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The study aims to explore the impact of substituting natural aggregates with up to 100 vol% of two types of PVC—soft PVC from electric wire coverings and hard PVC from water pipes—on the fresh properties, mechanical strength, physical performance and environmental impact of the limestone-calcined clay cement (LC3) system. Incorporating both hard and soft PVC reduced the flowability significantly. In addition, flexural and compressive strength of LC3 mortars declined by around 52% and 48% for H-PVC, and by around 64% and 87% for S-PVC mixes, respectively. However, completely substituting natural aggregates with PVC enhanced thermal insulation by up to 60% compared to control samples. The thermal conductivity of LC3 containing 100 S-PVC achieves the requirements of RILEM to be considered as an insulating lightweight cementitious material. PVC incorporation exhibited mixed effects on sound insulation. On the other hand, the total absorbed energy increased significantly by 290% and 305% for 100% hard (HPVC) and 100% soft (SPVC) samples, respectively, compared to the control sample. Conversely, the peak impact force slightly decreased for LC3 with HPVC, while LC3 with 100% SPVC showed a reduction of about 53%. The use of PVC aggregates remarkably diminished the carbon footprint of the selected mix designs, suggesting that recycling PVC into aggregate for cementitious materials offers a sustainable solution for both load-bearing (HPVC) and non-load-bearing (SPVC) applications.

About the project

Industrial carbon emissions significantly contribute to global warming, with ordinary Portland cement (OPC) production alone accounting for up to 8% of carbon dioxide emissions (Al-noaimat et al., 2023a; Al-noaimat and Akis, 2023). Different approaches were considered to decrease the impact of OPC on the environment, including partially replacing ordinary cement with supplementary cementitious materials (SCMs). Nevertheless, common SCMs, such as fly ash, GGBS, and silica fume, share the problem of limited availability in some regions of the world (Al-noaimat and Akis, 2023). A suitable alternative that has received the research spotlight lately because of its wide availability and high reactivity after activation is calcined clay. Due to their suitable chemical composition, calcined clay gains pozzolanic reactivity after calcination at elevated temperatures (600–900 C) (Al-noaimat and Akis, 2023; Fernandez et al., 2011). Zhang et al. (Zheng et al., 2022) investigated the feasibility of using low-grade calcined clay (containing 14.6% kaolinite) to replace up to 40% OPC to develop blended cement and found that the optimal replacement level was in the range of 10–20%.

One of the latest developments in construction materials is the limestone calcined clay cement (LC3) system as an approach to mitigate the CO2 emissions from OPC production since it allows the high replacement of OPC of up to 50% in the binder (Al-noaimat et al., 2023b; Chen et al., 2019a). At the same time, this system maintains a strength performance comparable to OPC while having better durability (Sharma et al., 2021). Most studies focused on investigating the properties of the LC3 system using medium/high-grade kaolinite clay as a calcined clay product. However, due to its relatively high price and limited access globally, researchers were focusing on utilising locally available clay instead. Although clays containing high kaolinite content are highly amorphous materials after calcination, locally available clays could exhibit pozzolanic reactivity after calcination depending on their mineralogical composition and react with available cement and portlandite (Zhou et al., 2017).

Polyvinyl chloride (PVC) is a thermoplastic polymer widely utilised in local and industrial applications, serving various purposes (Mohammed et al., 2019). Unlike other polymeric materials, PVC presents challenges in terms of recycling and, if mishandled, may release free chloride ions, posing a risk of environmental contamination (El-seidy et al., 2023a). The demand for plastic increased significantly due to its durability and affordable price, resulting in higher plastic waste generation worldwide (Subramanian, 2000). It was reported that the annual production of plastics exceeded 330 Mt from 2016 to 2020 (Lu et al., 2023), in which PVC accounts for around 12% of the total amount of plastics produced. In 2016, PVC production exceeded 45 Mt globally (El-seidy et al., 2023b). As the production of PVC increases, the amount of PVC waste generated increases. The worldwide market of PVC is expected to be more than 60 million tonnes by 2025 (Biçergil and At, 2023). This massive production rate, which is expected to grow further, underlines the importance of proposing suitable ways to reuse and/or recycle these materials rather than their disposal in landfills. Using recycled PVC as aggregates in mortar and concrete mixes offers a promising solution that can effectively reduce environmental waste and benefit the concrete industry. That is especially important because PVC is a common material found in landfills and takes a long time to biodegrade (Latroch et al., 2018). Previous studies have revealed the feasibility of using PVC as aggregates to develop lightweight cement and geopolymer concrete (El-seidy et al., 2023a; El-seidy et al., 2023b; Latroch et al., 2018; Kou et al., 2009; Gesoglu et al., 2017; Senhadji et al., 2015). Incorporating polymer aggregate to replace natural aggregates has been proven to negatively impact the mechanical performance of the cementitious mixture. However, the extent of this impact depends on the content and nature of the polymer used (Da Silva et al., 2014; Mouzoun et al., 2023). For instance, Kou et al. (2009) reported that replacing up to 45 vol% of natural aggregates with PVC in concrete improved chloride ion resistance and reduced drying shrinkage of concrete. El-Seidy et al. (El-seidy et al., 2023b) investigated the effect of replacing up to 100% of natural aggregates by volume with fine and coarse PVC separately on the different properties of alkali-activated materials (AAM). The authors found that although incorporating PVC in fine or coarse aggregate forms impairs the strength and durability performance of AAM, PVC aggregates can be used to reduce thermal conductivity, improving the thermal insulation of concrete. In addition, they found that replacing natural aggregates with PVC reduces CO₂ emission by around 37.7 kgCO₂-eq/tonne when using coarse PVC and around 483.5 kgCO₂-eq/tonne when replacing natural aggregates with fine PVC. In addition, recycling PVC in cementitious mixtures can prevent chloride from leaching into the environment (El-seidy et al., 2023b). Moreover, incorporating PVC was reported to be beneficial to improve the abrasion resistivity of concrete. In addition, it was reported the crack pattern changes in the presence of PVC, where the crack numbers, spacing and distribution are reduced in the presence of PVC (Mohammed et al., 2019). On the other hand, Mouzoun et al. (2023) found that substituting up to 15% sand with PVC wouldn't impact the strength performance, whereas higher replacement levels would decline the compression strength behaviour of the concrete. Boutlikht et al. (2024) found that incorporating low percentages, up to 15%, of recycled tube PVC on the properties of concrete and reported an enhancement in the compression strength of concrete, indicating the major role the substitution dosage plays in influencing the properties of concrete. Despite the various benefits PVC offers when incorporated in cementitious mixtures, many outstanding questions remain about how PVC would influence its thermal and acoustic performance when used in different cementitious systems. The limited research suggests that PVC's properties may vary significantly depending on factors like its particle size distribution, and interaction with the cement matrix, which leave gaps in understanding their suitability for specific applications as insulators or noise reduction elements in construction.

Recycling PVC as aggregates in mortars and concrete presents a suitable approach to protect the environment from pollution due to the presence of chlorine in its chemical composition. Although, many studies have investigated the effect of recycling PVC to replace natural sand in different cementitious mixtures, this study investigates the compatibility of recycling PVC in LC3 mixtures. This study mainly focuses on investigating the suitability and performance of PVC incorporation in LC3 systems made up of excavated London clay waste as calcined clay to develop a low-carbon mixture. To the best of the authors’ knowledge, no research has studied the feasibility of developing a low-carbon LC3 mixture that combines upcycled material as a binder and recycled aggregates in the same mix design. This study examines the effect of replacing 50 and 100% of natural aggregates with soft and hard PVC on the fresh properties, mechanical strength, physical properties and carbon footprint of LC3 mixtures. This study provides more insights into the feasibility of PVC in low-carbon cementitious mixtures as a natural aggregate alternative from an engineering and environmental perspective.