Sustainable Structural Lightweight Concrete with Recycled Polyethylene Terephthalate Waste Aggregate

Abstract

Plastic is a widely consumed material with a high decomposition time, occupying significant space in landfills and dumps. Thus, strategies to reuse plastic waste are imperative for environmental benefit. Plastic waste is a promising eco-friendly building material for cement-based composites due to its reduced specific gravity and thermal conductivity. However, this waste reduces the composites’ mechanical strength. This work aims to produce and evaluate lightweight concretes made with only lightweight aggregates and mostly recycled plastic aggregates. Initially, an optimized dosage approach for lightweight concrete is presented. The mixture proportion of the lightweight concrete was based on the performance of mortars with the complete replacement of natural aggregate by recycled polyethylene terephthalate (PET) aggregates. The PET aggregates showed irregular shapes, impairing workability and providing lightweight concretes with around 18% water absorption and 21% void index. However, the concretes presented significantly low-unit weight, approximately 1200 kg/m³. This work presented a structural lightweight concrete (ACI 213-R) using only lightweight aggregates and mostly plastic waste aggregate, with a compressive strength of up to 17.6 MPa, a unit weight of 1282 kg/m³, and an efficiency factor of 12.3 MPa·cm³/g. The study shows that with an optimum dosage, reusing plastic waste in concrete is a viable alternative contributing to environmental sustainability.

1. Introduction

Currently, plastic is an essential resource for the development of society. Compared to other materials such as steel and glass, plastic has advantages such as low production cost, high strength-to-weight ratio, ease of molding, and superior durability [1]. Plastic production has expanded widely in recent years due to its characteristics and potentiality. About 9.2 billion tons of plastic were produced worldwide from 1950 to 2017, representing more than a ton per inhabitant [2]. From 2000 to 2019, plastic production was higher than the sum of all previous years [3]. In 2022, the global demand for this material was 400 million tons (362 million tons of fossil-based plastics) [4]. This number may rise to 600 million tons by 2025 [5], an increase of approximately 70%.

Despite its high production, plastic has a short service life. Forty percent of all plastic becomes waste less than a month after production [2]. On the other hand, only 20% of plastic waste is collected for recycling, and part of this volume is discarded due to contamination or health and safety issues [3]. By 2017, only 6.5% and 9.8% of all plastic waste were recycled and incinerated, respectively, and more than half became waste [2]. The incineration of this waste causes the release of toxic substances into nature [6,7,8,9] with an energy efficiency rate of just 20%.

Consequently, most of this waste is disposed of in landfills, dumps, or oceans. Around 11 million tons of plastic waste enter the oceans annually, which could triple by 2040 if immediate action is not taken [10]. By 2050, 12 billion tons of plastic may end up in landfills or inappropriate dumps [11], occupying significant space [8], with a period of up to 400 years for complete decomposition [3].

Plastic waste is a considerable environmental liability due to its low biodegradability and large-scale production [6]. Recycling this waste reduces the environmental impacts associated with its production and use. In this process, around 65% of the energy contained in plastic waste is recovered [2]. Also, recycling provides the preservation of natural resources [1,12,13], a benefit that meets the global demand for sustainable practices that are less aggressive to the environment.

On the other hand, the construction sector is responsible for about 20 to 50% of natural resource consumption, 50% of total solid waste generation [14], 36% of energy consumption, and 37% of CO₂ emissions worldwide [15]. Also, mining river sand (the conventional fine aggregate for mortar and concrete) leads to various environmental impacts, including water pollution, riverbed geometry alterations, and flora and fauna damage [16]. So, using wastes as aggregates in cement-based composites presents an efficient solution for preserving natural resources [17]. Plastic waste has low thermal conductivity and specific gravity compared to conventional aggregates, making it a promising eco-friendly material. Thus, they can improve thermal performance [6,8,18,19,20] and reduce the weight of cement-based composites [6,7,8,9,11,19,21,22,23,24,25,26,27,28,29,30,31], which contributes to reducing energy and material consumption in buildings. Basha et al. [25] reported a 64% reduction in the thermal conductivity of concretes due to the replacement of natural coarse aggregate with recycled plastic aggregate. Similarly, Sosoi et al. [32] produced concretes with chopped PET fine aggregates, which showed a 27% reduction in thermal conductivity for a 60% replacement of natural fine aggregates. Furthermore, Resende et al. [20] reported an average of 5.0% reduction in energy consumption in Brazilian social housing prototypes by adopting PET-containing mortars for internal and external masonry coatings.

However, the use of recycled plastic aggregates in cement-based composites impairs their performance in the fresh [19,24,33] and hardened state [13,18,28,30,33,34,35]. This is mainly attributed to the hydrophobicity of the plastic aggregates, which provides a weak bond between them and the cement paste in the interfacial transition zone and, consequently, high porosity [6,18,19,33]. Moreover, using steel fibers to mitigate the loss of compressive strength in cement-based composites with plastic aggregates is not effective at room or elevated temperature exposure [13]. Lightweight concrete with the replacement content close to 100% of fine and/or coarse aggregates for recycled plastic aggregates generally do not reach the minimum strength of 17 MPa at 28 days [36] to be classified as structural concretes [6,24,25]. Belmokaddem et al. [6] evaluated concretes with 75% replacement of fine and medium aggregates (3–8 mm) for three different plastic aggregates: polyvinyl chloride (PVC), high-density polyethylene (HDPE), and polypropylene (PP). None of the mixes proposed presented compressive strength superior to 17 MPa at 28 days. Basha [25] produced structural lightweight concretes with 17 MPa compressive strength for the 100% replacement of natural coarse aggregates by recycled plastic aggregates and cement consumption of 370 kg/m³ (w/c = 0.40). However, for a cement consumption of 350 kg/m³ (w/c = 0.45), the concretes presented a compressive strength of 16 MPa.

Probably due to the undesirable effects of plastic aggregates on cement-based composites, the authors did not find additional works aiming to use simultaneously recycled plastic aggregates and another lightweight aggregate for developing a structural concrete composed entirely of fine and coarse lightweight aggregates. Addressing this knowledge gap, this current study evaluates the properties of fresh and hardened lightweight concretes, aiming for an optimal mixture design exclusively with lightweight aggregates and mostly plastic waste aggregates. So, the novelty of this work is related to understanding the feasibility and performance of lightweight concrete and incorporating these unconventional materials.

2. Materials and Methods

This research was divided into three stages: (i) physical and morphological characterization of the materials; (ii) development of lightweight mortars with recycled PET aggregates as a dosage approach for the production of lightweight concrete; and (iii) production and characterization of the fresh and hardened state of lightweight concrete.

2.1. Materials

This research used a Brazilian high early strength Portland cement CP V (ASTM Type III equivalent) with a specific gravity of 3.12 g/cm³ and bulk density of 0.74 g/cm³, respectively, to produce all the cement-based composites evaluated. A glass fiber waste (GFW), with a specific gravity and bulk density of 1.90 and 0.17 g/cm³, respectively, was added to the cement-based composites to provide greater cohesion and mitigate the low adhesion between plastic aggregates and cement paste. This strategy also aims to reduce the cracking trend since lightweight cement-based composites usually present higher drying shrinkage, higher brittleness, and lower flexural and tensile strength [37] than conventional cement-based composites.

Linear Alkyl Benzene Sodium Sulfonate (LAS), a biodegradable air-entraining admixture proposed by Mendes et al. [38], was used in all mortars in order to reduce the specific gravity of the composites. The reference mortar was produced with natural quartzite aggregate, a residue of the quartzite rock extraction process, which usually presents a higher silica content (quartz form) and lower amorphous and organic matter contents than natural river sand aggregate [39]. A lightweight reference mortar was produced with fine expanded clay (C) aggregate.

Two recycled PET fine aggregates supplied by the Polymer Laboratory (LabPol) of the Federal University of Espírito Santo were used: granular PET (gPET) and micronized PET (mPET). Figure 1 illustrates the production process for these aggregates.

The gPET is the recycling product of preform tubes, which are discarded from the plastic bottle production process, as they are unsuitable for the extrusion and blowing stages. The mPET is an intermediate product in manufacturing a polymeric resin to suppress ore dust, using dark-colored post-consumer PET bottles as raw materials [40].

The fine aggregates were physically characterized in terms of specific gravity (Anton Paar 5000 pycnometer, São Paulo, Brazil), bulk density (NBR 16972 [41]), particle size distribution (NM 248 [42]), and water absorption (NM 30 [43]). Table 1 and Figure 2 present the fine aggregates’ physical characteristics.

Quartzite was adopted as a reference aggregate because it has a particle size distribution similar to mPET, with D90 = 0.80 mm and D10 = 0.40 mm. The gPET aggregate has an approximately uniform granulometric distribution (D90 = 4.40 mm and D10 = 2.00 mm). mPET presents a better grain size distribution (D90 = 1.0 mm and D10 = 0.18 mm) than gPET aggregate. The fine expanded clay has a continuous particle size distribution, D90 = 1.90 mm, and the highest content of material finer than 75 μm, about 18%. Figure 3 shows the images of the aggregates obtained with an optical microscope (Kontrol IM-413, North Miami, FL, USA, with 20× magnification).

Expanded clay (CEC) was adopted as the coarse aggregate with particle sizes between 15 and 22 mm to produce lightweight concretes. The material has a specific gravity, bulk density, and water absorption of 0.60 g/cm³, 0.50 g/cm³, and 10%, respectively. A superplasticizer based on polycarboxylate ether was used to increase the plasticity of the concrete and reduce the water content.

2.2. Methods

2.2.1. Lightweight Mortars with Recycled PET Aggregates

Aiming to understand the PET aggregates effect and obtain the best mixture proportion for the lightweight concrete, mortars with the four different aggregates were produced initially. Based on preliminary studies, glass fiber waste incorporation was 2.00% by cement volume, and LAS incorporation was 0.20% by cement mass (for all the mixtures). To understand the influence of the morphology, particle size distribution, and specific surface of gPET and mPET aggregates on the mixtures’ rheology, a constant w/c ratio of 0.50 was adopted for all treatments. Thus, the workability was a response parameter.

The reference mixture adopted, with quartzite aggregate, was 1:3 by mass. A volumetric replacement of the reference aggregate by fine expanded clay and the recycled PET lightweight aggregates was performed, using bulk density as a conversion parameter. The PET aggregates’ particle size distribution does not fit within the lower and upper limits of ASTM C-33 [44] for fine aggregates for concrete. So, to assess the impact of PET aggregate packing on the properties of cement-based composites, various proportions of gPET and mPET aggregates were developed until reaching the limits indicated in the standard: the mixture G15M85 (Figure 4). The code G15M85 means that 15% of the total aggregate volume is gPET, while 85% is mPET aggregate. The same applies to other codes. Table 2 presents the mortars’ mixture design, and Figure 4 shows the blend of aggregates’ particle size distributions.

For each mixture, six cylindrical specimens, ∅ 5 cm × 10 cm, were produced. The specimens were mixed and cast following NBR 7215 [45] prescriptions and placed in a humid chamber (25 ± 2 °C; 90 ± 5% relative humidity) for initial curing for 24 h. Then, they were demolded and kept in a humid chamber for 7 days. Next, the specimens were placed in an oven at 60 °C for 24 h to remove free water.

Tests were carried out to determine the consistency index (NBR 13276 [46]) to evaluate the workability of the mixtures and bulk specific gravity following the method NBR 13280 [47]. The compressive strength test was performed at 8 days, following the method NBR 5739 [48]. These results made it possible to determine the optimal granulometric proportion between the PET aggregates for producing lightweight concrete mixtures.

2.2.2. Lightweight Concretes

For the lightweight concretes, a cement content of 445 kg/m³ was adopted, based on previous studies [13,24,30,34]. Considering the hydrophobicity of the PET aggregates, silica fume was added at 9.00% by cement volume (maximum content suggested by the manufacturer) as a strategy to increase the rate of hydrophilicity and fine materials in the lightweight concretes. Similar to the production of lightweight mortars, glass fiber waste was added at a content of 2.00% by cement mass to reduce the matrix cracking trend and provide greater cohesion. Two water-to-cement ratios, 0.50 and 0.47, were defined for producing lightweight concrete mixtures. The superplasticizer dosage was adjusted to maximize the mixtures’ workability, which was determined following NBR 16889 [49]. The volumetric proportion between cement and fine aggregates in lightweight concrete was the same as in mortars. Based on the mortar results, three blends of fine aggregates were adopted. The lightweight concrete mixture design content will be presented after the results and discussion of the lightweight mortars.

Ten specimens measuring ∅ 10 cm × 20 cm were molded (NBR 5738 [50]) for each mixture. The specimens were kept in a humid chamber (25 ± 2 °C; 90 ± 5% relative humidity) for 7 days and then placed in an oven at 60 °C for 24 h to remove free water. Table 3 presents the tests performed in the hardened concrete and the respective adopted standards.

Additionally, a method, based on Solak et al. [56], Abril et al. [57] and de Azevedo et al. [58], was proposed to evaluate the possible occurrence of coarse aggregate segregation in the concretes. Each specimen was sectioned longitudinally using a diamond saw in a wet section. Next, the sections were digitalized and treated by image processing software. In this stage, the coarse expanded clay aggregates turned black, while the rest of the image was erased and replaced by a white background. Next, each image was divided into five groups, each 4 cm high, and the percentage of area occupied by the coarse aggregate to the section was determined using Matlab software (R2017a-9.2).

3. Results and Discussion

3.1. Properties of Lightweight Mortars with Recycled PET Aggregates for the Dosage Proposition

Figure 5 shows the mortar’s flows, compressive strength at 8 days, and bulk specific gravity. The conventional reference mortar and the G15M85 mortar exhibited a lack of cohesion as they disintegrated under the impact of the flow table. Their spreads were considered 120 mm, corresponding to the cone trunk base diameter.

The mortars with recycled PET aggregates showed a bulk specific gravity of approximately 1.35 g/cm³, a 30% reduction compared to the REF. mixture, and a 12% decrease compared to the lightweight reference mortar (REF._C.). This result agrees with the specific gravity of recycled plastic aggregates, which is approximately 40% lower than the quartzite aggregate and 8.5% lower than the fine expanded clay aggregate.

The lightweight reference mixture (REF._C.), with fine expanded clay, showed an average flow of 136 mm, an increase of 13% compared to the reference mixture. The PET-containing mortars (except mixture G15M85) showed a flow increase compared to REF and REF._C. This result may be related to the zero-water absorption of plastic aggregates, allowing more free water in the mixtures, as previously reported by Spósito et al. [29], who observed a 9.14% gain in spreading in rendering mortars with a 20% replacement of river sand by recycled PET aggregates.

As the proportion of gPET aggregates increased (and mPET decreased), the mortar’s flow also enhanced, achieving the highest value (206 mm) in the mixture G70M30. That can be explained by the reduction in the surface area of the aggregates in the system, considering that the gPET has a higher fineness modulus and D₉₀ and, consequently, a smaller surface area than the mPET aggregate. Also, the smooth surface of the gPET aggregate decreases the inner friction in the cement paste and improves the fluidity and workability of mortars, as pointed out, by Choi et al. [59], who achieved a 16% increase in mortar flow for 100% replacement of natural aggregates by recycled plastic aggregates. The mixtures G85M15 and G100M0 presented a trend of reduction in the flow due to the lack of cohesion of these mixtures, which could be related to an approximately uniform particle size distribution of the aggregates’ blend.

The compressive strength results agree with the PET-containing mortar’s flow. The G70M30 blend exhibited the highest flow and average compressive strength, 16.8 mPa, at 8 days. Since all mortars have the same water content, higher fluidity leads to lower void content in the cement-based composite. So, this positively impacts its mechanical performance. The compressive strength of the G70M30 mixture is 14% higher than the second-best performance mixture, G55M45, and about 45% lower than the conventional reference mixture. The enhancement of the mPET content resulted in a reduction in the compressive strength results. The mixture G15M85, in which the fine aggregates’ particle size distribution fits into ASTM C-33 limits, exhibited the second-lowest average compressive strength, measuring 12.3 mPa. This value is 27% lower than the average compressive strength of the G70M30 mixture. This result may be related to the morphology of the mPET aggregate since more water and voids tend to accumulate on its rough surface. Consequently, a weakly bonded aggregate–cement paste is obtained [29].

3.2. Lightweight Concrete

3.2.1. Mixture Design

Despite the aggregates’ proportions of G55M45 and G70M30 not fitting into the limits specified by ASTM C-33 [44], the results indicate better performance in the fresh and hardened state among the PET-containing mortars evaluated. Considering also that the recycled PET aggregates present morphology, mineralogy, and chemical composition completely different from conventional aggregates, the definition of the optimal proportion of recycled PET fine aggregates was not associated with the normative parameters. Thus, G55M45 and G70M30 mixtures were used to develop and produce lightweight concrete mixtures.

The lightweight mortar results also indicated a better compressive strength with REF._C. than those with recycled PET aggregates. To mitigate the impact of the plastic aggregate’s hydrophobicity into the matrix and, consequently, improve the lightweight concrete rheology, pores distribution, and compressive strength, another proportion of the fine aggregate with 15% (in volume) fine expanded clay (hydrophilic aggregate), 55% gPET, and 30% mPET (called C-G55M30C15) was proposed. This new aggregate blend presents better-distributed grain sizes than the G-55M45 and G70M30 blends, as presented in Figure 6. Table 4 presents the mixture design of lightweight concretes, which included the coarse expanded clay aggregate.

3.2.2. Slump

Figure 7 presents the slump results and the chemical admixture content for the produced concretes. Lightweight concretes generally have lower slump values than conventional concretes due to the reduced specific gravity of their aggregates and, consequently, less deformation due to gravity [60]. As illustrated in Figure 3, the mPET aggregate presents a non-uniform and lamellar shape, which tends to provide greater friction with the cement paste compared to aggregates with a rounded and smooth shape, which reduces the workability of the concretes [9,23,61,62].

The slump results of the proposed lightweight concrete are superior to some related in the literature for fine or coarse natural aggregate replacement by plastic waste aggregates. Mohammed et al. [24] evaluated lightweight concretes with a cement consumption of 455 kg/m³, a w/c ratio of 0.52, and an 85% replacement of fine and coarse natural aggregates by PVC aggregates (two mixtures), and obtained a 0 and 20 mm slump, respectively. Harihanandh and Karthik [21] also obtained a 0 mm slump for concretes with cement consumption of 362 kg/m³ and a w/c of 0.45 for a 20% and 25% replacement of the natural fine aggregate by recycled PET aggregates. Hama and Hilal [61] observed the same for the 40% replacement of natural fine aggregates by recycled PET aggregates.

In contrast, in other studies [30,59], an increase in the workability of concrete is reported as a function of the increment in the content of recycled plastic aggregates. In these previous works [30,59], the plastic aggregates have a spherical shape and high hydrophobicity, allowing more free water in the mixtures. Thus, the workability of cement-based composites with recycled plastic aggregates is closely related to the aggregates’ shape.

3.2.3. Unit Weight

According to the Brazilian standard NBR 8953 [63], lightweight concretes must present a unit weight, the ratio between the mass of the dry sample and the total volume, including permeable and non-permeable pores, lower than 2000 kg/m³. The American technical report ACI 213-R [36] requires a unit weight between 1120 and 1920 kg/m³, while the European standard Eurocode 2 [64] specifies a bulk specific gravity between 800 and 2000 kg/m³. Thus, as seen in Figure 8, the developed mixtures met the requirement for lightweight concrete indicated by all mentioned references, with a unit weight between 1190 and 1280 kg/m³. In addition to the unit weight results of the lightweight concretes, Figure 8 presents all the limits mentioned.

The low unit weight of the concretes is attributed to the reduced specific gravity of the fine and coarse aggregates used (PET wastes, fine and coarse expanded clay aggregates) compared to conventional aggregates, and agrees with results presented in other studies [9,22,24,25,30,33]. Incorporating expanded clay coarse aggregate results in an additional reduction in unit weight of approximately 10%, when comparing the specific gravity of concretes with the results obtained for the PET-containing mortars. The unit weight values obtained are also lower than those reported by other authors, who partially replaced the natural aggregates with lightweight aggregates. Table 5 presents some results obtained in other studies.

The variation between the proportions of recycled PET fine aggregates (mixtures C-G70M30, C-G55M45) did not influence the concretes’ unit weight. Incorporating fine expanded clay (mixtures C-55M30C15) increased the unit weight for both w/c ratios adopted, compared to concretes made entirely of fine recycled PET fine aggregates. For a w/c ratio of 0.50, the C-G55M30C15 mixture showed a unit weight 7.6% higher than the C-G70M30 mixture, while for a w/c ratio of 0.47 the increase in this property was 10.0%. This result may be related to the better-distributed fine aggregate grain sizes of the C-G55M30C15 blend (Figure 6), which may have provided a better matrix packing and pore refinement and the higher specific gravity of fine expanded clay aggregate compared to recycled PET aggregates.

3.2.4. Water Absorption and Void Ratio

Figure 9 presents the water absorption and void ratio of the developed concretes. Using plastic waste as fine or coarse aggregates generally provides cement-based composites with a void ratio and water absorption superior to conventional concrete. The water absorption and void index tend to increase as a function of the plastic aggregate content in the cement composite [24,31,65,67], the size of the aggregates [31,33], and the w/c ratio adopted [31]. The lightweight concretes presented around 18% water absorption and a 21% void index, which may be an issue to long-term durability performance.

The C-G55M30C15 mixtures showed a water absorption and void index inferior to C-G70M30 and C-G55M45 concretes. The fine expanded clay aggregate’s hydrophilic characteristic, rounded shape, and the better-distributed particle size of the C-G55M30C15 blend compared with C-70M30 and C-55M45 mixtures could have provided better packing, pore refinement, and, consequently, lower void index. For the C-G55M30C15 (w/c = 0.50) mixture, a 20% and 13% reduction in the water absorption and void ratio, respectively, were verified for an enhancement of 8.4% in the unit weight, compared to the C-G70M30 mixture.

The water absorption and void index results agree with those obtained for unit weight since the mixture C-G55M30C15, for both w/c ratios, presented the lowest void index and water absorption and higher unit weight, indicating a greater packing density and pore refinement. On the other hand, the mixtures C-G70M30 (w/c = 0.47) and C-G55M45 (w/c = 0.50) presented the lowest unit weight and the highest void index and water absorption results.

At a w/c ratio of 0.50, reducing the gPET aggregate proportion from 70% to 55% had no significant impact on the water absorption and void index results, as they were similar within the standard deviation. However, at a w/c ratio of 0.47, the water absorption and void index parameters presented a relative decrease of 11% and 5%, respectively, from the C-G70M30 to the C-G55M45 mixture, which may be associated with the gPET aggregates’ smooth and cubic morphology, providing more free water (and consequently pores) in the lightweight concrete mixture. Also, the variation in the w/c ratio from 0.47 to 0.50 increased the water absorption of concretes for all mixtures, which is consistent with what was observed by other authors [31].

3.2.5. Ultrasonic Pulse Velocity

Figure 10 shows the results of the ultrasonic pulse velocity test for each mixture produced. The figure also presents the concrete quality classification as a function of ultrasonic pulse velocity, according to Feldman [68], as follows: very poor (1800 to 2100 m/s); poor (2100 to 3000 m/s); questionable (3000 to 3600 m/s); good (3600 to 4500 m/s) and excellent (>4500 m/s). This classification refers to conventional concrete with natural aggregates and, therefore, may not be representative of cement-based composites with plastic wastes. The C-G55M30C15 mixtures, at both w/c ratios, presented the highest ultrasonic pulse velocities. These results are consistent with the earlier results on unit weight, water absorption, and void index, reinforcing the indication of improved packing and pore refinement. Except for the mixture C-G55M45, the increment of the w/c ratio from 0.47 to 0.50 decreased the ultrasonic pulse velocity. This is caused by the excess of water in the pores, which evaporates and creates voids in the concrete in the hardened state [9,31,66].

The UPV results are similar to those reported in other studies, although these previous works adopted lower replacement rates of conventional aggregates by recycled plastic aggregates. Mohammed et al. [24] obtained ultrasonic pulse velocities of 3373 m/s and 2452 m/s for replacing 85% of fine and coarse natural aggregates with recycled PVC aggregates (cement = 455 kg/m³; w/c = 0.52). Senhadji et al. [34] produced concretes with a 70% replacement of fine and medium (3–8 mm) aggregates for PVC aggregates and obtained an ultrasonic pulse velocity of 3081 m/s at 28 days (cement = 400 kg/m³; w/c = 0.48). Qaidi et al. [66] produced concretes with a 50% substitution of fine aggregates for PET aggregates and reported an ultrasonic pulse velocity at 28 days of 3530 m/s (cement = 460 kg/m³; w/c = 0.40). So, despite the classification of poor quality of the produced lightweight concretes, their characteristics were similar to matrices with lower PET aggregate replacement rates and similar cement and water content (Table 5). This indicates that the dosage approach was adequate to enhance the properties and reduce the limitations of PET aggregates.

3.2.6. Compressive Strength, Tensile Strength, and Efficiency Factor

Figure 11 shows the compressive strength, efficiency factor, and tensile strength results of lightweight concrete mixtures. The efficiency factor is the ratio between the compressive strength and unit weight for each mixture.

Introducing fine expanded clay aggregate was an efficient strategy to improve the lightweight concrete mechanical properties since the C-G55M30C15 mixture showed superior compressive and tensile strength to those made only with fine PET aggregates. After 8 days, the C-G55M30C15 mixture presented a compressive strength 14.6% higher than the C-G70M30 mixture for a w/c of 0.50. For a w/c of 0.47, the difference is 23.9%. Compared to the C-G55M45 mixture, the compressive strength gain was 7.1% and 16.1%, for a w/c of 0.47 and 0.50, respectively. The C-G55M30C15 mixture, with a w/c ratio of 0.47, was classified as structural lightweight concrete according to the ACI 213-R technical report [36].

The increment of the concrete compressive strength by using fine expanded clay aggregate could be related to its hydrophilic nature, which suggests a more effective paste–aggregate interaction in the interfacial transition zone than that generated with the hydrophobic PET aggregates and an enhancement of the cement hydration products. Also, as presented in Figure 6, the mixture C-G55M30C15 presented better-distributed particle size than C-70M30 and C-55M45 mixtures, which could have contributed to a better packing of the concrete. In addition, the compressive strength results of the mixtures with fine expanded clay aggregate agree with the water absorption, void index, and ultrasonic pulse velocity results, pointing to the improvement of these properties due to the incorporation of fine expanded clay into the concrete.

The C-G70M30 and C-G55M45 mixtures’ lower compressive and tensile strength results than the C-G55M30C15 mixture may be related to three factors. The first one is the zero-water absorption capacity of the PET aggregates, which causes an accumulation of water in the interfacial transition zone and, consequently, a weak bond between the cement paste and the aggregates, reducing the compressive strength of the concretes [1,11,13,30,34,67]. The second one is the lower strength and rigidity of plastic aggregates compared to natural ones that can produce stress-concentration zones, adding to the propagation of cracks [1,34]. The last one is the high porosity of the matrices [1,11,34]. Then, the C-G70M30 and C-G55M45 mixtures, composed entirely of fine PET aggregates, were not classified as structural lightweight concretes. Although this may limit the application of these concretes for structural purposes, they can be applied to elements such as subfloors, masonry concrete blocks, and cement slabs, among others.

The compressive strength results obtained in this research, with the full use of lightweight fine and coarse aggregates, are similar to those obtained in other studies (presented in Table 5) with partial use of lightweight aggregates. Despite the current research showing lower compressive strength for some mixtures, the concretes were produced entirely with lightweight aggregates, mainly PET aggregates, allowing greater incorporation of waste and a reduction in the unit weight.

Furthermore, for a better understanding of the efficiency of the proposed dosage method, it is necessary to evaluate the efficiency factor obtained for the matrices produced. The concretes showed an efficiency factor between 9.7 and 12.3 mPa·cm³/g. These results are similar to those obtained by Moravia et al. [69], who produced lightweight concretes with coarse expanded clay, natural river sand, a cement consumption of 465 kg/m³, and a w/c ratio of 0.55. The authors reached an efficiency factor of 10.9 mPa·cm³/g, with a compressive strength of 17.6 mPa at 7 days. Also, as presented in Table 5, the literature concretes with the replacement of fine and/or coarse natural aggregate for plastic waste superior to 50% presented an efficiency factor between 3.0 and 11.2 mPa·cm³/g. The present research obtained similar and/or superior results, which points again to the efficiency of the dosage strategy adopted.

3.2.7. Evaluation of the Distribution of Coarse Expanded Clay Aggregate

Figure 12 presents the results obtained for evaluating the distribution of coarse expanded clay aggregates in lightweight concrete. Each specimen’s (10 cm × 20 cm) digitalized section was divided into five groups (4 cm each), and the area occupied by the coarse aggregate was determined by Matlab software. The percentage indicated in each section represents the content of expanded clay in this area. Four ranges of coarse expanded clay percentages to the total area of the sections were adopted. The coarse expanded clay aggregate has a specific gravity considerably lower than the mortar phase and a rounded shape, characteristics that could favor its vertical upward movement in the matrix. However, this phenomenon was not observed in the proposed mixtures, with its contents similar to the other sections.

Three of the mixtures produced (C-G70M30—w/c of 0.50, C-G55M30C15—w/c of 0.50, and C-G70M30—w/c of 0.47) had a variation superior to 10% in the coarse expanded clay content between sections, which can be considered a high variability. The C-G55M30C15 mixture, with a w/c of 0.47, presented the lowest variability between the analyzed sections. This result agrees with the previous results, considering that this mixture presented better mechanical performance, ultrasonic pulse velocity, a lower void index, and water absorption. Also, this result agrees with the findings of Solak et al. [56] that studied the influence of segregation on the compressive strength of lightweight concretes.

4. Conclusions

This work aimed to develop structural lightweight concretes with complete replacement of conventional aggregates by lightweight aggregates made of recycled PET and expanded clay. For this purpose, the mixture proportion of the lightweight concrete was based on the fresh and hardened state performance of mortars produced only with recycled PET aggregates. Given the results, the following conclusions were obtained:

• The dosage method, in which the mixture proportion of lightweight concrete was based on the performance of mortars produced with recycled PET aggregates, was efficient since it was possible to determine the optimum mixture proportion of aggregates, considering fresh and hardened state properties. Also, the lightweight concretes showed a mechanical performance similar to the respective mortars.

• The produced concretes showed higher voids and water absorption compared to conventional concretes. This is due to the plastic aggregates’ hydrophobicity and angular/lamellar shape, resulting in poor workability composites.

• The full use of lightweight fine and coarse aggregates in concretes provided a unit weight reduction superior to that reported by other authors, who partially replaced the aggregates.

• The ultrasonic pulse velocity of the concretes was considerably affected by the incorporation of PET aggregates due to the hydrophobic nature of these aggregates, which causes more voids, an accumulation of water in the interfacial transition zone, and, consequently, reduces the velocity of the propagation of the pulse.

• The concretes presented similar or superior compressive strength and efficiency factors compared to results reported in the literature for concretes with 50% or more replacement of fine/coarse aggregates by plastic aggregates. The C-G55M30C15 mixture (w/c = 0.47) was classified as structural lightweight concrete according to the ACI 213-R technical report, considering the unit weight and compressive strength parameters. These results also suggest that the dosage approach was efficient for improving the mechanical performance of the concretes.

• Incorporating a small volume of hydrophilic fine expanded clay aggregate proved to be an effective strategy to minimize the loss of mechanical performance due to using recycled PET aggregates without significantly enhancing the unit weight of the composites.

• The proposed evaluation of the distribution of the coarse aggregates in the specimens presented and its results are consistent with the mechanical results obtained. So, it proved to be an efficient method to evaluate the homogeneity of concrete specimens.

Using recycled PET aggregates as an eco-friendly building material in cement-based composites reduces their unit weight, which helps to develop lighter structures and reduces the consumption of natural resources. However, as presented in this research, the complete replacement of conventional aggregates by lightweight aggregates and, specifically, plastic waste considerably affects the concretes’ properties in the fresh and hardened state, impairing the workability, increasing the void index, increasing the water absorption, and lowering the mechanical strength. To equalize the environmental benefits with the mechanical performance of lightweight cement-based composites with plastic waste is, thus, a challenge to enable its broad application.

Author Contributions

Conceptualization, D.M.R., B.O.P., J.M.F.d.C. and R.A.F.P.; methodology, D.M.R., J.M.F.d.C. and R.A.F.P.; validation, D.M.R., L.C.B.C. and R.A.F.P.; formal analysis, D.M.R. and L.C.B.C.; investigation, D.M.R.; resources, D.R, L.C.B.C., B.O.P., G.d.R.G., J.M.F.d.C. and R.A.F.P.; writing—original draft preparation, D.M.R.; writing—review and editing, D.M.R., L.C.B.C., G.d.R.G., J.M.F.d.C. and R.A.F.P.; supervision, G.d.R.G., J.M.F.d.C. and R.A.F.P.; project administration, J.M.F.d.C., G.d.R.G. and R.A.F.P.; funding acquisition, R.A.F.P. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data used in this research have been properly cited and reported in the main text.

Acknowledgments

We gratefully acknowledge the Ministry of Science, Technology, Innovation and Communications, and the agencies CAPES, CNPq, and FAPEMIG for providing financial support. We are also grateful for the infrastructure and collaboration of the Research Group on Solid Waste—RECICLOS—CNPq.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Production of recycled PET aggregates.

Figure 2. Particle size distribution of aggregates [44].

Figure 3. (a) Quartzite (b) Fine expanded clay (c) gPET (d) mPET.

Figure 4. Particle size distribution curves of the aggregates for each blend studied [44].

Figure 5. Flow, compressive strength, and bulk specific gravity of the lightweight mortars.

Figure 6. Particle size distribution of fine aggregates in concrete mixtures [44].

Figure 7. Slump results and admixture content in lightweight concretes.

Figure 8. The unit weight of the concretes.

Figure 9. Void ratio and water absorption of the lightweight concretes.

Figure 10. Results of ultrasonic pulse velocity measurements in the lightweight concretes.

Figure 11. Compressive strength, tensile strength, and efficiency factor of the lightweight concretes.

Figure 12. Evaluation of the distribution of coarse aggregates into the specimens.

Table 1

Physical characterization of the aggregates.

Aggregate	Specific Gravity	Bulk Density (g/cm3)	Fineness Modulus	D90 * (mm)	Water Absorption
Granular PET	1.57	0.61	4.84	4.40	0.00%
Micronized PET	1.66	0.38	2.11	1.00	0.00%
Fine Expanded Clay	1.78	0.66	2.17	1.90	0.88%
Quartzite	2.68	1.37	2.44	0.80	0.70%
Coarse Expanded Clay	0.64	0.50	7.09	25.0	10.0%

* The D₉₀ parameter indicates the particle diameter corresponding to 90% of the accumulated volume passing in the particle size distribution, respectively.

Table 2

Mix design for lightweight mortars.

Mixture	Cement (g)	Quartzite (g)	C (g)	mPET (g)	gPET (g)	GFW (g)	Water (mL)	LAS (g)
REF	1000	3000	-	-	-	4.6	500	2.0
REFC	1000	-	1456	-	-	4.6	500	2.0
G15M85	1000	-	-	709	200	4.6	500	2.0
G35M65	1000	-	-	542	468	4.6	500	2.0
G55M45	1000	-	-	376	735	4.6	500	2.0
G70M30	1000	-	-	250	935	4.6	500	2.0
G85M15	1000	-	-	125	1135	4.6	500	2.0
G100M0	1000	-	-	0	1336	4.6	500	2.0

Table 3

Tests performed and methods.

Test	Method
Specific gravity, void ratio, and water absorption	NBR 9778 [51]
Ultrasonic pulse velocity (TICO—Proceq equipment)	NBR 8802 [52]
Compressive strength	NBR 5739 [48]
Tensile strength	NBR 7222 [53]
Efficiency factor	Rossignolo [54,55]

Table 4

Lightweight concrete mixtures.

Lightweight Concrete Mixtures (kg/m3)
Mixture	w/c	Cement	gPET	mPET	C	GFW	CEC	Water	Silica Fume
C-G70M30	0.50	445	415	111	-	2.0	199	222	31.4
	0.47	445	415	111	-	2.0	208	209	31.4
C-G55M45	0.50	445	326	167	-	2.0	215	222	31.4
	0.47	445	326	167	-	2.0	224	209	31.4
C-G55M30C15	0.50	445	326	111	97	2.0	201	222	31.4
	0.47	445	326	111	97	2.0	210	209	31.4

Table 5

Results of unit weight, compressive strength, and efficiency factor of lightweight concrete with plastic aggregates from other studies.

Reference	Plastic Type	Amount of Replacement Content	w/c	Cement (kg/m3)	Unit Weight (kg/m3)	Compressive Strength (mPa)	Efficiency Factor (mPa·cm3/g)
Akçaözoǧlu et al. (2013) [65]	PET	60%—Fine and coarse aggregate	0.50	500	1530	9.5	6.2
Senhadji et al. (2015) [34]	PVC	70%—Fine and medium (3–8 mm) aggregate	0.48	400	1751	17.7	10.1
Islam et al. (2016) [30]	melted waste PET	50%—Coarse aggregate	0.48	449	1970	19.7	10
Mohammed et al. (2019) [24]	PVC	85%—Fine aggregate	0.52	455	2055	16.4	8.0
Mohammed et al. (2019) [24]	PVC	85%—Coarse aggregate	0.52	455	1695	8.3	4.9
Belmokaddem (2020) [6]	PVC *	75%—Fine and medium (3–8 mm) aggregate	0.48	350	1700	12.0	7.1
Belmokaddem (2020) [6]	PP *	75%—Fine and medium (3–8 mm) aggregate	0.48	350	1380	5.2	3.8
Belmokaddem (2020) [6]	HDPE *	75%—Fine and medium (3–8 mm) aggregate	0.48	350	1318	4.0	3.0
Basha et al. (2020) [25]	PET	100%—Coarse aggregate	0.45	350	1497	16.0	10.7
Basha et al. (2020) [25]	PET	100%—Coarse aggregate	0.40	370	1518	17.0	11.2
Qaidi et al. (2023) [66]	PET	50%—Fine aggregate	0.40	460	1993	18.9	9.5

* PVC: polyvinyl chloride; PP: polypropylene; HDPE: high-density polyethylene.

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