3.1.2. Consistency Limits

The consistency limits of the soil used presented the plasticity index (PI) values in Table 2. Thus, it was possible to verify that the values were adequate, since the PI for the production of soil-cement bricks should be up to 18%, as shown in studies [45,46]. In addition, the results reached the reference values for the manufacture of soil-cement bricks, i.e., soils with the maximum limit of 45% for liquidity limit (LL) and 18% for plasticity limit (PL), as well as those recommended by the ABNT standard [35].


\* Liquidity Limit (LL), Plasticity Limit (PL) and Plasticity Index (PI).

Differently from the results of the soil (reference sample), the values of the PET-Soil cement mixtures showed decreased LL, PL and PI compared to natural soil. This occurred due to the addition of PET waste (sandy fraction), showing that with an increase in the percentage of addition, the greater the amount of water needed in the mixture to get out of its plastic state. This can be related to the workability of the mixture, i.e., the greater the amount of water added, the lower the workability of the mixture [47,48].

## 3.1.3. Moisture Content and Compaction Energy

The optimum compaction moisture results with the respective maximum densities of the mixtures are presented in Table 3.


**Table 3.** Optimum compaction humidity and maximum mixture densities.

The results in this table showed that with the increase of PET waste, there was an increase in the optimum moisture content of compaction and a decrease in the maximum specific weight. As for the decrease in maximum specific weight, this is due to the density value of the PET waste being lower than that of the natural soil. Moreover, the sample mass decreased as water was added to the compaction process because the waste did not have plastic properties. With this, as the water addition increased, it was possible to verify that the workability was decreasing, therefore this is also related to the compaction strength, as reported in study by Akinyele and Ajede [23]. The compaction curves of the mixtures are presented in Figure 4. Moreover, the curves proved (especially between 0% and 30%) the difference between the mixtures in relation to the addition of waste, i.e., the higher the percentage of addition, the greater the difficulty of reaching the optimum moisture content with increasing percentage of PET.

**Figure 4.** Compaction curves and optimum moisture of mixtures. Percentages (%) indicate the amount of water content in each mixture (0%, 10%, 20% and 30%).

3.1.4. Microstructural Characterization

A morphological analysis in Figure 5 by SEM showed that PET presents high surface area and irregularities in the shape and size of its particles. These are typical of different cutting and crushing processes of the waste, as shown in studies by Siqueira and Holanda [49]. As the particles present an irregular distribution, smaller sized particles can occupy the free spaces left by larger sized particles, forming particle packing [50].

**Figure 5.** Scanning Electron Microscopy (SEM) of PET 60× magnification.
