## 4 Conclusions  
A simple and efficient method has been developed to determine the dynamic Young's modulus through the measurement of the vibrations induced on the surface of the finished mortar specimens. The Arduino low-cost accelerometer sensor used in this work reliably collected the propagation velocity values of mechanical disturbances through the designed mortars. The correlation between the results obtained by the traditional method based on the emission and capture of ultrasonic waves and the alternative method designed with Arduino accelerometers has been established, observing  
Figure 13: Microscopies performed on the cement mortar sample made with recycled concrete aggregate and cement/aggregate at a ratio of 1:3 at the age of 28 days: **(a)** Mortar matrix; **(b)** Ettrinigite crystal.  
a strong positive linear relationship. The fact that this correlation is present allows us to consider future applications of this measurement technique, designed for the determination of dynamic Young's modulus at the laboratory level, in industries involved in the production of cement mortars who may wish to use this system as a tool for quality control.  
Our approach allows us to more deeply understand the characterization of mortars, and it has been verified that mortars made with aggregates from CDW obtain lower values of their dynamic Young's modulus, which translates into a lower flexural strength than mortars made with natural aggregate. Of the two aggregates studied in this work, the recycled aggregate from ceramic waste had the worst properties for the production of cement mortars. The cement/aggregate ratio is also decisive in terms of the final properties of cement/aggregate, with the ratio of 1:4 having the lowest deformation capacity regardless of the nature of the aggregate used. In this way, the results reflect that dosage with a cement/aggregate ratio of 1:3 by weight would be optimal for use in the preparation of masonry mortars.  
The study of the evolution of mechanical properties over time has shown that resistance to flexion and compression increases rapidly at the beginning of the process and tends to stabilize its value after nearly 56 days. Besides this, in all cases, it was found that mortars made with natural aggregate had higher strengths; additionally, mortars that were more abundant in cement (with a 1:3 ratio) showed better results. The progression in these resistances is related to the evolution of the dynamic Young's modulus, as measured in all the mixes. Besides this, the compressive strength results showed that it is possible to use this type of mortar--made with recycled aggregate--for the production of masonry mortars to be used in the execution of brick facades.  
In addition, it was also possible to determine the lower density of recycled aggregates and their higher absorption coefficients, especially in recycled ceramic aggregate. The greater adherence of mortars made with natural aggregate, which have less fineness, which increases in cement/aggregate ratios of 1:3 compared to 1:4, was also shown. In the shrinkage test, it was possible to verify that the stability of the volume during setting was higher in the mixes made with natural aggregate than with mortars made with recycled ceramic aggregate, presenting higher shrinkage values regardless of the cement/aggregate ratio.  
Conceptualization, D.F. and C.M.; methodology, D.F.; software, E.Y. and E.G.; validation, D.F., E.Y. and E.G.; formal analysis, D.F. and C.M.; investigation, E.G. and E.Y.; resources, C.M.; data curation, E.Y.; writing--original draft preparation, D.F. and E.Y.; writing--review and editing, E.Y.; visualization, D.F.; supervision, C.M.; funding acquisition, C.M. All authors have read and agreed to the published version of the manuscript. This research was funded by Universidad Politecnica de Madrid. The authors want to express their gratitude to all the companies and laboratories that have collaborated and provided their facilities to carry out this work. The authors declare no conflict of interest.