The chapter delves into the utilization of recycled mixed plastic fine aggregate (rMPFA) as a sustainable replacement for natural sand in cement concrete. It discusses the challenges and advantages of using mixed plastic waste, highlighting the potential to enhance thermal conductivity and reduce carbon footprint. The study examines the effects of rMPFA on compressive strength, water penetration, and environmental properties, offering a detailed analysis of the results. Notably, it emphasizes the viability of rMPFA in achieving comparable mechanical and durability properties to recycled single-type plastic aggregates, thus promoting a more efficient and practical waste recycling strategy.
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Abstract
The literature extensively examines the utilization of sorted single-type plastic waste from post-consumer waste streams as a sustainable substitute for natural sand in cement concrete. However, severe heterogeneity of plastic waste in municipal solid waste streams, including variations in polymer types, grades, shapes, sizes, and cross-contamination with other commingled waste materials, poses a significant challenge in adopting findings from prior research that necessitates high-purity single-type plastic waste for concrete applications. This paper reports the characterization of cement concrete incorporated with mixed plastic fine aggregate (rMPFA) containing an optimized blend of plastic types produced using a proprietary mixed plastic recycling process. Five concrete mixtures containing 0% (M0), 10% (M10), 20% (M20), 30% (M30), and 40% (M40) rMPFA by volume of natural sand were investigated in this study. The laboratory results show that concrete mixture M20 had comparable compressive strength and water penetration test results when compared to control mixture M0. Additionally, toxicity characterization of concrete mixture M20 demonstrated a reduction of heavy metals in the leachate solution when compared to control mixture M0. Furthermore, microplastic detection analysis results of concrete mixtures M0 and M20 were comparable and stable.
1 Introduction
Plastic waste constitutes a significant portion of municipal solid waste (MSW), making up approximately 10% to 15% of the total amount, with less than 10% currently being recycled [1]. Domestic and industrial entities generate plastic waste, primarily in the form of food packaging, trash bags, and plastic bottles. In Singapore, the annual plastic waste generation has risen sharply over the past two decades, from 546,537 tonnes in 2001 to 982,000 tonnes in 2021 [2]. Improper disposal of non-biodegradable plastic waste has become a pressing societal and environmental issue. Thus, converting plastic waste into valuable products can contribute to establishing a sustainable circular economy and achieving carbon neutrality.
One promising strategy for addressing this issue is recycling plastic waste as value-added materials for building and infrastructure, such as an alternative to natural sand. However, plastic waste in MSW streams is highly heterogeneous in terms of polymer types, grades, dimensions, and other commingled materials, including metal, paper, and cardboard. The non-homogeneity and contamination of mixed plastics are significant obstacles to recycling them as feedstock for building materials. Hence, past studies reported on the use of sorted single-type plastic waste such as, polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polycarbonate (PC), acrylonitrile butadiene styrene (ABS) in cement concrete as natural sand replacement [3] is cost-ineffective and difficult for implementation in practice.
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In response to these challenges, Khalil et al. [4] investigated the use of irregular mixed plastic waste as coarse aggregate in concrete materials, ranging from 15 to 45 vol%. Their study revealed that samples containing 45 vol% mixed plastic coarse aggregate can be classified as structural lightweight concrete, along with significant improvement in thermal conductivity. Similarly, Lee et al. [5] explored the use of 10, 15 and 20 vol% mixed plastic fine aggregates, made up of an optimized blend containing different plastic types, as a substitute for natural sand in cement concrete. They concluded that the effects of mixed plastic aggregate on the mechanical and durability properties are comparable to those of recycled plastic fine aggregate made of sorted single-type plastic waste. This suggests that with proper recycling techniques, the need for sorting mixed plastic waste to achieve high-purity homogeneous plastic recycling streams for use in cement concrete can be minimized, thus increasing the recovery efficiency of mixed plastic waste. Additionally, Lee et al. [6] studied the use of mixed plastic fiber as reinforcement, with similar polymer composition to plastic waste in MSW streams for fiber-reinforced concrete.
Presently, existing studies on the use of heterogeneous waste materials in building and infrastructural applications have largely been limited to the fresh, durability and mechanical properties. In addition to technical performance, the objective of this study is also to examine the effects of recycled mixed plastic fine aggregate (rMPFA) on the environmental properties of cement concrete designed for general concreting application at various natural sand (NS) replacement levels.
2 Materials and Methods
2.1 Materials
In this study, concrete mixtures were prepared with constituent materials conforming to SS EN 206 [7]. The raw ingredients include ordinary Portland cement (OPC, CEM Type I 42.5 N, specific gravity (SG) 3.15), NS (4 mm nominal size, SG 2.60) as fine aggregate, rMPFA made of an optimized concoction of multiple plastic types manufactured via a proprietary mixed plastic recycling facility (4 mm nominal size, SG 0.95) as fine aggregate replacement, natural gravel (NG, 20 mm nominal size, SG 2.65) as coarse aggregate, tap water and commercial chemical admixtures, i.e., retarder and water reducing superplasticizer.
Five concrete mixtures containing 0% (M0), 10% (M10), 20% (M20), 30% (M30), and 40% (M40) rMPFA by volume of NS content were designed in this study. The mix design of the concrete mixtures is shown in Table 1. The water/binder ratio, aggregate/binder ratio and dosages of chemical admixtures were kept constant to capture the effects of rMPFA content on the properties of all concrete mixtures investigated.
Table 1.
Mix design of concrete mixtures.
Mix No
OPC
Water
NG
NS
rMPFA
M0
1
0.45
2.5
1
0
M10
1
0.45
2.5
0.9
0.1
M20
1
0.45
2.5
0.8
0.2
M30
1
0.45
2.5
0.7
0.3
M40
1
0.45
2.5
0.6
0.4
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2.2 Sample Preparation
The dry ingredients namely, OPC, NS, NG and rMPFA were first mixed in a rotary mixer for 3 min. Tap water dosed with chemical admixtures was then added to the dry mixture and blended for an additional 3 min. The freshly mixed concrete mixtures were transferred and compacted into 150 mm cube formworks. After 24 h, the concrete samples were removed from the formworks and stored in a sheltered open space to air cure till its designated test dates.
2.3 Laboratory Tests
The compressive strength of the concrete samples was evaluated in accordance with BS EN 12390-3 [8] using 150 mm cube samples after 1, 7 and 28 days of curing. To evaluate water penetration, 150 mm cubic concrete specimens were tested according to the procedures outlined in BS EN 12390-8:2019 [9]. After 28 days of curing, one surface of each specimen was subjected to 500 kPa water pressure for 72 ± 2 h. The specimens were then split into two to measure the depth of water penetration.
The leachate solutions of concrete mixes were prepared using the standard batch leaching method for granular waste materials, as specified in BS EN 12457-1 [10]. The amount of heavy metal content in the solutions was determined in accordance with the methods outlined by the American Public Health Association [11, 12]. Microplastic detection analysis was performed by filtering the leachate solutions obtained from the standard batch leaching procedure through a 0.45 μm cellulose nitrate membrane. The residual particles on the filter membrane were collected with a surgical blade and then transferred to a diamond cell for analysis using microscopic Fourier-Transform Infrared Spectroscopy (FTIR). The collected particles were matched against a library for identification.
3 Results and Discussion
The compressive strength tests of different concrete mixtures were conducted at various curing ages, and the results are illustrated in Fig. 1. The findings indicated that the compressive strength trends of the five concrete mixtures remained relatively consistent over time. Concrete mixtures M0 and M40 demonstrated the highest and the lowest compressive strengths, respectively. This observation is consistent with past literatures reported on the effects of increasing recycled plastic content over the mechanical properties of concrete mixtures [5]. Additionally, the results showed that the compressive strength of concrete mixture M20, which contained 20 vol% rMPFA, decreased only slightly in comparison to control mixture M0.
Fig. 1.
Compressive strength of designed concrete mixtures.
Fig. 2.
Water penetration depth of designed concrete mixtures.
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The water penetration tests of the designed concrete mixtures were also conducted after 28 days of curing, and the results are displayed in Fig. 2. It is evident that concrete mixtures containing rMPFA demonstrated a rise in water penetration depth in contrast to the control mixture M0. This increase in water penetration depth could be linked to the poor adhesion bonds between rMPFA and the cementitious matrix, leading to an increase in porosity which allows for easier medium transport within the microstructure of the concrete mixtures.
From the preliminary investigation, concrete mixture M20 was selected for further examination of its environmental properties, compared to control mixture M0. The limit of reporting (LOR) and leaching test results are presented in Table 2. The computation of leachate concentration for various parameters is expressed as milligram equivalent mass of a parameter per kilogram of solid sample, with test results below the LOR indicated as <LOR value.
Most of the test parameters for concrete mixtures M0 and M20 showed similar concentrations below the LOR values, with the exception of Ba, F, Pb, Hg, and phenolic compounds. Concrete mixture M20 demonstrated lower concentrations of Ba, Pb, Hg, and phenolic compounds than control mixture M0. Further investigation revealed that the slight increase in F content was due to the presence of fluoride in natural aggregates rather than the addition of rMPFA.
Table 2.
Leaching test results (mg parameter/kg solid sample).
Parameters
LOR
M0
M20
Arsenic (As)
0.01
< 0.01
< 0.01
Barium (Ba)
1.00
3.53
3.23
Cadmium (Cd)
0.001
< 0.001
< 0.001
Chromium (Cr)
0.01
< 0.01
< 0.01
Copper (Cu)
0.01
< 0.01
< 0.01
Fluoride (F)
1.00
2.90
2.94
Lead (Pb)
0.01
0.02
0.01
Manganese (Mn)
0.10
< 0.01
< 0.01
Mercury (Hg)
0.001
0.002
< 0.001
Nickel (Ni)
0.01
< 0.01
< 0.01
Phenol
1.00
0.46
0.31
Selenium (Se)
0.01
< 0.01
< 0.01
Silver (Ag)
0.01
< 0.01
< 0.01
Zinc (Zn)
0.10
< 0.1
< 0.1
Figure 3 displays the FTIR spectra of residual particles found in the leachate solutions of the designed concrete mixtures. Through visual examination and matching with a FTIR library, a significant portion of the particles was identified as ground calcium carbonate, which is consistent with the hydration products formed within the cementitious matrix. Only a small amount of polymeric particles, such as kaolin, cellulose, PP and PET, were detected on the filter membrane. These particles are likely to have been introduced from external sources, such as materials storage and handling equipment, during the sample preparation process involving material mixing and transfer.
Fig. 3.
FTIR spectra of residual particles after filtration of leachate solutions derived from concrete mixtures.
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4 Conclusions
This paper presents a study on the technical performances and environmental properties of cement concrete incorporated with rMPFA as partial NS replacement, produced from a proprietary mixed plastic recycling process. Five different concrete mixtures containing rMPFA content ranged from 0 to 40 vol% of NS were characterized and the key findings are:
A higher rMPFA content in concrete mixtures led to lower compressive strength across all curing ages, well-aligned with findings reported in the literature.
Concrete mixtures containing rMPFA exhibited a slight increase in water penetration depth when benchmarked with the control mixture M0, possibly due to the increase in porosity caused by reduced adhesion interfacial bonds between rMPFA and the cementitious matrix.
Upon further analysis, concrete mixture M20 revealed improvements in leachate content with lower or comparable leachate concentration as control mixture M0. Despite an increase in porosity that eased transport within the microstructure of concrete mixtures, the addition of rMPFA had little influence on the mobility of the heavy metals.
The microplastic detection test results showed that the concrete mixture M20 with 20 vol% rMPFA exhibited little or no formation of microplastic, comparable to the control mixture M0.
This study primarily focused on the laboratory characterization of concrete mixtures containing rMPFA. Field investigation on concrete mixtures containing rMPFA should be carried out to characterize the short- and long-term durability performances, mechanical properties, and environmental behaviors under natural weathering conditions in actual operative environments. Further studies could also be carried out to produce recycle mixed plastic coarse aggregate of suitable particle size to substitute NG in concrete mixtures to further increase the recycled mixed plastic content. Finally, life cycle assessment could be performed to quantify the effects of recycled mixed plastic ingredients on the embodied carbon of concrete mixtures, which can provide valuable information for future development of greener concrete mixtures.
Acknowledgements
This research is supported by the National Research Foundation, Singapore, and National Environment Agency, Singapore, under its Closing the Waste Loop Funding Initiative (Award No. USS-IF-2019-2). The authors are grateful to Mr. Lim Yin Yen and the technical staff from the Centre for Urban Sustainability at the School of Applied Science, Temasek Polytechnic. The authors would also like to express their gratitude to Mr. Lim Guang Jie Jonathan, Dr. Wang Su, and the technical staff from Pan-United Corporation Pte. Ltd. for their tremendous support.
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