**1. Introduction**

Cement manufacturing can be identified as a major source of CO2 emissions, from production to emissions, making it the largest source of industrial emissions [1]. The cement industry is the second-largest source of CO2 emissions, accounting for 27% of CO2 emissions from the industrial sector and 8% of global CO2 emissions [2,3]. It is estimated that by 2050, cement production will increase by approximately 12–22% compared to 2014. Therefore, one way to reduce CO2 emissions is to replace cement with materials that use waste materials or industrial by-products to reduce CO2 emissions [4].

Blast-furnace slag is an industrial by-product of iron extraction. Depending on the cooling method, blast-furnace slag can be divided into air-cooled blast-furnace slag and ground-granulated blast-furnace slag (GGBFS), which is then dried and ground into a powder of comparable fineness to replace cement as a cementitious material [5]. While the chemical composition of GGBFS is very similar to that of Portland cement, its composition consists of varying proportions of lime and alumina. The cementitious properties of GGBFS are controlled by the type of ore, the type of flux used, and the contaminants in the charged coke. Magnesium, silicon, calcium, aluminum, and oxygen account for 95% of the total GGBFS content [6]. Therefore, proper use of ground-granulated blast-furnace slag (GGBFS) to replace cement can not only reduce cement use but also reduce slag emissions, and the properties of ground-granulated blast-furnace slag can also be used to improve its engineering properties.

In the 1950s, plastic or synthetic organic polymers were mass-produced and used. Although the rapid growth in the production of plastic man-made materials still does not surpass that of steel and cement materials widely used in civil construction, the impact

**Citation:** Lin, K.-T.; Hung, C.-C.; Wang, H.-Y.; Wen, F.-L. Polyethylene (PE) Waste Minimization Study of Cement Mortar with Adding PE Content under Different W/B Ratios. *Buildings* **2022**, *12*, 2117. https:// doi.org/10.3390/buildings12122117

Academic Editors: Huazhe Jiao and Abdelhafid Khelidj

Received: 16 October 2022 Accepted: 30 November 2022 Published: 2 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of plastic waste on the environment and how to eliminate it is still an important issue that cannot be ignored [7]. The largest market for plastics is packaging, where the use of single-use containers has grown so rapidly that the proportion of solid waste generated from the use of these containers has increased from less than 1% in 1960 for middle- and high-income countries to more than 10% in 2005 [8].

The vast majority of monomers used to make plastics, such as ethylene and propylene, are derived from fossil hydrocarbons. None of the common plastics are biodegradable [9]. The only way to permanently eliminate plastic waste is through destructive heat treatment, such as combustion or pyrolysis, which often causes secondary pollution of the environment due to the subsequent emission of CO2. If such waste is buried in a landfill, it will only accumulate in the natural environment and will not decompose [9,10]. Therefore, the near-permanent contamination of the natural environment by plastic waste is a growing problem [11].

Disposable or single-use paper containers are often coated with a plastic film, such as polyethylene (PE), on the inner surface to meet waterproof or oil-proof requirements. Therefore, most single-use paper containers are regarded as plastic waste and not waste paper. Recycling is an effective way to reuse or regenerate waste into useful products, materials, or components, especially with regard to recycling waste made from composite materials such as lunch boxes and beverage cups [12–14]. With the continuous increase in plastic waste, the inclusion of recycled plastic waste in building materials, such as bricks and concrete, has been studied by researchers [15,16]. In addition, after recycled plastic waste is combined with wood and other plant fibers, it can also be used in plastic–wood building materials [17].

The cement in concrete consumes 2% of global energy in the production process, and every ton of cement produced will emit 0.85 tons of carbon dioxide [18,19]. Therefore, many researchers continue to investigate sustainable, environmentally friendly, and costeffective alternatives. Recycled plastic waste can be transformed into a suitable shape or size and added to the concrete as an aggregate [20], which can reduce structural weight, increase design flexibility, reduce total construction costs, reduce the structural gravity load, and improve seismic performance. Due to its advantages, such as structural reaction under action and enhanced structural thermal insulation, it is one of the alternatives to decontaminate waste [16,21].

Plastic waste is incorporated into mortars and cements as aggregates, and most studies have focused on the microplastic and neoplastic range [11,22,23]. Taiwan has the secondhighest density of convenience stores in the world [24], and most disposable food packaging contains plastic waste composed of polyethylene (PE) components [14], a high-quality material with good chemical stability, resistance to impact, and low temperature resistance. According to the Environmental Protection Agency, in Taipei City alone, food delivery packaging increased by 85% in May 2021 [24], perhaps due to the impact of the COVID-19 outbreak. Usually, recycled food packaging is composed of different polymers and complex materials, which makes the separation of each material difficult [24]. It is necessary to separate the PE components in the packaging container through processes such as buoyancy sorting before recycling for reuse. The focus of this study is to investigate the durability of recycled polyethylene (PE) plastic waste added to cement mortar and the benefits of PE waste reuse under different W/B ratios.

PE polymers are one of many fibers, whether they are polymers or metals, and they are widely used in concrete engineering because of their advantages [25–28]. The compressive strength and toughness of concrete can be improved by adding steel fibers due to the high modulus of elasticity and stiffness [25]. Although adding steel fibers to concrete can improve the properties of concrete, the fiber content must be high. This increases the structural gravity load of concrete and has a balling effect during mixing, thus reducing workability [29,30]. There have been many studies on the use of PE as a substitute for some natural aggregates in cementitious materials. Basha et al. found that the amount of recycled plastic aggregate leads to a decrease in compressive strength, flexural strength, elastic modulus, and adhesive strength but is useful for concrete thermal insulation [23]. By incorporating high-density polyethylene (HDPE) and low-density polyethylene (LDPE) into concrete, Rumsys et al. found that the compressive strength of concrete is decreased [31]. Yoo and Kim found that replacing steel fibers with PE fibers results in a decrease in compressive strength due to uneven dispersion of high aspect ratio PE fibers [32].

Single-use paper containers or tableware coated with polyethylene (PE) plastic films for waterproofing and oil stain prevention have become indispensable in people's lives because of their low cost and convenience. After use, it is sent to a professional waste paper container treatment plant for recycling, and its plastic film can be reused. However, due to inaccurate recycling classification or high cost, it is mixed with waste paper and sent to general nonprofessional waste paper factories for processing. The separated plastic film will be identified as garbage and sent to incinerators for incineration, which increases the processing cost of waste paper factories, secondary air pollution, and energy consumption. In addition, in other studies related to the acquisition of plastic wastes to concrete, such as polyethylene terephthalate (PET), some researchers pointed out that the accumulation of this waste did not contribute to the improvement of concrete compressive strength [33–39]. However, the issue of the behavior of cement mortars containing PE wastes is new, and research in this context is limited. Research in this context should continue to highlight the important hardening and durability properties of concrete containing PE plastic wastes coating disposable containers. Therefore, this study used different W/B ratios and different contents of waste PE to make cement mortar, with the GGBFS content being fixed at 20%, to discuss the durability and the benefits of energy savings and carbon reduction. The preliminary study of cement mortar specimens was used to investigate the feasibility of waste reuse and to suggest the appropriate PE addition ratio for the reference of concrete mixture proportioning design, so as to achieve waste minimization by recycling PE film on the surface of disposable containers.
