**3. Results and Discussion**

This section presents the results derived from the experimental campaign proposed for this study and their discussion.

#### *3.1. Mechanical Characterization Tests*

Next, Figures 2 and 3 show the results of flexural and compressive strength tests carried out on the 4 × 4 × 16 cm prismatic specimens of plaster mortar.

**Figure 2.** (**a**) Flexural strength test according to UNE-EN-13279-2: 2014; (**b**) results derived from flexural strength test on the prepared mortars.

**Figure 3.** (**a**) Compressive strength test according to UNE-EN-13279-2: 2014; (**b**) results derived from compressive strength test on the prepared mortars.

Figure 2 shows the improvement in flexural strength of plaster materials by incorporating sand into their constitution. All mortars with the incorporation of aggregate without insulation exceeded the E0.8 plaster reference, with mortars made with natural

sand showing the highest values. On the other hand, it can be seen how the incorporation of expanded polystyrene residue with graphite in the manufacture of mortars reduces flexural strength. This is because preferential breakage points occur between EPS spheres and plaster mortar matrix, thus generating greater heterogeneity that negatively affects the mechanical behaviour of the material [55]. On the other hand, the incorporation of mineral wool residue improves the flexural strength of the hardened plaster mortar. This is in line with other studies that highlight the beneficial effect of the incorporation of fibres in the mortar matrix to improve its ductility and deformation capacity [56,57]. Regarding recycled aggregates, mortars that incorporate sand from concrete waste have better performance than mortars with aggregates from mixed ceramic waste.

Furthermore, Figure 3 shows how the incorporation of aggregates improves the compressive strength of plaster material, where mortars made with natural aggregates are the ones that presented the best results. Among mortars with recycled aggregate, the ones that incorporated RAcon obtained the greatest resistance. Finally, it can be seen how the incorporation of EPS residue also decreases the compressive strength of mortars, while the incorporation of mineral wool residue is not decisive in improving this mechanical property [58].

Table 10 shows the results derived from the analysis of variance (ANOVA) carried out to determine the effect of the factors included in the study on the mechanical behaviour of plaster mortars.


**Table 10.** Analysis of variance (ANOVA) for flexural and compressive strength.

As can be seen in Table 10, both in mechanical resistance to bending and in compressive strength of mortars, the two factors included in this study (type of aggregate and thermal insulation residue) are statistically significant, having *p*-values lower than the level of significance (α = 0.05).

Finally, Table 11 includes the results obtained for the multiple range test performed for the mechanical properties of mortars.

**Table 11.** Multiple range test for mechanical properties.


In the multiple range test shown in Table 11, it can be seen how there are significant differences for mechanical resistance to bending at all levels for the two factors analysed in this study. On the other hand, for mechanical compressive resistance, there are significant differences at all levels when we refer to the aggregate type of factor. However, in the case of the incorporation of thermal insulation residues, it cannot be affirmed that there are significant differences between plaster mortars without thermal insulation and those that incorporate mineral wool fibre, both types of mortars presenting greater resistance to statistically significant compression compared to plaster mortars with EPS.

#### *3.2. Physical Characterization Tests*

This section includes the tests for the physical properties of plaster mortars carried out in this work. These tests have also been carried out on 4 × 4 × 16 cm specimens, and include the following measurements: bulk density, Shore C surface hardness, longitudinal Young's modulus determined by ultrasound, and thermal conductivity coefficient. These are parameters that allow a characterization of the material to later define its possible uses and applications in the building sector.

Figure 4 shows the method used to perform physical characterization tests, and, in Table 12, the results obtained for each of the properties are presented.

**Figure 4.** (**a**) Shore C hardness test; (**b**) water absorption by capillarity test; (**c**) determination of the coefficient of thermal conductivity; (**d**) ultrasound test.


**Table 12.** Physical characterization tests of plaster mortars.

From the results presented in Table 12, it can be seen how all plaster mortars have a higher density than reference plaster E0.8 due to the incorporation of aggregates in their dosage. In addition, among the elaborated mortars, it can be observed how those that incorporate recycled ceramic aggregate are lighter and how density is reduced when EPS residues are added to the composition of mixtures [59]. On the other hand, absorption of water by capillarity is reduced in plaster mortars. Absorption is lower when mortars are made with natural aggregate compared to those made with recycled aggregate [43]. In addition, in this case, the incorporation of EPS in plaster mortars makes it difficult for the water to rise by capillarity in the materials studied.

In addition, the surface hardness is also increased with the incorporation of sand in plaster mixes, with mortars made with natural aggregate having greater hardness and density being lower in mortars that incorporate EPS. On the other hand, longitudinal Young's modulus determined by ultrasound is also increased in mortars. This is in accordance with the greater mechanical resistance to bending obtained by plaster mortars compared to the reference sample E0.8 [60]. This Young's modulus is greater in mortars made with natural aggregate compared to mortars that incorporate recycled aggregate, furthermore, it decreases when EPS is incorporated in the mortar manufacturing process, while when MW is incorporated, a significant decrease it is noticed. Finally, and in accordance with the results obtained for density, plaster mortars have a higher thermal conductivity than reference E0.8 plaster [61]. However, this thermal conductivity is reduced with the incorporation of thermal insulation residues in mortar mixtures, obtaining lower conductivity values for plaster mortars with EPS than those incorporating MW.

Table 13 shows the results obtained for the analysis of variance (ANOVA) performed for the physical characterization tests, while Table 14 shows the results obtained after performing the multiple range test.

As can be seen in Table 13, all the *p*-values were lower than the level of significance (α = 0.05), which implies that both factors included in the design of experiments, type of aggregate and type of insulation, are statistically significant for all the response variables analysed in physical properties of plaster mortars.


**Table 13.** Analysis of variance (ANOVA) for flexural and compressive strength.


**Table 14.** Multiple range test for physical properties.

Table 14 shows the composition of homogeneous groups after the multiple range test. This table shows how mortars with natural aggregate have a higher surface hardness and a higher Young's modulus, in accordance with Table 12, and how the incorporation of thermal insulation residues decreases the values obtained in these physical properties. In regard to thermal conductivity, mortars with recycled aggregate have better performance for this property, reducing this conductivity with the incorporation of thermal insulation waste and especially EPS [62]. Finally, with respect to apparent density, it can be seen how mortars with recycled aggregate are lighter. This affects the results of the compressive strength test, but it can also be seen that there are no differences between homogeneous groups in mortars with mineral wool insulation and without insulation. Likewise, in the absorption of water by capillarity, mortars with a natural aggregate present the best behaviour. It can also be observed how the incorporation of thermal insulation residues reduces the height reached by the water in this test with respect to mortars without insulation.

#### *3.3. Tests on Prefabricated Plates and Blocks*
