**1. Introduction**

The scarcity of natural resources and the generation of solid wastes without adequate disposal is of worldwide concern, which makes their reuse feasible in the civil construction sector, besides encouraging sustainable development and a circular economy [1–4]. The solid waste problem is of concern mainly in urbanized regions and developing countries where collection and disposal services have difficulties dealing with increasing amounts of waste [2]. As a result, waste is either disposed in open, uncontrolled dumps, accounting for 61% of the landfill sector's CO2 emissions, or burned in the open dumps, accounting for 40% of global waste [5]. Sustainable solid waste management has become a necessity for industries seeking to promote industrialization and sustainable development. Government regulations have become more stringent around the world, representing an accelerating

**Citation:** da Silva, T.R.; Cecchin, D.; de Azevedo, A.R.G.; Valadão, I.; Alexandre, J.; da Silva, F.C.; Marvila, M.T.; Gunasekaran, M.; Garcia Filho, F.; Monteiro, S.N. Technological Characterization of PET—Polyethylene Terephthalate—Added Soil-Cement Bricks. *Materials* **2021**, *14*, 5035. https://doi.org/10.3390/ma14175035

Academic Editor: Rossana Bellopede

Received: 3 August 2021 Accepted: 25 August 2021 Published: 3 September 2021

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**Copyright:** © 2021 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/).

factor in the adoption of reverse logistics initiatives, including in countries that have been facing difficulties in recycling processes, thus giving more space for the use of this waste within its life cycle [4].

Global plastic production is growing rapidly and by 2030 the world may produce about 619 million tons of plastic per year [6]. A study by Spósito et al. [7] emphasizes that post-consumer polyethylene terephthalate (PET) products have generated a growing interest regarding their recycling potential and their negative impacts on the environment, such as pollution and long degradation time. According to WWF [8], phasing out single-use plastics has the potential to reduce plastic demand by up to 40% by the year 2030. Thus, the current scenario requires a more sustainable route for the recycling of this waste, which if not realized will increase the environmental imbalance due to its non-biodegradability in nature [8,9].

The growing demand for sustainable products has encouraged several studies that search for alternative techniques regarding the reuse of waste in construction materials, such as mortars with sugarcane bagasse [10], cement pastes with açai fiber [11,12], blast furnace slag [13,14], construction and demolition waste [15], ceramic materials with rice ash water treatment plant sludge [16], pulp and paper industry sludge [17], construction and demolition waste [18], and agricultural waste [19], as well as concrete with plastic waste [20]. Therefore, research also highlights the reuse of plastic waste in construction materials [19,20] such as paving [21], mortar [7], concrete [19,20,22,23], fired clay blocks, and bricks [24], as well as unfired blocks and bricks [25,26], thus showing PET (Figure 1a) as an addition in the production of these materials.

**Figure 1.** Materials used: (**a**) PET waste; (**b**) soil-cement brick.

Among the various building materials available for waste addition stand soil-cement bricks (Figure 1b), the use of soil-cement bricks presents many advantages from the environmental point of view. There is no need for a burning process, which is associated with a reduction of greenhouse gas emissions and enhanced technological properties. Another advantage is the reduction of costs when these bricks are used for the execution of masonry due to the dismissal of the use of mortar to join the bricks of a fitting type [16,27]. Thus, this construction material has potential for use in small and medium sized buildings without structural function, in addition to having a low financial cost [28,29].

The soil-cement bricks allow the incorporation of waste in their composition and reduce costs up to 40% compared to traditional masonry, especially in low-income housing. In this way, the brick can be considered more sustainable in relation to the traditional brick [30]. In this context, it is also possible to verify the use of waste in soil-cement bricks as shown França et al. [28], who studied the durability of soil-cement bricks with incorporation limestone waste. The authors used 30%, 40% and 50% of waste for the manufactured soil-cement mixtures. The results verified that the incorporation of waste

rock was technologically feasible. The parameters studied for compressive strength, water absorption and durability showed superior performance of bricks with waste incorporation. Reis et al. [31] evaluated the incorporation of quartzite mining tailings in soil-cement bricks. The authors tested additions of quartzite tailings at 0%, 15% and 30%. The results showed that the compressive strength of the soil-cement bricks decreased with the addition of quartzite waste. However, the authors observed that the results of compressive strength and water absorption performed at 7 and 28 days demonstrated the possibility of using the waste without compromising the physical and mechanical properties required by the standard, thus verifying that the soil-cement brick is a viable technique for the disposal of this type of waste. Kongkajun et al. [32] evaluated soil-cement bricks with incorporation of construction waste (clay bricks) and fiber-cement industry sludge. The authors used 15 wt.% of Portland cement, 15 wt.% of sand and 70 wt.% of laterite to produce the bricks. The clay brick waste was added from 10% to 50 wt.% of laterite in the control samples. Sludge, on the other hand, was added at 5% and 10 wt.% to replace the total weight of the mixture in the control samples. The maximum obtainable substitution of laterite for clay brick waste was 50 wt.% in the mixture. Maximum compressive strength was achieved for the 10% replacement of laterite with clay bricks. Partial replacement of laterite with clay bricks improved the compressive strength of soil-cement bricks for load-bearing brick application. Although the incorporation of silt caused a reduction in the compressive strength of the brick samples compared to the samples prepared from the control sample, they still exceeded the Thailand community product standard. Increasing the percentage of sludge from 0% to 10 wt.% resulted in a significant decrease in thermal conductivity of 45% compared to the control formula. When using the sludge and clay bricks, the thermal conductivity and density of the bricks were further reduced, while their compressive strength and water absorption values were still satisfactory.

Kouamé et al. [33] verified the influence of shea butter wastes on the physical properties of cement-stabilized soil bricks. The authors used three local clay raw materials consisting mainly of kaolinite, quartz and micaceous phases, as well as shea butter waste and cement. In the mixtures tested, the amount of cement was kept constant (5%), while the amount of shea butter waste varied from 2% to 10%, replacing the soil. The results obtained by the authors showed that the presence of pores due to the shea butter waste influences the reduction of density and thermal conductivity of the bricks. A 25% decrease in thermal conductivity was verified for the samples with clay F, 16% for the samples with clay K, and 22% with clay Y. The authors concluded that the bricks showed good stiffness related to the presence of cementation phases. Therefore, for the samples with clay F and clay Y, the replacement rate of 6% by the shea butter waste was sufficient as compared to 8% for the formulations with K to obtain a physical property. Thus, they found that the shea butter waste offered good thermal insulation and good stiffness properties with a lower amount of cement used.

Vilela et al. [34] evaluated the incorporation of mining waste in soil-cement bricks for soil substitution at 10%, 20%, 30% and 40% of waste. As for the mechanical strength, the authors found that all treatments showed values above the required standards, with the minimum standard value being set at 2.0 MPa. The treatment with 10% mining waste presented the best results. The thermal conductivity showed a direct link with the density of the bricks since the increase in density (bricks with 40% mining waste) led to a material with a lower heat dissipation property. The study showed that the addition of mining waste to soil-cement bricks met all the required standards [34].

In this sense, the use of plastic waste can also be considered in studies such as [7], which showed that hydrated mortars produced with PET bottle waste replaced the fine aggregate in the mixture and suffered changes in properties in their fresh and hardened states. Reference [21] investigated the effects of PET waste on hardened properties in high strength concrete and the investigation showed the interference of high temperatures on concrete properties, in this case, the occurrence of material fragmentation and the release of greenhouse gases. Reference [22] evaluated PET blends for sidewalk sub-bases, highlighting bottles and food packaging taken from collection points and crushed into mixtures with two main constituents of waste materials or construction and demolition by-products: concrete aggregate and crushed brick.

Studies by [24] found that compared to normal concrete, high strength concrete has a failure mode and that the lack of ductility can be solved by using different types of plastic fibers. Akinyele et al. (2020) evaluated the incorporation of PET into fired blocks varied by 0%, 5%, 10%, 15% and 20% and found changes in the samples with respect to high temperature, compressive strength and water absorption. Reference [23] investigated concrete with added waste plastic by varying it at 0%, 5%, 10%, 15% and 20% and found that the concrete showed failure in shear, while in the hardened state it showed gradual reduction in the strength of the material as more granular plastic was added to the concrete mix.

According to [27], the addition of plastic waste in pressed blocks with a variation of 0%, 1%, 3%, and 7% showed that the compressive strength of the stabilized earth block without additives was low and that there was an initial increase in compressive strength with increasing plastic waste. The optimum compressive strength for the study was obtained for blocks containing 1% crushed plastic waste, whose particle sizes were less than 6.3 mm. The increase in compressive strength was 244.4% when compared to the block without the addition of plastic waste.

This paper aimed to evaluate the influence of the incorporation of polyethylene terephthalate (PET) waste in the properties of soil-cement bricks. The study emphasizes mainly the analysis of characterization of the materials used, since this type of brick has particularities for its manufacturing. Therefore, characterization of the soil was performed, as well as the PET waste, to see the relationship of both in the mixtures. The compaction curves were also highlighted, since most studies have difficulties regarding the optimum moisture content used in the production of mixtures for this type of masonry. Moreover, this work also presents the PET waste as an innovation since it is still minimally used and not often discussed in research on the subject of soil-cement.
