## 3 Results and Discussion  
### Evolution of the Dynamic Young's Modulus over Time  
Figure 5 shows the variation in the longitudinal waves' propagation speed, which was generated using ultrasound impulses transmitted through the different mixes. Furthermore, since the mortars are solid materials, the positive effect of the higher density of mortars made with aggregate is noted as being natural as it has a greater top speed of propagation through the ultrasonic waves [55]. Mortars made with recycled ceramic aggregate have the lowest propagation speed values. In all cases, mixes with a cement/aggregate ratio of 1:3 had the highest propagation speeds due to their greater compactness and cement content. Figure 6 shows the evolution of the dynamic Young's modulus against time. In all of the samples analyzed, the development of this property followed a logarithmic progression, with the highest values being found in the specimens made with natural aggregate--a result that is in accordance with the values obtained in Figure 5. Ceramic presented the lowest dynamic Young's modulus values; furthermore, cement/aggregate ratio dosages of 1:4 showed lower costs than their 1:3 ratio counterparts. Figure 5: Variation of the propagation speed of ultrasound (m/s) versus time (days) for the prepared mortar specimens. Figure 6: Evolution of the dynamic Young’s modulus (MOE) (GPa) versus time (days) for the prepared mortar specimens. Evolution of the Dynamic Young's Modulus and Its Relationship with the Measures of the Designed Equipment  
Figure 7 shows the evolution of the propagation speed of the longitudinal waves produced by impact through the mortar. These values were measured with the help of the Arduino sensor system designed for this study and in the same specimens as those in the ultrasound test. As can be seen in Figure 7, the behavior of the propagation velocity values calculated through the accelerometer measurements reflect a similar trend to the velocity measurements captured with the ultrasound equipment. On the other hand, Figure 8 presents the correlation between the measurements collected with the ultrasound method and the Arduino equipment with an accelerometer. The values of the regression lines obtained and their coefficient R\({}^{2}\) for each type of mortar are collected in Table 6.  
As can be seen in Figure 8, supported by Table 6, the speeds calculated through the time measurements collected with the MMA8451 sensor and the rates taken with the ultrasound equipment follow a strong positive correlation. Thus, it is possible--with the appropriate calibration of the accelerometer--to relate both measurements to the calculation of the dynamic Young's modulus in cement mortars. Furthermore, the ease of the execution of the test, as well as the low cost of the Arduino equipment, allow us to propose the widespread application of this measurement method  
Figure 8: Correlation between the speeds measured with ultrasound and the rates measured with the Arduino equipment. **(a)** Mortrars with a cement/aggregate ratio of 1:3; **(b)** mortars with a cement/aggregate ratio of 1:4. Figure 7: Evolution of the speed calculated through the time measurements collected with the Arduino equipment. as a control tool in mortar production plants and for the characterization of this type of material in laboratory tests. Evolution of the Dynamic Young's Modulus and Its Relation to the Physical Properties of the Processed Mortars  
Firstly, Table 7 presents some of the most relevant physical properties of cement mortars that were measured at the age of 28 days, as indicated by the UNE-EN 196-1 standard [53]. Analyzing Table 7, we can see that the adherence represented by the ability of the mortars to absorb normal stresses to the application surface is higher in mortars made with natural aggregate. This is due to the higher fineness content and impurities of aggregates from CDW, which decrease their adherence. Furthermore, it can be seen that the higher the cement content in the mix, the higher the adhesion values [56].