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Article

Valorization of Dredged Harbor Sediments through Lightweight Aggregate Production: Application of Waste Oyster Shells

by
Yee Cheng Lim
,
Chih-Feng Chen
,
Chiu-Wen Chen
* and
Cheng-Di Dong
*
Department of Marine Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung City 81157, Taiwan
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(6), 5466; https://doi.org/10.3390/su15065466
Submission received: 1 February 2023 / Revised: 2 March 2023 / Accepted: 7 March 2023 / Published: 20 March 2023
(This article belongs to the Special Issue Emerging Green Technologies for Solid Waste Management: A Contribution to Sustainable Environmental Practices and Circular Economy)
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Versions Notes

Abstract

:
The treatment and valorization of wastes such as dredged harbor sediments and oyster shells have become critical for environmental management. In order to promote waste valorization and resource sustainability, this study evaluated the feasibility of using harbor sediments and oyster shells for lightweight aggregate (LWA) production. The effects of the oyster shell content and sintering temperature on the sediment-based LWA properties, including particle density (PD), water absorption (WA), and crushing strength (CS), were investigated. The engineering applicability of the sediment-based LWAs was also assessed. The results showed that it was feasible to use harbor sediments admixed with oyster shells to produce LWAs that were suitable for engineering applications. The LWA properties were highly varied according to the sintering temperature and oyster shell content. Remarkably, the LWAs prepared with a 5–15% oyster shell content and sintered at 1125 °C were suitable for structural lightweight concrete (PD: 1.73–1.83 g/cm3, WA: 12.2–15.1%, CS: 7.2–10.4 MPa). The open porosity was a key factor affecting the particle density, water absorption, and crushing strength of the LWAs. Moreover, the leachability of toxic metals and chloride ions in the LWAs complied with the regulations for building materials in Taiwan. Waste oyster shells could be an excellent additive to lower the optimal sintering temperature required for sediment-based LWA production.

1. Introduction

Shipping plays an integral role in international trade. With the massive use of large and ultra-large container ships, maintaining the water depth of wharves and channels has become crucial for port operations [1]. Harbor sediment dredging is necessary to keep the required water depth for navigation [2]. Hundreds of millions of tons of dredged harbor sediments are generated annually worldwide [3]. Harbor sediments may contain potentially toxic metals and organic pollutants [4,5]. Thus, the disposal and treatment of dredged sediments have become an essential environmental engineering issue for public concern [6]. Commonly, the dredged sediments are dumped into officially designed ocean or land disposal sites. However, land disposal is often costly and requires a large amount of land. Ocean disposal may cause unexpected adverse impacts on marine ecosystems [7]. Therefore, some countries have developed sustainable methods to resolve the problem of sediment disposal in different fields, such as brick production [8], paving block production [9], pavement construction [10], beach replenishment, and land reclamation [11]. Some studies used dredged sediments, particularly lake and reservoir sediments, to form self-consolidating or non-structural concretes mixed with binders [12,13]. Dredged sediments have also been used as cementitious materials such as eco-friendly cement and geopolymer [14]. Improving the utilization of dredged harbor sediments can create a win-win situation by reducing the environmental impacts and increasing value-added products in the circular economy. However, dredged harbor sediments have a high content of chlorides and a high pollution potential, making their use in engineering applications without pretreatment difficult [15,16]. Their weak mechanical strength also limits the engineering applicability of sediments.
Taiwan has 13 commercial ports, 5 industrial ports, and 149 fishing ports. The disposal of a large amount of dredged sediment is required regularly. For Kaohsiung Port, the 15th largest international container port, approximately 0.5–1 million m3 of dredged sediments is generated annually [3]. Although the utilization of dredged sediments, such as through land reclamation, has increased in recent years, most dredged sediments are still managed via low-cost ocean disposal [15]. Considering their potential pollutant and high chloride contents, dredged sediments may be challenging to valorize under many restrictions and have a high risk of secondary pollution [16]. An increase in the cost required for processing may be inevitable. As a result, some studies have developed high-efficiency and low-cost methods to remove pollutants in dredged sediments and improve their usability in engineering applications [17,18].
Through thermal treatments, dredged sediments have been made into bricks, ceramic materials, and lightweight aggregates (LWAs) [19,20]. Among them, LWAs are widely used in construction engineering to prepare lightweight concrete (LWC) because of their low density, which can significantly reduce the dead weight of buildings, thereby reducing the costs of structure design and transportation. LWAs have the advantages of fire resistance and noise reduction when used in room partitions [21]. LWAs have also been used in agricultural techniques such as soilless cultivation [22]. In addition to natural LWAs (such as volcanic pumice), LWAs are mainly made of expanded clay through artificial sintering procedures [23]. Some studies have used industrial/agricultural wastes or by-products to prepare LWAs with suitable engineering applicability [24]. Sustainable development must be achieved by effectively utilizing waste instead of natural resources. Hence, the valorization of dredged harbor sediments through LWA production benefits the circular economy and promotes a sustainable community under the limitations of natural resources.
On the other hand, the oyster is a vital aquaculture product worldwide. Approximately 6.4 million tonnes of oysters are produced per year [25], and 18 thousand tonnes per year in Taiwan [26]. However, oyster shells, the by-product of oyster farming, represent up to 70% of the oyster’s weight. The disposal of oyster shells has generated an environmental issue that needs to be resolved [27]. In general, oyster shells are piled up near oyster fields, affecting the city’s appearance and leading to environmental sanitation problems. Developing means for the reuse of oyster shells could address the environmental concerns posed by oyster farming [28]. Oyster shells, mainly composed of calcium carbonate (CaCO3), serve as a good fluxing agent. The advantages of oyster shell additives include: (1) CaCO3 is decomposed into calcium oxide (CaO) and carbon dioxide (CO2) during the sintering process (Equation (1)). The release of CO2 induces the generation of more pores inside the aggregate [29]. Cao et al. [30] also reported that CaO addition was beneficial in producing more CO2 and made the LWA porous.
CaCO3 → CaO + CO2 ↑
The sintering temperature can be reduced through eutectic reactions. The melting points of SiO2 and Al2O3 are as high as 1670 and 2050 °C, respectively, while the melting point of the CaO–Al2O3–SiO2 system decreases to 1170 °C [31].
Therefore, this study used oyster shells as additives to admix with dredged sediment from Kaohsiung Port for LWA preparation. The objectives of this study were: (1) to evaluate the feasibility of LWA preparation by sintering dredged harbor sediments with oyster shells; (2) to examine the effects of the sintering temperature and oyster shell content on the properties of the sediment-based LWAs; and (3) to assess the engineering applicability of the sediment-based LWAs.

2. Materials and Methods

2.1. Raw Materials

Harbor sediments (HS) were collected from Kaohsiung Port in southern Taiwan (Figure 1). Oyster shells (OS) were collected from local cultural farms along the coast of western Taiwan. The HS and OS were dried, then ground with a high-speed desktop pulverizer (Rong Tsong Precision Technology RT-34, Taiwan) to pass through a 125 μm standard sieve and stored in PE bottles. The chemical composition of the raw materials was determined by an X-ray fluorescence spectrometer (Bruker S8 TIGER Series 2, USA). As shown in Table 1, the proportions of SiO2, Al2O3, and fluxing components (the sum of Fe2O3, K2O, CaO, Na2O, and MgO) in the HS were 62.0, 16.8, and 18.2%, respectively. We followed the Riley predictive scheme [32] for LWA production (Figure 2). The OS had a high proportion of fluxing components (94.9%) that may have enhanced the sintering reaction during LWA manufacturing.
The particle size distributions of the ground harbor sediments and oyster shells were determined using a laser diffraction technique (Beckman Coulter LS230, USA) and the wet sieving method, respectively. As shown in Figure 3, the ground sediment sample’s D10, D50 (median), and D90 were 1.7, 8.8, and 110 μm, respectively. The ground oyster shells were dominant in particle sizes less than 38 μm, representing up to 58.9%. The fineness of the raw materials is vital for LWA preparation, as it is beneficial to gas bubble wrapping and the sintering kinetics during the sintering process [24].
The thermal behaviors of the raw materials were determined using a simultaneous thermogravimetric/differential thermal (TG/DT) analyzer (Hitachi STA 7200, Japan) under a N2 atmosphere with a heating rate of 15 °C/min. As shown in Figure 4, the HS’s total weight loss (i.e., LOI) was approximately 3.9% when heated to 1000 °C, mainly attributed to the decomposition of organic matter and sulfide compounds and the dehydration of structural moisture [34]. The OS had relatively high weight loss (43.5%) compared to the HS, mainly attributed to the decomposition of calcium carbonate. A high LOI in the raw minerals is beneficial to density reduction for LWA production [35].

2.2. Aggregate Preparation

Aggregate pellets were prepared from the mixture of HS with different OS contents (0, 5, 10, 15, and 20%) and sintered at 1000, 1050, 1100, 1125, 1150, and 1175 °C. To precisely compare the engineering properties between different aggregates, consistency in the size and shape of each aggregate pellet was required. Hence, in this study we formed the aggregate samples using a mold. About 10 g of raw materials for each green pellet preparation was shaped into a cylindrical pellet (~2.0 cm length, ~1.6 cm diameter) using a PVC mold. The green pellet was pre-dried at 105 °C for 24 h and then sintered at a defined temperature for 15 min with a heating rate of approximately 15 °C/min using a muffle furnace (SNOL LSM01 6.2/1300, Lithuania). After sintering, the sintered aggregates (SAGs) cooled naturally in the furnace. Figure 5 presents a photo of the SAGs’ appearance. The SAGs sintered at 1175 °C are not shown because they melted and stuck to the sample holder.

2.3. Aggregate Performance Tests

The typical engineering properties of LWAs, including water absorption (WA), particle density (PD), and crushing strength (CS), were examined to evaluate the performance of the sintered aggregates (SAGs). Approximately 3–5 replicate samples were analyzed for each test. The WA was determined following the ASTM standard test method (ASTM C127-15) and calculated using Equation (2). The PD was determined by the pycnometer method and calculated using Equation (3) [19]. The CS was determined using an automatic compression testing machine (25 kN capacity, Summit, Taiwan) by pressing the individual SAG until cracking (Equation (4)).
WA = WSSD WOD × 100
PD = WOD ( WSSD + WW − WAW ) / Dw
C S = P c π D 2
Here, WA is the water absorption (%); WOD is the oven-dried weight of the SAG (g); WSSD is the weight of the saturated SAG with a dry surface after immersing in water for 24 h (g); PD is the particle density (g/cm3); WW is the weight of the pycnometer containing water (g); WAW is the weight of the pycnometer containing SAG and water (g); Dw is the water density (1 g/cm3); CS is the crushing strength of the individual SAG (MPa); Pc is the fracture loading force (kN); and D is the diameter of the SAG (cm).
In addition, the open porosity (Po), weight loss (WL), and bloating index (BI) of the SAG and green pellet prior to sintering were examined. The Po was calculated from the ratio of the particle density (PD) to the apparent density (AD) of the SAG using Equation (5) [36]. The WL and BI were determined by the weight and volume difference between the green pellet and the SAG, respectively (Equations (6) and (7)).
Po = ( 1 − PD AD ) × 100 ;   AD = WOD ( WOD + WW − WAW ) / Dw
WL = WG − WOD WG × 100
BI = VOD − VG VG × 100
Here, Po is the open porosity (%); PD is the particle density (g/cm3) defined by Equation (3); AD is the apparent density (g/cm3); WOD, WW, WAW, and Dw are defined in Equation (3); WL is the weight loss (%); WG is the weight of the green pellet (g); BI is the bloating index (%); VG and VOD are the volume (cm3) of the green pellet and SAG, respectively, which were measured by a digimatic caliper with a minimum indication of 0.01 mm (Mitutoyo CD-6” ASX, Japan).
The water-soluble chloride content of the SAG was determined following the standard method of the Bureau of Standards, Metrology, and Inspection, Taiwan (CNS 13407). Briefly, the crushed aggregate was submerged in deionized water (~18 MΩcm) with a solid/liquid ratio of 1/5 (20 g/100 mL) for 24 h. The dissolved chloride concentration of the supernatant was measured using an ion chromatography system (Dionex DX-120, USA).
The leachability of toxic metals in the SAG was determined based on the standard method of the Taiwan Environmental Protection Administration (NIEA R201.14C) using the toxicity characteristic leaching procedure (TCLP). Briefly, the crushed aggregate was mixed with 0.1 M acetate acid solution (pH 2.88) under a solid/liquid mass ratio of 1/20 using a rotary agitation apparatus (~30 rpm) for 18 h. After the leaching procedure, the metal concentrations (Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, Se, and Zn) of the TCLP leachate were measured using a polarized Zeeman flame atomic absorption spectrophotometer (Hitachi ZA-3300, Japan).

2.4. Statistical Analysis

Spearman’s correlation coefficients between the sintered aggregate properties and sintering conditions were analyzed using IBM SPSS Statistics software (version 20).

3. Results and Discussion

3.1. Effects of Sintering Temperature and Oyster Shell Content

As shown in Figure 6, the sintered aggregate (SAG) properties varied highly with the sintering temperature (Ts) and oyster shell (OS) content. The particle density (PD) of the aggregates showed an increasing trend with Ts when the temperature was above 1100 °C (Figure 6a). On the contrary, water absorption (WA) decreased sharply with a Ts between 1125 and 1150 °C. Remarkably, the WA reached a low level (3.5–4.0%) for the aggregates with 0–10% OS content (Figure 6b). However, with a Ts between 1000 and 1100 °C, the PD and WA varied slightly, indicating that when the temperature was lower than 1100 °C, the sintering reaction had not yet occurred. This result agreed with Ayati et al. [23], who suggested that 1100–1250 °C is the optimal Ts for the production of most clay LWAs.
Generally, raw materials with high LOI values induce a lower lightweight aggregate (LWA) particle density (PD) due to high weight loss during the sintering process [35]. As shown in Figure 6d, the aggregates with a higher OS content had a higher weight loss (WL) after sintering, regardless of the Ts. As a result, the aggregates with 20% OS had the lowest PD. In addition to the WL, the SAG’s volume change (shrinkage or bloating) is also a critical factor in controlling the PD [37]. Oyster shell primarily comprises calcium carbonate, which can be a fluxing component for sintering [29]. As shown in Figure 6e, the SAGs’ bloating index (BI) increased with the OS content; in particular, the aggregates with 10–20% OS contents exhibited volume expansion (BI: −4.0–11.9%, mean: 3.7%), except for when the Ts was above 1150 °C. Thus, oyster shell addition could enhance the bloating process for sediment-based LWA production. Moreover, when the Ts reached 1150 °C, the volume of the sintered aggregates showed shrinkage, resulting in a sharp increase in the PD (Figure 6a). The PD showed a significantly negative correlation with the BI (r = −0.78, p < 0.01) (Table 2). The PD of the aggregates sintered at 1150 °C was almost higher than 2.0 g/cm3, which meant that they did not belong to the LWA category, except for the aggregates with a 20% OS content (PD: 1.66 g/cm3).
On the other hand, a LWA’s water absorption (WA) is strongly affected by the formation of an impermeable vitrified layer. High sintering temperatures (Ts >1100 °C) typically melt silicate minerals in aggregates and form a viscous glassy layer [38]. When the glassy layer completely surrounds the aggregate to form a waterproof shell, it can prohibit external moisture from penetrating the aggregate and thus lowers the WA [39]. As shown in Figure 6b, the aggregates’ WA dropped substantially when the Ts increased from 1100 to 1150 °C. However, the addition of OS may have increased the internal porosity of the aggregates during the bloating process. As shown in Figure 7, under the same sintering temperature (1150 °C), the aggregate with a 20% OS content was porous, presenting larger pores and a rougher surface compared to the aggregate with a 10% OS content. The formation of larger pores could promote the water absorption capacity. Thus, the aggregates with a higher OS content showed a higher WA. Corresponding to the open porosity of the aggregates (Figure 6f), the aggregates with a higher OS content had a higher open porosity after sintering at a temperature lower than 1125 °C. A highly positive correlation (r = 0.98, p <0.01) between WA and Po was observed (Table 2).
In addition to the PD and WA, the crushing strength (CS) is a vital LWA property for practical applications, as a weak CS may limit the LWA’s suitability for construction applications. As shown in Figure 6c, the CS increased with the increase in the Ts and was relatively high (16.5–40.1 MPa) at a Ts of 1150 °C. The highest CS (38.4–40.1 MPa) was observed for the SAG with a 5–10% OS content, being ~2.4 times higher than that for the SAG with a 20% OS content. At a high Ts, the partially melted glassy phase can wrap the sediment particles to vitrify the aggregate structure, resulting in the CS increasing with the sintering temperature [39]. When the Ts reached 1150 °C, the SAG shrank obviously (Figure 6e) and caused the aggregate to become denser, increasing the resistance to fracture loading.
In contrast, the CS was relatively low (1.0–5.8 MPa) for the SAG sintered at 1000–1100 °C. The CS variation was highly correlated with the PD and BI (Table 2). In general, LWAs with a higher PD have a higher CS, and vice versa [37]. An appropriate level of OS addition (5–10%) increased the CS of the SAG (38.4–40.1 MPa), especially for a Ts of 1150 °C, probably due to the enhancement of the sintering reaction. However, the promotion of the bloating behavior by the OS addition may not have been conducive to the CS of the SAG with a 20% OS content (16.5 MPa) compared to that of the SAG without OS (18.5 MPa). The LWAs with higher volume bloating had a lower CS, because large internal pores weakened the strength of the aggregate structure [40]. As shown in Table 2, the Po showed a significant correlation with the properties of the SAG, indicating that the porous structure and pore sizes inside the aggregate were significant factors affecting the PD, WA, and CS of the LWAs.

3.2. Aggregate Performance and Possible Applications

Typically, particle density, water absorption, and compressive (or crushing) strength are key properties for the use of LWAs in practical engineering applications [23]. However, there is no official standard for individual aggregate properties. Chen et al. [41] reported that lightweight concrete made from a LWA with a CS of 6 MPa exhibited a compressive strength of up to 40 MPa. Souza et al. [42] referred to commercial LWA datasheets (Arlita Leca® and Argila Expandida®) and proposed the following five criteria: for high-strength LWC, PD < 2.0 g/cm3, WA = 0–20%, CS > 5.0 MPa; for structural LWC, PD < 2.0 g/cm3, WA = 0–20%, CS > 2.3 MPa; for non-structural LWC and lightweight mortars, PD < 2.0 g/cm3, WA = 0–34%, CS > 1.8 MPa; for geotechnical applications, PD < 2.0 g/cm3, WA = 10–38%, CS > 1.8 MPa; for gardening and acoustic and thermal insulation, PD < 2.0 g/cm3, WA = 10–38%, CS > 1.0 MPa.
The properties of the sintered aggregates prepared in this study could be categorized into four groups (Table 3). The aggregates prepared with a 5–15% OS content and sintered at 1000–1100 °C (Group A-1) had similar particle density (1.66–1.76 g/cm3) and water absorption (15.6–18.8%) values, with averages of 1.70 g/cm3 and 17.3%, respectively. Group A-1 had a relatively low crushing strength in the range of 1.0–5.8 MPa (mean: 2.7 MPa). The aggregates in this group could be cautiously applied in non-structural LWC, even though some of the crushing strength values matched the criteria of structural LWC. For the aggregates prepared with 20% OS and sintered at 1000–1100 °C (Group A-2), the water absorption increased to 21.0–22.0%, and the crushing strength dropped to 1.0–2.2 MPa. The low crushing strength made the aggregates in this group only suitable for acoustic and thermal insulation or gardening and geotechnical applications.
For the aggregates prepared with a 5–15% OS content and sintered at 1125 °C (Group B-1), the particle density and water absorption were in the range of 1.73–1.83 g/cm3 (mean: 1.77 g/cm3) and 12.2–15.1% (mean: 13.5%), respectively. The crushing strength of this group (7.2–10.4 MPa) was higher than that of Group A, making it possibly suitable for structural lightweight concrete. However, when the OS content reached 20% (Group B-2), the aggregates’ crushing strength dropped to 3.8 MPa, reducing their engineering applicability.
When the sintering temperature increased to 1150 °C (Group C-1), the water absorption of the aggregates (with OS contents of 5–10%) decreased sharply to a low level (3.5–8.0%). Meanwhile, the particle density was higher than or close to 2.0 g/cm3, indicating that the aggregates in Group C-1 were no longer in the LWA category, even though they had an extremely high crushing strength of up to 40.1 MPa. In contrast, the aggregates with a 20% OS content sintered at 1150 °C (Group C-2) had appropriate particle density (1.66 g/cm3) and water absorption (14.3%) values and a relatively high crushing strength (16.5 MPa). The sintered aggregates prepared in this study were suitable for lightweight concrete and were comparable to some commercial LWAs [23,43,44]. Group C-2 may be suitable for high-strength lightweight concrete based on the criteria suggested by Souza et al. [42]. In addition, compared to the Ts required for the sediment-based LWAs without additives (1200–1300 °C) and those with steel slag (1175 °C) [15], the Ts required in this study was lower. This indicated that waste oyster shells could be a good additive for LWA manufacture from dredged sediments.

3.3. Leachability of Toxic Metals and Chloride Ions

Besides the engineering properties, the leachability of toxic metals and chloride ions from LWAs should be kept at a low level to comply with official regulations and ensure safety in practical applications. As shown in Table 4, the water-soluble chloride contents of the LWAs did not exceed 25 mg/kg and were 124–228 times lower than those of the raw HS (5700 mg/kg) and OS (3100 mg/kg). This indicated that the sintering process could volatilize the chloride in the raw materials. A higher sintering temperature was more efficient in reducing the water-soluble chloride contents of the LWAs [19,45]. The LWAs prepared in this study aligned with the regulations for concrete construction in Taiwan (Cl- < 120 mg/kg), indicating their suitability for use as concrete aggregates in practical applications.
Moreover, the TCLP concentrations of leachable metals in the LWAs were in the range of <0.01, <0.01, <0.01, <0.01–0.07, <0.01–0.11, <0.01, 0.02–0.03, <0.01, <0.01, and <0.01–0.23 mg/L for Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, Se, and Zn, respectively, which complied with the regulations for building materials in Taiwan (Table 5). The sintering process led to the re-crystallizing of the silicate minerals in the sediments and the incorporation of metals into new crystal phases, resulting in low metal leachability [46,47]. González-Corrochano et al. [46] reported that the sintering process was effective in the immobilization of metals such as Cr, Ni, Cu, Zn, and Pb because these elements could become part of the structure of neo-formed spinel minerals or refractory minerals in the matrix of the LWA. Xu et al. [47] reported that high temperatures promoted the formation of Cd2SiO4, PbCrO4, and PbSi2O3, enhancing the solidification of metals in the LWA. This indicated that the use of the LWAs prepared in this study for construction applications would be environmentally friendly.

3.4. Economic and Environmental Considerations

The valorization of harbor sediments through lightweight aggregate (LWA) production could make important contributions to the economy and environment. In terms of the circular economy, the utilization of dredged harbor sediment, which was originally waste, to form lightweight aggregates could not only reduce the cost of ocean disposal, but also increase the additional income of port operations. In terms of environmental science, the utilization of dredged harbor sediments could reduce the possible adverse impacts of ocean disposal on marine ecosystems, and the reuse of oyster shells could also reduce the environmental sanitation issues caused by piling up oyster shells.
Due to Taiwan having no natural LTWA production, almost all LWA demand is imported from foreign countries. The valorization of dredged harbor sediments by forming them into LWAs could promote economy in civil engineering, even though the cost of the ocean disposal of dredged sediments (~15 USD/ton) is currently 3–5-fold cheaper than that of sintering treatment. Considering the adverse environmental impacts and the yearly increasing costs of ocean disposal, the valorization of dredged harbor sediments and waste oyster shells through LWA production will become increasingly viable.

4. Conclusions

The results achieved by this study are summarized as follows:
(1)
Dredged harbor sediments admixed with oyster shells could feasibly be sintered into lightweight aggregates (LWAs) with suitable engineering properties for construction applications.
(2)
The recommended optimal oyster shell content and sintering temperature are 5–15% and 1125 °C, respectively.
(3)
The oyster shell content and sintering temperature firmly controlled the LWA’s engineering properties (particle density, water absorption, and crushing strength). Briefly, the addition of oyster shells was beneficial in reducing the particle density but was conducive to water absorption, and higher sintering temperatures could keep the water absorption low and increase the crushing strength.
(4)
The sintering process could effectively mitigate the leachability of chloride ions and metals in the sediment-based LWAs, making the LWAs safe for building material applications.

Author Contributions

Conceptualization, C.-W.C. and C.-D.D.; methodology, Y.C.L.; validation, Y.C.L. and C.-F.C.; formal analysis, Y.C.L.; investigation, Y.C.L.; resources, C.-W.C.; data curation, C.-F.C.; writing—original draft preparation, Y.C.L.; writing—review and editing, C.-F.C.; supervision, C.-D.D.; project administration, C.-W.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Ministry of Science and Technology, Taiwan (MOST-107-2622-E-992-015-CC2).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgments

The authors thank CHC Resources Corporation for providing TG/DT analysis and crushing strength measurement instruments. We also express our deep appreciation for Tsing-Hai Wang (Yuan Ze University, Taiwan) for assisting with the XRF analysis. Additionally, we thank Gautam Divyashakti Sureshchandra (NKUST) for his diligent proofreading of this manuscript, and Zhen-Han Chen for his work in the lab.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Harbor sediment and oyster shell samples collection area.
Figure 1. Harbor sediment and oyster shell samples collection area.
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Figure 2. Ternary diagram of the Riley predictive scheme. The circles with different shades of gray represent the chemical composition of the original harbor sediment and the sediments with 5, 10, 15, and 20% oyster shells.
Figure 2. Ternary diagram of the Riley predictive scheme. The circles with different shades of gray represent the chemical composition of the original harbor sediment and the sediments with 5, 10, 15, and 20% oyster shells.
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Figure 3. Particle size distribution of the ground harbor sediment (a) and oyster shells (b). The shaded area represents the best particle size distribution range for LWA production recommended by Cougy [33].
Figure 3. Particle size distribution of the ground harbor sediment (a) and oyster shells (b). The shaded area represents the best particle size distribution range for LWA production recommended by Cougy [33].
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Figure 4. The TGA-DTA-DTG curves of (a) harbor sediment and (b) oyster shells, with a heating rate of 15 °C/min under a N2 atmosphere.
Figure 4. The TGA-DTA-DTG curves of (a) harbor sediment and (b) oyster shells, with a heating rate of 15 °C/min under a N2 atmosphere.
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Figure 5. Sintered aggregates prepared with different oyster shell contents (0, 5, 10, 15, and 20%) and sintering temperatures (1000, 1050, 1100, and 1150 °C) with a dwelling time of 15 min.
Figure 5. Sintered aggregates prepared with different oyster shell contents (0, 5, 10, 15, and 20%) and sintering temperatures (1000, 1050, 1100, and 1150 °C) with a dwelling time of 15 min.
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Figure 6. The properties of the sintered aggregates prepared with different sintering temperatures (1000, 1050, 1100, 1125, and 1150 °C) and oyster shell contents (0, 5, 10, 15, and 20%) with a dwelling time of 15 min: (a) particle density (PD); (b) water absorption (WA); (c) crushing strength (CS); (d) weight loss (WL); (e) bloating index (BI); and (f) open porosity (Po).
Figure 6. The properties of the sintered aggregates prepared with different sintering temperatures (1000, 1050, 1100, 1125, and 1150 °C) and oyster shell contents (0, 5, 10, 15, and 20%) with a dwelling time of 15 min: (a) particle density (PD); (b) water absorption (WA); (c) crushing strength (CS); (d) weight loss (WL); (e) bloating index (BI); and (f) open porosity (Po).
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Figure 7. The SEM images of the sintered aggregates with oyster shell contents of (a) 10% and (b) 20% under a sintering temperature of 1150 °C and dwelling time of 15 min.
Figure 7. The SEM images of the sintered aggregates with oyster shell contents of (a) 10% and (b) 20% under a sintering temperature of 1150 °C and dwelling time of 15 min.
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Table
Table 1. Chemical composition of the raw materials.
Table 1. Chemical composition of the raw materials.
SiO2Al2O3Fe2O3K2OCaONa2OMgOLOI
Harbor sediment62.016.87.24.02.52.32.23.9
Oyster shell2.10.80.40.292.60.90.843.5
Table
Table 2. Spearman’s correlation matrix for the properties of the sintered aggregates and sintering conditions.
Table 2. Spearman’s correlation matrix for the properties of the sintered aggregates and sintering conditions.
TsOSPDWACSWLBI
PD0.41 *−0.70 **
WA−0.67 **0.58 **−0.89 **
CS0.91 **−0.210.68 **−0.84 **
WL0.110.98 **−0.66 **0.52 **−0.12
BI−0.48 *0.60 **−0.78 **0.80 **−0.63 **0.53 **
Po−0.71 **0.52 **−0.84 **0.98 **−0.85 **0.45 **0.79 **
Ts: sintering temperature; OS: oyster shell content; PD: particle density; WA: water absorption; CS: crushing strength; WL: weight loss; BI: bloating index; Po: open porosity; ** significant at p < 0.01; * significant at 0.01 < p < 0.05; n = 25.
Table
Table 3. The properties of sintered aggregates prepared in this study.
Table 3. The properties of sintered aggregates prepared in this study.
GroupTs
(°C)		OS
(%)		PD
(g/cm3)		WA
(%)		CS
(MPa)		Application
A-11000–11005–151.66–1.7615.6–18.81.0–5.8Non-structural LWC
A-21000–1100201.57–1.6121.0–22.01.0–2.2Gardening and geotechnical uses
B-111255–151.73–1.8312.2–15.17.2–10.4Structural LWC
B-21125201.5820.03.8Acoustic and thermal insulation
C-111505–151.87–2.053.5–8.017.5–40.1Do not belong to LWA category
C-21150201.6614.316.5High-strength LWC
Table
Table 4. Water-soluble chloride contents (mg/kg) in the raw materials and the sintered aggregates.
Table 4. Water-soluble chloride contents (mg/kg) in the raw materials and the sintered aggregates.
Raw MaterialsSintered AggregatesLimits 2
HS 1OS 10% OS10% OS20% OS
Cl−5700310016, 4, 1 324, 7, 325, 12, 6120
1 HS: harbor sediments; OS: oyster shells; 2 regulations for concrete construction in Taiwan; 3 the data are presented in the order of the sintering temperature: 1000, 1100, and 1150 °C, respectively.
Table
Table 5. The TCLP leachable metal concentrations (mg/L) in the raw materials and the sintered aggregates.
Table 5. The TCLP leachable metal concentrations (mg/L) in the raw materials and the sintered aggregates.
Harbor SedimentSintered Aggregates with Ts 1 of 1100 °CLimits 2
0% OS 110% OS20% OS
Agnd 3ndndnd5.0
Asndndndnd5.0
Cd0.03ndndnd1.0
Cr0.05nd0.020.075.0
Cu0.710.110.09nd15.0
Hgndndndnd0.2
Ni0.280.030.020.02-
Pb0.05ndndnd5.0
Sendndndnd1.0
Zn6.750.230.08nd-
1 OS: oyster shells; Ts: sintering temperature; 2 regulations for building materials in Taiwan; -: not available; 3 nd: <0.01 mg/L.
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Lim, Y.C.; Chen, C.-F.; Chen, C.-W.; Dong, C.-D. Valorization of Dredged Harbor Sediments through Lightweight Aggregate Production: Application of Waste Oyster Shells. Sustainability 2023, 15, 5466. https://doi.org/10.3390/su15065466

AMA Style

Lim YC, Chen C-F, Chen C-W, Dong C-D. Valorization of Dredged Harbor Sediments through Lightweight Aggregate Production: Application of Waste Oyster Shells. Sustainability. 2023; 15(6):5466. https://doi.org/10.3390/su15065466

Chicago/Turabian Style

Lim, Yee Cheng, Chih-Feng Chen, Chiu-Wen Chen, and Cheng-Di Dong. 2023. "Valorization of Dredged Harbor Sediments through Lightweight Aggregate Production: Application of Waste Oyster Shells" Sustainability 15, no. 6: 5466. https://doi.org/10.3390/su15065466

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