of waste glass into the sediment can lead to more ra-pid sintering reaction. The sediment and waste glass contain 5.72%and 2.15% LOI components, respectively which may be relevant tothe bloating phenomena for expanding the pellets at high temper-atures. These LOI contents are too high to gradually expand thepellets in present study, thus the green pellets were pre-treatedto de-volatize fractional LOI to prevent the pellets from beingblasted into small pieces during subsequent sintering/bloating pro-cess at 10001150 C.

To effectively encapsulate the gases for bloating the pellets, sin-tering reaction should take place in time to form continuous phasebefore the gases escape from the pellets. To obtain a timely sinter-ing reaction, particle size distribution has been considered as animportant physical factor. Finer particles can ll in the void spaceamong coarse particles, whereby neck growth rate among particlesduring the high-temperature process can be rapid enough to formcontinuous phase. As depicted in Fig. 2, the sediment is relativelyner than the glass powder. More than 95% sediment powdersare

Sintering/bloating reactions have caused some transformationand formation of crystalline phases, as depicted by the XRD pat-terns in Fig. 5. Major crystalline phases found in the sedimentare silicon oxide (SiO2), corundum (a-Al2O3), and wusite (FeO).Minor crystalline phases present are calcium oxide (CaO), halite(NaCl), potassium oxide (K2O), sodium carbonate (Na2CO3), kyanite(Al2SiO5), sillimanite (Al2SiO5), anorthite (CaAl2Si2O8), and ensta-tite (MgSiO3). After the sintering/bloating reaction at temperatureranging from 1000 C to 1100 C, new crystalline phases (i.e., a-so-dium silicate (Na2Si2O5) and calcium silicate (CaSiO3)) are formedin the LWAs along with the disappearance of some crystallinephases including calcium oxide, sodium carbonate, sillimanite,and enstatite. Obviously, calcium oxide reacts with silicon oxideto form calcium silicates. Sodium carbonate rst decomposes intocarbon dioxide and sodium oxide which subsequently reacts withsilicon oxide to form the new crystalline phase a-sodium silicate.Because both calcium and sodium contents are ux component,the can react with silicon oxide to form new crystalline phases.This may explain why calcium oxide, halite, and sodium carbonateare not found in the XRD patterns from 1000 C to 1100 C LWAs.The other two ux components, wusite and potassium oxide crys-talline phases, become either barely recognizable or absent in the10001100 C diffraction patterns. In addition, the released carbondioxide contributes to the expansion of pellets. Additionally, Fig. 5reveals that higher temperature generally leads to lower crystal-line intensities, indicating that with more heat being provided tothe pellets, the crystalline size tends to gradually decrease.

After ring at 10001150 C, 4.67%6.51% sample weight waslost from green pellets (see Table 2). For sintered sediment (g/s = 0/100), the weight loss is 5.92%6.51% which is slightly greaterthan the sediment LOI (5.72%, see Table 1) because the latter was

138 Y.-L. Wei et al. /Marine PollutionFig. 5. XRD patterns from harbor sediment and lightweight aggregates preparedfrom mixtures with a sediment-to-waste glass ratio of 50/100 at 10001100 C.determined at a relatively lower temperature, 900 C. However,the TGA-weight loss of the sediment (g/s = 0/100) is 7.2%7.8% inthe 10001100 C range (see Fig. 4), which is greater than bothsediment LOI and LWA weight loss. The difference is attributedto different heating rate. The TGA experiment was performed ata very slow heating rate (10 C min1), while LWA preparationand LOI test were associated with rapid heating as the sampleswere directly exposed to the nal pre-set temperatures. Rapidheating during LWA preparation would cause instant sintering togenerate viscous liquid phases to encapsulate high-pressure gases,thereby forming LWAs. As to the effect of process temperature andaddition of glass on weight loss, the LWA weight loss generally in-creases with increasing temperature and decreasing glass content.

The generally acceptable particle density of LWAs is

Table 2Weight loss after ring the pellets consisting of various ratios of waste glass/sediment (g/s) at 10001150 C.

Temp. (C) Weight loss (%)

g/s 0/100 g/s 10/100 g/s 20/100 g/s 30/100 g/s 40/100 g/s 50/100

1000 5.92 5.55 5.46 5.15 4.80 4.761050 6.16 5.39 5.13 5.18 4.96 5.091100 5.98 5.64 5.41 4.85 5.25 4.671150 6.51 5.72 5.27 5.53 5.39

Table 3Effect of process temperature and waste glass/sediment ratio (g/s) on the particle density of LWAs.

Temp. (C) Particle density (kg m3)

g/s 0/100 g/s 10/100 g/s 20/100 g/s 30/100 g/s 40/100 g/s 50/100

1000 2.31 2.35 2.14 2.02 2.06 1.931050 2.33 2.24 1.93 1.87 1.71 1.731100 1.83 1.81 1.58 1.71 1.24 1.261150 1.16 1.11 1.18 0.92 0.79

Table 4Effect of process temperature and waste glass/sediment ratio (g/s) on the water absorption capacity of LWAs.

0/1

Y.-L. Wei et al. /Marine Pollution Bulletin 63 (2011) 135140 139(see Fig. 6). In general, the LWAs prepared under condition of lowertemperature and less glass addition are characterized by fewer and

Temp. (C) Water absorption (%)

g/s 0/100 g/s 10/100 g/s 2

1000 17.5 16.9 16.51050 13.2 11.3 10.51100 7.1 5.5 4.91150 5.6 5.1 3.7ner pores in their fractured cores (see upper left panels of Fig. 6);while the opposite condition leads to product containing largerpores (see lower right panels). Firing the pellets containing glassat 1100 C, the pores are so large that they even become intercon-nected due to the relatively low viscosity of the viscous phaseswhich could not withstand the bloating force from the high-pres-sure gases. As the heated pellets grow in size, the viscous phases

Fig. 6. Morphologies of fractured cores of the lightbecome thinner and their strength gets weaker. If this situationcontinues to proceed, the thin weak viscous phases may eventually

00 g/s 30/100 g/s 40/100 g/s 50/100

17.3 11.7 11.88.3 4.8 4.44.1 3.8 2.72.5 1.5 break; thereby the pores being interconnected. In contrast, ringthe pellets at lower temperatures would produce LWAs with moreisolated spherical pores in the fractured cores, and the inter-porecrystalline phases are thicker.

Furthermore, the leaching test of heavy metals from solidbuilding materials recycled from wastes (Taiwan EPA, 2010) wasperformed on the LWAs with a total solid/liquid ratio of approxi-

weight aggregates produced at 10001100 C.

mately 1/100, and the leaching concentrations of Pb, Cr, Zn, andMnfrom all LWA samples are 60.096, 60.013, 60.015, 60.024 mg L1,respectively. Its noteworthy that Taiwan has not imposed any reg-ulatory threshold on heavy metal leaching from constructionmaterials so far; how and what to be set as the regulatory thresh-old is a currently discussed issue among ofcials serving in variousTaiwan governmental ministries. For the practical application ofLWAs, because the LWAs contain 0.27%1.30% total chloride thatmight be corrosive to steel and because the strength of the LWAsreported in this manuscript can be as low as 22.6 kgf cm2, theLWAs reported in the present study are suggested to be limitedto non-structural usage in the absence of steel reinforcement, suchas being incorporated into lightweight concrete for room partition,gardening, and environmental application.

4. Conclusions

Because of its relatively low melting point, waste glass additivewhich is rich in sodium oxide and ux contents can effectively helpsinter to form continuous viscous phase to encapsulate gases forpreparing LWAs from harbor sediment.

Formed at higher temperature with more glass additive, theLWAs are less dense due to a decrease in the viscosity of viscouslayer and to a generation of richer bloating gases. With waste glass

Acknowledgment

We thank the National Science Council of Taiwan for the re-search fund through contract NSC 98-2221-E-029-003.

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140 Y.-L. Wei et al. /Marine Pollution Bulletin 63 (2011) 135140addition, the apparent particle density of LWAs prepared at1150 C decreases from 1.16 kg m3 to 0.79 kg m3.

All LWAs have a water absorption capacity