1. Introduction
The circular economy refers to a production process in industry in which the materials that contain waste are reincorporated repeatedly for the elaboration of new products and raw materials [
1]. Currently, the vast majority of generated construction and demolition waste (CDW) ends up being sent to a landfill—an action that contradicts the objectives of the 2030 horizon, where the importance of recycling and reusing all waste that can be recycled or reused is highlighted [
2].
The construction sector is a prevalent consumer of raw materials. The high use of aggregates for the production of mortars and concretes stands out to the extent that approximately 85% of this material is used for this purpose [
3]. However, for the aggregates to be used in the manufacture of mortars and concretes, they must have an adequate size and geometry. It is necessary to carry out crushing and separation processes for the CDW if recovery is to be efficiently achieved [
4]. Various factors come into play in this process—the origin of the waste, the treatment and washing of the aggregates, the separation at source, etc.—and crucially influence the quality of the final product obtained [
5,
6,
7]. Two types of recycled aggregates have been used to carry out this work: concrete and ceramic. Concrete recycled aggregate is obtained from specific structural waste recycling processes with a low content of ceramic and bituminous particles (it is considered to be recycled concrete aggregate if it contains more than 90% of concrete) [
8,
9]. On the other hand, recycled ceramic aggregate has a content of ceramic material exceeding 90% and is characterized by higher absorption and a lower density [
10].
A fundamental characteristic of mortars, which are known for their subsequent application on-site or their use as a restoration material, is their deformation capacity under the different stresses that may appear. It is well known that excessively rigid mortars vary the building’s structural behavior as a whole and increase the risk of cracks and other constructive pathologies [
11]. Thus, the determination of the Young’s modulus of construction materials is essential to improving our understanding of the relationship between the stresses and deformations of the content [
12]. This physical property becomes even more critical in the case of cement mortars made with recycled aggregate, as highlighted by some investigations. These have a negative influence, causing a decrease in the mechanical resistance to bending and compression, as well as more significant shrinkage during drying and lower adherence values [
13,
14].
Among the measurement techniques of Young’s modulus in the laboratory, we find static tests employing the application of a progressive load that allows the deformation produced when the material is under particular stress to be collected, along with dynamic tests. The best-known approach is the ultrasound method, which allows the dynamic Young’s modulus to be determined with the help of the propagation speed of the ultrasonic waves in the material under study [
15,
16]. Among the alternative techniques successfully applied in the last decades for the determination of this dynamic Young modulus in mortars, non-destructive tests consisting of the application and measurement of acoustic waves generated by the impact on the material have gained great importance [
17]. In this way, Rosell, J.R. and Cantalapiedra, I.R. have adapted the test methodology included in the UNE-EN ISO 12680-1 [
18] standard. To determine the dynamic Young’s modulus in refractory products, tailored in lime and cement mortars, a strong correlation between the results was obtained with this method, and the measurements were collected with the aid of strain gauges [
19,
20]; this correlation has also been studied in buildings in Italy, in which a strong relationship between the evolution of compressive strength and dynamic Young’s modulus was observed [
21]. In contrast, other studies have opted for the use of measurement techniques such as infrared thermography that allow imperfections and preferential breakpoints that decrease the elastic behavior of the material to be detected [
22,
23]. On the other hand, other studies have shown how water saturation in mortars decreases their static and dynamic mechanical strength [
24]. Finally, other authors have chosen to determine the influence of mixing water on the evolution of the resistance of mortars made with recycled aggregate, where a strong relationship was obtained between the age at which the binder material was tested and the increase of mechanical resistance [
25].
Over the past decade, many research projects have emerged that use Arduino technology in their measurement equipment. Barroca et al. have used low-cost equipment to monitor humidity and temperature properties within concrete structures and collect data in real-time [
26]. Some authors have used this equipment to determine material properties such as setting times using resistive sensors that vary their electrical conductivity as the material dries out [
27]. Craveiro et al. show that it is possible to use automated systems with Arduino to manufacture construction elements with pre-established thermo-mechanical performances [
28]. Furthermore, studies aimed at accurately monitoring and measuring the dynamic Young’s modulus are especially relevant. In this sense, the work of Panda et al. [
29] analyzed the mechanical properties in terms of deformation of some construction materials with the aim of extrapolating the results to large-scale printed concrete manufacturing [
30].
This work aims to study the evolution of the dynamic Young’s modulus in cement mortars made with recycled aggregate and its relationship with the material’s physical and mechanical properties. A further aim is to validate a new measurement system to determine the speed of wave propagation through the mortar based on low-cost Arduino sensors so that these measurements can be correlated with those collected by the traditional method of ultrasound.
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 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.