Figure 1 shows a specimen tested with its cross section before and after test. For the evaluation of the mechanical properties of the material, the Universal Shimadzu AGS-X machine from the Structure laboratory of the Surcolombiana University was used, and the specimens were tested according to the regulations required in each case. In the compression test, 10 specimens were tested as stipulated in ASTM D 695-15 Standard Test Method for Compressive Properties of Rigid Plastics (19). For the bending test, 10 specimens were used according to ASTM D 790-03 Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials (20), and for the tensile test, 5 samples were tested according to ASTM D 638-14 Standard Test Method for Tensile Properties of Plastics (21).

For evaluation of the physicochemical properties, the flame propagation test, Differential Scanning Calorimeter (DSC) and the absorption test were carried out. The flame propagation test was carried out on 5 specimens in the EATIC Laboratory of Grupo EPM in the city of Medellín, under the ASTM D 635-18 Standard Test Method for Rate of Burning and / or Extent and Time of Burning of Plastics in a Horizontal Position (22). For the Differential Scanning Calorimeter, 2 groups of samples were tested, some for analysis and others for reference in the EAFIT university laboratory according to the ASTM D 3418-15 (23) and the water absorption test was established for an immersion period of 24 hours, testing 5 specimens according to the ASTM D 570 Standard Test Method for Water Absorption of Plastics (24).

To carry out the research work, the procedure shown in figure 2 was considered. The results obtained in the laboratories (Compression-Absorption) were compared with the requirements established in the standard NTC 4205 Masonry units of fired clay, bricks and ceramic blocks (25) which defines the minimum requirements for non-structural masonry units.

The compression test was carried out according to the procedure defined in the ASTM D695-15 standard at a speed of 1.27mm/min; the flexural tests were carried out following the guidelines of the ASTM D790-03 standard applying a load at a speed of 1.4mm/min; the tension tests were carried out according to the procedure presented in the D638-14 standard, applying a speed of 0.5mm/min; the Absorption tests were carried out according to the ASTM D570-98 standard. The flammability test was carried out according to ASTM D635-18, which allows to determine the ignition time, the burning rate, and the heat release of plastic samples in a small-scale horizontal position, at a distance of 20±1 mm with time. 30 second ignition; 5 specimens were taken with approximate measurements of 15 cm long, 2.10 cm high and 1.20 cm thick.

Thermal analysis of the HDPE samples was carried out by means of Differential Scanning Calorimeter (DSC) tests, according to the procedure outlined in ASTM D 3418-15. This test makes it possible to determine the amount of heat that a material absorbs or releases when it is subjected to changes in temperature at a constant rate. To carry out the tests, 12 ± 1 mg of HDPE material were weighed by means of an analytical balance, then each one of the samples was placed in hermetically sealed containers, and placed in the differential scanning calorimeter (DSC) from TA Instruments Q200; The test consisted of heating the samples from 0 ° to 200 °C at a constant heating rate of 10 °C/min in a helium atmosphere with a purge rate of 70 ml/min.

Figure 3 shows the deformation suffered by the specimens HPPE-PP compound under a constant compression force as a function of time. As can be seen, the figure 3 shows a typical curve of a flexible plastic (26) that is characterized by having a first section of curve in a proportional region between stress-strain, in which the deformations are relatively small and are associated with the extension of the existing bonds between the atoms of the plastic molecules; then, it passes to a region where the material still behaves as elastic and its deformations are a consequence of the stretching of molecules that are wound on themselves, so the deformation is recoverable, although not instantaneously; finally, the curve shows a region where the deformations are no longer recoverable, since they are a consequence of the sliding of some molecules with respect to others.

Each specimen was subjected to axial compression load to failure, recording ultimate resistance. During the test it was determined that the compressive strength of the HDPE-PP compound varies between 21.3 MPa and 25.6 MPa with an average of 23.40 MPa, finding resistance values higher than those reported in other studies. Some authors have carried out similar test on materials with different characteristics; Alkine (2020), evaluated the resistance of compression of bricks of clay with 5% Polypropylene terephthalate (PET), finding a compression resistance average of 2,3 MPa (16); on the other hand, Martinez (2014) evaluated a materials with a composited 50% of cement and 50% of PET, finding than a compression resistance average of 2,39MPa (17); Archila (2017), tested recycled plastic wood through compressions tests, finding a resistance average 8,92MPa (27); and Flores (2019), carried out a characterization of the PET material as a material for masonry, reaching a compressive strength of 10.78 Mpa (28).

In Figure 4, the results obtained from the compressive strength for each test specimen were compared with the strength of non-structural fired clay bricks and concrete. Table 1 shows the minimum values reequipments for compressive strength for non-structural masonry units, as established in the Civil Engineering and Architecture Standard for fired clay masonry units (NTC 4205-2). Where HHB: Horizontal Hollow Bricks, VHB: Vertical Hollow Bricks, and SB: Solid.

As can be seen in table 1, for a vertical hollow bricks unit, the regulations require a minimum compressive strength of 10 MPa; According to the results of the tests carried out, the compressive strength of the HDPE-PP compound complies with all the specimens tested individually, indicating a high degree of compressive strength of the material. The above since this material is made of polyethylene, which provides better mechanical properties by having a higher density

For this test, 10 specimens were taken, of which, half were tested on the axis of greatest inertia (longitudinal axis) and the other half on the axis of least inertia (transverse axis).

The tested specimens were subjected to bending stresses at speed of 1.40 mm/min to determine their behavior and resistance to breakage; the results obtained are in the range of 15.06 MPa and 22.35 MPa, with an average value of 18.04 MPa for the section with the least inertia and a standard deviation of 2.95. For the section with the greatest inertia, values between 19.34 MPa and 24.68 MPa were obtained, with an average value of 21.20 MPa and a standard deviation of 2.20 with a confidence interval of 95%. Bearing in mind the above, it is assumed that the population from which the sample comes can be represented by the normal distribution. Higher flexural strength results were obtained compared to those reported by other authors, such as in Akinyele et all, 2020 who determined flexural strength values of 11.96 MPa for a dosage of 5% sodium terephthalate. polyethylene in addition to clay in fired bricks(16).

Regarding the research by Archilla et al, 2017 in recycled plastic wood specimens composed of high- and low-density polyethylene, results were obtained for bending stresses on the side with the least inertia of 13.01 Mpa (27). The conclusions reached Londoño & González Villa, 2009 indicate that for materials composed of low-density polyethylene, flexural strengths are reached on the longitudinal side of 14.01 MPa and on the transverse side of 11.61 MPa, with the values being much higher than the results achieved in this research, which indicates that the material composed of high-density polyethylene HDPE is an indicator of great rigidity and resistance (18). On the other hand, Flores (2019) determined that the flexural strength of PET as a material for masonry reaches a flexural strength of 126.7 MPa, finding higher values than those found in the present study (28).

The flexural strength should theoretically vary between 10% and 30% of the compressive strength (29), and the minimum compressive strength per unit according to NTC 4205 for non-structural masonry is 10 MPa. Bearing in mind the above, it is established that the minimum value of resistance to bending of the tested material must be 3 MPa. Therefore, it can be deduced that all the study specimens complied with the minimum required, given that the minimum flexural stress obtained was 15.1 MPa and the maximum was 24.7 MPa; this indicates that the material has a high rigidity when faced with the request of different loads applied perpendicular to its longitudinal axis (see figure 5).

According to Figure 5, the stress-strain relationship is established for each tested specimen. A characteristic behavior of elastic solids that store a certain amount of energy when subjected to minor stresses is detailed; while with a higher effort, they continuously deform like a fluid (30). In Figure 6, the results obtained from the flexural strength for the 10 specimens tested with non-structural fired clay bricks were compared to those of HDPE

This test applies the load perpendicular to its plane to equalize the behavior against similar stresses, generating bends parallel and perpendicular to the direction of the load, allowing to determine the maximum resistance achieved by the HDPE-PP compound before breaking. Figure 7b and 7c shows the procedure carried out to failure of the tensile test specimens.

According to figure 7a, two phases can be distinguished. At first, their stress-strain relationship has a linear elastic behavior, and then a loss of rigidity is observed until failure is reached, it is at this point, where the ductility of the material is reduced as that the load and the tensile modulus increase

It was found that the HDPE-PP compound has a tensile strength that is with a maximum value of 12.98 MPa and a minimum of 11.46 MPa. Its average of 12.20 MPa can be classified according to Méndez y Coreño as a flexible plastic, however, the determined elastic modulus of 590.75 MPa is considered to be high (31). The foregoing is presented by the interaction of the two polymeric materials that form the compound. On the one hand, polypropylene is characterized by its flexibility and mechanical resistance, and on the other hand, high-density polyethylene is characterized by having greater hardness and rigidity, thus allowing the compound to reach a high modulus.

The studies done by Pluijm were taken as a reference, in which he found tensile strength values that ranged between 1.5 and 3.5 MPa for clay and calcium silicate bricks (32). On the other hand, according to the experimental results of Sanchez and Mejía for conventional clay blocks, the value is between 0.35 and 1.4 MPa (33). The tensile strength results obtained in the HPDE-PP compound test are much higher than those presented by Pluijm and Sanchez, which represents the difference in the flexibility properties of masonry with respect to polymers (32, 33).

The flammability test allows analyzing the behavior of the HDPE - PP compound with respect to exposure to fire. Figure 8 shows the procedure that was carried out on the flammability test. Figure 8a presents the initial test conditions, and figure 8b show Drip start, flame withdrawal and determination of fire propagation/burning speed.

According to Hilado, 1968 one of the most important characteristics of plastics is the propagation of the flame, which can be defined as the speed of movement of a flame front under certain combustion conditions (34). This characteristic provides a measure of fire risk, since the spread of the flame on the surface can transmit fire to more flammable materials in the vicinity and, therefore, enlarge a conflagration. The flame spread of a material can be measured according to the burning rate, the burning distance of the flame, the flame spread factor and the height of the flame. Figure 9 shows the combustion rate and the ignition time for the 5 tested specimens.

According to the results, when the combustion reaches 25mm, the ignition time count starts and ends when it reaches a length of 100mm. The average burning speed obtained for the five samples was 9.64 mm/min; none of the samples tested self-extinguished, and continuous dripping was observed in all. The flame passed the 25 mm benchmark within 150 seconds in all samples, reaching an average ignition time of 505.2 seconds. The highest ignition time and the lowest burning rate show that the material can be classified as low flammability according to Suharty et al. (35).

Figure 10 shows the minimum absorption percentage for test piece 1 of 0.0436% and the maximum for test piece 2 of 0.4359%, with an average of 0.19%. This is related to the properties that characterize the HDPE material, as is non-hygroscopic, that is, water does not penetrate the material and remains on the surface. The variability in the results obtained is related to the internal structure of the material, the porosity present in the specimens is not uniform and varies according to each specimen tested.

The results of water absorption tests were relatively negligible, and this makes a material ideal for high humidity environments, because the increase in the mass of sub-merged water during the immersion time is minimal and does not affect its internal structure. According to Martínez et al. the absorption of water in materials constitutes a determining factor that limits its usefulness for construction, since this phenomenon can generate great dimensional deformation under the action of water (36).

On the other hand, the absorption results found in this study are similar to those reported by Krutibash et al (2020) who found that the absorption varies between 0.03% and 0.279% for compounds of 70% PP and 30% a glass/jute/sisal fiber (37). The results of the absorption test yielded values similar to those reported by Flores (2019), who found that the PET compounds yielded an absorption percentage of 0.27%, finding values similar to those found in this study (28).

Table 2 shows the maximum values of water absorption (%) in 24 hours of immersion, for non-structural masonry units in interior and exterior use, according to the provisions of the Civil Engineering and Architecture Standard for masonry units of fired clay (NTC 4205-2). Where HHB: Horizontal Hollow Bricks, VHB: Vertical Hollow Bricks, and SB: Solid Bricks.

According to the results obtained and when comparing with the water absorption requirements in 24 hours of immersion (average and individual maximum) of table 2, all the tested specimens are below the maximum values required in NTC 4205, which makes this material ideal for indoor and outdoor use (facades), as it will have good resistance to weathering, as it has high durability and resistance to atmospheric agents.

Figure 11 shows two peaks that show the solid-liquid phase transition and represent endothermic thermal events related to the melting points of two different materials that make up the sample. The first melting point corresponds to a temperature of 132.78 ° C, and the second is reached at a ap-proximate temperature of 161.8 ° C. The first melting point (132 ° C) is related to the melting temperature of HDPE and the second melting point (161.8 ° C) is related to the melting of poly-propylene. The results obtained in the thermal analysis coincide with that reported by Sutar et al. (2018) (38), who determined the DSC properties for specimens with different compositions of HDPE with PP; concluded that with a dosage of 80% HDPE and 20% PP, there are melting points of 134.8°C for HDPE and 162.2°C for PP. This behavior is due to the fact that PP crystallizes faster than HDPE, therefore, it slows down the nucleation of polymeric compounds.

In Figure 11 it is observed that, at a temperature of 80 ° C, a glass transition begins that is common in amorphous polymers, that is, polymers whose chains are not defined according to a crystalline ordering. That is, after 80°C, the compound becomes rubbery, losing its mechanical resistance properties. Asensio (2018) affirms that, by exceeding the glass transition temperature, the molecules change their hardness, density, rigidity and percentage of elongation; and each material has a different glass transition temperature, as it is an intrinsic property of each one (39).

On the other hand, according to the results of Figure 11, it is concluded that the material does not show degradation processes for a temperature range from 0 °C to 200 °C.