3.2.1. Compressive Strength

The compressive strength analysis of the bricks produced with different levels of in-corporation of PET waste showed a significant difference between the percentages, as well as between the ages. At seven days, the bricks made with 20% and 30% addition of PET waste showed the highest values of compressive strength, values that did not differ statistically between them. At 28 days, the highest average value was observed for the 20% addition (1.80 MPa), followed by 30% (1.45 MPa) and then the lowest values (0% and 10%) with no differences between them, as shown in Table 4.

**Table 4.** Compressive strength of samples at 7 and 28 days.


\* Val. \* Values followed by the same letter, uppercase in the column and lowercase in the row, do not differ at the level of 5% by Tukey's test.

According to the results, it was possible to observe that an increase in compressive strength occurred in bricks with different PET mixtures (20% and 30%). The samples with 20% PET reached the average value of compressive strength required by the ABNT standard NBR 15270-1 [51], which recommends 1.5 MPa for the total average of the samples. The mixtures with more PET had more water content. Since the PET particles were larger and the material was not porous, more water was available for cement hydration, which was probably the main reason why higher strengths were achieved, considering that the cementitious matrix does not have as good adhesion with PET [23].

The higher the concentration with smaller diameters of PET particles, the lower the probability of an increase in the number of voids and the direct interference in the strength of bricks [7]. In the case of 30% PET samples, Akinyele and Ajede [23] stated that the greater the amount of plastic, the greater the amount of water needed to improve workability;

in this case, it is possible to further state that the lack of water needed for hydration also correlated with lower strength. However, the authors state that this waste can be considered to replace fine aggregates up to 20% in cement-based materials. Additionally, the worse workability of the mixtures with a high amount of PET could be attributed not only to the higher particle size, but also to the rough and irregular shapes of the PET particles, as shown in the SEM images.

Regarding the reference sample (0% PET) and the samples of bricks with different incorporations of PET, they did not reach the average established by [43]. It is also estimated that they may have suffered interference in relation to cement hydration due to the type of cure used (humid chamber), damaging their strength. The incorrect hydration of the cement is generally due to the lack of water or humidity necessary for the cement to react, as shown in [49–52]. As for the ages, there was no difference for most of the treatments due to the cement used being CPV, which has greater strength gain at early ages in view of its better quality due to finer grinding.

#### 3.2.2. Water Absorption

The analysis of water absorption in the bricks in Table 5 showed that there was no significant difference between the mixtures (*p* < 0.05). Thus, it was found that the amount of PET incorporated in all mixtures did not interfere with water absorption.

**Table 5.** Water absorption of samples at 28 days.


\* Values followed by the same letter, uppercase letters in the column do not differ at the 5% level by Tukey's test.

According to the ABNT standard [43], the water absorption values should not present average values higher than 20%, nor individual values higher than 22%. Considering that PET is a material that does not absorb water, it is believed that it contributed to the bricks not presenting high water absorption. This was confirmed by Górak et al. [53] in studies of the effect of incorporating PET waste into cementitious composites, which indicated that the particle size of the waste has a significant effect on water absorption in relation to its porosity [54–57].

#### **4. Conclusions**

The soil used in the study, according to its particle distribution and sand characteristics, is considered suitable for use in the manufacturing of soil-cement bricks. Indeed, the soil with 57.1% sand fraction, 24.3% clay fraction and 18.6% silt fraction is classified as sandy clay soil. The PET waste corresponded to 99.9% of sandy particle fraction, with 85.5% coarse particles, 11.5% medium particles and 2.9% fine particles. It was possible to verify that as the addition of PET increased in the mixtures, the higher the content of sandy particles.

As for the optimal moisture content and compaction energy, it was observed that the natural soil (without waste addition) showed optimal moisture value and maximum specific weight satisfactory. For the mixtures, the greater the addition of PET (10%, 20% and 30%), the greater the optimum humidity of compaction, in addition to a decrease in maximum specific weight. This revealed that the higher the addition of water, the lower the workability of the mixture, thus interfering in the mechanical strength.

The evaluated PET waste can be classified as sandy grain size and non-plastic waste. Through its microstructure it was possible to verify the rough and irregular shapes of the PET due to its crushing and processing.

Based on the evaluation of the physical and mechanical tests of soil-cement bricks, it was possible to verify improvement in their properties with the incorporation of PET. The average compressive strength with the incorporation of 20% PET managed to reach the value of 1.80 MPa, i.e., above 1.50 MPa which is the minimum value established by the Brazilian standard. As for water absorption, the bricks showed satisfactory values and complied with the values established by the standard, i.e., not presenting average values higher than 20% or individual values higher than 22%.

For future studies related to the results of this work, it would be important to consider that although the amount of water in the mixture increased according to the amount of PET, it could compensate with an additional hydration of cement particles, suggesting higher strengths of the bricks.

Thus, the incorporation of PET in soil-cement bricks can be considered an alternative for non-structural applications, such as closing walls in building construction. Moreover, this study verifies that it is possible to reduce the environmental impacts of this type of waste, as demonstrated in the bricks made with 20% PET waste. However, it is necessary to consider further studies regarding the life cycle of this type of material, especially its final cycle and durability, in order to enhance applications and avoid greater environmental impacts.

**Author Contributions:** Conceptualization, M.T.M., M.G., T.R.d.S.; methodology, T.R.d.S. and J.A.; software, M.T.M.; validation, M.G.; formal analysis, F.C.d.S., S.N.M.; investigation, M.T.M., M.G., T.R.d.S.; resources, A.R.G.d.A.; data curation, F.C.d.S. and F.G.F.; writing—original draft preparation, I.V., T.R.d.S.; writing—review and editing, A.R.G.d.A. and S.N.M.; visualization, I.V. and F.G.F.; supervision, I.V., D.C. and A.R.G.d.A.; project administration, D.C., A.R.G.d.A. and S.N.M.; funding acquisition, S.N.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

